The SET protein promotes androgen production in testicular Leydig cells

Approximately 40% of middle-aged men exhibit symptoms of late-onset hypogonadism (LOH). However, the mechanism of androgen deficiency is still currently unclear. As shown in our previous studies, the SET protein is expressed in testicular Leydig cells and ovarian granule cells. This study was designed to investigate the effect of the SET protein on androgen production in Leydig cells. The AdCMV/SET and AdH1siRNA/SET adenoviruses were individually transduced into a cultured mouse Leydig cell line (mLTC-1) with or without human chorionic gonadotropin (HCG) stimulation in vitro. The primary mouse Leydig cells were used to confirm the main data from mLTC-1 cells. The SET protein was expressed in the cytoplasm and nucleus of mLTC-1 cells. Testosterone production was significantly increased in mLTC-1 cells overexpressing the SET protein compared with the control group (p < 0.05), whereas testosterone production was significantly decreased in the SET knockdown mLTC-1 cells (p < 0.05). Consistent with the testosterone levels, the expression levels of the steroidogenic acute regulatory (StAR) and cytochrome P450c17α-hydroxylase (CYP17a1) mRNAs and proteins synchronously changed according to the expression level of the SET protein. Interestingly, the expression of the SET protein was significantly increased in the mLTC-1 cells stimulated with 0.04 and 0.1 U/mL hCG. In the mLTC-1 cells transfected with AdH1siRNA/SET and concurrently stimulated with 0.1 U/mL hCG, both testosterone production and StAR expression were significantly lower than in the cells without SET knockdown (p < 0.05). In conclusion, the SET protein participates in regulating testosterone production by increasing the expression of StAR and CYP17a1, and it may be a downstream factor of the classic luteinizing hormone (LH)/luteinizing hormone receptor (LHR) signaling pathway. This study improves our understanding of the intracellular mechanism of testicular steroidogenesis and the pathophysiological mechanism of LOH in the aging male.