2175-P: Isoform-Specific Roles of Prolyl Hydroxylase Domain Proteins in Regulating Beta-Cell Function

Pancreatic β-cells secrete insulin in a biphasic manner via K_ATP-channel dependent and independent pathways. The K_ATP-channel independent pathway produces a rise in the NADPH/NADP⁺ ratio and cytosolic α-ketoglutarate (α-KG). Prolyl hydroxylase domain proteins (PHDs) belong to the α-KG-dependent dioxygenase superfamily and regulates stability of hypoxia-inducible factor α (HIF-α). Prolyl hydroxylation of HIF-α for ubiquitin-mediated proteasomal degradation requires sufficient levels of oxygen, iron and α-KG. We have previously shown pharmacological inhibition of PHDs reduces glucose-stimulated insulin secretion (GSIS), glucose utilization, and mitochondrial function in clonal β-cells. Glucose-induced changes in cytosolic α-KG may regulate PHD and thus β-cell function. In mouse pancreatic β-cells, PHD isoforms express unique subcellular localization: PHD1 is expressed exclusively in the cytosol, PHD2 is expressed in the cytosol and nucleus, and PHD3 is mainly expressed in the nucleus. To further explore the isoform-specific roles of PHD, we employed a β-cell specific knockout (β-PHD KO) mouse model for each of the PHD isoforms. β-PHD1 KO, β-PHD2 KO and β-PHD3 KO. β-PHD KO mice show no significant in vivo defects associated with glucose tolerance and insulin resistance; however, β-PHD1 KO and β-PHD2 KO mice have significantly increased plasma insulin compared to wild type (WT) controls (p<0.05). Consistent with our cell culture model of PHD knockdown, PHD1 and PHD3 regulate insulin secretion as both β-PHD1 KO and β-PHD3 KO mice show impaired GSIS. Only β-PHD1 KO mice have reduced beta-cell mass (p<0.01).

In conclusion, there are significant and unique effects observed with isoform-specific suppression of PHD.