Restricting glycolysis impairs brown adipocyte glucose and oxygen consumption

Abstract

During thermogenic activation, brown adipocytes take up large amounts of glucose. In addition, cold stimulation leads to an upregulation of glycolytic enzymes. Here we have investigated the importance of glycolysis for brown adipocyte glucose consumption and thermogenesis. Using siRNA-mediated knockdown in mature adipocytes, we explored the effect of glucose transporters and glycolytic enzymes on brown adipocyte functions such as consumption of glucose and oxygen. Basal oxygen consumption in brown adipocytes was equally dependent on glucose and fatty acid oxidation, whereas isoproterenol (ISO)-stimulated respiration was fueled mainly by fatty acids, with a significant contribution from glucose oxidation. Knockdown of glucose transporters in brown adipocytes not only impaired ISO-stimulated glycolytic flux but also oxygen consumption. Diminishing glycolytic flux by knockdown of the first and final enzyme of glycolysis, i.e., hexokinase 2 (HK2) and pyruvate kinase M (PKM), respectively, decreased glucose uptake and ISO-stimulated oxygen consumption. HK2 knockdown had a more severe effect, which, in contrast to PKM knockdown, could not be rescued by supplementation with pyruvate. Hence, brown adipocytes rely on glucose consumption and glycolytic flux to achieve maximum thermogenic output, with glycolysis likely supporting thermogenesis not only by pyruvate formation but also by supplying intermediates for efferent metabolic pathways.

Brown adipocyte glucose metabolism: a heated subject

“…In line with this, in subsequent studies, BAT 18 F‐FDG uptake in response to a beta‐3 adrenergic receptor agonist in UCP1 knockout mice was fully retained despite defective BAT thermogenesis upon the same pharmacological treatment . Furthermore, independent experiments on isolated brown adipocytes lacking UCP1 have all corroborated the in vivo imaging results . These findings clearly show that BAT glucose uptake can be dissociated from UCP1‐mediated thermogenesis.…”

Mitochondrial pyruvate carrier is required for optimal brown fat thermogenesis

“…In addition, it was previously recognized that cold exposure could lower blood glucose levels in adults (Martineau and Jacobs, 1989). The role of glucose uptake on metabolism has been explored in vitro using immortalized brown adipocytes where siRNAs targeting GLUT1, GLUT4, hexokinase, or pyruvate kinase (enzymes catalyzing the first and the last step of glycolysis) demonstrated the importance of glycolysis, as β3-AR agonist failed to increase glucose uptake and oxygen consumption in these cells (Winther et al, 2018). However, there is no adequate in vivo model demonstrating the importance of BAT glycolysis or glucose oxidation on adaptive thermogenesis.…”

Pyruvate kinase M2 represses thermogenic gene expression in brown adipocytes

“…Whole‐cell extracts were prepared as described . Glyceraldehyde 3‐phosphate dehydrogenase (GAPDH) and transcription factor II B (TFIIB) were both used as loading controls in Fig.…”