Interaction between the expression of retinol binding protein 4 and gonadotropin receptors in follicular granulosa cells of pigs

Retinol binding protein 4 (RBP-4) transports retinol from the liver to peripheral tissues, and the function is thought to be regulated by gonadotropins in the reproductive tissues. This study was conducted to measure the expression of RBP-4 in porcine ovaries and further investigate the interaction between the expression of RBP-4 and gonadotropin receptors in cultured granulosa cells. The expression of RBP-4 was detected by the immunohistochemical analysis of ovarian tissues and by western blot and real-time PCR analysis of the granulosa cells isolated from follicular fluids. The results showed that RBP-4 was located in the granulosa cell layer and inner theca layer of porcine follicles. Moreover, real-time PCR and western blot revealed that the expression levels of RBP-4 mRNA and protein in the granulosa cells treated with exogenous follicle-stimulating hormone (FSH) and luteinizing hormone (LH) were significantly higher than those in the controls (P < 0.05). Granulosa cells treated with RBP-4 also showed an increased expression of follicle-stimulating hormone receptor (FSHR) and luteinizing hormone receptor (LHR). Therefore, we conclude that RBP-4 might regulate the ovarian function through its interaction with gonadotropins.