Molecular effectors and modulators of hypericin-mediated cell death in bladder cancer cells.

Photodynamic therapy (PDT) is an anticancer approach utilizing a light-absorbing molecule and visible light irradiation to generate, in the presence of O(2), cytotoxic reactive oxygen species, which cause tumor ablation. Given that the photosensitizer hypericin is under consideration for PDT treatment of bladder cancer we used oligonucleotide microarrays in the T24 bladder cancer cell line to identify differentially expressed genes with therapeutic potential. This study reveals that the expression of several genes involved in various metabolic processes, stress-induced cell death, autophagy, proliferation, inflammation and carcinogenesis is strongly affected by PDT and pinpoints the coordinated induction of a cluster of genes involved in the unfolded protein response pathway after endoplasmic reticulum stress and in antioxidant response. Analysis of PDT-treated cells after p38(MAPK) inhibition or silencing unraveled that the induction of an important subset of differentially expressed genes regulating growth and invasion, as well as adaptive mechanisms against oxidative stress, is governed by this stress-activated kinase. Moreover, p38(MAPK) inhibition blocked autonomous regrowth and migration of cancer cells escaping PDT-induced cell death. This analysis identifies new molecular effectors of the cancer cell response to PDT opening attractive avenues to improve the therapeutic efficacy of hypericin-based PDT of bladder cancer.

Deficiency in apoptotic effectors Bax and Bak reveals an autophagic cell death pathway initiated by photodamage to the endoplasmic reticulum.

Efficient exploitation of cell death mechanisms for therapeutic purpose requires the identification of the molecular events committing cancer cells to death and the intracellular elements of the pro-death and pro-survival machinery activated in response to the anticancer therapy. Photodynamic therapy (PDT) is a paradigm of anticancer therapy utilizing the generation of reactive oxygen species to kill the cancer cells. In this study we have identified the photodamage to the sarco(endo)plasmic-reticulum Ca(2+)-ATPase (SERCA) pump and consequent loss in the ER-Ca(2+) homeostasis as the most apical molecular events leading to cell death in hypericin-photosensitized cells. Downstream of the ER-Ca(2+) emptying, both caspase-dependent and -independent pathways are activated to ensure cell demise. The induction of apoptosis as a cell death modality is dependent on the availability of proapopototic Bax and Bak proteins, which are essential effectors of the mitochondrial outer membrane permeabilization (MOMP) and subsequent caspase activation. In Bax(-/-)/Bak(-/-) cells a nonapoptotic pathway dependent on sustained autophagy commits the oxidatively damaged cells to death. These results argue that the decision to die in this paradigm of oxidative stress is taken upstream of Bax-dependent MOMP and that the irreversible photodamage to the ER induced by hypericin-PDT acts as a trigger for an autophagic cell death pathway in apoptosis-deficient cells.