Ovariectomy Increases Circulating Retinol-Binding Protein Concentrations Independently of Sex-Dependent Differences in Retinol Concentrations in Rats

Background: There is a sex-dependent difference in blood retinol and RBP concentrations, and plasma RBP is associated with insulin resistance.Objectives: We aimed to clarify sex-dependent variations in body concentrations of retinol and RBPs and their association with sex hor-mones in rats.Methods: Plasma and liver retinol concentrations and hepatic mRNA and plasma concentrations of RBP4 were analyzed in 3-and 8-wk-old male and female Wistar rats before and after sexual maturity (experiment 1) and in orchiectomized male Wistar rats (experiment 2) and ovariectomized female Wistar rats (experiment 3). Furthermore, the mRNA and protein concentrations of RBP4 in adipose tissue were measured in ovariectomized female rats (experiment 3).Results: There were no sex-dependent differences in liver retinyl palmitate and retinol concentrations; however, the plasma retinol con-centration was significantly higher in male rats than that in female rats after sexual maturity. Furthermore, the plasma retinol concentrations did not differ between the ovariectomized or orchiectomized rats and the control rats. Plasma Rbp4 mRNA concentrations were higher in male rats than those in female rats but not in castrated and control rats, a change consistent with plasma retinol concentration. Plasma RBP4 concentrations were also higher in male rats than those in female rats; however, unlike liver Rbp4 gene expression, plasma RBP4 con-centrations were 7-fold higher in the ovariectomized rats than those in the control rats. Moreover, the Rbp4 mRNA concentrations in inguinal white adipose tissue was significantly higher in the ovariectomized rats than those in the control rats and correlated with plasma RBP4 concentrations. Conclusions: Hepatic Rbp4 mRNA is higher in male rats through a sex hormone-independent mechanism, which may contribute to sex differences in blood retinol concentrations. Furthermore, ovariectomy leads to an increase in adipose tissue Rbp4 mRNA and blood RBP4 concentrations, which may contribute to insulin resistance in ovariectomized rats and postmenopausal women.

Automated summary

This study found that there are sex-dependent differences in blood retinol and RBP concentrations, which are associated with sex hormones. The expression of hepatic Rbp4 mRNA is higher in male rats, but there is no difference in castrated and control rats. Ovariectomy leads to increased Rbp4 mRNA and blood RBP4 concentrations in adipose tissue, which may contribute to insulin resistance in ovariectomized rats and postmenopausal women.