BRIEF REVIEWS
Sterol Regulatory Element-binding
Protein-1 as a Dominant Transcription
Factor for Gene Regulation of
Lipogenic Enzymes in the Liver
Hitoshi Shimano*

Sterol regulatory element-binding proteins (SREBPs) are basic helixloop-helix (bHLH) type transcription factors that control expression ofgenes involved in biosynthesis of cholesterol and fatty acids. Dietarystudies with normal, transgenic, and knockout mice have established
SREBP-1 as a dominant transcription factor regulating gene expression of lipogenic enzyme in the liver. Polyunsaturated fatty acids
inhibit hepatic lipogenic enzymes through suppressing SREBP-1.
Whereas SREBP-2 exerts sterol regulation through cleavage of themembrane-bound precursor protein to liberate the active nuclear form
into the nucleus, SREBP-1 controls lipogenic enzymes by self-regulating its own transcription level. Promoter analysis of the SREBP-1 gene
will be important to clarify the mechanism of nutritional regulation of
lipogenic genes. (Trends Cardiovasc Med 2000;10:275–278). © 2001,
Elsevier Science Inc.

Sterol regulatory element-binding proteins(SREBPs) are transcription factors thatbelong to the basic helix-loop-helix leucine
zipper (bHLH-Zip) family (Brown andGoldstein 1997 and 1999, Brown et al.
2000). In contrast to other members of thebHLH-Zip family, SREBPs are synthe-

Hitoshi Shimano is from the Department ofInternal Medicine, Institute of Clinical Medicine, Tsukuba, Ibaraki, Japan.
* Address correspondence to: Hitoshi Shimano, Department of Internal Medicine,
Institute of Clinical Medicine, 1-1-1 Tennodai, Tsukuba, Ibaraki, 305-8575 Japan. Fax:
181-298-53-3051; e-mail address: shimanotky@umin.ac.jp.
© 2001, Elsevier Science Inc. All rights
reserved. 1050-1738/01/$-see front matter

TCM Vol. 10, No. 7, 2000

sized as precursor proteins that remainbound to the endoplasmic reticulum andthe nuclear envelope in the presence ofsufficient sterol concentrations. Upon sterol deprivation, the precursor protein
undergoes a sequential two-step cleavage process to release the NH2 terminal
portion (Sakai et al. 1996). This mature
SREBP then enters the nucleus and activates the transcription of genes involvedin cholesterol and fatty acid synthesis bybinding to sterol regulatory elements(SREs) or to palindromic sequences called
E-boxes within their promoter regions(Kim et al. 1995, Wang et al. 1993). To
date, three SREBPs have been identified;
SREBP-1a and SREBP-1c (also known
as ADD1) are produced from a single genethrough the use of alternate promoters,

and SREBP-2 is transcribed from a separate gene (Hua et al. 1993, Tontonoz etal. 1993, Yokoyama et al. 1993). It has beenshown that all cultured cells analyzed todate mainly express SREBP-2 and the -1a
isoform of SREBP-1 whereas most organs, including the liver and adipose tissue, express predominantly SREBP-2
and the -1c isoform of SREBP-1 (Shimomura et al. 1997). Although SREBP1a and -1c share the same bHLH andregulatory domains, SREBP-1a is a stronger activator than SREBP-1c owing to alonger amino-terminal transactivation
domain and has a wider range of target
genes involved both in cholesterol andfatty acid synthesis (Shimano et al. 1996,1997). Cumulative lines of evidence, including studies with transgenic miceand diet studies with normal mice, established that SREBP-1c plays a role inregulating the transcription of genes involved in fatty acid synthesis whereas
SREBP-2 is involved in the transcription of cholesterogenic enzymes (Horton et al. 1998b).
• Studies from SREBP-1
Knockout Mice
Lipogenic enzymes which are involvedin energy storage through synthesis offatty acids and triglycerides are coordinately regulated at the transcriptional
level during different metabolic states(Goodridge 1987, Hillgartner et al. 1995).
Recent in vivo studies suggest that
SREBP-1c plays a crucial role in the dietary regulation of most hepatic lipogenic genes. These include studies of theeffects of the over-expression of SREBP1a and -1c on hepatic lipogenic gene expression in transgenic mice (Shimano etal. 1997, 1999), as well as physiologicalchanges of SREBP-1c protein in normal
mice after dietary manipulation such asplacement on fasting–refeeding regimens(Horton et al. 1998a). The similarly coordinated changes in SREBP-1c and lipogenic gene expression at fasting and275

refeeding were also observed in adipose
tissue (Kim et al. 1998).
To obtain a conclusive estimate of therole of SREBP-1 in dietary regulation ofhepatic lipogenic genes, we assessed theeffects of absence of SREBP-1a and -1c
proteins on the expression of these genes
in SREBP-1 knockout mice (Shimano
et al. 1999). Initially we used a fasting–refeeding treatment which is an established dietary manipulation for the induction of lipogenic enzymes. In the fasted
state, the mRNA levels of all lipogenic
enzymes were consistently low in bothwild-type and SREBP-1 null mice. However, absence of SREBP-1 severely impaired the marked induction of hepatic
mRNAs of fatty acid synthetic genes suchas acetyl-CoA carboxylase, fatty acid synthase (FAS), and stearoyl-CoA desaturase-1 (SCD-1) that was observed uponrefeeding in the wild-type mice. Furthermore, the refeeding responses of otherlipogenic enzymes (glycerol-3-phosphate
acyltransferase, ATP citrate lyase, malic
enzyme, glucose-6-phosphate dehydrogenase, and Spot14 (S14) mRNAs) werecompletely abolished in SREBP-1 knockout mice. In contrast, mRNA levels forcholesterol biosynthetic genes were paradoxically elevated in the refed SREBP1 knockout livers, accompanied by anincrease in nuclear SREBP-2 protein.
When fed a high carbohydrate diet for
14 days, the mRNA levels for these lipogenic enzymes were also strikingly lower
in SREBP-1 deficient mice than in wildtype mice (see Figure 1). These data basically confirmed that SREBP-1 plays acrucial role in the induction of lipogenesis, but not cholesterol biosynthesis, inliver when excess energy by carbohydrates is consumed. However, the extent
to which the gene regulation depends
upon SREBP-1 considerably differs among
the lipogenic genes and organs. For instance, glycolytic enzymes such as pyruvate kinase and glucokinase are sometimes classified as lipogenic enzymes, but
are not highly regulated by SREBP-1.
Gene expression of FAS and SCD-1 ishighly controlled by SREBP-1, but otherfactors such as USFs for FAS regulation
also seem to be involved. The cis-elements
which SREBPs bind and activate in the
promoters of lipogenic genes show aconsiderable diversity and do not completely match classic sterol regulatory
elements, but show some similarities to
SRE and/or E-boxes, and are currently

276

Figure 1. RNA levels of various genes involved in lipogenesis and cholesterogenesis as measured by Northern blot analysis of total RNA of livers from wild-type (WT) and SREBP-1 null(KO) mice fed a high carbohydrate diet (70% sucrose diet) for 2 weeks. A smaller SREBP-1
transcript from KO mice was derived from the disrupted SREBP-1 gene and produced an aberrant non-functional protein. Expression of lipogenic genes were highly induced by high carbohydrate diet. This induction was severely impaired in glycerol-3-phasphate acyltransferase(GPAT), glucose-6-phasphate dehydrogenase (G6PD), and S14 gene by SREBP-1 disruption.
Expression of acetyl-CoA carboxylase (ACC), fatty acid synthase (FAS), ATP citrate lyase (ACL)
stearoyl-CoA desaturase (SCD), 6-phosphogluconate dehydrogenase (6PG), and pyruvate kinase (PK) was less, but considerably impaired. In contrast, cholesterogenic genes including
SREBP-2, HMG CoA synthase, and reductase were roughly three-fold increased in SREBP-1
KO mice as compared to wild-type mice.

designated SRE-like sequences. A detailed promoter analysis is required forthese SRE-like sequences in lipogenic
gene promoters to clarify the differencesin response of each lipogenic gene to
SREBPs. Another interesting observation
was made in adipose tissue of refed
SREBP-1 knockout mice. The absenceof SREBP-1 did not severely affect overall gene expression of lipogenic enzymes

in fat as observed in the livers, suggesting that control of lipogenic gene expression might show some tissue specificity. The mechanism of the paradoxical
induction of cholesterogenic genes in
SREBP-1 knockout mice is not fully understood. It is presumably owing to a compensatory activation of SREBP-2 since cholesterol feeding partially suppressed thisinduction (data not shown). However,TCM Vol. 10, No. 7, 2000

overshooting of cholesterol synthetic genes
suggests that endpoint of this regulation
might not be cellular cholesterol content.
• Inhibition of Lipogenic Gene
Expression by Polyunsaturated
Fatty Acids (PUFA) throughSuppression of SREBP-1
Dietary manipulations are known to regulate gene expression of lipogenic genes.
Placement on a fat-free and high carbohydrate diet or refeeding with this dietafter fasting markedly induces lipogenic
gene expression in the livers. Meanwhile,
dietary polyunsaturated fatty acids (PUFA)
are negative regulators of hepatic lipogenesis that exert their effects primarily
at the level of transcription. Finally, studies
with the knockout mice established that
SREBP-1 plays a crucial role in the regulation of lipogenic gene expression in
the liver (Shimano et al. 1999). To explore
the possible involvement of SREBP-1 inthe suppression of hepatic lipogenesis by
PUFA, we challenged wild-type mice andtransgenic mice overexpressing a matureform of SREBP-1 in the liver with dietary PUFA (Yahagi et al. 1999). In theliver of wild-type mice, dietary PUFA
drastically decreased the mature, cleaved
form of SREBP-1 protein in the nucleus,
whereas the precursor, uncleaved formin the membranes was not suppressed.
The decreases in mature SREBP-1 paralleled those in mRNAs for lipogenic enzymes such as fatty acid synthase (FAS)
and acetyl-CoA carboxylase (ACC). In
the transgenic mice, dietary PUFA did not
reduce the amount of transgenic SREBP-1
protein, excluding the possibility that
PUFA accelerated the degradation of mature SREBP-1. The resulting sustained
expression of mature SREBP-1 almost
completely canceled the suppression of
lipogenic gene expression by PUFA in the
SREBP-1 transgenic mice. These results
demonstrate that the suppressive effectof PUFA on lipogenic enzyme genes in theliver is caused by a decrease in SREBP-1
expression. There are some other reports
describing similar results that diet containing fish oil decreased both hepatic
SREBP-1 mRNA and protein levels(Kim et al. 1999, Thewke et al. 1998, Worgall et al. 1998). In the light of suppression of lipogenesis, it does not matter
whether the fish oil belongs to n-3 or n-6
family of PUFA. In our studies, linoleate,
EPA and DHA were potent suppressors
TCM Vol. 10, No. 7, 2000

of SREBP-1, in this order of magnitude.
Fish oils, especially tuna oil enriched in
DHA, were generally stronger than eachpure PUFA. It is possible that some unknown component of fish oil might contribute to an additional effect of fish oil
on SREBP-1 expression.
Many investigators have analyzed thecis-acting elements in the promoter region
for carbohydrate stimulation and PUFA
suppression of lipogenic genes. In the caseof enzymes such as FAS, ACL, and pyruvate kinase (PK), glucose/insulin response
elements overlap with PUFA response regions. Especially the glucose/insulin and
PUFA response element in the FAS promoter has been shown to contain an
SREBP-binding site. The PUFA response
region in the mouse SCD1 promoter isalso reported to have an SREBP-bindingsite. Furthermore, PUFA suppressive element and SREBP binding site in S14 genepromoter were found to be identical.
These data are supportive of our findingthat carbohydrate stimulation and PUFA
suppression are mediated by a common
molecule, SREBP-1 (shown in Figure 2).

Because PUFA are known as activatorsof PPARs, the effects of PUFA throughPPARs were also examined. The findingthat fenofibrate did not affect the amountof mature SREBP-1 in the liver indicatesthat the suppressive effect of PUFA on
mature SREBP-1 is not mediated by
PPARa. This finding is compatible witha previous study using PPARa-null mice
which showed that PPAR was not required
for the PUFA-mediated inhibition ofeither FAS or S14 gene expression (Renet al. 1996). Future studies should focus
on the clarification of the mechanism bywhich PUFA modulates the cleavage of
SREBP-1 precursor protein in terms of theregulation of hepatic lipogenesis as wellas identification of the PUFA acceptor thatmediates PUFA suppression of SREBP-1c
promoter activity.
• SREBP-1c Promoter Analysis
and Future Prospects
Previous reports on the regulation of
SREBP-1c have all demonstrated the induction to be at the mRNA level (Foretz

Figure 2. Inhibition of lipogenic genes by PUFA (polyunsaturated fatty acids) through SREBP1c suppression PUFA suppresses both SREBP-1c gene expression and the activity to cleave
SREBP-1c precursor into its nuclear form to activate transcription of lipogenic genes. Consequently, SREBP binding sites (SRE-like sequences) correspond to PUFA responsible elements(PUFA-RE) in these gene promoters. SREBP-1c gene promoter contains SRE complex andoxysterol-inducible region. Neither of exact target sites for lipogenic signal to induce SREBP1c gene expression nor for PUFA to suppress SREBP-1c gene expression is currently identified.

277

et al. 1999, Hasty et al. 2000, Horton etal. 1998, Kim et al. 1999, Shimomura et al.
1999, Thewke et al. 1998, Worgall etal. 1998, Yahagi et al. 1999, ). In contrast
to SREBP-2, which mediates sterol regulation completely at the cleavage level
through interaction with SCAP and site-1
protease, SREBP-1c controls the transcriptional regulation of lipogenic enzymes
by self-regulating the nuclear concentration of its mature form, which is highly
correlated to its precursor and mRNA levels. This raises a possibility that the sterolregulated cleavage system might not be so
specific to SREBP-1c as to SREBP-2, andmakes the promoter analysis on SREBP1c gene important. Our initial analysisof the mouse SREBP-1c promoter identified two functional regions (AmemiyaKudo et al. 2000). One region is the SRE
complex that confers a response to
SREBPs. Upstream of this region, there
was an oxysterol-inducible region, details of which remains unknown (Figure 2).
Further studies are needed to identify
the regions of carbohydrate-responsive
elements and PUFA-suppressive elementsin the SREBP-1c promoter. Recently, we
have reported that SREBP-1c is regulated by glucose at the transcriptional
level in a well-differentiated hepatic cellline, H2-35 cells (Hasty et al. 2000). Preliminarily, we observed suppression of
SREBP-1c nuclear protein in these cellsby addition of EPA to the medium. This
cell line would help in the understandingof dietary regulation of lipogenic genes
through regulation of SREBP-1 expression in the liver.

• Acknowledgments
I greatly thank my cowokers: Naoya Yahagi, Michiyo Amemiya-Kudo, Alyssa H.
Hasty, and Tomohiro Yoshikawa for main
contributions to this work, Nobuhiro
Yamada, Shun Ishibashi, J. L. Goldsteinand M.S. Brown for continuous support.

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