Alternative Complement Pathway Regulates Thermogenic Fat Function via Adipsin and Adipocyte-derived C3aR1 in a Sex-dependent Fashion

Thermogenesis in beige/brown adipose tissues can be leveraged to combat metabolic disorders such as type 2 diabetes and obesity. The complement system plays pleiotropic roles in metabolic homeostasis and organismal energy balance with canonical effects on immune cells and non-canonical effects on non-immune cells. The adipsin/C3a/C3aR1 pathway stimulates insulin secretion and sustains pancreatic beta cell mass. However, its role in adipose thermogenesis has not been defined. Here, we show that Adipsin knockout mice exhibit increased energy expenditure and white adipose tissue (WAT) browning. C3a, a downstream product of adipsin, is generated from complement component 3 and decreases Ucp1 expression in subcutaneous adipocytes. In addition, adipocyte-specific C3aR1 knockout male mice show enhanced WAT thermogenesis and increased respiration. In stark contrast, adipocyte-specific C3aR1 knockout female mice display decreased brown fat thermogenesis and are cold intolerant. Female mice express lower levels of Adipsin in thermogenic adipocytes and adipose tissues than males. C3aR1 is also lower in female subcutaneous adipose tissue than males. Collectively, these results reveal sexual dimorphism in the adipsin/C3a/C3aR1 axis in regulating adipose thermogenesis. Our findings establish a newly discovered role of the alternative complement pathway in adipose thermogenesis and highlight sex-specific considerations in potential therapeutic targets for metabolic diseases. ### Competing Interest Statement The authors have declared no competing interest.