Housing at 27 °C, a Near-Thermoneutral Temperature, Exacerbates Metabolic Dysfunction-Associated Steatohepatitis in C57BL/6J Mice.

Metabolic dysfunction-associated steatohepatitis (MASH), a progressive form of metabolic dysfunction-associated steatotic liver disease, affects approximately 3-5% of the global population and lacks effective therapies. This study examined the impact of near-thermoneutral housing on MASH progression in mice fed a fast-food diet (FFD) enriched in fat, sucrose, and cholesterol. Male C57BL/6J mice were housed under standard (22 °C) or near-thermoneutral (27 °C) conditions and fed chow or FFD for 20 weeks. Body weight, composition, liver pathology, hepatic gene expression, plasma lipids, and adipose thermogenic and inflammatory markers were assessed. FFD increased body weight, fat mass, and lean mass at both temperatures. Near-thermoneutral housing markedly exacerbated MASH in FFD-fed mice, with increased steatosis, fibrosis, inflammation, and elevated liver enzymes. Hepatic lipogenic and fibrosis-related gene expression was upregulated, alongside higher plasma total cholesterol and very-low-density lipoprotein levels. Near-thermoneutral housing also impaired brown adipose tissue thermogenesis, reducing uncoupling protein 1 and other thermogenic genes, and modestly altered inguinal white adipose tissue inflammation and thermogenic signaling. These findings demonstrate that near-thermoneutral housing accelerates MASH progression, accompanied by increased hepatic lipid accumulation, fibrosis, and inflammation and reduced adipose thermogenic signatures. These findings identify environmental temperature as a critical modifier of MASH pathophysiology and suggest that enhancing adipose thermogenesis may offer therapeutic potential.