INTRODUCTION: Brown adipose tissue morphology and function vary significantly between sexes. Recently we identified adipsin, also known as complement factor D (Cfd), as a sex differentially expressed gene in mouse BAT. However, little is known about the function ofadipsin in BAT. Here, we aim to unravel the role of adipsin in BAT differentiation and function.

METHODS: siRNA silencing of adipsin (siCfd) and other factors of the complement pathway was performed in the mouse brown adipocyte cell line T37i before differentiation. Following differentiation, cells were stimulated with norepinephrine or insulin. Additionally, cells were treated with a C3aR agonist during differentiation. Differentiation of the cells was assessed with Oil red O staining and qPCR was performed to assess the expression of brown adipocyte target genes.

RESULTS: Adipsin silencing decreased lipid droplet accumulation indicated by Oil red O staining. SiCfd treatment also strongly reduced mRNA expression of Ucp1 (21.3-fold), Adipoq (6.3-fold) and Ap2 (6.2-fold) (all P<0.0001). Furthermore, preliminary results showed that norepinephrine and insulin stimulated Ucp1 expression was lost upon Cfd silencing. Since adipsin activates the cleavage of complement 3 (C3) into C3a, we next studied whether knockdown of C3aR, the receptor of C3a, also affected brown adipocyte differentiation. Indeed, silencing of C3aR also significantly inhibited adipocyte differentiation, resulting in a 4-fold lower Ucp1, 5.9-fold lower Adipoq, and 5.8-fold lower Ap2 mRNA expression, all P<0.01. Similarly, preliminary results showed that treatment with a C3aR agonist accelerated adipogenesis, reflected by increased expression of adipogenesic markers at day 3, 5, 7 of differentiation and comparable levels at endpoint. 

CONCLUSION: Our results show that adipsin stimulates differentiation of brown adipocytes and further suggest that the alternative complement factor pathway plays a role in adipogenesis. Further studies are needed to determine whether this pathway could be a potential target for obesity treatment.