Comparison of steroidogenic potencies of homologous and heterologous gonadotropins in rat and mouse Leydig cells

Abstract

The in vitro steroidogenic potencies have been determined in rat and mouse Leydig cells for two homologous human gonadotropins, lutropin (hLH) and choriogonadotropin (hCG), and two heterologous gonadotropins, hCGβ wild-type and the product of an hCGβ clone containing a premature termination codon at position 122, each associated with a co-expressed bovine α subunit. hCG was found to be more potent than hLH in rat, but not mouse Leydig cells, and the heterologous gonadotropin containing the truncated hCGβ subunit was equipotent to that with hCGβ wild-type in both rat and mouse Leydig cells. Persistent steroidogenesis was determined by measuring testosterone production following pre-incubation with each of the above four gonadotropins and with ovine LH, subsequent washing of the cells, and re-incubation in the absence and presence of additional hormone. Interesting differences were found with the five gonadotropins in rat and mouse Leydig cells. The testosterone response to all gonadotropins in rat Leydig cells was essentially the same whether or not additional hormone was added after the initial cell incubation and washing. In contrast, only hLH and hCG yielded identical responses in mouse Leydig cells in the presence and absence of additional hormone, testosterone production being invariably lower with ovine LH and the two expressed heterologous gonadotropins unless they were present in the second incubation. In summary, the hCGβ C-terminal sequence from residues 122–145 makes no discernible contribution to the in vitro potency in rat or mouse Leydig cells. Since rat Leydig cells are more sensitive to hCG than hLH, the amino acid differences in the core region of hCGβ compared to hLHβ, or the carbohydrate differences between the two β subunits, probably contribute to this difference in potency. Such differences, however, are not manifested in mouse Leydig cells, presumably reflecting subtle differences in the receptors or in the intracellular coupling mechanisms. Lastly, the results indicate that the α subunit may exert a modulatory role on the level of steroidogenic persistence.