RESUMO
The mammalian stress protein Hsp105α is expressed constitutively and is further induced under stress conditions, whereas the alternative spliced form, Hsp105ß is only expressed during mild heat shock. We previously reported that Hsp105α is localized mainly in the cytoplasm, whereas Hsp105ß is localized in the nucleus. Consistent with the different localization of these proteins, Hsp105ß but not Hsp105α induces the expression of the major stress protein Hsp70. We here identified N-myc and Stat interactor (Nmi), as an Hsp105ß-binding protein by yeast two-hybrid screening. Immunoprecipitation and pull-down assay showed that Nmi interacts with Hsp105ß in vivo and in vitro. Luciferase reporter gene assay and Western blotting showed that Nmi enhanced both the Hsp105ß-induced phosphorylation of Stat3 and the Hsp105ß-induced activation of the hsp70 promoter in a manner that is dependent on the Stat3-binding site, which results in an increase in Hsp70 protein levels. Most importantly, mild heat shock-induced Hsp70 expression, which is dependent on Hsp105ß, is suppressed by knockdown of endogenous Nmi. These results suggest that Nmi has a role as a positive regulator of Hsp105ß-mediated hsp70 gene expression along the Stat3 signaling pathway.
