Fructose impairs brown adipogenesis by promoting thyroid hormone resistance in differentiated brown adipocytes

Excessive fructose consumption is increasingly linked to obesity, diabetes, non-alcoholic fatty liver disease (NAFLD), and cardiovascular disease, yet its impact on brown adipocytes remains poorly defined. Here, we investigated how fructose alters brown adipogenesis with a focus on thyroid hormone receptor (THR) signaling. Differentiation of mouse brown preadipocytes in fructose-containing medium resulted in markedly reduced lipid droplet size and number, impaired expression of adipogenic markers (PPARγ, FABP4), and a loss of the brown adipocyte identity protein UCP-1. Whereas triiodothyronine (T3) and the THRβ-selective agonist resmetirom dose-dependently enhanced UCP-1 expression in glucose medium, neither T3 nor resmetirom restored UCP-1 in fructose medium, revealing thyroid hormone resistance. Mechanistically, fructose specifically downregulated THR but not its heterodimer partner, retinoid X receptor (RXR), via ubiquitin-mediated degradation. Overexpression of THR rescued UCP-1 in glucose medium and promoted dynamic lipid turnover but failed to restore UCP-1 or lipid remodeling in fructose medium, indicating a defect in THR transcriptional regulation. Our secretome proteomics revealed that both the sugar source and thyroid hormone receptor (THR) overexpression strongly remodel the secreted protein landscape of brown adipocytes. The data indicate that fructose both reconfigures metabolic and signaling networks and blunts oxidative resilience, while excess THR activity in fructose conditions disrupts energy metabolism and increases the risk of protein misfolding. All these could contribute to THR dysfunction and impair adipogenesis. Collectively, these findings identify fructose as a potent inhibitor of brown adipogenesis, suppressing THR-dependent UCP-1 expression by inducing thyroid hormone resistance through a defect in THR-mediated transcriptional regulation. This mechanism links dietary fructose to impaired brown adipocytes and metabolic imbalance.