Dietary clofibrate stimulates the formation and size of estradiol-induced breast tumors in female August-Copenhagen Irish (ACI) rats.

Administration of 0.4% clofibrate in the diet stimulated estradiol (E(2))-induced mammary carcinogenesis in the August-Copenhagen Irish (ACI) rat without having an effect on serum levels of E(2). This treatment stimulated by several-fold the NAD(P)H-dependent oxidative metabolism of E(2) and oleyl-CoA-dependent esterification of E(2) to 17beta-oleyl-estradiol by liver microsomes. Glucuronidation of E(2) by microsomal glucuronosyltransferase was increased moderately. In contrast, the activity of NAD(P)H quinone reductase 1 (NQO1), a representative monofunctional phase 2 enzyme, was significantly decreased in liver cytosol of rats fed clofibrate. Decreases in hepatic NQO1 in livers of animals fed clofibrate were noted before the appearance of mammary tumors. E(2) was delivered in cholesterol pellets implanted in 7-8-week-old female ACI rats. The animals received AIN-76A diet containing 0.4% clofibrate for 6, 12 or 28 weeks. Control animals received AIN-76A diet. Dietary clofibrate increased the number and size of palpable mammary tumors but did not alter the histopathology of the E(2)-induced mammary adenocarcinomas. Collectively, these results suggest that the stimulatory effect of clofibrate on hepatic esterification of E(2) with fatty acids coupled with the inhibition of protective phase 2 enzymes, may in part, enhance E(2)-dependent mammary carcinogenesis in the ACI rat model.

Melanoma mouse model implicates metabotropic glutamate signaling in melanocytic neoplasia.

To gain insight into melanoma pathogenesis, we characterized an insertional mouse mutant, TG3, that is predisposed to develop multiple melanomas. Physical mapping identified multiple tandem insertions of the transgene into intron 3 of Grm1 (encoding metabotropic glutamate receptor 1) with concomitant deletion of 70 kb of intronic sequence. To assess whether this insertional mutagenesis event results in alteration of transcriptional regulation, we analyzed Grm1 and two flanking genes for aberrant expression in melanomas from TG3 mice. We observed aberrant expression of only Grm1. Although we did not detect its expression in normal mouse melanocytes, Grm1 was ectopically expressed in the melanomas from TG3 mice. To confirm the involvement of Grm1 in melanocytic neoplasia, we created an additional transgenic line with Grm1 expression driven by the dopachrome tautomerase promoter. Similar to the original TG3, the Tg(Grm1)EPv line was susceptible to melanoma. In contrast to human melanoma, these transgenic mice had a generalized hyperproliferation of melanocytes with limited transformation to fully malignant metastasis. We detected expression of GRM1 in a number of human melanoma biopsies and cell lines but not in benign nevi and melanocytes. This study provides compelling evidence for the importance of metabotropic glutamate signaling in melanocytic neoplasia.

Progressive appearance of pigmentation in amelanotic melanoma lesions.

We previously described a transgenic mouse line (TG-3) that spontaneously develops pigmented cutaneous melanoma. The generation of several albino mice that developed amelanotic melanoma has also been reported. In this report, we describe an unanticipated result with crosses between C57BL/6-c2j and TG-3 mice. C57BL/6-c2j has the same genetic background as TG-3 (C57BL/6), except for a single base mutation (nucleotide 291) in the tyrosinase locus, resulting in albino coat colour. Only albino F2 mice generated from (TG-3 x C57BL/6-c2j) F1s were selected for further studies. Mice that contained the transgene showed a very high incidence of tumor development as early as 4-6 weeks of age. Raised amelanotic tumors developed on the ear pinnae and perianal region in young F2 albino mice, similar phenotypes as those described earlier for the other albino inbred strains. However, with time, these amelanotic tumors not only increased in size, but unexpectedly developed foci of dark pigmentation. DNA sequence analysis on reverse transcriptase-polymerase chain reactions (RT-PCRs) of tyrosinase mRNA showed that the original tyrosinase mutation was still present in the tumors, indicating that no reversion at this nucleotide had occurred in the tumors. Two different tyrosinase activity assays were used and tyrosinase activity was detected in most tumor samples. Furthermore, Western blot analysis demonstrated various levels of tyrosinase protein in ear tumor samples. These results suggest that tyrosinase and/or melanin are not directly involved in the establishment of melanoma, but that late events occurring within the tumors may generate some tyrosinase activity and production of melanin.