Indole Derivatives Isolated from Brown Alga Sargassum thunbergii Inhibit Adipogenesis through AMPK Activation in 3T3-L1 Preadipocytes

Seaweed, a popular and abundant food ingredient mainly consumed in Asian countries, is a good source of bioactive compounds with anti-obesity effects. However, the anti-obesity effects of Sargassum thunbergii have not yet been established. In this study, we isolated six indole derivatives (STCs)-indole-2-carboxaldehyde (STC-1), indole-3-carboxaldehyde (STC-2), indole-4-carboxaldehyde (STC-3), indole-5-carboxaldehyde (STC-4), indole-6-carboxaldehyde (STC-5), and indole-7-carboxaldehyde (STC-6)-from S. thunbergii and evaluated their inhibitory effects on adipocyte differentiation in 3T3-L1 cells. We found that STC-1 and STC-5 resulted in non-toxic inhibition of the differentiation of 3T3-L1 adipocytes and thus selected these compounds for further study. STC-1 and STC-5 significantly inhibited lipid accumulation and downregulated the expression of peroxisome proliferator-activated receptor-γ (PPARγ), CCAAT/enhancer-binding protein α (C/EBPα), and sterol regulatory element-binding protein 1c (SREBP-1c) in a dose-dependent manner. The specific mechanism mediating the effects of STC-1 and STC-5 was shown to be AMP-activated protein kinase (AMPK) activation. Our results demonstrated the inhibitory effect of STC-1 and STC-5 on adipogenesis through the activation of the AMPK signal pathway. Together, these findings suggested that STC-1 and STC-5 may be effective candidates for the prevention of obesity or obesity-related diseases.