Lipid droplets (LDs) provide a reservoir for triacylglycerol storage and are a central hub for fatty acid trafficking and signaling in cells. Lipolysis promotes mitochondrial biogenesis and oxidative metabolism via a SIRT1/PGC-1a/PPARa-dependent pathway through an unknown mechanism. Herein, we identify that monounsaturated fatty acids (MUFAs) allosterically activate SIRT1 toward select peptide-substrates such as PGC-1a. MUFAs enhance PGC-1a/ PPARa signaling and promote oxidative metabolism in cells and animal models in a SIRT1-dependent manner. Moreover, we characterize the LD protein perilipin 5 (PLIN5), which is known to enhance mitochondrial biogenesis and function, to be a fattyacid-binding protein that preferentially binds LDderived monounsaturated fatty acids and traffics them to the nucleus following cAMP/PKA-mediated lipolytic stimulation. Thus, these studies identify the first-known endogenous allosteric modulators of SIRT1 and characterize a LD-nuclear signaling axis that underlies the known metabolic benefits of MUFAs and PLIN5.

Hepatic steatosis is defined by the accumulation of lipid droplets (LDs). Once thought to be only inert energy storage depots, LDs are increasingly recognized as organelles that have important functions in hepatocytes beyond lipid storage. The lipid and protein composition of LDs is highly dynamic and influences their intrinsic metabolism and signaling properties, which ultimately links them to the changes in hepatic function. This concise review will highlight recent discoveries in LD biology and highlight unique aspects of hepatic LDs and their role in liver disease.

Hepatic acyl-CoA thioesterase 1 (ACOT1) catalyzes the conversion of acyl-CoAs to fatty acids (FAs) and CoA. We sought to determine the role of ACOT1 in hepatic lipid metabolism in C57Bl/6J male mice 1 week after adenovirus-mediated Acot1 knockdown. Acot1 knockdown reduced liver triglyceride (TG) as a result of enhanced TG hydrolysis and subsequent FA oxidation. In vitro experiments demonstrated that Acot1 knockdown led to greater TG turnover and FA oxidation, suggesting that ACOT1 is important for controlling the rate of FA oxidation. Despite increased FA oxidation, Acot1 knockdown reduced the expression of peroxisome proliferator–activated receptor α (PPARα) target genes, whereas overexpression increased PPARα reporter activity, suggesting ACOT1 regulates PPARα by producing FA ligands. Moreover, ACOT1 exhibited partial nuclear localization during fasting and cAMP/cAMP-dependent protein kinase signaling, suggesting local regulation of PPARα. As a consequence of increased FA oxidation and reduced PPARα activity, Acot1 knockdown enhanced hepatic oxidative stress and inflammation. The effects of Acot1 knockdown on PPARα activity, oxidative stress, and inflammation were rescued by supplementation with Wy-14643, a synthetic PPARα ligand. We demonstrate through these results that ACOT1 regulates fasting hepatic FA metabolism by balancing oxidative flux and capacity.

Peroxisome proliferator‐activated regulator alpha (PPARα) is a major regulator of fasting lipid metabolism. PPARα induces transcription of cytosolic Acyl‐CoA Thioesterase 1 (ACOT1), which hydrolyzes acyl‐CoAs into long‐chain free fatty acids and Coenzyme A. However, the physiological role of ACOT1 in lipid metabolism is not known. Thus, the objective of this research was to determine the contribution of hepatic ACOT1 to lipid metabolism and signaling. To test our objectives, we injected C57/Bl6 mice with an adenovirus harboring shRNA targeted to ACOT1 or a scrambled control. One week post‐injection, mice were fasted overnight and sacrificed. Compared to control mice, ACOT1 knockdown resulted in an increase in serum β‐hydroxybutyrate, a marker of fatty acid oxidation, and an increase in serum triglyceride, indicating a possible increase in very‐low density lipoprotein secretion. These changes were not accompanied by a significant change in liver triglyceride. In addition, genes involved in mitochondrial biogenesis were decreased in response to ACOT1 knockdown suggesting that ACOT1 alters cell signaling to influence gene expression. In summary, these results show that ACOT1 plays an important role in regulating hepatic lipid metabolism and may serve an important novel role in trafficking of lipids to influence cell signaling.

Acyl-CoA thioesterase 1 (ACOT1) catalyzes the hydrolysis of long-chain acyl-CoAs to free fatty acids and CoA and is typically upregulated in obesity. Whether targeting ACOT1 in the setting of high-fat diet–induced (HFD-induced) obesity would be metabolically beneficial is not known. Here we report that male and female ACOT1KO mice are partially protected from HFD-induced obesity, an effect associated with increased energy expenditure without alterations in physical activity or food intake. In males, ACOT1 deficiency increased mitochondrial uncoupling protein-2 (UCP2) protein abundance while reducing 4-hydroxynonenal, a marker of oxidative stress, in white adipose tissue and liver of HFD-fed mice. Moreover, concurrent knockdown (KD) of UCP2 with ACOT1 in hepatocytes prevented increases in oxygen consumption observed with ACOT1 KD during high lipid loading, suggesting that UCP2-induced uncoupling may increase energy expenditure to attenuate weight gain. Together, these data indicate that targeting ACOT1 may be effective for obesity prevention during caloric excess by increasing energy expenditure.