World Journal of Pharmaceutical Research SJIF Impact ... · Sweety Sihag* and Neha Wadhwa...

www.wjpr.net Vol 7, Issue 1, 2018. 1285

Sihag et al. World Journal of Pharmaceutical Research

ROLE OF P53: TUMOR SUPPRESSOR GENE

Sweety Sihag* and Neha Wadhwa

Department of Chemistry and Biochemistry, CCS Haryana Agricultural University, Hisar

(Haryana) - 125004, India.

ABSTRACT

The p53 gene like the Rb gene, is a tumor suppressor gene, i.e., its

activity stops the formation of tumors. If a person inherits only one

functional copy of the p53 gene from their parents, they are

predisposed to cancer and usually develop several independent tumors

in a variety of tissues in early adulthood. This condition is rare and is

known as Li-Fraumeni syndrome. However, mutations in p53 are

found in most tumor types and so contribute to the complex network of

molecular events leading to tumor formation. The p53 gene has been

mapped to chromosome 17. In the cell, p53 protein binds DNA, which

in turn stimulates another gene to produce a protein called p21 that

interacts with a cell division-stimulating protein (cdk2). When p21 is

complexed with cdk2 the cell cannot pass through to the next stage of cell division. Mutant

p53 can no longer bind DNA in an effective way and as a consequence the p21 protein is not

made available to act as the 'stop signal' for cell division. Thus cells divide uncontrollably

and form tumors. Help with unraveling the molecular mechanisms of cancerous growth has

come from the use of mice as models for human cancer, in which powerful 'gene knockout'

techniques can be used. The amount of information that exists on all aspects of p53 normal

function and mutant expression in human cancers is now vast, reflecting its key role in the

pathogenesis of human cancers. It is clear that p53 is just one component of a network of

events that culminate in tumor formation.

KEYWORDS: p 53 protein, stop signal, cell divison, gene Knockout.

World Journal of Pharmaceutical Research SJIF Impact Factor 7.523

Volume 7, Issue 1, 1285-1300. Review Article ISSN 2277– 7105

Article Received on

21 Nov. 2017,

Revised on 11 Dec. 2017,

Accepted on 01 January 2018

DOI: 10.20959/wjpr20181-10270

*Corresponding Author

Sweety Sihag

Department of Chemistry

and Biochemistry, CCS

Haryana Agricultural

University, Hisar (Haryana)

- 125004, India.



INTRODUCTION: WHAT ARE TUMOR SUPPRESSOR GENES?

• Tumor suppressor genes are normal genes that slowdown cell division, repair DNA

mistakes, and tell cells when to die (a process known as apoptosis or programmed cell

death).

• When tumor suppressor genes don’t work properly, cells can grow out of control, which

can lead to cancer. About 30 tumor suppressor genes have been identified, includes p53.[1]

• P53 Gene

• THE p53 GENE is a tumor suppressor gene.

• Its activity stops the formation of tumors.

• If a person inherits only one functional copy of the p53 gene from their parents, they are

predisposed to cancer and usually develop several independent tumors in a variety of

tissues in early adulthood. This condition is rare and is known as Li-Fraumeni syndrome.

However, mutations in p53 are found in most tumor types and so contribute to the

complex network of molecular events leading to tumor formation.[2]

• The p53 gene has been mapped to chromosome 17.

• Its molecular 53 kilodalton protein.

• In the cell, p53 protein binds DNA, which in turn stimulates another gene to produce a

protein called p21 that interacts with a cell division-stimulating protein (cdk2). When p21

is complexed with cdk2 the cell cannot pass through to the next stage of cell division.



Mutant p53 can no longer bind DNA in an effective way, and as a consequence the p21

protein is not made available to act as the 'stop signal' for cell division. Thus cells divide

uncontrollably and form tumors.

Mutations in the p53 gene are found in a greater percentage of tumors than any other gene

mutation. The situation with the p53 gene is complicated by the fact that mutation can result

in The loss of tumor suppressor function.

1. Oncogene activity including a dominant negative effect which overides the influence of

the wild type gene.

In the Li-Fraumeni syndrome, there is a germ-line mutation of the p53 gene resulting in a

high incidence of cancer particularly tumors of the adrenal cortex, breast and brain.[1]

P53 can bind to DNA Stabilized by Zn2+

.

The sequence-specific DNA-binding domain of p53 is localized between amino acid residues

102 and 292.

It is protease-resistant and independently folded domain containing a Zn2+

ion that is required

for its DNA-binding activity.



The discovery of p53

Studies of SV40-transformed cells show that a 55-kDa protein is coprecipitated with the

large-T antigen. This association was shown to be the result of an in vivo association between

the two proteins. It was then postulated that this protein could be encoded by the cellular

genome. (It should be kept in mind that no middle-T was found for SV40 and that the

molecular weight of this protein was similar to that of polyoma middle-T antigen). Linzer and

Levine found that the 54-kDa protein was overexpressed in a wide variety of murine SV40

transformed cells, but also in uninfected embryonic carcinoma cells. A partial peptide map

from this 54-kDa protein was identical among the different cell lines, but was clearly

different from the peptide map of SV40 large-T antigen. It was then postulated that SV40

infection or transformation of mouse cells stimulates the synthesis or stability of a cellular

54-kDa protein.[4]

P53: GATEKEEPER OF GENOME

A gene which encodes a protein that regulates all growth can disable to cause potential

cancerous cells to destroy themselves. The gene is an antioncogen.

• ‘P53 guardian of the genome´.

• Transcription factor.

• Key tetrameric protein in mammalian cell

• Regulates critical cellular function involving the G1and G2cell-cycle checkpoints in

response to DNA damage and apoptosis induced by certain stimuli, such as DNA

damaging agents and hypoxia. Inhibits and prevents tumor growth.[2]



• LOCATION

The tumor suppressor gene p53 is located at chromosomes region17p13 and is one of the

most frequently mutated gene in human cancers.

Post-translational modification of p53

The p53 protein is subject to a variety of post-translational modifications.

Phosphorylation and acetylation of p53 generally results in its stabilization and

accumulation in the nucleus, followed by activation. Several protein kinases can

phosphorylate p53.

Mutant p53 is generally phosphorylated and acetylated at sites that are known to stabilize

wild type p53 and could cause accumulation of dysfunctional p53 functioning as an

oncogene.

Overexpression of MDM2 E3 ubiquitin ligase results in the deactivation of p53 in many

tumors.

Role of Acetylation

• The «latent» (inactive) form, the protein is constitutively unstable and adopts a

conformation in which the extreme C-terminal domains hinder the interactions of the

DNA-binding domain with its target.[4]



• Posttranslational modification, acetylation change in conformations promote

DNA binding activation of p53.

• Several lysines in the C-terminus are covalently modified by acetylation, including lysine

320, 373 and 382.

• Acetylation occurs in response to many forms of DNA-damage.

1. Acetylation may contribute to stabilise p53 by concealing lysines used as target sites for

ubiquitin, therefore inhibiting degradation.

2. Acetylation may induce conformational rearrangements of the C- terminus, increasing

DNA binding capacity.

3. Acetylation may play a role in the regulation of compartmentalization of p53 between

nucleus and cytoplasm.

p53 REULATION

The p53 protein is a transcriptional regulator that has been associated with blocking cell cycle

progression and inducing apoptosis in some systems.

These effects may be mediated by the products of genes whose expression is enhanced by

the p53 protein including the p21WAF1/Cip1

gene and the Bax gene.

The p21WAF1/Cip1

is known to be an inhibitor of cyclin-dependent kinase activity and can

block cell cycle progression.

The Bax protein is a promoter of apoptosis. The p53 gene is activated by DNA damage. It

is thought to be important in normal cells to slow the cell cycle when DNA is damaged to

permit DNA repair before the DNA is replicated.



Failing this it may be preferable for the cell to die rather than perpetuate a damaged

genome. Some of the action of the p53 gene on DNA repair may be mediated by

activation of the Growth Arrest DNA damage gene, GADD45.

The function of the p53 protein can be inhibited by binding to the product of the mdm-2 gene.

This may constitute part of a feedback loop because the mdm-2 gene is activated by the p53

protein. When the mdm-2 gene is overexpressed as in some sarcomas it serves as an

oncogene by supressing the function of the p53 protein.[5]

Regulation of p53 Protein Degradation

• Much of the activation of p53 is achieved through p53 protein stabilization.

• Alongwith ubiquitin-proteasome Mdm2 plays a pivotal role.

• The binding of Mdm2 to p53 promotes the ubiquitination of p53 and its subsequent

degradation by the proteasome.[6]

• Covalent Modifications of p53 DNA damage induces covalent modifications of p53 and

Mdm2, particularly phosphorylation (indicated by (P)). Phosphorylation within the

Mdm2-p53 binding interface can block binding and thereby protect p53 from degradation

P53 and Oncogenic signals[7]

• Deregulated oncoproteins induce the synthesis of ARF, which binds to Mdm2 and prevents

its action.



p53–beta-catenin crossregulatory network

p53–beta-caten Induction of p53 by deregulated beta-catenin is strictly dependent on the

ARF protein, product of another important tumor suppressor.

Deregulated beta-catenin elevates the production of ARF mRNA.

The resultant ARF protein binds Mdm2, blocking its ability to promote the ubiquitination

and degradation of p53.[8]

This block probably relies on multiple mechanisms, including direct inhibition of

Mdm2’s E3 ubiquitin ligase activity, physical sequestration of Mdm2 in the cell nucleolus

away from p53 and interference with a postubiquitination step required for Mdm2-

mediated p53 degradation in the proteasome.

When this happens in response to beta-catenin deregulation, the cell phenotype can be

altered in a p53-dependent manner, resulting in an antiproliferative Effect.

In the absence of a functional p53 pathway, beta-catenin is rendered free to exert its

oncogenic effects on the affected cell.

In human tumors, this is often achieved through mutation or deletion of the p53 gene in

crossregulatory network.



p53–Akt crossregulatory network[9]

Akt is a well-established antiapoptotic protein

Activation of Akt dependent on PI3-kinase (PI3 K)

Akt can engage in direct protein–protein interactions with Mdm2. Furthermore, it can

phosphorylate Mdm2 on at least two residues, serines 166 and 186. This phosphorylation

required for the translocation of Mdm2 from the cytoplasm into the nucleus, where it can

target p53 for inactivation and degradation.

p53 positively regulates the expression of the PTEN tumor-suppressor gene

PTEN, responsible for inactivation and loss in human cancer, encodes a

phosphatidylinositide phosphatase, which counteracts the action of PI3 K.

PTEN serves to prevent the activation of Akt, and facilitate apoptosis



p53 also represses the expression of the catalytic subunit of PI3 K.

PI3 K is activator of Akt

The inhibitory effect of p53 lead to Akt inactivation, which may cooperate with the

induction of PTEN and the degradation of Akt to achieve effective p53-mediated

attenuation of Akt function.

Stress activate te p53

FUNCTIONS OF p53

• 1) Cell growth arrest

• 2) DNA repair

• 3) Apoptosis ±programmed cell death

• p53 protein binds in sequence specific manner to sites (p53-Response elements) in certain

genes (p53-Target Genes) such as WAF-1, BAX, MDM2 etc as a transcription factor.

• Resulting regulatory protein checks the cell cycle and directly initiates DNA-damage

repair or cell destruction (Apoptosis) based on the degree of DNA damage.



CELL CYCLE ARREST

Suppression of cell transformation is mediated by specific binding of p53 tetramers to

DNA at its recognition motifs in the promoter of the wild-type p53-activated fragment

(WAF1) gene (synonyms is p21 gene), which codes for a universal inhibitor (p21, or

CDKI) of the cyclindependent kinases that govern cell cycle progression.

When levels of p21 inhibitor rise, the cyclin/CDK complexes it binds to can no longer

phosphorylate Rb proteins (retinoblastoma tumor suppressor protein family).

Underphosphorylated Rb sequesters the E2F transcription factors required for producing

the DNA synthesis machinery and the cell cycle is thus blocked prior to S-phase.

Regulation of this G1/S boundary is a critical checkpoint in the cell cycle and is

potentially inhibited by p21.[10]

P53 and the cell cycle



P53 arrests the cell cycle primarily by upregulating p21 (Cip1/Waf-1), which inactivates

CDK/cyclin.[5]

P53 can also activate apoptosis.

Block DNA Synthesis

p21 inhibitor may also interfere with DNA synthesis directly by binding to proliferating

cell nuclear antigen(PCNA).

PCNA is an essential factor in DNA replication.

A second gene under transcription control by p53 affecting cell cycle kinetics is GADD45

(growth arrest DNA damage).

which encodes a protein that, like p21, inhibits DNA synthesis by binding to PCNA.

Cell Death

In response to DNA damage, p53 can trigger exit from the cell cycle and chromosomal

disintegration by an active enzymatic process of cell death (apoptosis).

The equilibrium of bax and bc1-2, two principal and opposing protein components of

apoptosis regulation that form neutralizing heterodimer complexes, may be shifted by p53

in favor of cell death.

p53 increases levels of the apoptosis-promoting factor BAX, which has the p53

recognition motif in its promoter and represses levels of the apoptosis- blocking protein

bc1-2.

• The p53-induced activation of target genes may result in the induction of growth arrest

either before DNA replication in the G1 phase of the cell cycle or before mitosis in the

G2 phase.

• The growth arrest enables the repair of damaged DNA.

• By programmed cell death, which is often referred to as apoptosis according to its

morphological appearance, the cells damaged beyond repair are eliminated thus

preventing the fixation of DNA damage as mutations.

• Because these processes ensure genomic integrity or destroy the damaged cell, p53 has

been called the guardian of the genome

P53 and Apoptosis[8]

1. P53 protein starts a pathway that releases cytc from mitochondria



2. This cytc comlexes with protein Apaf-1 and together they activate caspases-9

3. Effector caspases starts a pathway that results in cleavage of cell constituents: DNA etc.

4. Later phagocytosis of these remaining components by macrophages mark the end of

apoptosis

The role of caspase

• During apoptosis, the cell is killed by a class of cystein proteases called caspases. More

than 10 caspases have been identified. Some of them (e.g., caspase 8 and 10) are involved

in the initiation of apoptosis, others (caspase 3, 6 and 7) execute the death order by

destroying essential proteins in the cell. The apoptotic process can be summarized as

follows:

1. Activation of initiating caspases by specific signals

2. Activation of executing caspases by the initiating caspases which can cleave inactive

caspases at specific sites.

3. Degradation of essential cellular proteins by the executing caspases with their protease

activity.

Caspase Activation

Comparison between active and inactive forms of caspases. Newly produced caspases are

inactive. Specifically cleaved caspases will dimerize and become active.



Role Of Apoptosis

In each of these diverse areas implicates immense potential manipulation of apoptosis to treat

disease. Research is already underway to harness apoptosis as a therapeutic tool in modern

medicine.[10]

Possibilities include

Control of malignant disease

Delay of premature senescence in neurodegenerative disease

Regulation of inflammatory disease

Treatment of autoimmune disorders

P53: the guardian of genome



DNA Repair

DNA repair prevents the accumulation of mutations.

Every time a cell prepares to divide into 2 new cells, it must duplicateits DNA.

This process is not perfect, and copying errors sometimes occur.

Fortunately, cells have DNA repair genes, which make proteins that proofread DNA. But

if the genes responsible for the repair are faulty, then the DNA can develop abnormalities

that may lead to cancer.

Thus p53 plays a pivotal role in DNA repair and thus combating cancer.

• P53as a biomarker in alzheimer disease

• Fibroblasts derived from AD patients expressed an altered conformational status of p53

and were less sensitive to p53-dependent apoptosis compared to fibroblasts from non-AD

subjects. Results from research show the potential of p53 as a biomarker in AD

CONCLUSION

p53 guards two gates: a gate to life and a gate to death.



Sensing damage of DNA, p53 can initiate two processes to isolate the damaged cell and

prevent its uncontrolled growth.

P53 having the capacity to modulate various cellular processes including growth arrest,

apoptosis, senescence, differentiation, and DNA repair.

Mutation in p53 responsible for uncontrolled cell division and produces cancer cell.

REFERENCES

1. Boon Wee Keng and D. Aguda Baltazar (2006) Akt versus p53 in a Network of

Oncogenes and Tumor Suppressor Genes Regulating Cell Survival and Death Biophys J.,

91(3): 857–865.

2. Oren Moshe (1999) Regulation of the p53 Tumor Suppressor protein J. Biol. Chem., 274:

36031-36034.

3. The p53 tumor suppressor protein Genes and Disease [Internet] NCBI.

4. Bates S, Phillips AC, Clark PA, Stott F, Peters G, Ludwig RL, Vousden KH. (1998)

p14ARF links the tumour suppressors RB and p53. Nature, 395: 124-125.

5. Bell S, Klein C, Muller L, Hansen S, Buchner J. (2002). p53 contains large unstructured

regions in its native state. J Mol Biol, 322: 917-927.

6. Bischoff JR, Kirn DH, Williams A, Heise C, Horn S, Muna M, Ng L, Nye JA, Sampson-

Johannes A, Fattaey A, McCormick F. (1996). An adenovirus mutant that replicates

selectively in p53-deficient human tumor cells. Science, 274: 373-376.

7. Blagosklonny, MV. (2002). P53: An ubiquitous target of anticancer drugs. International

Journal of Cancer, 98: 161-166.

8. McCormick F. (2001). Cancer gene therapy: fringe or cutting edge? Nat Rev Cancer, 1:

130-141.

9. Strachan T, Read AP. (1999). Human Molecular Genetics 2. Ch. 18, Cancer Genetics

10. Vogelstein B, Lane D, Levine AJ. (2000). Surfing the p53 network. Nature, 408: 307-310.

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http://www.ncbi.nlm.nih.gov/pubmed/?term=Aguda%20BD%5Bauth%5D

World Journal of Pharmaceutical Research SJIF Impact ... · Sweety Sihag* and Neha Wadhwa...

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