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.