Beyond their contribution as fundamental building blocks of life, branched‐chain amino acids (BCAAs) play a critical role in physiologic and pathologic processes. Importantly, BCAAs are associated with insulin resistance, obesity, cardiovascular disease, and genetic disorders. However, several metabolome‐wide studies in recent years could not attribute alterations in systemic BCAAs as the sole driver of endocrine perturbations, suggesting that a snapshot of global BCAA changes does not always reveal the underlying modifications. Because enzymes catabolizing BCAAs have a unique distribution, it is plausible that the tissue‐specific roles of BCAA‐catabolic enzymes could precipitate changes in systemic BCAA levels, flux, and action. We review the genetic and pharmacological approaches dissecting the role of BCAA‐catabolic enzyme dysfunctions. We summarized emerging evidence on BCAA metabolic intermediates, tissue specificity of BCAA‐catabolizing enzymes, and crosstalk between different metabolites in driving metabolic maladaptation in health and pathology. This review substantiates the understanding that tissue‐specific dysfunction of the BCAA‐catabolic enzymes and accumulating intermediary metabolites could act as better surrogates of metabolic imbalances, highlighting the biochemical communication among the nutrient triad of BCAAs, glucose, and fatty acid.—Biswas, D., Duffley, L., Pulinilkunnil, T. Role of branched‐chain amino acid–catabolizing enzymes in intertissue signaling, metabolic remodeling, and energy homeostasis. FASEB J. 33, 8711–8731 (2019). http://www.fasebj.org

Autotaxin (ATX) is an adipokine that generates the bioactive lipid, lysophosphatidic acid (LPA). ATX-LPA signaling has been implicated in diet-induced obesity and systemic insulin resistance. However, it remains unclear whether the ATX-LPA pathway influences insulin function and energy metabolism in target tissues, particularly skeletal muscle, the major site of insulin-stimulated glucose disposal. The objective of this study was to test whether the ATX-LPA pathway impacts tissue insulin signaling and mitochondrial metabolism in skeletal muscle during obesity. Male mice with heterozygous ATX deficiency (ATX) were protected from obesity, systemic insulin resistance, and cardiomyocyte dysfunction following high-fat high-sucrose (HFHS) feeding. HFHS-fed ATX mice also had improved insulin-stimulated AKT phosphorylation in white adipose tissue, liver, heart, and skeletal muscle. Preserved insulin-stimulated glucose transport in muscle from HFHS-fed ATX mice was associated with improved mitochondrial pyruvate oxidation in the absence of changes in fat oxidation and ectopic lipid accumulation. Similarly, incubation with LPA decreased insulin-stimulated AKT phosphorylation and mitochondrial energy metabolism in C2C12 myotubes at baseline and following palmitate-induced insulin resistance. Taken together, our results suggest that the ATX-LPA pathway contributes to obesity-induced insulin resistance in metabolically relevant tissues. Our data also suggest that LPA directly impairs skeletal muscle insulin signaling and mitochondrial function.

For rapid tumor growth, cancer cells often reprogram the cellular metabolic processes to obtain enhanced anabolic precursors and energy. The molecular changes of such metabolic rewiring are far from established. Here we explored the role of mTOR (mechanistic target of rapamycin), which serves as a key regulator of cell growth, proliferation and survival, in the metabolic reprograming of cancer cells. When we inhibited mTOR in human hepatocellular carcinoma (HCC) and renal cell carcinoma (RCC) cells, using pharmacologic inhibitors or by RNA interference, we noticed shuttle of the glycolytic flux to gluconeogenesis pathway along with reduction in cellular proliferation and survival. Augmentation of gluconeogenesis was mechanistically linked to upregulation of the key gluconeogenic enzymes PCK1 and G6PC expressions, enhanced lactate dehydrogenase activity and glucose-derived lipogenesis without causing any attenuation in mitochondrial function. Interestingly, concomitant knocking down of PCK1 and not G6PC along with mTOR pathway could overcome the inhibition of cancer cell proliferation and survival. These observations were validated by identifying distinctive diminution of PCK1 and G6PC expressions in human HCC and RCC transcriptome data. Significant correlation between mTOR-dependent upregulation of PCK1 and cell death in different cancer cell lines further emphasizes the physiological relevance of this pathway. We reveal for the first time that inhibition of mTORC2 and consequent redistribution of glycolytic flux can have a prosurvival role in HCC and RCC cancer cells only in the presence of downregulation of gluconeogenesis pathway genes, thus identifying novel pivots of cancer cell metabolic rewiring and targets for therapy.

Branched-chain α-keto acids (BCKAs) are catabolites of branched-chain amino acids (BCAAs). Intracellular BCKAs is cleared by branched-chain ketoacid dehydrogenase (BCKDH), which is sensitive to inhibitory phosphorylation by BCKD kinase (BCKDK). Accumulation of BCKAs is an indicator of defective BCAA catabolism and has been correlated with glucose intolerance and cardiac dysfunction. However, it is unclear whether BCKAs directly alter insulin signaling and function in the skeletal and cardiac muscle cell. Furthermore, the role of excess fatty acids (FA) in perturbing BCAA catabolism and BCKA availability merits investigation. By using immunoblot and UPLC MS/MS to analyze the hearts of fasted mice, we observed decreased BCAA catabolizing enzyme expression and increased circulating BCKAs, but not BCAAs. In mice subjected to diet-induced obesity (DIO), we observed similar increases in circulating BCKAs with concomitant changes in BCAA catabolizing enzyme expression only in the skeletal muscle. Effects of DIO were recapitulated by simulating lipotoxicity in skeletal muscle cells treated with saturated FA, palmitate. Exposure of muscle cells to high concentrations of BCKAs resulted in inhibition of insulin-induced AKT phosphorylation, decreased glucose uptake and mitochondrial oxygen consumption. Altering intracellular clearance of BCKAs by genetic modulation of BCKDK and BCKDHA expression showed similar effects on AKT phosphorylation. BCKAs increased protein translation and mTORC1 activation. Pretreating cells with mTORC1 inhibitor rapamycin restored BCKAs effect on insulin-induced AKT phosphorylation. This study provides evidence for FA mediated regulation of BCAA catabolizing enzymes, BCKA content and highlights the biological role of BCKAs in regulating muscle insulin signaling and function.

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Transcription factor EB (TFEB) is a master regulator of lysosomal biogenesis and autophagy with critical roles in several cancers. Lysosomal autophagy promotes cancer survival through the degradation of toxic molecules and the maintenance of adequate nutrient supply. Doxorubicin (DOX) is the standard of care treatment for triple-negative breast cancer (TNBC); however, chemoresistance at lower doses and toxicity at higher doses limit its usefulness. By targeting pathways of survival, DOX can become an effective antitumor agent. In this study, we examined the role of TFEB in TNBC and its relationship with autophagy and DNA damage induced by DOX. In TNBC cells, TFEB was hypo-phosphorylated and localized to the nucleus upon DOX treatment. TFEB knockdown decreased the viability of TNBC cells while increasing caspase-3 dependent apoptosis. Additionally, inhibition of the TFEB-phosphatase calcineurin sensitized cells to DOX-induced apoptosis in a TFEB dependent fashion. Regulation of apoptosis by TFEB was not a consequence of altered lysosomal function, as TFEB continued to protect against apoptosis in the presence of lysosomal inhibitors. RNA-Seq analysis of MDA-MB-231 cells with TFEB silencing identified a down-regulation in cell cycle and homologous recombination genes while interferon-γ and death receptor signaling genes were up-regulated. In consequence, TFEB knockdown disrupted DNA repair following DOX, as evidenced by persistent γH2A.X detection. Together, these findings describe in TNBC a novel lysosomal independent function for TFEB in responding to DNA damage.

Adipose triglyceride lipase (ATGL) maintains an optimum mitochondrial function putatively by generating cognate ligands for peroxisome proliferator-activated receptor α (PPARα), which, together with PPARγ coactivator-1α (PGC1α), regulate muscle mitochondrial biogenesis. However, the cross-talk between ATGL and PPARα in skeletal muscle mitochondrial metabolism and its implication in chronological aging is poorly understood. The role of ATGL in muscle mitochondrial metabolism was studied by overexpressing and depleting the gene and studying its downstream effect in cultured myotubes and in murine skeletal muscle. We found that PPARα directly induces ATGL expression during myogenesis. Overexpression of ATGL significantly enhanced while depletion of ATGL attenuated mitochondrial oxidative phosphorylation and fatty acid oxidation without alteration in mitochondrial content, and it rendered PPARα and PGC1α redundant in promoting mitochondrial oxidative function. However, ATGL did not alter PPARα-dependent lipid accumulation and insulin sensitivity. In middle-aged rats, ATGL expression was higher and correlated with PPARα expression and sustained fatty acid oxidation in oxidative soleus muscle. Fenofibrate feeding further induced ATGL expression selectively in this muscle compartment. These findings illustrate that PPARα and ATGL constitute a regulatory pathway in skeletal muscle, suggesting their role as a mitochondrial metabolic reserve.-Biswas, D., Ghosh, M., Kumar, S., Chakrabarti, P. PPARα-ATGL pathway improves muscle mitochondrial metabolism: implication in aging.

Background: Predicting relapses of post-operative complications in obese patients who undergo cardiac surgery is significantly complicated by persistent metabolic maladaptation associated with obesity. Despite studies supporting the linkages of increased systemic branched-chain amino acids (BCAAs) driving the pathogenesis of obesity, metabolome wide studies have either supported or challenged association of circulating BCAAs with cardiovascular diseases (CVDs). Objective: We interrogated whether BCAA catabolic changes precipitated by obesity in the heart and adipose tissue can be reliable prognosticators of adverse outcomes following cardiac surgery. Our study specifically clarified the correlation between BCAA catabolizing enzymes, cellular BCAAs and branched-chain keto acids (BCKAs) with the severity of cardiometabolic outcomes in obese patients pre and post cardiac surgery. Methods: Male and female patients of ages between 44 and 75 were stratified across different body mass index (BMI) (non-obese = 17, pre-obese = 19, obese class I = 14, class II = 17, class III = 12) and blood, atrial appendage (AA), and subcutaneous adipose tissue (SAT) collected during cardiac surgery. Plasma and intracellular BCAAs and BC ketoacids (BCKAs), tissue mRNA and protein expression and activity of BCAA catabolizing enzymes were assessed and correlated with clinical parameters. Results: Intramyocellular, but not systemic, BCAAs increased with BMI in cardiac surgery patients. In SAT, from class III obese patients, mRNA and protein expression of BCAA catabolic enzymes and BCKA dehydrogenase (BCKDH) enzyme activity was decreased. Within AA, a concomitant increase in mRNA levels of BCAA metabolizing enzymes was observed, independent of changes in BCKDH protein expression or activity. BMI, indices of tissue dysfunction and duration of hospital stay following surgery correlated with BCAA metabolizing enzyme expression and metabolite levels in AA and SAT. Conclusion: This study proposes that in a setting of obesity, dysregulated BCAA catabolism could be an effective surrogate to determine cardiac surgery outcomes and plausibly predict premature re-hospitalization.