Regulation of heat shock protein synthesis by gonadotropins in cultured granulosa cells.

Primary cultures of granulosa cells can be stimulated to produce large amounts of progesterone by gonadotropins. This stimulation is associated with significant changes in the expression of several major proteins, as revealed by two-dimensional gel electrophoresis. These changes include a decrease in the synthesis of actin cytoskeleton proteins and an increase in the synthesis of a few abundant proteins, one of which is a mammalian heat shock protein, hsp90. Under culture conditions that have previously been shown to bring about the maturation of granulosa cells into progesterone-producing cells (i.e. treatment with gonadotropins or cAMP or by disrupting the actin cytoskeleton with cytochalasin), an increased synthesis of hsp90 could be demonstrated. Freshly isolated granulosa cells isolated from PMSG-treated animals synthesize hsp90 at a much higher level than cells isolated from diethylstilbestrol-treated rats. Kinetic studies have shown that granulosa cells isolated from diethylstilbestrol- or PMSG-treated rats synthesize high levels of hsp90 if maintained in culture in the presence of gonadotropins, but rapidly decrease hsp90 synthesis in the absence of gonadotropins and increase the synthesis of actin cytoskeleton proteins. Furthermore, in cells cultured for 48 h in the presence of cytochalasin-B followed by incubation for 24 h in the absence of the drug, the synthesis of hsp90 and several other proteins characteristic of mature granulosa cells decreased, while that of the actin cytoskeleton increased. In vitro translation assays and Northern blot analyses suggest that hsp90 synthesis in gonadotropin-stimulated cells may be regulated by mRNA translational efficiency. Taken together with recent findings in which hsp90 was identified in complex with cytoplasmic steroid receptors and the hormonal regulation of hsp90 content in target tissues, the results support the notion that hsp90 plays a role in the control of steroid hormone action.