When GLP-1 hits the liver: a novel approach for insulin resistance and NASH.

NONALCOHOLIC FATTY LIVER DISEASE (NAFLD) encompasses a spectrum ranging from simple steatosis to steatohepatitis (NASH), increasing fibrosis and eventually, cirrhosis (22). Importantly, NASH accompanied by fibrosis and severe inflammation is the most relevant predictor for disease progression to cirrhosis and hepatocellular carcinoma (21). NAFLD is tightly associated with obesity, insulin resistance (IR), and overt type 2 diabetes (T2D) and represents a manifestation and driver of the metabolic syndrome (5, 7, 20). While the best therapy to improve insulin sensitivity is diet combined with exercise, these are difficult to achieve for the majority of patients, requiring at least adjunctive pharmacological therapies (5, 20). A novel class of pharmacological agents that appear to be devoid of unwanted side effects, including weight gain, are drugs that increase the action of glucagon-like peptide-1 (GLP-1) directly (GLP-1 mimetics, such as exenatide) or indirectly via inhibition of dipeptidyl peptidase-4 (DPP-4), an enzyme that is ubiquitously expressed on the cell surface and that rapidly degrades GLP-1 to an inactive peptide. Exenatide and DPP-4 inhibitors have been widely validated to treat T2D, and their potential to effectively treat NAFLD and NASH attracts increasing attention (8, 22, 23). Moreover, experimental models that mimic human NAFLD and especially NASH are constantly improved to serve as a preclinical platform to evaluate novel therapeutic strategies. Mouse models of NAFLD are based on genetic mutations, such as ob/ob or db/db mice, or diet, such as the methionine/choline-deficient (MCD) diet (1). However, none of these fully reflect the phenotype of human NAFLD and especially NASH. The more recent models that exploit either a high-fat/trans-fat-enriched and high fructose diet (28) diet, or a high-fat diet with low-dose streptozotocin (which induces mild insulin deficiency not matching insulin demand) (14) much better resemble human NASH pathogenesis and histopathology including fibrosis. The study by Trevaskis et al. (29) examined another modification of these novel NASH models, i.e., mice deficient in leptin-signaling (ob/ob) or their wild-type controls fed a high trans-fat (HTF), high fructose, and high cholesterol diet. The authors also assessed the beneficial effect the GLP-1R agonist AC3174 in this model. They found that ob/ob mice fed this diet developed more severe pathophysiologic features of fibrotic NASH than wild-type mice. This is unexpected, since intact leptin signaling is considered important for the activation of fibrogenic hepatic stellate cells and myofibroblasts (here jointly termed HSC) (10), and leptin deficient ob/ob mice are resistant to hepatic fibrosis. Thus leptin has been shown to induce TNFand IL-6 in the regenerative response to hepatic injury (12) and to promote HSC activation via hedgehog signaling (4). Accordingly, ob/ob mice in which fibrogenesis was induced with thioacetamide (TAA), carbon tetrachloride, or an MCD diet, displayed an attenuated hepatic profibrogenic TGFand procollagen type I mRNA expression, but these transcript levels were restored to increased levels after injection of leptin (11). Similarly, a leptin antagonist ameliorated TAA-induced liver fibrosis (6). However, hepatic fibrogenesis can be triggered and maintained by several chemokines and cytokines produced by immune and other cells, and does not solely rely on leptin signaling (18). In the present model, fibrogenesis could have been accelerated by a more excessive fat accumulation due to leptin deficiency and an unopposed generation of oxidative stress, which is considered a central driver of hepatocyte apoptosis and fatty liver inflammation (20). Notably, the present diet contained HTF, an important contributor to the development of hepatic steatosis and steatohepatitis (28), and high cholesterol (2%). The addition of 0.15–2% cholesterol to a steatogenic diet has previously been shown to exacerbate hepatic free cholesterol accumulation, which leads to severe steatosis, hepatocyte injury, macrophage recruitment, and liver fibrosis (3, 24, 27). Recently, Teratani et al. (27) provided a functional link between high dietary cholesterol and enhanced inflammation and fibrosis, demonstrating that in BL6 mice dietary cholesterol (1%) upregulated Toll-like receptor 4 on HSC, which suppressed the inhibitory TGFpseudorecepter Bambi, leading to increased TGFsignaling, HSC activation, and fibrosis. Notably, the postulated mechanism was Kupffer celland leptinindependent. This may explain the development of fibrosis in leptin-deficient ob/ob mice by the HTF, high-fructose and highcholesterol diet in the present paper. Taken together, reflecting the rare prevalence of leptin deficiency in man and the multiple and often divergent activities of leptin signaling in obesity and NASH, the relevance of ob/ob mice as preclinical model for human NASH is limited, and a model based on life style changes and metabolic alterations, such as a steatogenic, and, if possible, also fibrogenic diet is desirable. The model described by Trevaskis et al. (29) appears to come a step closer to such a humanized NASH model. However, a more rigorous fibrosis assessment would have been welcome, including hydoxyproline determination as a quantitative measure of collagen accumulation, picrosirius red staining for collagen, collagen morphometry on representative liver sections, and a broader quantitative PCR analysis of transcripts related to fibrogenesis, and fibrolysis, as well as more biochemical, histological, and transcript markers of lipid metabolism and inflammation (2, 17). Another novel aspect of the present paper is the effect of AC3174, a GLP-1 receptor (GLP-1R) agonist, in the novel NASH model applied to wild-type, ob/ob and GLP-1 receptor deficient (GLP-1RKO) mice. AC3174 is a peptide analogue of the GLP-1R agonist exenatide, with a leucine for methionine at position 14. GLP-1 lowers plasma glucose (9), mainly via its insulinotropic activity (29) coupled to improved insulin sensiAddress for reprint requests and other correspondence: D. Schuppan, Molecular and Translational Medicine, Dept. of Medicine I, Johannes Gutenberg Univ., Langenbeckstr. 1, 55131 Mainz, Germany (e-mail: [email protected]). Am J Physiol Gastrointest Liver Physiol 302: G759–G761, 2012; doi:10.1152/ajpgi.00078.2012. Editorial Focus