Citation: Kim, J.-d.; Zhou, T.; Zhang, A.; Li, S.; Gupte, A.A.; Hamilton, D.J.; Fang, L. AIBP Regulates Metabolism of Ketone and Lipids but Not Mitochondrial Respiration. Cells 2022, 11, 3643. https://doi.org/10.3390/ cells11223643 Academic Editors: Justin R. DiAngelo and Alexis Nagengast Received: 17 October 2022 Accepted: 15 November 2022 Published: 17 November 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). cells Article AIBP Regulates Metabolism of Ketone and Lipids but Not Mitochondrial Respiration Jun-dae Kim 1,† , Teng Zhou 1,† , Aijun Zhang 2,3 , Shumin Li 2 , Anisha A. Gupte 2,3 , Dale J. Hamilton 2,3,4 and Longhou Fang 1,3,4, * 1 Center for Cardiovascular Regeneration, Department of Cardiovascular Sciences, Houston Methodist Research Institute, 6550 Fannin St., Houston, TX 77030, USA 2 Center for Bioenergetics, Houston Methodist Research Institute, 6550 Fannin St., Houston, TX 77030, USA 3 Department of Medicine, Houston Methodist, Weill Cornell Medicine Affiliate, 6550 Fannin St., Houston, TX 77030, USA 4 Weill Cornell Medical College, Cornell University, 407 E 61st St., New York, NY 10065, USA * Correspondence: lhfang@houstonmethodist.org; Tel.: +713-363-9012; Fax: +713-363-9782 † These authors contributed equally to this work. Abstract: Accumulating evidence indicates that the APOA1 binding protein (AIBP)—a secreted protein—plays a profound role in lipid metabolism. Interestingly, AIBP also functions as an NAD(P)H- hydrate epimerase to catalyze the interconversion of NAD(P)H hydrate [NAD(P)HX] epimers and is renamed as NAXE. Thus, we call it NAXE hereafter. We investigated its role in NAD(P)H-involved metabolism in murine cardiomyocytes, focusing on the metabolism of hexose, lipids, and amino acids as well as mitochondrial redox function. Unbiased metabolite profiling of cardiac tissue shows that NAXE knockout markedly upregulates the ketone body 3-hydroxybutyric acid (3-HB) and lipid-associated metabolites α-linolenic acid and deoxycholic acid. Paralleling greater ketone levels, ChemRICH analysis of the NAXE-regulated metabolites shows reduced abundance of hexose despite similar glucose levels in control and NAXE-deficient blood. NAXE knockout reduces cardiac lactic acid but has no effect on the content of other NAD(P)H-regulated metabolites, including those associated with glucose metabolism, the pentose phosphate pathway, or Krebs cycle flux. Although NAXE is present in mitochondria, it has no apparent effect on mitochondrial oxidative phosphorylation. Instead, we detected more metabolites that can potentially improve cardiac function (3-HB, adenosine, and α-linolenic acid) in the Naxe −/− heart; these mice also perform better in aerobic exercise. Our data reveal a new role of NAXE in cardiac ketone and lipid metabolism. Keywords: AIBP/NAXE; NAD(P)HX epimerase; cardiac tissue; untargeted metabolite profiling; mitochondrial respiration 1. Introduction NAXE is a secreted protein and, as indicated by its name, binds APOA1 and the APOA1-containing high-density lipoprotein (HDL) [1,2]. NAXE accelerates cholesterol efflux to endothelial cells (ECs), macrophages, microglia, and cancer cells [3–5]. In addition, NAXE can regulate lipid rafts independent of cholesterol efflux but dependent on the Rho family member CDC42 [6]. Its control of lipid raft abundance contributes to NAXE regula- tion of vascular biology [7,8], inflammatory responses [3,9–12], and anti-tumor function [5]. Systemic NAXE knockout mice show increased retinal angiogenesis in development and greater adult angiogenesis following hindlimb ischemia [8]. In contrast to humans [13], no apparent neurological defects were observed in the global NAXE-deficient mice. In addition to its role in lipid metabolism, NAXE has epimerase activity required for the repair of a hydrated form of NAD(P)H [14]. The cellular NAD(P)H can be hy- drated by glyceraldehyde 3-phosphate dehydrogenase (GAPDH) [15–17] or by heat shock (>40 ◦ C) and becomes a hydrated form known as NAD(P)HX [18,19]. The hydrated form Cells 2022, 11, 3643. https://doi.org/10.3390/cells11223643 https://www.mdpi.com/journal/cells