274-OR: The Malic Enzyme-1 Links Pyruvate Carboxylase–Mediated Anaplerosis to Redox State in Liver

Redox balance is critical for numerous cellular functions, including substrate oxidation, biosynthesis, and antioxidant defenses. Redox homeostasis is dysregulated in many metabolic diseases, including obesity and Type 2 diabetes. In addition to its role in replenishing TCA cycle intermediates, we previously showed that pyruvate carboxylase (PC) -mediated anaplerosis is necessary to regulate redox state, including the NADPH/NADP⁺ ratio and antioxidant capacity in liver, though the mechanism is unclear. We hypothesized that PC elicits effects on redox by modifying ratios of metabolites involved in redox mediated exchange reactions. Malic enzyme-1 (Me1) catalyzes the reversible, NADPH dependent, decarboxylation of malate to pyruvate. This enzyme plays a vital role in recycling malate and providing NADPH during lipogenesis. We show here that Me1 mediates redox by sensing TCA cycle function through the mass action of malate and pyruvate. We tested the role of Me1, as a sensor of anaplerosis in the TCA cycle and regulator of redox, using acute Me1 knockdown (Me1KD) in liver-specific PC knockout (LPCKO) mice. Stable isotope tracers were used to examine anaplerosis, gluconeogenesis, and pyruvate cycling fluxes, and targeted metabolomics were used to evaluate tissue energy status and redox. Anaplerosis, pyruvate cycling, and gluconeogenesis were lower in the absence of liver PC but were unaffected by Me1KD. Loss of PC decreased the malate/pyruvate ratio, congruent with impaired conversion of pyruvate to TCA cycle intermediates, which resulted in a concomitant decrease in the NADPH/NADP⁺ ratio. Acute removal of Me1 in LPCKO mice normalized NADPH/NADP⁺ and restored the malate/pyruvate ratio. Additionally, mitochondrial NAD⁺/NADH and FAD⁺/FADH redox states were more oxidized in LPCKO liver and were normalized by loss of Me1. Thus, despite not altering the aforementioned fluxes, Me1 transmits redox state in response to TCA cycle function by sensing alterations in the malate/pyruvate ratio.