A new era of understanding in vivo metabolic flux in thermogenic adipocytes

Abstract

Nonshivering thermogenesis by brown adipose tissue (BAT) is an adaptive mechanism for maintaining body temperature in cold environments. BAT is critical in rodents and human infants and has substantial influence on adult human metabolism. Stimulating BAT therapeutically is also being investigated as a strategy against metabolic diseases because of its ability to function as a catabolic sink. Thus, understanding how brown adipocytes and the related brite/beige adipocytes use nutrients to fuel their demanding metabolism has both basic and translational implications. Recent advances in mass spectrometry and isotope tracing are improving the ability to study metabolic flux in vivo. Here, we review how such strategies are advancing our understanding of adipocyte thermogenesis and conclude with key future questions.

Figure 1.. Overview of Brown Fat Anatomy and Thermogenesis

(A) Model depicting major brown adipose tissue (BAT) depots in mice. (B) UCP1-dependent Thermogenesis occurs in mitochondria. Nutrients such as glucose and lipids are mainly converted to acetyl-CoA, which powers the TCA cycle and the electron transport chain. Brown adipocyte mitochondria contain UCP1 on the inner mitochondrial membrane, which dissipates the electrochemical proton gradient to generate heat. Emerging roles for BCAAs, succinate, and glutamine as thermogenic fuels have been reported but are less understood. (C) Brown adipocyte dynamics. Brown adipocyte morphology dramatically changes with temperature. At thermoneutrality, brown adipocyte lipid droplets coalesce and UCP1 is low. Upon acute cold, lipid droplets start to breakdown as UCP1 rises. Most mice are maintained in cool-adapted (room temperature) conditions, in which BAT is active, has medium to high levels of UCP1, and lipid droplets multi-locular and of variable sizes. With more severe cold adaptation, lipid droplets persist but are much smaller and more uniform in size, and UCP1 is at its highest expression level.

Figure 2.. Overview of arteriovenous (AV) sampling.

(A) AV metabolomics blood sampling strategy. Blood is collected from both the artery nourishing a tissue and a major vein draining the tissue. Subtracting a metabolite’s venous concentration from arterial concentration can be used to determine the net uptake and release of a metabolite, and when corrected for blood flow rate, metabolic flux of a metabolite across a tissue can be inferred. By performing AV metabolomics across many tissues, evidence of inter-organ metabolite exchange can also be obtained. (B) In mice, BAT venous blood can be obtained from the Sulzer’s vein, and arterial blood from the left ventricle. (C) Incorporating recent metabolomics, stable isotope tracing, and arteriovenous metabolomics data with existing studies, a model emerges indicating that glucose, when available, is the preferred BAT fuel. In fed mice eating a standard chow diet (in which carbohydrates are plentiful), acute cold stimulates a moderate uptake of circulating nutrients into BAT because it mainly relies on its intracellular lipid stores for fuel. During chronic cold adaptation, glucose and lactate are more important sources of carbon, while amino acid uptake also greatly increases relative to the other conditions, providing an additional carbon source and a major source of increased nitrogen. Lipid uptake is relatively similar across these conditions. (D) However, BAT must be flexible in its fuel utilization. When mice are fasting or consuming a lipid rich diet i.e., when glucose is less available, a greater proportion of BAT carbon flux comes from lipids. It is important that BAT maintain fuel flexibility to defend body temperature regardless of nutrient availability.

Figure 3.. Potential advantages of using a glucose-based fuel economy during prolonged adaptive thermogenesis.

(A) Left - During acute cold exposure, intracellular lipid droplets are a major source of acetyl-CoA for the TCA cycle and uncoupled respiration while glucose uptake mildly increases. Glucose derived pyruvate could contribute to the acetyl-CoA pool, boosting the TCA cycle and ROS production. Alternatively, or in addition, with such a large influx of mitochondrial acetyl-CoA coming from fatty acid oxidation, there is evidence to suggest that pyruvate could also be used anaplerotically by pyruvate carboxylase (PC) to make oxaloacetate (OAA) that could help support the citrate synthase reaction (which catalyzes the condensation of acetyl-CoA with OAA) or other OAA fates, such as aspartate production. Notably, there could be different populations of mitochondria, some that use pyruvate in the pyruvate dehydrogenase reaction, some that use the PC reaction (see Outstanding Questions). Regardless, acute BAT thermogenesis may be more dependent overall on lipid oxidation and UCP1-dependent heat production. Right - During chronic cold adaptation, brown fat metabolism may reprogram to a more glucose-based fuel economy if glucose is plentiful. There are several advantages of doing this: (1) Glycolysis is a significant source of ATP; (2) glycolytic intermediates support the pentose phosphate pathway, a source of NADPH, GSH and nucleotide precursors; (3) glycolytic intermediates support glycerol-3-phosphate production, which provides electron donors and precursors for phospholipids and TAGs; (4) mitochondrial exported metabolites can be used in anabolic reactions--one example being citrate, which supports de novo synthesis of lipids for oxidation, signaling metabolites, and lipid building blocks; (5) interestingly, many of the pathways that use glucose also contain an ATP hydrolysis step, which collectively may provide a significant source of UCP1-independent heat production. This could lower the overall demand for oxygen and relieve pressure on the mitochondria, preventing excessive ROS and organelle damage. (B) The conventional view of BAT thermogenesis is that heat is mainly produced via UCP1. Several studies suggest that UCP1-indepenent pathways may also contribute to heat production in certain cases. One interesting possibility is that during chronic cool or cold adaptation, BAT increases the use of UCP1-independent pathways because it provides better fuel economy and supports homeostatic processes important to the overall health of the tissue.