Apoptotic brown adipocytes enhance energy expenditure via extracellular inosine

Abstract

Brown adipose tissue (BAT) dissipates energy1,2 and promotes cardiometabolic health3. Loss of BAT during obesity and ageing is a principal hurdle for BAT-centred obesity therapies, but not much is known about BAT apoptosis. Here, untargeted metabolomics demonstrated that apoptotic brown adipocytes release a specific pattern of metabolites with purine metabolites being highly enriched. This apoptotic secretome enhances expression of the thermogenic programme in healthy adipocytes. This effect is mediated by the purine inosine that stimulates energy expenditure in brown adipocytes by the cyclic adenosine monophosphate–protein kinase A signalling pathway. Treatment of mice with inosine increased BAT-dependent energy expenditure and induced ‘browning’ of white adipose tissue. Mechanistically, the equilibrative nucleoside transporter 1 (ENT1, SLC29A1) regulates inosine levels in BAT: ENT1-deficiency increases extracellular inosine levels and consequently enhances thermogenic adipocyte differentiation. In mice, pharmacological inhibition of ENT1 as well as global and adipose-specific ablation enhanced BAT activity and counteracted diet-induced obesity, respectively. In human brown adipocytes, knockdown or blockade of ENT1 increased extracellular inosine, which enhanced thermogenic capacity. Conversely, high ENT1 levels correlated with lower expression of the thermogenic marker UCP1 in human adipose tissues. Finally, the Ile216Thr loss of function mutation in human ENT1 was associated with significantly lower body mass index and 59% lower odds of obesity for individuals carrying the Thr variant. Our data identify inosine as a metabolite released during apoptosis with a ‘replace me’ signalling function that regulates thermogenic fat and counteracts obesity.

A purinergic call to arms from dying brown adipocytes

“…In principle, this can be achieved by reversing the cellular scenecence, or cessation of cell division, that occurs in thermogenic adipocyte precursors during aging [13]. Writing in Nature, Niemann et al [14] performed a comprehensive set of experiments starting with the relatively neglected subject of apoptosis, or programmed cell death, in thermogenic brown adipocytes. Remarkably, it took just 3 days of inactivating brown adipose tissue thermogenesis by housing mice at their thermoneutral temperature of 30 o C to detect apoptotic cells.…”

“…While apoptosis has been shown to remain elevated in brown adipocytes after prolonged periods of thermoneutrality (4 weeks) [15], it would have been interesting to see if this is also the case in diet-induced obese mice as well as in aged mice. Niemann et al [14] next asked the intriguing question of what metabolites apoptotic brown adipocytes could release. Through a combination of targetted and untargetted metabolomic approaches, 3 purine metabolites (AMP, inosine and hypoxanthine) were identified to be at 5-10-fold higher concentrations in the media of brown adipoyctes made apoptotic by radiation (UV light) or ________________________ Comment on Niemann B, Haufs-Brusberg S, Puetz L, Feickert M, Jaeckstein MY, Hoffmann A, Zurkovic J, Heine M, Trautmann EM, Müller CE, Tönjes A, Schlein C, Jafari A, Eltzschig HK, Gnad T, Blüher M, Krahmer N, Kovacs P, Heeren J, Pfeifer A (2022).…”

“…Nevertheless, the data strongly support the idea that the increased release of ATP from dying brown adipocytes serves as the starting point for increased extracellular inosine production. Niemann et al [14] then proceeded by asking if the metabolites released from apoptotic brown adipocytes communicate with neighbouring healthy brown adipocytes in a paracrine fashion. To do so, an elegant experiment was designed in which the media of apoptotic brown adipocytes was applied onto healthy brown adipocytes.…”

“…Consistent with the increase in PKA signalling caused by inosine in brown adipocytes, its thermogenic effects were lost in A2A and A2B receptor knockout mice. Having established a clear acute thermogenic effect of inosine, Niemann et al [14] then asked if chronic inosine treatment promotes a negative energy balance. Two approaches were taken in which inosine was administered for 4 weeks to mice either before or after the onset of highfat diet-induced obesity.…”

“…These results suggest that inosine mainly targets peripheral tissues to achieve its effects on body weight. At this junction of the manuscript, Niemann et al [14] were ideally placed to ask how extracellular levels of inosine are regulated by adipocytes. Instead of focusing on Pannexin1 channels that mediate ATP release by apoptotic cells [16], focus was placed on the equilibrative nucleoside transporters 1 and 2 (ENT 1 and 2, respectively).…”