Expression and glycogenic effect of glycogen-targeting protein phosphatase 1 regulatory subunit G_L in cultured human muscle

Glycogen-targeting PP1 (protein phosphatase 1) subunit G_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_L mRNA levels in primary cultured human myotubes are similar to those in freshly excised muscle, unlike subunits G_M (gene PPP1R3A) or PTG (protein targeting to glycogen; gene PPP1R3C), which decrease strikingly. In cultured myotubes, expression of the genes coding for G_L, G_M and PTG is not regulated by glucose or insulin. Overexpression of G_L activates myotube GS (glycogen synthase), glycogenesis in glucose-replete and -depleted cells and glycogen accumulation. Compared with overexpressed G_M, G_L has a more potent activating effect on glycogenesis, while marked enhancement of their combined action is only observed in glucose-replete cells. G_L does not affect GP (glycogen phosphorylase) activity, while co-overexpression with muscle GP impairs G_L activation of GS in glucose-replete cells. G_L enhances long-term glycogenesis additively to glucose depletion and insulin, although G_L does not change the phosphorylation of GSK3 (GS kinase 3) on Ser⁹ or its upstream regulator kinase Akt/protein kinase B on Ser⁴⁷³, nor its response to insulin. In conclusion, in cultured human myotubes, the G_L gene is expressed as in muscle tissue and is unresponsive to glucose or insulin, as are G_M and PTG genes. G_L activates GS regardless of glucose, does not regulate GP and stimulates glycogenesis in combination with insulin and glucose depletion.