Poor Nutrition Leading to Potential Clinically Relevant Pathologies: The Role of Epigenetics and Myostatin Signalling

Abstract

The increasingly prevalent 'Western style' diet is typified by an excessive intake of high caloric foods with little or poor nutritional value. Notably, the branched-chain amino acids (BCAAs) and free fatty acids (FFAs) micronutrients have been linked with diet induced progression of metabolic syndrome. Branched-chain amino acids concentrations are increased in the plasma of obese and type 2 diabetes mellitus (T2DM) patients and may be potential inducers of initial hyperglycemia and insulin resistance (IR). Specifically, the BCAA leucine (leu) has been highlighted as potentially significant for stimulating the secretion of insulin and promoting IR as a consequence. However, seemingly contradictory evidence suggests that leu may also be protective against the development of IR and T2DM. By examining the molecular mechanism underpinning the effects of leu on hepatic glucose metabolism (the central physiological mediator of systemic glucose homeostasis) I have arrived at a potentially parsimonious explanation allowing for a reconciliation of the many disparate observations about leu that exist in this literature. Specifically, leu supplementation is initially linked with a beneficial hepatic sequestration of 'circulating‘ glucose. However, this results in a consequential hepatic accumulation of triglycerides (TGs), through the activation of the glucose transporter solute carrier family 2 (SLC2A2), its transcriptional factor forkhead box A2 (FOXA2) and downstream genes. Thus, as a consequence of persistent dietary intake, the resulting metabolic ‗trade-off‘ in which enhanced glucose uptake in the shorter term leads to elevated TGs in the longer term, could account for the pathological appearance of intra-hepatic fat droplets and progression of fatty liver disease that is often associated with IR and metabolic syndrome. Notably, my studies link leu supplementation with intracellular signalling through the key metabolic regulator myostatin (MSTN) and the associated AMP-kinase (AMPK). There is a growing body of evidence that MSTN activity enhances systemic insulin sensitivity and prevents obesity. Further, I have identified a strong epigenetic component to this regulation and specifically, identify miR-143, miR-92b* and miR-335 as instrumental to the leu-dependent induction of hepatic MSTN, SLC2A2 and FOXA2. I have also explored the molecular basis of hepatic metabolic shifts resulting from free fatty acid (palmitate) supplementation. Multiple clinical studies identify palmitate as likely the primary FFA contributing to obesity driven by dietary excess. Again MSTN dependent signalling is found to be central to these responses, as are specific miRNA species, and in particular miR-335. A key terminal effector in this signalling cascade is the hepatokine fetuin A (FetA), which is linked pathologically with clinical obesity and promotes peripheral IR by reducing insulin sensitivity. My in vitro work establishes a mechanistic link between FFA supplementation and hepatokine induction, a central element of which is the epigenetic regulation of MSTN. To further test the validity of these latter observations and in an attempt to translate these lab bench observations to the clinical setting, I have also explored these pathways in the context of pre and post bariatric surgery. Currently the clinical intervention of last resort for morbid obesity, bariatric surgery encompasses a range of surgical procedures designed to lower dietary intake. Whilst each surgical approach results in similar levels of initial weight loss, the longer term outcomes would seem to vary particularly with regard to the remission of metabolic syndrome and associated complications. The current consensus from prospective studies of post surgery T2DM remission suggest that gastric bypass (GBP) is superior to other modalities such as sleeve gastrectomy (SG), However, relatively little is known about the mechanistic basis of the metabolic shifts resulting from these types of surgery, although FetA hepatokine expression has been described as a possible biomarker of efficacy. I have defined a specific set of circulating miRNAs for which post-surgery expression is strongly correlated with both decreased body mass index (BMI) and glycated hemoglobin (HbA1c) level, but only following GBP and not SG. Of these, miRNA-355 is one of the principal regualtors of MSTN that I had identified in vitro in the context of MSTN dependent FetA expression following palmitate supplementation, suggesting that epigenetically regulated hepatokine expression may indeed be of clinical significance to the physiological changes initiated by GBP surgery at least. It remains to be seen why similar changes are not observed with SG, but clearly implies mechanistic differences in the biological responses initiated by the different surgical approaches. Collectively, the work presented herein illustrates the centrality of epigenetic regulation and of the MSTN-dependent signalling axis in underpining pivotal metabolic shifts invoked by two distinct micronutrient stimuli (BCAA and FFA), each of which is linked with deleterious consequences of dietary excess. Further, the relevance of these findings to understanding the observed features of metabolic syndrome is encouraged by the first steps that have been taken to recapitulate the initial in vitro observations in the context of clinical patients.