A critical role for cAMP response element-binding protein (CREB) as a Co-activator in sterol-regulated transcription of 3-hydroxy-3-methylglutaryl coenzyme A synthase promoter.
3-Hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) synthase, a key regulatory enzyme in the pathway for endogenous cholesterol synthesis, is a target for negative feedback regulation by cholesterol. When cellular sterol levels are low, the sterol regulatory element-binding proteins (SREBPs) are released from the endoplasmic reticulum membrane, allowing them to translocate to the nucleus and activate SREBP target genes. However, in all SREBP-regulated promoters studied to date, additional co-regulatory transcription factors are required for sterol-regulated activation of transcription. We have previously shown that, in addition to SREBPs, NF-Y/CBF is required for sterol-regulated transcription of HMG-CoA synthase. This heterotrimeric transcription factor has recently been shown to function as a co-regulator in several other SREBP-regulated promoters, as well. In addition to cis-acting sites for both SREBP and NF-Y/CBF, the sterol regulatory region of the synthase promoter also contains a consensus cAMP response element (CRE), an element that binds members of the CREB/ATF family of transcription factors. Here, we show that this consensus CRE is essential for sterol-regulated transcription of the synthase promoter. Using in vitro binding assays, we also demonstrate that CREB binds to this CRE, and mutations within the CRE that result in a loss of CREB binding also result in a loss of sterol-regulated transcription. We further show that efficient activation of the synthase promoter in Drosophila SL2 cells requires the simultaneous expression of all three factors: SREBPs, NF-Y/CBF, and CREB. To date this is the first promoter shown to require CREB for efficient sterol-regulated transcription, and to require two different co-regulatory factors in addition to SREBPs for maximal activation. (+info)
A novel splicing isoform of mouse sterol regulatory element-binding protein-1 (SREBP-1).
We cloned a cDNA encoding the NH2-terminal portion of mouse SREBP-1. The deduced amino acid sequence was 76% and 90% identical to human and hamster SREBP-1, respectively. We found out a novel splicing isoform of mouse SREBP-1 that lacks 42 amino acid residues composing a PEST sequence observed in unstable proteins. It has been reported that SREBP-1 is rapidly turned over in the nucleus. Although this isoform was not a dominant isoform, it might be possible that the produced protein functions differently from other isoforms including a complete PEST sequence. (+info)
Disruption of LDL receptor gene in transgenic SREBP-1a mice unmasks hyperlipidemia resulting from production of lipid-rich VLDL.
Transgenic mice that overexpress the nuclear form of sterol regulatory element binding protein-1a (SREBP-1a) in liver (TgBP-1a mice) were shown previously to overproduce cholesterol and fatty acids and to accumulate massive amounts of cholesterol and triglycerides in hepatocytes. Despite the hepatic overproduction of lipids, the plasma levels of cholesterol ( approximately 45 mg/dl) and triglycerides ( approximately 55 mg/dl) were not elevated, perhaps owing to degradation of lipid-enriched particles by low-density lipoprotein (LDL) receptors. To test this hypothesis, in the current studies we bred TgBP-1a mice with LDL receptor knockout mice. As reported previously, LDLR-/- mice manifested a moderate elevation in plasma cholesterol ( approximately 215 mg/dl) and triglycerides ( approximately 155 mg/dl). In contrast, the doubly mutant TgBP-1a;LDLR-/- mice exhibited marked increases in plasma cholesterol ( approximately 1,050 mg/dl) and triglycerides ( approximately 900 mg/dl). These lipids were contained predominantly within large very-low-density lipoprotein (VLDL) particles that were relatively enriched in cholesterol and apolipoprotein E. Freshly isolated hepatocytes from TgBP-1a and TgBP-1a;LDLR-/- mice overproduced cholesterol and fatty acids and secreted increased amounts of these lipids into the medium. Electron micrographs of livers from TgBP-1a mice showed large amounts of enlarged lipoproteins within the secretory pathway. We conclude that the TgBP-1a mice produce large lipid-rich lipoproteins, but these particles do not accumulate in plasma because they are degraded through the action of LDL receptors. (+info)
ADD1/SREBP-1c is required in the activation of hepatic lipogenic gene expression by glucose.
The transcription of genes encoding proteins involved in the hepatic synthesis of lipids from glucose is strongly stimulated by carbohydrate feeding. It is now well established that in the liver, glucose is the main activator of the expression of this group of genes, with insulin having only a permissive role. While ADD1/SREBP-1 has been implicated in lipogenic gene expression through temporal association with food intake and ectopic gain-of-function experiments, no genetic evidence for a requirement for this factor in glucose-mediated gene expression has been established. We show here that the transcription of ADD1/SREBP-1c in primary cultures of hepatocytes is controlled positively by insulin and negatively by glucagon and cyclic AMP, establishing a link between this transcription factor and carbohydrate availability. Using adenovirus-mediated transfection of a powerful dominant negative form of ADD1/SREBP-1c in rat hepatocytes, we demonstrate that this factor is absolutely necessary for the stimulation by glucose of L-pyruvate kinase, fatty acid synthase, S14, and acetyl coenzyme A carboxylase gene expression. These results demonstrate that ADD1/SREBP-1c plays a crucial role in mediating the expression of lipogenic genes induced by glucose and insulin. (+info)
Co-stimulation of promoter for low density lipoprotein receptor gene by sterol regulatory element-binding protein and Sp1 is specifically disrupted by the yin yang 1 protein.
Sterol regulation of gene expression in mammalian cells is mediated by an interaction between the cholesterol-sensitive sterol regulatory element-binding proteins (SREBPs) and promoter-specific but generic co-regulatory transcription factors such as Sp1 and NF-Y/CBF. Thus, sterol-regulated promoters that require different co-regulatory factors could be regulated independently through targeting the specific interaction between the SREBPs and the individual co-regulatory proteins. In the present studies we demonstrate that transiently expressed yin yang 1 protein (YY1) inhibits the SREBP-mediated activation of the low density lipoprotein (LDL) receptor in a sensitive and dose-dependent manner. The inhibition is independent of YY1 binding directly to the LDL receptor promoter, and we show that the same region of YY1 that interacts in solution with Sp1 also interacts with SREBP. Furthermore, other SREBP-regulated genes that are not co-regulated by Sp1 are either not affected at all or are not as sensitive to the repression. Thus, the specific interaction that occurs between SREBPs and Sp1 to stimulate the LDL receptor promoter is a specific target for inhibition by the YY1 protein, and we provide evidence that the mechanism can be at least partially explained by the ability of YY1 to inhibit the interaction between SREBP and Sp1 in solution in vitro. The LDL receptor is the key gene of cholesterol uptake, and the rate-controlling genes of cholesterol synthesis are stimulated by the concerted action of SREBPs along with coregulators that are distinct from Sp1. Therefore, repression of gene expression through specifically targeting the interaction between SREBP and Sp1 would provide a molecular mechanism to explain how cholesterol uptake can be regulated independently from cholesterol biosynthesis in mammalian cells. (+info)
YY1 is a negative regulator of transcription of three sterol regulatory element-binding protein-responsive genes.
Ying Yang 1 (YY1) is shown to bind to the proximal promoters of the genes encoding 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) synthase, farnesyl diphosphate (FPP) synthase, and the low density lipoprotein (LDL) receptor. To investigate the potential effect of YY1 on the expression of SREBP-responsive genes, HepG2 cells were transiently transfected with luciferase reporter constructs under the control of promoters derived from either HMG-CoA synthase, FPP synthase, or the LDL receptor genes. The luciferase activity of each construct increased when HepG2 cells were incubated in lipid-depleted media or when the cells were cotransfected with a plasmid encoding mature sterol regulatory element-binding protein (SREBP)-1a. In each case, the increase in luciferase activity was attenuated by coexpression of wild-type YY1 but not by coexpression of mutant YY1 proteins that are known to be defective in either DNA binding or in modulating transcription of other known YY1-responsive genes. In contrast, incubation of cells in lipid-depleted media resulted in induction of an HMG-CoA reductase promoter-luciferase construct by a process that was unaffected by coexpression of wild-type YY1. Electromobility shift assays were used to demonstrate that the proximal promoters of the HMG-CoA synthase, FPP synthase, and the LDL receptor contain YY1 binding sites and that YY1 displaced nuclear factor Y from the promoter of the HMG-CoA synthase gene. We conclude that YY1 inhibits the transcription of specific SREBP-dependent genes and that, in the case of the HMG-CoA synthase gene, this involves displacement of nuclear factor Y from the promoter. We hypothesize that YY1 plays a regulatory role in the transcriptional regulation of specific SREBP-responsive genes. (+info)
SREBP-1 binds to multiple sites and transactivates the human ApoA-II promoter in vitro : SREBP-1 mutants defective in DNA binding or transcriptional activation repress ApoA-II promoter activity.
-Screening of an expression human liver cDNA library resulted in the isolation of several cDNA clones homologous to sterol regulatory element-binding protein-1 (SREBP-1) that recognize the regulatory element AIIAB and AIIK of the human apoA-II promoter. DNaseI footprinting of the apoA-II promoter using SREBP-1 (1 to 460) expressed in bacteria identified 5 overall protected regions designated AIIAB (-64 to -48), AIICD (-178 to -154), AIIDE (-352 to -332), AIIHI (-594 to -574), and AIIK (-760 to -743). These regions contain inverted E-box palindromic or direct repeat motifs and bind SREBP-1 with different affinities. Transient cotransfection experiments in HepG2 cells showed that SREBP-1 transactivated the -911/29 apoA-II promoter 3.5-fold as well as truncated apoA-II promoter segments that contain 1, 2, 3, or 4 SREBP binding sites. Mutagenesis analysis showed that transactivation by SREBP was mainly affected by mutations in element AIIAB. Despite the strong transactivation of the apoA-II promoter by SREBP-1 we could not demonstrate significant changes on the endogenous apoA-II mRNA levels of HepG2 cells after cotransfection with SREBP-1 or in the presence or absence of cholesterol and 25-OH-cholesterol. An SREBP-1 mutant lacking the amino-terminal activation domain bound normally to its cognate sites and repressed the apoA-II promoter activity. Repression was also caused by specific amino acid substitutions of Leu, Val, or Gly for Lys359, which affected DNA binding. Repression by the DNA binding-deficient mutants was abolished by deletion of the amino-terminal activation domain (1 to 90) of SREBP-1. Overall, the findings suggest that the wild-type SREBP-1 can bind and transactivate efficiently the apoA-II promoter in cell culture. SREBP-1 mutants lacking the activation domain bind to their cognate sites and directly repress the apoA-II promoter whereas mutants defective in DNA binding indirectly repress the apoA-II promoter activity, possibly by a squelching mechanism. (+info)
Signal transduction from the endoplasmic reticulum to the cell nucleus.
The endoplasmic reticulum (ER) serves several important functions. Cholesterol, an essential component of cellular membranes, is synthesized on the ER surface. Inside the organelle, proteins destined for secretion or transport to the cell surface are folded and become glycosylated. Because these processes are essential for cell viability, a disturbance in ER function presents significant stress to the cell. In response to ER stress, three distinct signal transduction pathways can be activated. Two of these, the unfolded protein response and the ER-overload response, respond to disturbances in protein processing. The third, the sterol regulatory cascade, is activated by depletion of cholesterol. This review summarizes the recent advances in our understanding of these ER-nuclear signal transduction pathways. In addition, it points to novel regulatory mechanisms discovered in these pathways, which may be widely used in other systems. (+info)