Ribozyme-mediated Suppression of the G Protein γ7Subunit Suggests a Role in Hormone Regulation of Adenylylcyclase Activity

Human HEK 293 cells present a simple and tractable system to directly test the hypothesis that the G protein gamma subunits contribute to the specificity of receptor signaling pathways in vivo. To begin to elucidate the functions of the individual gamma subunits in these cells, a ribozyme strategy was used to specifically inactivate the mRNA encoding the gamma7 subunit. A phosphorothioated DNA-RNA chimeric hammerhead ribozyme was constructed and analyzed for specificity toward the targeted gamma7 subunit. In vitro cleavage analysis of this ribozyme revealed a highly efficient cleavage activity directed exclusively toward the gamma7 RNA transcript. In particular, this ribozyme did not result in cleavage of the gamma12 RNA transcript, which is 75% identical to the gamma7 RNA transcript. Using a transient transfection assay, in vivo analysis of this ribozyme showed a specific reduction in both the mRNA and protein expression of the gamma7 subunit in HEK 293 cells. Coincident with this loss in gamma7 subunit, there was a specific reduction in the protein expression of the beta1 subunit, suggesting that the beta1 and gamma7 subunits may functionally interact to form a betagamma dimer in vivo. Functional analysis of the consequences of ribozyme-mediated suppression of the gamma7 subunit expression indicated that it was associated with significant attenuation of isoproterenol-, but not prostaglandin E1-, stimulated adenylylcyclase activity. Suppression of the gamma7 subunit expression had no effect on carbachol- and ATP-mediated stimulation of phosphatidylinositol turnover. Taken together, these results not only indicate the feasibility of using the ribozyme technology to determine the roles of individual gamma subunits in receptor-G protein-effector pathways in vivo, but they point to a specific role of the gamma7 subunit in the regulation of adenylylcyclase activity in response to isoproterenol.