Follitropin (FSH) and a phorbol ester stimulate the phosphorylation of the FSH receptor in intact cells.

Using a cell line stably transfected with the rat follitropin (FSH) receptor cDNA we demonstrate that the FSH receptor becomes phosphorylated when cells are exposed to FSH. Since binding of FSH to its receptor results in an increase in cAMP and inositol phosphate accumulation, we examined the potential involvement of protein kinase A and C in mediating receptor phosphorylation. Stimulation of protein kinase A does not appear to be necessary because hFSH-induced receptor phosphorylation was minimally impaired in a cell line that overexpresses cAMP phosphodiesterase. Moreover, stimulation of the protein kinase A pathway with other agonists result in minimal phosphorylation of the FSH receptor. Stimulation of the protein kinase C with a phorbol ester did result in an increase in receptor phosphorylation, and down-regulation of the protein kinase C decreased, but did not abolish, the FSH-induced receptor phosphorylation. The possible impact of phosphorylation on the functions of the receptor was examined by testing if conditions that lead to phosphorylation decrease the ability of FSH to stimulate cAMP synthesis. Our data show that as with the addition of FSH, addition of a phorbol ester also results in a decrease in the ability of FSH to stimulate cAMP synthesis.