THE REGULATION OF GLYCOGEN-SYNTHASE BY PROTEIN PHOSPHATASE-1 IN 3T3-L1 ADIPOCYTES - EVIDENCE FOR A POTENTIAL ROLE FOR DARPP-32 IN INSULIN ACTION

The stimulation of glycogen-targeted protein phosphatase 1 (PP1), glyc ogen synthase, and glycogen synthesis by insulin was examined during t he differentiation of 3T3-L1 fibroblasts into adipocytes. Insulin trea tment barely changed the low levels of glycogen synthesis measured in fibroblasts. Following differentiation into adipocytes, insulin increa sed glycogen synthesis up to 40-fold. After further culturing of the a dipocytes for a week, insulin stimulated glycogen accumulation 700-fol d, Differentiation of 3T3-L1 cells also resulted in the increased expr ession of glycogen synthase and in increases in both total glycogen sy nthase activity and -fold stimulation by insulin, While the levels of PP1 protein were unchanged by differentiation, PP1 specific activity d ecreased over 60%, although sensitivity to insulin treatment was augme nted. Concurrently, levels of the PP1 inhibitor protein DARPP-32 were dramatically induced upon 3T3-L1 adipogenesis. DARPP-32 in both 3T3-L1 and primary rat adipocytes was exclusively localized to the particula te fractions, including the glycogen enriched pellet. PP1 activity fro m 3T3-L1 adipocytes exhibited a kinetic lag in vitro, which was not pr esent in fibroblast extracts, Insulin pretreatment of the adipocyte ce lls overcame the in vitro lag in PP1 activity, resulting in up to 5-fo ld stimulation of PP1 activity being measured at early assay time poin ts, These results suggest that in 3T3-L1 adipocytes, DARPP-32 may main tain glycogen-targeted PP1 activity in a low basal state, priming the phosphatase for stimulation by insulin.