Resistance of cancers to immunologic cytotoxicity and adoptive immunotherapy via X-linked inhibitor of apoptosis protein expression and coexisting defects in mitochondrial death signaling.

The ability of cancers to evade immune surveillance and resist immunotherapy raises a fundamental question of how tumor cells survive in the presence of a competent immune system. Studies to address this question have primarily focused on mechanisms by which tumor cells avoid recognition by or induce tolerance in the immune system. However, little is known about whether cancer cells also acquire an intrinsic ability to resist killing by immune effectors. We find that cancer cells enhance their ability to withstand an attack by cytotoxic immune effector cells via acquisition of specific genetic alterations that interfere with the shared mitochondrial death signaling pathway entrained by granzyme B, IFN-gamma, and Apo2 ligand/tumor necrosis factor-related apoptosis inducing ligand (Apo2L/TRAIL), three key mediators of immunologic cell-mediated cytotoxicity. We show that the coexistence of specific mitochondrial signaling defects (either deletion of Bax, overexpression of Bcl-x(L), or deletion of Smac) with expression of X-linked inhibitor of apoptosis protein decreases the sensitivity of cancer cells to IFN-gamma/Apo2L/TRAIL- or granzyme B-induced apoptosis, lymphocyte-mediated cytotoxicity in vitro, and adoptive cellular immunotherapy in vivo. Conversely, negating X-linked inhibitor of apoptosis protein expression or function in tumor cells with defective mitochondrial signaling enables direct activation of caspase-3/-7 by granzyme B or Apo2L/TRAIL, and restores their susceptibility to immunologic cytotoxicity. These findings identify an important mechanism by which cancers evade elimination by immune effector cells and suggest that cancer immunotherapy might be improved by concurrent strategies to alleviate or circumvent the intrinsic mitochondrial death signaling defects that help cancer cells resist immunologic cytotoxicity.

Elimination of hepatic metastases of colon cancer cells via p53-independent cross-talk between irinotecan and Apo2 ligand/TRAIL.

The majority of colorectal cancers have lost/inactivated the p53 tumor suppressor gene. Using isogenic human colon cancer cells that differ only in their p53 status, we demonstrate that loss of p53 renders tumor cells relatively resistant to the topoisomerase I inhibitor, irinotecan. Whereas irinotecan-induced up-regulation of the proapoptotic proteins PUMA and Noxa requires p53, we find that irinotecan inhibits Janus kinase 2 (JAK2)-signal transducer and activator of transcription 3 and 5 (STAT3/5) signaling in both p53-proficient and p53-deficient tumor cells. We show that irinotecan inhibits JAK2-STAT3/5-dependent expression of survival proteins (Bcl-x(L) and XIAP) and cooperates with Apo2 ligand/tumor necrosis factor-related apoptosis-inducing ligand (Apo2L/TRAIL) to facilitate p53-independent apoptosis of colon cancer cells. Whereas xenografts of p53-deficient colon cancer cells are relatively resistant to irinotecan compared with their p53-proficient counterparts, combined treatment with irinotecan and Apo2L/TRAIL eliminates hepatic metastases of both p53-proficient and p53-deficient cancer cells in vivo and significantly improves the survival of animals relative to treatment with either agent alone. Although the synergy between chemotherapy and Apo2L/TRAIL has been ascribed to p53, our data demonstrate that irinotecan enhances Apo2L/TRAIL-induced apoptosis of tumor cells via a distinct p53-independent mechanism involving inhibition of JAK2-STAT3/5 signaling. These findings identify a novel p53-independent channel of cross-talk between topoisomerase I inhibitors and Apo2L/TRAIL and suggest that the addition of Apo2L/TRAIL can improve the therapeutic index of irinotecan against both p53-proficient and p53-deficient colorectal cancers, including those that have metastasized to the liver.