Multistage chemical carcinogenesis in mouse skin

Skin tumors in mice can be induced by the sequential application of a subthreshold dose of a carcinogen (initiation phase) followed by repetitive treatment with a noncarcinogenic tumor promoter. The initiation phase requires only a single application of either a direct acting carcinogen or a procarcinogen which has to be metabolized before being active and is essentially an irreversible step which probably involves a somatic cell mutation. There is a good correlation between the skin tumor initiating activites of several polycyclic aromatic hydrocarbons (PAH) and their ability to bind covalently to epidermal DNA. Laboratory results suggest that bay region diol-epoxides are the ultimate carcinogenic form of PAH carcinogens. Potent inhibitors and stimulators of PAH tumor initiation appear to affect the level of the PAH diol-epoxide reacting with specific DNA bases. Reecent data suggests that the tumor promotion stage involves at least three important steps: (1) the induction of embryonic looking cells (dark cells) in adult epidermis; (2) an increased production of epidermal prostaglandins and polyamines; (3) sustained proliferation of dark cells. Retinoic acid specifically inhibits step two whereas the anti-inflammatory steriod fluocinolone acetonide is a potent inhibitor of steps one and three. The mechanism and the importance of a specific sequence for each step in chemical carcinogenesis in mouse skin are detailed

Specificity of interaction between carcinogenic polynuclear aromatic hydrocarbons and nuclear proteins: widespread occurrence of a restricted pattern of histone-binding in intact cells

Metabolic activation of benzo(a)pyrene (B(a)P) produces a number of potentially reactive metabolites. The endproducts of one metabolic pathway, 7,8-dihydroxy-9,10-oxy-7,8,9,10-tetrahydro-B(a)P (BPDE) are responsible for essentially all DNA adduct formation in animal cells treated with B(a)P, and a particular stereoisomer, designated (+)-anti-BPDE is thought to be the ultimate carcinogenic derivative of B(a)P. In hamster embryo cell nuclei treated with (+)-anti-BPDE, two of the histones of the nucleosomal core, H3 and H2A, are covalently modified, while the remaining core histones, H4 and H2B, are essentially unmodified. All four purified core histones, however, serve as targets. 7,12-dimethylbenz(a)anthracene and 3-methylcholanthrene show the same pattern of histone binding in hamster embryo cells. Treatment of mouse embryo cells with (/sup 3/H)-BPDE results in covalent binding of the hydrocarbon to histones H3 and H2A among the many cellular targets, while histones H2B and H4 are not bound. Similar binding patterns are seen in mouse embryo cells, a permanent murine, fibroblastic cell line, and a human mammary epithelial cell line, T47D, treated with (/sup 3/H)B(a)P. Again, the histones are unevenly labeled, displaying the H3 and H2A pattern. Histone-binding in the human cells may also be mediated by BPDE. Similar BPDE binding patterns were observed in other murine and human cell lines and in primary cultures of murine epidermal epithelial cells. The restriction of histone H2B and H4 binding appears to be general when intact cultured cells are studied. This specificity was not observed in a mixed reconstituted system in which rat liver microsomes were used to activate B(a)P. This finding reinforces reservations concerning the use of microsomal systems to probe the interactions of carcinogens with macromolecules and the relationships of adduct formation with the processes of carcinogenesis. (ERB