Chemical Carcinogenesis

neoplasia

abnormal proliferation of cells

neoplasm

abnormal mass of tissue resulting from neoplasia

benign neoplasm

slow tissue growth, local proliferation, no invasion of adjacent tissues, no metastases, no recurrence after ablation

malignant neoplasm

rapid proliferation, invasion of neighboring structures, capable of metastases, recurrences after surgery

metastases

secondary growth derived from primary malignant neoplasm

tumor

lesion characterized by swelling/increase in size, now considered to be synonymous with neoplasm (exception is leukemia, which results in accumulation of white blood cells and is not a mass of tissue)

cancer

malignant neoplasm

carcinogen

a physical or chemical agent that causes or induces neoplasia

genotoxic carcinogen

direct interaction with DNA to produce mutation

nongenotoxic neoplasm

modify gene expression without direct DNA interaction

• promote cell proliferation

• alter chromatin

main groups of carcinogenic factors

1. primary determining factors:

   1. chemical substances: polyaromatic hydrocarbons, alkylating agents, inorganic metals, natural compounds or pollutants

   2. physical agents: non-ionizing and ionizing radiation

   3. carcinogenic transformation by viruses

2. secondary determining factors:

   1. hereditary determinism

3. favoring factors

   1. exogenous and endogenous risk factors

   2. geographic region, nutrition, sex, age, etc

Classification of chemical carcinogens

genotoxic carcinogens: includes direct action (ultimate) carcinogens and indirect action carcinogens (procarcinogens)

• ultimate carcinogens: no need for metabolic bioactivation

• procarcinogens: majority of chemical carcinogens, produce a carcinogenic metabolite

nongenotoxic carcinogens: work in tandem with genotoxic, but exposure to just nongenotoxic does not cause cancer (need a mutation)

• co-carcinogen: can enhance carcinogenic effect of other substances

• act as promoter in tissues where initiation stage of carcinogenesis has occurred

genetic (somatic) mutation theory

• origin of cancer is due to structural anomolies of genes that regulate cell growth and differentiation

• genetic changes can be hereditary or occur in the course of life

• main points are (1) spontaneous mutation and (2) mutation by external factors

mutation theory - spontaneous mutation

• consequence of lesions that must occur frequently enough to overcome the cell’s capacity to repair DNA damage

• can have same cancer-inducing potential as those caused by environmental exogenous agents

• potential sources:

  • DNA instability: DNA polymerases missing nitrogenous base (abasic position) leading to replication errors and mismatched bases

  • mutagenesis from free oxygen radicals

  • errors in DNA replication

mutation theory - external factors

• disruption of genetic stability

  • inherited mutation in tumor suppressor gene followed by second mutation acquired during life

    • Wilms tumors, retinoblastoma, p53, BRCA1, p16 gene

  • chromosomal anomalies

    • Down syndrome (trisomy 21) and Klinefelter syndrome (XXY) → acute leukemia

    • D deletion syndrome (deletion of long arm of Ch 13) → chronic myeloid leukemia

• presence of chromosomal anomalies in cancer cells has been detected in almost all malignant tumors

  • high degree of aneuploidy

• correlation between mutagenicity and carcinogenicity

cancer genes

• tumor suppressor genes: antiproliferation genes

  • proteins involved in cell cycle checkpoints

  • defects in genes release cells from normal division controls

  • defect in both copies of gene necessary to produce effect

• proto-oncogenes: proliferation genes

  • proteins that regulate cell survival or apotosis

  • defect in genes can cause unregulated growth

  • defect in one copy of gene necessary to produce effect

  • oncogenes are the modified versions that we don’t want

multistage carcinogenesis

• period of time between the damage to chromosomal DNA by a carcinogenic chemical and the appearance of neoplastic cells can be divided into three stages: initiation, promotion, and progression

multistage carcinogenesis: initiation

• nonreversible

• single treatment can induce mutation

• starts with interaction of carcinogen with chromosomal DNA (DNA modification, genotoxic), inducing an adduct (mutation)

• biological mechanisms of repair

  • direct reversal: adduct can be identified by specific endonucleases that dissect damaged DNA strand and uses complementary strand as template for replacement

  • excision repair: other enzymes can excise a muted single base: N-glycosylase, AAE, exonuclease, DNA-polymerase, lygase

  • postreplication repair: DNA polymerase skips the damaged area that cannot be replicated. integrity of newly synthesized DNA strand is restored by exchange between homologous filaments

  • non-homologous end joining

• mitosis is delayed to try and repair DNA (one cell division necessary to lock in mutation)

• initiation of a cell is finalized with the impossibility of repairing the DNA lesion

• modification is not enough to produce cancer:

  • initiated cells can remaining G0

  • initiated cells may possess mutations incompatible with viability or normal function and be deleted through apoptosis

  • initiated cells may undergo cell division resulting in proliferation

multistage carcinogenesis - promotion

• multiple cell divisions necessary

• clonal expansion of the initiated cell

• reversible

• multiple treatments necessary

• threshold

1. chronic genetic alterations of the initiated cell determine the neoplastic transformation

2. promoter is applied several times after administration of an initiating carcinogen

3. “complete” carcinogens can induce cancer without need for subsequent action of a promoter

4. promoters act by alteration of normal growth processes (alteration of transcriptions control)

5. tumor promotion is associated with epigenetic factors

6. promoted cell no longer recognizes differentiation signals that would remove it from population (decrease in apoptosis)

7. promoters do not bind to DNA, main target is cell membrane to stimulate signaling pathways (nongenotoxic)

multistage carcinogenesis - progression

• DNA modification

• genotoxic event

• mutation, chromosome disarrangement

• irreversible

• number of treatments unknown

1. marked malignancy and tendency to induce changes that cause the death of the host

2. cells are characterized by genetic changes, gene alterations, and rearrangements (including karyotype alterations)

3. tumor is phenotypically characterized by rapid proliferation, invasive properties, and biochemical and morphological changes. Extraordinary instability of karyotype.

Failures required for chemical carcinogenesis

1. failure of DNA repair (genotoxic carcinogens)

2. failure of apoptosis (facilitates mutation and clonal expansion)

3. failure to terminate cell proliferation (enhanced mitotic activity)

main events at genomic level

1. physical factors, chemical factors, viruses, genetic factors

2. change in somatic cell genome

3. proto-oncogene activation, tumor suppressor gene inhibition

4. expression of altered genes, non-expression of regulating genes

5. malignant tumor

nongenotoxic carcinogens

• increase transcriptional activation of proto-oncogenes or decrease transcriptional activation of tumor suppressor genes

• epigenetics: modification of the activation of gene expression caused by mechanisms other than changes in underlying DNA sequence. changes may be passed on through cell divisions for remainder of cell’s life and multiple generations

• include post-transcriptional gene silencing, histone modification, and DNA methylation

chromatin remodeling

• post-traslational modification of the amino acids in histone proteins

  • acetylation

  • methylation

  • ubiquitylation

  • phosphorylation

  • sumoylation

DNA methylation

• mainly addition at CpG sites within promoter region of the gene

• promoter methylation weakens the interaction between DNA and transcription factor

Hypermethylation of CpG islands

• tumor suppressor genes

• cell cycle control genes

• DNA mismatch repair genes

• Hormone receptors

• tissue and cell adhesion molecules

microRNAs

• downregulate gene expression by base-pairing with the 3’ UTR of target messenger RNAs

• most transcribed by RNA Pol II to generate stemloop containing primary miRNA (pri-miRNA)

• pri-miRNA contain 5’ cap structures, are polyadenylated, and may undergo splicing

• most encoded in introns and miRNA processing occurs before mRNA splicing

miRNA processing

results in miRNA:miRNA* duplex (not 100% complementary) that is then separated into single stranded miRNA within RNA-induced silencing complex (RISC)

role of miRNA

• help maintain and define cell types by dampening types expression of unwanted transcripts. transcription of target may be turned off while miRNA transcription is being turned on

• transcriptional regulation of mRNA is primary, miRNA regulation is secondary mechanism

• binding of miRNAs to target transcripts inhibits translation and can lead to sequestration or degradation of the mRNA

• miRNAs have been implicated in important cellular processes:

  • development

  • cell proliferation

  • cell differentiation

  • apoptosis

miRNA targeting and expression

• prediction of miRNA targets in challenging due to mismatch binding potential

• number of miRNAs bound mRNA will regulate how efficiently translation occurs

• a few miRNAs can bind to hundreds of mRNAs, which allows regulation of many processes

• overexpression of miRNAs that target tumor expression → prevent tumor expression gene from being translated → loss of function phenotype

• inhibit miRNAs that bind to proto-oncogene mRNA → translate more copies of proto-oncogene → gain of function

miRNAs and cellular stress

• hypoxia

• hutrient deprivation

• DNA damage

• oncogenic stress

• xenobiotic exposure

• cardiac pressure overload

• can participate in carcinogenesis by:

  • overexpression of miRNAs → loss of function

  • repression of miRNAs → gain of function