The regulation of GLUT5 and GLUT2 activity in the adaptation of intestinal brush-border fructose transport in diabetes

Abstract

The adaptation of d-fructose transport in rat jejunum to experimental diabetes has been studied. In vivo and in vitro perfusions of intact jejunum with d-fructose revealed the appearance of a phloretin-sensitive transporter in the brush-border membrane of streptozotocin-diabetic rats which was not detectable in normal rats. The nature of the transporters involved was investigated by Western blotting and by d-fructose transport studies using highly purified brush-border and basolateral membrane vesicles. GLUT5, the major transporter in the brush-border membrane of normal rats, was not inhibited by d-glucose or phloretin. In contrast, GLUT2, the major transporter in the basolateral membrane of normal rats, was strongly inhibited by both D-glucose and phloretin. In brush-border membrane vesicles from diabetic rats, GLUT5 levels were significantly enhanced; moreover the presence of GLUT2 was readily detectable and increased markedly as diabetes progressed. The differences in stereospecificity between GLUT2 and GLUT5 were used to show that both transporters contributed to the overall enhancement of d-fructose transport measured in brush-border membrane vesicles and in vitro isolated loops from diabetic rats. However, overall d-fructose uptake in vivo was diminished. The underlying mechanisms and functional consequences are discussed.

Characterisation of glucose transporters in the intact coronary artery endothelium in rats: GLUT-2 upregulated by long-term hyperglycaemia

“…7). This is the first report of its kind with regard to the endothelium, but our results are consistent with the dramatic increases in GLUT-2 expression seen in rat intestinal enterocytes [59] and kidney proximal tubule cells [60] in response to STZ-induced diabetes. The presence of GLUT-2, and its increase in diabetes, would be expected to have profound effects on the ability of the endothelial cell to regulate its glucose uptake.…”

Stimulation of fructose transport across the intestinal brush-border membrane by PMA is mediated by GLUT2 and dynamically regulated by protein kinase C

“…Thus we have observed that GLUT5 levels in the brush-border membrane were enhanced almost 3-fold and that GLUT2 was readily detectable at the brush-border membrane in the jejunum of diabetic rats [5]. Crucially, GLUT2 was accessible to fructose in the luminal perfusate and was fully functional.…”

“…In many cell lines, PKC isoenzymes can regulate the extracellular signalregulated kinase\mitogen-activated protein kinase (ERK\MAP kinase) pathway, while in others the phosphatidylinositol 3-kinase pathway can regulate PKC isoenzymes, notably PKCξ. Both these pathways are regulated by insulin, which we have shown is involved in the regulation of both intestinal glucose and fructose transport [5,[9][10][11]. Moreover, in pancreatic islets, perfusion with glucose causes rapid translocation and activation of PKCα to the plasma membrane [12].…”

“…The systems for the luminal perfusion of jejunal loops in itro and in i o have been described in detail previously [5,11,13]. Because PKC activators and inhibitors take some time to become effective in whole tissue and because there is a limit to the time of viability for isolated loops in itro, the following procedure for the treatment of jejunum with drugs was adopted.…”

“…GLUT5 in rat is specific for fructose and is not inhibited by glucose even at ratios as high as 100 : 1 or by phloretin. GLUT2 on the other hand transports glucose as well as fructose and is inhibited by phloretin [5,6].…”