Regulatory mechanisms of intracellular distribution of Na+-dependent glucose transporter and the role in recovery from cellular injury

Exposure of cells or organs to sublethal stress induces the expression of some heat-shock proteins (Hsp), including Hsp70. In porcine renal LLC-PK1 cells, heat stress (HS) elevates Hsp70 expression and Na+-dependent glucose transport. We examined whether Na+-dependent glucose transporter (SGLT1) interacts with Hsp70 to elevate SGLT1 activity and whether SGLT1 activation is involved in the recovery from HS injury. HS (42degreesC for 3 h) elevated SGLT1 activity and expression of SGLT1 in the apical membrane fraction. HS increased the maximal transport rate (V-max), but did not affect the apparent affinity constant (K-m) for glucose. The HS-induced SGLT1 activation was inhibited by anti-transforming growth factor (TGF)-beta1 antibody. Furthermore, transfection of anti-Hsp70 antibody into the cells inhibited SGLT1 activation. These results suggest that HS induces TGF-beta1 secretion, and then Hsp70 forms a complex with SGLT1 and increases the distribution of SGLT1 in the apical membrane. Next, we examined the effect of HS on plasma membrane integrity. Accumulation of calcein, a membrane-impermeable fluorescent dye, was decreased by HS and then returned to basal level. This recovery was inhibited by phloridzin, a selective SGLT inhibitor, and nonmetabolizable glucose analogues. Anti-TGF-beta1 antibody also inhibited the recovery of calcein accumulation. Taken together, the present results show that HS elevates SGLT1 activity mediated via the TGF-beta1 signaling pathway and that the increase in glucose uptake is necessary to repair plasma membrane injury.