Expression and glycogenic effect of glycogen-targeting protein phosphatase 1 regulatory subunit G sub(L) in cultured human muscle

Glycogen-targeting PP1 (protein phosphatase 1) subunit G sub(L) (coded for by the PPP1R3B gene) is expressed in human, but not rodent, skeletal muscle. Its effects on muscle glycogen metabolism are unknown. We show that G sub(L) mRNA levels in primary cultured human myotubes are similar to those in freshly excised muscle, unlike subunits G sub(M) (gene PPP1R3A) or PTG (protein targeting to glycogen; gene PPP1R3C), which decrease strikingly. In cultured myotubes, expression of the genes coding for G sub(L), G sub(M) and PTG is not regulated by glucose or insulin. Overexpression of G sub(L) activates myotube GS (glycogen synthase), glycogenesis in glucose-replete and -depleted cells and glycogen accumulation. Compared with overexpressed G sub(M), G sub(L) has a more potent activating effect on glycogenesis, while marked enhancement of their combined action is only observed in glucose- replete cells. G sub(L) does not affect GP (glycogen phosphorylase) activity, while co-overexpression with muscle GP impairs G sub(L) activation of GS in glucose-replete cells. G sub(L) enhances long-term glycogenesis additively to glucose depletion and insulin, although G sub(L) does not change the phosphorylation of GSK3 (GS kinase 3) on Ser super(9) or its upstream regulator kinase Akt/protein kinase B on Ser super(473), nor its response to insulin. In conclusion, in cultured human myotubes, the G sub(L) gene is expressed as in muscle tissue and is unresponsive to glucose or insulin, as are G sub(M) and PTG genes. G sub(L) activates GS regardless of glucose, does not regulate GP and stimulates glycogenesis in combination with insulin and glucose depletion.