The mechanistic and therapeutic implications of tumor hypoxia in the TRAIL signaling pathway

As tumors grow, oxygen supplied by local blood vessels fails to reach remote tumor cells. Cancer cells respond to limiting oxygen by stimulating the formation of their own vascular supply. However, blood vessels formed by tumors are often abnormal and nonfunctional. The result is that the majority of solid tumors contain regions with low or inconsistent oxygen availability - termed tumor hypoxia. Hypoxic stress provides a selective pressure for the enrichment of tumor cells with a more aggressive and metastatic phenotype as well as a loss of apoptotic potential; which poses a significant barrier to treatment of solid tumors. Hypoxic tumor cells are largely resistant to traditional ionizing radiation and chemotherapies via well defined resistance mechanisms involving activation of multidrug resistant pumps, decreased proliferation and the lack of O2-dependent fixation of therapy-induced DNA damage. We were interested in how tumor hypoxia may affect sensitivity of cancer cells to treatment with therapies targeting the tumor-necrosis factor related apoptosis inducing ligand (TRAIL) family of membrane bound death receptors. We found that hypoxia has a significant effect on the relative sensitivity of human cancer cells treated with TRAIL by altering the expression of mediators of the apoptotic signaling pathway. We went on to identify a mechanism by which hypoxia activates TRAIL signaling in cancer cells with wild-type p53, through p53-mediated repression of the anti-apoptotic protein c-FLIP. In addition, we identified mechanisms by which hypoxia promotes TRAIL resistance in cancer cells lacking wild-type p53. Attempts to reverse hypoxia-induced apoptotic resistance by combining TRAIL with chemotherapy or Tirapazamine (TPZ) failed to significantly affect TRAIL resistance in hypoxic cells. Therefore, we screened a library of small molecules against hypoxia-induced TRAIL-resistant cells which identified a class of structurally related compounds (SLMs) which re-sensitize hypoxic cells to TRAIL-induced apoptosis. Further studies were carried out with in vivo mouse models using real-time optical imaging to analyze the consequence of tumor hypoxia on TRAIL efficacy with or without the addition of SLMs. By performing real-time imaging of tumor vasculature we discovered that one of the consequences of hypoxic tumor cell resistance was increased angiogenesis. However, by combining SLMs with TRAIL tumor angiogenesis was inhibited. These findings demonstrate that tumor hypoxia can significantly alter apoptotic signaling as well as the efficacy of TRAIL targeted therapies. Our results suggest that hypoxia-targeted adjuvants may be required to optimize therapeutic regiments which include TRAIL in some human tumors, specifically those lacking p53