Expression of small stress proteins (shsp) enhances the survival of mammalian cells exposed to heat or oxidative injuries. Recently, we have shown that the expression of shsp from different species, such as human hsp27, Drosophila hsp27 or human alphaB‐crystallin protected murine L929 cells against cell death induced by tumor necrosis factor (TNFalpha), hydrogen peroxide or menadione. Here, we report that, in growing L929 cell lines, the presence of these shsp decreased the intracellular level of reactive oxygen species (ROS). shsp expression also abolished the burst of intracellular ROS induced by TNFalpha. Several downstream effects resulting from the TNFalpha‐mediated ROS increment, such as NF‐kappaB activation, lipid peroxidation and protein oxidation, were inhibited by shsp expression. We also report that the expression of these different shsp raised the total glutathione level in both L929 cell lines and transiently transfected NIH 3T3‐ras cells. This phenomenon was essential for the shsp‐mediated decrease in ROS and resistance against TNFalpha. Our results therefore suggest that the protective activity shared by human hsp27, Drosophila hsp27 and human alphaB‐crystallin against TNFalpha‐mediated cell death and probably other types of oxidative stress results from their conserved ability to raise the intracellular concentration of glutathione.

We have previously shown that the small heat shock protein HSP27 inhibited apoptotic pathways triggered by a variety of stimuli in mammalian cells. The present study demonstrates that HSP27 overexpression decreases U937 human leukemic cell sensitivity to etoposide-induced cytotoxicity by preventing apoptosis. As observed for Bcl-2, HSP27 overexpression delays poly(ADP-ribose)polymerase cleavage and procaspase-3 activation. In contrast with Bcl-2, HSP27 overexpression does not prevent etoposide-induced cytochrome c release from the mitochondria. In a cell-free system, addition of cytochrome c and dATP to cytosolic extracts from untreated cells induces the proteolytic activation of procaspase-3 in both control and bcl-2-transfected U937 cells but fails to activate procaspase-3 in HSP27-overexpressing cells. Immunodepletion of HSP27 from cytosolic extracts increases cytochrome c/dATP-mediated activation of procaspase-3. Overexpression of HSP27 also prevents procaspase-9 activation. In the cell-free system, immunodepletion of HSP27 increases LEDH-AFC peptide cleavage activity triggered by cytochrome c/dATP treatment. We conclude that HSP27 inhibits etoposide-induced apoptosis by preventing cytochrome c and dATP-triggered activity of caspase-9, downstream of cytochrome c release.

Heat shock proteins (HSP) are a subset of the molecular chaperones, best known for their rapid and abundant induction by stress. HSP genes are activated at the transcriptional level by heat shock transcription factor 1 (HSF1). During the progression of many types of cancer, this heat shock transcriptional regulon becomes co-opted by mechanisms that are currently unclear, although evidently triggered in the emerging tumor cell. Concerted activation of HSF1 and the accumulation of HSPs then participates in many of the traits that permit the malignant phenotype. Thus cancers of many histologies exhibit activated HSF1 and increased HSP levels that may help to deter tumor suppression and evade therapy in the clinic. We review here the extensive work that has been carried out and is still in progress aimed at: (1) understanding the oncogenic mechanisms by which HSP genes are switched on, (2) determining the roles of HSF1 / HSP in malignant transformation and, (3) discovering approaches to therapy based on disrupting the influence of the HSF1 controlled transcriptome in cancer.