Donald B. Jump

Fatty Acid Regulation of Gene Transcription

ABSTRACT Dietary fat has a dual role in human physiology: a) it functions as a source of energy and structural components for cells; b) it functions as a regulator of gene expression that impacts lipid, carbohydrate, and protein metabolism, as well as cell growth and differentiation. Fatty acid effects on gene expression are cell-specific and influenced by fatty acid structure and metabolism. Fatty acids interact with the genome through several mechanisms. They regulate the activity or nuclear abundance of several transcription factors, including PPAR, LXR, HNF-4, NFκB, and SREBP. Fatty acids or their metabolites bind directly to specific transcription factors to regulate gene transcription. Alternatively, fatty acids indirectly act on gene expression through their effects on a) specific enzyme-mediated pathways, such as cyclooxygenase, lipoxygenase, protein kinase C, or sphingomyelinase signal transduction pathways; or b) pathways that involve changes in membrane lipid/lipid raft composition that affect G-protein receptor or tyrosine kinase-linked receptor signaling. Further definition of these fatty acid-regulated pathways will provide insight into the role dietary fat plays in human health and the onset and progression of several chronic diseases, like coronary artery disease and atherosclerosis, dyslipidemia and inflammation, obesity and diabetes, cancer, major depressive disorders, and schizophrenia.

Dietary polyunsaturated fatty acids and regulation of gene transcription.

Dietary polyunsaturated fatty acids (PUFAs) are a source of energy and structural components for cells. PUFAs also have dramatic effects on gene expression by regulating the activity or abundance of four families of transcription factor, including peroxisome proliferator activated receptor (PPAR) (α, β and γ), liver X receptors (LXRs) (α and β), hepatic nuclear factor-4 (HNF-4)α and sterol regulatory element binding proteins (SREBPs) 1 and 2. These transcription factors play a major role in hepatic carbohydrate, fatty acid, triglyceride, cholesterol and bile acid metabolism. Non-esterified fatty acids or fatty acid metabolites bind to and regulate the activity of PPARs, LXRs and HNF-4. In contrast, PUFAs regulate the nuclear abundance of SREBPs by controlling the proteolytic processing of SREBP precursors, or regulating transcription of the SREBP-1c gene or turnover of mRNASREBP-1c. The n3 and n6 PUFAs are feed-forward activators of PPARs, while these same fatty acids are feedback inhibitors of LXRs and SREBPs. Saturated fatty acyl coenzyme A thioesters activate HNF-4α, while coenzyme A thioesters of PUFAs antagonize HNF-4α action. Understanding how fatty acids regulate the activity and abundance of these and other transcription factors will likely provide insight into the development of novel therapeutic strategies for better management of whole body lipid and cholesterol metabolism.

N-3 polyunsaturated fatty acid regulation of hepatic gene transcription

Purpose of review The liver plays a central role in whole body lipid metabolism and adapts rapidly to changes in dietary fat composition. This adaption involves changes in the expression of genes involved in glycolysis, de-novo lipogenesis, fatty acid elongation, desaturation and oxidation. This review brings together metabolic and molecular studies that help explain n-3 (omega-3) polyunsaturated fatty acid regulation of hepatic gene transcription. Recent findings Dietary n-3 polyunsaturated fatty acid regulates hepatic gene expression by targeting three major transcriptional regulatory networks: peroxisome proliferator-activated receptor α, sterol regulatory element binding protein-1 and the carbohydrate regulatory element binding protein/Max-like factor X heterodimer. 22: 6,n-3, the most prominent n-3 polyunsaturated fatty acid in tissues, is a weak activator of peroxisome proliferator-activated receptor α. Hepatic metabolism of 22: 6,n-3, however, generates 20: 5,n-3, a strong peroxisome proliferator-activated receptor α activator. In contrast to peroxisome proliferator-activated receptor α, 22: 6,n-3 is the most potent fatty acid regulator of hepatic sterol regulatory element binding protein-1. 22: 6,n-3 suppresses sterol regulatory element binding protein-1 gene expression while enhancing degradation of nuclear sterol regulatory element binding protein-1 through 26S proteasome and Erk1/2-dependent mechanisms. Both n-3 and n-6 polyunsaturated fatty acid suppress carbohydrate regulatory element binding protein and Max-like factor X nuclear abundance and interfere with glucose-regulated hepatic metabolism. Summary These studies have revealed unique mechanisms by which specific polyunsaturated fatty acids control peroxisome proliferator activated receptor α, sterol regulatory element binding protein-1 and carbohydrate regulatory element binding protein/Max-like factor X function. As such, specific metabolic and signal transduction pathways contribute significantly to the fatty acid regulation of these transcription factors and their corresponding regulatory networks.

Regulation of hepatic fatty acid elongase and desaturase expression in diabetes and obesity

Fatty acid elongases and desaturases play an important role in hepatic and whole body lipid composition. We examined the role that key transcription factors played in the control of hepatic elongase and desaturase expression. Studies with peroxisome proliferator-activated receptor α (PPARα)-deficient mice establish that PPARα was required for WY14643-mediated induction of fatty acid elongase-5 (Elovl-5), Elovl-6, and all three desaturases [Δ5 desaturase (Δ5D), Δ6D, and Δ9D]. Increased nuclear sterol-regulatory element binding protein-1 (SREBP-1) correlated with enhanced expression of Elovl-6, Δ5D, Δ6D, and Δ9D. Only Δ9D was also regulated independently by liver X receptor (LXR) agonist. Glucose induction of l-type pyruvate kinase, Δ9D, and Elovl-6 expression required the carbohydrate-regulatory element binding protein/MAX-like factor X (ChREBP/MLX) heterodimer. Suppression of Elovl-6 and Δ9D expression in livers of streptozotocin-induced diabetic rats and high fat-fed glucose-intolerant mice correlated with low levels of nuclear SREBP-1. In leptin-deficient obese mice (Lepob/ob), increased SREBP-1 and MLX nuclear content correlated with the induction of Elovl-5, Elovl-6, and Δ9D expression and the massive accumulation of monounsaturated fatty acids (18:1,n-7 and 18:1,n-9) in neutral lipids. Diabetes- and obesity-induced changes in hepatic lipid composition correlated with changes in elongase and desaturase expression. In conclusion, these studies establish a role for PPARα, LXR, SREBP-1, ChREBP, and MLX in the control of hepatic fatty acid elongase and desaturase expression and lipid composition.

Sterol response element-binding protein 1c (SREBP1c) is involved in the polyunsaturated fatty acid suppression of hepatic S14 gene transcription.

Polyunsaturated fatty acids (PUFA) suppress hepatic lipogenic gene transcription through a peroxisome proliferator activated receptor α (PPARα)- and cyclooxygenase-independent mechanism. Recently, the sterol response element-binding protein 1 (SREBP1) was implicated in the nutrient control of lipogenic gene expression. In this report, we have assessed the role SREBP1 plays in the PUFA control of three hepatic genes, fatty acid synthase, L-pyruvate kinase (LPK), and the S14 protein (S14). PUFA suppressed both the hepatic mRNASREBP1 through a PPARα-independent mechanism as well as SREBP1c nuclear content (nSREBP1c, 65 kDa). Co-transfection of primary hepatocytes revealed a differential sensitivity of the fatty acid synthase, S14, and LPK promoters to nSREBP1c overexpression. Of the three promoters examined, LPK was the least sensitive to overexpressed nSREBP1c. Promoter deletion and gel shift analyses of the S14 promoter localized a functional SREBP1c cis-regulatory element to an E-box-like sequence (−139TCGCCTGAT−131) within the S14 PUFA response region. Although overexpression of nSREBP1c significantly reduced PUFA inhibition of S14CAT, overexpression of other factors that induced S14CAT activity, such as steroid receptor co-activator 1 or retinoid X receptor α, had no effect on S14CAT PUFA sensitivity. These results suggest that PUFA regulates hepatic nSREBP1c, a factor that functionally interacts with the S14 PUFA response region. PUFA regulation of nSREBP1c may account for the PUFA-mediated suppression of hepatic S14 gene transcription.

The Role of Liver X Receptor-α in the Fatty Acid Regulation of Hepatic Gene Expression

Liver X receptors (LXR) α and β play an important role in regulating the expression of genes involved in hepatic bile and fatty acid synthesis, glucose metabolism, as well as sterol efflux. Studies with human embryonic kidney 293 cells indicate that unsaturated fatty acids interfere with oxysterols binding to LXR and antagonize oxysterol-induced LXRα activity. In this report, we evaluated the effects of unsaturated fatty acids on LXR-regulated hepatic gene expression. The LXR agonist, T1317, induced mRNAs encoding sterol regulatory element-binding protein 1c (SREBP-1c) and two SREBP-1c-regulated lipogenic genes, e.g. fatty-acid synthase and the S14 protein in primary hepatocytes. Treatment of hepatocytes with eicosapentaenoic acid (20:5n-3) suppressed these mRNAs in the absence and presence of T1317. The cis-regulatory elements targeted by T1317 were not required for fatty-acid suppression of FAS or S14 promoter activity. In contrast to SREBP-1-regulated lipogenic genes, 20:5n-3 had no effect on the T1317 induction of ABCG5 or ABCG8 in the rat hepatoma cell line, FTO-2B. These two genes require LXR but not SREBP-1c for their expression. Feeding rats a diet supplemented with fish oil suppressed hepatic SREBP-1c-regulated genes and induced PPARα-regulated genes but had no effect on the LXR-regulated transcripts, CYP7A1, ABCG5, or ABCG8. Transfection studies, using either full-length hLXRα or a chimera containing only the LXRα ligand binding domain, indicate that a wide array of unsaturated fatty acids had little effect on LXRα activity in primary hepatocytes or FTO-2B. These studies suggest that LXRα is not a target for unsaturated fatty acid regulation in primary rat hepatocytes or in liver. Thus, oxysterol/LXR-mediated regulation of transcripts involved in bile acid synthesis or sterol efflux appear insensitive to dietary unsaturated fatty acids. The unsaturated fatty acid suppression of SREBP-1 and its targeted lipogenic genes is independent of LXRα

Docosahexaenoic acid (DHA) and hepatic gene transcription.

The type and quantity of dietary fat ingested contributes to the onset and progression of chronic diseases, like diabetes and atherosclerosis. The liver plays a central role in whole body lipid metabolism and responds rapidly to changes in dietary fat composition. Polyunsaturated fatty acids (PUFA) play a key role in membrane composition and function, metabolism and the control of gene expression. Certain PUFA, like the n-3 PUFA, enhance hepatic fatty acid oxidation and inhibit fatty acid synthesis and VLDL secretion, in part, by regulating gene expression. Our studies have established that key transcription factors, like PPARalpha, SREBP-1, ChREBP and MLX, are regulated by n-3 PUFA, which in turn control levels of proteins involved in lipid and carbohydrate metabolism. Of the n-3 PUFA, 22:6,n-3 has recently been established as a key controller of hepatic lipid synthesis. 22:6,n-3 controls the 26S proteasomal degradation of the nuclear form of SREBP-1. SREBP-1 is a major transcription factor that controls the expression of multiple genes involved fatty acid synthesis and desaturation. 22:6,n-3 suppresses nuclear SREBP-1, which in turn suppresses lipogenesis. This mechanism is achieved, in part, through control of the phosphorylation status of protein kinases. This review will examine both the general features of PUFA-regulated hepatic gene transcription and highlight the unique mechanisms by which 22:6,n-3 impacts gene expression. The outcome of this analysis will reveal that changes in hepatic 22:6,n-3 content has a major impact on hepatic lipid and carbohydrate metabolism. Moreover, the mechanisms involve 22:6,n-3 control of several well-known signaling pathways, such as Akt, Erk1/2, Gsk3beta and PKC (novel or atypical). 22:6,n-3 control of these same signaling pathways in non-hepatic tissues may help to explain the diverse actions of n-3 PUFA on such complex physiological processes as visual acuity and learning.

Fatty acid regulation of hepatic lipid metabolism.

Purpose of reviewTo discuss transcriptional mechanisms regulating hepatic lipid metabolism. Recent findingsHumans who are obese or have diabetes (NIDDM) or metabolic syndrome (MetS) have low blood and tissue levels of C20–22 polyunsaturated fatty acids (PUFAs). Although the impact of low C20–22 PUFAs on disease progression in humans is not fully understood, studies with mice have provided clues suggesting that impaired PUFA metabolism may contribute to the severity of risk factors associated with NIDDM and MetS. High fat diets promote hyperglycemia, insulin resistance and fatty liver in C57BL/6J mice, an effect that correlates with suppressed expression of enzymes involved in PUFA synthesis and decreased hepatic C20–22 PUFA content. A/J mice, in contrast, are resistant to diet-induced obesity and diabetes; these mice have elevated expression of hepatic enzymes involved in PUFA synthesis and C20–22 PUFA content. Moreover, loss-of-function and gain-of-function studies have identified fatty acid elongase (Elovl5), a key enzyme involved in PUFA synthesis, as a regulator of hepatic lipid and carbohydrate metabolism. Elovl5 activity regulates hepatic C20–22 PUFA content, signaling pathways (Akt and PP2A) and transcription factors (SREBP-1, PPARα, FoxO1 and PGC1α) that control fatty acid synthesis and gluconeogenesis. SummaryThese studies may help define novel strategies to control fatty liver and hyperglycemia associated with NIDDM and MetS.

Omega-3 Fatty Acid Supplementation and Cardiovascular Disease

Epidemiological studies on Greenland Inuits in the 1970s and subsequent human studies have established an inverse relationship between the ingestion of omega-3 fatty acids [C20–22 ω 3 polyunsaturated fatty acids (PUFA)], blood levels of C20–22 ω 3 PUFA, and mortality associated with cardiovascular disease (CVD). C20–22 ω 3 PUFA have pleiotropic effects on cell function and regulate multiple pathways controlling blood lipids, inflammatory factors, and cellular events in cardiomyocytes and vascular endothelial cells. The hypolipemic, anti-inflammatory, anti-arrhythmic properties of these fatty acids confer cardioprotection. Accordingly, national heart associations and government agencies have recommended increased consumption of fatty fish or ω 3 PUFA supplements to prevent CVD. In addition to fatty fish, sources of ω 3 PUFA are available from plants, algae, and yeast. A key question examined in this review is whether nonfish sources of ω 3 PUFA are as effective as fatty fish-derived C20–22 ω 3 PUFA at managing risk factors linked to CVD. We focused on ω 3 PUFA metabolism and the capacity of ω 3 PUFA supplements to regulate key cellular events linked to CVD. The outcome of our analysis reveals that nonfish sources of ω 3 PUFA vary in their capacity to regulate blood levels of C20–22 ω 3 PUFA and CVD risk factors.

Fatty Acid–Regulated Transcription Factors in the Liver

Fatty acid regulation of hepatic gene transcription was first reported in the early 1990s. Several transcription factors have been identified as targets of fatty acid regulation. This regulation is achieved by direct fatty acid binding to the transcription factor or by indirect mechanisms where fatty acids regulate signaling pathways controlling the expression of transcription factors or the phosphorylation, ubiquitination, or proteolytic cleavage of the transcription factor. Although dietary fatty acids are well-established regulators of hepatic transcription factors, emerging evidence indicates that endogenously generated fatty acids are equally important in controlling transcription factors in the context of glucose and lipid homeostasis. Our first goal in this review is to provide an up-to-date examination of the molecular and metabolic bases of fatty acid regulation of key transcription factors controlling hepatic metabolism. Our second goal is to link these mechanisms to nonalcoholic fatty liver disease (NAFLD), a growing health concern in the obese population.