Regulation of rat heat shock factor 2 expression during the early organogenic phase of embryogenesis

A central step in the transcriptional regulation of heat shock protein (hsp) genes is the binding of the heat shock factor (HSF) to the upstream heat shock elements (HSEs). In vertebrates, HSF2 has been suggested to mediate the transcriptional regulation of hsp gene expression during development and differentiation. The expression levels of HSF2 were shown to vary widely among fully developed mouse organs. However, there exists limited information on the regulation of HSF2 expression during the inductive stage of organ formation in mammalian development. In this study, we have cloned the rat HSF2 cDNA and examined embryos for HSF2 expression from days 9.5 (E9.5) to 15.5 (E15.5) of gestation that correspond to the period when the major organ primordia are being actively established. We show that rat HSF2 has 94.6 and 96.3% identity to mouse HSF2 in nucleotide and amino acid sequences, respectively. By establishing a competitive RT-PCR, we show that about 503.6 pg of HSF2 mRNA were present per microgram of embryonic RNA in the primitive streak stage E9.5 embryos. The amounts of HSF2 mRNA then gradually decreased, resulting in an approximately 300-fold reduction in E15.5 embryos. The amounts of HSF2 mRNA in the embryos were found to be closely correlated with those of HSF2 protein and their HSE-binding activities. To our knowledge, this is the first detailed report on the structure and regulation of the rat HSF2 during the early organogenic period of mammalian embryogenesis.