NIK is required for NF-κB-mediated induction of BAG3 upon inhibition of constitutive protein degradation pathways

Recently, we reported that induction of the co-chaperone Bcl-2-associated athanogene 3 (BAG3) is critical for recovery of rhabdomyosarcoma (RMS) cells after proteotoxic stress upon inhibition of the two constitutive protein degradation pathways, that is, the ubiquitin-proteasome system by Bortezomib and the aggresome-autophagy system by histone deacetylase 6 (HDAC6) inhibitor ST80. In the present study, we investigated the molecular mechanisms mediating BAG3 induction under these conditions. Here, we identify nuclear factor-kappa B (NF-kappa B)-inducing kinase (NIK) as a key mediator of ST80/Bortezomib-stimulated NF-kappa B activation and transcriptional upregulation of BAG3. ST80/Bortezomib cotreatment upregulates mRNA and protein expression of NIK, which is accompanied by an initial increase in histone H3 acetylation. Importantly, NIK silencing by siRNA abolishes NF-kappa B activation and BAG3 induction by ST80/Bortezomib. Furthermore, ST80/Bortezomib cotreatment stimulates NF-kappa B transcriptional activity and upregulates NF-kappa B target genes. Genetic inhibition of NF-kappa B by overexpression of dominant-negative I kappa B alpha superrepressor (I kappa B alpha-SR) or by knockdown of p65 blocks the ST80/Bortezomib-stimulated upregulation of BAG3 mRNA and protein expression. Interestingly, inhibition of lysosomal activity by Bafilomycin A1 inhibits ST80/Bortezomib-stimulated I kappa B alpha degradation, NF-kappa B activation and BAG3 upregulation, indicating that I kappa B alpha is degraded via the lysosome in the presence of Bortezomib. Thus, by demonstrating a critical role of NIK in mediating NF-kappa B activation and BAG3 induction upon ST80/Bortezomib cotreatment, our study provides novel insights into mechanisms of resistance to proteotoxic stress in RMS.