Deletion of the von Hippel-Lindau gene in pancreatic beta cells impairs glucose homeostasis in mice.

Defective insulin secretion in response to glucose is an important component of the beta cell dysfunction seen in type 2 diabetes. As mitochondrial oxidative phosphorylation plays a key role in glucose-stimulated insulin secretion (GSIS), oxygen-sensing pathways may modulate insulin release. The von Hippel-Lindau (VHL) protein controls the degradation of hypoxia-inducible factor (HIF) to coordinate cellular and organismal responses to altered oxygenation. To determine the role of this pathway in controlling glucose-stimulated insulin release from pancreatic beta cells, we generated mice lacking Vhl in pancreatic beta cells (betaVhlKO mice) and mice lacking Vhl in the pancreas (PVhlKO mice). Both mouse strains developed glucose intolerance with impaired insulin secretion. Furthermore, deletion of Vhl in beta cells or the pancreas altered expression of genes involved in beta cell function, including those involved in glucose transport and glycolysis, and isolated betaVhlKO and PVhlKO islets displayed impaired glucose uptake and defective glucose metabolism. The abnormal glucose homeostasis was dependent on upregulation of Hif-1alpha expression, and deletion of Hif1a in Vhl-deficient beta cells restored GSIS. Consistent with this, expression of activated Hif-1alpha in a mouse beta cell line impaired GSIS. These data suggest that VHL/HIF oxygen-sensing mechanisms play a critical role in glucose homeostasis and that activation of this pathway in response to decreased islet oxygenation may contribute to beta cell dysfunction.

Hypoxia-inducible transcription factors stabilization in the thick ascending limb protects against ischemic acute kidney injury.

Hypoxia-inducible transcription factors (HIF) protect cells against oxygen deprivation, and HIF stabilization before ischemia mitigates tissue injury. Because ischemic acute kidney injury (AKI) often involves the thick ascending limb (TAL), modulation of HIF in this segment may be protective. Here, we generated mice with targeted TAL deletion of the von Hippel-Lindau protein (Vhl), which mediates HIF degradation under normoxia, using Tamm-Horsfall protein (Thp)-driven Cre expression. These mice showed strong expression of HIF-1α in TALs but no changes in kidney morphology or function under control conditions. Deficiency of Vhl in the TAL markedly attenuated proximal tubular injury and preserved TAL function following ischemia-reperfusion, which may be partially a result of enhanced expression of glycolytic enzymes and lactate metabolism. These results highlight the importance of the thick ascending limb in the pathogenesis of AKI and suggest that pharmacologically targeting the HIF system may have potential to prevent and mitigate AKI.

Vascular endothelial growth factor induces chemotaxis and proliferation of microglial cells.

Vascular endothelial growth factor (VEGF) is an angiogenic peptide that is produced in the brain after ischemia, injury or in malignant gliomas. Since these pathological conditions are associated with the infiltration of microglial cells, we investigated the expression of VEGF receptors (VEGFR) and possible effects of VEGF on cultivated microglial cells. As shown by reverse transcription-polymerase chain reaction and immunocytochemistry, rat microglial cells as well as the murine cell line BV-2 express the VEGFR-1, but not VEGFR-2. Murine VEGF induced 3H-thymidine incorporation into DNA of murine and rat microglial cells as well as chemotaxis in Boyden chamber assays. However, VEGF did not alter the phosphorylation of mitogen-activated protein kinases and only slightly that of the kinase Akt. These results show that microglial cells are targets for VEGF which induces migration and proliferation of these immunocompetent cells in the brain.

Functional significance of vascular endothelial growth factor receptor expression on human glioma cells.

Vascular endothelial growth factor (VEGF) is one of the most important angiogenesis factors. In many tumors, VEGF plays a pivotal role for their vascularization and is necessary to supply the malignant tissue with oxygen and nutrients. However, VEGF receptors (VEGFR) have recently been detected also on some tumor cells, and autocrine mitogenic effects of VEGF have been suspected. Since glioma cells are known to produce large amounts of VEGF, we investigated VEGFR-expression and effects of VEGF on glioma cells. The three glioma cell lines and eight glioma cells cultivated from WHO grade IV gliomas investigated strongly expressed VEGF121 and VEGF165, but weakly either VEGFR-1 or -2, sometimes for both, as evidenced by reverse transcription-polymerase chain reaction (RT-PCR) and immunocytochemistry. Quantitative RT-PCR revealed a 1000- to 50-fold lower expression of VEGFR than in cultivated human umbilical vein endothelial cells. In two glioma cell lines analyzed, VEGF induced a weak tyrosine phosphorylation of the VEGFR, but downstream signal transduction effects on the mitogen-activated protein kinases p42/p44 or transcription factors like AP-1 or NFKB were within the background of the methods. In accordance, VEGF or the VEGFR agonists VEGF-D or placenta growth factor (P1GF) did not produce significant effects on glioma cell proliferation or VEGF production. We conclude that despite a low expression of VEGFR in some glioma cells functional effects are low and autocrine growth stimulatory effects within a glioma are minor.