DUAL COUPLING AND DOWN-REGULATION OF HUMAN FSH RECEPTOR IN CHO CELLS

The FSH receptor is a member of the family of G protein-coupled recept ors that activate adenylyl cyclase. The binding of agonist to cell sur face receptors leads to a reduction in the intensity of the response t o continuous stimulation, a process that is usually referred to as des ensitization. Although the exact mechanism is not fully understood, th e molecular cloning of the FSH receptor has made it possible to study desensitization in transfected cell lines. In this experiment FSH-indu ced desensitization was studied using Chinese hamster ovary cells expr essing a functional human FSH receptor (CHO-FSHR cells). Stimulation o f the CHO-FSHR cells with 10 ng/ml human FSH resulted in a decreased s ensitivity to a second FSH stimulation. This decrease in FSH-induced c AMP production was observed within 2 h, and exposure of cells to FSH f or 20 h led to a 70-80 % inhibition of cAMP formation. Moreover, the d esensitization effect observed in CHO cells was mimicked by forskolin and, therefore, was mediated by cAMP. Incubation of cells with I-125-F SH showed an efficient internalization of the ligand in the CHO-FSHR c ells. The CHO-FSHR cells rapidly internalized approximately 30% of the receptor-associated I-125-FSH by 2 h and 50% by 4 h. The responsivene ss of individual CHO-FSHR cells to FSH was studied and administration of human FSH(30 ng/ml) induced a rapid rise in cytosolic calcium, reac hing a peak at 6 sec. The data that human FSH can increase intracellul ar calcium in cells transfected with the FSH receptor cDNA reveal the possibility for the human FSH receptor to couple to both adenylyl cycl ase and phospholipase C cascades.