Dysregulated Glutamate Transporter SLC1A1 Propels ......2020/12/01 · Jing Ling6, Qi Li6, Yadi...

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Dysregulated Glutamate Transporter SLC1A1 Propels Cystine Uptake via Xc-

for Glutathione Synthesis in Lung Cancer

Wenzheng Guo1, 9 ‡

, Kaimi Li1, 10,‡

, Beibei Sun3, ‡

, Dongliang Xu1,2

, Lingfeng Tong2, Huijing Yin

1,

Yueling Liao1,2

, Hongyong Song1,2

, Tong Wang1,2

, Bo Jing1,2

, Min Hu1,2

, Shuli Liu4, Yanbin Kuang

5,

Jing Ling6, Qi Li

6, Yadi Wu

8, Qi Wang

5, Feng Yao

7, Binhua P. Zhou

8, ,Shu-Hai Lin

11*, Jiong

Deng1,2,3,*

1Key Laboratory of Cell Differentiation and Apoptosis of Chinese Minister of Education, Shanghai

Jiao Tong University School of Medicine, Shanghai, China;

2Shanghai Key Laboratory for Tumor Microenvironment and Inflammation, Shanghai Jiao Tong

University School of Medicine, Shanghai, China;

3Translational Medical Research Center, Shanghai Jiao Tong University, Shanghai, China;

4Department of Oral and Maxillofacial–Head and Neck Oncology, the Ninth People’s Hospital,

College of Stomatology, Shanghai General Hospital, Shanghai Jiao Tong University School of

Medicine, Shanghai, China;

5Department of Respiratory Medicine, The Second Affiliated Hospital, Dalian Medical University,

Dalian, China

6Department of Oncology, Shanghai General Hospital, Shanghai Jiao Tong University School of

Medicine, Shanghai, China

7Department of Thoracic Surgery, Shanghai Chest Hospital; Shanghai Jiao Tong University,

Shanghai, China

8Department of Molecular and Cellular Biochemistry, Markey Cancer Center, University of

Kentucky College of Medicine, Lexington, KY, USA

9Department of Laboratory Medicine, Shanghai East Hospital, Tongji University School of Medicine,

Shanghai, China;

10Department of Pathology, Molecular Pathology Research Center, Peking Union Medical College

Hospital, Chinese Academy of Medical Sciences and Peking Union Medical College, Beijing, China

11State Key Laboratory of Cellular Stress Biology, Innovation Center for Cell Signaling Network,

School of Life Sciences, Xiamen University, Xiamen, Fujian 361102, China.

‡These authors contribute equally to this work

*Correspondence: Jiong Deng, Shanghai Jiao Tong University School of Medicine, 280 South

Chongqing Rd., 2-801, Shanghai 200025, China; Tel: (08621)-64666338; Email:

Research. on August 30, 2021. © 2020 American Association for Cancercancerres.aacrjournals.org Downloaded from

Author manuscripts have been peer reviewed and accepted for publication but have not yet been edited. Author Manuscript Published OnlineFirst on November 23, 2020; DOI: 10.1158/0008-5472.CAN-20-0617

http://cancerres.aacrjournals.org/

2

[email protected]; Shu-Hai Lin, State Key Lab of Cellular Stress Biology, School of Life

Sciences, No. 4221-120 South Xiangan Rd., Xiamen, Fujian 361102, China; Tel; (086)-

15021534583; Email: [email protected]

Running title: SLC1A1 facilitates cystine uptake

Key words: Glutathione, Glutamate, SLC1A1, GPRC5A, Lung tumorigenesis

Competing interests

The authors declare that they have no competing interests

Funding

This work was supported by grants from National Nature Science Foundation of China 81620108022

(JD), 91957120 (SHL), 91129303(JD), 91729302 (JD), 81572759 (JD), 81902338 (WZG) and

81672911 (SHL).

Abbreviations

GSH: glutathione; SLC1A1: a sodium-dependent glutamate carrier; ROS: reactive oxygen species;

GCL: glutamate-cysteine ligase; EAAT3 /EAAC1: excitatory amino acid carrier 1; GPRC5A: G

protein coupled receptor family C group 5 type A; NSCLC: non-small cell lung cancer; COPD:

chronic obstructive pulmonary disease; MTEC: mouse tracheal epithelia cells; TCA cycle:

Tricarboxylic acid cycle; xCT: x-Cystine-glutamate-transporter.



mailto:[email protected]



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Abstract

Cancer cells need to generate large amounts of glutathione (GSH) to buffer oxidative stress during

tumor development. A rate-limiting step for GSH biosynthesis is cystine uptake via a

cystine/glutamate antiporter Xc-. Xc- is a sodium-independent antiporter passively driven by

concentration gradients from extracellular cystine and intracellular glutamate across the cell

membrane. Increased uptake of cystine via Xc- in cancer cells increases the level of extracellular

glutamate, which would subsequently restrain cystine uptake via Xc-. Cancer cells must therefore

evolve a mechanism to overcome this negative feedback regulation. In this study, we report that

glutamate transporters, in particular SLC1A1, are tightly intertwined with cystine uptake and GSH

biosynthesis in lung cancer cells. Dysregulated SLC1A1, a sodium-dependent glutamate carrier,

actively recycled extracellular glutamate into cells, which enhanced the efficiency of cystine uptake

via Xc- and GSH biosynthesis as measured by stable isotope-assisted metabolomics. Conversely,

depletion of glutamate transporter SLC1A1 increased extracellular glutamate, which inhibited

cystine uptake, blocked GSH synthesis, and induced oxidative stress-mediated cell death or growth

inhibition. Moreover, glutamate transporters were frequently upregulated in tissue samples of non-

small cell lung cancer patients. Taken together, active uptake of glutamate via SLC1A1 propels

cystine uptake via Xc- for GSH biosynthesis in lung tumorigenesis.

Significance: Cellular GSH in cancer cells is not only determined by upregulated Xc- but also by

dysregulated glutamate transporters, which provides additional targets for therapeutic intervention.




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Introduction

Cancer cells are facing two emergent needs, an increased demand of metabolic building blocks

for growing biomass, and an increased demand of protection against reactive oxygen species (ROS).

Compared to normal cells, cancer cells experience increased environmental stresses, including

oxygen or nutrient deficiency, low pH, mediators of inflammation, and oxidative stress from ROS

(1). ROS can be generated from increased metabolism within tumor cells, and induced by pro-

inflammatory stimuli and TNFα in the microenvironment (2). High ROS levels in the tumor

microenvironment can be detrimental, since ROS exerts oxidative stress that can trigger senescence

or programmed cell death (1, 3). Thus, the metabolic profile of tumor cells, especially cancer stem

cells, must be reprogrammed to counteract the toxicity of ROS.

Glutathione (GSH) is the most abundant antioxidant within all cells. GSH is synthesized in a

two-step process. First, glutamate-cysteine ligase (GCL) catalyzes the formation of γ-

glutamylcysteine from glutamate and cysteine, which is the rate limiting step. Second, glutathione

synthetase couples glycine to γ-glutamylcysteine to form GSH. The availability of cysteine, from a

cystine transporter, is considered rate limiting for GSH synthesis. System Xc-, a Na

+-independent

cystine/glutamate antiporter composed of a light-chain subunit (xCT or SLC7A11) and a heavy-

chain subunit (CD98hc or SLC3A2), mediates the extracellular cystine influx for exchange of

intracellular glutamate efflux. Recent studies showed that CD44v could interact with and stabilize

xCT at the plasma membrane, which emphasizes the importance of GSH synthesis in maintaining the

stemness of cancer stem cells (4). Thus, the efficiency of cystine uptake via system Xc- is crucial for

the GSH-dependent antioxidant system (5). Xc- antiporter is driven by the concentration gradients of

glutamate and cystine across plasma membrane, which transports one cystine into the cell in the

expense of exporting one glutamate out. Thus, extracellular glutamate acts as a competitive inhibitor

for cystine uptake via system Xc- (6). Because high concentration of extracellular glutamate inhibit

cystine uptake via Xc-, it remains a paradox how cancer cells enhance cystine uptake while expel

glutamate in exchange for cystine. Presumably, cancer cells should evolve a mechanism to import

extracellular glutamate for maintaining a glutamate gradient across membrane, which can facilitate

cystine uptake via Xc- system for GSH biosynthesis.

Excitatory amino acid carrier 1 (EAAC1, or EAAT3), is a member of the EAAT family of high-

affinity, sodium-dependent glutamate carriers encoded by solute carrier family 1 member 1

(SLC1A1) (7). It is generally accepted that 3 Na+ ions and 1 H

+ are co-transported while 1 K

+ is

counter-transported with each glutamate molecule (8). EAAT family has five members, EAAT1-5.




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These proteins are “high-affinity” glutamate transporters. SLC1A1 (EAAT3) is mainly expressed in

the brain, but also in the kidney and the intestinal mucosa. Within the brain, SLC1A1 serves as the

predominant glutamate transporter. SLC1A1 uptakes synaptic released glutamate, the excitatory

neurotransmitter, in the inter-neuronal cleft. Deficiency in uptake of extracellular glutamate leads to

oxidative stress and exocitotoxicity (9). Slc1a1-deficient mice show age-dependent brain atrophy,

learning/memory dysfunction, and reduced brain GSH levels (10). Thus, SLC1A1 is important for

neuronal GSH synthesis. SLC1A1 is also present at low level in muscle and lung. However, the role

of glutamate transporters in cancer cells is largely unknown.

Lung cancer is the leading cause of cancer death worldwide (11). Revealing the mechanism of

lung tumorigenesis requires an animal model that resembles the pathological features of human lung

cancer. G protein coupled receptor family C group 5 type A (GPRC5A; also known as RAIG1), a

newly identified lung tumor suppressor gene, is preferentially expressed in lung tissues (12-14).

Gprc5a gene knockout (ko) in mice leads to development of spontaneous lung adenocarcinoma (13,

14), which is associated with chronic lung inflammation, persistent activation of NF-κB, EGFR, and

STAT3 signaling (15-17). Importantly, GPRC5A is repressed in most of non-small cell lung cancer

(NSCLC) and all of chronic obstructive pulmonary disease (COPD) (13, 18). Thus, Gprc5a-ko mice

provide a unique mouse model that resembles lung cancer development in human.

In this study, we investigated the metabolic reprogramming of lung tumors in Gprc5a-ko mouse

model. Unexpectedly, we found that glutamate transporter SLC1A1, dysregulated in lung cancer

cells, is tightly intertwined with cystine uptake and GSH biosynthesis. Our study demonstrates that

upregulation of glutamate transporter SLC1A1, via increasing glutamate influx, facilitates cystine

uptake and GSH biosynthesis for lung tumor development.




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Materials and Methods

Mice and tumorigenicity

Gprc5a-knockout (ko) mice, from Dr. R. Lotan (University of Texas M.D. Anderson Cancer Center),

were generated in a mixed background of 129sv × C57BL/6 as described previously (13). Mice were

maintained according to a protocol approved by Shanghai Jiao Tong University School of Medicine

Animal Care and Use Committee [experimental animal use permission No: SYXK (Shanghai) 2008-

0050] in the specific pathogen-free animal facility in the university. Eight-week-old wild-type

(Gprc5a+/+

) and Gprc5a-/-

mice were received 2 weekly i.p. injections of NNK (100 mg/kg of body

weight) (Midwest Research Institute, Kansas City, MO) dissolved in saline solution (0.9% NaCl) or

saline alone. Ten to twelve months later, mice were sacrificed, one lube of lung was fixed in paraffin

for H&E staining analysis, the rest lung tissues were homogenized in liquid nitrogen for extraction of

protein and RNA(17).

Derivation of Primary Mouse Cells

Primary mouse lung cancer cells (SJT-1601) were isolated from NNK-induced lung tumor of

Gprc5a-ko mouse. Cells were digested into single cells and seed the cells onto the palate over 3 days,

then passing on the living cells for passaging. Unless otherwise stated, SJT-1601 cell were cultured

in a humidified 37°C atmosphere of 21% oxygen, 5% carbon dioxide, in high glucose DMEM

(Invitrogen) supplemented with 10% fetal bovine serum (Gibco), penicillin and streptomycin.

Cell lines and Cell Culture

Mouse tracheal epithelia cells (MTEC) were obtained from normal tracheal tissue of 3-week-old

wild-type (wt) and Gprc5a-ko mice (C57 BL/6 X129sv) as described (15, 16). The MTEC cells were

cultured in K-SFM supplemented with epithelia growth factor (EGF, 5 ng/mL) and bovine pituitary

extract (50 mg/mL, Invitrogen). The established mouse lung tumor cell line, named SJT-1601, was

derived from the NNK-induced lung tumor of a Gprc5a-ko mouse. Human embryonic kidney cells,

HEK293T, and NSCLC cells (H292G, Calu-1 and HCC827) were tested and authenticated by DNA

typing at Shanghai Jiao Tong University Analysis Core. The 16HBE (Human bronchial epithelial

cells) cells were as described previously (19). HBEC cells were cultured in serum-free medium, and

all other cells were cultured in DMEM essential medium with 10% fetal calf serum, at 37℃ in a

humidified incubator in an atmosphere of 95% air and 5% CO2.

Tumorigenicity in nude mice




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Eight-week-old nude mice were injected with A549-shNC/SLC1A1 (1.5x106 cells) combined with

matrigel. Two weeks later, the volume of tumors were measured and further analyzed.

Reagents

Recombinant mouse fibroblast growth factor-basic (FGF, PMG0034) was from Gibco by life

technologies. D-Luciferin (S7763) was from Selleck.cn (Houston, TX, USA). EGF (E9644), Insulin

(I3536), BSA (V900933), Dulbecco,s Modified Eagle

,s Medium-high glucose (D0422-100ML),

GSH-MEE (#G1404), vitamin E analog (α-Tocopherol) (#T3251) and L-Buthionine-sulfoximine

(BSO)(B2515) were from Sigma-Aldrich. Matrigel Matrix (354262) was from Corning.

B27(1639356) was from Gibico. L-Glutamic acid (D316BA0011) was from BBI Life Sciences. L-

Glutamine (13

C5, 99%) (CLM-1822-H-0.5), L-Glutamic Acid (13

C5, 99%) (CLM-1800-H-1),

[13

C6,15

N2]-Cystine (CNLM-4244-H-PK) and D-Glucose (13

C6, 99%) (CLM-1396-5) were from

Cambridge Isotope Laboratories. Annexin V-FITC reagent (C1062S), ROS kit (S0033) was from

Beyotime.

Antibodies

The following antibodies were used: Anti-SLC1A1 antibody (12686-1-AP) for IHC was from

Proteintech. Anti-SLC1A1 antibody (#14501), Anti-cleaved-PARP (#5625), Anti-Phospho-p38

MAPK (#4511) and Phospho-p38 (#8690T) were from Cell Signaling Technology. Anti-RAI3

(Gprc5a) (sc-98884) was from Santa Cruz. Anti-β-ACTIN-HRP (PM053-7) was from MBL. Anti-

SLC1A1 antibody (#14501) was from Abcam. The antibodies were diluted according to

manufacturers’ instructions.

Soft Agarose

A 2% agarose solution was combined with DMEM-10%FBS (1:3 v/v; final concentration, 0.7%)

and added to 24-well plates (0.5 mL per well), and allowed to solidify for 10 minutes at 4oC. MTEC

or NSCLC cells (500 cells in 50 l medium) were added over the solidified agarose. A top 0.5 mL

agarose layer was added over the cells, consisting of the 2% agarose solution combined with DMEM

-10%FBS (1:6 v/v; final concentration, 0.35%) along with matrigel (1:30 v/v). The plates were

incubated in 37℃, 5% CO2 incubator. After 2 weeks, established colonies were counted and

photographed.

Sphere culture assay




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Mouse tracheal epithelial cells and mouse lung cancer cell were cultured in sphere assay medium:

DMEM/F12 medium with 0.4% BSA, 20ng/mL EGF, 20ng/mL FGF, 50ug/mL insulin and B27.

Low-adherent plates were used to optimize sphere formation with 1000 cells in 24-well plates or

2000 cells in 6-well plates. The media was changed every three days with sphere formation complete

in two to three weeks. To avoid necrosis, care was taken to limit sphere size.

Western blot

Cells were lysed with RIPA buffer or SDS buffer, and equal amounts of the protein samples were

mixed with loading buffer and boiled for 5 minutes. Proteins were resolved by SDS-PAGE and

transferred to nitrocellulose (NC) membranes. Non-specific binding sites were blocked with 5% (w/v)

non-fat dry milk in TBST, and the membranes were incubated overnight on a shaking platform at

4oC with the specific primary antibodies diluted in 5% BSA with 0.05% sodium azide. HRP-

conjugated secondary antibody was incubated at room temperature for 1h. Finally, the proteins were

visualized by exposure with Immobilon Western Reagents. All antibodies were diluted for use

according to manufacturers’ instructions.

Clonogenic assay

A549-shNC/SLC1A1 or Calu-1-shNC/SLC1A1 cells (600 cells per 6 cm dish) were seed in a culture

plate. The plates were incubated for 2 weeks in 37℃, 5% CO2 incubator. After washing the plate

twice with PBS, incubate with crystal violet for 15 minutes. Rinse with running water and take

pictures.

The FACS detection assay

The FACS protocol of detection the apoptosis of A549 cells using Annexin V was performed as

previously described (20) . The FACS protocol of detection of ROS of A549 cells was performed as

previously described(21) .

Living cell count

A549-shNC/SLC1A1 or Calu-1-shNC/SLC1A1 cells (2 x 105 cells) were seed in 6 cm dish, using

different media according to experimental conditions, regular DMEM (C), glutamine-depleted

medium (Gln-), and replenish of glutamate (Glu) in glutamine-depleted medium (Gln-Glu+). The

plates were incubated for 48h in 37℃, 5% CO2 incubator. Digesting cells and counting the living

cells using the Invitrogen™ Countess™ II FL (ThermoFisher SCIENTIFIC).




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RNA Extraction from Cells and qRT-PCR Analysis

Total RNA was isolated from cultured cells using an RNA simple Total Kit (TIANGEN). 1μg RNA

was reverse-transcribed using a FastQuant RT Kit (TIANGEN). Resulting cDNA was then diluted

1:20, and amplified with qPCR using 2XSYBR Green qPCR Master Kit (Bimake.cn) reagents in an

ABI StepOne Plus detection system. Cycling conditions: Heat ramp 95°C x 10min, extension (95°C

x 30s, 60°C x 30s,72°C x 30s) x 40 cycles, melt curve 95°C x 15s, 60°C x 1min, 95°C x 15s with

0.3°C increments. Fold change gene expression was calculated by normalization to GAPDH using

formula of 2-ΔΔCT.

Immunohistochemistry (IHC)

A tissue microarray composed of tumor and adjacent normal tissue was stained to identify SLC1A1

and GPRC5A proteins. The IHC protocol and score method were performed as previously described

(22). All antibodies were diluted for use according to manufacturers’ instructions.

Transfection and stably transfected cells

HEK293T cells were transfected with lenti-shRNA-SLC1A1, luciferase reporter plasmids and lenti-

SLC1A1 through PEI (plasmid: PEI 1:4) transfection reagents, and the media replaced 4-6 h post-

transfection. The lentivirus medium collected after 48 hs was used to infect MTEC cells and NSCLC

cells. Stably transfected cells were selected by puromycin (2 g/mL) until all cells fluoresced green

(GFP-fusion protein in the vector).

Glutamate Analysis

The level of glutamate in the MTEC cell line and in the lung tumor cell line pellets was detected

using the Glutamate Colorimetric Assay Kit, and performed according to the methods provided in

the kit.

Quantitative real-time (Q)-PCR

Human lung cancer tissues samples were obtained from Shanghai Chest Hospital, Shanghai Jiao

Tong University (Shanghai, China). Cells or tissues were lysed with Trizol, then total RNA was

extracted with RNA Extract Kit and cDNA were prepared from 1.5ug of total RNA using Fast Quant

Kit. All mRNA were detected by ABI 7300 real-time PCR machine; the PCR primers are presented

in Supplementary Table1.

Aldefluor assay and analyzing the ALDH positive cell by FACS




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The Aldefluor assay Kit was used to analyze the population of high ALDH enzyme positive cells.

A549, Calu-1 and SJT-1601 cells (4 x105 cells/mL) were suspended in Aldefluor assay buffer with

the activated reagent, then dividing into two aliquots containing ALDH substrate (BAAA, 5ul/2 x105

cells). Negative control aliquots were treated with a specific ALDH inhibitor (DEAB, 5l/2 x105).

Aliquots were incubated for 45 minutes at 37oC. The gates were established using the negative

controls cells stained with DEAB. Finally, the data was analyzed in the software.

Stable isotope-assisted metabolomics in vitro and in vivo

For the cell metabolic flux analysis, A549 shNC/SLC1A1 cells and Calu1 shNC/SLC1A1 cells were

cultured to 90% density, then changed to glutamine-deficient medium with 2 mM [U-13

C5]-

Glutamine or 2 mM [U-13

C5]-Glutamate or 0.26 mM [13

C6,15

N2]-Cystine. After 12h, the labeled cells

from each sample were harvested in 1.6 mL 80% (v/v) methanol solution. For comparison, cells were

also cultured in glucose-deficient medium with 15mM [U-13

C6]-Glucose and were harvested in 1.6

mL 80% (v/v) methanol solution as outlined above.

For the metabolic flux analysis in vivo, on the first day, mice were surgically intubated through the

jugular veins and allowed to recover for 2 to 3 days. Infusions were performed using [U-13

C5]-

Glutamine as the tracer into mice using a microfluidic pump connected to the cannula for 6 h

(2mg/kg/min). Lung tumor and para-cancer normal tissue were isolated, and then quenched in liquid

nitrogen and store at -80 °C for mass spectrometric analysis.

Statistics

Data were analyzed using the software SPSS Statistics (IBM, Version 19) Data are presented as the

mean + standard deviation. The differences of results were compared using two-tailed paired t-test

assuming unequal distribution. Multiple group comparisons used one-way analysis of variance

(ANOVA). p-value <0.05 was considered to be statistically significant.

Other methods, including GC-MS analysis, UHPLC-qTOF-MS analysis, Quantification of Cystine

in condition medium, Untargeted metabolomics by liquid chromatography (LC)-MS/MS, In vivo

biodistribution study in mouse model, In vivo FITC-Glutamate distribution study in mouse models,

Pathway analysis and tissues metabolomics, refer to Supplementary Methods




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Results

Dysregulated GSH synthesis and glutamate transporter Slc1a1 are associated with lung

tumorigenesis in Gprc5a-ko mice

To determine the mechanism of lung tumorigenesis, we applied carcinogen-induced lung tumor

model in Gprc5a-ko mice for characterization. Wild-type (WT) and Gprc5a-ko (KO) mice were i.p

injected with NNK (100 mg/kg body weight, once a week for twice) at two months of age; lung

tissues were harvested 12 months later. All of Gprc5a-ko (KO-NNK-14m) mice (12 of 12, 100%)

developed lung cancer, whereas none of wild-type (WT-NNK-14m mice) mice (0 of 12) did (Fig. 1A)

(13). Next, comprehensive metabolomics analysis was performed to characterize the metabolic

profile in the lungs of all groups (Fig. 1B). Notably, the GSH level is dramatically increased in the

Gprc5a-ko-NNK14m group, the only group with lung tumor, compared to the other groups (Fig. 1C).

In addition, glutamate is moderately increased in lung tissues from Gprc5a-ko mice compared to

those from WT mice (Fig. 1D). Among other intermediate metabolites of glutathione synthesis,

glutathione-oxidized (GSSG), S-methylglutathione, glycine, cysteine, and cystine were slightly

increased in lung tissues of Gprc5a-ko-NNK14m mice compared to those of WT-NNK14m mice

(Supplementary Fig. S1A-E), whereas other metabolites, such as cysteine-glutathione disulfide,

glutamine, 5-oxoproline and ophthalmate, were of no significant difference (Supplementary Fig.

S1F-I). Taken together, increased level of glutathione is highly associated with lung tumorigenesis in

Gprc5a-ko mice.

To determine the mechanisms involved, we performed RNAseq analysis using mouse

tracheal epithelial cells (MTEC) from wild-type (WT) and Gprc5a-ko (KO) mice. Pathway analysis

showed that the most dramatically changed pathways are the metabolic pathways (Supplementary

Fig. S2A). Among the differentially expressed genes in amino acid metabolism, we noticed that

Slc1a1 was significantly upregulated in MTEC-KO compared to MTEC-WT (Fig. 1E). Upregulated

Slc1a1 was validated by Western blot (Fig. 1F) and quantitative real-time (Q)-PCR analysis,

respectively (Fig. 1G-H). Slc1a1 encodes EAAC1, a member of the high-affinity glutamate

transporters, playing an essential role in transporting glutamate across plasma membranes in neurons.

To validate the biochemical role of upregulated Slc1a1, we examined the glutamate level in MTEC

cells using a glutamate colorimetric assay. Indeed, glutamate concentration was significantly higher

in MTEC-KO cells than in MTEC-WT (Fig. 1I and Supplementary Fig. S2B). Consistently, the

glutamate level in condition medium (CM) of MTEC-KO cells is significantly lower than that of

MTEC-WT (Fig. 1J), suggesting that MTEC-KO cells uptake more glutamate from medium than

MTEC-WT cells do. Thus, upregulated Slc1a1 enhances glutamate uptake in Gprc5a-ko lung




12

epithelial cells. Notably, upregulated Slc1a1 was also found in mouse lung tumor cell line SJT-1601

that was derived from Gprc5a-ko mouse lung tumor (Fig. 1K), supporting that lung tumorigenesis is

beneficial from upregulated Slc1a1. Re-expression of Gprc5a in SJT-1601 cells inhibits Slc1a1 (Fig.

1K), suggesting that upregulated Slc1a1 is resulted from Gprc5a-deficiency. Upregulated SLC1A1

was also observed in human lung cancer cell line, whereas overexpression of GPRC5A inhibited

SLC1A1 expression in A549 cells (Supplementary Fig. S2C). To determine Slc1a1 expression in

vivo, we examined Slc1a1 levels in lung tissues via immunohistochemistry (IHC) staining. We found

that, SLC1A1 IHC scores in S/TB, but not the alveolar region, was significantly upregulated in

Gprc5a-ko (KO) mouse lungs compared to those of wild-type (WT) ones, both in 2m and 14m-NNK

groups (Fig. 1L-M). Taken together, these results suggest that increased GSH and upregulated

glutamate transporter Slc1a1 are associated with lung tumorigenesis in Gprc5a-ko mice.

Upregulated SLC1A1 is essential for increased glutamate uptake, GSH synthesis, and the

malignant phenotype of lung cancer cells

To correlate the expression profile of SLC1A1 in human lung cancer cells, we examined the levels of

SLC1A1 and xCT in a panel of human non-small cell lung cancer (NSCLC) cell lines. Western blot

analysis showed that xCT was upregulated in all NSCLC cell lines, whereas SLC1A1 was

upregulated in a subset of NSCLC cell lines (H292G, Calu-1, HCC827, A549), compared to that in

normal human lung epithelial cell lines, HBEC (Fig. 2A). To determine the biological roles of

SLC1A1, we silenced SLC1A1 using short hairpin (sh) RNA (shSLC1A1) in A549 cells (Fig. 2B)

and Calu-1 cells (Supplementary Fig. S2D). SLC1A1-knockdown (KD) significantly reduced the

colony formation (Fig. 2C), anchorage-independent growth (Fig. 2D), and 3D-sphere formation (Fig.

2E) in A549 and Calu-1 cells. These results suggest that upregulated SLC1A1 is essential for the

malignant phenotypes of human lung cancer cells.

To determine the biochemical role of SLC1A1, we examined glutamate transportation and

GSH synthesis in these cell lines. Liquid chromatography-tandem mass spectrometry (LC-MS/MS)

analysis showed that glutamate and GSH levels were significantly reduced in A549-shSLC1A1 cells

compared to control cells, A549-non-specific control (NC) (Fig. 2F-G). GSH is known to buffer

intracellular ROS for maintaining redox homeostatsis in cells. To determine if GSH synthesis is

required for maintaining the malignant features of the lung cancer cells, we examined the roles of

GSH via BSO, an inhibitor of GSH synthesis, on A549 cells. The results showed that BSO treatment

significantly suppressed stem cell-like subpopulation, in A549 cells (Fig. 2H), and SJT-1601 cells

(Supplementary Fig. S3A). BSO treatment also reduced the number and size of colonies of A549




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(Supplementary Fig. S3B). These findings suggest that GSH is required for maintaining the

malignant features of lung tumor cells, similar to previous reports (27, 28). Next, we examined the

effects of SLC1A1-knockdown on Aldh+ cells, a stem cell-enriched subpopulation. FACS analysis

showed that Aldh+ population was significantly reduced in A549-siSLC1A1 cells (1.26%) compared

to that in A549-siNC cells (3.11%) (Fig. 2I-J). These observations suggest that SLC1A1 is required

for maintaining the stemness of these cancer cells. Moreover, tumorigenicity of A549-shSLC1A1

cells was significantly reduced compared that of A549-shNC cells (Fig. 2K). Taken together, these

results suggest that SLC1A1 overexpression is essential for maintaining the malignant phenotypes of

lung cancer cells both in vitro and in vivo.

SLC1A1-mediated glutamate uptake alleviates glutamine deficiency-induced ROS and cell

growth inhibition

GSH is an anti-ROS molecule. Generally, GSH level is inversely correlated with oxidative stress

within cells. By examining ROS level, we found that ROS was significantly increased in A549-si-

SLC1A1 cells compared to that in A549 si-NC cells (Fig. 3A). Consistently, silenced SLC1A1 by

siRNA resulted in increased ROS targets (cleaved-PARP, p-P38) (9) as compared to NC in both

A549 and Calu-1 lung cancer cells (Fig. 3B). This suggests that SLC1A1 knockdown reduces GSH

and increases oxidative stress, leading to reduced stem cell subpopulation and increased programmed

cell death.

Intracellular glutamate is largely converted from extracellular glutamine, imported via

glutamine transporters. Upregulated glutamate transporter in tumor cells suggests that there is a

functional link between SLC1A1 upregulation and the malignant phenotypes. To determine the role

of extracellular glutamate on tumor cells, we examined the growth of tumor cells by culturing them

in glutamine-depleted medium with or without exogenous glutamate. Depletion of glutamine in

medium (Gln-) inhibited the growth in A549-shNC cells (Fig. 3C). However, addition of exogenous

glutamate in medium (Gln-Glu+) restored the growth in A549-shNC cells (Fig. 3C), but failed to do

so in A549-sh-SLC1A1 cells (Fig. 3C). This finding suggests that glutamate transporter SLC1A1 is

essential for cell growth in the glutamine-depleted medium but supplied with glutamate (Gln-Glu+).

Consistently, depletion of glutamine in the medium (Gln-) reduced ALDH+ cell; whereas addition of

exogenous glutamate in the medium (Gln-Glu+) largely restored ALDH+ cell population in A549-

siNC and Calu-1-siNC cells; however, this effect was lost in A549-siSLC1A1 and Calu-1-siSLC1A1

cells (Fig. 3D, Supplementary Fig. S3C-D). These findings suggest that SLC1A1 is essential for

restoration of ALDH+ cell population when extracellular glutamate is available. To determine the




14

role of SLC1A1 on programmed cell death, we examined Annexin V, an apoptotic marker, in cells.

The results showed that, following culture in the medium (Gln-Glu+), Annexin V levels were

significantly increased in A549-siSLC1A1 and Calu-1-siSLC1A1 cells, in comparison to those of si-

NC control cells (Fig. 3E-F, Supplementary Fig. S4A-B). These observations suggest that SLC1A1-

mediated uptake of glutamate inhibits apoptotic subpopulation, a type of programmed cell death, in

the medium (Gln-Glu+).

Increased ROS is implicated to induce various forms of programmed cell death and growth

inhibition in cells (1, 9). Next, we asked if glutamine depletion-induced programmed cell death and

proliferation inhibition were resulted from increased ROS. Indeed, ROS was increased in si-

SLC1A1-C cells compared to si-NC-C A549 cells in medium (Gln-Glu+) (Fig. 3G-3H); whereas

addition of reducing agent GSH-MEE or α-Tocopherol significantly reduced ROS in A549-si-

SLC1A1, but did not in A549-si-NC (Fig. 3G-H). These findings suggest that SLC1A1-mediated

glutamate uptake is essential for buffering ROS induced via glutamine depletion. In addition, we also

examined the effects of SLC1A1 overexpression in 16HBE, an immortalized normal lung epithelial

cell line. The results showed that the ROS level was significantly reduced in 16HBE-SLC1A1 cells

in comparison to that in 16HBE-V cells when cultured in medium (Gln-Glu+) (Fig. 3I,

Supplementary Fig. S4C). These results suggest that, SLC1A1 overexpression confers 16HBE cells

the extra-ability to buffer ROS.

To determine the biological impact of SLC1A1 deficiency-induced ROS, we examined the

total living cell number, the net outcome of all forms of cell death and growth inhibition, in medium

with or without glutamine (Gln), or glutamate (Glu). Depletion of glutamine in medium (Gln-) in 48

hours leads to significantly reduced cell numbers, compared to those in normal medium (C), in both

A549-shNC and A549-shSLC1A1 cells (Fig. 3J); importantly, replenish of glutamate (Glu) in

glutamine-depleted medium (Gln-Glu+) largely restored the proliferation of A549-shNC cells, but

not that of A549-shSLC1A1 cells (Fig. 3J). Consistently, clonogenic assay (culture for 2 weeks)

showed that, glutamine-depletion in medium (Glu-) completely eliminated the colonies forming

activity of A549 cells; whereas replenish of glutamate in Gln-depleted medium (Gln-Glu+) largely

restored the colony forming activity in A549-shNC cells, but not that of A549-shSLC1A1 cells (Fig.

3K). These findings suggest that, glutamine depletion-induced cell death and/or growth inhibition

can be largely restored by exogenous glutamate, which is dependent on SLC1A1.

To determine if SLC1A1-mediated biological functions were resulted from the impact of ROS,

we examined the role of reducing agents on cell growth at various conditions. Culture of A549-




15

shSLC1A1 cells in Gln-Glu+ medium resulted in significantly inhibited cell growth as compared to

that of A549-shNC cells. Importantly, treatment of A549-shSLC1A1 cells with reducing agents,

GSH-MEE and α-Tocopherol, largely restored the cell growth; whereas the cell growth of A549-

shNC cells were not further increased (Fig. 3L). These findings suggest that SLC1A1 knockdown

confers the susceptibility to ROS-mediated cell death and/or proliferation inhibition in Gln-Glu+

medium.

Active glutamate uptake via SLC1A1 propels passive cystine uptake and GSH synthesis in lung

cancer cells

Xc- is a Na

+-independent antiporter that is driven by the concentration gradients of cystine and

glutamate across membrane. If extracellular glutamate were high, the glutamate gradient across

membrane would be reduced, leading to decreased cystine import. We hypothesized that, glutamate

uptake via SLC1A1, which reduce extracellular glutamate, is to maintain a high concentration

gradient of glutamate across membrane, which propels passive cystine uptake via antiporter Xc-. To

determine if the glutamate imported via SLC1A1 is indeed used for GSH synthesis, we performed a

stable isotope tracing analysis and assessed the fate of [U-13

C5]-glutamine-derived GSH (29-31) (Fig.

4A). Of note, the labeling of GSH (M5 isotopologue; five-labeled carbons), glutamate (M5

isotopologue) and metabolic intermediate -glutamylcysteine (M5 isotopologue) from the glutamine-

carbon was significantly reduced in A549-shSLC1A1 cells compared with those from control A549-

shNC cells (Fig. 4B-D). In addition, metabolic intermediates involved in the synthesis of GSH, such

as GSSG and pyroglutamate were also reduced in A549-shSLC1A1 cells compared with the

metabolites from A549-shNC cells (Supplementary Fig. S5A-B). These findings suggest that the

glutamate uptake via SLC1A1 facilitates GSH synthesis. To rule out the possibility that altered

uptake of labeled glutamine was due to reduced cell proliferation, we also examined the

incorporation of the metabolites by using a tracer [U-13

C6]-glucose in the same experimental

condition. The results showed that labeled glutamate (M5 isotopologue), GSH (M5 isotopologue)

and intermediate γ-glutamylcysteine (M5 isotopologue) derived from labeled glucose carbon

exhibited no significant change between two cell lines (Supplementary Fig. S5C-E). Thus, SLC1A1

facilitates GSH synthesis via glutamine uptake.

SLC1A1 is a glutamate transporter, and its impact on glutamine uptake must be indirect. To

determine the role of SLC1A1 on glutamate uptake, we examined GSH synthesis using [U-13

C5]-

labeled glutamate in Calu-1-shSLC1A1 and -shNC cells (Figure. S2D). 13

C-labeled GSH (M5

isotopologue) metabolites were measured 6 h after treatment with [U-13

C5]-glutamate in medium




16

(Gln-). GSH (M5 isotopologue) from 13

C-labeled glutamate carbons were significantly reduced in

Calu-1-shSLC1A1 cells compared to vector control, Calu-1-shNC (Fig. 4E). Consistently, labeled

glutamate (M5 isotopologue) from [U-13

C5]-glutamate was also significantly reduced in Calu-1-

shSLC1A1 compared to control cells (Fig. 4F). And 13

C-labeled glutamate carbons in the condition

medium (CM) of Calu-1-shSLC1A1 cells was higher than that in the CM of Calu-1-shNC cells (Fig.

4G). These results suggest that glutamate uptake by Calu-1-shSLC1A1 cells was reduced as

compared to that by Calu-1-shNC cells. In comparison, the 13

C incorporation in the intermediate

metabolites, aspartate, malate and succinate of TCA cycle were not changed in Calu-1-shSLC1A1

cells (Fig. 4H-I, Figure S5F). Glutamine depletion in medium did not induce significant apoptotic

cells in 6 h in A549 and Calu-1 cells (Fig. 3E-F), suggesting that reduced GSH and Glu are not due

to cell death. Taken together, SLC1A1 promotes glutamate uptake for GSH synthesis rather than

feeding the TCA cycle anaplerosis. For further characterization, we also examined GSH metabolites

in 16HBE and 16HBE-SLC1A1 cells. 13

C-labeled GSH (M5 isotopologue) from labeled glutamate

carbons was significantly increased in 16HBE-SLC1A1 cells compared to the parental cell line (Fig.

4J). This result suggests that SLC1A1 overexpression enhances GSH synthesis in 16HBE cells.

Taken together, SLC1A1 enhances GSH synthesis via glutamate uptake.

To determine the role of SLC1A1 on cystine uptake for GSH synthesis, we performed stable

isotope-assisted metabolomics using [13

C6-15

N2]-labeled cystine as the tracer in Calu-1 and A549

cells (Supplementary Fig. S5H). 13

C-labeled GSH (M4 isotopologue) was significantly decreased in

Calu-1-siSLC1A1 cells compared to that in control Calu-1-siNC cells (Fig. 4K), suggesting that

SLC1A1 is essential for efficient cystine uptake. The M8 isotopologue of GSSG were also decreased

in Calu-1-siSLC1A1 cells (Supplementary Fig. S5G), supporting the notion. Similarly, [13

C3-15

N1]-

labeled GSH was also reduced in A549-siSLC1A1 cells compared to A549-siNC cells (Fig. 4L).

Consistently, [13

C6-15

N2]-labeled cystine in condition media (CM) of Calu-1-siSLC1A1 and A549-

siSLC1A1 cells were significantly higher than those in control si-NC cells (Fig. 4M), suggesting that

SLC1A1-knockdown reduces cystine uptake from medium (Supplementary Fig. S5H). Thus,

glutamate transporter SLC1A1 facilitates cystine uptake for GSH synthesis in lung cancer cells.

Lung tumors preferentially uptake glutamate for GSH synthesis in vivo

Cellular metabolic processes can be greatly influenced by tissue microenvironment in vivo (31). To

determine the roles of glutamate uptake and GSH synthesis in vivo, we performed an experiment

tracing [U-13

C5]-glutamine in lung tumors from Gprc5a-ko mice. Gprc5a-ko mice develop lung

tumors by 14 months of age (5a-ko-NNK14m) (13, 14). Before sacrifice, [U-13

C5]-glutamine (2




17

mg/kg/min, 6h) was injected by jugular vein catheterization to these mice (31). The lung tissues were

harvested after 6 h infusion, and metabolites from lung tumors and normal lung tissues were

measured (Fig. 5A). The results showed that M5 isotopologues of GSH and glutamate derived from

labeled-glutamine carbon were significantly increased in lung tumors compared to adjacent normal

lung tissues (Fig. 5B-C). On contrary, the M5 isotopologues of GSSG and glutamine from the

labeled glutamine carbon were similar in tumors and adjacent lung normal tissues (Supplementary

Fig. S6A-B). Taken together, these results suggest that GSH and glutamate are preferentially

synthesized and used for the growth and survival of lung tumors in vivo.

To determine the fate of glutamate, we designed fluorescent isothiocyanate (FITC)-labeled

glutamate (5-FITC-(Acp)-Glu) to track the location of lung tumors in mouse lung tumor model.

Gprc5a-ko (KO-NNK-14m) mice developed lung tumors, which were detectable by MRI imaging

(red circle, Fig. 5D). Importantly, glutamate-FITC via i.p. injection was much brighter in tumor

regions than in adjacent normal tissues (Fig. 5E). Light images (red arrow, Fig. 5F) and HE staining

(Fig. 5G) confirmed the location of tumors. This suggests that lung tumor tissues prefer glutamate

uptake compared to normal lung tissues. In experimental metastasis model, mouse lung tumor cells,

SJT-1601-luc, were i.v. injected in nude mice. Three weeks later, lung tumors were developed, as

imaged by luciferase activity via i.p. injection luciferin (Fig. 5H) or FITC-Glutamate via tail vain

injection. Luminescence imaging showed that metastatic tumors were formed in mouse lungs (Fig.

5I, below). Importantly, FITC-glutamate also showed strong imaging in tumor tissues in lungs,

which is largely overlapped with luciferase images (Fig. 5I, upper), both in mice injected with SJT-

1601-luc at 106 (Fig. 5I, right) or 10

5 (Fig. 5I, left). To determine if SLC1A1 expression is essential

for glutamate uptake in tumor tissue in vivo, we examined the uptake of FITC-glutamate in tumors

derived from A549-shNC and A549-shSLC1A1 cells. To compare glutamate uptake by tumors with

similar size, we subcutaneously inoculated 1.5x106 A549-shNC cells and 3x10

6 A549-shSLC1A1

cells in nude mice. When similar size of tumors were formed (in 2 weeks), FITC-Glutamate was

injected via tail vein. Two hours later, tumors were isolated for fluorescence photography. The

results showed that the tumor mass from A549-shNC cells exhibited about two-fold higher FITC

intensity than that from A549-shSLC1A1 cells, suggesting that SLC1A1 knockdown significantly

reduces uptake of FITC-glutamate in lung tumor (Supplementary Fig. S6C-D). Thus, SLC1A1 is

essential for efficient FITC-glutamate uptake in tumor tissues.

Dysregulated glutamate transporters are prevalent in NSCLC tissues




18

To determine if SLC1A1 is upregulated in human lung cancer, we measured mRNA from 16 paired

lung non-small lung cancer (NSCLC) tissues and normal tissues by using Q-PCR. The result showed

that SLC1A1 expression was upregulated in a subset (7/16) of lung tumor tissues compared to

adjacent normal tissues, but not in other samples (Fig. 6A). We reasoned that other glutamate

transporters might perform similar function as SLC1A1. Then, we examined the mRNA levels of

other glutamate transporters, SLC1A2, SLC1A3. The results showed that most of lung tumors (12/16)

expressed either one or more types of the glutamate transporters tested (Fig. 6A-C). To further

characterize the expression of SLC1A1 and GPRC5A in human lung tumors, we performed IHC

staining using a tissue microarray of human lung tumor and adjacent normal tissue (n=76 pairs).

GPRC5A was highly expressed in adjacent-normal tissue compared to lung tumor tissue; whereas

SLC1A1 was highly expressed in most of lung tumor tissues compared to adjacent-tumor tissues

(Fig. 6D). The IHC score showed that there is an inverse correlation between GPRC5A and SLC1A1

on average (Fig. 6E-F), supporting that GPRC5A deficiency contributes SLC1A1 upregulation in

NSCLCs. In addition, we measured the GSH and glutamine levels in 30 pairs of freshly isolated

NSCLC and adjacent normal lung tissues. The results showed that GSH and glutamine were

significantly higher in lung tumor tissues than that in normal lung tissues (Fig. 6G-H), supporting

that GSH synthesis is greatly enhanced in tumor tissues.

To extend this observation further, we collected data on GPRC5A and SLC1A1 expression

from The Human Protein Atlas at www.proteinatlas.org, and found that there is an inverse

correlation between two genes, which is consistent with our original data (Supplementary Fig. S7A-

D). We also analyzed published data from TCGA; lung cancer patients that were divided into two

groups according to the level of SLC1A1 expression (32-34). The analysis showed that patients with

a high expression of SLC1A1 showed poor survival compared to those with low expression (Fig. 6I),

supporting the promotion role of SLC1A1 in lung tumor progression. Taken together, these data

suggest that glutamate transporters, in particular SLC1A1, are frequently upregulated in lung cancer

tissues, which predicts poor survival.

Discussion

In this study, we show that upregulated glutamate transporter SLC1A1 is essential for

increased uptake of glutamate and cystine for GSH synthesis, which contributes to the malignant



http://www.proteinatlas.org/


19

phenotypes of lung cancer cells. Consistently, carcinogen-induced lung tumors and metastatic tumors

exhibited increased uptake of glutamate compared to adjacent normal lung tissues, which can be

used as a potential diagnosis marker for lung cancer. Finally, upregulated glutamate transporters, in

particular SLC1A1, as well as GSH synthesis, are found in NSCLC samples, supporting the role in

tumorigenesis.

Lung tumorigenesis is associated with chronic inflammation (35, 36) and COPD (37-40).

Inflammation has been linked to a high level of ROS production (41). To buffer the high level of

ROS, malignant cells must gain the extra ability to generate a high level of GSH since the regular

program of GSH synthesis under physiological conditions is inadequate for the need in lung

tumorigenesis (42). In this study, we found that glutamate transporters, in particular SLC1A1, are

overexpressed in NSCLC tissues. Glutamate transporters SLC1A1-5 are mostly found in neuronal

cells, to uptake extracellular glutamate released in cleft, which prevents excitotoxic damage.

SLC1A1 is also critical in glutathione synthesis in neurons (43). System Xc- imports cystine while

exporting glutamate in a 1:1 ratio. Thus, extracellular glutamate acts as a competitive inhibitor for

cystine uptake via system Xc-. Because SLC1A1-mediated glutamate uptake reduces extracellular

glutamate, it would facilitate cystine uptake. Consistent with this model, it was reported that transient

over-expression of SLC1A1 (EAAT3) in hippocampal HT22 cells increases intracellular GSH in the

presence of high glutamate concentrations, and protected host cells from oxidative glutamate toxicity

(44-46). Thus, although cystine is considered as the rate-limiting amino acid (47), the glutamate

gradient across membrane is crucial for efficient cystine uptake and GSH synthesis, especially in

cancer cells that need high level of GSH. Consistent with concept, other members of glutamate

transporters were found to be upregulated in those cancer cells when SLC1A1 was not upregulated.

It remains to determine the role of other SLC1A family members that are also implicated in human

tumors although the mechanisms remain unclear (48). Previously, we showed that chronic

inflammation was associated with lung tumorigenesis in Gprc5a-ko mouse mice; and Gprc5a-

deletion leads to aberrant activation of NF-κB and STAT3 signaling in lung epithelial cells (15, 49,

50). It remains to be determined what mechanisms are involved in upregulation of glutamate

transporters, such as SLC1A, in tumor cells. Nevertheless, it appears that upregulated glutamate

transporters, in particular SLC1A1, a program used in neuron, have been hijacked by lung tumor

cells for cancer progression. GSH synthesis in normal cells is sufficient for homeostasis, which is

independent of SLC1A1. However, metabolism in cancer cells is dysregulated, which generates extra

high level of ROS. Our assumption is that SLC1A1-mediated glutamate uptake confers to synthesize

an extra-level of GSH for buffering increased ROS in cancer cells.




20

Analogues of glutamate, such as 4-[18

F]fluoro L-glutamate (BAY 85-8050), have been

reported to be used for tumor imaging (51). PET imaging with excellent tumor visualization and

high tumor to background ratios was achieved in preclinical ectopia tumor models via subcutaneous

inoculation. However, the hypothetic explanation theory proposed is very different from ours, in

which the authors assumed that xCT is responsible for the preference of glutamate uptake in cancer

cells. In this study, we applied FITC-Glutamate tracing assay in orthortropic tumor model, in which

lung tumors preferentially uptake glutamate compared to normal lung tissues. Moreover, we showed

that SLC1A1 is essential for preferential glutamate uptake in tumor tissues in vivo. Thus, we

conclude that it is glutamate transporter, such as SLC1A1, rather than xCT, that is mainly

responsible for preferential glutamate uptake.

In general, cancer stem cells, metastatic cancer cells and cancer cells associated with relapse,

are more resistant to oxidative stress provoked by chemo- or radiotherapy than non-stem-like cancer

cells, whereas increased GSH has been linked to stem-like features (47, 52, 53). xCT overexpression

and high GSH synthesis have been identified in cancer stem cells (4, 54). Here, we report that the

glutamate transporters, in particular SLC1A1, are highly expressed to maintain a high level of GSH

synthesis. The efficacy of cancer therapies, including radiotherapy and anticancer drugs, is

attributable in part to the production of ROS and the consequent induction of oxidative stress in

cancer cells, it is therefore expected that targeting of antioxidant systems in cancer stem cells, such

as glutamate transporter SLC1A1 and xCT, in combination of chemo- or radiotherapy, would

improve the efficacy of cancer treatment. Taken together, dysregulated glutamate transporter

SLC1A1 is essential for increased glutamate uptake, which facilitates cystine uptake and GSH

synthesis for lung tumorigenesis (Fig. 7).




21

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Figure Legends

Figure 1. Increased GSH synthesis and upregulated glutamate transporter Slc1a1 are

associated with lung tumor development in Gprc5a-ko mice. (A) Schema of lung tumor

development in Gprc5a-ko mouse model (above). Both wild-type (WT) and Gprc5a-ko (KO) mice

were i.p. injected with the carcinogen NNK at two months of age. Lung tissues were harvested at 14

months. Tumor incidences for each group are indicated (left), and representative HE staining images

of lung tissues are presented (right). (B) Global metabolomic profiles were identified for each group

(n=6/group) of mouse lung tissues collected from WT and KO mice as indicated. (C and D) The

relative concentration of glutamate and glutathione-reduced (GSH) in mouse lung tissues from the

different treatment groups were determined by the metabolomics analysis. (E) Gene expression

heatmap of the GSH synthesis pathway in MTEC-KO vs MTEC-WT cells through an mRNA array

analysis. (F) Western blot analysis of Slc1a1 and Gprc5a expression in MTEC cells. (G and H)

Relative mRNA of Slc1a1 and Gprc5a in MTEC cells were analyzed by Quantitative real time (Q)-

PCR assay. (I and J) The concentration of glutamate in the cellular pellet and condition medium of

MTEC cells was measured using the Glutamate Colorimetric Assay Kit. (K) Western blot analysis of

Slc1a1 expression in the mouse lung cancer cell SJT-1601-V and in SJT-1601-Gprc5a transfectants.

(L and M) Representative images of SLC1A1 staining by immunohistochemistry (IHC) in mouse

lung tissues from different treatment groups as indicated. IHC stain intensity scores of the different

treatment groups are shown. * p<0.05. Error bars represent SEM. When not mentioned, differences

are not significant.

Figure 2. Upregulated SLC1A1 is essential for increased glutamate uptake, GSH synthesis, and

the malignant phenotype of lung cancer cells. (A) Western blot analysis of SLC1A1 and xCT

expression in human lung cancer cell lines as indicated. (B) Western blot showing SLC1A1

expression in A549-shNC and A549-shSLC1A1 cells. (C) Representative images of A549-shNC and

A549-shSLC1A1, Calu-1-shNC and Calu-1-shSLC1A1 clones formed via clonogenic analysis. (D

and E) Representative images of the colonies formed from the indicated cell lines in soft agrose and

3D-Sphere formation. (F and G) The concentrations of cellular glutamate and GSH in A549-shNC

and A549-shSLC1A1 cells. (H) FACS analysis of the Aldh1+, stem cell–enriched population, of

A549-C and A549-BSO cells (I) Western blot analysis of A549 cells transfected with either control

si-NC or si-SLC1A1. (J) FACS analysis of the Aldh1+, stem cell–enriched population, of A549-siNC

and A549-si-SLC1A1 cells. (K) Tumor volumes were assessed in A549-shNC/shSLC1A1 cells.

Nude mice were injected with cells (1.5x106 cells, n=5), mixed in Matrigel, and tumor volumes of




25

the different treatment groups were measured as indicated. * p<0.05. Error bars represent SEM.

When not mentioned, differences are not significant.

Figure 3. SLC1A1-mediated glutamate uptake alleviates glutamine deficiency-induced ROS

and cell growth inhibition. (A) The level of ROS was detected in A549 si-SLC1A1 and A549 si-

NC cells (B) Western blot assay showed the expression of ROS target genes (c-PARP, p-P38). (C)

Cell proliferation for A549 and A549-shSLC1A1 cells in a glutamine-deficient growth medium

(DMEM), Gln (-), with (+) or without (-) glutamate (Glu). (D) FACS showed the ALDH+ cell

population for A549-siNC and A549-siSLC1A1 in a glutamine-deficient growth medium and with or

without glutamate. (E and F) FACS showed the annexin V cell population of A549 and Calu-1 cells

in Gln-Glu+ medium in different time. (G and H) The levels of ROS in A549 siNC and A549-

siSLC1A1 cells that had been treated without or with GSH-MEE (2 mM) or α-Tocopherol (100g/ml)

in glutamine deficient medium added with glutamate (Med:Gln-Glu+). (I) The ROS levels were

detected in 16HBE-V and 16HBE-SLC1A1 cells in Gln-Glu+ medium (Top: Western blot showed

the expression of SLC1A1 in 16HBE). (J) Total cell numbers of A549 shNC/SLC1A1 cells in

various growth medium, DMEM (C), glutamine deficient (Gln-) and glutamine deficient but added

with glutamate (Med:Gln-Glu+), after culture for 48 hours. (K) Clonogenic assay, culture of A549

shNC/SLC1A1 cells in various growth medium (DMEM, Gln-, Gln-Glu+) for 2 weeks. (L) Total cell

number of A549 sh-NC/SLC1A1 cells following treatment with GSH-MEE or α-Tocopherol for 48

hours. * p<0.05. ** p<0.01. Difference is not significant (NS). Error bars represent SEM.

Figure 4. Glutamate uptake via SLC1A1 is essential for GSH synthesis and the stem-like

features of lung cancer cells. (A) The schema of tracing glutathione (GSH) synthesis with [U-13

C5]-

glutamine. (B-D) Isotopologue spectral analysis of 13

C-labeled GSH (M5), glutamate (M5) and -

glutamylcysteine (M5) using [U-13

C5]-glutamine as the tracer in vitro. (E and F) Isotopologue

spectral analysis of 13

C-labeled GSH (M+5), and glutamate (M+5) in Calu1-shNC, and -shSLC1A1

cells using [U-13

C5]-glutamate as the tracer in vitro. (G) Isotopologue spectral analysis of 13

C-labeled

glutamate accumulated in the condition medium of Calu1-shNC and -shSLC1A1 cells. (H and I)

Isotopologue spectra analysis of 13

C-labeled aspartate (M+4) and 13

C-labeled malate (M+4) in

Calu-1-shNC and Calu-1-shSLC1A1 cells using [U-13

C5]-glutamate as the tracer in vitro. (J)

Isotopologue spectra analysis of 13

C-labeled GSH (M+5) in 16HBE and -SLC1A1 cells using [U-

13C5]-glutamate as the tracer in vitro. (K) Isotopologue spectral analysis of

13C-labeled GSH (M+4)

in Calu1-siNC and Calu1-siSLC1A1 using 13

C6-15

N2-cystine as the tracer in vitro. (L) Isotopologue




26

spectral analysis of 13

C-labeled GSH (M+4) in A549-siNC and A549-siSLC1A1 using 13

C6-15

N2-

cystine as the tracer in vitro. (M) UPLC/MS showed the concentration of labeled cystine in condition

media (CM) of A549 and Calu1 cells. * p<0.05. Error bars represent SEM. When not mentioned,

differences are not significant.

Figure 5. Lung tumors preferentially uptake glutamate for GSH synthesis in vivo

(A) The schema of tracing GSH synthesis pathway with [U-13

C5]-glutamine in vivo. (B and C)

Isotopologue spectral analysis of 13

C-labeled GSH (M5) and glutamate (M5) using [U-13

C5]-

Glutamine as the tracer in vivo. (D) MRI imaging detected the lung tumor developed from Gprc5a-ko

(KO-NNK-14m) mice. (E-G) Fluorescence (FITC-(Acp)-Glutamate (0.5mg/kg/min, 6h)), light and

HE-staining detected the lung tumor developed from Gprc5a-ko (KO-NNK-14m) mice. (H)

Biological luminescence (luciferase) images of control mouse and SJT-1601-luc mouse model. (SJT-

1601-luc cell: 1x105). (I) Different exposure biological luminescence (luciferase) and FITC-

fluorescence images of control mouse and SJT-1601-luc mouse model. Bottom: luciferin. Top:

FITC-fluorescence. * p<0.05. Error bars represent SEM. When not mentioned, differences are not

significant.

Figure 6. Glutamate transporters are upregulated in NSCLC tissues. (A-C) Relative mRNA

expression of SLC1A1 (A), SLC1A2 (B), SLC1A3 (C) in a set of human lung tumors compared with

adjacent normal lung tissues. (D) The expression of SLC1A1 and GPRC5A by IHC staining in a

human lung tissue chip that includes tumors and adjacent normal tissues (n=76 pairs). (E and F) IHC

scores of SLC1A1 and GPRC5A from the tissue chip were as indicated. (G and H) Metabolic

analysis of GSH and glutamine in human lung tumors (n=30) compared to normal tissues (n=30). (I)

The Kaplan-Meier 10-year RFS (relapse-free survival) curves of SLC1A1-high expression and

SLC1A1-low expression in lung cancer patients from an TCGA dataset.

Figure 7. The proposed model: upregulated glutamate transporter SLC1A1 propels cystine

uptake via Xc- for GSH synthesis in lung cancer cells.




Published OnlineFirst November 23, 2020.Cancer Res Wenzheng Guo, Kaimi Li, Beibei Sun, et al. Uptake via Xc- for Glutathione Synthesis in Lung CancerDysregulated Glutamate Transporter SLC1A1 Propels Cystine

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