Revisiting protein acetylation and myocardial fatty acid oxidation.

In the 20th century, our knowledge of posttranslational modifications (PTMs) and their impact on protein function/enzyme activity was largely confined to that of protein phosphorylation and their regulation via kinases and phosphatases. However, as our scientific tools have become more sophisticated, and as we have advanced our knowledge of cellular/molecular biology, which has further been augmented by the sequencing of the human and mouse genomes, we are now aware that protein function can be influenced by a variety of PTMs. This includes glycosylation, sumoylation, sulfation, and, of relevance to this particular Editorial Focus, acetylation, to name a few. Illustrating the predominance of protein phosphorylation regarding our overall knowledge of protein PTMs, there are nearly nine times as many published entries dealing with protein phosphorylation than there are entries dealing with protein acetylation on the Swiss-Prot database (8), which curates sequence information on the proteome from the published literature (2). Of interest, extensive evidence supports protein acetylation as a critical regulator of energy metabolism (4, 11). Indeed, the substrate for protein acetylation, acetyl CoA, is the common metabolic intermediate linking oxidative metabolism of all exogenous and endogenous fuel sources within our bodies (15), and the vast majority of cellular acetyl CoA is localized within mitochondria (7), the organelle that drives energy production. Because the heart is the most metabolically demanding organ on a per gram basis (15), while containing the highest cellular levels of CoA alongside the liver, of which the vast majority is present as acetyl CoA (13), protein acetylation is now widely recognized as a major regulator of myocardial energy metabolism. In a recent article in the American Journal of Physiology-Heart and Circulatory Physiology, Thapa et al. (14) demonstrated that acetylation of the fatty acid oxidation enzymes short-chain acyl CoA dehydrogenase (SCAD), longchain acyl CoA dehydrogenase (LCAD), and -hydroxyacyl CoA dehydrogenase ( HAD) is increased in hearts of obese mice. Furthermore, Thapa et al. attributed these increases in SCAD, LCAD, and HAD acetylation to increased expression/ activity of amino acid synthesis 5-like 1 (Gcn5l1). Of particular interest, these increases in SCAD, LCAD, and HAD acetylation were associated with increased enzyme activity. Finally, to confirm a key mechanistic role of Gcn5l1 in explaining their observations, the authors also generated various stable Gcn5l1 knockdown cell lines in H9c2 myoblasts, which decreased SCAD/LCAD acetylation and subsequent SCAD/ LCAD activity, ultimately reducing fatty acid oxidation rates in vitro. The authors’ findings are particularly intriguing considering that current dogma often presumes that protein acetylation reduces enzyme activity/function (4, 11). In the context of fatty acid oxidation, a number of studies have demonstrated that sirtuin 3 (SIRT3), a nicotinamide adenine dinucleotide-dependent deacetylase, increases fatty acid oxidation by deacetylating LCAD. Seminal findings from Hirschey and colleagues (5) have shown that mice with a whole body deficiency for SIRT3 (SIRT3 / ) have reduced fatty acid oxidation rates in the heart, skeletal muscle, liver, and brown adipose tissue (BAT). Reduced BAT fatty acid oxidation likely accounts for the impaired cold tolerance in SIRT3 / mice, which cannot maintain core body temperature to the same extent as their wild-type littermates (5). Moreover, the reduced peripheral tissue fatty acid oxidation rates in SIRT3 / mice are associated with an exacerbation of high-fat diet-induced obesity, hepatic steatosis, and insulin resistance (6). Reasons for the discrepancies between these previous studies and the work of Thapa and colleagues (14) remains enigmatic but could be explained by tissue-specific differences or enzymatic differences controlling acetylation/deacetylation of LCAD and other fatty acid oxidation enzymes. In the work of Hirschey and colleagues (5, 6), much of the phenotype was attributed to changes in hepatic and BAT fatty acid oxidation, whereas the study by Thapa and colleagues (14) focused on myocardial fatty acid oxidation. In addition, the increased acetylation of LCAD, SCAD, and HAD in the hearts of obese mice was attributed to increased expression/activity of Gcn5l1 (14) versus SIRT3 deficiency accounting for the increased acetylation of LCAD in peripheral tissues of SIRT3 / mice (5, 6). As the aforementioned studies simply measured overall acetylation of LCAD and not specific lysine residues within LCAD, it is possible that the acetylated lysine residues regulated by Gcn5l1 and SIRT3 within LCAD are different, thereby producing opposing actions on LCAD activity. Moreover, not all lysine residues within LCAD that are acetylated would be expected to impact LCAD activity in the same manner, while certain acetylated lysine residues within LCAD may not be important for controlling fatty acid oxidation. Thus, if Gcn5l1 and SIRT3 do indeed acetylate or deacetylate different lysine residues within LCAD, respectively, they may not have the same end result on fatty acid oxidation. As such, the relative activities of Gcn5l1 versus SIRT3 in a given tissue may dictate whether acetylation increases or decreases fatty acid oxidation in that tissue. With regard to tissue-specific regAddress for reprint requests and other correspondence: J. R. Ussher, 2-020C, Katz Group Centre for Pharmacy and Health Research, Faculty of Pharmacy and Pharmaceutical Sciences, Univ. of Alberta, Edmonton, AB, Canada T6G 2E1 (e-mail: [email protected]). Am J Physiol Heart Circ Physiol 313: H617–H619, 2017; doi:10.1152/ajpheart.00303.2017.