RBP4 regulates androgen receptor expression and steroid synthesis in Sertoli cells from Bactrian camels.

Retinol-binding protein (RBP4) plays an important role in the transport and metabolism of retinol. In addition, RBP4 contributes to testicular homeostasis, including maintenance of spermatogenesis and synthesis of androgens that mediate their physiological functions through the androgen receptor. RBP4 in Sertoli cells regulates testosterone and dihydrotestosterone synthesis and secretion, although the mechanisms have yet to be revealed. In this study, we examined the expression and function of RBP4 in Sertoli cells isolated from Bactrian camels. qRT-PCR analysis of various Bactrian camel tissues revealed high expression of RBP4 in the testis and epididymis. To examine RBP4 function, Sertoli cells isolated from testes were transfected with an RBP4 overexpression plasmid or RBP4-targeting siRNA. RBP4 overexpression resulted in significant inhibition of transcription and translation of the steroidogenic enzymes 3βHSD and SRD5A1 concomitant with a significant decrease in androgen receptor expression and dihydrotestosterone secretion. Conversely, RBP4 knockdown significantly increased the expression of 3βHSD, SRD5A1 and androgen receptor and enhanced the secretion of dihydrotestosterone and testosterone. These data reveal a novel role for RBP4 in regulating steroid synthesis in Sertoli cells from Bactrian camels.