Identification of novel pathogenic roles of BLZF1/ATF6 in tumorigenesis of gastrointestinal stromal tumor mediated by Golgi-localized mutant KIT

Background: Gastrointestinal stromal tumors (GISTs) frequently show KIT mutations, accompanied by overexpression and aberrant localization of mutant KIT (MT-KIT). However, it remains unclear how MT-KIT contributes to GIST pathogenesis. Methods To evaluate the expression, localization and stability of wild type KIT (WT-KIT) and MT-KIT, we performed western blotting, biotinylation assays and confocal microscopic analysis using GIST and colon cancer cells. Colocalization of MT-KIT with GRB2, P85, or BLZF1 was examined using immunoprecipitation and confocal microscopy. We screened various WT- and MT-KIT expressing cancer cell lines to identify a regulator of unfolded protein response (UPR) in GISTs. The effect of ATF6 inhibitors were examined in a tetrazolium-based MTT assay and GIST xenograft models. Immunohistochemistry analysis of ATF6 was performed using GIST tissues. Results We discovered that MT-KIT initiates downstream signaling in the Golgi complex. BLZF1 was identified as a novel MT-KIT-binding partner that tethers MT-KIT to the Golgi complex. Sustained activation of ATF6, which belongs to the UPR family, alleviates endoplasmic reticulum (ER) stress by upregulating chaperone expression, including HSP90, which assists in MT-KIT folding. BLZF1 knockdown and ATF6 inhibition suppressed both imatinib-sensitive and -resistant GIST in vitro . ATF6 inhibitors further showed potent antitumor effects in GIST xenografts, and the effect was enhanced with ER stress-inducing drugs. ATF6 activation was frequently observed in 67% of patients with GIST (n = 42), and was significantly associated with poorer relapse-free survival ( P  = .033). Conclusions Our findings show that GIST bypasses ER quality control (QC) and ER stress-mediated cell death via UPR activation and uses the QC-free Golgi to initiate signaling.