Selective light-induced modulation of Bcl-XL and Bax expressions in indocyanine green-loaded U937 cells: effects of continuous or intermittent photo-sensitization with low IR-light using a 805-nm diode laser

The present study is concerned with the effects of an IR diode laser source emitting at 805 nm on a human leukaemic cell strain from a histiocytic lym phoma (U937) pre-loaded with the dye indocyanine green (ICG). The first aim of this work was the assessment of the earliest cellular defense events oc curring upon ICG photosensitization. To this purpose, photosensitization wa s performed at low ICG concentration and low light energy density. The seco nd aim was the comparative evaluation of the effects produced by continuous or fragmented irradiation. Independent of the irradiation method employed (continuous or fragmented), we could demonstrate that cells are forced to a poptosis, as indicated by the appearance in cell extracts of the 85-kDa pro teolytic fragment of the poly(ADP-ribose)polymerase (PARP). This proteolysi s, however, could be entirely prevented by a specific Caspase-3 inhibitor b enzyloxycarbonyl-Asp-Glu-Val-Aspfluoromethylketone (Z-DEVD-fmk). Indeed the only perceptible change in the expression of pro/antiapoptotic proteins pr oduced by continuous photostimulation was a small, albeit reproducible, inc rease in Bar. In contrast, when the photostimulation was achieved by means of several consecutive pulses, we observed not only a remarkable increase i n Bar but also a noticeable abatement in Bcl-XL expression. The potential i nvolvement of singlet oxygen in this process has been directly demonstrated by ICG photo-mediated oxidation of dithiothreitol in water. It has also be en demonstrated that this oxidation is apparently more efficient when ICG i s photostimulated by light pulses. (C) 2000 Elsevier Science S.A. All right s reserved.