Is the Adiponectin-AMPK-Mitochondrial Axis Involved in Progression of Nonalcoholic Fatty Liver Disease?

Nonalcoholic fatty liver disease (NAFLD) is the most common form of liver disease, affecting nearly a third of the population.1 NAFLD is more than simply a “hepatic disease”; rather, it is part of a dysmetabolic state with contributions by the metabolically active organ triad comprised of the liver, adipose tissue, and skeletal muscle towards hepatic disease. Despite the focus on the liver, cardiovascular events are the major cause of mortality in NAFLD and there is enormous interest to identify the mechanistic link between NAFLD and cardiovascular events.2 The initial trigger of excess caloric intake contributes to peripheral insulin resistance in the skeletal muscle and adipose tissue with elevated circulating fatty acids.3 Increased hepatocyte uptake of fatty acids is followed by cellular partitioning into either β-oxidation or esterification to triglycerides.4 β-Oxidation of fatty acids occurs in both peroxisomes and mitochondria that generate acetyl CoA that is oxidized by way of the Kreb's cycle in the mitochondrial matrix. Mitochondrial dysfunction in NAFLD contributes to the shift of fatty acids from oxidation into the esterification and export pathways as well as generation of reactive oxygen species.5 Impaired insulin signaling, substrate metabolism, and mitochondrial function contribute to varying extents toward the development and progression of NAFLD. Integration of the excess nutrient and metabolic regulatory pathways is mediated by adiponectin and AMP-activated kinase (AMPK) and their crosstalk could potentially contribute to the progression of NAFLD. In this issue, Handa et al.6 report a novel adiponectin-AMPK axis that affects mitochondrial function in the pathogenesis of NAFLD. Adiponectin, an adipocytokine, is a central regulatory link between insulin resistance, disordered substrate oxidation, and mitochondrial dysfunction in multiple organs.7 Adiponectin expression is highly specific to adipose tissue, but is expressed in the liver and skeletal muscle also.8 Of the different circulating isoforms of adiponectin, full-length adiponectin binds to adiponectin receptor 2 (AdipoR2), expressed primarily in the liver; and the globular domain trimer (lacking the N terminal domain) binds preferentially to adiponectin receptor 1 (AdipoR1), expressed primarily in the skeletal muscle. Adiponectin receptor signaling regulates substrate metabolism by activation of several key players in cellular energy management including AMPK, the sirtuins, and nuclear receptor peroxisome proliferator-activated receptor alpha (PPARα).9, 10 Additional hepatic salutary effects of adiponectin include antiinflammatory and antifibrotic effects. Despite increased understanding of the pathogenesis and progression of NAFLD, a number of questions remain, not the least of which are the mechanisms of progression, and identification of potential molecular targets for therapy. Handa et al.6 report the results of a series of very elegant in vivo studies in the liver and adipose tissue of a murine model fed a diet rich in omega-6 fatty acids that replicates the spectrum of NAFLD from steatosis to nonalcoholic steatohepatitis (NASH). Mechanistic in vitro studies in a murine hepatocyte cell line as well as in primary hepatocytes complement the in vivo data. They demonstrate that adiponectin depletion is a direct consequence of weight gain, and plays a critical regulatory role in the development and progression of NAFLD. Their studies provide answers to two specific questions: why does plasma adiponectin decrease with progression of NAFLD, and is there a mechanistic relation between reduced adiponectin and progression of NAFLD. Using a standard murine model of insulin resistance, in the Leprdb/db mice fed a high-fat diet they demonstrated that hypoadiponectinemia resulted in reduced AMPK activation and lower acyl CoA carboxylase (ACC), the immediate downstream target of AMPK. Since AMPK activation is a cellular response to activate oxidative phosphorylation, reduced adiponectin acts by way of blunted cellular energy sensing mechanisms.10 Additionally, the authors demonstrate a novel link between adiponectin and mitochondrial biogenesis6 that is a paradigm shift in understanding the mechanism of mitochondrial dysfunction in NASH.5 These studies from the Kowdley laboratory6 help identify the potential mechanisms for a number of clinical and molecular observations in patients with NAFLD who have an increased adipose tissue mass and reduced plasma adiponectin.9 Since adiponectin is synthesized primarily by adipocytes, it has been a challenge to explain low circulating adiponectin levels despite an expansion of adipose tissue mass. There has been controversy regarding the mechanism of low circulating adiponectin and the hyperinsulinemia of NAFLD, since insulin directly stimulates adiponectin biosynthesis and secretion.11 In vitro studies in NIH 3T3 L1 adipocytes showed increased adiponectin in the culture medium in response to insulin. In contrast, in vivo, hyperinsulinemia consistently is associated with hypoadiponectemia.7 Physiologically, insulin inhibits adipose tissue FOXO1 that in turn represses its downstream target PPARγ, a transcriptional inducer of the adiponectin gene.12 In NAFLD, it is believed that insulin resistance is accompanied by FOXO1 activation with reduced adiponectin transcription. There is, however, very limited evidence to support this mechanistic hypothesis. Additional contributors to the reduced adiponectin include inflammation-induced oxidative stress in adipose tissue. The very interesting direct studies by Handa et al. in this issue demonstrate an increased inflammatory response of adipose tissue. These data suggest that expression of adiponectin is inversely related to a number of inflammatory adipocytokines such as nuclear factor kappa B (NFκB), interleukin (IL)-6, and tumor necrosis factor alpha (TNFα).6 The critical question that remains as to how adiponectin functions as an integrated metabolic regulator is clarified by the investigators by studies in liver and adipose tissue, as well as direct studies in hepatocytes.6 NAFLD and specifically NASH in vivo is associated with reduced liver AMPK activation by way of impaired adiponectin signaling. Low adiponectin is accompanied by reduced AdipoR2 expression and decreased phosphorylation of AMPK. Previous studies have suggested that AMPK phosphorylation by adiponectin was mediated by way of APPL1 (leucine zipper motif). Handa et al. show that LKB1 expression was reduced in NASH, suggesting a novel adiponectin-AdipoR2-LKB1-AMPK signaling pathway. Additional in vivo studies in genetic mouse models with knockout and transgenic overexpression of these genes are necessary to confirm these in vitro studies on this novel pathway. Whether activation of AMPK by alternative mechanisms including pharmacological intervention using 5-amino-imidazole-4-carboxamide (AICAR) can overcome the hypoadiponectinemia is yet to be resolved, but does provide an exciting therapeutic option. In addition to demonstrating the biochemical basis of fat accumulation by way of increased ACC activity due to reduced AMPK activation, mitochondrial biogenesis was also affected. Conflicting reports on disordered hepatic mitochondrial function and increased oxidative stress have been reported in NASH but its mechanism is not entirely clear.5, 13 The present study provides evidence of regulation of mitochondrial dynamics by adiponectin.6 Even though the authors did not show direct evidence of mitochondrial dysfunction by either oxygen consumption ratio or substrate oxidation, their data reiterate that mitochondrial dysfunction is an essential component of NASH and is a direct effect of reduced adiponectin function. The mechanisms by which adiponectin regulates mitochondrial dynamics and function need to be elucidated. The skeletal muscle is the third and major component of the metabolically active organ triad and sarcopenic obesity occurs in metabolic syndrome, insulin-resistant states, and fatty liver. Myostatin, a skeletal muscle myokine of the transforming growth factor beta (TGFβ) superfamily, causes skeletal muscle loss and increased fat mass. There is increasing evidence of increased myostatin expression in insulin-resistant states14, 15 and blocking myostatin protects against both diet-induced obesity and fatty liver.16 Adiponectin binds to skeletal muscle AdipoR1 and regulates peripheral insulin sensitivity as well as fatty acid oxidation.8, 9 One critical question is whether the loss of muscle mass or altered signaling responses in the skeletal muscle contributes to the development and progression of NAFLD is an area of intense research interest.17 These data lead to the question of whether myostatin and adiponectin crosstalk18 and, if so, does this crosstalk occur at the transcriptional, posttranslational, or receptor level? Finally, is the skeletal muscle the elusive link between the heart and the liver? Increased exercise and focus on improved skeletal muscle function does reduce cardiovascular risk and our pathogenic and therapeutic focus in the management of the metabolic syndrome and NAFLD should include targeting the skeletal muscle. The present study by Handa et al. is also of interest in the context of a number of novel adiponectin responses that include: ceramidase activity,19 suppression of gluconeogenic enzymes, independent of AMPK by way of the APPL1 pathway,8 and interaction of adiponectin with the suppressor of glucose from autophagy protein.20 These exciting discoveries will help our understanding of progression of NAFLD, identify novel mechanisms, and permit development of molecular therapeutic targets. In summary, there is compelling evidence that adiponectin is a key mediator of the liver-skeletal muscle axis (Fig. 1). The study in this issue6 provides evidence of the central role of adiponectin-mediated, AMPK-dependent regulation of signaling responses in vivo in NAFLD and specifically in NASH. There is increasing recognition of the considerable adverse effects of the thiazolidinedione class of insulin sensitizers that have effects beyond adiponectin. Studies focusing on the development of adiponectin analogs or inducers as well as adiponectin sensitizers including the recently described small molecule AdipoR agonist (AdipoRon) are likely to provide novel therapeutic options for NAFLD. David Schumick from Medical Illustrations at the Cleveland Clinic assisted with the figure. Srinivasan Dasarathy, M.D.