Progress in Lipid Research 40 (2001) 439–452
www.elsevier.com/locate/plipres

Review

Sterol regulatory element-binding proteins (SREBPs):
transcriptional regulators of lipid synthetic genes
Hitoshi Shimano*
Department of Internal Medicine, Institute of Clinical Medicine, University of Tsukuba, 1-1-1 Tennodai,
Tsukuba, Ibaraki 305-8575, Japan

Abstract
Roles of sterol regulatory element-binding proteins (SREBPs) have been established as lipid synthetictranscription factors especially for cholesterol and fatty acid synthesis. SREBPs have unique characteristics. Firstly, they are membrane-bound proteins and the N-terminal active portions enter nucleus toactivate their target genes after proteolytic cleavage, which requires sterol-sensing molecule, SREBP-activating protein (SCAP) and is crucial for sterol-regulation. Secondly, they bind and activate sterol-regulatory (SREs) containing promoters as well as some E-boxes, which makes SREBPs eligible to regulate awide range of lipid genes. Finally, three isoforms, SREBP-1a, –1c, and –2 have diﬀerent roles in lipid
synthesis. In vivo studies using transgenic and knockout mice suggest that SREBP-1 seems to be involvedin energy metabolism including fatty acid and glucose/insulin metabolism, whereas SREBP-2 is speciﬁc tocholesterol synthesis. Future studies will be focused on understanding molecular mechanisms sensing cellular sterol and energy states where SREBPs are deeply involved. # 2001 Elsevier Science Ltd. All rights
reserved.

Contents
1. Introduction ........................................................................................................................................................... 440
2. Molecular aspects of SREBPs ................................................................................................................................ 440
2.1. Structure of SREBPs (Fig. 1) ........................................................................................................................ 440
2.2. Cleavage of SREBPs for activation (Fig. 2) .................................................................................................. 441
2.3. SRE and E-box: DNA binding sites of SREBPs........................................................................................... 443
2.4. Target genes of SREBPs................................................................................................................................ 443
2.5. Cofactors for SREBPs................................................................................................................................... 443
2.6. Repression of genes through SRE ................................................................................................................. 444* Fax: +81-298-63-2170.
E-mail address: shimano-tky@umin.ac.jp (H. Shimano).
0163-7827/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved.
PII: S0163-7827(01)00010-8

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3. Isoforms of SREBP family..................................................................................................................................... 445
3.1. SREBP-1a,-1c, and -2 (Fig. 1) ....................................................................................................................... 445
3.2. Diﬀerences among SREBP isoforms in target DNA speciﬁcity .................................................................... 445
4. In vivo functions of SREBPs ................................................................................................................................. 445
4.1. SREBP transgenic mice ................................................................................................................................. 445
4.2. SREBP knockout mice [72] ........................................................................................................................... 446
4.3. Nutritional regulation of lipogenic enzyme genes by SREBP-1 .................................................................... 446
4.4. The eﬀects of insulin/glucose on SREBP-1 expression.................................................................................. 447
5. Promoter analysis of SREBP-1c (Fig. 3)................................................................................................................ 447
5.1. Presence of SRE and oxysterol-inducible region in SREBP-1c promoter ..................................................... 447
5.2. LXR/RXR activation of SREBP-1c: a new link between cholesterol and fatty acid regulation................... 448
6. ADD1/SREBP-1c in adipogenesis ......................................................................................................................... 448
7. Transcriptional regulation of lipid synthesis by SREBP-2 and -1c in the liver (Fig. 4) ......................................... 449
8. Future aspects of SREBPs; crosstalk of transcription factors for lipid metabolism .............................................. 450
Acknowledgements...................................................................................................................................................... 450
References ................................................................................................................................................................... 450

1. Introduction
Cholesterol and fatty acids are primary components of cellular membranes. Growing cells needto synthesize both cholesterol and fatty acids according to their demand for growth. In the differentiated tissues such as liver and endocrine organs, cholesterol biosynthetic pathway is linked
to synthesis of bile acids and steroid hormones, respectively. Synthesis for fatty acids and triglycerides, often referred to as lipogenesis, is an energy storage system specialized to lipogenicorgans such as liver and adipose tissues. In contrast to cholesterol synthesis, which is tightly
regulated by a feedback system to maintain cellular cholesterol levels, fatty acid synthesis is driven primarily by the availability of carbohydrates and the actions of hormones such as insulin(reviewed in [1, 2]). Both pathways are nutritionally controlled at the transcriptional level. Recent
evidence suggests that despite these diﬀerent patterns of regulation, both biosynthetic pathways
are controlled by a common family of transcription factors designated sterol regulatory elementbinding proteins (SREBPs) (reviewed in [3]). In this review, we focus on recent progress inunderstanding the molecular basis on physiological functions of SREBPs which have now been
established as global lipid synthetic regulators.

2. Molecular aspects of SREBPs
2.1. Structure of SREBPs (Fig. 1)
SREBPs were puriﬁed as nuclear factors that bind to the sterol regulatory element (SRE)common to LDL receptor and HMG CoA synthase genes [4,5]. Cloning and sequence of SREBP
genes revealed their unique aspect as membrane-bound transcription factors (6). SREBPs areVersion 7, No. pages 14

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structurally composed of four domains with two membrane-spanning regions. Both amino- andcarboxyl-terminal portions of the proteins project into the cytoplasm. The N-terminal domain of
approximately 480 amino acids is a basic-helix-loop-helix leucine zipper (bHLHZip) protein anda functionally active portion as a transcription factor. This segment begins with an acidic transactivation domain that clusters acidic residues. When this segment is deleted in transfection
studies, the proteins retain the ability to bind to the relevant DNA sequence, but they lose theirability to activate transcription [7]. The acidic NH2-terminal sequence is followed by a glycine,
serine, proline, and glutamine rich region whose function is unknown. These sequences are followed by the bHLH-Zip domain. SREBPs are believed to form a homodimer to bind to an SRE.
What makes SREBPs a unique membrane-bound transcription factor is the subsequent sequence
consisting of a hydrophobic membrane-spanning sequence, a hydrophilic loop region, a second
membrane-spanning sequence, and a long carboxyl-terminal region. SREBP forms a complex
with WD domain of SREBP cleavage activating protein (SCAP) through this carboxyl-terminal
region (Fig. 2). Therefore, this region of SREBP should be involved in sterol-regulation, and thus
is named a carboxyl regulatory domain [8,9].
2.2. Cleavage of SREBPs for activation (Fig. 2)
Unlike other members of this class, SREBPs are produced as membrane-bound precursors that
require cleavage by a two-step proteolytic process in order to release their amino-terminal bHLHZip-containing domain (nuclear SREBP) into the nucleus to activate their target genes [10,11].
Molecular mechanisms for cleavage of SREBPs in sterol regulation have been elegantly andextensively analyzed by Goldstein and Brown [3,12–14]. Upon sterol deprivation, the membranebound SREBP precursor protein undergoes a sequential two-step cleavage process to release theamino-terminal portion SREBP [10,11]. This nuclear SREBP, then enters the nucleus and activates the transcription of genes involved in cholesterol and fatty acid synthesis by binding tosterol regulatory elements (SREs) or to palindromic sequences called E-boxes within their promoter regions. SREBP precursor and SCAP form a complex on the rough endoplasmic reticulum(ER) membrane [9]. SCAP is a multiple membrane-spanning protein containing a consensus
sequence for a sterol-sensing domain which is shared by HMG CoA reductase, Niemann-Pick C1
disease gene product (NPC1), and Patched [8,15]. Therefore, SCAP has been suggested to function as a cholesterol sensor. SCAP is prerequisite for cleavage of SREBPs [16,17]. When sterol isdepleted, the SREBP-SCAP complex targets to golgi where site-1 and site-2 proteases await. Site-1

Fig. 1. Domain structure of SREBP family.

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protease, activated by auto-catalysis, cleaves SREBP at the loop region, followed by the second
cleavage in the ﬁrst membrane spanning region by site-2 protease to liberate the N-terminal of
SREBP [16,18–21]. Importin b is involved in this nuclear transport of SREBP [22]. The SCAP
presumably goes back to rough ER to meet another SREBP molecule. When sterol is abundant,
SREBP-SCAP complex is retained at the rough ER and no cleavage occurs. Therefore, cellular
sterol regulation entirely depends upon the sterol-sensing and cleavage system. The molecularmechanism of this sterol-sensing system is currently a most intriguing enigma. Some oxysterols
such as 25-OH cholesterol eﬃciently inhibit this sterol-regulated, inter-organella traﬃcking of the
SREBP-SCAP complex. It is a tempting speculation that an unknown sterol sensor protein which
could be an acceptor of oxysterol might bind to the sterol sensing domain of SCAP to make thecomplex retain to rough ER.
In contrast to this sterol regulation, sterol-independent cleavage of SREBP has been also suggested. Caspase-3 (CPP32) can cleave SREBP to liberate nuclear forms in apoptosis [23].
TNF-alpha involving neutral shpingomyelinase and ceramide, is also known to activate
SREBP, presumably through the same cleavage process [24,25]. Physiological relevance of thissterol-independent cleavage is currently unknown.
In another report, unsaturated fatty acids mediate regulate cleavage of SREBP-1, but not
SREBP-2 [26]. These data might open up a new regulation of fatty acid metabolism mediated by
SREBP-1 at the cleavage level in contrast to sterol regulation by SREBP-2. The key to the understanding of this distinct regulation by SREBP-1 and-2 is the lipid sensing mechanism of SCAP.

Fig. 2. SREBP-SCAP traﬃcking and sterol regulation.

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2.3. SRE and E-box: DNA binding sites of SREBPs
After entry into nucleus, nuclear SREBP binds to the speciﬁc DNA sequence in the promotersof its target genes. The cis-elements are named as sterol regulatory elements (SREs) because they
were identiﬁed as regulatory elements common to HMG CoA synthase and reductase and LDL
receptor gene promoters. The original SRE sequence was ATCACCCCAC [4]. In the initial
mutational analysis, SRE was thought to be tightly strict to the original sequence with a permissive diversity of only a couple of bases [4].
Another unique feature of SREBPs is that they can bind to and activate classic palindromic Eboxes (CAXXTG) containing promoters as well as nonpalindromic sterol regulatory elements(SREs) [27]. In the screening of SREBP binding sites from random DNA sequences, ADD1, a rat
homologue picked up ATCACGTGAT most preferably [27]. The tyrosine residue in the basic
region of SREBPs where regular bHLH proteins have an ariginine residue, is a key to this dual
binding speciﬁcity of SREBPs [27]. Mutant SREBPs where ariginine was substituted for the tyrosine lost a binding speciﬁcity to the SRE, but retains to the E boxes [27]. Presumably, this dual
binding speciﬁcity makes SREBPs eligible to have a broad spectrum of target genes covering bothcholesterogenic and lipogenic genes. The 2.3 A resolution co-crystal structure of the DNA-binding
portion of SREBP-1a bound to an SRE revealed a quasi-symmetric homodimer with an asymmetric DNA-protein interface [28]. Our recent data on binding speciﬁcity of SREBP to SRE andE-box indicate that SREBPs have higher aﬃnity and lower capacity for SRE than E-box [110].
2.4. Target genes of SREBPs
The current list of SREBP target genes is shown in Table 1. Basically, they are all involved inlipid synthesis and roughly categorized as cholesterol synthetic genes and lipogenic enzyme genes.
Genes involved in cholesterol synthesis contain SRE(s) which are identical or relatively similar tothe original SRE sequence found in the LDL receptor gene. Meanwhile, sequences of SREBP
binding and activation sites in lipogenic genes vary considerably and are tentatively designatedSRE-like sequences [29].
2.5. Cofactors for SREBPs
In the neighboring (usually 15 bases around the SRE) sequence of SREBP target gene promoters, NF-Y site or Sp1 site is usually found [5,30–59]. NF-Y and Sp1 directly bind to SREBP
and seem involved in recruitment of basic transcription machinery including CBP (58). Transactivation domains of SREBP-1a and -2 binds to CBP, but SREBP-1c does not (58).
In some SREBP target genes, SREBP activation of gene expression can be negatively regulatedby Yin Yang-1 zinc ﬁnger transcription factor (YY1). YY1 can repress SREBP activation of
HMG CoA synthase, FPP synthase, and LDL receptor genes by displacing NF-Y, which isessential for SREBP activation of these genes [49,60]. Another mechanism is also proposed in
YY1 repression of HDL receptor (SRBI) and steroidogenic acute regulatory protein (StAR)
genes: YY1 binds to SREBP-1a with a high aﬃnity and interferes SREBP binding to the SREs inthese genes [61,62]. These transfection studies suggest involvement of YY1 in SREBP gene regulation, but physiological relevance of YY1 in this context needs to be tested in vivo.

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2.6. Repression of genes through SRE
SREBPs are basically thought to activate their target genes. However, there are some reportsthat SREBPs play a role in repression of SRE-containing genes such as microsomal transfer
protein (MTP) [63] and caveolin [39]. MTP and caveolin are involved in regulating cellularcholesterol content by assembly and secretion of lipoproteins, and cholesterol eﬄux, respectively.
Therefore, repression of these genes by SREBPs could be a part of sterol regulation. Binding to
SREBP site and displacing some other positive regulators speciﬁc to these genes can be a possible
mechanism of their SREBP repression.

Table 1
Genes activated by SREBPsa
Gene
Cholesterol metabolism
LDL receptor
HMG CoA synthase
HMG CoA reductase
Farnesyl diphosphate synthase
Squalene synthase
SREBP-2Lanosterol 14a-demethylase (Cyp51)
Fatty acid metabolism
Acetyl CoA carboxylase
Fatty acid synthase
Stearoyl CoA desaturase-1 and 2
AcylCoA binding protein (Diazepam)
SREBP1
ATP citrate lyase
Malic enzyme
PPAR gamma
Acetyl CoA synthease
Triglyceride synthesis
Glycerol-3-phosphate acyltransferase

Plasma lipoprotein metabolism
Lipoprotein lipase
HDL receptor (SRBI)SRE promoter sequence

Reference

ATCACCCCAC
CTCACCCCAC
GCCACCCTAC
ACCGCACCATCTCTCACCAC
CTCACACGAG
ATCACGCCAG
CTAGTGTGAG
ATCACCCCAC
ATCACCTCAG

[101]
[102]

CCAT - - - - TCAC
GCCACGCCAC
GTCAGCCCATAGCAGATTGTG
CTCGCCCGAG
CTCACCCGAG
TCAGGCTAG
TCACCCGTCGGTG
ATCACTTGAGATTCATGTGACATATCACTCCAC

[103]
[34]
[104]
[35]
[105]

[38]
[106]
[50,107][107][55][54,56]
[110]
[98]
[111]

CTCAGCCTAG
CTCACCCCAG
GACACCCCAG[37]

CTCCCCCCAA
GCCACCTGCA

[108]
[52]

a
Other genes whose promoters are involved in SREBP binding includes StARP [62], CETP [109], and neutral cholesterol ester hydrolase, [44]. In contrast, MTP [63], caveolin [39] are reported to be repressed by SREBPs.

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3. Isoforms of SREBP family
3.1. SREBP-1a,-1c, and -2 (Fig. 1)
To date, three SREBPs have been identiﬁed; SREBP-1a and SREBP-1c produced from a single
gene through the use of alternate promoters, and SREBP-2 from a separate gene [6,64–66].
SREBP-1a and -1c are identical except the NH2-terminal transactivation domains. The rat
homologue of SREBP-1c, named ADD1, was cloned independently as a protein which binds toE-boxes, and presumably promotes adipocyte diﬀerentiation [67]. All actively growing cultured
cells so far studied produce predominantly SREBP-1a and SREBP-2, whereas most organs
including liver from adult animals predominantly synthesize SREBP-1c and SREBP-2 [68]. Consistent with high similarity between N-terminal portions of the isoforms, all three SREBPs arecapable of activating each of the known target genes, although with diﬀering eﬃciencies (discussed later). SRBP-1c is weaker than SREBP-1a and SREBP-2 due to its shorter transactivation
domain [69,70]. The caboxyl-terminal regulatory domains of SREBP-1 and -2 are relatively lessconserved as compared to other portions of SREBPs, suggesting some diﬀerence between
SREBP-1 and -2 in a sterol regulation manner.
3.2. Diﬀerences among SREBP isoforms in target DNA speciﬁcity
As discussed later, various lines of evidence from in vivo studies suggested that SREBP-1 ismore active for lipogenic genes whereas SREBP-2 is more speciﬁc to cholesterogenic genes [69–74]. Generally, cholesterogenic genes contain classic SREs in their promoters while SREBP
binding sites in the promoters of lipogenic genes vary including E-boxes. Consistently, our recent
transfection studies indicated that SREBP-1a, -1c and -2 have a diﬀerent speciﬁcity for activationof SRE-containing promoters and E-box containing promoters [110]. In summary, SREBP-1aand -2 are equally active for SRE-reporter gene (SRE-luciferase) whereas SREBP-1c is essentiallyinactive. In contrast, SREBP-1a and -1c are active for E-box while SREBP-2 is inactive. All
SREBP isoforms can bind and activate SRE-like sequences in lipogenic gene promoters, althoughwith diﬀerent eﬃciencies. These diﬀerent activities of SREBP isoforms for target DNAs canpartially explain their diﬀerent in vivo functions.

4. In vivo functions of SREBPs
4.1. SREBP transgenic mice
To gain insight into the distinct roles of each SREBP isoform in vivo, transgenic mice thatoverexpress truncated, active nuclear forms of human SREBP-1a, -1c, or-2 in the liver,were produced and characterized [69,74]. The diﬀerent SREBP-overexpressing transgenic
animals showed diﬀerent patterns of increase in hepatic synthesis and accumulation ofcholesterol and/or fatty acids. These data suggest that the SREBP-1c is more selective in activating fatty acid biosynthetic genes while SREBP-2 is more speciﬁc for controlling cholesterol biosynthesis. Overexpression of nuclear SREBP-1a profoundly activated both cholesterol

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and fatty acid synthetic genes, resulting in the most striking phenotype of a huge fatty liverwith the accumulation of large amounts of triglycerides and cholesteryl esters [71]. Verysimilar phenotypic changes in the liver were observed in transgenic mice overexpressing dominantpositive type of mutated SCAP [75]. Induction of a whole battery of lipogenic genes in thistransgenic mouse liver was the ﬁrst observation that SREBP-1 could be involved in lipogenic
genes.
4.2. SREBP knockout mice [72]SREBP1 knockout mice were partially embryonic lethal (70–90% of expected number ofhomozygotes). They die at day 10.5–11.5 (p.c.). The cause of their lethality remains unknown.
Livers in surviving homozygote at ad labium showed a signiﬁcant decrease in fatty acid synthesis
with a trend in decreases in mRNA levels of fatty acid synthetic genes. Unexpectedly, SREBP-1
deﬁcient mice showed increased mRNA levels in cholesterol synthetic enzyme genes, accompanying a signiﬁcant increase in cholesterol synthesis. This was due to activation of SREBP-2 tocompensate for the absence of SREBP-1 for an unknown endpoint at the expense of overshooting
cholesterol synthesis. SREBP2 knockout mice were completely lethal at the day earlier than 8(p.c.). SREBP-1 could not compensate for absence of SREBP-2, presumably due to its diﬀerent
way of regulation.
4.3. Nutritional regulation of lipogenic enzyme genes by SREBP-1
The ﬁrst evidence of diﬀerent regulation of SREBP-1 and -2 emerged from in vivo study with
HMG CoA reductase inhibitor (lovastatin) and bile acid lesin (Colestipol) [76]. When hamsters
were put on a diet containing these anti-hypercholesterolemic drugs which deplete cholesterol in
the liver, the nuclear SREBP-2 protein was upregulated to restore hepatic cholesterol content
as sterol regulation. In contrast, mRNA, precursor, and nuclear protein levels of SREBP-1
were decreased, suggesting that in vivo regulation of SREBP-1 was not in a sterol-regulatory
manner.
Lipogenic enzymes, which are involved in energy storage through synthesis of fatty acids andtriglycerides, are coordinately regulated at the transcriptional level during diﬀerent metabolicstates [1,2]. Recent in vivo studies demonstrated that SREBP-1c plays a crucial role in the dietary
regulation of most hepatic lipogenic genes; these include studies of the eﬀects of over-expressionof SREBP-1 on hepatic lipogenic gene expression as described [69,72] and physiological changesof SREBP-1c protein in normal mice after dietary manipulation,such as placement on high carbohydrate diets, polyunsaturated fatty acid-enriched diets, and fasting-refeeding regimens [77–81]
The similar coordinated changes in SREBP-1c and lipogenic gene expression upon fasting andrefeeding were also observed in adipose tissue [82]. Finally, we established a role of SREBP-1 innutritional regulation of lipogenic genes by showing a severely impaired nutritional induction of
lipogenic gene expression in SREBP-1-null mice [73]. Previous reports on the regulation of
SREBP-1c have all demonstrated the induction to be at the mRNA level. Up-regulation ofhepatic SREBP-1 mRNA was observed in rodent livers on a fasting-refeeding regimen, a chronichigh carbohydrate diet, and in primary hepatocytes with a high glucose medium [80,81,83,84]. In
contrast, down-regulation of SREBP-1c was observed in livers from fasted rodents, insulin-

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depleted diabetic rats with streptzotocin treatment, and from mice on a diet containing polyunsaturated fatty acids [77–81,85].
4.4. The eﬀects of insulin/glucose on SREBP-1 expression
It is known that both glucose and insulin are required for the production of fatty acids via the
induction of hepatic lipogenic enzymes. Several lines of evidence suggest that the roles of glucose
and insulin in this action are mediated by induction of SREBP-1c. In support of a role for
SREBP-1 in the induction of lipogenic genes by insulin, diﬀerent groups have shown that
SREBP-1c is up-regulated by insulin in vivo and in primary hepatocyte cultures [85–87]. Theseobservations raised a possibility that SREBP-1c could be a metabolic mediator of insulin eﬀect in
the liver [88]. It is interesting to explore the relationship insulin-receptor signaling and SREBP-1c
induction. PI3 kinase inhibitors decrease nuclear and precursor SREBP-1c proteins [84]. In contrast, we and other groups showed that glucose can induce SREBP-1c expression [83,84]. It has
been shown that glucose must be metabolized for this eﬀect on lipogenic gene induction and that
insulin could only be permissive to this glucose action [89,90]. Further studies are required toclarify the mechanism by which insulin/glucose induces SREBP-1c expression.

5. Promoter analysis of SREBP-1c (Fig. 3)
5.1. Presence of SRE and oxysterol-inducible region in SREBP-1c promoter
Transcriptional regulation of lipogenic enzymes is controlled by the amount of SREBP-1c
mRNA. This notion prompted us to analyze the promoter of SREBP-1c to understand the regulation of SREBP-1c itself, and thus, that of lipogenic enzymes [55]. A cluster of putative bindingsites of several transcription factors composed of NF-Y site, E-box, sterol-regulatory element,
and Sp1 site were located at 90 bp of the SREBP-1c promoter. Luciferase reporter gene and gel
shift assays indicated that the NF-Y site and the SRE in the SRE complex are responsible for
SREBP activation. This implicates an autoloop production of SREBP-1c through the SRE complex,

Fig. 3. Activation of SREBP-1c promoter.

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possibly leading to an overshoot phenomenon in induction of SREBP-1c and its downstream
genes in the livers of refed mice [81]. We also found another region that is upregulated byoxysterols upstream of the SRE complex. This region was identiﬁed as LXR/RXR bindingsite later (next section). The presence of the SRE complex and a sterol-inducible region in
the same promoter suggests a novel regulatory link between cholesterol and fatty acid synthesis(Fig. 3).
5.2. LXR/RXR activation of SREBP-1c: a new link between cholesterol and fatty acid regulation
We took an expression cloning strategy to seek a factor that activates SREBP-1c promoter [91].
We cloned and identiﬁed LXR a and b as strong activators of SREBP-1c promoter [91]. Deletionand mutation analysis on the SREBP-1c promoter identify two sets of new LXR binding sites(LXR elements) which are very similar to an LXRE which has been recently identiﬁed in ATPbinding cassette (ABC1) gene promoter [92–94]. Either LXR, RXR, or their respective ligands
can synergistically activate SREBP-1c promoter. Recent reports on the eﬀects of pharmacological
LXR ligands and mice deﬁcient in LXR a, b or both also indicated that LXR/RXR activated
SREBP-1c expression [95,96]. Both group observed that this activation of SREBP-1c by LXR/RXR is accompanied by a substantial increase in nuclear SREBP-1c, leading to activation of its
target lipogenic genes [95]. LXR has been recently established as an oxysterol receptor and plays
a key role in dealing with an excess amount of free cholesterol in the cells by enhancing cholesterol eﬄux or bile acid synthesis. Since SREBP-1c can activate fatty acid synthesis, activation of
SREBP-1c by the oxysterol receptor could be another cellular adaptation for excess cholesterol
by enhancing formation of cholesterol esters. This might suggests a new link between cholesteroland fatty acid metabolism. As shown in Fig. 3, SREBP-1c promoter can be activated regardlessof sterols, ensuring lipogenesis or cholesterol ester formation.

6. ADD1/SREBP-1c in adipogenesis
SREBP-1c was also cloned as a transcription factor for genes involved in adipogenesis, anddesignated adipocyte determination and diﬀerentiation factor 1 (ADD1) [67]. ADD1/SREBP-1c
has been suggested to be involved in adipogenesis by activating PPAR gamma, a master gene for
adipogenesis [97]. It was reported that ADD1 activates promoters of PPAR gamma 1 and 3, anddirectly induces transcription of PPAR gamma [98]. More interestingly, ADD1 induces production or secretion of some lipid molecule which can be a ligand for PPAR gamma [99]. This lipid
molecule could be an endogenous ligand for PPAR gamma and a key for adipogenesis. However,
the role of ADD1/SREBP-1c in adipogenesis and lipogenesis in adipose tissue seems to be very
complex and not straightforward. SREBP-1 knockout mice have slightly less fat, but adipogenesis and lipogenesis were grossly abnormal, although it could be explained by a compensatory
activation of SREBP-2 [72,73]. Overexpression of nuclear SREBP-1c in a fat-speciﬁc mannerusing aP2 promoter in transgenic mice impaired adipose tissue diﬀerentiation, causing decreased
amount of fat and severe insulin resistance and hyperinsulinemia [100]. These apparently opposing observations lead us to speculate that timing and amount of ADD1/SREBP-1c expression
might be important for adipogenesis.

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7. Transcriptional regulation of lipid synthesis by SREBP-2 and -1c in the liver (Fig. 4)
Fig. 4 depicts a current diagram for diﬀerent functions of SREBP-2 and SREBP-1c in the regulation of lipid synthesis in the liver. Both SREBP-2 and -1c are subjected to the cleavage-system
by SCAP, S1P, and S2P. If SCAP is disrupted, no cleavage of SREBP-1 or -2 occurs in the cultured
cells [17], suggesting that there is no other cleavage system at least in the cultured system. Then, inthe presence of suﬃcient amount of cholesterol, is lipogenesis shut down because cleavage of
SREBP-1 is cancelled? Our recent ﬁndings using a well-diﬀerentiated liver cell line showed thatcleavage of SREBP-1 was less sensitive to sterol-suppression than SREBP-2, suggesting that
SREBP-1 could have a chance to liberate its active form into nucleus even in the presence ofsterols to activate lipogenic genes by increasing its own amount [84]. In another report, unsaturated fatty acids mediate and regulate cleavage of SREBP-1, but not SREBP-2 [26]. These data
might open up a new regulation of fatty acid metabolism mediated by SREBP-1 in contrast to
sterol regulation by SREBP-2. The key to an understanding of this distinct regulation by SREBP1 and-2 is the lipid sensing mechanism of SCAP. Through these complex ways of SREBP regulation at diﬀerent levels, SREBP-2 controls cholesterol synthesis at the cleavage system, whereas
SREBP-1c regulates lipogenesis mainly by changing SREBP-1c mRNA level.

Fig. 4. Diﬀerent regulations of lipid synthetic genes by SREBP-2 and -1c in the liver.

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8. Future aspects of SREBPs; crosstalk of transcription factors for lipid metabolism
The list of SREBP target genes is expanding. More detailed analysis on unknown target genes
using DNA microarray or DNA chip technology will help our understanding of SREBP functions.
It is necessary to elucidate a sterol-sensing system and identify signaling molecules for cellularcholesterol amount. The SCAP–SREBP-2 complex should play a key role in this mechanism.
Identiﬁcation of energy sensing molecules is another interesting topic. SREBP-1 should be
involved in this mechanism and play a role as a mediator for energy storage.
Recent progress in the studies on transcriptional regulation of cholesterol and fatty acid metabolism unveiled several key players; SREBP-2 for cholesterol synthesis, LXR/RXR for cholesterol catabolism, SREBP-1 for fatty acid synthesis, and PPARa for fatty acid degradation. These
factors might interact with each other to exert their own functions more eﬃciently. For instance,
nutritional regulation of SREBP-1c and PPARa are reciprocal and the eﬀects of polyunsaturated
fatty acids on these are opposite. LXR/RXR activates SREBP-1c, suggesting oxysterol involvement in fatty acid metabolism. Cholesterol and energy metabolism may be regulated in a complex
manner by mutual interaction of these transcription factors. Future studies on SREBPs should befocused on the crosstalk of these lipid transcription factors.

Acknowledgements
I greatly thank my co-workers: Naoya Yahagi, Michiyo Amemiya-Kudo, Alyssa H. Hasty, andTomohiro Yoshikawa for the main contribution to our work presented in this review, andNobuhiro Yamada, Shun Ishibashi, J.L. Goldstein and M.S. Brown for continuous support.
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