Claudins play a role in normal and tumor cell motility

Background: Claudins are key integral proteins of the tight junction. Although they play an essential role in controlling paracellular diffusion in epithelia, increasing evidence supports a role for these proteins in non-barrier forming activities. To elucidate a potential function for claudins outside of their traditional role in tight junctions, subcellular localization of claudin-4 was determined in normal mammary epithelial cells as well as breast and ovarian cancer cell lines and the effects of a claudin mimic peptide on cell motility were determined. Results: Immunofluorescence revealed that claudin-4 was localized along cellular projections. Using a fluorescent peptide that mimics a conserved sequence in the second extracellular loop of a set of claudin subtypes, that includes claudin-4, exposure of this loop to the extracellular environment was confirmed in non-polarized cells. This peptide inhibited cell motility when normal mammary epithelial cells as well as breast and ovarian tumor cells were subjected to a wound healing assay. Knockdown of claudin-4 also inhibited cell motility and the mimic peptide had no effect on motility in the claudin-4 knockdown cells. This effect on motility was seen when cells were grown on collagen, but not when cells were grown on non-physiological cell adhesive or fibronectin. Conclusion: The second extracellular loop of claudins is able to interact with the extracellular environment to promote normal and tumor cell motility when it is not associated with tight junction structures.