High Expression of Macrophage Stimulating Protein (MSP) Correlates with Survival Benefit in Malignant Pleural Mesothelioma

ABSTRACT Introduction RON/MST1R is a member of the MET tyrosine kinase family and has a putative role in several cancers. Macrophage stimulating protein (MSP) is the only known ligand for RON/MST1R. The MSP-RON signalling pathway has been implicated in a variety of cellular functions such as macrophage activity and wound healing. We have previously identified MST1R/RON as frequently activated in MPM, and high positivity for RON staining was an independent predictor of favourable prognosis. Methods A panel of mesothelioma cell lines were screened for the expression of MSP and RON at the mRNA and protein level. The proliferative response of Ju77, H226 and Met5A (non-malignant transformed human pleural mesothelial cells) to MSP treatment was determined. A phospho-kinase array was utilised to detect the downstream signalling pathways activated upon MSP stimulation. The effect of two MST1R/RON inhibitors (a) a pre-clinical monoclonal antibody (RON8, Imclone) and (b) a small molecule inhibitor on proliferation and migration was assessed. In addition, a series of MPM TMAs were stained for MSP and macrophage markers. Results MSP and MST1R expression varied between the mesothelioma cell line panel at both the mRNA and protein level. Treatment with MSP reduced the proliferative capacity of the Met5A cell, with a modest effect on the Ju77 MPM line. However, MSP stimulation modified the expression of the SRC family of kinases. In terms of targeting MST1R/RON, the small molecule inhibitor resulted in a significant decrease in proliferation and migration. Although treatment with RON8 had no effect on proliferation, it did affect the migration capacity of the MPM cells. High expression of MST1R/RON or MSP correlated with better survival by univariate analysis. In multivariate analysis, MSP was identified as an independent prognosticator for survival in MPM. We observed no correlation with macrophage (CD 68) staining and survival. Conclusion RON /MST1R comprises of a number of isoforms, the most common of which are flRON (full length) and sf (short form). Our results indicate that although high levels of RON and MSP correlate with increased survival, in vitro observations would indicate that this may be isoform dependant. Experiments are ongoing to further elucidate the RON-MSP axis in MPM, including in vivo studies. Disclosure All authors have declared no conflicts of interest.