Expression levels of PEPT1 and ABCG2 play key roles in 5-aminolevulinic acid (ALA)-induced tumor-specific protoporphyrin IX (PpIX) accumulation in bladder cancer

Abstract

A detection method widely used of late in cancer surgery is 5-aminolevulinic acid-based photodynamic diagnosis (ALA-PDD), which relies on the tumor-specific accumulation of photosensitizing protoporphyrin IX (PpIX) after the administration of ALA. In this regard, we recently reported that peptide transporter PEPT1 and human ATP-binding cassette transporter ABCG2 are key players in regulating intracellular PpIX levels. In the present study, we re-evaluated in vivo the expression of genes involved in the porphyrin biosynthesis pathway. Using quantitative real-time (qRT)-PCR, we measured the mRNA levels in a clinical specimen of bladder cancer from a patient who had been subjected to ALA-PDD. We confirmed that PEPT1 and ABCG2 are major contributors to the regulation of tumor-specific PpIX accumulation. qRT-PCR analysis revealed a predominantly high level of PEPT1 mRNA and a very low level of ABCG2 mRNA in the bladder cancer, corresponding to the roles of these genes in vitro. These findings were further confirmed by immunohistochemical studies with PEPT1- and ABCG2-specific antibodies. The induction of PEPT1 gene and the suppression of ABCG2 gene expression are among the key molecular mechanisms underlying tumor-specific PpIX accumulation after the administration of ALA in bladder cancer.