PARENT SESSION
3:45 PM to 5:15 AM
Sunday, April 21, 2002
Mini-Symposium 4
Experimental Therapeutics
Room: Nevada 6-7
, Co-Chair: Gupta, Anjali¹; Lawrence, Theodore²¹University of Pennsylvania, Philadelphia, PA²University of Michigan, Ann Arbor, MI

(MS04-2) Low-Dose Fractionated Radiation Potentiates the Effects of Paclitaxel in Wild-Type and Mutant p53 Head and Neck Tumor Cell Lines.

Arnold, Suzanne², Valentino, Joseph², Chendil, Damodaran¹, Das, Anindita¹, Mohiuddin, Mohammed¹, Ahmed, Mansoor¹, Dey, Swatee^*,1, ² Department of Clinical Sciences, Lexington, KY¹ Department of Radiation Medicine, Lexington, KY

ABSTRACT-
In this study we used p53 wild-type and p53-mutant squamous cell carcinoma of head and neck cell lines: (1) to evaluate the presence of hyper-radiation sensitivity (HRS) regions and compare it with p53 status; (2) to compare the Paclitaxel potentiating effect of single dose fraction (>1Gy) versus low dose fractionated radiation (LDFRT - <1Gy) and (3) to understand the molecular mechanism of LDFRT mediated effects. Both cell lines exhibited the presence of HRS region irrespective of p53 status. Compared to SCC-61 cells, SQ-20B cells were resistant to radiation (RT) and Paclitaxel (P) alone. SCC-61 cells showed similar degree of radiosensitization to Paclitaxel [enhancement ratio (ER)=1.78], when compared to SQ-20B cells (ER = 1.51). Interestingly, low doses of 0.5Gy four fractions significantly increased the ER for both mutant p53 SQ-20B cells (ER=3.27) and wild-type SCC-61 cells (ER=4.6). In SCC-61 cells, single RT or P or combined treatments functionally induced p53 and its effector genes (p21^waf1/cip1 and bax). Whereas, induction of p53 and its effector genes in SQ-20B were not evident clearly in response to these treatments, with complete absence of p21^waf1/cip1 protein. Interestingly, bcl-2 protein levels were elevated in SQ-20B cells and in SCC-61 cells, the bcl-2 level remained constant before and after treatments. In SQ20B cells, LDFRT alone caused significant induction of bax protein with no changes in bcl-2, 3 and 6 hours after the last fraction of 0.5Gy LDFRT treatment. NFB, a pro-survival transcription factor and transactivator of bcl-2, was found to be upregulated by single 2 Gy dose radiation, whereas, LDFRT treatments failed to induce NFB in SQ20B cells. Paclitaxel alone and in combination with LDFRT caused downregulation of bcl-2 with no changes in bax and failed to induce NFB activity. In wild-type p53 SCC61 cells, the kinetics of bax, bcl-2 and NFB were similar to that of SQ20B cells as demonstrated by Western blot analysis and gel-shift assay. Thus, the pro-apoptotic signaling initiated by LDFRT can be attributed to significant increase in the enhancement ratios. These findings strongly suggest that LDFRT, at HRS dose, can be used in combination with chemotherapeutic drug to overcome the anti-apoptotic and pro-survival effects of bcl-2 and NFB, respectively.

KEYWORDS: Paclitaxel, Low-Dose Radiation, Hyper Radiation Sensitivity, NFB