Cellular FLIP is expressed in cardiomyocytes and down-regulated in TUNEL-positive grafted cardiac tissues

c-FLIP is a natural homologue of caspase 8, and may antagonize activation of death pathways mediated by FADD. c-FLIP is highly expressed in the heart, and a recent report suggests that c-FLIP may protect against certain types of myocyte death. The present study was designed to define the expression patterns of c-FLIP in the heart. The expression pattern of c-FLIP in end-stage human hearts, and rat cardiomyocyte grafting models was analyzed by in situ hybridization, immunohistochemistry and TUNEL assay. In addition, to determine whether Fas-dependent pathway is active in cardiomyocytes in vitro, we examined whether activated monocytes can kill neonatal cardiomyocytes in a co-culture system. c-FLIP mRNA and protein were abundantly expressed in normal cardiomyocytes from failing human heart. In animal models, c-FLIP protein was absent in TUNEL-positive grafted cardiomyocytes. Double staining demonstrated that c-FLIP-positive cells rarely had fragmented DNA, while TUNEL-positive cells rarely contained c-FLIP. Finally, activated monocytes induced death of neonatal rat cardiomyocytes via the Fas/FasL system. Loss of c-FLIP expression correlates with cardiomyocyte cell death. We hypothesize that diminished c-FLIP expression may predispose cardiomyocytes to apoptotic death.