STIMULATORY AND INHIBITORY EFFECTS OF IRON ON PHOTODYNAMIC INACTIVATION OF LEUKEMIA-CELLS

The influence of exogenous iron on merocyanine 540 (MC540)-sensitized photoinactivation of leukemia cells has been investigated. Irradiation of murine L1210 or human HL-60 cells (similar to 10(6)/mL in 1% serum /RPMI medium) with broadband visible light in the presence of MC540 (2 mu M) resulted in a progressive loss of clonally assessed cell viabil ity. When added to cells 30 min before irradiation, the low polarity c helate, ferric 8-hydroxyquinoline [Fe(HQ)2, 0.5 mu M] stimulated dye-s ensitized photokilling, whereas high polarity chelates such as ferric 8-hydroxyquinoline-5-sulfonate [Fe(HQS)(2), 0.5 mu M] or ferric ethyle nediaminetetraacetate (Fe . EDTA, 0.5 mu M) had no effect. A striking reversal of Fe(HQ)(2)-enhanced photokilling was observed upon increasi ng the preirradiation incubation time with Fe(HQ)(2) such that a marke d resistance (relative to non-iron-treated controls) was evident after 24 h. Cells exposed for 24 h to Fe(HQS)(2) or Fe . EDTA showed simila r or even greater resistance to photokilling. Like phototoxicity, H2O2 -induced cytotoxicity was enhanced after a 30 min exposure of cells to Fe(HQ)(2) but strongly repressed after 24 h. Immunoblot (western) ana lysis, using a polyclonal antibody to ferritin, revealed that cells ex posed to Fe(HQ)(2) for 24 h contained at least 12 times as much ferrit in heavy chain as non-Fe(HQ)(2)-treated controls. Preincubating cells with emetine, an inhibitor of protein synthesis, prevented both ferrit in induction and the development of hyperresistance. These findings, a long with the observation that exogenous apoferritin protected L1210 c ells against photokilling, suggest a possible role for ferritin in iro n-stimulated photoresistance. It is conceivable that in photodynamic t reatment of tumors, certain cells might resist inactivation via this m echanism, a possibility that has not been recognized heretofore.