We examined the effects of reducing agents on the expression of heat s
hock protein 27 (hsp27), alpha B crystallin, and hsp70 in C6 rat gliom
a cells in response to stress. Cells were exposed to arsenite (100 mu
M for 1 h) in the presence of dithiothreitol at various concentrations
(0.03-2 mM), and the accumulation of all three proteins was markedly
stimulated in cells that had been exposed to arsenite in the presence
of a low concentration (0.03-0.1 mM) of dithiothreitol. Stimulation of
these arsenite-induced responses was also observed in the presence of
0.1 mM 2-mercaptoethanol or 0.05 mM dithioerythritol. The enhanced ex
pression of mRNAs for hsp27, alpha B crystallin and hsp70, as well as
the prolonged activation of heat shock transcription factor 1 (HSF1),
were also observed in cells that had been treated with arsenite in the
presence of 0.05 mM dithiothreitol. The arsenite-inducible expression
of the three proteins was completely suppressed when dithiothreitol w
as present at concentrations above 1 mM during the stress period, alth
ough delayed activation of the binding to a heat shock element (HSE) b
y phosphorylated HSF was observed in these cells, Exposure of cells fi
rst to arsenite for 1 h and then to dithiothreitol resulted in a very
effective suppression of the arsenite-inducible responses, and the res
ponses were inhibited even by a low concentration of dithiothreitol. T
hese results suggest that the signal transduction pathway for the arse
nite-induced expression of hsps involves at least two redox-sensitive
steps: (i) a process that is stimulated by mild reducing power during
the stress period; and (ii) a process that is followed by the activati
on of HSF and is very sensitive to suppression by a reducing agent.