ABCG2 expression is related to low 5-ALA photodynamic diagnosis (PDD) efficacy and cancer stem cell phenotype, and suppression of ABCG2 improves the efficacy of PDD

Abstract

Photodynamic diagnosis/therapy (PDD/PDT) are novel modalities for the diagnosis and treatment of cancer. The photosensitizer protoporphyrin IX is metabolized from 5-amino vulinic acid (5-ALA) intracellularly, and PDD/PDT using 5-ALA have been approved in d matologic malignancies and gliomas. However, the molecular mechanism that defines t efficacy of PDD/PDT is unknown. In this study, we analyzed the functions of ATP-bindin cassette (ABC) transporters in PDD using 5-ALA. Most of the human gastrointestinal ca line cells examined showed a homogenous staining pattern with 5-ALA, except for the p creatic cancer line PANC-1, which showed heterogeneous staining. To analyze this hete geneous staining pattern, single cell clones were established from PANC-1 cells and the expression of ABC transporters was assessed. Among the ABC transporter genes exam ined, ABCG2 showed an inverse correlation with the rate of 5-ALA-positive staining. PA 1 clone #2 cells showed the highest level of ABCG2 expression and the lowest level of 5 ALA staining, with only a 0.6% positive rate. Knockdown of the ABCG2 gene by small int fering RNAs increased the positive rate of 5-ALA staining in PANC-1 wild-type and clone cells. Interestingly, PANC-1 clone #2 cells showed the high sphere-forming ability and tumor-formation ability, indicating that the cells contained high numbers of cancer stem (CSCs). Knockdown or inhibition of ABCG2 increased the rate of 5-ALA staining, but did decrease sphere-forming ability. These results indicate that gastrointestinal cancer cell l expressing high levels of ABCG2 are enriched with CSCs and show low rates of 5-ALA staining, but 5-ALA staining rates can be improved by inhibition of ABCG2.