Intratumor administration of interleukin 13 receptor-targeted cytotoxin induces apoptotic cell death in human malignant glioma tumor xenografts.

Apoptosis is not only essential for homeostasis in normal cells but also in cancer cells, in which it is associated with cell death mechanisms caused by novel therapeutics. We have previously reported that interleukin-13 receptors (IL-13R) are constitutively overexpressed on a majority of human malignant glioma cell lines and primary cell cultures. In addition, we have reported that IL-13 cytotoxin, comprised of human IL-13 and a mutated form of Pseudomonas exotoxin, is highly and specifically cytotoxic to these cells and can lead to pronounced antitumor activity in malignant glioma tumors in animal models. However, the molecular mechanisms of tumor cytotoxicity induced by IL-13 cytotoxin are poorly understood. In this study, we demonstrate that glioma tumors undergo apoptotic cell death on intratumoral administration of IL-13 cytotoxin. This conclusion was made based on (a) time-dependent induction of several proapoptotic molecules, such as caspases (caspase-3, -8, and -9) in tumors; (b) cleavage of procaspase-3 and poly(ADP-ribose) polymerase (PARP); and (c) the release of cytochrome c from mitochondria to the cytosol on injection of IL-13 cytotoxin in U251 glioblastoma tumors established in immunodeficient animals. These indicators of two major pathways of apoptosis were detected in tumors even though IL-13 cytotoxin was no longer present in tumors. In addition, we found that inducible nitric oxide was expressed in tumors in a time-dependent manner with primary localization in infiltrating phagocytes after treatment with IL-13 cytotoxin. These studies demonstrate that IL-13 cytotoxin mediates apoptotic death of glioma cells, resulting in regression of established tumors. Our studies will help unravel the molecular pathways of cell death associated with tumor regression and provide additional insight and define apoptosis as possible surrogate marker of tumor response.