The molecular basis of the differential sensitivity of normal hematopoietic stem cells and of
leukemia,
lymphoma, and
neuroblastoma cells to
merocyanine 540 (MC540)-mediated
photodynamic therapy (
PDT) is not yet completely understood. While the capacity to bind
dye molecules appears to be the major determinant of a cell's susceptibility of MC540-mediated
PDT, we here present evidence that under certain experimental conditions a cell's capacity to repair MC540-mediated photodynamic damage is also an important factor. Two parameters, temperature and intracellular
glutathione (GSH) content, were varied to investigate the role of cellular defense mechanisms in the
dye-sensitized photoinactivation of normal murine granulocyte/macrophage progenitors (CFU-GM) and K562, L1210, and
melphalan-resistant L1210/L-PAM1
leukemia cells. When exposed to MC540 and light at room temperature, the three
leukemia cell lines bound similar amounts of
dye and accumulated similar amounts of
lipid hydroperoxide (LOOH) but differed markedly in their sensitivity to MC540-mediated
PDT. Performing MC540-mediated
PDT at 4 degrees C instead of at room temperature reduced
dye binding and LOOH generation and enhanced cytotoxicity in some but not all cell lines. A brief (< or = 120 minutes) incubation at 37 degrees C immediately following MC540-mediated
PDT accelerated the decay of LOOH in all leukemic cell lines and reduced cell kill by about 2 log in both CFU-GM and
leukemia cells. The effect of post-
PDT incubation at 37 degrees C on LOOH decay was most pronounced in K562 and least pronounced in L1210/L-PAM1 cells, whereas its effect on cell survival was less pronounced in L1210 cells than in the remaining cell types. L1210/L-PAM1 cells whose GSH content had been reduced from 8.2 to 1.6 micrograms/mg
protein by incubation with
buthionine sulfoximine recovered from potentially lethal photodynamic damage as rapidly as untreated L1210/L-PAM1 cells and more rapidly than wild-type L1210 cells with a GSH content of 4.5 micrograms/mg
protein. Thus, with regard to capacity of L1210/L-PAM1 cells to recover from photodynamic damage, the cells' enhanced capacity to synthesize GSH appeared more decisive than intracellular GSH levels per se. Taken together, these data suggest that temperature-dependent cellular defense mechanisms are significant determinants of a cell's susceptibility to MC540-mediated
PDT. The data emphasize the need for temperature control during and immediately after the photochemical purging of autologous bone marrow or peripheral blood stem cells.