Abstract 952: Protein kinase C theta (PKCθ) and c-Jun regulate proliferation through cyclin D1 in KIT-independent gastrointestinal stromal tumors

Abstract Oncogenic KIT or PDGFRA receptor tyrosine kinase (RTK) mutations are compelling therapeutic targets in gastrointestinal stromal tumors (GISTs), and the KIT/PDGFRA kinase inhibitor, imatinib, is standard of care for patients with metastatic GIST. However, most patients eventually develop clinical resistance to imatinib and other KIT/PDGFRA kinase inhibitors due to acquisition of secondary mutations in the KIT kinase domain, genomic amplification of KIT, or activation of alternate oncoproteins. Approximately 10% of KIT-dependent GIST metastases lose KIT expression at time of clinical progression during imatinib therapy. We performed whole transcriptome sequencing in KIT-expressing GIST vs. KIT-negative GIST, and thereby identified strong cyclin D1 expression only in KIT-negative GISTs. KIT-negative GISTs showed complete loss of the GIST biomarker, PKCθ, and restoration of PKCθ expression in these cells resulted in loss of Cyclin D1 expression. Lentivirus-mediated CCND1 (Cyclin D1) short hairpin RNA (shRNA) knockdown resulted in profound anti-proliferative and pro-apoptotic effects in KIT-negative GIST cell lines, and was associated with activation (dephosphorylation) of the tumor suppressor Rb and upregulation of the p27 CDK inhibitor. c-Jun expression levels paralleled the Cyclin D1 expression in the GIST lines, and c-Jun shRNA knockdown resulted in downregulation of cyclin D1 in the KIT-negative GIST lines, and was accompanied by profound anti-proliferative effects, including p27 upregulation, and pro-apoptotic effects as evidenced by PARP cleavage. By contrast, cyclin D1 and c-Jun silencing showed little anti-proliferative and pro-apoptotic effects in KIT-positive, KIT-dependent GIST lines. These findings show that cyclin D1 and c-Jun overexpression serve a crucial oncogenic role in KIT-independent GISTs, including those enabling clinical resistance to imatinib or sunitinib targeted kinase inhibitor therapies. Our findings also show that PKCθ downregulation and c-Jun upregulation coordinately enable cyclin D1 overexpression in KIT-independent GISTs. These novel findings highlight the relevance of c-Jun/cyclin D1 as novel therapeutic targets in GISTs that have lost KIT oncoprotein expression. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 952. doi:10.1158/1538-7445.AM2011-952