Overexpression of Heat Shock Factor 1 Masks the Heavy Metal Response of the Heat Shock Protein 70 (hsp70) Gene Promoter

Abstract

Transcription of the heat shock protein 70 (hsp70) gene is induced by heavy metals including Cd and Zn, probably through the interaction between the heat shock elements (HSE) and heat shock factor 1 (HSF1) binding to it. We studied the mechanism of this transcriptional activation by means of electrophoretic mobility shift assay (EMSA) and transient transfection assay. In HeLa cells, the DNA-binding activity of HSF1 was activated by Zn in a concentration-dependent manner similar to that for hsp70 gene expression, suggesting the direct contribution of HSF1 to the metal-induced transcriptional activation. To facilitate the analysis of the metal-regulatory mechanism, we attempted to establish a system that can reproduce the metal activation of overexpressed recombinant HSF1. In cells transfected with an HSF1 expression vector, strong HSE-binding activity was detected in EMSA, and reporter gene expression driven by the hsp70 gene promoter was markedly increased. However, no Zn response was observed in either assay. The overexpressed HSF1 appeared to cause the constitutive activation of reporter expression, which is an intriguing feature that might reflect an aspect of the hsp70 gene regulation.