The role of G2-block abrogation, DNA double-strand break repair and apoptosis in the radiosensitization of melanoma and squamous cell carcinoma cell lines by pentoxifylline.

PURPOSE

To examine the role of G2-block abrogation, DNA repair inhibition and apoptosis in the enhancement of radiotoxicity by pentoxifylline.

MATERIALS AND METHODS

The influence of pentoxifylline on radiotoxicity was assessed by colony assay in TP53 wild-type Bell and mutant MeWo melanoma, and in TP53 wild-type 4197 and mutant 4451 squamous cell carcinoma (SCC) cell lines. G2-block abrogation was assessed by flow cytometry. Induction of DNA damage and repair was measured over a dose range of 0-100 Gy by constant field gel electrophoresis (CFGE). The Annexin-V binding assay was used to identify apoptotic cells.

RESULTS

Pentoxifylline, when combined with irradiation, significantly increased radiotoxicity in the TP53 mutant MeWo and 4451 cell lines by radiotoxicity enhancement factors of 3 and 14.5 respectively. No radiosensitization was seen in the TP53 wild-type Be11 and 4197 cells. When the drug was added after irradiation at the time of maximum G2-block expression, no radiosensitization was seen in any of the four cell lines. CFGE analyses showed that pentoxifylline effectively suppressed DNA double-strand break (DSB) repair in all four cell lines, as indicated by 20 h repair inhibition factors of 1.4-2.4. Pentoxifylline did not increase apoptosis in any of the four cell lines.

CONCLUSION

These data suggest that radiosensitization by pentoxifylline is not a consequence of G2-block abrogation alone, but that inhibition of DSB repair plays a role in certain cell types.