Radiation schedule for the cell culture and xenograft experiments. Human prostate cancer cells were either plated or injected subcutaneously into the flanks of the right hind legs of athymic nude mice. After 24 h (in vitro) or tumors reached a tumor size of 5 mm x 5 mm (in vivo), tumors were irradiated with different fractionation regimens including a single dose of 10 Gy at day 5, 10 fractions of 1 Gy (2 fractions per day), or 5 fractions of 2 Gy (1 fraction per day). Unirradiated cells or tumors were used as control.
ARTICLE ABSTRACT
Implementing targeted drug therapy in radio-oncologic treatment regimens has greatly improved the outcome of cancer patients. However, the efficacy of molecular targeted drugs such as inhibitory antibodies or small molecule inhibitors essentially depends on target expression and activity, which both can change during the course of treatment. Radiotherapy has previously been shown to activate prosurvival pathways, which can help tumor cells to adapt and thereby survive treatment. Therefore, we aimed to identify changes in signaling induced by radiation and evaluate the potential of targeting these changes with small molecules to increase the therapeutic efficacy on cancer cell survival. Analysis of “The Cancer Genome Atlas” database disclosed a significant overexpression of AKT1, AKT2, and MTOR genes in human prostate cancer samples compared with normal prostate gland tissue. Multifractionated radiation of three-dimensional–cultured prostate cancer cell lines with a dose of 2 Gy/day as a clinically relevant schedule resulted in an increased protein phosphorylation and enhanced protein–protein interaction between AKT and mTOR, whereas gene expression of AKT, MTOR, and related kinases was not altered by radiation. Similar results were found in a xenograft model of prostate cancer. Pharmacologic inhibition of mTOR/AKT signaling after activation by multifractionated radiation was more effective than treatment prior to radiotherapy. Taken together, our findings provide a proof-of-concept that targeting signaling molecules after activation by radiotherapy may be a novel and promising treatment strategy for cancers treated with multifractionated radiation regimens such as prostate cancer to increase the sensitivity of tumor cells to molecular targeted drugs. Mol Cancer Ther; 17(2); 355–67. ©2017 AACR.See all articles in this MCT Focus section, “Developmental Therapeutics in Radiation Oncology.”
