PEPCK-M expression in mouse liver potentiates, not replaces, PEPCK-C mediated gluconeogenesis

Abstract

Background & Aims Hepatic gluconeogenesis helps maintain systemic energy homeostasis by compensating for discontinuities in nutrient supply. Liver specific deletion of cytosolic phosphoenolpyruvate carboxykinase (PEPCK-C) abolishes gluconeogenesis from mitochondrial substrates, deregulates lipid metabolism and affects TCA cycle. While, mouse liver almost exclusively expresses PEPCK-C, humans equally present a mitochondrial isozyme (PEPCK-M). Despite clear relevance to human physiology, the role of PEPCK-M and its gluconeogenic potential remain unknown. Here, we test the significance of PEPCK-M in gluconeogenesis and TCA cycle function in liver-specific PEPCK-C knockout and WT mice. Methods The effects of the overexpression of PEPCK-M were examined by a combination of tracer studies and molecular biology techniques. Partial PEPCK-C re-expression was used as a positive control. Metabolic fluxes were evaluated in isolated livers by NMR using 2H and 13C tracers. Gluconeogenic potential, together with metabolic profiling, were investigated in vivo and in primary hepatocytes. Results PEPCK-M expression partially rescued defects in lipid metabolism, gluconeogenesis and TCA cycle function impaired by PEPCK-C deletion, while ~10% re-expression of PEPCK-C normalized most parameters. When PEPCK-M was expressed in the presence of PEPCK-C, the mitochondrial isozyme amplified total gluconeogenic capacity, suggesting autonomous regulation of oxaloacetate to phosphoenolpyruvate fluxes by the individual isoforms. Conclusions We conclude that PEPCK-M has gluconeogenic potential per se, and cooperates with PEPCK-C to adjust gluconeogenic/TCA flux to changes in substrate or energy availability, hinting at a role in the regulation of glucose and lipid metabolism in human liver.

Pharmacological inhibition of S6K1 increases glucose metabolism and Akt signalling in vitro and in diet-induced obese mice

“…However, PF-4708671 failed to significantly reduce PEPCK or G6Pase protein expression despite lower mRNA expression of these enzymes. It is possible that G6Pase and PEPCK proteins are only reduced in specific cellular compartments [38][39][40]. Indeed, we have used a PEPCK1-specific antibody that recognises the cytosolic isoform, whereas PEPCK2 is predominantly a mitochondrial form.…”

A Role for Mitochondrial Phosphoenolpyruvate Carboxykinase (PEPCK-M) in the Regulation of Hepatic Gluconeogenesis

“…Rodents should not be considered PEPCK-M knockouts especially when interpreting data from PEPCK-C knock-out mice (11)(12)(13)39). It is important to point out that both isoforms depend on mitochondrial metabolism.…”

“…Whole body loss of PEPCK-C increases hepatic TCA cycle intermediates and reduces oxidation of glucose, acetate, and glutamate between 65 and 80% (12,28). Recently, 100-fold hepatic overexpression of PEPCK-M in liver-specific knockouts of PEPCK-C had no measureable consequence in vivo and only a marginal improvement (ϳ8%) of PEP gluconeogenesis in vitro (39). The interpretation was that PEPCK-M has a minimal supportive role in gluconeogenesis.…”

Aryl Hydrocarbon Receptor Activation by Dioxin Targets Phosphoenolpyruvate Carboxykinase (PEPCK) for ADP-ribosylation via 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD)-inducible Poly(ADP-ribose) Polymerase (TiPARP)

“…A possible role for PEPCK-M in insulin secretion has been suggested (31). A recent report (32) showed that overexpression of PEPCK-M in mice in which hepatic PEPCK-C was deleted partially rescued defects in glucose production. Furthermore, the two enzymes were shown to be independently regulated.…”