GLUT2 protein at the rat proximal tubule brush border membrane correlates with protein kinase C (PKC)-βl and plasma glucose concentration

Abstract

Aims/hypothesis GLUT2 is the main renal glucose transporter upregulated by hyperglycaemia, when it becomes detectable at the brush border membrane (BBM). Since glucoseinduced protein kinase C (PKC) activation in the kidney is linked to diabetic nephropathy, we investigated the effect of glycaemic status on the protein levels of PKC isoforms α, βI, βII, δ and ɛ in the proximal tubule, as well as the relationship between them and changes in GLUT2 production at the BBM. Methods Plasma glucose concentrations were modulated in rats by treatment with nicotinamide 15 min prior to induction of diabetes with streptozotocin. Levels of GLUT2 protein and PKC isoforms in BBM were measured by western blotting. Additionally, the role of calcium signalling and PKC activation on facilitative glucose transport was examined by measuring glucose uptake in BBM vesicles prepared from proximal tubules that had been incubated either with thapsigargin, which increases cytosolic calcium, or with the PKC activator phorbol 12-myristate,13-acetate (PMA).Results Thapsigargin and PMA enhanced GLUT-mediated glucose uptake, but had no effect on sodium-dependent glucose transport. Diabetes significantly increased the protein levels of GLUT2 and PKC-βI at the BBM. Levels of GLUT2 and PKC-βI correlated positively with plasma glucose concentration. Diabetes had no effect on BBM levels of α, βII, δ or ɛ isoforms of PKC. Conclusions/interpretation Enhanced GLUT2-mediated glucose transport across the proximal tubule BBM during diabetic hyperglycaemia is closely associated with increased PKC-βI. Thus, altered levels of GLUT2 and PKC-βI proteins in the BBM may be important factors in the pathogenic processes underlying diabetic renal injury.

Experimental type II diabetes and related models of impaired glucose metabolism differentially regulate glucose transporters at the proximal tubule brush border membrane

“…2A). As previously demonstrated (Marks et al 2003;Goestemeyer et al 2007), exposure to streptozotocin induced hyperglycaemia (Table 1). Interestingly, while GLUT2 and PKC-βI expression at the BBM was significantly increased, SGLT1 and SGLT2 levels were unaffected 1-week after streptozotocin injection (Fig.…”

“…The present study used animal models of diabetes and models of diet-induced obesity and insulin resistance of differing aetiology to assess the effects of insulin resistance and hyperglycaemia on expression of glucose transporters at the renal BBM. Our previous findings have shown that expression of GLUT2 and its regulator, PKC-βI, are increased in a model of established type I diabetes (Marks et al 2003) and that their expression levels are correlated with changes in glucose concentration within the pathophysiological range (Goestemeyer et al 2007). …”

“…Adaptation of glucose transport at the BBM has been documented in diabetes; in particular, the facilitated transporter, GLUT2, is readily detectable at the BBM in animals 2-4 weeks after induction of type I diabetes (Marks et al 2003). Indeed, expression of GLUT2 shows a positive correlation with blood glucose concentrations, and it has been proposed that increased renal GLUT2 expression at the BBM is mediated by protein kinase C-βI (PKC-βI), because concentrations of this signalling molecule also show a positive correlation with both the expression of GLUT2 and blood glucose concentrations (Goestemeyer et al 2007). Although there are conflicting reports on the impact of diabetes on SGLT-mediated transport (Marks et al 2003;Albertoni Borghese et al 2009), inhibitors of SGLT2 have been demonstrated to increase glycosuria and reduce hyperglycaemia in type II diabetes (Han et al 2008).…”

The proximal tubule in the pathophysiology of the diabetic kidney

“…Moreover, STZ-diabetes can target GLUT2 protein (but not GLUT1) to the brush border membrane of proximal tubules (87). The latter may be linked to protein kinase C PKC␤1 activation (49,107,108) and implicates facilitative glucose transport in the increased glucose reabsorption across the apical membrane in the diabetic kidney (Fig. 1B).…”

Reduced cortical renal GLUT1 expression induced by angiotensin-converting enzyme inhibition in diabetic spontaneously hypertensive rats

“…In the early S1 segment, where the bulk of filtered glucose is reabsorbed, the low affinity/ high capacity glucose transporters, SGLT2 and GLUT2 are co-expressed in the luminal brush border membrane and in the basolateral membrane, respectively. Increases in the cortical GLUT2 gene expression have been extensively reported in diabetes (9)(10)(11)(12)(13)(14)(15), and are important for renal glucose reabsorption maintenance in this condition, since high blood and interstitial glucose concentrations may lower the outwardly directed glucose gradient from tubule to blood (11). GLUT1 protein is also detected in the outer renal cortex, where it is not related to the tubule epithelial cells, but to the mesangial cells (16).…”