Endogenously released Smac is insufficient to mediate cell death of human lung carcinoma in response to etoposide.

Cytotoxic agents eliminate tumor cells via different mechanisms including apoptosis, although this process is not equally efficient in all kinds of cancer cells. Thus, small cell lung carcinomas (SCLCs) are more sensitive than non-small cell lung carcinomas (NSCLCs) to therapy-induced killing. During apoptosis, several apoptogenic proteins release from the mitochondria. Among these proteins is Smac/DIABLO. Overexpression of Smac effectively potentiates apoptosis by neutralizing the caspase-inhibitory function of the inhibitors of apoptosis proteins (IAPs). However, the physiological relevance of endogenously released Smac in the promotion of malignant cell death is still unclear. Analysis of a panel of human lung cancer cell lines revealed that there is no altered Smac expression in NSCLC and SCLC that might initially impair the drug-induced cell death. Upon engagement of the mitochondrial pathway of apoptosis, etoposide provoked cytosolic accumulation of Smac along with cytochrome c and loss of the mitochondrial membrane potential. Most of these events as well as nuclear apoptotic changes required caspase activation in SCLC, but not in NSCLC. Unexpectedly, pan-caspase inhibition had no effect on Smac release. Co-treatment of SCLC with the IAP-binding peptide Smac-N7 enhanced etoposide-induced apoptosis in a concentration-dependent manner, whereas Smac downregulation by small interfering RNA (siRNA) did not influence caspase-3/-7 activities, nuclear morphological changes, DNA fragmentation, and plasma membrane integrity. Release of cytochrome c and mitochondrial protease Omi/HtrA2 is still detectable at these conditions. These data suggest that Smac deficiency may be compensated for by action of redundant determinants to kill cancer cells. Thus, translocation of endogenous Smac into cytosol does not play a critical role in cell death of human lung carcinoma after etoposide treatment.