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 (approximately 10(6)/mL in 1% serum/RPMI medium) with broadband visible light in the presence of MC540 (2 microM) resulted in a progressive loss of clonally assessed cell viability. When added to cells 30 min before irradiation, the low polarity chelate, ferric 8-hydroxyquinoline [Fe(HQ)2, 0.5 microM] stimulated dye-sensitized photokilling, whereas high polarity chelates such as ferric 8-hydroxyquinoline-5-sulfonate [Fe(HQS)2, 0.5 microM] or ferric ethylenediaminetetraacetate (Fe.EDTA, 0.5 microM) had no no effect. A striking reversal of Fe(HQ)2-enhanced photokilling was observed upon increasing the preirradiation incubation time with Fe(HQ)2 such that a marked 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 similar 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) analysis, using a polyclonal antibody to ferritin, revealed that cells exposed to Fe(HQ)2 for 24 h contained at least 12 times as much ferritin heavy chain as non-Fe(HQ)2-treated controls. Preincubating cells with emetine, an inhibitor of protein synthesis, prevented both ferritin induction and the development of hyperresistance. These findings, along with the observation that exogenous apoferritin protected L1210 cells against photokilling, suggest a possible role for ferritin in iron-stimulated photoresistance.(ABSTRACT TRUNCATED AT 250 WORDS)