Synergy Between Peroxisome Proliferator-Activated Receptor γ Agonist and Radiation Therapy in Cancer

Abstract

Approximately 60% of newly diagnosed cancer patients receive radiotherapy to accomplish neoadjuvant, definitive, adjuvant or palliative purpose as required. However, radiotherapy may trigger host-mediated proangiogenic and proinflammatory effects that lead to tumor recurrence. Angiogenesis and inflammation are crucial processes, through which tumor microenvironment (TME) influences tumor growth, invasiveness and metastasis. Peroxisome proliferator-activated receptor γ (PPARγ) agonist rosiglitazone has anti-inflammatory, antiangiogenic and antineoplastic activities from preclinical data. We hypothesize that rosiglitazone and radiation therapy synergistically amplify the antitumor effect, which is based on the evidence that rosiglitazone remodels tumor vasculature and limits tumor-associated macrophages (TAMs) infiltration. The aim of this study is to evaluate the synergistic effect of PPARγ agonist rosiglitazone combined with radiation therapy. Each female Balb/c mouse was inoculated with 5X105 murine colon carcinoma cell line CT26 or breast cancer 4T1. When tumor size approximately reached 250-300 mm3, 12Gy in a fraction as hypofractionated irradiation therapy (HFRT) or 12Gy in 4 daily fractions of 3Gy as conventional fractionated irradiation therapy (CFRT) was delivered to the tumor, meanwhile administration of 100mg/kg rosiglitazone was started daily for 2 weeks. Tumors were harvested according to the schedule. Flow cytometry, ELISA, immunofluorescence, western blot, and so forth were performed to test the synergistic mechanisms. We presented that rosiglitazone has antiangiogenic effect by inhibiting VEGF-VEGFR2 signaling pathways. Immunofluorescence work identified that rosiglitazone can remodel tumor vasculature contributing to decrease of microvessel density, promotion of pericyte coverage, reduction of TAMs infiltration, and even further accompanied improvement of intratumoral hypoxia and vessel perfusion compared with the control (P<0.05). 4T1 or CT26 tumor-bearing mice treated with rosiglitazone in combination with HFRT or CFRT showed a significant reduction in lesion size and lung metastases compared with the control (P<0.001), rosiglitazone(P<0.001) or radiotherapy group (P<0.05). Analysis of ELISA, immunofluorescence and flow cytometry showed that tumor-derived chemokine CCL2 in reaction to HFRT facilitates the recruitment of migrating CD11b+ myeloid monocytes and TAMs into irradiated sites that initiates vasculogenesis and allows tumor recurrence after HFRT. We found that rosiglitazone decreased CCL2 and suppressed CD11b+ myeloid monocytes and TAMs infiltrations after HFRT, which delayed tumor recurrence. Rosiglitazone may elicit normalization of tumor vasculature and limit TAMs infiltration, and thereby delay tumor progression with a fresh perspective. Rosiglitazone, as an adjuvant therapy to HFRT, inhibits CCL2 mediating tumor recurrence.