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)