A central role for cyclic AMP, but not progesterone, in luteinizing hormone receptor down-regulation in the granulosa cell.

Abstract

Although luteinizing hormone (LH) is known to down-regulate its own receptor in several gonadal cell types, the mechanisms underlying this process are poorly understood. To elucidate these mechanisms we have examined the role of cAMP and progesterone in LH-stimulated down-regulation of the LH receptor, using cultured granulosa cells as a model. LH receptors were induced by culturing the cells with follicle-stimulating hormone for 2 days, and once induced, could be down-regulated by a brief exposure to LH. Down-regulation also occurred when cells were cultured with activators of adenylate cyclase, inhibitors of phosphodiesterase, or analogues of cAMP. Cholera toxin and N6,O2'-dibutyryladenosine 3':5'-cyclic monophosphate, like LH, decreased the number of LH receptors, without affecting affinity for 125I-human chorionic gonadotropin (hCG). The extent of receptor loss after treatment with LH plus cholera toxin was no greater than that caused by LH alone. LH, hCG, and deglycosylated hCG, which binds to the LH receptor but has little bioactivity, caused down-regulation, and their relative capacity to cause down-regulation was highly correlated with their relative capacity to stimulate cAMP production. Indirect evidence suggested that maximal down-regulation requires activation of adenylate cyclase for at least 3 h. Consistent with this idea, a 3-h exposure to dibutyryl cAMP caused near-maximal down-regulation. Progesterone secretion was enhanced by all agents that caused down-regulation of the LH receptor; however, there was little correlation between progesterone secretion and down-regulation. Furthermore, maximal down-regulation occurred when progesterone secretion was inhibited greater than 99% with cyanoketone. These data indicate that cAMP, but not progesterone, plays a central role in LH receptor down-regulation in the granulosa cell and that elevation of intracellular cAMP levels for 3 h is both necessary and sufficient to trigger maximal down-regulation.