Leptin and peroxisome proliferator activated receptor alpha: understanding their contribution towards normalising the programmed phenotype in the peripheral tissues of IUGR offspring
University of Southampton, School of Biological Sciences,
In a rat model of intrauterine growth restriction (IUGR) induced by maternal global undernutrition, adult offspring are obese with associated metabolic disturbances. These metabolic abnormalities are all augmented by feeding a high calorie postnatal diet and reversed by neonatal leptin treatment. Evidence is now accumulating which indicates that altered epigenetic regulation and gene expression may underpin the relationship between the early life environment and metabolic disturbances in adult life. Therefore, to determine the mechanism responsible for the alterations in energy balance in the IUGR rat, this study investigated the effect of maternal diet, neonatal leptin treatment and a postnatal high fat diet on the expression and DNA methylation of genes involved in energy balance in the liver and adipose tissue of adult offspring. These genes included the peroxisome proliferator activated receptors (PPARs) and their target genes; acyl-coA oxidase (AOX), carnitine palmitoyl transferase-1 (CPT-1) and lipoprotein lipase (LPL).
Real time PCR indicated that the expression of several key genes involved in energy balance, including PPAR?, PPAR? and their target genes, was not altered by maternal diet or postnatal diet in the liver or adipose tissue of these offspring. However, in adipose tissue, neonatal leptin treatment resulted in an increase in the expression of most genes tested, including PPAR?, PPAR? and their target genes. The increased PPAR? and LPL would facilitate the uptake of fatty acids into the adipocyte, whilst the upregulation of PPAR? and its target genes AOX and CPT-1, not normally expressed in adipocytes, would direct fatty acids taken up towards the ?-oxidation pathway instead of storage. This would imply that the fat cell had transformed from a fat storing cell to a fat metabolising cell. Gene expression data therefore indicated that the phenotypic changes induced by neonatal leptin treatment, i.e. the reduced weight gain, could be due to increased expression of PPAR?, PPAR? and their target genes in adipose tissue: Furthermore, the effects of this are persistent, due to the specific period of leptin administration during neonatal development. There was, however, no evidence of altered DNA methylation in the promoter regions measured which could account for these persistent effects.
To investigate mechanisms underlying the regulation of the PPAR? promoter by leptin, the rat PPAR? promoter was mapped, cloned and characterised. As part of this process, six alternatively spliced variants were identified; one from adipose tissue (P1), two from the liver (P2, P3), one from the heart (P4) and two from the kidney (P5, P6). These transcripts were found to differ in their 5’untranslated region due to tissue specific promoter usage and alternative transcription start sites. The liver and adipose specific promoters were cloned and characterised using a reporter gene strategy. They were shown to differ in their basal activity, response to known activators of transcription and to neonatal leptin treatment. The regulation of the PPAR? promoter by leptin was investigated and shown to function via a non-canonical mechanism requiring both signal transducer and activators of transcription (Stat3) and specificity protein-1 (Sp1), which act at a unique region of the liver specific P2 promoter. The adipose specific P1 promoter was shown to be unresponsive to leptin treatment. Furthermore, real time PCR with primers specific to the P1 and P2 PPAR? transcripts indicated that the increased PPAR? expression seen in leptin treated offspring was due to an increase in the P2 specific transcript, not the P1 transcript. This indicated that the neonatal leptin treatment facilitated a selective switch in promoter usage to increase the expression of PPAR? and its target genes in a tissue in which they are not normally expressed, thus inducing an altered metabolism within the adipocytes of these offspring.
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