APOPTOSIS INDUCTION BY PHOTODYNAMIC TREATMENT WITH ENDOGENOUS PROTOPORPHYRIN IX? Short title: Apoptosis by ALA-PDT?

v Apoptosis can be induced in several cell lines by photodynamic treatment with different photosensitizers including aminolevulinic acid (ALA)-stimulated endogenous protoporphyrin IX (PpIX). This PpIX is successfully used as tumor marker in diagnosis, and owns good sensitizing properties for tumor therapy (PDT). The aim of the study was to investigate cell death mechanisms and its consequences for tumor treatment with PpIX, since the mode of cell death seems to influence the efficiency o f the therapy. The no-response to a therapeutical approach is often caused by lack o f apoptosis induction. On the other hand, the mode of cell death during primary tumor eradication seems to have a strong impact on those immune reactions, which are required in order to prevent tumor recurrence: the immune system is more effectively activated by necrotic cell death than by apoptosis. We could show in apoptosis detection studies supported by cell cycle and gene expression analysis that light-activated, endogenous PpIX induces mainly necroses and only a smaller apoptotic fraction in fibroblast cultures. This difference to studies in the literature describing mainly apoptosis induction could be partly explained by switching between apoptosis and necrosis, dependent on the intracellular amount o f ATP concentration. It is very likely that in ALA-PDTdamaged cells maintaining a cellular energy level above a threshold, an apoptotic program is induced in mitochondria. The execution proceeds via impairment of the mitochondrial functions, cytochrome c release and caspases activation to the morphological appearance o f apoptosis.