TRAIL/TRAIL-R in hematologic malignancies

Defects in apoptosis are observed in many cancer cell types and contribute in a relevant way to tumorigenesis. Apoptosis is a complex and well-regulated cell death program that plays a key role in the control of cell homeostasis, particularly at the level of the hematopoietic system. Apoptosis can be initiated through two different mechanisms involving either activation of the death receptors (extrinsic pathway) or activation of a mitochondrial apoptotic process (intrinsic pathway). Among the various death receptors a peculiar role is played by TNF-related apoptosis-inducing ligand (TRAIL)-receptors (TRAIL-Rs) and their ligand TRAIL. TRAIL recently received considerable interest for its potent anti-tumor killing activity, sparing normal cells. Here, we will review the expression and the abnormalities of TRAIL/TRAIL-R system in hematologic malignancies. The large majority of primary hematologic tumors are resistant to TRAIL-mediated apoptosis, basically due to the activation of anti-apoptotic signaling pathway (such as NF-kappaB), overexpression of anti-apoptotic proteins (such as FLIP, Bcl-2, XIAP) or expression of TRAIL decoy receptors or reduced TRAIL-R1/-R2 expression. Strategies have been developed to bypass this TRAIL resistance and are based on the combination of TRAIL with chemotherapy or radiotherapy, or with proteasome or histone deacetylase or NF-kappaB inhibitors. The agents used in combination with TRAIL either enhance TRAIL-R1/-R2 expression or decrease expression of anti-apoptotic proteins (c-FLIP, XIAP, Bcl-2). Many of these combinatorial therapies hold promise for future developments in treatment of hematologic malignancies.