The Effects Of HSP70 In TGF-β Signal Transduction

The transforming growth factorβ(TGF-β) superfamily are a group of secretedpolypeptides that regulate a diverse array of developmental and biological processes,including cell proliferation, differentiation, apoptosis, and specification of developmental fate.As the most important elements, Smad2 interacts with other cofactors in the nucleus toinfluence transcriptional activation. Frequently, the interactions between Smad2 and itsbinding priteins control the signal transduction.In this study, we adopted a glutathione-S-transferase (GST) pull down assay usingzebrafish Smad2 (zSmad2) as the bait to detect potential Smad2 binding proteins. Then wereport that the 70-kDa heat-shock protein (HSP70) driectly interacts with Smad2, and theassociation of these two proteins is conserved between different species. Subsequently, theARE reporter assays support the conclusion that overexpression of HSP70 has a repressivefunction on TGF-β/Smad2 signal transduction.Next, we found that HSP70 was localized exclusively in the cytoplasm in HaCaT cellsregardless of TGF-βexistence. And the association of these two proteins did not significantlychange after activating TGF-βsignaling by expressing the constitutively active type I TGF-βreceptor, caALK5. Moreover, the receptor-dependent phosphorylation of Smad2 wasmarkedly decreased by HSP70 expression. Furthermore, the TGF-βinduced Smad2 nucleartranslocation was evidently disturbed by overexpression of HSP70.More important, HSP70 blocks TGF-β-induced epithelial-mesenchymal transition (EMT)in HaCat cells. Our findings reveal an essential role of HSP70 in TGF-β-induced epithelialmesenchymaltransition (EMT) by impeding Smad2 phosphorylation.HSP70 is highly expressed in cancer cells. However, TGF-βsuppresses spontaneoustumorigenesis. According to our findings, high expression of HSP70 in cells blocks TGF-β-induced phosphorylation of Smad proteins, leading to resistance to TGF-βstimulus andpromotion of tumorigenesis.