Focus
Focus
Scott L. Friedman
Division of Liver Diseases, Mount Sinai School of Medicine, New York, NY, United States
See Article, pages 933–999
Going with the flow – Autophagy and fatty liver
The cellular pathway of autophagy has emerged from the periphery of biomedicine into the spotlight. Literally meaning ‘selfeating’, 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 [1,2]. Strictly, there are three types of autophagy: macroautophagy, microautophagy, and chaperone-mediated
autophagy [3,4], 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 doublemembrane 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], who demonstrated that hepatocytes consume intracellular lipid when they are starved. This advance
has led to a broader focus on how autophagy regulates cellular
Received 24 January 2013; accepted 24 January 2013
q
DOI of original article: http://dx.doi.org/10.1016/j.jhep.2013.01.011.
E-mail address: scott.friedman@mssm.edu
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]. 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]. 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], 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]; in these models, genetic reconstitution
of autophagy attenuates features of the disease [9].
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
Open access under CC BY-NC-ND license.
Journal of Hepatology 2013 vol. 58 j 845–846
Focus
autophagy is altered in human fatty liver diseases. Additionally,
the link between autophagy and insulin resistance needs further
clarification. Because insulin inhibits autophagy [10], 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]; 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
846
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.
Conflict of interest
The author declared that he does not have anything to disclose
regarding funding or conflict of interest with respect to this
manuscript
References
[1] Mizushima N, Yoshimori T, Ohsumi Y. The role of Atg proteins in
autophagosome formation. Annu Rev Cell Dev Biol 2011;27:107–132.
[2] Mizushima N, Komatsu M. Autophagy: renovation of cells and tissues. Cell
2011;147:728–741.
[3] Levine B, Mizushima N, Virgin HW. Autophagy in immunity and inflammation. Nature 2011;469:323–335.
[4] Levine B, Kroemer G. Autophagy in the pathogenesis of disease. Cell
2008;132:27–42.
[5] Singh R, Kaushik S, Wang Y, Xiang Y, Novak I, Komatsu M, et al. Autophagy
regulates lipid metabolism. Nature 2009;458:1131–1135.
[6] Perlmutter DH. Alpha-1-antitrypsin deficiency: importance of proteasomal
and autophagic degradative pathways in disposal of liver disease-associated
protein aggregates. Annu Rev Med 2011;62:333–345.
[7] Hidvegi T, Ewing M, Hale P, Dippold C, Kemp CB, Maurice N, et al. An
autophagy-enhancing drug promotes degradation of mutant {alpha}1antitrypsin Z and reduces hepatic fibrosis. Science 2010;329 (5988):
229–232.
[8] Ding WX, Li M, Chen X, Ni HM, Lin CW, Gao W, et al. Autophagy reduces
acute ethanol-induced hepatotoxicity and steatosis in mice. Gastroenterology 2010;139:1740–1752.
[9] Yang L, Li P, Fu S, Calay ES, Hotamisligil GS. Defective hepatic autophagy in
obesity promotes ER stress and causes insulin resistance. Cell Metab
2010;11:467–478.
[10] Liu HY, Han J, Cao SY, 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;284:31484–31492.
[11] Hernandez-Gea V, Ghiassi-Nejad Z, Rozenfeld R, Gordon R, Fiel MI, Yue Z,
et al. Autophagy releases lipid that promotes fibrogenesis by activated
hepatic stellate cells in mice and in human tissues. Gastroenterology
2012;142:938–946.
Journal of Hepatology 2013 vol. 58 j 845–846