Histone H3.3 chaperone HIRA is required for acquired tolerance by rendering stress-responsive genes poised for prospective lethal stresses

Appropriate responses to environmental challenges are imperative for the survival of all living organisms. Exposure to low-dose stresses is recognized to yield increased cellular fitness, a phenomenon termed hormesis. However, our molecular understanding of how cells respond to low-dose stress remains profoundly limited. Here we report that histone variant H3.3-specific chaperone, HIRA, is required for acquired tolerance, where low-dose heat stress exposure confers resistance to subsequent lethal heat stress. We found that human HIRA activates stress-responsive genes, including HSP70, by depositing histone H3.3 following low-dose stresses. These genes are also marked with histone H3 Lys-4 trimethylation and H3 Lys-9 acetylation, both active chromatin markers. Moreover, depletion of HIRA greatly diminished acquired tolerance, both in normal diploid fibroblasts and in HeLa cells. Collectively, our study revealed that HIRA is required for eliciting adaptive stress responses under environmental fluctuations and is a master regulator of stress tolerance. To test the effects of knocking down the histone chaperone HIRA on whole transcripts, total RNA was extracted from HIRA-knockdown (shHIRA) and control (shLuc) cells and hybridyzed on Affymetrix microarrays. We sought to obtain differentially expressed genes between those cells.