In addition to drawing attention to

In addition to drawing attention to Ser454 of ACL as a phosphosite that is regulated by both BDK and PPM1K, our phospho-proteomics screen identified several additional sites in other proteins. For example, Ser25, Ser29, and Ser79 of the lipogenic enzyme acetyl-coA carboxylase 1 (ACC1) were found to be less phosphorylated in BT2-treated compared with vehicle-treated ZFR. Ser25 and Ser29 are known to be phosphorylated in response to insulin, when ACC1 activity is high (Haystead and Hardie, 1988, Haystead et al., 1988), whereas Ser79 is the highly studied 5′ AMP-activated protein kinase regulatory site that inhibits ACC1 activity (Munday et al., 1988). Thus, while these data suggest that BT2 might mediate some of its effects through regulation of ACC1 phosphorylation and activity, the net effect of these multiple changes in phosphorylation on enzyme activity remains to be determined for ACC1, as well as the other candidate phosphoproteins listed in Figures 2B and 2C. In conclusion, our findings shed new light on mechanisms underlying the strong relationship between elevated BCAA and cardiometabolic diseases (Newgard, 2017), by showing that the BDK/PPM1K kinase/phosphatase pair regulate both BCAA and lipid metabolism. What remains unclear is if BCAA represent a causal factor, a biomarker, or both. Our current interpretation is that BCAA are both a biomarker of dysregulated lipid metabolism (due in part to mechanisms defined in the current paper), and also a causal agent for metabolic diseases, at least in some contexts. Our own work has shown that supplementation of BCAA into high fat diets fed to normal rats exacerbates insulin resistance (Newgard et al., 2009), whereas, conversely, our group and another have shown that BCAA restriction in ZFR or normal mice improves insulin sensitivity (Fontana et al., 2016, White et al., 2016). Very recent human genetics studies add complexity to the story. Thus, on the one hand, these studies show that genetic variants that cause insulin resistance and dyslipidemia in human populations are strongly associated with increased circulating BCAA, but variants that directly raise BCAA do not exhibit “reverse causality” with insulin resistance (Lotta et al., 2016, Mahendran et al., 2017, Wang et al., 2017). However, two of the studies also show that variants that increase BCAA, including in the PPM1K locus, strongly increase risk for T2D. Thus, we view the human genetic studies as consistent with a causal role for BCAA in development of T2D, if not insulin resistance. The potential translational significance of the present work is further highlighted by our finding that manipulation of the BDK:PPM1K ratio to favor PPM1K via BT2 treatment or PPM1K overexpression lowers liver TG levels and blood glucose excursions in highly obese and insulin-resistant ZFR. Thus, our study introduces regulation of ACL by BDK and PPM1K as part of a regulatory node, that, when modulated, contributes to simultaneous improvements in lipid, glucose and amino phospholipase inhibitor metabolism, even in the absence of weight loss.
STAR★Methods
Acknowledgments The studies reported here were supported by NIH grants PO1-DK58398 (to C.B.N.), DK100425 (to M.A.H), DK083439 (to M.L.), DK62306, and DK92921 (to D.T.C.), and K08HL135275 (to R.W.M.), as well as a grant from the Welch Foundation (to D.T.C.) and a Pathways Award from the American Diabetes Association 1-16-INI-17 (to P.J.W.). A fellowship performed by A.L.L in the Newgard lab was supported by a sponsored research grant from Pfizer. The authors are grateful to Dr. J. Will Thompson and the Duke Proteomics and Metabolomics Shared Resource for assistance with the phosphoproteomics studies.
Modern Challenges in Cardiovascular and Metabolic Health Require New Therapeutic Strategies A combination of human genetic factors, overnutrition, and a sedentary lifestyle promotes derangements in cholesterol and triglyceride metabolism; these can manifest as one or more risk factors associated with increased probability of developing a number of life-threatening metabolic and/or cardiovascular diseases. The importance of maintaining cholesterol homeostasis in humans is strongly supported by both epidemiologic cohort studies and meta-analyses of multiple Mendelian and statin randomized trials that clearly demonstrate a causal association between elevated plasma levels of low-density lipoprotein cholesterol (LDL-C; see Glossary) (hypercholesterolemia) and atherosclerotic cardiovascular disease (ASCVD) risk 1, 2. While a causal association for circulating triglyceride levels is less clear 3, 4, aberrations in liver triglyceride metabolism also manifest as other metabolic ASCVD risk factors including insulin resistance, Type 2 diabetes (T2D), and nonalcoholic fatty liver disease (NAFLD)5, 6. Moreover, NAFLD also poses an independent health challenge as it is now the most common cause of chronic liver disease in the Western world and a leading cause of liver-related morbidity and mortality worldwide [7]. Of relevance, neither ASCVD nor NAFLD is adequately addressed by currently available treatment options. There are no FDA-approved therapies for NAFLD, and because many patients are not effectively treated for lipid disorders with the current standard of care, ASCVD remains the leading cause of death and disability in the Western world [8]. As such, therapeutic strategies that target cholesterol and triglyceride metabolism are required to provide patients with more potent LDL-C reduction regimens and therapeutic options to treat NAFLD. ATP-citrate lyase (ACL) is an enzyme uniquely positioned at the intersection of nutrient catabolism, and cholesterol and fatty acid biosynthesis. In this review, we discuss emerging evidence supporting the notion that ACL-derived acetyl-coenzyme A (CoA) serves not only as carbon precursor for cholesterol and fatty acid biosynthesis, but also as a key metabolic checkpoint used by cells to sense nutrient availability and coordinate metabolic adaptions. We raise key remaining questions regarding the potential role of ACL in controlling lipid metabolism, mitochondrial biogenesis, apoptosis, and inflammation by influencing protein acetylation. We also review findings from recent preclinical studies, and Mendelian and clinical randomized trials, that suggest that ACL is in a strategic position in metabolism; it may provide a unique therapeutic opportunity to treat hypercholesterolemia, and potentially address the overlapping pathophysiology that exists between ASCVD and NAFLD. Finally, we highlight how over the next few years the continued clinical investigation of bempedoic acid (BA) will potentially establish whether lowering LDL-C by targeting ACL may constitute a new viable strategy to reduce ASCVD risk and possibly treat other associated metabolic disorders.