The Bowman-Birk protease inhibitor enhances clonogenic cell survival of ionizing radiation-treated nucleotide excision repair-competent cells but not of xeroderma pigmentosum cells

Purpose : The radioprotective effect of the Bowman-Birk protease inhibitor (BBI) was previously shown to result from a TP53 dependent mechanism. Whether this effect involves specific DNA repair mechansims is now tested. Material and methods : Normal human fibroblasts were pre-treated with BBI before exposure to X-rays, UVB or to chemical agents (bleomycin, N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), cisplatin). These agents were chosen because of their ability to induce different spectra of DNA damage. The radiometric agent bleomycin primarily induces double-strand breaks (dsb), which are repaired by recombination; MNNG results in alkylated bases which are repaired by base excision repair (BER); cisplatin results in DNA-crosslinks which are repaired mainly by nucleotide excision repair (NER); and finally UVB generates thymine dimers and thymine-cytosine-6-4 products which are also repaired by NER. Cell survival was analysed by colony formation assay and DNA dsb by constant field gel electrophoresis. The combined effect of BBI and X-rays was also tested for XP-fibroblasts, which are defective in NER. Results : For normal human fibroblasts the radioprotective effect of BBI was clearly found by using a delayed plating procedure. The radioprotective effect was found to be unrelated to an altered induction or repair of radiation-induced DNA dsb. Pre-treatment with BBI did not affect cell killing after exposure to bleomycin or MNNG, but resulted in a significant protection of cells exposed to cisplatin or UVB. These results indicate that pre-treatment with BBI did not alter recombination repair or BER, but was able to modify NER. The latter finding was supported by the observation made for XP-cells, where pre-treatment with BBI failed to result in radioprotection after exposure to ionizing radiation. Conclusions : On the basis of these data it is proposed that the radioprotective effect of BBI is the result of an improved nucleotide excision repair mechanism.