MET RECEPTORS INDUCE SAM68-DEPENDENT CELL MIGRATION BY ACTIVATION OF ALTERNATE ERK FAMILY MEMBERS.

The hepatocyte growth factor (HGF)/Met receptor signaling pathway is deregulated in diverse human malignancies and plays a central role in oncogenesis, tumor progression and invasive cancer cell growth. Additionally, altered expression and splicing (i.e. inclusion of variable exon-5 (v5)) of the cell adhesion marker, CD44, is associated with advanced cancer phenotypes. We sought to understand how HGF regulates CD44v5 expression. The immortalized non-tumorigenic keratinocyte (HaCaT) cell line, abundantly expresses both the tyrosine kinase growth factor receptor, c-Met, and the transmembrane glycoprotein CD44v5. HGF stimulation increased HaCaT cell migration and CD44v5 protein expression. HGF-dependent CD44v5 up-regulation required activation of the ERK1/2 MAPK module and phosphorylation of Sam68, a protein involved in RNA processing, splicing, and CD44 variant 5 inclusion. Knock-down of Sam68 or treatment of cells with the MEK inhibitor (U0126) blocked CD44v5 expression and cell migration in response to HGF. Similar to HaCaT cells, highly migratory MDA-MB-231 breast cancer cells also required Sam68 expression for HGF-induced migration. However, in these cells, HGF-induced migration occurred independently of ERK1/2 activation or CD44v5 (or v6) expression, but instead required ERK5 signaling. Phospho-mutant, but not wt-Sam68, blocked HGF-induced cell migration in both cell types. These results suggest that Sam68 acts as a convergence point for ERK-dependent inputs governing the expression of cell-specific gene targets. Future experiments will be aimed at elucidation of Sam68-regulated (splice variants) genes required for HGF-induced breast cancer cell migration. Blockade of Sam68, a key mediator of cell migration, may provide a new avenue for therapeutic inhibition of metastatic cancers.