1. NON-ORGAN-DIRECTED TOXICITY

2. TARGET ORGAN TOXICITY

Molecular Mechanisms of Toxicology

Definitions• Neoplasia is often defined as the presence or formation

of new, abnormal growth of tissue.• The resulting neoplastic lesion is referred to as a neoplasm.• Both benign and malignant neoplasms can be induced by

chemical carcinogens.

• Benign tissue is not cancer. Although the cell growth is moderately increased, the cells do not invade nearby tissue or spread to other parts of the body

• Malignant tissue is cancer. The cancer cells divide out of control. They can invade and destroy nearby healthy tissue. Also, cancer cells can break away from the tumor they form and enter the bloodstream and lymphatic system

• Metastasis: the spread of cancer beyond the organ of origin

Neoplasms

• For benign neoplasms, the tissue of origin is frequently followed by the suffix “oma”; for example, – a benign fibrous neoplasm would be termed fibroma, and – a benign glandular epithelium termed an adenoma.

• For Malignant neoplasms from– Epithelial origin are called carcinomas while – those derived from mesenchymal origin are referred to as

sarcoma. Thus, a malignant neoplasm of fibrous tissue would be a fibrosarcoma, from bone would be an ostoesarcoma. a malignant from the liver would be a hepatocellular carcinoma, from skin would be referred to as a squamous cell carcinoma.

Tumor• A swelling of a part of the body, generally without

inflammation, caused by an abnormal growth of tissue, whether benign or malignant. (Table 8-3).

MULTISTAGE CARCINOGENESIS

• initiation• promotion• progression

Initiation• Tumor initiation is defined as "a process in which

normal cells are changed so that they are able to form tumors". It is the first phase in tumor development. Mutagens, substances that cause cancer can be tumor initiators.A process that is defined as a stable, heritable change. This stage is a rapid, irreversible process that results in a

carcinogen-induced mutational event.

Chemical and physical agents that function at this stage are referred to as initiators or initiating agents.

Initiating agents lead to genetic changes including mutations and deletions. initiating carcinogens are classified as

Chemical carcinogens that covalently bind to DNA and form adducts that result in mutations are initiating agents. chemicals polycyclic hydrocarbons and nitrosamines,

biological agents such as viruses, and physical agents such as X-rays and UV light.

Initiating agents

Promotion

The stage of promotion requires repeated applications of a second compound, called the promoter, which by itself is incapable of inducing carcinogenesis.

Reversed order of application of these compounds—that is, first promoter, then initiator—does not produce tumors.

Both exogenous and endogenous agents that function at this stage are referred to as tumor promoters.

Tumor promoters are not mutagenic and generally are not able to induce

tumors by themselves; rather they act through several mechanisms involving gene expression changes that result in sustained cell proliferation, either through increases in cell proliferation and/or the inhibition of apoptosis.

Nongenotoxic carcinogens frequently function at the tumor promotion stage. The growth of preneoplastic lesions requires repeated applications of or continuous exposure to tumor-promoting compounds.

Tumor promoters in general show organ-specific effects, e.g.,

a tumor promoter of the liver, such as phenobarbital will not function as a tumor promoter in the skin or other tissues.

Resulting from conditions within the organism rather than externally caused.

Externally caused rather than resulting from conditions within the organism.

Progression

the progression stage is an irreversible stage in that neoplasm formation, whether benign or malignant, occurs

The final stage of the carcinogenesis process, progression, involves the conversion of preneoplastic lesions into neoplastic cancer.

chemicals that function as progressor agents are usually capable of causing chromosomal abnormalities.

Tumor progression is the third and last phase in tumor development. This phase is characterised by increased growth speed and invasiveness of the tumor cells. As a result of the progression, phenotypical changes occur.

In this stage, due to increase in DNA synthesis cell proliferation in the preneoplastic lesions, additional genotoxic events may occur resulting in additional DNA damage including chromosomal aberrations and translocations.

Progression cont……

Complete carcinogens have the ability to function at the initiation, promotion, and progression stages and hence by definition have genotoxic properties.

carcinogenA substance capable of causing cancer in

living tissue.A carcinogen is an agent, chemical or physical, that

causes or induces neoplasia.

Carcinogens can be chemicals, viruses, hormones, radiation,or solid materials.GenotoxicNongenotoxic (epigenetic)

Carcinogen cont…..Genotoxic, meaning that they interact

physically with DNA to damage or change its structure.

DNA adduct is a piece of DNA covalently bonded to a (cancer-causing) chemical. This process could be the start of a cancerous cell, or carcinogenesis

Nongenotoxic (epigenetics) carcinogens are agents that do not directly interact with DNA and are believed to enhance tumor development by affecting gene expression

Carcinogen cont…..

• Large numbers of chemicals were tested for carcinogenic potential in the 1970-1990s– Maximum Tolerated Doses (MTD) were used.– 60% of carcinogens were genotoxic– 40% of carcinogens were nongenotoxic– Some chemicals were single site, single species

carcinogens– Others were multisite, multispecies carcinogens

History of Chemical Carcinogenesis

Squamous cell carcinoma

polycyclic aromatic hydrocarbons

Toxic Effects ofHydrocarbons and

Alcohols

Aliphatic Hydrocarbons:

Non-Halogenated (Hexane)

Halogenated:

Chlorinated Hydrocarbons (Trichloroethylene)

Brominated Hydrocarbons (Halothane)

Fluorinated Hydrocarbons (Methoxyflurane)

Cyclic Hydrocarbons (Cyclohexane)

Alcohols (Ethanol)

Polycyclic aromatic hydrocarbons (PAHs)

The first chemically identified carcinogens

PAHs are one of the most widespread organic pollutants. In addition to their presence in fossil fuels they are also formed by incomplete combustion of carbon-containing fuels such as wood, coal, diesel, fat, tobacco .they are common environmental contaminants.

The PAHs are chemically inert, and require metabolism to exert their biologic effects

Polycyclic Aromatic Hydrocarbons: Benzo(a)pyrine

Absorption

In humans, the major routes of uptake of PAH are thought to be through

1. the lungs and the respiratory tract after inhalation of PAH-containing aerosols or of particulates to which a PAH, in the solid state, has become to be absorbed

2. the gastro-intestinal tract after ingestion of contaminated food or water

3. the skin as a result of contact with PAH-bearing materials.

Distribution

Owing to the high lipophilicity of this class of compounds, their bioavailability after ingestion and inhalation must be considered to be significant:

4. detectable levels of PAH occur in almost all internal organs,

5. organs rich in adipose tissue can serve as storage depots from which the hydrocarbons are gradually released,

6. the GI tract contains high levels of hydrocarbon and metabolites, even when PAH are administered by other routes, as a result of mucociliary clearance and swallowing or hepatobiliary excretion.

Polycyclic Aromatic Hydrocarbons: Benzo(a)pyrine con…..

This is a multi-step process, it involves the following:

1. The first reaction is an epoxidation. initial epoxidation (cytochrome P450, CYP1A1 is an inducible enzyme),

2. The secend reaction is an hydration. 7,8-epoxide, is the subject of epoxide hydrolases to form stereoisomeric dihydrodiols.

Metabolism of benzo(a)pyrene

• dihydrodiols are converted further to the 7,8-dihydrodiol-9,10-epoxide. The terminal oxidase is cytochrome P-450 (CYP1A1). The diol epoxide can exist in 4 stereoisomeric forms of which the key

carcinogenic product is benzo(a)pyrene-r-7,t-8-diol-t-9,10-epoxide. The arene ring of benzo[a]pyrene-7,8-diol 9,10-oxide opens spontaneously at

the 10 position, giving a highly reactive carbonium ion that can form a covalent addition product (i.e., adduct) with cellular macromolecules, including DNA

Metabolism of benzo(a)pyrene cont….

Epoxide: consisting of, or containing an oxygen atom joined to two different groups that are themselves joined to other groups

Cytochrome P450, family 1, subfamily A, polypeptide 1 is a protein that in humans is encoded by the CYP1A1 gene. The protein is a member of the cytochrome P450 super family of enzymes

inducible Enzyme: An enzyme that is normally present in minute quantities within a cell, but whose concentration increases dramatically when a substrate compound is added

Schematic diagram showing the mechanism through which exposure to polycyclic aromatic hydrocarbons is thought to cause cancer

Rundle, Mutat Res 600(1-2):23-36 (2006)

Several DNA-adducts can be formed, the most abundant being at the exocyclic amino group of deoxyguanosine ([7R]-N2-[10-{7,8,9-trihydroxy-7,8,9,10-tetrahydro-benz[a]pyrene} yl] - deoxyguanosine; BPdG) (B)

Cancer Classification :PAHs are likely carcinogenic to humans based on sufficient evidence of carcinogenicity from studies in experimental animals.

Human studies: There is inadequate evidence for the carcinogenicity of PAHs in humans. Workers exposed to

numerous PAHs developed skin tumors. Dermal exposure to coal tar containing PAHs have been associated with increased incidences of skin tumors in humans

Exposures to other chemical mixtures that contain PAHs, such as cigarette smoke, coal tar and have been associated with increased incidences of lung cancer in humans.

experimental animals:Target organs (sites): stomach, lung, liver, skin, mammary gland