Investigating the effects of a high carbohydrate, high fat diet on the ghrelin-serotonin 2C receptor pathway in the brain

Obesity is a global pandemic with more than 13% of the adult population classified as obese and 39% overweight. Strategies to treat and prevent the energy imbalance that causes obesity could be developed by understanding the gene expression and epigenetic changes that drive appetite and energy regulation. Ghrelin is a stomach and brain-secreted peptide hormone that stimulates appetite and energy balance. It is activated by the enzyme ghrelin-O-acyltransferase (GOAT) and is mediated by the ghrelin receptor (GHSR1a). Serotonin (5-HT) is another stomach-secreted hormone that regulates appetite by stimulating post-meal satiety and reducing food intake through binding to the 2C serotonin receptor (5-HT2CR) in the brain. Plasma ghrelin levels are increased and plasma 5-HT levels are decreased by a high fat diet in rats, however the expression of ghrelin, GOAT, GHSR1a and 5-HT2CR by the brain in response to a long term high carbohydrate and high fat diet (HCHF), representative of the ‘Western’ diet, has not been explored. This study investigated the effects of diet and diet composition change (reverting from a HCHF diet back to a standard corn starch (CS) diet) on the expression of the ghrelin and 5-HT2CR pathways in the rat brain using real time reverse transcriptase polymerase chain reaction (RT-PCR) and Western Blot (Immunoblot). This study also investigated whether the HCHF diet could influence the epigenome of human colon cells via changes in the gut microbiome. There was not a consistent upregulation or down-regulation of ghrelin, GOAT or GHSR in response to the HCHF diet however there was a consistent and significant decrease in the expression of both GOAT and GHSR when the diet was reverted back to the standard diet. The expression of 5-HT2CR was more consistent with all rats fed the HCHF diet demonstrating a non-significant decrease in the expression of the 5-HT2CR and three out of four rats demonstrating a non-significant increase in response to reverting back to the standard diet. In order to investigate the effects of the HCHF-induced microbiome population change on colon cells (independent of other diet-induced changes), colon contents from rats fed the HCHF and CS were cultured under anaerobic conditions and then in vitro co-cultured with human colon cells. RT-PCR analysis of the expression of epigenetic modifying enzymes in both groups demonstrated a twelve-fold increase in the expression of deleted in azoospermia (DAZ) interacting zinc finger protein 3 (DZIP3) in response to the HCHF. DZIP3 is a chromatin modifying enzyme and thus may influence changes to the epigenome. In summary, these findings support a role for brain-expressed GOAT and GHSR1a in reducing the ability of ghrelin to stimulate appetite and for brain-expressed 5-HT2CR to induce satiety in the brain when HCHF food is removed. This study also provides pilot data that HCHF can modify the epigenome of colon cells via changes in the gut microbiome