1,721,039 research outputs found
Cross-talk between the signals hypoxia and glucose at the glucose response element of the L-type pyruvate kinase gene
No full textThe signals oxygen and glucose play an important role in metabolism, angiogenesis, tumorigenesis, and embryonic development. Little is known about an interaction of these two signals. We demonstrate here the cross-talk between oxygen and glucose in the regulation of L-type pyruvate kinase (L-PK) gene expression in the liver. In the liver the periportal to perivenous drop in O-2 tension was proposed to be an endocrine key regulator for the zonated gene expression. In primary rat hepatocyte cultures the expression of the L-PK gene on mRNA and on protein level was induced by venous pO(2), whereas its glucose-dependent induction occurred predominantly under arterial pO(2). It was shown by transient transfection of L-PK promoter luciferase and glucose response element (Glc(PK)RE) SV40 promoter luciferase gene constructs that the modulation by O-2 of the glucose-dependent induction occurred at the Glc(PK)RE in the L-PK gene promoter. The reduction of the glucose-dependent induction of the L-PK gene expression under venous pO(2) appeared to be mediated via an interference between hypoxia inducible factor-1 (HIF-1) and upstream stimulating factor at the Glc(PK)RE. The glucose response element also functioned as an hypoxia response element which was confirmed in cotransfection assays with Glc(PK)RE luciferase gene constructs and HIF-1 alpha expression vectors. Furthermore, it was found by gel shift and supershift assay that HIF-1 alpha and USF-1 or USF-2 could bind to the Glc(PK)RE. Our findings implicate that the cross-talk between oxygen and glucose might have a fundamental role in the regulation of several physiological and pathophysiological processes
Perivenous localization of insulin receptor protein in rat liver, and regulation of its expression by glucose and oxygen in hepatocyte cultures
No full textInsulin stimulates glucose utilization in the liver, which occurs mainly in the less aerobic, perivenous, zone. Accordingly, the insulin receptor protein was predominantly expressed in this area, although the insulin receptor mRNA was homogeneously distributed. In hepatocyte cultures venous O-2 partial pressure (pO(2)) induced insulin receptor protein expression. High glucose concentrations enhanced insulin receptor protein under arterial and venous pO(2). The induction of insulin receptor protein by venous pO(2) would explain its zonated expression
Regulation of the hypoxia-dependent plasminogen activator inhibitor I expression by MAP kinases
No full textMitogen-activated protein kinases (MAPKs) and protein kinase B (PKB) mediate growth and stress signals and have been implicated in the hypoxic response. Under hypoxic conditions, the expression of plasminogen activator inhibitor-1 (PAI-1) is mainly controlled by the hypoxia-inducible factor HIF-1. However, the role of MAPKs and PKB in HIF-1-mediated PAI-1 regulation is not clear. Treatment with the p38 inhibitor SB203580 and the PI3K inhibitor LY294002, but not with the MEK1 inhibitor PD98059, abrogated hypoxia-dependent PAI-1 induction in HepG2 cells. Consistently, overexpression of PKB or of the p38 upstream kinases MKK6 and MKK3 and of JNK, but not of ERK, enhanced PAI-1 mRNA levels. In MKK3-, MKK6- and PKB-expressing cells luciferase (Luc) activities from a hypoxia-inducible PAI-1-Luc construct or from a HIF-dependent Luc construct and, concomitantly, HIF-1alpha protein levels were enhanced. These findings indicate that p38- and PKB-dependent signalling pathways contribute to enhanced PAI-1 levels in the hypoxic response
Stimulation by portal insulin of intestinal glucose absorption via hepatoenteral nerves and prostaglandin E-2 in the isolated, jointly perfused small intestine and liver of the rat
No full textInsulin infused into the portal vein acutely enhanced intestinal glucose and galactose absorption via the sodium-dependent glucose cotransporter-1 in the isolated, jointly perfused small intestine and liver of the rat. Atropine and tetrodotoxin infused into the superior mesenteric artery completely prevented the portal insulin-dependent increase in intestinal glucose absorption, and carbachol caused an increase similar to that of portal insulin. Thus, a signal was transmitted against the bloodstream in a retrograde direction from the portal vein to the small intestine via hepatoenteral cholinergic nerves. The intracellular messenger in the enterocytes was cAMP, and the link between the muscarinic receptors, which do not increase cAMP concentrations, and adenylate cyclase was found to be prostaglandin E-2
Modulation of glucokinase expression by hypoxia-inducible factor 1 and upstream stimulatory factor 2 in primary rat hepatocytes
No full textGlucokinase (GK) is the key enzyme of glucose utilization in liver and is localized in the less aerobic perivenous area. Until now, the O-2-responsive elements in the liverspecific GK promoter are unknown, and therefore the aim of this study was to identify the O-2-responsive element in this promoter. We found that the GK promoter sequence -87/-80 matched the binding site for hypoxia inducible factor 1 (HIF-1) and upstream stimulatory factor (USF). In primary rat hepatocytes we could show that venous pO(2) enhanced HIF-1alpha and USF-2a levels, both of which activated GK expression. Furthermore, transfection experiments revealed that the GK sequence -87/-80 mediated the HIF-1alpha or USF-2-dependent activation of the GK promoter. The binding of HIF-1 and USF to the GKHRE was corroborated by electrophoretic mobility shift assay (EMSA). However, the maximal response to HIF-1alpha or USF was only achieved when constructs with the -87/ -80 sequence in context with a 39-36 bp native GK promoter sequence containing a hepatocyte nuclear factor 4 (HNF-4) binding site were used. HIF-1alpha and HNF-4 additively activated the GK promoter, while USF-2 and HNF-4 together did not show this additive activation. Thus, HIF-1 and USF may play differential roles in the modulation of GK expression in response to O-2
ATP-DEPENDENT TRANSPORT OF AMPHIPHILIC CATIONS ACROSS THE CANALICULAR MEMBRANE MEDIATED BY P-GLYCOPROTEIN
No full textNormal kinetics of intestinal glucose absorption in the absence of GLUT2: Evidence for a transport pathway requiring glucose phosphorylation and transfer into the endoplasmic reticulum
No full textGlucose is absorbed through the intestine by a transepithelial transport system initiated at the apical membrane by the cotransporter SGLT-1; intracellular glucose is then assumed to diffuse across the basolateral membrane through GLUT2. Here, we evaluated the impact of GLUT2 gene inactivation on this transepithelial transport process. We report that the kinetics of transepithelial glucose transport, as assessed in oral glucose tolerance tests, was identical in the presence or absence of GLUT2; that the transport was transcellular because it could be inhibited by the SGLT-1 inhibitor phlorizin, and that it could not be explained by overexpression of another known glucose transporter. By using an isolated intestine perfusion system, we demonstrated that the rate of transepithelial transport was similar in control and GLUT2(-/-) intestine and that it was increased to the same extent by cAMP in both situations. However, in the absence, but not in the presence, of GLUT2, the transport was inhibited dose-dependently by the glucose-6-phosphate translocase inhibitor S4048. Furthermore, whereas transport of [C-14]glucose proceeded with the same kinetics in control and GLUT2(-/-) intestine, [C-14]3-O-methylglucose was transported in intestine of control but not of mutant mice. Together our data demonstrate the existence of a transepithelial glucose transport system in GLUT2(-/-) intestine that requires glucose phosphorylation and transfer of glucose-6-phosphate into the endoplasmic reticulum. Glucose may then be released out of the cells by a membrane traffic-based pathway similar to the one we previously described in GLUT2-null hepatocytes
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