Prolyl-hydroxylase 3 maintains [beta]-cell glucose-sensing under metabolic stress

Aims: Prolyl-4 hydroxylase domain protein 3 (PHD3) is an alpha ketoglutarate-dependent dioxygenase involved in the oxygen-dependent regulation of cell phenotype. While PHD3 has been reported to suppress insulin sensitivity in the liver, little is known about effects of the enzyme in insulin-secreting β-cells.

Methods: βPHD3−/− mice were generated by crossing the Ins1Cre driver line with animals bearing a floxed Egln3 gene (encoding PHD3). Ca2+ fluxes, ATP/ADP dynamics, insulin secretion and metabolic tracing were assessed using Fluo8, Perceval, HTRF assay and GC–MS, respectively.

Results: PHD3 loss under standard chow did not affect insulin secretion, Ca2+ fluxes and ATP/ADP ratios, and this was mirrored by normal glucose homeostasis in vivo. After four weeks HFD feeding, however, βPHD3−/− mice were glucose-intolerant, despite improved glucose-stimulated insulin secretion (GSIS) from isolated islets. 13C6 mass isotopomer distribution analysis of HFD βPHD3−/− islets showed an increase of glucose incorporation into m+3 lactate, indicating reduced input of glycolysis into the tricarboxylic acid (TCA) cycle. The following observations were suggestive of a switch to utilisation of fatty acids in HFD βPHD3−/− islets: 1) ATP/ADP responses to glucose were halved; 2) glucose-driven ATP/ADP ratios could be rescued by inhibiting the fatty acid transporter CPT1; 3) glucose incorporation into lipid pools was decreased; and 4) chronic incubation with fatty acid to supply acetyl-CoA to the TCA cycle was able to amplify GSIS. By eight weeks HFD, βPHD3−/− islets presented with markedly impaired Ca2+ and insulin responses to glucose.

Summary: Specific loss of PHD3 in β-cells leads to dependence on fatty acid metabolism, eventually leading to insulin secretory failure. Thus, PHD3 might be a pivotal component of the β-cell glucose-sensing machinery by disallowing use of fatty acids as a primary fuel source under metabolic stress.