Abstract #2292: Cytosolic phospholipase A2: A novel molecular target for tumor sensitization to radiation therapy (2009)

We have recently identified cytosolic phospholipase A2 (cPLA 2 ) as a key component in the vascular endothelium response to therapeutic doses of ionizing radiation (2-5 Gy). Radiation-induced activation of cPLA 2 leads to downstream phosphorylation of the pro-survival kinases Akt and ERK1/2. This results in enhanced radioresistance of tumor vasculature which can significantly diminish overall cancer treatment success rates. In our present study, we provide evidence that chemical inhibition of cPLA 2 with AACOCF 3 can increase the radiation-induced destruction of tumor blood vessels and may significantly improve the response of radioresistant lung tumors to radiation therapy. Using a tumor model of Lewis Lung Carcinoma (LLC), we demonstrated that a combined treatment of AACOCF 3 and radiation inhibited tumor growth by 75% as compared to untreated tumors. Treatment with either radiation or drug alone failed to produce a significant delay in tumor growth (2.6 days and 1.8 days, respectively.) However, mice treated with AACOCF 3 and radiation demonstrated a significant tumor growth delay of 12.2 days. Tumor blood flow analysis revealed that treatment with AACOCF 3 resulted in a 1.6-fold decrease in tumor blood flow as compared to untreated mice (7.5% vs. 12%). This decrease was further potentiated in mice that received AACOCF 3 followed by radiation (6% vs. 12%). To investigate whether cPLA 2 inhibition affects tumor vascularity, tumor sections were stained with anti-von Willebrand Factor antibody. Treatment with AACOCF 3 resulted in a significant decrease in tumor blood vessel number (4%) compared to untreated mice (10%). The most pronounced reduction in vascularity, however, was observed in mice treated with AACOCF 3 followed by radiation (2%). TUNEL staining revealed that this blood flow attenuation and reduction in vascularity is associated with the induction of apoptosis within tumor vascular endothelial cells. In addition, treatment with AACOCF 3 suppressed radiation-induced Akt phosphorylation in irradiated tumors. To determine whether AACOCF 3 affects cellular viability in irradiated tumor cells as well as vascular endothelial cells, we performed clonogenic survival assays. Treatment with AACOCF 3 enhanced radiation-induced cell death among 3B11, but not LLC cells. At 2 Gy, 3B11 cells pretreated with AACOCF 3 exhibited a 30% decrease in survival in comparison to irradiated cells treated with vehicle alone. Western immunoblot analysis revealed that the effects of AACOCF 3 on 3B11 cell viability may be due to changes in radiation-induced pro-survival signaling. At 3 minutes post-irradiation, AACOCF 3 prevented radiation-induced ERK1/2 phosphorylation in 3B11 cells, but not in LLC. Taken together, these findings identify cPLA 2 as a critical component of the tumor vascular response to ionizing radiation and suggest that cPLA 2 is a novel molecular target for tumor sensitization to radiation therapy. Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr 2292.