CCBIO seminar: Hani Gabra
OPCML and the regulation of receptor tyrosine kinase network: Focus on AXL
Ovarian Cancer Action Research Centre, Department of Surgery and Cancer, Imperial College London, UK
OPCML, a GPI-anchored tumour suppressor gene, is inactivated by somatic methylation in multiple cancers. We previously identified this gene by LOH (loss of heterozygosity) mapping and demonstrated that it was inactivated by somatic methylation in 80% of ovarian cancers and 44% of all cancers. Restoring OPCML expression by stable transfection suppressed in vitro growth and in vivo tumourigenicity. We hypothesized that OPCML might abrogate growth signalling pathways. In SKOV-3 and PEO1, ovarian cancer cell lines with no expression of OPCML, we demonstrated that OPCML negatively regulates a specific repertoire of receptor tyrosine kinases (RTKs), AXL, EPHA2, FGFR1, FGFR3, HER2 and HER4, and reciprocally, OPCML siRNA and shRNA knockdown in normal ovarian surface epithelial cells up-regulates these same RTKs, with no effect on RTKs EPHA10, FGFR2, FGFR4, EGFR, HER3, VEGFR1 and VEGFR3. shRNA knockdown shows that loss of OPCML accelerates the growth of normal ovarian surface epithelial cells in vitro.
In particular, OPCML modulates the receptor tyrosine kinase AXL, which is shown to promote metastasis in ovarian, lung, breast and hepatocellular carcinoma. Upon addition of the AXL ligand GAS6, OPCML preferentially binds to the activated form of the receptor and alters its membrane distribution; thereby reducing oncogenic signalling downstream of the RTK and preventing GAS6/AXL mediated increase in single cell motility, collective cell migration and spheroid invasion. A recombinant modified OPCML-like protein without a GPI anchor, signal peptide or glycosylation was constructed and expressed in E. coli. rOPCML was then injected intraperitoneally in two murine intraperitoneal models of ovarian cancer (nude mouse A2780 and SKOV3) and demonstrated profound inhibition of tumour weight, ascites volume and peritoneal dissemination compared with BSA control.
In summary, the OPCML tumour suppressor mediates its suppressor function by systems- level negative regulation of at least 5 RTKs, and a recombinant modified derivative is a potent tumour suppressor protein therapeutic in vitro and in vivo that recapitulates the in vitro mechanism.
Chairperson: Helga Birgitte Salvesen, CCBIO