Cell-type-dependent access of HSF1 and HSF4 to αB-crystallin promoter during heat shock

Epithelial cells and fibroblasts both express heat shock transcription factors, HSF1 and HSF4, yet they respond to heat shock differentially. For example, while HSP70 is induced in both cell types, the small heat shock protein, alpha B-crystallin gene (CRYAB) that contains a canonical heat shock promoter, is only induced in fibroblasts. A canonical heat shock promoter contains three or more inverted repeats of the pentanucleotide 5'-nGAAn-3' that make the heat shock element. It is known that, in vitro, promoter architecture (the order and spacing of these repeats) impacts the interaction of various heat shock transcription factors (HSFs) with the heat shock promoter, but in vivo relevance of these binding preferences so far as the expression is concerned is poorly understood. In this report, we first establish cell-type-dependent differential expression of CRYAB in four established cell lines and then working with adult human retinal pigment epithelial cells and NIH3T3 fibroblasts and employing chromatin immunoprecipitation, attempt to relate expression to promoter occupancy by HSF1 and HSF4. We show that HSF4 occupies only CRYAB and not HSP70 promoter in epithelial cells, while HSF1 occupies only HSP70 promoter in both cell types, and cryab promoter, only in heat shocked fibroblasts; HSF4, on the other hand, is never seen on these two promoters in NIH3T3 fibroblasts. This comparative analysis with CRYAB and HSP70 demonstrates that differential heat shock response is controlled by cell-type-dependent access of HSFs (HSF1 and HSF4) to specific promoters, independent of the promoter architecture.