Expression of cyclin-dependent kinase 5 and associated cyclins in Leydig and Sertoli cells of the testis

In this study, we examined the expression and subcellular localization of cyclin-dependent kinase 5 (Cdk5), cyclin D1, and cyclin E in Leydig and Sertoli cell lines that were cultured with 7.5, 1.0, 0.5, or 0% serum (mixture of a 2:1 ratio of horse serum and fetal bovine serum) and in the developing rat testis to verify the possible functions of Cdk5, cyclin D1, and cyclin E in the testis. The abundance of Cdk5 and cyclin E in the Leydig cell line, TM3, was significantly reduced at low serum concentrations. In contrast, serum concentration had no effect on Cdk5 and cyclin E levels in the Sertoli cell line, TM4. Cyclin D1 was detected by western blot analysis in TM4 cells only, and its abundance was serum dose dependent. The kinase activity of Cdk5 in TM3 and TM4 cells that were cultured at various serum concentrations coincided with the levels of Cdk5 expression. Immunohistochemical staining for Cdk5 and cyclin E revealed nuclear and cytoplasmic distribution. both in TM3 and TM4 cells. Moreover, cyclin D1 immunoreactivity was only detected in TM4 cells. In the developing rat testis, Cdk5 expression was most prominent at 2 and 3 weeks after birth. Cyclin D1 was strongly expressed at 1 and 2 weeks in premature rat testes. On the other hand, cyclin E was highly expressed in the adult testis. Immunohistochemical localization of Cdk5, cyclin D1, and cyclin E in 1-week-old and adult rat testes revealed expression in both Leydig and Sertoli cells. Our results suggest that Cdk5 in TM3 and Leydig cells of the testis might play a role in cell cycle regulation, whereas Cdk5 in TM4 and Sertoli cells of the adult testis might have some additional functions besides control of proliferation.