Poldip2 is an Oxygen-sensitive Mitochondrial Protein that Controls Oxidative/glycolytic Metabolism Balance and Proteasome Activity

Abstract

The polymerase delta interacting protein 2 (Poldip2) is a nuclear-encoded mitochondrial protein of unknown function. We recently reported that Poldip2 is repressed under hypoxia and that its deficiency results in repressed mitochondrial function and increased glycolytic activity. However, the mechanisms responsible for this metabolic reprograming and its consequences are unknown. In this study, we show that Poldip2 controls the lipoylation and consequently the activity of the pyruvate and α-ketoglutarate dehydrogenase complexes by a mechanism that involves Clp-protease complex-mediated degradation of Acyl-CoA Synthetase Medium-Chain Family Member 1 (ACSM1), a lipoate-activating enzyme required for the utilization of lipoic acid derived from the salvage pathway. With regards to the increased glycolytic activity, we found that Poldip2 deficiency inhibits PHDs inducing HIF-1α-dependent upregulation of Hexokinase 2. As a result, we observed an increase in the flux through the Hexosamine biosynthetic pathway (HBP), a glucose metabolic pathway that produces uridine diphosphate N-acetylglucosamine, the sugar donor for the O-GlcNAc transferase (OGT). The OGT-catalyzed transfer of O-GlcNAc is a post-translational modification implicated in cell signaling. Interestingly we found that Poldip2 deficiency induces the inhibition of the ubiquitin proteasome system (UPS) by OGT-dependent O-GlcNAcylation of the 19S regulatory unit of the 26S proteasome regulatory subunit 4 (PRS4). Additionally, we demonstrated that Poldip2-mediated inhibition of the UPS is required for the hypoxia-induced expression of connective tissue growth factor (CTGF), a cytokine that participates in multiple pathologies including fibrotic diseases and cancer. In all, we demonstrate that Poldip2 is an oxygen sensitive protein that regulates the activity of key enzymes of the Krebs cycle by coupling proteasome activity to mitochondrial function and fibrotic responses under hypoxia.