Stimulation of electrogenic glucose transport by glycogen synthase kinase 3.

Glycogen synthase kinase 3 GSK3β participates in a wide variety of functions including regulation of glucose metabolism. It is ubiquitously expressed including epithelial tissues. However, whether GSK3β participates in the regulation of epithelial transport is not known. The present study thus explored whether GSK3β influences the Na(+)-coupled transport of glucose. To this end, SGLT1 was expressed in Xenopus oocytes with or without GSK3β and glucose-induced current (I(g)) determined by dual electrode voltage clamp. In Xenopus oocytes expressing SGLT1 but not in water-injected oocytes glucose induced an inwardly directed I(g), which was significantly enhanced by coexpression of GSK3β. According to chemiluminescence and confocal microscopy, GSK3β increased the SGLT1 protein abundance in the oocyte cell membrane. To explore whether GSK3β sensitivity of SGLT1 participates in the regulation of electrogenic intestinal glucose transport, Ussing chamber experiments were performed in intestinal segments from gene-targeted knockin mice with mutated and thus PKB/SGK-resistant GSK3α,β (gsk3(KI)), in which the serine of the PKB/SGK phosphorylation site was replaced by alanine, and from wild type mice (gsk3(WT)). The glucose-induced current was significantly larger in gsk3(KI) than in gsk3(WT) mice. The present observations reveal a novel function of GSK3, i.e. the stimulation of Na(+)-coupled glucose transport.