GROWTH-HORMONE TRANSGENES REGULATE THE EXPRESSION OF SEX-SPECIFIC ISOFORMS OF 3-BETA-HYDROXYSTEROID DEHYDROGENASE/DELTA-5-]4-ISOMERASE IN MOUSE-LIVER AND GONADS

The sexually dimorphic pattern of GH secretion regulates the expression of several steroidogenic enzymes in rat liver, including a male-specific 3beta-hydroxysteroid dehydrogenase/DELTA5-->4-isomerase (3betaHSD). Recently, we identified male-specific isoforms of immunoreactive 3betaHSD in mouse liver [42 kilodaltons (kDa)] and gonads (47 kDa). To test whether GH can regulate the expression of these murine 3betaHSDs, endogenous forms of 3betaHSD were studied in transgenic mice expressing heterologous GH transgene products. Mice from five transgenic lines were used; two expressed GH transgenes encoding the phosphoenolpyruvate carboxykinase (PEPCK) promoter fused to either the human (h) GH (somatogenic and lactogenic) or bovine (b) GH (somatogenic) structural genes, and three expressed GH transgenes encoding the mouse metallothionein-1 (MT1) promoter fused to the hGH, hGH variant (hGHv), or bGH structural genes. Control mice were normal nontransgenic littermates. Expression of a male-specific (42 kDa) isoform of hepatic 3betaHSD is dramatically suppressed in all transgenic mouse lines, as detected on Western immunoblots, without affecting a 47-kDa isoform expressed in livers of both male and female mice. This negative regulation was not observed in mouse kidney, which normally expresses two 3betaHSD isoforms (in both sexes) with molecular masses similar to those in liver. Considering that PEPCK and MT1 promoters direct expression of GH fusion genes in both tissues, the inhibition of hepatic, but not renal, 3betaHSD immunoreactivity suggests that GH affects sex-specific, rather than tissue-specific, expression of 3betaHSD. As in the liver, sex-specific expression of 3betaHSD in the testis is also suppressed by heterologous GH, but with one notable difference. Only human-derived GH (MT1-hGH and MTI-hGHv) effectively inhibits expression of the 47-kDa sex-specific isoform of testicular 3betaHSD, without affecting the 44-kDa isoform expressed in gonads of both male and female mice. These results suggest that the negative effects of heterologous GH on sex-specific 3betaHSDs may be mediated by PRL receptors in the testis and GH receptors in the liver. PEPCK-GH transgenes had little effect on testicular 3betaHSD, possibly because this promoter (unlike MT1) is relatively inactive in this tissue. In the liver of male transgenics (PEPCK-hGH), loss of the sex-specific (42-kDa) 3betaHSD has little effect on the K(m) for dehydroepiandrosterone (DHEA; 11 muM) compared with that in normal controls (16 muM). By contrast, in the liver of female transgenics (PEPCK-hGH), in which there is little effect on 3betaHSD immunoreactivity, the apparent K(m) for DHEA is markedly reduced (27 muM) compared with that in normal controls (62 muM). The testis is similar to the liver, in that loss of the sex-specific (47-kDa) testicular 3betaHSD in male transgenics has no effect on 3betaHSD activity (control, 0.17 muM; MTI-hGH, 0.15 muM; MT1-bGH, 0.17 muM). These results suggest that sex-specific isoforms of 3betaHSD in the liver and testis may not metabolize DHEA. Clearly, heterologous GH transgene products have dramatic inhibitory effects on the sex-specific expression of 3betaHSD in mouse liver and gonads. More striking, however, is the dissociation of these effects of heterologous GH on 3betaHSD activity and immunoreactivity. This reveals a complexity and diversity of mouse 3betaHSDs that is not understood and invokes regulatory and possibly functional correlations with hepatic steroid hydroxylases.