No cross-resistance between ALA-mediated photodynamic therapy and nitric oxide

Photodynamic therapy (PDT) interactions with nitric oxide (NO) are not well understood. In this work, we attempted to elucidate whether NO cytotoxicity and PDT from aminolevulinic acid (ALA) have independent cell damage mechanisms. We employed the murine mammary adenocarcinoma cell line LM3 and its NO-resistant variant LM3-SNP obtained after successive exposures to sodium nitroprusside (SNP). No cross-resistance was found between NO cytotoxicity and ALA-PDT; LM3-SNP cells were not more resistant to ALA-PDT than the parental line, instead they were more sensitive. We also induced resistance to ALA-PDT in LM3-SNP cells after multiple cycles of photodynamic treatment. We isolated two clones, identified as Clon 1 and Clon 3, which were 9.2 and 12.5 times more resistant to ALA-PDT than the parental lines, showing that resistance to NO did not interfere in the development of PDT resistance. In addition, the sensitivity to NO decreased in Clon 1 and increased in Clon 3, but they did not show any modifications in NO production. All the cell lines have similar GSH content and GSH transferases activities. However, GSSG content is markedly lower in LM3-SNP, Clon 1, and Clon 3 compared to parental LM3 line and consequently GSH/GSSG ratios are also higher. Our results suggest that different degrees of NO resistance of tumours would not correlate with resistance to PDT.