Coordinate regulation of glycolytic and lipogenic gene expression by polyunsaturated fatty acids

Fatty acids and fibrates are potent inducers of transcription of the phosphenolpyruvate carboxykinase gene in adipocytes

Cytosolic phosphoenolpyruvate carboxykinase (PEPCK) plays a critical role in adipose tissue glyceroneogenesis. We have previously shown that transcription of the PEPCK gene was stimulated by isoprenaline and retinoic acid in 3T3-F442A adipocytes. We also showed that oleate increased PEPCK mRNA. Here, we analysed the effect that fatty acids of various chain lengths and unsaturation degrees exerted on PEPCK gene expression in 3T3-F442A adipocytes. When maintained in serum-free, glucose-free medium, differentiated cells responded to unsaturated long-chain fatty acids by a large increase in PEPCK mRNA whereas saturated fatty acids were inefficient. A maximum fivefold stimulation by oleate was attained at 4 h of treatment with 1 mM fatty acid bound to albumin in a 6:1 ratio. The poly-unsaturated very long-chain fatty acid all-cis-4,7,10,13,16,19-docosahexaenoic acid (C22:6) was even more potent and produced a tenfold increase. The expression of the genes encoding glycerol-3-phosphate dehydrogenase, hormone-sensitive lipase or actin remained unaffected by oleate exposure. A 4-h treatment by the hypolipidemic drug clofibrate, 0.5-2 mM, also produced a large (3-9-fold) increase in PEPCK mRNA. When used at non-saturating concentrations, oleate and clofibrate acted in an additive manner. At maximally effective concentrations, additivity was lost, suggesting that fatty acids and fibrates might act through similar mechanisms. Nuclear transcription experiments showed that oleate and clofibrate stimulated the transcription rate of the gene. 3T3-F442A cells were stably transfected with a plasmid containing the base pairs -2100 to +69 of the PEPCK gene promoter fused to the chloramphenicol acetyltransferase gene. These differentiated stable transfectants responded to oleate and clofibrate by a specific increase in chloramphenicol acetyltransferase activity. Adipocytes express various isoforms of peroxisome-proliferator-activated receptors that can be activated by fibrates and fatty acids. Potential recognition sequences for peroxisome-proliferator-activated receptors are present in the -2100 to +69 fragment of the PEPCK gene promoter. Thus, this gene represents an ideal molecular target for understanding the complex transcriptional control exerted by fatty acids and peroxisome proliferators.

Regulation of rat liver apolipoprotein A-I, apolipoprotein A-II and acyl-coenzyme A oxidase gene expression by fibrates and dietary fatty acids

The regulation by fibrates and dietary fatty acids of the hepatic gene expression of apolipoproteins (apo) A-I and A-II, the major protein constituents of high-density lipoproteins, as well as of acyl-CoA oxidase, the rate-limiting enzyme of the peroxisomal beta-oxidation pathway, was studied in vivo in the rat and in vitro in primary cultures of rat hepatocytes. In primary hepatocytes, different fibrates decreased apo A-I and increased acyl-CoA oxidase mRNA levels, whereas apo A-II mRNA only decreased in level after treatment with fenofibric acid, but not after bezafibrate, gemfibrozil or Wy-14643 treatment. Treatment with fenofibric acid counteracted the increase in apo A-I mRNA levels observed after dexamethasone or all-trans retinoic acid treatment, whereas simultaneous addition of fenofibric acid together with all-trans retinoic acid or dexamethasone resulted in a superinduction of acyl-CoA oxidase mRNA. Addition of the n-3 polyunsaturated fatty acids (PUFAs), docosanohexaenoic acid and eicosanopentaenoic acid, or the fatty acid derivative alpha-bromopalmitate, decreased apo A-I and increased acyl-CoA oxidase mRNA in a dose-dependent and time-dependent manner, whereas apo A-II mRNA did not change significantly. Nuclear run-on experiments demonstrated that fenofibric acid and alpha-bromopalmitate decreased apo A-I and increased acyl-CoA oxidase gene expression at the transcriptional level. When rats were fed isocaloric diets enriched in saturated fat (hydrogenated coconut oil), n-6 PUFAs (safflower oil) or n-3 PUFAs (fish oil), a significant decrease in liver apo A-I and apo A-II mRNA levels was only observed after fish oil feeding. Compared to feeding low fat, liver acyl-CoA oxidase mRNA increased after fat feeding, but this effect was most pronounced (twofold) in rats fed fish oil. Results from these studies indicate that fish oil feeding reduces rat liver apo A-I and apo A-II gene expression, similar to results obtained after feeding fenofibrate. Fibrates and n-3 fatty acids (and the fatty acid derivative, alpha-bromopalmitate) down-regulate apo A-I and induce acyl-CoA oxidase gene expression through a direct transcriptional action on the hepatocyte. In contrast, only fenofibric acid, but not the other fibrates or fatty acids tested, decrease apo A-II gene expression in vitro.

Effects of fatty acids on hepatic gene expression

Polyunsaturated fatty acids (PUFA) have dramatic effects on hepatic lipid metabolism by regulating the transcription of specific genes encoding enzymes involved in glycolysis and lipogenesis. The S14 gene, a putative lipogenic protein, has been used as a model to define the molecular basis of PUFA action on hepatic gene expression. We have shown that PUFA-regulated hepatic transcription factors target cis-regulatory elements located between -220 and -80 bp upstream from the 5' end of the S14 gene. Peroxisomal proliferators (PP) also have dramatic effects on hepatic lipid metabolism through effects on gene expression. The mechanism of PP action is mediated, at least in part, through nuclear receptors, i.e. PP activated receptor (PPAR). We found that the potent PP, i.e. WY14,643, suppressed mRNAS14 and the activity of an S14CAT fusion gene in cultured primary hepatocytes. Preliminary mapping studies showed that WY14,643 cis-regulatory elements were located either within the S14 proximal promoter (-290 and +19), the S14 TRE (-2900 to -2500) or both regions. Gel shift analysis showed that PPAR did not bind S14 promoter elements. These studies suggest that PUFA- and PP-regulated factors may share common cis-acting elements within the S14 promoter. However, if PUFA control of S14 gene transcription is mediated by PPAR, this mechanism does not involve direct interaction of PPAR with the S14 proximal promoter.

Fatty acids in cell signalling: modulation by lipid binding proteins

Long-chain fatty acids and several of their metabolites have now been shown to be involved as primary or secondary messengers in specific cell signalling pathways. In view of their extremely low aqueous solubility, the extracellular as well as intracellular transport of these compounds is assumed to be facilitated by specific lipid binding proteins, such as cytoplasmic fatty acid-binding protein (FABP). In this paper a survey is given on the biological significance and possible modulatory action of intracellular lipid binding proteins for fatty acid-mediated signal transduction pathways.