Ghrelin and fibrogenesis: Relief for a hungry liver
Authors: Fabio Marra
Journal: Journal of Hepatology (2011)

COMMENTARY ON: Ghrelin attenuates hepatocellular injury and liver fibrogenesis in rodents and influences fibrosis progression in humans.Moreno M, Chaves JF, Sancho-Bru P, Ramalho F, Ramalho LN, Mansego ML, Ivorra C, Dominguez M, Conde L, Millán C, Marí M, Colmenero J, Lozano JJ, Jares P, Vidal J, Forns X, Arroyo V, Caballería J, Ginès P, Bataller R. Hepatology 2010;51(3):974–985.http://www.ncbi.nlm.nih.gov/pubmed/20077562. COMMENTARY ON: Ghrelin attenuates hepatocellular injury and liver fibrogenesis in rodents and influences fibrosis progression in humans. Moreno M, Chaves JF, Sancho-Bru P, Ramalho F, Ramalho LN, Mansego ML, Ivorra C, Dominguez M, Conde L, Millán C, Marí M, Colmenero J, Lozano JJ, Jares P, Vidal J, Forns X, Arroyo V, Caballería J, Ginès P, Bataller R. Hepatology 2010;51(3):974–985. http://www.ncbi.nlm.nih.gov/pubmed/20077562. For many years, liver cell biologists have concentrated almost exclusively on hepatocytes. It is only in the last two decades that the identification of the cellular and molecular mechanisms of fibrogenesis has directed the attention of several investigators to the role played by hepatic myofibroblasts and by the complex networks involving parenchymal and non-parenchymal cells [[1]Friedman S.L. Mechanisms of hepatic fibrogenesis.Gastroenterology. 2008; 134: 1655-1669Abstract Full Text Full Text PDF PubMed Scopus (2209) Google Scholar]. Even more recently, many groups investigating hepatic fibrogenesis have started to look outside the liver, in order to understand if signals derived from other systems may affect the hepatic ‘wound healing’ response that leads to tissue fibrosis. Adipose tissue is a clear example of how changes in extrahepatic sites may influence the fibrogenic process, as highlighted by clinical studies that have indicated the detrimental role played by fat accumulation on the development and progression of chronic liver diseases [[2]Diehl A.M. Hepatic complications of obesity.Gastroenterol Clin North Am. 2010; 39: 57-68Abstract Full Text Full Text PDF PubMed Scopus (40) Google Scholar]. As a molecular counterpart, leptin and adiponectin, which are increased and decreased in obese patients, respectively, play opposite roles in the modulation of the fibrogenic process [[3]Marra F. Bertolani C. Adipokines in liver diseases.Hepatology. 2009; 50: 957-969Crossref PubMed Scopus (396) Google Scholar]. At a different level, the bone marrow has been shown to provide the liver with fibrogenic cells, or to limit the development of fibrosis with the contribution of different cell types [[4]Alison M.R. Islam S. Lim S. Stem cells in liver regeneration, fibrosis and cancer: the good, the bad and the ugly.J Pathol. 2009; 217: 282-298Crossref PubMed Scopus (193) Google Scholar]. Ghrelin, the focus of the paper by Moreno et al. [[5]Moreno M. Chaves J.F. Sancho-Bru P. Ramalho F. Ramalho L.N. Mansego M.L. et al.Ghrelin attenuates hepatocellular injury and liver fibrogenesis in rodents and influences fibrosis progression in humans.Hepatology. 2010; 51: 974-985Crossref PubMed Scopus (100) Google Scholar], expands the list of extrahepatic factors, which modulate the fibrogenic process. Ghrelin is mainly produced by enteroendocrine cells of the gastric mucosa and secreted in the circulation, to exert hormonal action on different tissues [[6]Chen C.Y. Asakawa A. Fujimiya M. Lee S.D. Inui A. Ghrelin gene products and the regulation of food intake and gut motility.Pharmacol Rev. 2009; 61: 430-481Crossref PubMed Scopus (192) Google Scholar]. Originally identified for its role in the release of growth hormone and as a potent stimulant of appetite, ghrelin is also implicated in development, gastrointestinal motility, sleep, memory, depression, and cancer [[6]Chen C.Y. Asakawa A. Fujimiya M. Lee S.D. Inui A. Ghrelin gene products and the regulation of food intake and gut motility.Pharmacol Rev. 2009; 61: 430-481Crossref PubMed Scopus (192) Google Scholar]. Its actions are mediated, at least in part, by the growth hormone secretagogue receptor 1a (GHS-R1a), which belongs to the seven-transmembrane domain superfamily. Nonetheless, there are actions of ghrelin that are mediated by a yet unrecognized receptor. Moreno et al. [[5]Moreno M. Chaves J.F. Sancho-Bru P. Ramalho F. Ramalho L.N. Mansego M.L. et al.Ghrelin attenuates hepatocellular injury and liver fibrogenesis in rodents and influences fibrosis progression in humans.Hepatology. 2010; 51: 974-985Crossref PubMed Scopus (100) Google Scholar] have used a panel of experimental approaches in rodents and humans to provide evidence that ghrelin has antifibrotic effects in the liver (Fig. 1). In rats subjected to ligation of the common bile duct, a well-established model of fibrosis, both recombinant ghrelin and a specific agonist of GHS-R1a resulted in a significant reduction of fibrotic tissue accumulation. This was associated with modulation of hepatic gene expression in agreement with the observed antifibrotic effects. Complementary data were obtained in mice defective in the ghrelin gene, where the absence of endogenous ghrelin resulted in more marked fibrosis upon chronic carbon tetrachloride administration. The cellular and molecular mechanisms of the anti-fibrogenic action of ghrelin were also investigated in detail. An important feature of animals treated with ghrelin was the lower degree of hepatocellular injury, highlighted by dedicated experiments using an acute toxic model of liver injury. The ability of ghrelin to counteract tissue damage is in line with the protective roles reported in other tissues [7Jacob A. Shah K.G. Wu R. Wang P. Ghrelin as a novel therapy for radiation combined injury.Mol Med. 2010; 16: 137-143Crossref PubMed Scopus (25) Google Scholar, 8Zhou X. Xue C. Ghrelin inhibits the development of acute pancreatitis and nuclear factor kappaB activation in pancreas and liver.Pancreas. 2009; 38: 752-757Crossref PubMed Scopus (55) Google Scholar]. In the liver, ghrelin affected several components of tissue injury, namely inflammation, apoptosis, and generation of oxidative stress-related molecules (Fig. 1). These actions, which by themselves may justify a reduced fibrogenic stimulus, were associated with direct effects on many, but not all, the profibrogenic features of activated stellate cells, thus providing another level of modulation of the tissue repair process. Interestingly, while activation of the GHS-R1a was sufficient to reproduce the effects of ghrelin, it is possible that other, yet unknown, membrane receptors contributed to the observed effects. This study is a brilliant example of translational research, where the role of a putative factor and its mechanisms of action were studied in cultured cells, animal models, and finally in humans. In fact, circulating levels of ghrelin were lower in patients with advanced fibrosis and a polymorphism of the ghrelin gene segregated individuals with faster fibrosis progression during chronic HCV infection. Thus, these data provide strong pre-clinical evidence for a possible interventional study evaluating the role of ghrelin as an antifibrotic agent. The good news is that this peptide is already being investigated in clinical trials for several disorders in humans (clinicaltrials.gov, accessed November 10, 2010), and its safety profile could be less of a concern compared to other ‘experimental’ anti-fibrogenic therapies. Interestingly, treatment with ghrelin has also been suggested for patients with end-stage liver disease, where anorexia and malnutrition are common. The effects of ghrelin on fibrosis are another piece of the puzzle that puts together metabolic regulation and the wound healing response. In recent years, leptin, a potent regulator of food intake and lipid partitioning, has emerged as an important stimulus to hepatic fibrogenesis, with actions on hepatic stellate cells and other non-parenchymal cells [9Marra F. Leptin and liver tissue repair: do rodent models provide the answers?.J Hepatol. 2007; 46: 12-18Abstract Full Text Full Text PDF PubMed Scopus (44) Google Scholar, 10Wang J. Leclercq I. Brymora J.M. Xu N. Ramezani-Moghadam M. London R.M. et al.Kupffer cells mediate leptin-induced liver fibrosis.Gastroenterology. 2009; 137: 713-723Abstract Full Text Full Text PDF PubMed Scopus (161) Google Scholar]. The actions of leptin on the hypothalamus are contrary to those of ghrelin, resulting in the stimulation of neuronal circuits that reduce appetite, and the observed anti-fibrogenic effects of ghrelin identify an additional condition in which these two hormones have opposing effects. Adiponectin and leptin, which also have opposite actions in terms of fibrogenesis [[3]Marra F. Bertolani C. Adipokines in liver diseases.Hepatology. 2009; 50: 957-969Crossref PubMed Scopus (396) Google Scholar], are mutually regulated in hepatic stellate cells [[11]Ding X. Saxena N.K. Lin S. Xu A. Srinivasan S. Anania F.A. The roles of leptin and adiponectin: a novel paradigm in adipocytokine regulation of liver fibrosis and stellate cell biology.Am J Pathol. 2005; 166: 1655-1669Abstract Full Text Full Text PDF PubMed Scopus (223) Google Scholar], and it will be interesting to investigate whether ghrelin or leptin affect their reciprocal expression or that of their receptors. Along these lines, leptin prevents ectopic fat deposition through central and peripheral mechanisms, these latter include activation of hepatic AMP-activated protein kinase (AMPK) [[3]Marra F. Bertolani C. Adipokines in liver diseases.Hepatology. 2009; 50: 957-969Crossref PubMed Scopus (396) Google Scholar]. In contrast, ghrelin has been found to favor hepatic steatosis and to inhibit hepatic AMPK activation [[12]Barazzoni R. Bosutti A. Stebel M. Cattin M.R. Roder E. Visintin L. et al.Ghrelin regulates mitochondrial–lipid metabolism gene expression and tissue fat distribution in liver and skeletal muscle.Am J Physiol Endocrinol Metab. 2005; 288: E228-E235Crossref PubMed Scopus (198) Google Scholar]. AMPK activation by a given ligand is cell-specific and AMPK has been recently identified as an anti-fibrogenic pathway in HSC [[13]Caligiuri A. Bertolani C. Guerra C.T. Aleffi S. Galastri S. Trappoliere M. et al.Adenosine monophosphate-activated protein kinase modulates the activated phenotype of hepatic stellate cells.Hepatology. 2008; 47: 668-676Crossref PubMed Scopus (116) Google Scholar]. Thus, the possibility that at least some of the effects of ghrelin are mediated through activation of this pathway deserves future investigation. Soluble factors targeting the liver from distant sites act as circulating hormones. Nonetheless, components of a wide number of these systems are also expressed within the liver, suggesting that at least some of their actions may be mediated by local ligand generation. Moreno et al. [[5]Moreno M. Chaves J.F. Sancho-Bru P. Ramalho F. Ramalho L.N. Mansego M.L. et al.Ghrelin attenuates hepatocellular injury and liver fibrogenesis in rodents and influences fibrosis progression in humans.Hepatology. 2010; 51: 974-985Crossref PubMed Scopus (100) Google Scholar] found that the ghrelin gene is expressed in normal and fibrotic human livers, and that both hepatocytes and HSC contribute to its expression. Nonetheless, other cells within the liver may express ghrelin, and additional work is required to elucidate better the cellular sources of this factor in the normal and inflamed tissue. Hepatic expression of ghrelin is reminiscent of data previously reported for other ‘systemic’ factors, such as the renin-angiotensin system, and several adipokines, including leptin and adiponectin [11Ding X. Saxena N.K. Lin S. Xu A. Srinivasan S. Anania F.A. The roles of leptin and adiponectin: a novel paradigm in adipocytokine regulation of liver fibrosis and stellate cell biology.Am J Pathol. 2005; 166: 1655-1669Abstract Full Text Full Text PDF PubMed Scopus (223) Google Scholar, 14Bataller R. Sancho-Bru P. Gines P. Lora J.M. Al-Garawi A. Sole M. et al.Activated human hepatic stellate cells express the renin-angiotensin system and synthesize angiotensin II.Gastroenterology. 2003; 125: 117-125Abstract Full Text Full Text PDF PubMed Scopus (288) Google Scholar]. Interestingly, intrahepatic expression of ghrelin was found to directly correlate with expression of profibrogenic genes, and patients with a haplotype of the promoter that is transcriptionally more active had more severe fibrosis. Although these findings sound surprising based on the anti-fibrogenic effects of ghrelin in animal models, they raise the hypothesis that the liver may be resistant to the effects of ghrelin, despite its increase in local expression. Resistance to leptin has been shown in obese patients and its mechanisms partially unraveled [[15]Myers M.G. Cowley M.A. Munzberg H. Mechanisms of leptin action and leptin resistance.Annu Rev Physiol. 2008; 70: 537-556Crossref PubMed Scopus (820) Google Scholar], and additional data on ghrelin are awaited. The data provided by Moreno et al. [[5]Moreno M. Chaves J.F. Sancho-Bru P. Ramalho F. Ramalho L.N. Mansego M.L. et al.Ghrelin attenuates hepatocellular injury and liver fibrogenesis in rodents and influences fibrosis progression in humans.Hepatology. 2010; 51: 974-985Crossref PubMed Scopus (100) Google Scholar] advocate the administration of recombinant ghrelin or stimulation of its signaling as a possible treatment strategy for liver fibrosis. However, antagonism of ghrelin’s action has been suggested for the therapy of other conditions, such as obesity and fatty liver. In fact, ghrelin-deficient mice were less susceptible to obesity, and in conditions of excess adiposity, reduction of ghrelin-mediated signal may reduce hyperphagia and lead to weight loss [16Tschop M. Smiley D.L. Heiman M.L. Ghrelin induces adiposity in rodents.Nature. 2000; 407: 908-913Crossref PubMed Scopus (3411) Google Scholar, 17Wortley K.E. del Rincon J.P. Murray J.D. Garcia K. Iida K. Thorner M.O. et al.Absence of ghrelin protects against early-onset obesity.J Clin Invest. 2005; 115: 3573-3578Crossref PubMed Scopus (321) Google Scholar]. As obesity and steatosis are well-known risk factors for the progression of chronic liver diseases, the possibility that chronic ghrelin administration could favor these conditions must be taken into account. Conversely, antagonism of ghrelin’s action, proposed for the treatment of obesity, may favor the progression of fibrosis in a population with a high prevalence of nonalcoholic liver disease and in general more susceptible to accelerated fibrogenesis [[18]Lanthier N. Horsmans Y. Leclercq I.A. The metabolic syndrome: how it may influence hepatic stellate cell activation and hepatic fibrosis.Curr Opin Clin Nutr Metab Care. 2009; 12: 404-411Crossref PubMed Scopus (46) Google Scholar]. Another point that will deserve further clarification is related to the observed mitogenic effect of ghrelin on cultured cancer cells, including hepatocellular carcinoma cells [[19]Murata M. Okimura Y. Iida K. Matsumoto M. Sowa H. Kaji H. et al.Ghrelin modulates the downstream molecules of insulin signaling in hepatoma cells.J Biol Chem. 2002; 277: 5667-5674Crossref PubMed Scopus (252) Google Scholar]. As a treatment of fibrosis is expected to encompass a long period of time, and fibrogenic liver diseases are preneoplastic conditions, the possibility that ghrelin promotes the appearance or progression of hepatocellular carcinoma is a critical issue. In spite of these caveats, it is from well-conducted and methodologically sound work like the one performed by Moreno et al. that new approaches for the treatment of fibrosis are likely to be developed. A post-translational modification of ghrelin results in acylation of serine-3 with an eight-carbon fatty acid, octanoate. While des-acyl ghrelin, the peptide devoid of the acyl moiety, has been long considered a non-functional form of ghrelin, recent data indicate that it actually participates in the regulation of food intake, gastrointestinal physiology, and metabolism, with actions that are in general opposite to those elicited by ghrelin [[6]Chen C.Y. Asakawa A. Fujimiya M. Lee S.D. Inui A. Ghrelin gene products and the regulation of food intake and gut motility.Pharmacol Rev. 2009; 61: 430-481Crossref PubMed Scopus (192) Google Scholar]. This may have relevance also for hepatic fibrosis, considering that des-acyl ghrelin represent the most abundant form secreted in the circulation. Ghrelin O-acyltransferase (GOAT) is the enzyme which catalyzes acylation of des-acyl ghrelin to provide ghrelin. Expression of GOAT is limited to tissues that secrete ghrelin, and the reaction is highly specific for this peptide [[6]Chen C.Y. Asakawa A. Fujimiya M. Lee S.D. Inui A. Ghrelin gene products and the regulation of food intake and gut motility.Pharmacol Rev. 2009; 61: 430-481Crossref PubMed Scopus (192) Google Scholar]. Thus, future investigations should take into account the regulation of GOAT, and interference with its activity is currently being considered to modulate the ghrelin system. The complexity of the system is further demonstrated by the existence of two additional products of the ghrelin gene, des-Gln14-ghrelin, which is created by alternative splicing, and binds GHS-R1a and obestatin. The pathophysiological role of obestatin in the regulation of metabolism and other conditions is being actively investigated. The author declared that he does not have anything to disclose regarding funding or conflict of interest with respect to this manuscript. Work in Dr. Marra’s laboratory is supported by grants from the European Community’s Seventh Framework Programme (FP7/2007-2013) under grant agreement n° HEALTH-F2-2009-241762 for the project FLIP. Associazione Italiana per la Ricerca sul Cancro (AIRC), and Istituto Toscano Tumori (ITT).