Cross-regulation between G-protein-mediated pathways: Stimulation of adenylyl cyclase increases expression of the inhibitory G-protein, G_iα2

The hormone-sensitive adenylyl cyclase system is under dual control, receiving both stimulatory and inhibitory inputs. Guanine nucleotide-binding regulatory proteins (G-proteins) transduce signals from cell surface receptors to effectors such as adenylyl cyclase. Hormonal stimulation is propagated via G_s, inhibition by G_i. Persistent (24-h) activation of the stimulatory pathway of adenylyl cyclase by the diterpene forskolin or the β-adrenergic agonist isoproterenol in S49 mouse lymphoma cells enhanced the effects of somatostatin mediated via the inhibitory pathway of adenylyl cyclase. Stimulating cells with forskolin or isoproterenol for 24 h resulted in a 3-fold increase in the steady-state levels of G_iα2 and a 25% decline in G_sα, as quantified by immunoblotting. Within 12 h of stimulation of adenylyl cyclase, G_iα2 mRNA levels increased 4-fold, measured by DNA-excess solution hybridization. G_sα mRNA levels, in contrast, increased initially (25%), but then declined to 75% of control. In S49 variants that lack functional protein kinase A (kin^-), stimulation by isoproterenol failed to alter G_iα2 expression at either the protein or the mRNA levels. A 3-fold increase in relative synthesis rate and no change in the half-life (∼80 h) of G_iα2 was observed in response to forskolin stimulation. Although G_sα synthesis increased (70%) modestly in response to forskolin stimulation, the half-life of G_sα actually decreased from 55 h in naive cells to 34 h in treated cells. Thus, the two G-protein-mediated pathways controlling adenylyl cyclase display "cross-regulation." Persistent activation of the stimulatory pathway increases G_1α2 mRNA and expression. Transiently elevated G_sα mRNA levels are counterbalanced by a reduction in the half-life of the protein.