ES/RP 531 Fundamentals of Environmental Toxicology Fall 2005 ESRP531 Lect 15 Cancer.doc Page 1 of 14 October 17, 2005 Lecture 15: Carcinogenicity (Soft Tissue Effects) I. Mutagenicity, Tumorigenicity, Carcinogenicity A. Mutagenicity--changes in the genetic material (DNA lesion) in nucleus of cell; usually involves change in bases of DNA or reaction with bases (for ex., alkylation) that could be potentially transmitted to new cells. 1. DNA lesions are normal and occur quite frequently; however, cell nucleus has repair mechanisms for excising the lesion a. Recall the biochemical steps for genotype to phenotype expression: 1. Replication (DNA---->DNA) a. A highly active process during cell division; b. This process is arguably most vulnerable to mutations. 2. Transcription (DNA---->RNA) 3. Translation (DNA---->amino acids/proteins) a. This step translates the genotype to the phenotype. 2. If the lesion is not repaired, it can be transmitted to new DNA during cell division. 3. Normal metabolism has been estimated to cause 100,000 and 10,000 lesions per cell per day in rats and humans, respectively (Ames and Gold 1993) B. Clastogenicity 1. Chromosomal breakages and fragments 2. Not necessarily related to gene mutation; more likely due to binding of toxicant or metabolite to histone proteins or other proteins associated with DNA; strains caused by binding could cause abnormal breakage of the chromosome. a. Histones are the chief proteins of chromatin. They act as spools around which DNA winds and they play a role in gene regulation. Histones are found in the nuclei of eukaryotic cells. C. Tumorigenicity--uncontrolled cellular proliferation leading to formation of a mass of undifferentiated fast growing cells (tumor) 1. The mechanisms proposed for tumor formation have been observed to be either due to a mutagenic effect or a nonmutagenic effect associated with cell toxicity. a. A compound causing a mutagenic effect is called a genotoxin. b. A nongenotoxic compound associated with tumor formation is said to have an epigenetic effect (an epigenotoxin?). 2. A tumor actually starts as a neoplasm (“new cell growth”) (See Figures 1 & 2) a. An undifferentiated population of cells in an organ, known as stem cells, divides as part of the organ developmental process and replaces cells that are lost during differentiation (cell death is normal); b. The stem cells may be the sites most susceptible to damage from chemical exposure; older more differentiated cells may be comparatively unaffected. 1. When a cell has a mutation, it will normally repair it, but in some cases these mistakes persist.ES/RP 531 Fundamentals of Environmental Toxicology Fall 2005 ESRP531 Lect 15 Cancer.doc Page 2 of 14 2. Normally one mistake will not affect functioning of the cell, but during the next generation of cells (i.e., the ones produced during cell division), other mistakes can accumulate; these mistakes may not affect the normal physiological functions of the cell, but the tissue morphology may begin to change (the neoplasm). a. Eventually enough damage accumulates across several cell generations to cause the cells to be transformed and not function as originally programmed. b. Mutations or other adverse effects in normal development of stem cells lead to problems in the more differentiated intermediate cells; thus, the stem cells can accumulate enough damage (genetic or other physiologic damage) to become transformed to tumor like cells. (See Figure 2) Figure 1. Sequence of oncogenesis (formation of tumor cells/growths). Note the two distinct stages in tumor production: neoplastic transformation (known also as initiation) and neoplastic development (known as promotion and progression). The sequence of processes are functional regardless of whether a compound is directly mutagenic, and thus genotoxic, or it causes “faulty” cells through cellular toxicity or other affects on cell cycling and control (for example, inhibition of apoptosis). Notes that neoangiogenesis is the recruitment of new blood vessels into a tumor (necessary for supplying nutrition to the rapidly dividing cells) (based on Williams 2001). 3. High doses probably lead to cell death and chronic cell division in an attempt to replace dead cells; in other words, high doses can overwhelm ability to detoxify contaminant and thus it is more available to cause physiological damage; other possibilities areES/RP 531 Fundamentals of Environmental Toxicology Fall 2005 ESRP531 Lect 15 Cancer.doc Page 3 of 14 interactions with specific receptors (see discussion below under carcinogen classification) a. More probability of mutations because of repair mistakes, especially if cells suffering toxicity (mitogenic theory of tumorigenicity) Figure 2. One mutation in a cell, whether due to alkylation by a toxicant mutagen or due to failure to effect repair (as a result of a high dose insult or other form of cellular toxicity) can be passed on to following generations of cells. However, the mutated or affected DNA does not lead to a malignant cancer cell. Several other changes in DNA in subsequent cell generations are required before the cell loses its structure (de-differentiates) and begins to take on the characteristics of a tumor cell. 4. Studies by Cohen and Ellwein (1990) show that exposure of rats to 2-acetyl aminofluorene (2-AAF), a known mutagen, causes increased cell populations of bladder and liver cells that are directly related to duration of exposure (see Figure 3 with exposure at 18, 24, 33 mos.) and dose (see Figure 4 with doses from 45 - 150 ppm) a. Thus, even mutagens depend on cell proliferation in the process of tumor formationES/RP 531 Fundamentals of Environmental Toxicology Fall 2005 ESRP531 Lect 15 Cancer.doc Page 4 of 14 Figure 3. Tumor prevalence in at two organ sites (---- liver tumors; _____ bladder tumors) in relation to dose of 2-aminoacetylfluorene. Note that the dose response relationship differs for the two organ sites, suggesting that the mechanism is different (one is a genotoxic effect and the other is an epigenic effect). Copied from Cohen and Ellwein (1990). 1. Note that the liver seems to be more sensitive to the effects of 2-AAF when fed to mice for 33 months; however, tumor incidence drops significantly with shorter durations of exposure. 2. On the other hand, the bladder seems more tolerant of doses less than 25 ppm, but it responds quickly to