Evidence for independent control at the mRNA and protein levels of cellular retinol binding protein 1 in rat Sertoli cells

Cellular retinol binding protein 1 (CRBP1) is the cytosolic carrier for ret inol. It is expressed in many tissues, but the concentrations vary consider ably. In Sertoli cells from immature rat testis, CRBP1 is highly expressed. The results of the present study show that regulation of CRBP1 expression at the protein level appears to be independent of regulation at the mRNA le vel. In Sertoli cells from prepubertal 19-day-old rats, CRBP1 mRNA is stron gly induced for up to 72 h by the presence of serum factors. Ln contrast, t reatment of the cells with cAMP analogues led to a rapid reduction in mRNA to quantities less than 5% of control values. However, the changes in CRBP1 mRNA did not lead to similar changes in the concentration of CRBP1 protein during 72 h of observation. Similarly, treatment of cells from 32- and 44- day-old rats with serum led to increased CRBP1 mRNA, whereas cAMP treatment resulted in a decrease in CRBP1 mRNA. Again, no changes were observed in t he concentration of CRBP1 protein. Furthermore, co-incubation of Sertoli ce lls from 19-day-old rats with purified pachytene spermatocytes or round spe rmatids resulted in an increase in mRNA for CRBP1. However, comparable chan ges in CRBP1 protein concentrations were not observed. Neither cAMP nor ser um changed the fraction of CRBP1 mRNA that was associated with polysomes. A s a possible explanation for some of the results, pulse-chase experiments s howed that the rate of CRBP1 degradation in cultured Sertoli cells is decre ased by cAMP. It is proposed that these changes at the level of protein tur nover contribute to the maintenance of stable concentrations of CRBP1 even when the corresponding mRNA concentrations vary markedly.