Tissue-dependent expression of heat shock factor 2 isoforms with distinct transcriptional activities

Heat shock factor 2 (HSF2) functions as a transcriptional regulator of heat shock protein gene expression in mammalian cells undergoing processes of differentiation and development. Our previous studies demonstrated high regulated expression and unusual constitutive DNA-binding activity of the HSF2 protein in mouse testes, suggesting that HSF2 functions to regulate heat shock protein gene expression in spermatogenic cells. The purpose of this study was to test whether HSF2 regulation in testes is associated with alterations in the HSF2 polypeptide expressed in testes relative to other mouse tissues. Our results show that mouse cells express not one but two distinct HSF2 proteins and that the levels of these HSF2 isoforms are regulated in a tissue-dependent manner. The testes express predominantly the 71-kDa HSF2-α isoform, while the heart and brain express primarily the 69- kDa HSF2-β isoform. These isoforms are generated by alternative splicing of HSF2 pre-mRNA, which results in the inclusion of an 18-amino-acid coding sequence in the HSF2-α mRNA that is skipped in the HSF2-β mRNA. HSF2 alternative splicing is also developmentally regulated, as our results reveal a switch in expression from the HSF2-β mRNA isoform to the HSF2-α isoform during testis postnatal development. Transfection analysis shows that the HSF2-α protein, the predominant isoform expressed in testis cells, is a more potent transcriptional activator than the HSF2-β isoform. These results reveal a new mechanism for the control of HSF2 function in mammalian cells, in which regulated alternative splicing is used to modulate HSF2 transcriptional activity in a tissue-dependent manner.