Chemical constituents of Alnus japonica and their anti-adipogenic activities

Abstract

Obesity causes many health problems and is known to be associated with an excessive growth of adipocyte mass tissue by increasing the number and size of adipocytes differentiated from preadipocytes. The inhibition of adipogenesis has been considered as an effective treatment for obesity. In the course of searching for anti-adipogenic drug condidates from nature sources using 3T3-L1 cells, it was found that the 80 % MeOH extract of Alnus japonica Steud. fruits, Alnus hirsuta Turcz. var. sibirica (Spach) H. Ohba leaves, Alnus firma Sieb. et Zucc. barks (Betulaceae) showed significant inhibitory activity on adipocyte differentiation. These Alnus species has been used for hemorrhage, burn injuries, anti-pyretic fever, diarrhea, and alcoholism in traditional Korean medicine. The various types of plant secondary metabolites such as flavonoids, triterpenoids, tannin, phenols, steroids, and diarylheptanoids have been reported from Alnus species. In the present study, thirty five diarylheptanoids, ten flavonoids, nine tannins, seven triterpenoids, five phenolic compounds and two sterols were isolated by the bioactivity-guided isolation over three Alnus species. Among the compounds AJ1, AJ2, AJ12, AJ37, AJ41, AH2, and AF19 were newly reported from the nature. Among these compounds, diarylheptanoids showed the most significant anti-differentiation effect than other on 3T3-L1 cells. These results showed that the presence of carbonyl group at C-3 in heptane chain is an important structural determinant for the activity, and additions of hydroxyl group in benzene ring and

substitution of glucosyl moiety, α,β-unsaturated ketone moiety also affect their biological activity. Compounds AJ9, AJ10, and AJ13 showed much better potent activity than other diarylheptanoids and induced the down-regulation of adipocyte fatty acid binding protein (aP2), lipoprotein lipase (LPL), and leptin gene as well as the peroxisome proliferator-activated receptors gamma (PPARγ) and CCAAT/enhancer-binding protein alpha (C/EBPα) genes. Compounds AJ9 and AJ10 also inhibited the expression of sterol regulatory element binding protein 1 (SREBP1), steroyl-coenzyme A desaturase 1 (SCD-1), and fatty acid synthase (FAS), target gene of lipogenesis, while compound AJ13 slightly affected these genes. In addition, compounds AJ9, AJ10, and AJ13 promoted lipolysis on mature adipocyte. The measurement of cell proliferation on 3T3-L1 cells revealed that compounds which showed potent anti-differentiation activity effectively decreased preadipocytes proliferation. Among them, compounds AF17-19, lupane type triterpenoids, showed the most anti-proliferative activity in concentration-dependent manner and decreased cell viability through inducing apoptosis. Consequently, the present study suggests that various constituents isolated from three Alnus species prove beneficial for decreasing adipose tissue volume, targeting different stages of the adipocyte life cycle.