High glucose activates Hypoxia Inducible Factor 1 alpha (HIF1alpha) and HIF2alpha in cultured rat β-cells

Background: We previously reported that the mRNA levels of most glycolytic enzymes and other HIF target genes are markedly up-regulated in rat islets cultured in 30mM glucose (G30) instead of G5. Hypothesis: High glucose, which increases O2 consumption in β-cells, may induce hypoxia and thereby cause glucotoxicity. We therefore tested whether overnight culture in high glucose activates HIF in whole rat islets and INS1-cell monolayer. Results: High glucose significantly increased the protein levels of HIF1α, HIF2α and their dimerization partner HIF1β (ARNT) in INS1-cell nuclear extracts, and up-regulated HIF target gene mRNA levels (Gapdh, AldoA, LdhA, Adrenomedullin). These glucose effects, which were confirmed in rat islets, were mimicked by 6h exposure to a low pO2 (1%) or by 18h treatment with 100µM CoCl2, a known activator of HIF. In contrast, they were inhibited by a high pO2 (60%), siRNAs directed against both HIFα isoforms, or agents that reduce the glucose stimulation of β-cell O2 consumption by inhibiting Ca2+ influx and insulin secretion (diazoxide, nimodipine). Conclusion: Our results suggest that high glucose triggers hypoxia not only in whole rat islets but also in INS1-cell monolayer. The role of hypoxia in in vitro β-cell glucotoxicity merits further investigation