Epidermal Growth Factor Desensitizes the Gonadotropin-Responsive Adenylyl Cyclase in Membranes Isolated from MA-10 Ley dig Tumor Cells and Luteinized Rat Ovaries*

In a previous publication we showed that addition of mouse epidermal growth factor (mEGF) to MA-10 Leydig tumor cells or rat luteal cells leads to an attenuation of the elevated rate of cAMP accumulation provoked by subsequent addition of hCG. Those studies also suggested that this was due to a decrease in the hCG-activated adenylyl cyclase activity, but formal proof of this hypothesis was not presented. The experiments presented herein were conducted to investigate the mechanisms responsible for this phenomenon and show that our initial suggestion was correct, because we can show that mEGF attenuates the hCG-activated adenylyl cyclase in membranes from MA-10 cells or luteinized rat ovaries. More importantly, however, the establishment of a cell-free system in which mEGF attenuates adenylyl cyclase allows us to conduct detailed investigations on the molecular mechanisms that underly this phenomenon. Using this system we have been able to show that the ability of mEGF to attenuate adenylyl cyclase activity 1) is specific for the hCG-activated enzyme, since it cannot be detected when the enzyme is activated with other ligands, such as vasoactive intestinal peptide or isoproterenol; 2) does not appear to be due to a reduction in the activity of the catalytic subunit of adenylyl cyclase or the stimulatory GTP-binding protein (Gs), because mEGF does not affect the stimulation of adenylyl cyclase by other effectors, and it has little or no effect on the ability of Gs to restore isoproterenol sensitivity to the adenylyl cyclase of S49 cyc- membranes; and 3) optimal expression of the mEGF effect can only be obtained in the presence of ATP or GTP.