CHARACTERIZATION OF SKIN TUMOR PROMOTION BY MIREX - STRUCTURE-ACTIVITY-RELATIONSHIPS, SEXUAL DIMORPHISM AND PRESENCE OF HA-RAS MUTATION

In the present study we have compared the tumor-promoting activity of the non-phorbol ester-type skin tumor promoter, mirex, a halogenated c ycloalkane pesticide, to the following: (i) chlordane, a halogenated c ycloalkene pesticide; (ii) 1,1-bis (4-chlorophenyl)-2,2,2-trichloroeth ane (DDT), a halogenated bridged aromatic pesticide; and (iii) kepone, a halogenated cycloalkane pesticide, which only differs from mirex by the substitution of two chlorine atoms with an oxygen atom. Topical a pplication of 200 nmol mirex three times weekly for 20 weeks to 7,12-d imethylbenz[a]anthracene (DMBA)-initiated female mouse skin produced a pproximately 16 tumors/mouse with a 96% incidence of tumor bearing mic e. Neither chlordane (2 mumol) or DDT (5 mumol) promoted tumors in DMB A-initiated mouse skin after three times weekly application for 20 wee ks. Unexpectedly, DMBA-initiated mice treated with 250 nmol kepone thr ee times weekly for 20 weeks did not develop any tumors, demonstrating that the replacement of two chlorine atoms by an oxygen atom results in loss of the skin tumor-promoting activity of mirex. To further char acterize mirex-induced skin tumor promotion, male mice were initiated with a single topical application of 200 nmol DMBA and promoted topica lly three times weekly for 20 weeks with 200 nmol mirex. As compared t o female mice, male mice demonstrated (i) 70% fewer tumors/mouse, (ii) decreased incidence of tumor bearing mice, (iii) increased time to fi rst tumor and (iv) increased latency. To determine the role of ovarian hormones in the increased sensitivity of female mice, mice were initi ated with DMBA, ovariectomized (OVX) 2 weeks later and then promoted w ith mirex. OVX mice exhibited 70% fewer tumors/mouse and a 40% decreas e in incidence of tumor-bearing mice as compared to controls. Finally, > 90% of DMBA-initiated/mirex-promoted papillomas from male mice and female mice demonstrated a mutated Ha-ras gene with an A --> T transve rsion in the middle base of the 61st codon. Collectively, these data i ndicate that the tumor-promoting ability of mirex is highly structure specific, and ovarian hormones are a factor in the increased sensitivi ty of female mice to the skin tumor-promoting ability of mirex. Furthe rmore, mirex appears to clonally expand epidermal cells with a mutated Ha-ras oncogene.