Original Article Ectopic expression of new alternative splice variant of Smac/DIABLO increases mammospheres formation

Smac-α is a mitochondrial protein that, during apoptosis, is translocated to the cytoplasm, where it nega- tively regulates members of the inhibitor of apoptosis (IAP) family via the IAP-binding motif (IBM) contained within its amino-terminus. Here, we describe a new alternative splice variant from Smac gene, which we have named Smac-e. Smac-e lacks both an IBM and a mitochondrial-targeting signal (MTS) element. Smac-e mRNA exhibits a tissue- specific expression pattern in healthy human tissues as well as in several cancer cell lines. The steady-state levels of endogenous Smac-e protein is regulated by the proteasomal pathway. When ectopically expressed, this isoform presents a cytosolic localization and is unable to associate with or to regulate the expression of X-linked Inhibitor of apoptosis protein, the best-studied member of IAP family. Nevertheless, over-expression of Smac-e increases mam- mosphere formation. Whole genome expression analyses from these mammospheres show activation of several pro-survival and growth pathways, including Estrogen-Receptor signaling. In conclusion, our results support the functionality of this new Smac isoform.