Despite advances in our understanding of the ways in which nutrient oversupply and triacylglycerol (TAG) anabolism contribute to hepatic steatosis, little is known about the lipases responsible for regulating hepatic TAG turnover. Recent studies have identified adipose triglyceride lipase (ATGL) as a major lipase in adipose tissue, although its role in the liver is largely unknown. Thus, we tested the contribution of ATGL to hepatic lipid metabolism and signaling. Adenovirus-mediated knockdown of hepatic ATGL resulted in steatosis in mice and decreased hydrolysis of TAG in primary hepatocyte cultures and in vitro assays. In addition to altering TAG hydrolysis, ATGL was shown to play a significant role in partitioning hydrolyzed fatty acids between metabolic pathways. Although ATGL gain and loss of function did not alter hepatic TAG secretion, fatty acid oxidation was increased by ATGL overexpression and decreased by ATGL knockdown. The effects on fatty acid oxidation coincided with decreased expression of peroxisome proliferator-activated receptor a (PPAR-a) and its target genes in mice with suppressed hepatic ATGL expression. However, PPAR-a agonism was unable to normalize the effects of ATGL knockdown on PPAR-a target gene expression, and this suggests that ATGL influences PPAR-a activity independently of ligandinduced activation. Conclusion: Taken together, these data show that ATGL is a major hepatic TAG lipase that plays an integral role in fatty acid partitioning and signaling to control energy metabolism. (HEPATOLOGY 2011;53:116-126)

SUMMARY Hepatic lipid droplet (LD) catabolism is thought to occur via cytosolic lipases such as adipose triglyceride lipase (ATGL) or through autophagy of LDs, a process known as lipophagy. We tested the potential interplay between these metabolic processes and its effects on hepatic lipid metabolism. We show that hepatic ATGL is both necessary and sufficient to induce both autophagy and lipophagy. Moreover, lipophagy is required for ATGL to promote LD catabolism and the subsequent oxidation of hydrolyzed fatty acids (FAs). Following previous work showing that ATGL promotes sirtuin 1 (SIRT1) activity, studies in liver-specific SIRT1−/− mice and in primary hepatocytes reveal that SIRT1 is required for ATGL-mediated induction of autophagy and lipophagy. Taken together, these studies show that ATGL-mediated signaling via SIRT1 promotes autophagy/lipophagy as a primary means to control hepatic LD catabolism and FA oxidation.

Sirtuin 1 (SIRT1), an NAD+-dependent protein deacetylase, regulates a host of target proteins, including peroxisome proliferator–activated receptor (PPAR)-γ coactivator-1α (PGC-1α), a transcriptional coregulator that binds to numerous transcription factors in response to deacetylation to promote mitochondrial biogenesis and oxidative metabolism. Our laboratory and others have shown that adipose triglyceride lipase (ATGL) increases the activity of the nuclear receptor PPAR-α, a PGC-1α binding partner, to promote fatty acid oxidation. Fatty acids bind and activate PPAR-α; therefore, it has been presumed that fatty acids derived from ATGL-catalyzed lipolysis act as PPAR-α ligands. We provide an alternate mechanism that links ATGL to PPAR-α signaling. We show that SIRT1 deacetylase activity is positively regulated by ATGL to promote PGC-1α signaling. In addition, ATGL mediates the effects of β-adrenergic signaling on SIRT1 activity, and PGC-1α and PPAR-α target gene expression independent of changes in NAD+. Moreover, SIRT1 is required for the induction of PGC-1α/PPAR-α target genes and oxidative metabolism in response to increased ATGL-mediated lipolysis. Taken together, this work identifies SIRT1 as a critical node that links β-adrenergic signaling and lipolysis to changes in the transcriptional regulation of oxidative metabolism.

The ubiquitin-proteasome pathway has emerged as an important regulatory mechanism governing the activity of several transcription factors. While estrogen receptor ␣ (ER␣) is also subjected to rapid ubiquitinproteasome degradation, the relationship between proteolysis and transcriptional regulation is incompletely understood. Based on studies primarily focusing on the C-terminal ligand-binding and AF-2 transactivation domains, an assembly of an active transcriptional complex has been proposed to signal ER␣ proteolysis that is in turn necessary for its transcriptional activity. Here, we investigated the role of other regions of ER␣ and identified S118 within the N-terminal AF-1 transactivation domain as an additional element for regulating estrogen-induced ubiquitination and degradation of ER␣. Significantly, different S118 mutants revealed that degradation and transcriptional activity of ER␣ are mechanistically separable functions of ER␣. We find that proteolysis of ER␣ correlates with the ability of ER␣ mutants to recruit specific ubiquitin ligases regardless of the recruitment of other transcription-related factors to endogenous model target genes. Thus, our findings indicate that the AF-1 domain performs a previously unrecognized and important role in controlling ligandinduced receptor degradation which permits the uncoupling of estrogen-regulated ER␣ proteolysis and transcription.The ubiquitin-proteasome pathway contributes to the control of transcription through the ubiquitination and regulated degradation of multiple components of the transcriptional machinery (10, 34). Among these components is a large list of transactivators whose activity can be related to their proteolytic degradation. For many, particularly those that possess acidic activation domains, such as VP16 and c-myc, sequence elements essential for proteasome-mediated proteolysis reside within transactivation domains (33,43). Several members of the nuclear receptor superfamily are substrates for the ubiquitin-proteasome pathway (11,19,21,24,26,29,37,50,52,55), the first identified being estrogen receptor ␣ (ER␣) (1, 13, 36). ER␣ possesses two transactivation domains, AF-1 and AF-2, which reside in the N terminus and C terminus of the receptor, respectively. These activation domains are bridged by a conserved DNA binding domain and a hinge region responsible for receptor nuclear localization.The transcriptional activity of AF-2 is strictly ligand dependent, but the AF-1 is not; thus, AF-2 received much attention for analysis of the relationship between estrogen-stimulated proteolysis and transcription. AF-2 is highly structured, consisting of 12 ␣-helices that adopt an active conformation upon agonist binding, which exposes a hydrophobic surface where coactivator proteins bind (6). It has been shown that mutations of residues critical for AF-2-mediated transactivation disrupt proteolysis (27, 53). E6-AP, a ubiquitin ligase (35), and the TRIP1/Rpt6/SUG1 (28, 42) subunit of the 19S regulatory cap of the proteasome bind to ER␣ through the coactiva...

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.

Following acute hepatic injury, the metabolic capacity of the liver is altered during the process of compensatory hepatocyte proliferation by undefined mechanisms. In this study, we examined the regulation of de novo lipogenesis by cyclin D1, a key mediator of hepatocyte cell cycle progression. In primary hepatocytes, cyclin D1 significantly impaired lipogenesis in response to glucose stimulation. Cyclin D1 inhibited the glucose-mediated induction of key lipogenic genes, and similar effects were seen using a mutant (D1-KE) that does not activate cdk4 or induce cell cycle progression. Cyclin D1 (but not D1-KE) inhibited the activity of the carbohydrate response element-binding protein (ChREBP) by regulating the glucose-sensing motif of this transcription factor. Because changes in ChREBP activity could not fully explain the effect of cyclin D1, we examined hepatocyte nuclear factor 4α (HNF4α), which regulates numerous differentiated functions in the liver including lipid metabolism. We found that both cyclins D1 and D1-KE bound to HNF4α and significantly inhibited its recruitment to the promoter region of lipogenic genes in hepatocytes. Conversely, knockdown of cyclin D1 in the AML12 hepatocyte cell line promoted HNF4α activity and lipogenesis. In mouse liver, HNF4α bound to a central domain of cyclin D1 involved in transcriptional repression. Cyclin D1 inhibited lipogenic gene expression in the liver following carbohydrate feeding. Similar findings were observed in the setting of physiologic cyclin D1 expression in the regenerating liver. In conclusion, these studies demonstrate that cyclin D1 represses ChREBP and HNF4α function in hepatocytes via Cdk4-dependent and -independent mechanisms. These findings provide a direct link between the cell cycle machinery and the transcriptional control of metabolic function of the liver.

No abstract

No abstract