BAG-1M regulates keratin-associated Hsp70 chaperoning of aPKC in intestinal cells under inflammatory signaling

Abstract

Atypical PKC (ι/λ and ζ) is a key player in the acquisition of epithelial polarity and participates in other signaling cascades including NF-kB control. This kinase is post-translationally regulated by Hsp70-mediated refolding. Previous work has shown that such a chaperoning activity is specifically localized on keratin intermediate filaments. This work was performed with the goal of identifying the molecule(s) blocking Hsp70 activity on keratin filaments in inflammation. A transcriptional screen allowed us to focus on BAG-1, a multi-functional protein which assists Hsp70 in nucleotide exchange, but also blocks its activity at higher concentrations. We found the BAG-1M isoform upregulated 3 fold under TNFα stimulation in Caco-2 cells and up to 6 fold in mouse enterocytes under DSS colitis. BAG-1M but not other isoforms, was found to copurify with intermediate filaments and block Hsp70 activity in the keratin fraction but not in the soluble fraction within the range of concentrations found in epithelial cells. BAG-1M constitutive expression decreased p-aPKC. BAG-1 knockdown, conversely, blocked the TNFα-induced decreased levels of p-aPKC. We conclude that BAG-1M mediates Hsp70 inhibition downstream of NF-kB.