Focus
Authors: Scott L. Friedman
Journal: Journal of Hepatology (2013)

Pharmacological promotion of autophagy alleviates steatosis and injury in alcoholic and non-alcoholic fatty liver conditions in miceJournal of HepatologyVol. 58Issue 5PreviewPharmacological approaches can potentially improve fatty liver condition in alcoholic and non-alcoholic fatty liver diseases. The salutary effects of reducing lipid synthesis or promoting lipid oxidation have been well reported, but the benefits of increasing lipid degradation have yet to be well explored. Macroautophagy is a cellular degradation process that can remove subcellular organelles including lipid droplets. We thus investigated whether pharmacological modulation of macroautophagy could be an effective approach to alleviate fatty liver condition and liver injury. Full-Text PDF The cellular pathway of autophagy has emerged from the periphery of biomedicine into the spotlight. Literally meaning ‘self-eating’, our understanding of autophagy has evolved with a history in the scientific literature similar to what previously occurred with the study of apoptosis – first described as a morphologic curiosity, it is now clear that autophagy, like apoptosis, is a tightly regulated pathway that maintains cellular homeostasis, with important disease implications relevant to cancer, inflammation and immunity, genetic diseases, and fibrosis, among others. Also like apoptosis, a cascade of specific effector proteins has been uncovered that link autophagic pathways to changes in cell structure and function. Collectively known as “ATG” proteins (with 30 identified to date), these autophagic mediators integrate a number of convergent signals to either accelerate intracellular recycling and cell survival, or sometimes provoke cell death [1Mizushima N. Yoshimori T. Ohsumi Y. The role of Atg proteins in autophagosome formation.Annu Rev Cell Dev Biol. 2011; 27: 107-132Crossref PubMed Scopus (2209) Google Scholar, 2Mizushima N. Komatsu M. Autophagy: renovation of cells and tissues.Cell. 2011; 147: 728-741Abstract Full Text Full Text PDF PubMed Scopus (4070) Google Scholar]. Strictly, there are three types of autophagy: macroautophagy, microautophagy, and chaperone-mediated autophagy [3Levine B. Mizushima N. Virgin H.W. Autophagy in immunity and inflammation.Nature. 2011; 469: 323-335Crossref PubMed Scopus (2478) Google Scholar, 4Levine B. Kroemer G. Autophagy in the pathogenesis of disease.Cell. 2008; 132: 27-42Abstract Full Text Full Text PDF PubMed Scopus (5647) Google Scholar], but macroautophagy is the focus of most human disease studies, where its primary role is to degrade intracellular organelles and protein aggregates that are too large to be broken down by the proteasome. Morphologically, autophagy is comprised of series of well-defined steps: formation of a double-membrane around an organelle to generate an autophagosome, followed by its fusion with a lysosome to form an autolysome, and then degradation of the contents by lysosomal enzymes. While the breadth of autophagy’s roles is still not fully appreciated, its most cogent evolutionary function is to preserve cellular energy homeostasis. There is a basal level of autophagy in normal cells that provides a quality control mechanism to clear damaged organelles or protein aggregates. However, autophagy is induced in the midst of inadequate nutrient supply or stress. Thus, starvation is the classic trigger that initiates autophagy to provide fuel through consumption of intracellular substrates. In this context, the interest in autophagy in studies of liver was greatly accelerated by a seminal study by Mark Czaja and colleagues in 2009 [[5]Singh R. Kaushik S. Wang Y. Xiang Y. Novak I. Komatsu M. et al.Autophagy regulates lipid metabolism.Nature. 2009; 458: 1131-1135Crossref PubMed Scopus (2730) Google Scholar], who demonstrated that hepatocytes consume intracellular lipid when they are starved. This advance has led to a broader focus on how autophagy regulates cellular homeostasis in hepatocytes, and how this pathway contributes to disease, or might be exploited to establish new treatments. The most immediate liver disease-relevance of autophagy is in alpha1-antitrypsin disease, where increased autophagic flux occurs in cells that have accumulated the mutant protein, and where inhibition of autophagy greatly delays alpha1-antitrypsin degradation [[6]Perlmutter D.H. Alpha-1-antitrypsin deficiency: importance of proteasomal and autophagic degradative pathways in disposal of liver disease-associated protein aggregates.Annu Rev Med. 2011; 62: 333-345Crossref PubMed Scopus (92) Google Scholar]. Based on this finding, David Perlmutter and colleagues demonstrated that carbamazepine, an anti-convulsant CNS drug known to promote autophagy, can further accelerate hepatocellular clearance and improve degradation of the mutant protein in a mouse model of alpha1-antitrypsin [[7]Hidvegi T. Ewing M. Hale P. Dippold C. Kemp C.B. Maurice N. et al.An autophagy-enhancing drug promotes degradation of mutant {alpha}1-antitrypsin Z and reduces hepatic fibrosis.Science. 2010; 329: 229-232Crossref PubMed Scopus (487) Google Scholar]. This finding has led to a phase 2 clinical trial using carbamazepine in patients with severe alpha1-antitrypsin liver disease (ClinicalTrials.gov NCT01379469). In this month’s Journal, an article by Lin and colleagues weaves these two threads from the Czaja and Perlmutter studies together by exploring the treatment of fatty liver due to chronic ethanol feeding or high fat diet by stimulating autophagy. Building on their earlier study that demonstrated increased autophagy in mice acutely fed ethanol [[8]Ding W.X. Li M. Chen X. Ni H.M. Lin C.W. Gao W. et al.Autophagy reduces acute ethanol-induced hepatotoxicity and steatosis in mice.Gastroenterology. 2010; 139: 1740-1752Abstract Full Text Full Text PDF PubMed Scopus (406) Google Scholar], here Lin et al. report that either carbamazepine or rapamycin (which also stimulates autophagy, by suppressing mTOR activity) enhance macroautophagy in cultured hepatocytes and reduce steatosis, injury and ALT elevation in vivo following acute or chronic ethanol feeding. In contrast, chloroquine, which blocks autophagy, exacerbates steatosis and injury in these animals. Similarly, in mice fed a high fat diet for 12 weeks, carbamazepine or rapamycin significantly reduces steatosis, hepatic and serum triglycerides and insulin resistance. Because carbamazepine had no effect on several genes regulating lipogenesis and fatty acid oxidation, the findings are consistent with its benefiting fatty liver by increasing autophagy, although more studies are needed to establish this effect on autophagy as the primary mechanism of action, including more extensive metabolic characterization of mice treated with the drug. The findings from this study are highly translational and reinforce the potential utility of stimulating autophagic flow to improve fatty liver disease. Moreover, in animal models of NAFLD there is a relative deficiency of autophagy, the mechanism of which is not known [[9]Yang L. Li P. Fu S. Calay E.S. Hotamisligil G.S. Defective hepatic autophagy in obesity promotes ER stress and causes insulin resistance.Cell Metab. 2010; 11: 467-478Abstract Full Text Full Text PDF PubMed Scopus (964) Google Scholar]; in these models, genetic reconstitution of autophagy attenuates features of the disease [[9]Yang L. Li P. Fu S. Calay E.S. Hotamisligil G.S. Defective hepatic autophagy in obesity promotes ER stress and causes insulin resistance.Cell Metab. 2010; 11: 467-478Abstract Full Text Full Text PDF PubMed Scopus (964) Google Scholar]. There are several gaps and a caveat that need to be addressed before fully embracing autophagy stimulation as a therapy for fatty liver disease, however. First, there is scant evidence that autophagy is altered in human fatty liver diseases. Additionally, the link between autophagy and insulin resistance needs further clarification. Because insulin inhibits autophagy [[10]Liu H.Y. Han J. Cao S.Y. Hong T. Zhuo D. Shi J. et al.Hepatic autophagy is suppressed in the presence of insulin resistance and hyperinsulinemia: inhibition of FoxO1-dependent expression of key autophagy genes by insulin.J Biol Chem. 2009; 284: 31484-31492Crossref PubMed Scopus (312) Google Scholar], the hyperinsulinemia typical of NAFLD may be provoking autophagy deficiency rather than the other way around, although autophagy may modify insulin sensitivity as well. Further studies are also needed to definitely link the beneficial effects of carbamazepine to its induction of autophagy. Gene array studies combined with pathway analysis might uncover additional downstream responses to carbamazepine that could help suggest new therapeutic targets and drugs. From a commercial perspective, because carbamazepine and rapamycin are both available as generic formulations, it would be difficult to attract a commercial sponsor to support trials for fatty liver disease using these drugs alone. However, because they are both established molecular entities, the drugs might be used in combination with a novel experimental agent to enhance efficacy in at least one arm of a potential multi-arm clinical trial. Alternatively, since carbamazepine is primarily used as an agent to treat neurologic diseases and has a number of adverse effects, careful structure-activity studies might uncover novel derivatives of the drug that preserve the autophagy-inducing activity while eliminating the CNS effects. This strategy would have the added benefit of creating a new molecular entity that could entice pharmaceutical companies to support its development. A final point is that while most studies suggest that enhancing autophagy should reduce the risk of neoplasia, at least a few indicate that the pathway also amplifies hepatitis virus replication and can enhance tumor cell metabolism, so vigilance for these untoward effects would be warranted if clinical trials are conducted, especially in patients with cirrhosis, who are at higher risk for cancer. An additional caveat concerns the collateral impact of manipulating hepatic autophagy on non-hepatocytes. Specifically, we have demonstrated that autophagy in hepatic stellate cells drives hepatic fibrogenesis in vivo since animals with stellate cell-specific defects in autophagic signaling have reduced fibrosis following toxic liver injury [[11]Hernandez-Gea V. Ghiassi-Nejad Z. Rozenfeld R. Gordon R. Fiel M.I. Yue Z. et al.Autophagy releases lipid that promotes fibrogenesis by activated hepatic stellate cells in mice and in human tissues.Gastroenterology. 2012; 142: 938-946Abstract Full Text Full Text PDF PubMed Scopus (462) Google Scholar]; presumably, autophagy in this context provides the fuel necessary to support the increased energy demands of stellate cell activation. In fact, in our studies inhibition of autophagy with chloroquine enhances toxic liver injury due to CCl4 in mice, however the animals have less fibrosis (unpublished results). Therefore, efforts to stimulate autophagy in order to improve fatty liver disease must be mindful of the potential for increasing fibrosis, although if the fibrogenic signals from injured hepatocytes are sufficiently attenuated by autophagy induction, this may not be a significant concern. On the other hand, cell-specific inhibition of autophagy in stellate cells might be an attractive antifibrotic strategy, provided that collateral effects on other cell types (including endothelial cells, inflammatory cells as well as hepatocytes) are minimized. In summary, the emergence of autophagy as a pathway central to hepatic homeostasis is highlighted by the findings of Lin et al. in this issue of the Journal. ‘Going with the flow’ of autophagy merits serious consideration as a new strategy for the treatment of fatty liver disease. The author declared that he does not have anything to disclose regarding funding or conflict of interest with respect to this manuscript