C) Imunoblotting evaluations of EGFR, ERBB2, ERBB4, MET, and AXL expression and/or activation in ovarian cancer primary frozen tumors (p1-p15). is dependent on an individual kinase oncoprotein or the coordinated activity of multiple kinases. We hypothesized that a coordinated network of multi-RTK activation is important for the tumorigenesis of ovarian cancers. Results Herein, we demonstrate co-activation of multiple RTKs (EGFR, ERBB2, ERBB4, MET and/or AXL) in individual ovarian cancer cell lines and primary tumors. We also show that coordinate inhibition of this multi-kinase signaling has substantially greater effect on ovarian cancer proliferation and survival, compared to inhibition of individual activated kinases. The inhibition of this multi-RTK signaling by HSP90 suppression results in profound pro-apoptotic and anti-proliferative effects, and is associated with the inactivation of RTK downstream PI3-K/AKT/mTOR and RAF/MAPK signaling. Conclusion These studies suggest that anti-multiple RTK strategy could be useful in the treatment of ovarian cancer. Keywords: Ovarian Cancer, Tyrosine Kinases, coactivation, HSP90 Background Ovarian cancer is a leading cause of cancer death among women in Western Europe and the United States, which has the highest mortality rate of all gynecologic malignancy [1,2]. Ovarian cancer histologic subtypes include epithelioid (serous, endometrioid, mucinous, clear cell and undifferentiated) and non-epitheliod [3], of which the epithelioid subtype accounts for 90% of ovarian malignancies [4]. Although more than 70% patients have increased 5-year survival rates after surgery followed by chemotherapy and second-line therapies [5], the low overall cure rates and the intolerable side effects of systemic chemotherapy asks for the development of novel and more effective pharmacological interventions. An improved understanding of ovarian cancer biology – including crucial growth factor signaling pathways – is needed for the identification of biologically rational targets for novel therapies. The increasing evidences suggest that receptor tyrosine kinase (RTK) activation participates in the oncogenic progression from nonneoplastic mesothelial lining of the ovaries or the fallopian tube epithelium to epithelial ovarian cancer. Epidermal growth factor receptor (EGFR) is amplified in approximately 4%-22% of ovarian cancer and activating EGFR mutations is rare with a frequency of 4% or less [6-8]. EGFR upregulation is detected in ~60% ovarian cancer and associated with increased tumor cell proliferation, advanced tumor grades and poor patient prognosis [6,7]. Furthermore, the EGFR small molecular inhibitors gefitinib and erlotinib inhibited EGFR-mediated AKT and MAPK phosphorylation and decreased tumor cell proliferation in some ovarian cancer cell lines and tumor xenograft models [3]. ERBB2 overexpression and amplification are present in a subset of epithelial ovarian cancer and serous carcinoma [9,10]. Anti-ERBB2 Trastuzumab and lapatinib inhibited the proliferation and tumor growth in ovarian cancers with ERBB2 PGF upregulation [3,9,11]. More recently, an activated ERBB3/NRG1 autocrine loop has been demonstrated to support tumor cell proliferation in a subset of primary ovarian cancers and ovarian cancer cell lines [12]. The MET receptor tyrosine kinase and its ligand (hepatocyte growth factor, HGF) are highly expressed in ovarian cancers, and MET inactivation by small molecular inhibitor and siRNA reduced tumor burden and metastasis in nude mice with ovarian cancer [13,14]. EPHA2 is overexpressed in many types of human cancer but is absent in normal epithelial tissues [15]. EPHA2 inhibition by dasatinib or a novel immunoconjugate containing an anti-EPHA2 monoclonal antibody linked to a chemotherapeutic agent, shows antitumor activity against EPHA2-positive DBCO-NHS ester 2 ovarian cancer cell lines and mouse tumor models [15,16]. Platelet derived growth factor receptor (PDGFR) is expressed in 50-80% of ovarian cancers [17]. High expression of PDGFR has been correlated with aggressive tumor phenotypes including high proliferation index and advanced histologic grade [18]. PDGFR inactivation by both RNAi and a neutralizing antibody, results in significant anti-proliferative effects in ovarian cancer cells [19]. High expression of VEGF (vascular endothelial growth factor) and its receptors (VEGFR-1, -2, and-3) has been associated with poor prognosis in ovarian cancer [20,21]. Anti-angiogenic Pazopanib or sunitinib suppressed tumor growth in preclinical ovarian cancer models [2]. The AXL receptor tyrosine kinase protein, and its ligand Gas 6 (growth arrest-specific gene 6) are expressed significantly higher in ovarian cancers than in normal ovaries, although its role in the tumorigenesis of ovarian cancer needs further studies [22]. In addition, numerous evidences have indicated the association between TP53 mutations in ovarian cancer and prognosis. Most high-grade serous carcinomas are characterized by TP53 mutations and lack of mutations of KRAS, BRAF, or ERBB2 [23]. Mutant p53 DBCO-NHS ester 2 is almost invariably present and plays a crucial role in the molecular pathogenesis of high grade serous carcinoma [24]. In recent years, RTK-targeted cancer therapies – for example, anti-ERBB2 in breast cancer [25], DBCO-NHS ester 2 anti-KIT and PDGFA in gastrointestinal stromal tumors (GISTs) [26], anti-BCR-ABL in chronic myelogenous leukemia [27] and anti-EGFR in non-small-cell lung cancer [28] – have seen.