title: Photo-activation of hypericin with low doses of light promotes apparent photo-resistance in human histiocytic lymphoma U937 cells.
abstract: We have observed that exposure of U937 cells, pre-incubated for 18 h with 0.2 microM hypericin, to 599 nm laser radiation with a fluence of 2.5 J/cm(2) renders them insensitive to higher light doses. In fact, pre-sensitized cells appear to be fully resistant to light doses that normally determine massive cellular apoptosis in experimental photo-dynamic therapy. The appearance of the photo-resistance, as measured by evaluating the changes in levels of expression of pro and anti apoptotic proteins, PARP fragmentation and cell viability is exclusively observed with exposure to light doses not exceeding 5-6 J/cm(2). Above this energy, necrosis replaces apoptosis upon photo-stimulation of U937 cells. Here, we describe the appearance of photo-resistance in hypericin-loaded U937 cells, but could not fully unravel the molecular mechanism underlying this process. However, the observed stimulation of the expression of the HSP-70 protein upon photo-induced stress may suggest its involvement in this process.
authors.affiliation: Dipartimento di Biologia e Patologia Cellulare e Molecolare 'Luigi Califano' and CEOS-CNR, University of Naples, Federico II, Via S. Pansini 5, 80131, Naples, Italy.
authors.forename: V
authors.initials: V
authors.lastname: Paba
authors.forename: M
authors.initials: M
authors.lastname: Quarto
authors.forename: L
authors.initials: L
authors.lastname: Varriale
authors.forename: E
authors.initials: E
authors.lastname: Crescenzi
authors.forename: G
authors.initials: G
authors.lastname: Palumbo
mesh_terms: D000873:Anthracenes; D017209:Apoptosis / Q000187:drug effects*; D016923:Cell Death / Q000187:drug effects / Q000528:radiation effects; D004307:Dose-Response Relationship, Radiation; D019008:Drug Resistance, Neoplasm; D018840:HSP70 Heat-Shock Proteins / Q000187:drug effects / Q000528:radiation effects; D006801:Humans; D009336:Necrosis; D010569:Perylene / Q000031:analogs & derivatives* / Q000378:metabolism / Q000494:pharmacology* / Q000528:radiation effects*; D010789:Phototherapy; D011832:Radiation Injuries; D020298:U937 Cells / Q000187:drug effects / Q000473:pathology
chemical_list: D000873:Anthracenes; D018840:HSP70 Heat-Shock Proteins; D010569:Perylene; C004965:hypericin