Mammalian cells are equipped with elaborate systems for protection against the toxicity of reactive
oxygen and
nitrogen species and electrophiles that are constant dangers to the integrity of their
DNA. Phase 2
enzymes (e.g.,
glutathione transferases,
NAD(P)H:
quinone reductase) and
glutathione synthesis are widely recognized as playing major protective roles against electrophilic
carcinogens, but their
antioxidant functions have attracted far less attention. The cytotoxicities of four oxidative stressors (
menadione,
tert-butyl hydroperoxide, 4-hydroxynonenal, and
peroxynitrite) for human adult
retinal pigment epithelial cells (ARPE-19) were quantified by measuring the concentration dependence of cell death and were expressed as the median effect dose (D(m)) for each
oxidant.
After treatment of ARPE-19 cells for 24 h with 0-5 microM concentrations of
sulforaphane (the powerful Phase 2
enzyme inducer isolated from broccoli), the toxicities of the
oxidants were markedly reduced as shown by 1.5- to 3-fold increases in D(m) values. The magnitude of protection was a function of the nature of the
oxidants and the concentrations of both the
oxidants and
sulforaphane. Protection was prolonged and persisted for several days after removal of
sulforaphane before returning to control levels. The
sulforaphane-dependent increases in specific activities of cytosolic
quinone reductase and the
glutathione levels were highly significantly correlated with the degree of protection as measured by D(m) values.
Antioxidant protection was also demonstrated for human HaCaT keratinocytes and L1210 murine
leukemia cells. It is therefore highly likely that the multifaceted and prolonged
antioxidant protection provided by
sulforaphane is a general phenomenon that is mediated through induction of the Phase 2
enzyme response.