Gastrointestinal stromal tumor (GIST) is a prototype of mutant KIT oncogene-driven tumor. Prolonged tyrosine kinase inhibitor (TKI) treatment may result in a resistant phenotype through acquired secondary KIT mutation. Heat shock protein 90 (HSP90AA1) is a chaperone protein responsible for protein maturation and stability, and KIT is a known client protein of HSP90AA1. Inhibition of HSP90AA1 has been shown to destabilize KIT protein by enhancing its degradation via the proteasome-dependent pathway. In this study, we demonstrated that NVP-AUY922 (AUY922), a new class of HSP90AA1 inhibitor, is effective in inhibiting the growth of GIST cells expressing mutant KIT protein, the imatinib-sensitive GIST882 and imatinib-resistant GIST48 cells. The growth inhibition was accompanied with a sustained reduction of both total and phosphorylated KIT proteins and the induction of apoptosis in both cell lines. Surprisingly, AUY922-induced KIT reduction could be partially reversed by pharmacological inhibition of either autophagy or proteasome degradation pathway. The blockade of autophagy alone led to the accumulation of the KIT protein, highlighting the role of autophagy in endogenous KIT turnover. The involvement of autophagy in endogenous and AUY922-induced KIT protein turnover was further confirmed by the colocalization of KIT with MAP1LC3B-, acridine orange- or SQSTM1-labeled autophagosome, and by the accumulation of KIT in GIST cells by silencing either BECN1 or ATG5 to disrupt autophagosome activity. Therefore, the results not only highlight the potential application of AUY922 for the treatment of KIT-expressing GISTs, but also provide the first evidence for the involvement of autophagy in endogenous and HSP90AA1 inhibitor-induced KIT degradation.

Purpose: KIT mutations, the most prevalent genetic event in gastrointestinal stromal tumors (GIST), are associated with malignant features and poor prognosis. Aggressive GISTs possess a high propensity to spread to the liver. This study aimed to explore the role of KIT mutations in GIST liver metastasis.Experimental Design: A total of 170 GISTs were used to determine the association between KIT mutations and liver metastasis. Immunohistochemistry was performed to assess the correlation of KIT mutations with CXCR4 and ETV1 expression. Genetic and pharmacologic methods were used to study the regulation of CXCR4 and ETV1 by KIT mutations.Results: Codons 557 and 558 in KIT exon 11 were deletion hot spots in GISTs. KIT exon 11 deletions involving codons 557-558 were highly associated with liver metastasis. Overexpression of mutant KIT with exon 11 codons 557-558 deletion (KIT D557-558) increased GIST cell motility and liver metastasis. Mechanistically, overexpression of KIT D557-558 in GIST cells increased ETV1 and CXCR4 expression. CXCR4 knockdown counteracted KIT D557-558-mediated cell migration. Moreover, KIT D557-558-induced CXCR4 expression could be abolished by silencing ETV1. The chromatin immunoprecipitation assay showed that ETV1 directly bound to the CXCR4 promoter. After ERK inhibitor PD325901 treatment, the upregulation of ETV1 by KIT D557-558 was prevented. In addition, KIT exon 11 codons 557-558 deletion enhanced CXCL12-mediated GIST cell migration and invasion.Conclusions: KIT exon 11 557-558 deletion upregulates CXCR4 through increased binding of ETV1 to the CXCR4 promoter in GIST cells, which thus promotes liver metastasis. These findings highlighted the potential therapeutic targets for metastatic GISTs.