The generation of photoproducts of psoralen (POPs) might be relevant in cell death induced by psoralen plus UVA, namely PUVA, which is a recognized effective treatment for cutaneous T-cell lymphoma, chronic graft-versus-host disease, and psoriasis. We investigated the occurrence of POP-induced cell death and the underlying mechanisms. POPs were produced by irradiating a psoralen solution with UVA. Jurkat cells treated in the dark with these mixtures died mainly through an apoptotic mechanism. POPs were separated by high-performance liquid chromatography (HPLC), and cells were added with each of these fractions. A total of 2 dimers of psoralen and 6-formyl-7-hydroxycoumarin (FHC) were identified in the apoptogenic fractions. Apoptosis was preceded by mitochondrial dysfunction caused by the opening of the mitochondrial permeability transition pore (PTP). In fact, both mitochondrial depolarization and cell death were prevented by the PTP inhibitor cyclosporin A (CsA). PTP opening was also documented in isolated mitochondria added with POP, suggesting that apoptosis is caused by a direct effect of POP on mitochondria. In fact, FHC alone induced PTP opening and CsA-inhibitable cell death of Jurkat cells, whereas nontransformed T lymphocytes were resistant. Along with identifying novel apoptogenic molecules, the present results indicate that POP generation directs transformed cells to apoptosis. IntroductionThe combination of psoralens and UVA irradiation, which is commonly referred to as PUVA, represents a therapeutic approach useful in the treatment of cutaneous T-cell lymphoma (CTCL) 1 as well as skin diseases such as psoriasis and vitiligo. 2 In particular, the combination of interferon ␣-2a with oral photochemotherapy (PUVA) resulted in 70% complete remission in patients with CTCL. 1 A modified PUVA protocol is called extracorporeal photopheresis. This treatment involves the mechanical separation of mononuclear white cells, which are treated with psoralen and exposed to UVA light, and then returned to the patient. 3 Besides CTCL, extracorporal photopheresis is a recognized effective treatment modality in chronic graft-versus-host disease. 4 Different mechanisms have been related to PUVA cytotoxicity. 5 Besides the generation of reactive oxygen species, psoralen has been shown to bind covalently to and cross-link DNA. 6 Laskin and coworkers demonstrated that mammalian cells have specific highaffinity receptor sites for psoralens, distinct from DNA, but their molecular characterization has been not elucidated yet. 7 More recently, PUVA treatment has been reported to cause cell death by apoptosis, 8 and we have demonstrated that the underlying mechanism involves mitochondrial dysfunction caused by opening of the permeability transition pore (PTP). 9 On the other hand, upon UVA irradiation, psoralen photoproducts (POPs) are generated, which are likely to contribute to PUVA cytotoxicity. 10 In fact, POP were reported to elicit a wide variety of effects, such as oxidation of proteins and unsaturated lipids, modulation...