The pivotal role of the axl/gas6 signaling node in the mesenchymal subtype of ovarian cancer, and its modulation by the tumor suppressor OPCML
Creators: name: Antony, Jane
Publisher: name: Imperial College London
Year: 2016

description: Epithelial Ovarian Cancer (EOC) is a complex and lethal gynecological malignancy as demonstrated by the heterogeneity in gene expression molecular subtypes (GEMS). To address the GEMS-specific lethality in ovarian cancer, we highlight the relevance of how a receptor tyrosine kinase (RTK) differs in the signaling pattern, functional consequences, and therapeutic implications in the context of GEMS. An enrichment analysis of the human kinome among the ovarian cancer GEMS shows that AXL is the top-ranking RTK for the EMT-driven, poor prognosis Mes subtype. Interestingly, AXL is also expressed, at a lower rank, in the epithelial, better prognosis Epi-A subtype. Upon ligand stimulation with Gas6, specific to Mes, there is recurrent temporal activation of the extracellular regulated kinase (ERK) signaling downstream of AXL. Reverse Phase Protein Analysis (RPPA) and proximity ligation assays further show that Gas6/AXL signaling transactivates other RTKs such as cMET, EGFR, and HER2 in Mes, exclusively. This RTK crosstalk in concert with the sustained activation of FRA1 downstream to ERK, coincides with the induction of the EMT driver Slug in Mes but not Epi-A. This suggests that the intrinsic property of GEMS influences the complexity and duration of signal propagation. This signal amplification downstream to the Gas6/AXL axis alludes to pathway addiction in Mes. Functionally, the recurrent ERK activation results in a motile and invasive phenotype synonymous to Mes and EMT. This further sensitizes Mes to a selective AXL inhibitor, R428, by attenuating the RTK-ERK activation, reducing the in vitro motility and invasion, and inhibiting the in ovo tumor growth in the chick chorion-allantoic membrane (CAM) assay at a lower GI50 dose compared to Epi-A. Furthermore, silencing AXL in Mes reverses the mesenchymal phenotype and abolishes tumor formation in SCID mice. Our results imply that the mesenchymal state is more sensitive to AXL inhibition, and the RTK crosstalk along the EMT gradient rewires the system, thereby drastically sensitizing it to RTK node inhibition. The epithelial state might retain a linear signaling axis and shows less therapeutic advantage to targeting AXL. Stratifying ovarian cancer patients based on the GEMS and EMT states followed by targeting AXL is promising in a prospective clinical setting. The second part of this project is modulation of AXL by the GPI-anchored tumor suppressor OPCML (Opioid-binding Protein/Cell-adhesion Molecule Like). OPCML is abrogated in 83% of sporadic EOCs and 44% of all human cancer by somatic methylation. OPCML is a potent tumor suppressor that severely attenuates tumorigenesis by down modulating a spectrum of RTKs, including, but not limited to FGFR1, FGFR3, HER2, HER4 and EPHA2. Here we show that OPCML interacts with and represses AXL. OPCML preferentially binds to the activated form of AXL and alters its membrane distribution, thereby preventing downstream oncogenic signaling and Gas6/AXL-mediated increase in migration and invasion. OPCML potentiated the effect of AXL inhibitor R428, increasing signaling inhibition and cell death. This completely novel mechanism of regulation of RTKs may be a general mechanism for coordinated regulation of signaling, and may contribute to pro-oncogenic cancer signaling through epigenetic inactivation of OPCML.