REGULATION OF SEX HORMONE-BINDING GLOBULIN SECRETION AND GENE-EXPRESSION BY CYCLOHEXIMIDE IN-VITRO

The role of protein synthesis in sex hormone-binding globulin (SHBG) s ecretion and gene expression was studied in HepG2 cell cultures. Inhib ition of protein synthesis by cycloheximide suppressed SHBG levels. Tr iiodothyronine and estradiol increased SHBG production, and cyclohexim ide reduced their effects to an extent which correlated with the degre e of suppression obtained with the drug alone. Insulin decreased SHBG production, and the effect of the treatment with insulin and cyclohexi mide together did not differ from that with cycloheximide alone. Cyclo heximide did not, alone or with the hormones, decrease SHBG levels mor e markedly extra- than intracellularly. Therefore, cycloheximide does not impair the secretion of SHBG which is synthesized in the presence of the drug. In contrast to SHBG protein levels, cycloheximide increas ed SHBG mRNA levels. When the effect of cycloheximide on the rate of S HBG mRNA decay was tested, the drug was found to extend the half-life of SHBG mRNA. Of the hormones, insulin decreased and triiodothyronine modestly increased SHBG mRNA levels, whereas estradiol had no clear ef fect. Treatment with cycloheximide together with any of the hormones r esulted in an increase in SHBG mRNA levels. We conclude that protein s ynthesis inhibition does not impair the secretion of SHBG produced und er such conditions, but stabilizes SHBG mRNA by removing some hepatic protein species involved in the regulation of its degradation.