Integrin Splice Variant β4E: Characterization of a Novel Membrane Receptor in Epithelial Cancers

Although much progress has been made over the past 100 years, metastatic cancer is still a devastating and lethal disease, accounting for over 90% of cancer-related deaths. Metastasis is a complex process that requires cells to detach from the primary tumor and survive harsh conditions in the tumor microenvironment and shear stress forces encountered during vascular and lymphatic dissemination. One family of cell adhesion receptors involved in this process are the laminin-binding integrin receptors. Specifically, this report investigates the role of the novel integrin α6β4E splice variant in epithelial cancer progression. Unlike the canonical β4C subunit, integrin β4E lacks the cytoplasmic region required to form stable adhesion complexes and contains a unique 114 amino acid sequence. Taking advantage of the unique β4E transcript, we have identified expression patterns in five epithelial cancers and cancer cell lines. Notably, we have determined that this novel splice variant is a functional integrin receptor that heterodimerizes with the α6 subunit, utilizes laminin 332, increases migration in PC3N cells and associates with the lateral binding partner, tetraspanin CD151. Integrin α6β4E was found to be the preferred α6-binding partner in colon and pancreatic cancer 3D spheroids with increased cell-cell adhesions. This phenotype is reversible when 3D spheroids are challenged to reform cell-ECM adhesions. Immunofluorescence analysis detected integrin α6β4E co-distribution patterns with the known cell-cell adhesion protein E-cadherin in colon and pancreatic cancer spheroids. We also discovered integrin α6β4E on the edge of cells grown in 2D outside of hemidesmosome-like structures. Remarkably, non-cancerous cell lines also induce α6β4E expression when challenged to increase cell-cell contacts or when treated with simvastatin. A survey of pancreatic cancer cell lines showed differential invasion properties in cells expressing integrin α6β4E and reinforce previous work that suggests the receptor functions to maintain cancer clusters. We have also demonstrated that cleavage of the integrin α6 protein, which removes the extracellular ligand-binding domain, is required for invasive network formation. Taken together, these data have identified pro-metastatic roles for integrin receptor variants