Since obtaining my PhD in 1982 I have maintained continuous funding for research into aspects of lipid and lipoprotein metabolism. To date this has amounted to over £3 million, in addition to my association with the two successful bids for equipment totalling approximately £4 million from the Joint Infrastructure Fund.
In recent years my research interests have focused on the mechanisms whereby dietary lipids regulate lipid and lipoprotein metabolism. Using the Golden Syrian Hamster as a model of human lipoprotein metabolism we have demonstrated that the effects of specific fatty acids on plasma lipoprotein concentrations is associated with changes in the expression of key genes in the liver (1-4). We have also shown interactive effects of dietary fatty acids and cholesterol in modulating lipoprotein metabolism (5). More recently we have focused on the specific mechanisms whereby fatty acids and cholesterol regulate gene expression. We have just completed a series of experiments investigating the regulation of the expression of the microsomal triglyceride transfer protein (MTP). MTP plays an essential role in the assembly of Very Low Density Lipoprotein (VLDL) and we have previously shown its expression to be regulated by fatty acids and cholesterol (2-3). Our analysis of the MTP promoter indicates the presence of three putative DR1 transcription factor binding sites. Such sites are capable of binding a range of transcription factors including the peroxisomal proliferator activated receptors (PPARs) and hepatic nuclear factor 4 (HNF4). Using reporter gene-promoter constructs in which each of the sites are mutated we have demonstrated that two of these sites are regulated by over-expression of both HNF4 and PPARs (alpha and gamma). Electrophoretic mobility shift assays have demonstrated the ability of both sites to bind both classes of transcription factor and chromatin immunoprecipitation has suggested binding of both PPARa and HNF4 to the MTP promoter in rat hepatocytes. Our current hypothesis is that while the interaction of HNF4 with the MTP promoter is essential for basal transcription of the MTP gene, PPARs may play a significant role in metabolic/dietary regulation of the expression of the gene. Some of this data, has been recently presented at conferences (6-7) and a full manuscript is currently in preparation.
Another major area of current research in my laboratory has been in the role of sterol regulatory element binding proteins in mediating the effects of dietary lipids on expression of genes involved in lipid and lipoprotein metabolism. The three SREBP isoforms are believe to be master regulators of the expression of genes involved in lipid metabolism (8) Conventionally the SREBP1c isoform is believed to be involved in regulating lipogenic gene expression while SREBP2 regulates expression of genes associated with cholesterol metabolism (including the LDL receptor). SREBP 1a has been suggested to influence the expression of both sets of genes though its expression in liver is very low. However, recent work from our laboratory has suggested that the SREBPs may be more "promiscuous" than previously believed. We have preliminary evidence to suggest that diets rich in saturated fatty acids and cholesterol may down-regulate the expression of some lipogenic genes (fatty acid synthase (FAS) and acetyl CoA carboxylase (ACC)) but not others (stearoyl CoA desaturase-1 (SCD1) and lipoprotein lipase (LPL)) through an interaction of their promoters with SREBP2. In fact, the expression of SCD and LPL is up-regulated by such diets, an effect we attribute to another transcription factor, the Liver X receptor (LXR). It is of note that the promoters of all four of these genes containing sterol regulatory protein and LXR binding sites and we will shortly submit a grant application to further investigate the differential regulation of their expression and promiscuity of the SREBPs.
We have also being investigating the effects of conjugated linoleic acid (CLAs) on lipid and lipoprotein metabolism in the hamster. Recently (manuscript in preparation) we have looked at the impact of low dose dietary supplementation with pure CLA isomers, against a background of both low and high dietary fat. These studies confirm a modest effect of the t10, c12-CLA isomer in reducing adipose tissue deposition and suggest that the mechanism may, at least be in part, through a specific SREBP1c mediated down-regulation of adipose tissue LPL expression.
Working in an environment which is at the interface of animal science and human nutrition has provided the opportunity for collaborations investigating factors influencing the nutritional quality of animal products. In one such project the ovine Stearoyl Coenzyme A desaturase (SCD) gene, responsible for the production of monounsaturated fatty acid, was cloned (10). We then looked at the impact of specific hormones and nutrients on the expression of this gene and how this influences the fatty acid composition of sheep tissues (11-13). It has now become clear that SCD may play an important role in regulating overall fat deposition in animals and inhibition of SCD activity has suggested as a possible treatment for obesity. However, our recent work in the hamster has suggested that this may be associated with potentially adverse effects on plasma lipoprotein and hepatic function (submitted for publication).
My interest in the impact of dietary fatty acids on lipoprotein metabolism has led to a significant collaboration with Professor Dale Bauman (Cornell University) and Dr Adam Lock (University of Vermont). We have shown that butter, produced from milk specifically enriched in c9, t11-CLA (rumenic acid, RA) reduces the concentration of atherogenic lipoproteins in the plasma of cholesterol -fed hamsters compared to standard butter (15). This was despite the fact that such milk was also enriched in the trans monoeic fatty acid, vaccenic acid (t11C18:1, VA). This led to a follow-up study comparing the effects of VA with elaidic acid (t9C18:1, EA) and partially hydrogenated vegetable oil (PHVO). Surprisingly, both pure trans isomers, reduced cholesterol compared with PHVO. This effect was associated with a relative down-regulation of hepatic LDL receptor expression. This work will be submitted for publication within the next month.
An increasing focus of my research is the impact of the nutritional environment encountered during fetal life, as a determinant of lifelong metabolic capacity and risk of disease. We have shown that a low protein diet in pregnancy leads to hypertriglyceridemia and insulin resistance in the offspring at 18 months of age (16). These phenotypic changes were accompanied by age-related changes in mRNA and protein expression of the transcription factors SREBP-1c, ChREBP, PPAR a and PPARg and their respective downstream target genes, ACC-1, FAS, L-PK and MCAD. The findings indicate that prenatal protein restriction programmes development of a metabolic syndrome-like phenotype that develops only with senescence. More recently we have shown that a similar low protein maternal diet leads to an approximate doubling of the extent of atherosclerotic lesions in the aorta of apoE*3 Leiden transgenic mice (a strain known to develop diet -dependent atherosclerosis) offspring fed for 3 months on an atherogenic diet. This was associated with increases in both plasma cholesterol and triacylglycerol. Using an atherosclerosis array we have identified a number changes in gene expression in these offspring, including down-regulation of the hepatic LDL receptor. This work has been submitted for publication. A similar experiment looking at the impact of a maternal high fat diet (designed to have a similar fatty acid composition to a typical "Western" diet) is nearing completion. In collaboration with Dr Adam Lock (University of Vermont) we have applied for funding (USDA) to investigate the impact of different sources (dairy vs hydrogenated fat) of trans fatty acids in the maternal diet. Further applications to investigate the molecular mechanisms underlying these effects and the potential for using protective fatty acid (n-3 PUFA, CLA) to reverse such "fetal programming" are under preparation.
1) Bennett, AJ, Billett, MA, Salter, AM, Mangiapane, EH, Bruce, JS, Anderton, KL, Marenah, CB, Lawson, N & White, DA (1995) Modulation of hepatic apolipoprotein-b, 3-hydroxy-3-methylglutaryl-coa reductase and low-density-lipoprotein receptor messenger-RNA and plasma-lipoprotein concentrations by defined dietary fats - comparison of trimyristin, tripalmitin, tristerin and triolein. Biochem J 311, 167-173
2) Bennett, AJ, Billett, MA, Salter, AM & White, DA (1995) regulation of hamster hepatic-microsomal triglyceride transfer protein messenger-RNA levels by dietary fats. Biochem. Biophys. Res. Commun. 212, 473-478
3) Bennett, AJ, Bruce, JS, Salter, AM, White, DA & Billett, MA (1996) Hepatic microsomal triglyceride transfer protein messenger RNA concentrations are increased by dietary cholesterol in hamsters. FEBS Lett. 394, 247-250
4) Salter, AM, Mangiapane, EH, Bennett, AJ, Bruce, JS, Billett, MA, Anderton, KL, Marenah, CB, Lawson, N & White, DA (1998) The effect of different dietary fatty acids on lipoprotein metabolism: concentration-dependent effects of diets enriched in oleic, myristic, palmitic and stearic acids. Br. J. Nutr. 79, 195-202
5) Billett,MA, Bruce, JS, White,DA, Bennett,AJ, Salter,AM (2000) Interactive effects of dietary cholesterol and different saturated fatty acids on lipoprotein metabolism. Br J Nutr 84, 439-447
6) Vallim, T. Salter, A & Bennett, A (2006) Transcriptional regulation of the microsomal triacylglycerol transfer protein by members of the nuclear hormone receptor superfamily. Nutrition Society Summer Meeting, Aberdeen, 2006 Abstract OC037
7) Vallim, T. Salter, A & Bennett, A (2007). Transcriptional control of the microsomal triglyceride transfer protein by members of the nuclear hormone receptor superfamily. Keystone Symposium-Nuclear Receptor Pathways to Metabolic Regulation, 2007 Poster #323
8) Salter AM & Tarling E (2007) Regulation of gene transcription by fatty acids. Animal 1: 1314-1320
9) Ward, RJ, Travers, MT, Richards, SE, Vernon, RG, Salter, AM, Buttery, PJ & Barber, MC (1998) Stearoyl-CoA desaturase mRNA is transcribed from a single gene in the ovine genome. Biochim. Biophys. Acta-Lipids Lipid Metab. 1391, 145-156
10) Daniel,ZCTR, Richards,SE, Salter,AM & Buttery, PJ (2004) Insulin and dexamethasone regulate stearoyl-CoA desaturase mRNA levels and fatty acid synthesis in ovine adipose tissue. J Anim Sci 82, 231-237
11) Daniel,ZCTR, Wynn,RJ, Salter,AM & Buttery,PJ (2004) Differing effects of forage and concentrate diets on the oleic acid and conjugated linoleic acid content of sheep tissues: The role of stearoyl-CoA desaturase. J Anim Sci 82, 747-758
12) Daniel,ZCTR, Salter,AM, Buttery,PJ (2004) Vitamin A regulation of stearoyl-Co A desaturase mRNA levels and fatty acid composition in sheep tissues. Anim Sci 78, 237-243
13) Lock AL, Horne CA, Bauman DE, Salter AM (2005) Butter naturally enriched in conjugated linoleic acid and vaccenic acid alters tissue fatty acids and improves the plasma lipoprotein profile in cholesterol-fed hamsters. Journal of Nutrition, 135:1934-1939
14) Erhuma, A, Salter, AM, Sculley, DV, Langley-Evans, SC & Bennett, A. (2007) Prenatal exposure to a low protein diet programmes disordered regulation of lipid metabolism in the ageing rat. Am J Physiol. 292: E1702-E1714
15) Yates Z, Tarling EJ, Langley-Evans, SC and Salter, AM (2008) Maternal undernutrition programmes atherosclerosis in the apo E*3 Leiden mouse. Submitted to Circulation