Activation of extracellular signal-regulated kinase during silibinin-protected, isoproterenol-induced apoptosis in rat cardiac myocytes is tyrosine kinase pathway-mediated and protein kinase C-dependent

Aim: To investigate the mechanism of silibinin-protected isoproterenol-induced apoptosis in rat cardiac myocytes. Methods: The viability of rat cardiac myocytes was measured by MTT method. The apoptotic ratio was measured by terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling. Protein kinase C (PKC) activity assay was carried out according to the instructions of the PepTag non-radioactive protein kinase C assay kit. Western blot analysis was used to evaluate the level of Ras, Raf-1 and mitogen-activated protein kinase (MAPK) expression. Results: The protective effects of silibinin were significantly suppressed by inhibitors, including genistein, manumycin A and GW5074 [inhibitors for protein tyrosine kinases (PTK), Ras and Raf-1, respectively]. The exposure of rat cardiac myocytes to isoproterenol alone caused decreased PKC activity, which was prevented by pretreatment with silibinin dose-dependently. Simultaneously, the increased expression of Ras and Raf-1 activated by silibinin were blocked by the PKC inhibitor, stauroporine. In addition, the extracellularly responsive kinase (ERK) inhibitor, PD98059, suppressed silibinin-protected apoptosis, whereas the p38 MAPK inhibitor, SB203580, protected cardiac myocytes from isoproterenol-induced injury, and the c-Jun N-terminal kinase (JNK) inhibitor, SP600125 had no protective effects. Furthermore, Western blot analysis showed that the expression of phosphorylated ERK was increased by silibinin, the expression of phos-phorylated p38 MAPK was decreased and total ERK, p38, JNK and phosphorylated JNK MAPK did not change after treatment with both isoproterenol and silibinin. Furthermore, pretreatment of cardiac myocyte with PKC, Ras and Raf inhibitors significantly blocked ERK phosphorylation. Conclusion: Silibinin is suggested to protect isoproterenol-induced rat cardiac myocyte apoptosis by activating the tyrosine kinase pathway, PKC and MAPK pathways.