Adiponectin inhibits leptin signalling via multiple mechanisms to exert protective effects against hepatic fibrosis.

Hepatic fibrosis, results from excess deposition of extracellular matrix (ECM) proteins such as type I collagen, and is characteristic of chronic liver injury, regardless of etiology [1]. The accumulation of excess ECM leads to the formation of fibrous scar tissue that causes portal hypertension, and ultimately chronic liver failure. Hepatic fibrosis is considered part of the wound-healing program in response to liver injury and may be reversed if the initiating insult is resolved sufficiently early, but protracted fibrosis, specifically that which accompanies nonalcoholic fatty liver disease (NAFLD), may lead to cirrhosis and potentially hepatocellular carcinoma [2, 3]. Hepatic stellate cells (HSC) of healthy liver are normally quiescent, residing in the Space of Disse. They serve as storage repositories for retinoic acid in the form of retinyl esters. During the wound-healing response to chronic liver injury, however HSCs are ‘activated’, becoming proliferative and increasing their production of ECM proteins [4]. Although other hepatic cell types may also display fibrogenic properties, the activated HSC appears to be the primary actor responsible for e excessive ECM present in the fibrotic liver [3]. Leptin, the 16kDa adipocytokine product of the ob gene, known largely for its role in influencing hypothalamic control of appetite, insulin secretion, and glucose metabolism, displays multiple profibrogenic properties [5, 6]. Synthesized and secreted by white adipose tissue, leptin promotes expression of α2 (I) collagen [7, 8], the excessive deposition of which is a key feature of hepatic fibrosis. Leptin also suppresses the expression and activity of the collagen-degrading matrix metalloproteinase-1 (MMP-1) [9-11], and promotes expression of tissue inhibitor of metalloproteinase-1 (TIMP-1) [9], an important negative regulator of this MMP. Leptin also promotes maintenance of HSCs in the ‘activated’ phenotype by stimulating their proliferation and suppressing apoptosis [12]. Evidence supporting a role for leptin in promoting hepatic fibrogenesis is provided by Saxena et al’s observation that lean mice, when compared with their leptin non-producing ob/ob counterparts, display elevated deposition of collagen in the carbon tetrachloride (CCl4) model of hepatic fibrosis [8] while the CCl4-treated ob/ob mice failed to develop liver fibrosis. Leptin signal transduction is conducted via a Janus-Activated Kinase 2 (Jak2)/Signal Transducer and Activator of Transcription 3 (Stat3) tyrosine kinase pathway, initiated by leptin binding at the cell surface to the long form leptin receptor, Ob-Rb [13, 14]. Leptin binding to Ob-Rb results in activation of the receptor-associated Jak2 by autophosphorylation [15, 16]; Jak2 subsequently activates Ob-Rb by phosphorylation at Tyr985 (Y985) and Tyr1138 (Y1138) [17]. Phosphorylation of Ob-Rb at Tyr1138 is required to induce binding by Stat3 [17], which is then also phosphorylated and activated by Jak2 [18]. Once activated, Stat3 can form homodimers and translocate to the nucleus, where it acts in concert with other cellular and microenvironment-specific factors, ultimately regulating transcription of respective target genes. Leptin-stimulated transcriptional activation of both type I collagen and TIMP-1 involve Jak/Stat signal transduction [9]. Adiponectin is a 30kDa protein that, like leptin, is synthesized and secreted primarily by white adipose tissue. Adiponectin circulates in the blood as low, medium, or high molecular weight oligomers, and its serum levels correlate inversely with body fat [5, 19]. Two receptors that propagate signal transduction in response to adiponectin have been identified: AdipoR1, expressed primarily in skeletal muscle and AdipoR2, expressed abundantly in liver [19]. Adiponectin signal transduction in the liver is conducted primarily through activation of adenosine monophosphate-activated kinase (AMPK) by phosphorylation at Thr172, although there is also evidence of a role for peroxisome proliferator-activated receptor alpha (PPAR-α) mediated signaling in response to adiponectin [20, 21]. LKB1 has been identified as an upstream activator of AMPK, but the molecular events involved in adiponectin signal transduction have not been completely defined [22], particularly as related to biological actions not related to the regulation of cellular energy stores and insulin sensitivity. Whereas leptin generates profibrogenic effects, multiple studies indicate that adiponectin is antifibrogenic, although the mechanisms of this protective effect have not yet been described [5]. Overexpression of adiponectin in rat HSCs suppresses proliferation and reduces expression of proliferating cell nuclear antigen (PCNA) and α smooth muscle actin (αSMA), a marker for HSC activation [23]. Treatment with adiponectin also stimulates apoptosis in activated, but not quiescient HSCs, suggesting that adiponectin may act to maintain HSCs in the quiescient state, thereby reducing their fibrogenic actions [23]. In humans, reduced serum adiponectin levels are associated with several negative physiological consequences, including poor liver histology, inflammation, and fibrosis [5, 24-26]. Moreover, mice overexpressing adiponectin via adenoviral delivery are less vulnerable than lac z-expressing mice to CCl4-induced hepatic fibrosis [27]. Extending this paradigm, recent experiments from our laboratory demonstrate that adiponectin knockout mice (Ad−/−) are more susceptible to CCl4-induced hepatic fibrosis than wild-type mice, and that Ad−/− mice are also more vulnerable to leptin-induced fibrosis [6, 10]. A potential clue from these experiments suggests that a plausible mechanism explaining the increased vulnerability of the Ad−/− mice to leptin-mediated hepatic fibrosis is their comparatively decreased expression of the Suppressors of Cytokine Signaling 3 (SOCS-3) protein, an important negative regulator of leptin signaling. These data imply a functional loss of the ability of Ad−/− mice to inhibit leptin signaling, leading to the enhanced hepatic fibrosis observed therein. Although leptin and adiponectin clearly represent a mutually antagonistic paradigm regulating hepatic extracellular matrix deposition, very little is known about the molecular mechanisms of crosstalk between these two important pathways. In the current study, we investigated mechanisms by which adiponectin can antagonize leptin signaling to provide protection against leptin-stimulated hepatic fibrosis.