New Aspects of Silibinin Stereoisomers and their 3-O-galloyl Derivatives on Cytotoxicity and Ceramide Metabolism in Hep G

Ceramide as a second messenger is a key regulator in apoptosis and cytotoxicity. Ceramide-metabolizing enzymes are ideal target in cancer chemo-preventive studies. Neutral sphingomyelinase (NSMase), acid ceramidase (ACDase) and glucosyl ceramide synthase (GCS) are the main enzymes in ceramide metabolism. Silymarin flavonolignans are potent apoptosis inducers and silibinin is the most active component of silymarin. This study evaluated the effects of silybin A, silybin B and their 3-O-gallyl derivatives (SGA and SGB) at different concentrations (0-200 micro molar) on ceramide metabolism enzymes in Hep G2 hepatocarcinoma cell line. Cell viability, caspase-3 and 9 activities, total cell ceramide and the activities of ACDase, NSMase and GCS were evaluated. Under silibinin derivatives treatments, cell viability decreased and the activities of caspase-3 and 9 increased in a dose dependent manner among which SGB was the most effective one (P<0.05). Total cell ceramide and the activity of NSMase, the enzyme which elevates ceramide level, increased by silibinin derivatives. Furthermore, the activities of removing ceramide enzymes (ACDase and GCS) decreased efficiently. The galloyl esterification increased the activity of silibinin isomers. Consequently, this study reveals new sibilinin effects on ceramide metabolism and potential strategies to enhance the antineoplastic properties of this compound.