The photosensitizer protoporphyrin IX, endogenously accumulated from the precursor aminolevulinic acid (ALA) as well as other less photodynamically effective intermediates, is a successful agent in photodynamic therapy. Despite encouraging clinical results, the basic mechanisms leading to cell death are not fully understood. To elucidate these fundamentals, the alteration of the gene expression pattern in the squamous cell carcinoma cell line A-431 was studied at different time-points after photodynamic treatment with ALA by cDNA-array technique. Cells were incubated for 16 h with 100 µg/ml ALA and irradiated with a fluence of 3.5 J/cm2 resulting in 50% survival until 8 h post treatment. RNA was isolated at 1.5, 3, 5 and 8 h post treatment and from 3 controls (untreated, light only and dark), radioactively labeled by reverse transcription with 33P-dCTP and hybridized onto macroarray PCR filters containing PCR products of 2135 genes, which were selected for relevance in carcinogenesis, stress response and signal transduction. Verification of observed expression changes was carried out by real-time RT-PCR. We found a strong induction of expression of the immediate early genes c-jun and c-fos as well as decreased expression of genes involved in proliferation such as myc, genes involved in apoptosis such as Fas associated via death domain (FADD) and the fibronectin gene for cell adhesion. An apoptosis induction rate of not more than 30% as proved by apoptosis detection assays and caused by PpIX localization in the membrane was reflected by the expression profile.

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