Chronic somatostatin treatment induces enhanced forskolin-stimulated cAMP accumulation in wild-type S49 mouse lymphoma cells but not in protein kinase-deficient mutants.
Many different types of cells exhibit a supersensitivity of adenylate cyclase after chronic treatment with inhibitory drugs; this phenomenon is manifested by enhanced cAMP accumulation upon removal of the inhibitory drug. Acute treatment of wild-type S49 cells with the somatostatin analog SMS 201-995 (SMS) results in inhibition of cAMP accumulation. We have found that chronic SMS treatment of S49 cells results in enhanced isoproterenol- and forskolin-stimulated cAMP accumulation after removal of the SMS. The forskolin-stimulated cAMP synthetic rate was about 57% higher in SMS-pretreated cells (14.22 +/- 1.02 pmol of cAMP/10(6) cells/min) than in untreated control cells (9.08 +/- 0.84 pmol of cAMP/10(6) cells/min). The time course of forskolin-stimulated intracellular cAMP accumulation is complex, with desensitization of cAMP synthesis and marked egress of cAMP from the cells. We have modeled the forskolin-stimulated cAMP time course to a simple function incorporating the initial synthetic rate and rate constants for desensitization and elimination (degradation plus egress). The mathematical modeling suggests that the difference in forskolin-stimulated cAMP time courses between control and SMS-pretreated cells can be explained on the basis of a difference in initial synthetic rates. We tested the hypothesis that the SMS-induced change in forskolin-stimulated cAMP accumulation is triggered by the decrement in the concentration of intracellular cAMP caused by SMS. We studied two independently isolated mutants of S49 cells that are devoid of cAMP-dependent protein kinase activity (kin-). Although SMS acutely inhibits cAMP accumulation in both kin- mutants, neither mutant exhibited an enhanced forskolin-stimulated cAMP synthetic rate after chronic SMS treatment. These results suggest that cAMP-dependent protein kinase is important in the induction of adenylate cyclase supersensitivity in wild-type S49 cells. The mechanistic signal for induction of supersensitivity may be the decreased cAMP accumulation that occurs in response to stimulation of inhibitory receptors, although other hypothetical mechanisms may be invoked.
Thomas
J M
JM
Department of Medicine, Stanford University School of Medicine, Palo Alto.
Hoffman
B B
BB
D000067956:Adenylyl Cyclase Inhibitors; D000818:Animals; D005576:Colforsin / Q000494:pharmacology*; D000242:Cyclic AMP / Q000378:metabolism*; D007545:Isoproterenol / Q000494:pharmacology; D008223:Lymphoma / Q000378:metabolism; D051379:Mice; D011494:Protein Kinases / Q000502:physiology*; D013004:Somatostatin / Q000494:pharmacology*; D014407:Tumor Cells, Cultured
D000067956:Adenylyl Cyclase Inhibitors; D005576:Colforsin; D013004:Somatostatin; D000242:Cyclic AMP; D011494:Protein Kinases; D007545:Isoproterenol