The role of the proto-oncogene Kit in gametogenesis and steroidogenesis.

摘要

The Kit receptor tyrosine kinase and its cognate ligand kit ligand, Kitl, have critical roles in hematopoiesis, melanogenesis, and gametogenesis. In gametogenesis roles for Kit are known in primordial germ cells and in oocyte and spermatogonial development. Although Kit is known to be expressed in Leydig cells, no role for Kit is known in this cell type. The purpose of this thesis was to investigate a role for Kit in Leydig cells.; To examine the effects of kitL on murine male steroidogenesis, the Leydig cells of two to three month old C57B16/J wild type mice were isolated and stimulated with kit ligand (kitL). Testosterone synthesis was monitored by testosterone radioimmunoassay (RIA). Mutant mice (designated kitY719F/Y719F or Y719F for short) lacking the ability to signal through the p85 subunit of the phosphatidylinositol 3-kinase pathway after kit stimulation were also studied. Results revealed that kitL had a stimulatory effect on testosterone biosynthesis in wild type mice whereas cells from mutant mice were unaffected. Use of the fungal metabolite wortmannin to block the phosphatidylinositol 3-kinase signaling pathway reduced testosterone biosynthetic abilities in wild type mice, indicating that kitL-mediated testosterone biosynthesis works through the PI3-kinase pathway.; Isolated Leydig cell fractions from Y719F mutant mice had the ability to synthesize higher than wild type levels of testosterone. Serum testosterone levels, however, were unchanged compared to wild type levels. Since luteinizing hormone (LH) levels in the mutant were found to be higher than wild type, and LH is known to stimulate Leydig cell proliferation, a model is proposed in which testosterone levels are controlled by a combination of Leydig cell death and mitosis. Leydig cells of Y719F mutant animals are unable to respond as effectively to kitL stimulation, initially have lower levels of testosterone synthesis and high levels of Leydig cell apoptosis, effectively lowering testosterone in the serum. By a feedback mechanism, the lower serum testosterone levels cause the anterior pituitary to secrete additional luteinizing hormone, leading ultimately (in 16 week animals) to wild type levels of serum testosterone and an observed hyperplastic Leydig cell phenotype.