Secreted-Osteopontin Contributes to Brown Adipogenesis In Vitro via a CD44-Dependent Pathway

Osteopontin (OPN), a secreted glycoprotein, is involved in various pathophysiological processes including immune response, inflammation, tumor formation, and metabolism. OPN exists in 2 forms, secreted-OPN (sOPN) and intracellular-OPN (iOPN). While they might have different biological activities, it remains largely unknown whether sOPN and iOPN induce the differentiation of brown adipocytes. To test this possibility, 3T3-L1 cells were induced by DMI induction with or without recombinant human OPN (rhOPN, 10, 50, 100, 200 μM), respectively. Meanwhile, another batch of 3T3-L1 cells were infected with Ad-GFP-ap2-OPN and followed by DMI differentiation. Subsequently, the infected cells were treated with either anti-CD44 antibody or immunoglobulin G (Ig G). Accumulation of lipid droplets was visualized by Oil red O staining and protein levels were assayed by western blotting analysis. The results showed that sOPN and not rhOPN, notably increased the accumulation of lipid droplets and the expression of brown adipocyte-related genes. Moreover, neutralization of CD44 partially abrogated the effects induced by sOPN. These data demonstrate that sOPN and not rhOPN has the capacity to induce the differentiation of white preadipocytes into brown adipocytes through a CD44-dependent mechanism. The findings might provide a potential target for sOPN to combat obesity.