CREating a SCAP-less liver keeps SREBPs pinned in the ER membrane and prevents increased lipid synthesis in response to low cholesterol and high insulin

Hypoxia activates SREBP2 through Golgi disassembly in bone marrow-derived monocytes for enhanced tumor growth

Bone marrow-derived cells (BMDCs) infiltrate hypoxic tumors at a pre-angiogenic state and differentiate into mature macrophages, thereby inducing pro-tumorigenic immunity. A critical factor regulating this differentiation is activation of SREBP2-a well-known transcription factor participating in tumorigenesis progression-through unknown cellular mechanisms. Here, we show that hypoxia-induced Golgi disassembly and Golgi-ER fusion in monocytic myeloid cells result in nuclear translocation and activation of SREBP2 in a SCAP-independent manner. Notably, hypoxia-induced SREBP2 activation was only observed in an immature lineage of bone marrow-derived cells. Single-cell RNA-seq analysis revealed that SREBP2-mediated cholesterol biosynthesis was upregulated in HSCs and monocytes but not in macrophages in the hypoxic bone marrow niche. Moreover, inhibition of cholesterol biosynthesis impaired tumor growth through suppression of pro-tumorigenic immunity and angiogenesis. Thus, our findings indicate that Golgi-ER fusion regulates SREBP2-mediated metabolic alteration in lineage-specific BMDCs under hypoxia for tumor progression.

A mutation in transmembrane protein 135 impairs lipid metabolism in mouse eyecups

Aging is a significant factor in the development of age-related diseases but how aging disrupts cellular homeostasis to cause age-related retinal disease is unknown. Here, we further our studies on transmembrane protein 135 (Tmem135), a gene involved in retinal aging, by examining the transcriptomic profiles of wild-type, heterozygous and homozygous Tmem135 mutant posterior eyecup samples through RNA sequencing (RNA-Seq). We found significant gene expression changes in both heterozygous and homozygous Tmem135 mutant mouse eyecups that correlate with visual function deficits. Further analysis revealed that expression of many genes involved in lipid metabolism are changed due to the Tmem135 mutation. Consistent with these changes, we found increased lipid accumulation in mutant Tmem135 eyecup samples. Since mutant Tmem135 mice have similar ocular pathologies as human age-related macular degeneration (AMD) eyes, we compared our homozygous Tmem135 mutant eyecup RNA-Seq dataset with transcriptomic datasets of human AMD donor eyes. We found similar changes in genes involved in lipid metabolism between the homozygous Tmem135 mutant eyecups and AMD donor eyes. Our study suggests that the Tmem135 mutation affects lipid metabolism as similarly observed in human AMD eyes, thus Tmem135 mutant mice can serve as a good model for the role of dysregulated lipid metabolism in AMD.

Early pregnancy dyslipidemia is associated with placental DNA methylation at loci relevant for cardiometabolic diseases

Aim: To identify placental DNA methylation changes that are associated with early pregnancy maternal dyslipidemia. Materials & methods: We analyzed placental genome-wide DNA methylation (n = 262). Genes annotating differentially methylated CpGs were evaluated for gene expression in placenta (n = 64). Results: We found 11 novel significant differentially methylated CpGs associated with high total cholesterol, low-density lipoprotein cholesterol and triglycerides, and low high-density lipoprotein cholesterol. High triglycerides were associated with decreased methylation of cg02785814 (ALX4) and decreased expression of ALX4 in placenta. Genes annotating the differentially methylated CpGs play key roles in lipid metabolism and were enriched in dyslipidemia pathways. Functional annotation found cis-methylation quantitative trait loci for genetic loci in ALX4 and EXT2. Conclusion: Our findings lend novel insights into potential placental epigenetic mechanisms linked with maternal dyslipidemia. Trial Registration: ClinicalTrials.gov, NCT00912132.

Prenatal Alcohol Exposure: Profiling Developmental DNA Methylation Patterns in Central and Peripheral Tissues

Background: Prenatal alcohol exposure (PAE) can alter the development of neurobiological systems, leading to lasting neuroendocrine, neuroimmune, and neurobehavioral deficits. Although the etiology of this reprogramming remains unknown, emerging evidence suggests DNA methylation as a potential mediator and biomarker for the effects of PAE due to its responsiveness to environmental cues and relative stability over time. Here, we utilized a rat model of PAE to examine the DNA methylation profiles of rat hypothalami and leukocytes at four time points during early development to assess the genome-wide impact of PAE on the epigenome and identify potential biomarkers of PAE. Our model of PAE resulted in blood alcohol levels of ~80-150 mg/dl throughout the equivalent of the first two trimesters of human pregnancy. Hypothalami were analyzed on postnatal days (P) 1, 8, 15, 22 and leukocytes at P22 to compare central and peripheral markers. Genome-wide DNA methylation analysis was performed by methylated DNA immunoprecipitation followed by next-generation sequencing. Results: PAE resulted in lasting changes to DNA methylation profiles across all four ages, with 118 differentially methylated regions (DMRs) displaying persistent alterations across the developmental period at a false-discovery rate (FDR) < 0.05. In addition, 299 DMRs showed the same direction of change in the hypothalamus and leukocytes of P22 pups at an FDR < 0.05, with some genes overlapping with the developmental profile findings. The majority of these DMRs were located in intergenic regions, which contained several computationally-predicted transcription factor binding sites. Differentially methylated genes were generally involved in immune function, epigenetic remodeling, metabolism, and hormonal signaling, as determined by gene ontology analyses. Conclusions: Persistent DNA methylation changes in the hypothalamus may be associated with the long-term physiological and neurobehavioral alterations in observed in PAE. Furthermore, correlations between epigenetic alterations in peripheral tissues and those in the brain will provide a foundation for the development of biomarkers of fetal alcohol spectrum disorder (FASD). Finally, findings from studies of PAE provide important insight into the etiology of neurodevelopmental and mental health disorders, as they share numerous phenotypes and comorbidities.

Skin-specific regulation of SREBP processing and lipid biosynthesis by glycerol kinase 5

The recessive N-ethyl-N-nitrosourea-induced phenotype toku is characterized by delayed hair growth, progressive hair loss, and excessive accumulation of dermal cholesterol, triglycerides, and ceramides. The toku phenotype was attributed to a null allele of Gk5, encoding glycerol kinase 5 (GK5), a skin-specific kinase expressed predominantly in sebaceous glands. GK5 formed a complex with the sterol regulatory element-binding proteins (SREBPs) through their C-terminal regulatory domains, inhibiting SREBP processing and activation. In Gk5^toku/toku mice, transcriptionally active SREBPs accumulated in the skin, but not in the liver; they were localized to the nucleus and led to elevated lipid synthesis and subsequent hair growth defects. Similar defective hair growth was observed in kinase-inactive GK5 mutant mice. Hair growth defects of homozygous toku mice were partially rescued by treatment with the HMG-CoA reductase inhibitor simvastatin. GK5 exists as part of a skin-specific regulatory mechanism for cholesterol biosynthesis, independent of cholesterol regulation elsewhere in the body.

Stearoyl-CoA desaturase 1 expression is downregulated in liver and udder during E. coli mastitis through enhanced expression of repressive C/EBP factors and reduced expression of the inducer SREBP1A

Background

Stearoyl-CoA desaturase 1 (SCD1) desaturates long chain fatty acids and is therefore a key enzyme in fat catabolism. Its synthesis is downregulated in liver during illnesses caused by high levels of circulating lipopolysaccharide (LPS). SCD1 expression is known to be stimulated under adipogenic conditions through a variety of transcription factors, notably SREBP1 and C/EBPα and −β. However, mechanisms downregulating SCD1 expression during illness related reprograming of the metabolism were unknown. Escherichia coli elicited mastitis is an example of such a condition and was found to downregulates milk and milk fat synthesis. This is in part mediated through epigenetic mechanisms. We analyzed here mechanism controlling SCD1 expression in livers and udders from cows suffering from experimentally induced E. coli mastitis.

Results

We validated with RT-qPCR that SCD1 expression was reduced in these organs of the experimental cows. They also featured decreased levels of mRNAs encoding SREBP1a but increased levels for C/EBP α and −β. Chromatin accessibility PCR (CHART) revealed that downregulation of SCD1 expression in liver was not caused by tighter chromatin compaction of the SCD1 promoter. Reporter gene analyses showed in liver (HepG2) and mammary epithelial (MAC-T) model cells that overexpression of SREBP1a expectedly activated the promoter, while unexpectedly C/EBPα and −β strongly quenched the promoter activity. Abrogation of two from among of the three C/EBP DNA-binding motifs of the promoter revealed that C/EBPα acts in cis but C/EBPβ in trans. Overexpressing truncated C/EBPα or −β factors lacking their repressive domains confirmed in both model cells the direct action of C/EBPα, but not of C/EBPβ on the promoter.

Conclusions

We found no evidence that epigenetic mechanism remodeling the chromatin compaction of the SCD1 promoter would contribute to downregulate SCD1 expression during infection. Instead, our data show for the first time that C/EBP factors may repress SCD1 expression in liver and udder rather than stimulating as it was previously shown in adipocytes. This cell type specific dual and opposite function of C/EBP factors for regulating SCD1 expression was previously unknown. Infection related activation of their expression combined with downregulated expression of SREBP1a explains reduced SCD1 expression in liver and udder during acute mastitis.

Electronic supplementary material

The online version of this article (doi:10.1186/s12867-016-0069-5) contains supplementary material, which is available to authorized users.

Alterations in microRNA expression linked to microcystin-LR-induced tumorigenicity in human WRL-68 Cells

Microcystin-LR (MC-LR) is a cyclic heptapeptide that acts as a potent hepatotoxin and carcinogen. However, the mechanism of its carcinogenic action remains undetermined. In this study, MC-LR was used to induce the malignant transformation of the WRL-68 cell line. Alterations in microRNA (miRNA) expression in the transformed cell were analyzed to determine the role of miRNAs in MC-LR-induced carcinogenesis. Cultured WRL-68 cells (labeled 25MC10) were continuously exposed to a low concentration (10 μg/L) of MC-LR for 25 passages. Compared with the mock-treated parental cells, the induced 25MC10 cells exhibited a higher growth rate, resistance to serum-induced terminal differentiation, and tumorigenicity in a nude mouse xenograft test. A pilot miRNA expression array analysis was conducted on the 25MC10 cells, followed by validation of select miRNAs by RT-PCR. We found that the onco-miRNAs miR-21 and miR-221 displayed upregulated expression while the liver-specific miR-122 was downregulated. These results suggest that chronic MC-LR exposure alters the miRNA expression profile of WRL-68 cells and causes phenotypic transformation. We propose that characteristic miRNA alterations could be used as molecular targets for the development of environmental water monitoring methods.

Sterol regulatory element binding protein and dietary lipid regulation of fatty acid synthesis in the mammary epithelium

The lactating mammary gland synthesizes large amounts of triglyceride from fatty acids derived from the blood and from de novo lipogenesis. The latter is significantly increased at parturition and decreased when additional dietary fatty acids become available. To begin to understand the molecular regulation of de novo lipogenesis, we tested the hypothesis that the transcription factor sterol regulatory element binding factor (SREBF)-1c is a primary regulator of this system. Expression of Srebf1c mRNA and six of its known target genes increased ≥2.5-fold at parturition. However, Srebf1c-null mice showed only minor deficiencies in lipid synthesis during lactation, possibly due to compensation by Srebf1a expression. To abrogate the function of both isoforms of Srebf1, we bred mice to obtain a mammary epithelial cell-specific deletion of SREBF cleavage-activating protein (SCAP), the SREBF escort protein. These dams showed a significant lactation deficiency, and expression of mRNA for fatty acid synthase (Fasn), insulin-induced gene 1 (Insig1), mitochondrial citrate transporter (Slc25a1), and stearoyl-CoA desaturase 2 (Scd2) was reduced threefold or more; however, the mRNA levels of acetyl-CoA carboxylase-1α (Acaca) and ATP citrate lyase (Acly) were unchanged. Furthermore, a 46% fat diet significantly decreased de novo fatty acid synthesis and reduced the protein levels of ACACA, ACLY, and FASN significantly, with no change in their mRNA levels. These data lead us to conclude that two modes of regulation exist to control fatty acid synthesis in the mammary gland of the lactating mouse: the well-known SREBF1 system and a novel mechanism that acts at the posttranscriptional level in the presence of SCAP deletion and high-fat feeding to alter enzyme protein.

The unique role of proprotein convertase subtilisin/kexin 9 in cholesterol homeostasis

The LDL receptor (LDLR) plays an essential role in the regulation of plasma (LDL) cholesterol concentrations by virtue of its ability to clear plasma LDL. Down-regulation of the LDLR by proprotein convertase subtilisin/kexin 9 (PCSK9) has recently emerged as a regulatory mechanism that controls plasma LDL cholesterol concentrations. Studies in which PCSK9 is over-expressed in mice, have demonstrated that PCSK9, by enhancing hepatic LDLR degradation, decreases the availability of the LDLR for LDL uptake, resulting in increased plasma LDL cholesterol levels. However, PCSK9 has also recently been shown to mediate down-regulation of surface receptors other than the LDLR, suggesting that it may have much broader roles than initially thought.

Regulation of sterol regulatory element-binding transcription factor 1a by human chorionic gonadotropin and insulin in cultured rat theca-interstitial cells

Theca-interstitial (T-I) cells of the ovary synthesize androgens in response to luteinizing hormone (LH). In pathological conditions such as polycystic ovarian syndrome, T-I cells are hyperactive in androgen production in response to LH and insulin. Because cholesterol is an essential substrate for androgen production, we examined the effect of human chorionic gonadotropin (hCG) and insulin on signaling pathways that are known to increase cholesterol accumulation in steroidogenic cells. Specifically, the effect of hCG and insulin on sterol regulatory element-binding transcription factor 1a (SREBF1a) required for cholesterol biosynthesis and uptake was examined. Primary cultures of T-I cells isolated from 25-day-old rat ovaries responded to hCG and insulin to increase the active/processed form of SREBF1a. The hCG and insulin significantly reduced insulin-induced gene 1 (INSIG1) protein, a negative regulator of SREBF processing. Furthermore, an increase in the expression of selected SREBF target genes, 3-hydroxy-3-methylglutaryl-coenzyme A reductase (Hmgcr) and mevalonate kinase (Mvk), was also observed. Protein kinase A (PRKA) inhibitor completely abolished the hCG-induced increase in SREBF1a, while increasing INSIG1. Although the hCG-induced depletion of total and free cholesterol was abolished by aminoglutethimide, the stimulatory effect on SREBF1a was not totally suppressed. Treatment with 25-hydroxycholesterol abrogated the effect of hCG on SREBF1a. Inhibition of the phosphatidylinositol 3-kinase pathway did not block the insulin-induced increase in SREBF1a, whereas mitogen-activated protein kinase inhibition reduced the insulin response. These results suggest that the increased androgen biosynthesis by T-I cells in response to hCG and insulin is regulated, at least in part, by increasing the expression of sterol response element-responsive genes by increasing SREBF1a.

Cloning, comparative characterization of porcine SCAP gene, and identification of its two splice variants

Sterol responsive element binding protein (SREBP) cleavage-activating protein (SCAP) is the key regulator of activation of SREBPs, which stimulate most enzymes in cholesterol and lipid synthesis. In order to investigate the molecular basis of lipid metabolism in the pig, a unique model for fat deposition, we isolated and characterized the porcine SCAP. The 4,096-bp full-length porcine SCAP cDNA contains an open reading frame of 3,840 bp. The predicted SCAP protein consists of 1,280 amino acids of 55-92% identity with its vertebrate counterparts. The porcine SCAP gene consists of at least 19 exons and 18 introns, which span over 13 kb of the genome. The porcine SCAP gene was mapped to chromosome 13q21-22 using a porcine-rodent somatic cell hybrid panel. Comparison of SCAP genomic structures from various species revealed intron losses in porcine, Tetraodon and fugu SCAP, and intron gains in cow and chicken SCAP. Moreover, we isolated two novel splicing SCAP variants with 193-bp (variant 2) in-frame deletion from testis and a variant with 291-bp (variant 3) in-frame deletion from liver and muscle, which may affect the function of the porcine SCAP. In conclusion, the intron gains and losses appear to have contributed to the shape of the modern SCAP family. The splice variants detected, first to be reported in any species, may be involved in the particulars of the fat metabolism in the pig. Our data lay foundation for further study of SCAP function in this species.

Cellular stress reactions as putative cholinergic links in Alzheimer's disease

Alzheimer's disease involves normal cellular aging and chronic cellular stress events, leading to interrelated changes in gene expression and subsequent neurodegeneration. Premature death of cholinergic neurons and the formation of amyloid fibrils separately initiated the cholinergic and amyloid hypotheses of Alzheimer's disease. Here, we present evidence to the fact that these two distinct phenomena both associate with specific changes in acetylcholinesterase (AChE) gene expression within cholinergic neurons. For example, calcium misregulation promotes aberrant transcription and pro-apoptotic events, as well as AChE-induced modifications in cellular signal cascades. These reciprocally intercept with AChE regulation at the Endoplasmic Reticulum, modifying AChE gene expression, folding and signaling. Altered AChE properties may reflect changes in the enzymatic and/or non-enzymatic features of the multiple AChE splice variants. Under chronic cellular stress, aberrant AChE regulation may thus facilitate apoptotic pathways, promoting plaque formation, cognitive impairments and degeneration of cholinergic nerve cells.

The agouti-related protein and its role in energy homeostasis

The melanocortin system plays an important role in the regulation of energy homeostasis. The Agouti-related protein (AGRP) is a natural antagonist of the action of alpha-melanocyte stimulating hormone (alpha-MSH) at the melanocortin receptors (MCR). AGRP is upregulated by fasting while intracerebroventricular injections of synthetic AGRP lead to increased appetite and food intake. Transgenic mice overexpressing AGRP are also hyperphagic and eventually become obese. AGRP is, therefore, a significant regulator of energy balance and a candidate gene for human fatness. Indeed, humans with common single nucleotide polymorphisms (SNPs) in the promoter or the coding region are leaner and resistant to late-onset obesity than wild-type individuals. AGRP is also expressed in the periphery. Recent studies show that AGRP in the adrenal gland is upregulated by fasting as much as it is in the hypothalamus. These data open up the possibility for a wider role by AGRP not only in food intake but also in the regulation of energy balance through its actions on peripheral tissues. This review summarizes recent advances in the biochemical and physiological properties of AGRP in an effort to enhance our understanding of the role this powerful neuropeptide plays in mammalian energy homeostasis.

Spatial distribution and function of sterol regulatory element-binding protein 1a and 2 homo- and heterodimers by in vivo two-photon imaging and spectroscopy fluorescence resonance energy transfer

Sterol regulatory element-binding proteins (SREBPs) are a subfamily of basic helix-loop-helix-leucine zipper proteins that regulate lipid metabolism. We show novel evidence of the in vivo occurrence and subnuclear spatial localization of both exogenously expressed SREBP-1a and -2 homodimers and heterodimers obtained by two-photon imaging and spectroscopy fluorescence resonance energy transfer. SREBP-1a homodimers localize diffusely in the nucleus, whereas SREBP-2 homodimers and the SREBP-1a/SREBP-2 heterodimer localize predominantly to nuclear speckles or foci, with some cells showing a diffuse pattern. We also used tethered SREBP dimers to demonstrate that both homo- and heterodimeric SREBPs activate transcription in vivo. Ultrastructural analysis revealed that the punctate foci containing SREBP-2 are electron-dense nuclear bodies, similar or identical to structures containing the promyelocyte (PML) protein. Immunofluorescence studies suggest that a dynamic interplay exists between PML, as well as another component of the PML-containing nuclear body, SUMO-1, and SREBP-2 within these nuclear structures. These findings provide new insight into the overall process of transcriptional activation mediated by the SREBP family.

Selective association of sterol regulatory element-binding protein isoforms with target promoters in vivo

The mRNAs for all three members of the sterol regulatory element-binding protein (SREBP) family are widely expressed, and the proteins are highly similar. They have potential to both hetero- and homodimerize through their bHLHLZ domains, so it has been difficult to definitively study the role of each one apart from the other two. In the current study, we have utilized cell lines that express only one functional SREBP and the chromatin immunoprecipitation technique to analyze individual SREBP binding to three specific target genes: hydroxymethylglutaryl-CoA reductase (Red), fatty acid synthase (FAS), and squalene synthase (SQS). Our studies show that SREBP-2 binds to promoters for all three genes, and in agreement with the original report using these cells, all three mRNAs are also induced. In the line expressing only SREBP-1a, mRNAs for Red and FAS are induced, but SQS is not. Chromatin immunoprecipitation also shows that SREBP-1a is recruited efficiently to Red and FAS promoters but not to SQS. This observation indicates SREBP-2 selectively binds the SQS promoter and is sufficient to explain the lack of SQS mRNA induction in the SREBP-1a-expressing cells. SREBP-1c protein was not stably recruited to any SREBP target promoter despite being fully active in DNA binding when purified from extracts of the corresponding cells. This is also sufficient to explain the lack of SREBP target gene induction by the singular expression of SREBP-1c. We also show that whereas SREBP-1a and -2 proteins interact efficiently with transcriptional co-activators that modify cellular chromatin, SREBP-1c does not. Taken together, our data support a model suggesting that chromatin modification is required during the initial stage of specific site recognition by SREBPs in native chromatin in vivo.

Thyroid hormone regulation and cholesterol metabolism are connected through Sterol Regulatory Element-Binding Protein-2 (SREBP-2)

High affinity uptake of serum-derived low density lipoprotein (LDL) cholesterol is accomplished through the LDL receptor in the liver. In mammals, thyroid hormone depletion leads to decreased LDL receptor expression and elevated serum cholesterol. The clinical association in humans has been known since the 1920s; however, a molecular explanation has been lacking. LDL receptor levels are subject to negative feedback regulation by cellular cholesterol through sterol regulatory element-binding protein-2 (SREBP-2). Here we demonstrate that the SREBP-2 gene is regulated by thyroid hormone and that increased SREBP-2 nuclear protein levels in hypothyroid animals results in thyroid hormone-independent activation of LDL receptor gene expression and reversal of the associated hypercholesterolemia. This occurs without effects on other thyroid hormone-regulated genes. Thus, we propose that the decreased LDL receptor and increased serum cholesterol associated with hypothyroidism are secondary to the thyroid hormone effects on SREBP-2. These results suggest that hypercholesterolemia associated with hypothyroidism can be reversed by agents that directly increase SREBP-2. Additionally, these results indicate that mutations or drugs that lower nuclear SREBP-2 would cause hypercholesterolemia.

Dietary cholesterol opposes PUFA-mediated repression of the stearoyl-CoA desaturase-1 gene by SREBP-1 independent mechanism

Stearoyl-CoA desaturase (SCD) catalyzes the rate-limiting step in the cellular synthesis of monounsaturated fatty acids, mainly oleate (18:1) and palmitoleate (16:1), which are the major monounsaturated fatty acids of membrane phospholipids, cholesteryl esters, waxes, and triglycerides. The mouse expresses three well-characterized SCD genes (SCD1, 2, and 3). SCD1 is the main isoform expressed in the liver of mice. Previous in vivo studies have shown that the transcriptional repression by n-3 and n-6 polyunsaturated fatty acids (PUFAs) and the induction by cholesterol of the SCD1 gene are dependent on the maturation of the sterol regulatory element-binding protein-1c (SREBP-1c). We studied the regulation of SREBP-1, SCD1, and other SREBP-1 target genes when a high cholesterol diet is combined with PUFA as n-6 PUFA rich soybean oil (SO), or n-3 PUFA rich fish oil (FO). While the PUFA/cholesterol (PUFA/CH) diets repressed the maturation of the SREBP-1, the SCD1 mRNA levels, and protein and enzyme activity were induced. Compared with PUFA diets, hepatic cholesterol ester and triglyceride were enriched with 16:1 and 18:1 monounsaturated fatty acids in mice fed PUFA/CH diets. Total plasma cholesterol levels were not altered but plasma triglycerides were reduced in SO/CH-fed mice compared with SO-fed mice. The mRNA for SREBP-1 was increased by the PUFA/CH diet but the mRNA levels of SREBP-1 target genes such as fatty acid synthase and LDL receptor were decreased, indicating that the main control of PUFA-mediated suppression of SREBP-1 target genes is the maturation of SREBP-1. This study demonstrates that cholesterol overrides the PUFA-mediated repression of the SCD1 gene and regulates SCD1 gene expression through a mechanism independent of SREBP-1 maturation.

Sterol regulatory element-binding proteins (SREBPs) as regulators of lipid metabolism: polyunsaturated fatty acids oppose cholesterol-mediated induction of SREBP-1 maturation

Cellular cholesterol and fatty acid metabolism in mammals is controlled by a family of transcription factors called sterol regulatory element-binding protein isoforms, three of which (SREBP-1a, 1c, and 2) are well characterized. These proteins, which are synthesized as precursors, are inserted into the endoplasmic reticulum (ER) membrane with both the amino and carboxylic acid domains facing the cytosolic face of the membrane. In sterol-deficient cells, proteolytic cleavage of SREBPs occurs, thereby releasing their N-terminal mature and active forms and enabling them to enter the nucleus, where they bind to the sterol regulatory response element (SRE) and/or E-box sequences and activate genes involved in cholesterol, triglyceride, and fatty acid biosynthesis. Of the three SREBP isoforms, SREBP-1c gene expression is induced by cholesterol and repressed by polyunsaturated fatty acids (PUFA). We have examined the changes in SREBP-1c mRNA and protein levels as well as the mRNA levels of several SREBP-1c target genes when a high-cholesterol diet is combined with diets rich in PUFA of the n-6 series. Our studies show that PUFA oppose the cholesterol-mediated SREBP-1 maturation without affecting the cholesterol-mediated increase of SREBP-1c mRNA and precursor protein. The decrease in SREBP-1 mature protein paralleled the decrease in mRNAs for genes of fatty acid and cholesterol biosynthesis, such as HMG-CoA synthase and fatty acid synthase, but interestingly gene expression of stearoyl-CoA desaturase 1 (SCD1) was instead induced. These studies suggest that the main point of control of PUFA-mediated suppression of lipogenic gene expression is the inhibition of SREBP-1 maturation. The studies also reveal that the induction of SCD1 gene expression by cholesterol occurs through a mechanism independent of SREBP-1 maturation.