Identification and characterization of promoter elements responsible for the induction of the albumin gene by heat shock in early embryonic rat liver

We had reported earlier that the expression of albumin increases upon heat shock in embryonic rat liver cells at about 12-13 days of gestation. Here, we report on the identification of heat shock elements (HSEs) within -450 bp of the rat albumin promoter using chloramphenicol acetyl transferase (CAT) assays done with the extracts from H4II-E-C3 cells transfected with plasmids carrying the CAT reporter gene under the control of different deletion fragments of the rat albumin promoter. Gel retardation assays done with synthetic oligonucleotides representing putative HSEs in the rat albumin promoter and H4II-E-C3 cell extracts show that the heat shock factors bind this region in a sequence-specific and reversible manner. Super-shift assays demonstrated that the HSEs present in the rat albumin promoter are bound by HSF1 and not by HSF2. This effect of heat shock on the expression of rat serum albumin is seen only in the liver and is not observed in other tissues, suggesting that HSF-mediated activation of albumin gene cannot overcome the negative regulatory factors present in other tissues. In addition to the HSEs, we have identified a putative GAGA factor binding site in the rat albumin promoter at -228 bp to -252 bp position. These GAGA repeats are bound in a sequence-specific and reversible manner by two factors in a nonstressed cell, whereas only one of these two factors continues to bind the GAGA repeats under heat shock conditions. The physiological significance of these results is discussed.