Serine/threonine-protein kinase B-raf
positive regulation of D-glucose transmembrane transport
Stimulation of the Na(+)-coupled glucose transporter SGLT1 by B-RAF.
Gain of function mutations of B-RAF, a serine/threonine protein kinase may lead to development of tumor cells. As tumor cells mainly utilize glucose as fuel, their survival critically depends on their ability to accumulate glucose from extracellular space. The Na(+)-coupled glucose transporter SGLT1 accomplishes concentrative cellular glucose uptake against a chemical glucose gradient and thus even at low extracellular glucose concentrations. SGLT1 contributes to glucose uptake in several tumor cells. The present study thus explored whether B-RAF activates SGLT1. To this end, SGLT1 was expressed in Xenopus oocytes with or without additional coexpression of B-RAF and electrogenic glucose transport was determined by dual electrode voltage clamp. In SGLT1-expressing oocytes but not in oocytes injected with water the addition of glucose to the extracellular bath generated a current (I(g)), which was significantly increased following coexpression of wild-type B-RAF. According to kinetic analysis, coexpression of B-RAF enhanced the maximal transport rate without significantly modifying the affinity of the carrier. According to chemiluminescence and confocal microscopy experiments, B-RAF enhanced the Na(+)-coupled glucose transporter SGLT1 protein abundance in the cell membrane. Exposure of the Xenopus oocytes to Brefeldin A (5μM), an inhibitor of vesicle insertion, was followed by a decline of I(g), which was higher in oocytes expressing SGLT1 together with B-RAF than in oocytes expressing SGLT1 alone. In conclusion, B-RAF upregulates SGLT1 activity, an effect requiring vesicle insertion into the cell membrane.