MINIREVIEWFat paradox in liver disease
Hide Tsukamoto
Department of Pathology, Keck School of Medicine of the University of Southern California and the Departmentof Veterans Affairs Greater Los Angeles Healthcare System, Los Angeles, CA, USA(Received for publication on September 7, 2005)(Revised for publication on October 6, 2005)(Accepted for publication on October 13, 2005)

Abstract. Alcoholic liver disease (ALD) is characterized by accumulation of neutral lipids in hepatocytes leading to micro and macro-vesicular steatosis and balloon cell degeneration. Hypercaloric
alimentation and resultant obesity also cause similar changes as evident in non-alcoholic fatty liverdisease (NAFLD). Thus, accumulation of lipids in hepatocytes is a pathologic hallmark of ALD andNAFLD. In contrast, quiescent hepatic stellate cells (HSC) are characterized by the intracellular content of not only vitamin A but also triglycerides, and HSC activation is associated with depletion ofthese lipids. In fact, our recent work demonstrates that adipogenic/lipogenic transcriptional regulation
rendered by PPARg, LXRa, and SREBP-1c is essential for the maintenance of the fat-storing, quiescence phenotype of HSC. Expression of these adipogenic transcription factors is lost in activated HSC
and the treatment of the cells with the adipocyte differentiation cocktail or ectopic expression ofPPARg or SREBP-1c causes a reversal of activated cells to the quiescent phenotype. In steatotic livers
from ALD and NAFLD mouse models, the expression of these adipogenic transcription factors isinduced while the normal control livers lack such expression. Thus, adipogenic regulation is essentialfor HSC quiescence while it makes hepatocytes steatotic. Interestingly, under the adipogenic conditions of ALD and NAFLD, HSC are still activated to cause ﬁbrosis. This fat paradox in hepatocytes
and HSC highlights contrasted signiﬁcance of fat in these two cell types that depend on each other for
their homeostatic control. It further suggests, activated HSC in steatotic livers may have defective insulin signaling or lipogenic regulation. (Keio J Med 54 (4): 190–192, December 2005)
Key words: Ito cells, hepatic stellate cells, PPARg, adipocyte

conﬁrmed the depletion of PPARg in HSC nuclear
extracts from ﬁbrotic livers and a correction of this defect by the TZD treatment in vivo.

PPARg as a Molecular Target ofAnti-liver Fibrosis Modality
In 2000, three independent publications appearedthat reported inhibition of activated hepatic stellate
cells (HSC) with ligands for peroxisome proliferatoractivated receptor g (PPARg) in vitro.1,4,5 Our report
also demonstrated that HSC activated both in vitro andin vivo have reduced expression of PPARg and that thetreatment of culture-activated HSC with the PPARg
ligands restores its expression.1 These results raised
a possibility that PPARg may serve as a therapeutictarget for liver ﬁbrosis. This notion was subsequently
tested by Galli et al, who demonstrated the therapeutic
efﬁcacy of two thiazolidinedione (TZD) derivatives(pioglitazone and rosiglitazone) in two toxic and onecholestatic models of liver ﬁbrosis.6 This study also

Ectopic Expression of PPARg inCulture-activated HSC
TZDs are known to have multiple biological effects
including suppression of inﬂammation and cell proliferation, pro-apoptotic effects, besides the aformentioned anti-ﬁbrotic effects. These effects may or may
not be mediated via PPARg. A PPARg independent
effect was unequivocally demonstrated by Chawla et al,
who showed neither macrophage differentiation nor
anti-inﬂammatory effects of TZD are abrogated inmacrophages derived from PPARg null mouse embryonic stem cells.7 Thus, it is critical to determine whether

Reprint requests to: Dr. Hide Tsukamoto, 1333 San Pablo Street, MMR-412, Los Angeles, CA 90033, USA, e-mail: htsukamo@usc.edu
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Fig. 1 A schematic diagram depicting analogy between preadipocyte-adipocyte transdifferentiation and HSC activation. As required for
adipocyte differentiation, a group of transcription factors including C/EBPs, PPARg, LXRa, and SREBP-1 are essential for the maintenanceof adipogenic, quiescent phenotype of HSC. HSC was once termed ‘‘Fettspeicherungszellen: fat-storing cells’’ by Professor T. Ito upon hischaracterization of the cells in 1951 and 1956 (see references 8 and 9). This fat-storing phenotype of HSC needs to be re-visited.

PPARg per se is important for the demonstrated antiﬁbrotic effects. To this end, our laboratory used anadenoviral vector to ectopically express PPARg in
culture-activated HSC.2 This study demonstrated that
expression of PPARg itself is sufﬁcient to reverse themorphological and biochemical characteristics of activated HSC. In particular, PPARg-transduced cells areable to accumulate vitamin A in the presence of retinol
and palmitate and this effect is blocked by co-infection
with a vector expressing a dominant negative mutant ofPPARg. As a mechanistic insight into the observed reversal, PPARg is shown to decrease the binding ofJunD to the AP-1 site. This effect is not mediated byreduced expression of JunD or suppressed JNK activity.
But it is due to JunD-PPARg interaction as demonstrated by co-immunoprecipitation and glutathione Stransferase pull-down analysis. Further, the use of deletion constructs revealed that the DNA binding regionof PPARg is the JunD interaction domain.
Adipogenic Transcriptional Regulation in HSC
The above ﬁndings solidiﬁed the notion that PPARg
serves as an important therapeutic target for liver ﬁbrosis. They also supported a more fundamental notionthat differentiation of HSC is similar to that of adipocytes. This notion is based on several known common-alities. Firstly, both quiescent HSC and adipocytes store
lipids while activated HSC and pre-adipocytic ﬁbroblast
are devoid of lipid storage. Secondly, both quiescent
HSC and adipocytes express extracellular matrix proteins of the basement membrane while activated HSC
and preadipocytes mainly produce interstitial collagens.
Thirdly, soluble factors shown to activate HSC suchas PDGF, TGFb, TNFa, leptin, etc, are also known
mediators that inhibit adipocyte differentiation. Lastly,PPARg is now shown to serve as a key transcription
factor for HSC quiescence as it is considered as a master regulator for adipogenesis. If HSC are similar toadipocytes, HSC quiescence should be facilitated by agroup of adipogenic transcription factors besides
PPARg. These factors include CREB, C/EBPa, b andd, LXRa, and SREBP-1c. In fact, these factors andPPARg have positive-forward and cross-inductive regulation in order to orchestrate full transcriptional activation required for adipocyte differentiation. Thus, totest our notion of the similarity between HSC andadipocytes, we examined the expression of these putative adipogenic transcription factors in quiescent andculture-activated HSC. Indeed, quiescent and fullydifferentiated HSC express C/EBPa and b, an active
form of SREBP-1c, and LXRa and their levels rapidly
decline as HSC become activated and lose their lipid
content in culture.3 Coordinately, adipocyte-speciﬁc

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genes that are mostly down stream of PPARg andSREBP-1c are expressed at the higher levels in quiescent HSC including adipsin and resistin.
Gain of Adipogenic Transcriptional Factors Restores
HSC Quiescence
To test the causal relationship between HSC transdifferentiation (activation) and the loss of adipogenictranscription factors, we have used three different
approaches of gain of function: 1) the treatment ofactivated HSC with the adipocyte differentiation cocktail (insulin, dexamethasone, isobutylmethylxanthine);2) ectopic expression of PPARg as previously performed by an adenoviral vector; and 3) virally mediatedectopic expression of SREBP-1c. In summary, all threemanipulations reversed morphologic and biochemical
characteristics of culture-activated HSC to quiescent
HSC while restoring the expression of the adipogenictranscription factors.3 These results are in support ofthe central hypothesis that HSC quiescence is maintained by adipogenic transcriptional regulation as
depicted in Fig. 1.
Paradoxical Effects of Fat on Hepatocytes and HSC
Our ﬁndings to date support that the ‘‘fat-storing’’
phenotype is good for HSC quiescence and antiﬁbrogenesis. However, fat accumulation in hepatocytes
causes fatty liver as exempliﬁed by alcoholic liverdisease (ALD) and non-alcoholic fatty liver disease(NFLD). In fact, if we analyze the expression of adipogenic transcription factors described above in fatty
livers of ALD and NFLD animal models, they areupregulated (unpublished results). These results suggest that adipogenic transcriptional regulation may
participate in the genesis of fatty liver. Interestingly,
under these adipogenic conditions, HSC are still activated and collagen gene expression increased. This

Tsukamoto H: Fat paradox in liver diseaseleads to an intriguing question as to why HSC are resistant to adipogenic signals that make hepatocytes
steatotic in these models. An answer to this question
may provide novel insights into not only paradoxicalsigniﬁcance of fat in hepatocytes and HSC but also differential sensitivity of these two cell types to adipogenic
environments in pathologic conditions.
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