Abstract 2638: Galectin-3 activates the receptor tyrosine kinase Tyro3 and stimulates ERK signaling, cell survival and migration in human head and neck cancer cells
Authors: Hafizi Sassan , Kafri Nour Al 
Published: August 2019

Abstract
The beta-galactoside-binding protein galectin-3 (Gal-3) has been shown to influence the tumorigenic behaviour of cancer cells, and elevated serum levels of Gal-3 have been linked to tumor metastasis in several different cancers. Previously, Gal-3 was shown to be a ligand MerTK, a member of the TAM (Tyro3, Axl, MerTK) subfamily of receptor tyrosine kinases (RTKs) - these RTKs are implicated in tumorigenesis and/or enabling tumor chemoresistance in many different cancers. However, it is not known whether Gal-3 is also a ligand for the other two TAM RTKs, nor what signaling pathways it activates to regulate cancer cell functions. This study is aimed at determining the ligand properties of Gal-3 against Tyro3 and Axl RTKs in human cancer cells and the oncogenic signal pathways linked to them. The cultured human head and neck squamous cell carcinoma (HNSCC) cell line SCC-25 was determined by western blot and qPCR to express both Axl and Tyro3, but not MerTK. Recombinant Gal-3 and the TAM ligands Gas6 and ProS1 were added to serum-starved cells over a time-period of minutes and cells lysed to determine activation/phosphorylation of RTKs and intracellular signal mediators by western blotting. Longer incubations were performed to measure cell viability (MTS assay) and cell migration rate (scratch assay). All experiments were carried out a minimum three times. The known TAM ligands Gas6 and ProS1 stimulated TAM RTKs differentially in SCC-25 cells: Gas6 activated Axl strongly within minutes, with ProS1 having a weak effect. In contrast, ProS1 was the strongest ligand for Tyro3, activating it by fourfold, followed by Gal-3 at threefold and Gas6 at twofold. All three exogenous proteins also rapidly stimulated ERK1/2 phosphorylation by two-fold, whereas only Gas6 and Gal-3 stimulated Akt phosphorylation. All three proteins significantly maintained cell survival during acute induction of apoptosis by staurosporine. However, only Gas6 conferred a survival effect over long-term serum starvation-induced cell death. In addition, Gal-3 significantly stimulated cell migration during Axl blockade with the small molecule Axl inhibitor BGB324, further suggesting a role for Tyro3 in this effect. These results have shown that Gal-3 is a ligand for Tyro3 RTK, and that Gal-3 promotes cell survival and migration in HNSCC cells, coupled with activation of both the related signal pathways ERK and Akt. The data therefore reveal a new mechanism of Tyro3 activation; however, it remains to be determined whether Gal-3 interacts with the glycosyl chain on the RTK or else via other interfaces. Finally, cancer therapeutic strategies targeting Gal-3 action and/or cell surface glycosylation may be viable approaches in further research.
Citation Format: Sassan Hafizi, Nour Al Kafri. Galectin-3 activates the receptor tyrosine kinase Tyro3 and stimulates ERK signaling, cell survival and migration in human head and neck cancer cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 2638.