Down-regulation of gonadotropin receptors in a murine Leydig tumor cell line.

The murine Leydig tumor cell line, MLTC-1, has specific cell surface receptors for human chorionic gonadotropin (hCG), which are coupled to adenylate cyclase. When the cells were exposed to hCG, there was a loss of these receptors (down-regulation), which was both dose- and time-dependent. Down-regulation was inhibited by lowering the temperature, removing bound hormone or blocking protein synthesis with cycloheximide. Down-regulation was found to be a biphasic event. The initial phase was dependent upon the binding of hormone and had a half-time of approximately 3 h. The second phase occurred around 8 h after exposing the cells to the hormone and was apparently independent of bound hCG. It was related to increases in cyclic AMP since it could be mimicked by incubating the cells with choleragen or dibutyryl cyclic AMP and isobutylmethylxanthine. The reappearance of hormone receptors began approximately 24-32 h after the initial exposure to hCG and was complete within 48 h. Adenylate cyclase activity in membranes from control and down-regulated cells responded equally well to Mn2+ and NaF indicating that neither the catalytic or regulatory component (G/F) of the adenylate cyclase system had been lost during downregulation. Cholate extracts of control and down-regulated cells also were equally effective at reconstituting isoproterenol-stimulated adenylate cyclase activity in S49 cyc-membranes (which lack a functional G/F). Thus, down-regulation did not impair the ability of G/F to couple receptors to the catalytic component of adenylate cyclase.