Expression and localization of inhibitor of apoptosis proteins in normal human tissues

The family of inhibitor of apoptosis (IAP) proteins can suppress apoptosis induced by a variety of triggers. Among the IAPs, cIAP1, cIAP2, and XIAP have been characterized as inhibitors of specific caspases, and their expression, together with that of survivin, has been shown in several studies to play a role as tumor marker and prognostic factor for the survival of patients with cancer. Although survivin is usually not expressed in normal adult tissues, cIAP1, cIAP2, and XIAP have been found broadly expressed at messenger RNA level within normal cells. Here, we report an immunohistochemical study in a comprehensive panel of normal human tissues, and we confirm at the protein level the wide expression of IAPs. These results are consistent with a physiological role of IAPs in normal cells. Moreover, we show that IAPs' expression levels and localization patterns differ depending on the cell lineage. The variable subcellular localization of the IAPs within different cell types suggests that compartmentalization may contribute to regulate their function. The physiological role of these proteins should be further investigated to help tailor IAP-targeted therapeutic strategies for patients with cancer and circumvent possible side effects.