Radiosensitization of HT-29 cells and xenografts by the nitric oxide donor DETANONOate

Background Mechanisms of radioresistance in rectal cancer remain unclear. Objectives To determine mechanisms of radioresistance in rectal cancer cells and to assess the role of the nitric oxide donor DETANONOate as a radiosensitizing agent. Methods Survival was determined by clonogenic assays, apoptosis by PARP‐1 cleavage, and phenotypic differences by Western blot analysis. SCID mice bearing HT‐29 xenografts were treated with ionizing radiation (IR) [2.0 Gy × 5], DETANONOate [0.4 mg/kg i.p.], or combination treatment. Results Colorectal cancer HT‐29‐p53‐null cells were resistant and HCT‐116‐p53 wild‐type cells sensitive to IR, which correlated with cleaved PARP‐1. Increased levels of p21 occurred in HCT‐116 cells, while Bcl‐2 and survivin were elevated in HT‐29 cells. Radiosensitization was achieved with a substantial elevation of cleaved PARP‐1 in DETANONOate‐HT‐29‐treated versus control cells, which was accompanied by elevation of p21, p27, and BAX, and a concomitant decrease in Bcl‐2. SCID mice bearing HT‐29 xenografts demonstrated a 37.6%, 51.1%, and 70.1% inhibition in tumor growth in mice receiving IR, DETANONOate, and combination treatment versus control, respectively. Conclusions Radioresistant HT‐29 cells are p53‐null and have substantially decreased levels of p21. DETANONOate radiosensitized HT‐29 cells in vitro and in vivo by an additive effect in apoptosis. J. Surg. Oncol. 2009;100:149–158. Published 2009 Wiley‐Liss, Inc.