Selenium treatment of the
doxorubicin-resistant cell line, U-1285dox, derived from human
small cell carcinoma of the lung, resulted in massive apoptosis. This effect appeared maximal at 2 days after addition of
selenite. The apoptosis was
caspase-3 independent as revealed by Western blot analysis, activity measurement and by using
caspase inhibitors. Induction of apoptosis was significantly more pronounced and occurred after addition of lower concentrations of
selenite in the
doxorubicin-resistant cells compared to the parental
doxorubicin-sensitive cells. High levels of
selenite caused
necrosis in the
doxorubicin-sensitive cells. Analysis of enzymatic activity (
insulin reduction) of
thioredoxin reductase (TrxR) and TrxR
protein concentration, measured by ELISA, revealed increasing activity and
protein levels
after treatment with increasing concentrations of
selenium. Maximum relative increase was induced up to 1 microM in both sublines and at this
selenium level the concentrations of TrxR measured as
insulin reducing activity or ELISA immunoreactivity were nearly identical. Increasing concentrations of
selenite up to 10 microM resulted in increased activity and concentration of TrxR in the sensitive subline but decreasing levels in the resistant subline. The level of truncated Trx (tTrx) was higher in the resistant U-1285dox cells but the level did not change with increasing
selenite concentrations. Our results demonstrate pronounced selective
selenium-mediated apoptosis in
therapy-resistant cells and suggest that redox regulation through the
thioredoxin system is an important target for
cancer therapy.