Signal transduction mechanism of cell death-role of kinase cascade.

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To understand molecular mechanism of many neurodegenerative disorders, we should elucidate how neurons and muscle cells die and the signals for cell death in such diseases. Accumulating evidences have suggested that apoptotic cell death can occur in patients' brain from some neurodegenerative disorders. We have been involved in the study of the intracellular signal transduction pathway of apoptotic cell death in terms of intracellular protein kinase casccade. In this project, we tried to develope the model system where cells die through apoptotic mechanism in neuronal culture systems and examine the intracellular signal transduction pathway of apoptotic cell death. For these purposes, we found that L6 myocytes are killed by HMG-CoA reductase inhibitor (HCRI), widely used medicine for the treatment of hypercholesteroraemia, involving the induction of apoptotic mechanism. Hydrophobic derivative of HCRIs, simvastatin kills L myocytes at the concentration of 30 ug/ml and causes internucleo … More somal DNA fragmentation and tyrosine phosphorylation reaction of several cellular proteins. Herbimycin A or genistein, a potent inhibitor of protein tyrosine kinase activity prevented simvastatin-induced apoptotic cell death. In the next step, we tried to identify the protein which got tyrosine phosphorylated in response to simvastatin treatment and succeeded in identifying one of such proteins. Phospholipase C-gamma 1 was found to be tyrosine-phosphorylated in simvastatin-treated cells. Tyrosien phosphorylation of PLC-gamma1 was evident as early as 2 min after addition of simvastatin into culture medium and reached maximum at 10 min after its addition. Heretofore, PLC-gamma1 is activated when it gets tyrosine-phosphorylated, although tyrosine phosphorylation is not the only way for the activation. Therefore, we tried to check whether simvastatin induces PI turnove in simvastatin treated cells. Intracellular concentration of IP3 increased up to 3-fold than the basal level at 10 min after addition of simvastatin. U73122, a potent inhibitor of PLC,clear inhibited simvastatin-induced cell death. These results indicate the special role of tyrosine phosphoryaltion and PLC activity in apoptotic cell death of muscle cells. Less