A Role for Guanyl Nucleotide-Binding Regulatory Protein β- andγ -Subunits in the Expression of the Adrenocorticotropin Receptor

Mutant Y1 mouse adrenocortical tumor cells, isolated on the basis of their resistance to the growth-inhibitory effects of forskolin, arise from single mutational events. These mutants present complex phenotypes in which the activity of Gβ/γ is impaired, ACTH receptor gene expression is markedly diminished, and ACTH-responsive adenylyl cyclase activity is lost. In this study, we have tested the hypothesis that the impairment in Gβ/γ activity is responsible for the loss of ACTH receptor gene expression and ACTH-responsive adenylyl cyclase activity. Transfection of one of the mutant clones with expression vectors encoding either Gβ1 or Gβ2 together with Gγ2 increased ACTH receptor expression and restored ACTH-responsive adenylyl cyclase activity. Interestingly, either Gβ2 or Gγ2 alone was effective. These results thus support the hypothesis that the impairment in Gβ/γ activity is responsible for the loss of ACTH receptor expression. A luciferase reporter plasmid driven by the proximal promoter region of the mouse ACTH receptor gene was expressed poorly in the mutants compared with parental Y1 cells, suggesting that the Gβ/γ defect compromised transcriptional activity at the proximal promoter region of the ACTH receptor gene.