Hormonal regulation of carcinogenesis and gene expression in the mouse liver

In mice, the sex difference in susceptibility to hepatocarcinogenesis results from the tumor promoting activity of testosterone and from the inhibition of tumor promotion by ovarian hormones. I investigated the role of growth hormone in the sex-dependent regulation of susceptibility, because sex hormones are known to regulate the temporal pattern of growth hormone secretion and subsequent gender differences in hepatic gene expression. I found that, following perinatal treatment with the carcinogen N,N-diethylnitrosamine, wild type males and females developed 11- to 59-fold more liver tumors than growth hormone-deficient C57BL/6J-lit/lit (B6-lit/ lit) mutant mice. I subsequently bred the little mutation onto the more susceptible C57BR/cdJ (BR) and C3H/HeJ (C3H) genetic backgrounds to assess the generality of growth hormone's effect on liver tumorigenesis. In addition, to test the hypothesis that growth hormone mediates tumor promotion by testosterone, I analyzed tumor development in intact and gonadectomized mice of the three strains, wild type or lit/lit. I found that growth hormone-deficiency suppressed liver tumor development to the same absolute level on each genetic background, demonstrating that strain- and sex-dependent effects on susceptibility to liver carcinogenesis require wild type levels of growth hormone. Because gonadal sex hormones and pituitary growth hormone have dramatic effects on hepatic gene expression and susceptibility to hepatocarcinogenesis, a complete analysis of the hormonal effects on gene regulation in the liver may give insights into the hormonal regulation of cancer susceptibility. Although growth hormone-dependent hepatic expression has been demonstrated for several genes, the systematic screening of this hormone's effect on hepatic gene expression, has not been reported. To identify growth hormone-regulated genes in the liver, I used three common gene expression screens, (i) Representational Difference Analysis (RDA), (ii) microarray hybridization, and (iii) EST frequency analysis, to compare gene expression in the adult livers of C57BL/6J males and C57BL/6J- lit/lit males. These methods yielded overlapping and non-overlapping sets of growth hormone-regulated genes. I followed these studies by using quantitative RT-PCR to characterize several genes identified by the different techniques for their sex- and growth hormone-dependent regulation of gene expression in three strains of mice that vary greatly in susceptibility to liver cancer.