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SREBPs are membrane bound transcription factors that play a central role in regulating lipid production in all metazoans studied. This work has revealed the intricate machinery responsible for regulating the release of SREBPs from the membrane in response to cellular need for lipids. This machinery includes two proteases, an escort factor and retention factors, and is localized to intracellular membranes (Brown and Goldstein, 1999;McPherson and Gauthier, 2004). Each of these components is necessary to ensure regulated release of SREBPs from the membrane and, thus, access to the nucleus. Nuclear access is not the end of the story, however.
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Other work has focused on additional proteins needed to form the final transcriptionally active complex, once SREBPs reach the nucleus. These cofactors include CBP (the cAMP response element binding protein [CREB] binding protein), a related protein, p300 (Oliner et al., 1996), Sp1, Sp3, (Athanikar et al., 1997), NFY (Ericsson et al., 1996), and the large, multicomponent activatorrecruited cofactor (ARC) complex (or the metazoan Mediator complex) (Naar et al., 1999).
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Delineating events at SREBP target promoters more fully, Yang et al. (2006) focused on a single subunit of the large ARC complex. They report that interaction between ARC105 and SREBPs is selective; they detected no interaction between ARC105 and other transcription factors, such as the cellular myeloblast transforming factor (c-Myb) or CREB. Similarly, SREBPs did not bind to other ARC subunits tested. Thus interaction between ARC105 and SREBP is not simply a general phenomenon of the transcriptional machinery.
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This selectivity is perhaps surprising. ARC105 was first identified as an essential component of the complex required for TGFb signaling via Smad2/3-Smad4 binding (Kato et al., 2002). In the present study, the authors show that the SREBPinteracting domain of ARC105 (a domain that does not bind c-Myb or CREB) is structurally similar to the KIX domain of CBP (a domain that does bind c-Myb and CREB). Changing just two residues in the third a helix of ARC105 to the cor-responding residues in CBP (Ile 64 /Tyr; Asp 68 /Lys) substantially improved the ability of ARC105 to interact with both c-Myb and CREB.
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Interaction between ARC105 and SREBPs is functionally significant; when the authors used an siRNA strategy to reduce the abundance of ARC105 transcript in cultured cells, the transcription of SREBP-responsive genes was greatly reduced while transcription of several other, non-SREBP-dependent genes remained unaffected. This indicated that the ARC complex could still function with other transcription factors even when levels of ARC105 were artificially low. This is consistent with the selectivity observed in the binding studies. Yang et al. (2006) then used chromatin immunoprecipitation assays to demonstrate joint occupancy of target promoters by SREBP and ARC105.
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Experiments conducted in the nematode, C. elegans, whose genome harbors orthologs of both SREBP (SBP-1) and ARC105 (MDT-15), confirmed the physiological relevance of the interaction between ARC105 and SREBP. Disruption of the expression of either gene by RNAi resulted in highly similar phenotypes, including growth defects, infertility, shortened lifespan, and reduced fat storage. A clue to the direct cause of the defects
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in RNAi-treated animals came from analysis of their fatty acids. The ratio of stearate (18:0) to oleate (18:1) increased in RNAi-treated worms compared to vectortreated worms, suggesting a deficit in fatty acid desaturation. The abundance of transcripts encoding stearoyl CoA desaturases (fat-6 and -7) was diminished in animals treated with RNAi against SBP-1 or MDT-15. When these transcripts were themselves targeted by RNAi, phenotypes similar to the SBP-1 and MDT-15 knockdowns were observed.
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The authors then tested the ability of dietary supplementation with fatty acids to ameliorate the phenotypes observed in RNAi-treated worms. In contrast to stearate, supplementation with oleate afforded substantial rescue. However, oleate feeding reversed the growth defect in MDT-15 and SBP-1 knockdown worms less strongly than it reversed the growth defect in the fat-6 and -7 knockdown animals. Therefore, most, but not all, of the defects associated with disruption of SBP-1 transcription result from diminished desaturase activity and the consequent deficit in the supply of oleate. This differs from the case in Drosophila where the desaturase genes do not appear to be targets of dSREBP and lethality of dSREBP null mutants can be substan-tially rescued by stearate or oleate (as well as other fatty acids; Kunte et al., 2006). Nevertheless, feeding worms oleate effectively ameliorates the similar phenotypes resulting from disruption of MDT-15, the SBP-1 transcription factor, or of two of its targets, fat-6 and -7.
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SREBP-mediated transcriptional activation of important target genes such as the low density lipoprotein receptor (LDLR), fatty acid synthase (FAS), and 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) synthase (in mammals) and the Stearoyl CoA desaturases fat-6 and -7 (in worms) cannot occur without the concomitant presence of ARC105. These results demonstrate the crucial, selective role of by extension,ARC105) in regulating fatty acid synthesis in animals. Without ARC105, normal lipid homeostasis is disrupted and the expected undesirable consequences follow.
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The complicated end-product feedback regulation of SREBP activation is well known. Like SREBPs, ARC105 is a crucial player in many aspects of the transcriptional regulation of lipid metabolism. The selective interaction described by Yang et al. (2006) raises questions. From the point of view of cellular and organismal metabolism, the most intriguing question is: What, if anything, does ARC105 contribute to metabolic regulation of lipid metabolism? Why does transcription by SREBP require a specific component of the ARC/Mediator complex rather than the components that serve c-Myb and CREB?
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It will be intriguing to learn if the activity of ARC105 is regulated by metabolic needs. Is the level of ARC105 present in the nucleus coordinated with levels of nuclear SREBPs and, if so, what mechanisms might be employed to achieve this? ARC105 does not appear to be a target of SREBPs, for example (Horton et al., 2003). Equally interesting will be seeing whether ARC105 is required for all SREBP-mediated transcription or for only a subset of target promoters. Does this putative subset require that different ARC-component isoforms interact with SREBPs? If so, what impact would this have on lipid metabolism? Yang et al. (2006) show that ARC105 interacts more strongly with SREBP-1a than with either SREBP-1c or SREBP-2. Future efforts may determine to what extent these differing affinities play a role in transcriptional regulation. ARC105 does not function in isolation, of course. It is part of the ARC complex, many components of which are found in modules that vary depending on the transcription factor with which the ARC complex interacts (Naar et al., 2001). Do other ARC subunits vary in concert with ARC105 or is it alone in determining whether the ARC complex interacts with SREBPs?
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A final implication of the work considered here is that small molecules that alter the interaction between SREBPs and ARC105 might be novel therapeutic agents for the treatment of various disorders of lipid metabolism such as hyperlipidemias or metabolic syndrome.
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1 1 Department of Molecular Genetics University of Texas Southwestern Medical Center 5323 Harry Hines Boulevard Dallas, Texas 75390 Selected reading Athanikar, J.N., Sanchez, H.B., and Osborne, T.F. (1997). Mol. Cell. Biol. 17, 5193-5200. Brown, M.S., and Goldstein, J.L. (1999). Proc. Natl. Acad. Sci. USA 96, 11041-11048. Ericsson, J., Jackson, S.M., and Edwards, P.A. (1996). J. Biol. Chem. 271, 24359-24364. Interaction between the ARC/Mediator complex (aqua) and SREBPs (yellow) occurs through binding of the KIX domain of ARC105 to an amino-terminal region of SREBP (indicated in red). SREBPs bind to sterol regulatory elements in the upstream region of target genes (SRE). The ARC complex also interacts (curved arrow) with the COO --terminal domain of RNA polymerase II (Pol II; pink). General transcription factors (GTFs; blue) and TFIID (green) are also indicated. Selective interaction between ARC105 and SREBPs occurs in nematodes and in human cells but its regulatory significance is unclear at present. This essential interaction could confer an additional level of control on the transcriptional regulation of lipid metabolism. The species indicated by a question mark (?) highlights the possibility that other, perhaps unknown, protein(s) may serve functions analogous to ARC105 in mediating interactions between the ARC complex and SREBPs in circumstances different from those described by Yang et al. (2006). Preventing interaction between ARC105 and SREBPs should disrupt lipid metabolism, including the synthesis of fatty acids and, in mammals, cholesterol. (Adapted from Yang et al. [2006]).