Abstract 3037: Tumor cell death triggered by sustained IRES inhibition is a population-wide event involving a quorum-sensing mechanism in which putative tumor stem cells are eliminated along with their progeny
Authors: Vaklavas Christos , Grizzle William E. , Choi Hyoungsoo , Meng Zheng , Zinn Kurt R. , Blume Scott W. 
Published: July 2016

The internal ribosome entry site (IRES) is a feature contained within the 5’-untranslated region of the mRNA which allows the efficiency with which that mRNA is translated to be regulated independently of the general rate of protein synthesis in the cell. It appears that tumor cells rely on IRES-mediated translation to promote their own survival under adverse microenvironmental conditions, ultimately contributing to chemoresistance, relapse, and metastasis. Our lab has identified and characterized a series of compounds (prototype IRES inhibitors) which selectively modulate IRES function. Here we examine the phenotypic consequences of sustained IRES inhibition in triple-negative breast carcinoma and glioblastoma.

72 hours continuous exposure to the IRES inhibitor results in sudden loss of viability affecting &gt;99.9% of the tumor cell population. Cell death is a consequence of the inhibition of synthesis of critical IRES-driven proteins, and not a typical cytotoxic response. The treated cells exhibit prominent features of terminal differentiation, with marked gains in cytoskeletal organization and planar polarity, extensive intercellular networking, and formation of tight junctions or neuronal processes. In addition to depletion of Myc protein, specific changes in IGF1R, connexin 43, BiP, and CHOP also correlate with phenotypic outcome. The highly undifferentiated triple-negative breast tumor and glioblastoma cells apparently cannot tolerate the transition to a fully differentiated state, or the restoration of normal homeostatic mechanisms allows the abnormalities that cause these cells to be malignant to be recognized. Once triggered, cell death is rapid and comprehensive. PARP cleavage is not observed, caspase 3/7 activation is limited, and no protection afforded by Z-VAD-fmk. The cells show no evidence of recovery even following prolonged incubation in the absence of compound. In contrast, normal human mammary epithelial cells (which are already differentiated) are resistant to IRES inhibition.

The synchronized cell death response to IRES inhibition is a highly orchestrated event, with the population of malignant cells becoming physically linked together and responding as a unit, in contrast to typical stochastic processes such as apoptosis, programmed necrosis, and autophagy, where cell death decisions are made at the level of the individual cell. Importantly, tumor stem cells, which typically are resilient to conventional cytotoxic or targeted therapeutic regimens, appear to be vulnerable to IRES inhibition.

Together, these findings support the concept that IRES-mediated translation is critical for maintenance of the undifferentiated phenotype. IRES inhibition potentially represents a new strategy for treatment of undifferentiated tumor types such as triple-negative breast cancer and glioblastoma.

Citation Format: Christos Vaklavas, William E. Grizzle, Hyoungsoo Choi, Zheng Meng, Kurt R. Zinn, Scott W. Blume. Tumor cell death triggered by sustained IRES inhibition is a population-wide event involving a quorum-sensing mechanism in which putative tumor stem cells are eliminated along with their progeny. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3037.