BATokines as Biomarkers of Brown Fat in Humans

Overview

Brown adipose tissue (BAT) burns excess calories to produce heat in response to environmental cold. Rapidly growing evidence from rodent and human studies suggests that the presence and activation of brown fat are far more beneficial for whole body metabolism and cardiometabolic health than previously appreciated. Despite the clear associations between brown fat and metabolic health, we lack both: cost-effective means of detecting brown fat in humans as well as comprehensive insights into how brown fat facilitates metabolism on a molecular level in humans. Emerging evidence suggests that the benefits of brown fat activation are mediated, at least in part, by secretion of specific molecules into the bloodstream which signal to metabolically active organs such as skeletal muscle, liver and brain. A number of these so-called brown adipokines (or BATokines) have now been discovered in mice and shown to positively impact glucose homeostasis, liver and muscle function. Human deep-neck brown fat biopsies reveal that \>1000 molecules could potentially be secreted from brown fat, and \>400 are released by human brown fat cells in a dish, representing a major opportunity for discovery of high translational value. Here, we aim to identify a screen of first potential blood biomarkers of brown fat in healthy young humans. This will be achieved by analyzing plasma proteins in subjects with 'inactive brown fat' (warm) and 'activated brown fat' (3-hr cold exposure, cooling vests) using high-throughput technologies (SOMAscan and O-link) to identify temperature-sensitive brown fat-enriched molecules. This preliminary data will guide a larger follow up study in which we envision studying lean and obese (insulin sensitive and insulin resistant) subjects of various age groups and race/ethnicity. Human BATokines identified here will become primary targets for manipulation in experimental animals to assess their therapeutic potential against obesity, T2D, and associated diseases. Additionally, since current methods of brown fat detection in human rely on deep neck biopsies or costly 18-FDG-PET/CT scans, identification of blood biomarkers of brown fat would offer a cost-effective and non-invasive alternative for prediction of metabolic health in humans.

Fat plays a fundamental role in regulating metabolic health due to its capacity to store excess calories (white adipose tissue; WAT) or burn them to produce heat (brown adipose tissue, BAT). While white fat cells are located in deposits beneath the skin (subcutaneous) or around the internal organs (visceral) and expand with obesity, brown fat cells reside predominantly in the deep neck area and have evolved to turn fuel into heat (a process termed thermogenesis) when activated by environmental cold. This calorie-burning property of BAT suggest it is a type of 'good fat' which may protect us from conditions such as obesity. A wealth of mouse data confirms that activation of BAT via cold exposure provides significant health benefits to experimental animals, including protection from diet-induced obesity and improved glucose metabolism. BAT has only recently been detected in adult humans, which has led to efforts to understand its physiology and investigate whether its activity can be manipulated to treat metabolic diseases. Notably, in addition to storing or burning fat, adipose tissue secretes a large number of hormone-like factors into the bloodstream, known to affect blood glucose levels and insulin sensitivity (adiponectin, adipsin) as well as appetite and energy expenditure (leptin). Although many such factors have been identified in white fat, little is known about the proteins secreted from brown fat, particularly in humans. In a recent study of 52,000 patients with 18-FDG-PET/CT scans to identify the presence of brown fat, we found that BAT+ individuals had significantly reduced odds of type 2 diabetes (T2D), coronary artery disease, and congestive heart failure, compared to matched individuals who did not have brown fat. A smaller independent study demonstrated that short-term activation of BAT with cooling jackets significantly improved insulin sensitivity in T2D patients. These findings collectively reveal that BAT carries extraordinary potential to impact metabolic health in humans. Metabolic benefits linked with BAT cannot be explained by the generation of heat (thermogenesis) alone. Emerging evidence suggests that BAT secretes specific molecules into the bloodstream. A number of these so-called brown adipokines (or BATokines) have now been discovered in mice and shown to positively impact whole body glucose metabolism and liver function, reducing the susceptibility to metabolic disease. However, the 'secretome' of brown fat has not been well studied in humans, and hence the relevance of these BATokines to our physiology remains unclear. In support of this unexplored research avenue, gene expression data from deep-neck BAT biopsies reveal that \>1,000 molecules could potentially be secreted from BAT in humans, and 431 were identified as being released from human BAT cells in a dish, but a comprehensive validation of these proteins in blood samples of participants with activated BAT is lacking. The aim of this study is to identify a panel of plasma proteins induced by activation of brown fat in young healthy human participants and provide pilot data for a larger biomarker study. We will employ novel unbiased multiplex tools to identify up to 7,000 unique proteins of various abundances, in subjects before and after cooling. Using these data, we will be able to identify common circulating factors that correlate with BAT activation, and subsequently compare them with pre-existing gene expression data to find polypeptides secreted, shedded or otherwise released specifically by BAT. While this study will focus on the identification of proteins, we also have the potential to survey small molecule metabolites secreted into the circulation. This list of BATokines will become instrumental for validation and retro-translation of mouse data from our lab and the larger scientific community interested in the metabolic benefits conveyed by BAT. In addition to the discovery of potential therapeutic targets, the blood screen may become a valuable data platform for clinical biomarkers of brown fat. Since current methods of BAT detection in human rely on deep neck biopsies and/or costly 18-FDG-PET/CT scans, identification of blood factors that circulate proportionally to BAT activity or correlate with the presence or amount of BAT would offer a cost-effective, non-invasive alternative in human participants.