Thioredoxin is transcriptionally induced upon activation of heat shock factor 2

Heat shock gene expression is differentially regulated in cells exposed to stress stimuli and in cells undergoing processes of differentiation and development. Regulation of the classical heat shock response is mediated by heat shock factor 1 (HSF1), whereas heat shock factor 2 (HSF2) is activated in certain differentiating cells, for example during hemin-mediated differentiation of human K562 erythroleukemia cells. Hence, the signaling pathways leading to induction of heat shock gene expression upon different stimuli are likely to be distinct. We have used RNA arbitrarily primed polymerase chain reaction to identify genes that are differentially regulated upon activation of HSF1 and HSF2. In this study, we report that thioredoxin (TRX) expression is induced in K562 cells in response to hemin in an HSF2-dependent manner. Increased TRX expression was primarily detected on the transcriptional level, subsequently leading to elevated TRX mRNA and protein levels. Hemin treatment caused no reduction in cellular glutathione concentrations, indicating that the increased TRX expression was not due to oxidative stress. Studies using cell lines where overexpression of the HSF2-beta isoform represses HSF2 activation implied that active HSF2 is required for transcriptional induction of TRX. Unlike HSF2, activation of HSF1 did not induce TRX expression. Taken together, our results suggest that HSF1 and HSF2 may regulate distinct target genes, and activation of HSF2 could be involved in the regulation of TRX expression during hemin-mediated differentiation of K562 cells.