β-casein-derived peptides, produced by bacteria, stimulate cancer cell invasion and motility

In colon cancer, enteric bacteria and dietary factors are major determinants of the microenvironment but their effect on cellular invasion is not known. We therefore incubated human HCT‐8/E11 colon cancer cells with bacteria or bacterial conditioned medium on top of collagen type I gels. Listeria monocytogenes stimulate cellular invasion through the formation of a soluble motility‐promoting factor, identified as a 13mer β‐casein‐derived peptide (HKEMPFPKYPVEP). The peptide is formed through the combined action of Mpl, a Listeria thermolysin‐like metalloprotease, and a collagen‐associated trypsin‐like serine protease. The 13mer peptide was also formed by tumour biopsies isolated from colon cancer patients and incubated with a β‐casein source. The pro‐ invasive 13mer peptide‐signalling pathway implicates activation of Cdc42 and inactivation of RhoA, linked to each other through the serine/threonine p21‐ activated kinase 1. Since both changes are necessary but not sufficient, another pathway might branch upstream of Cdc42 at phosphatidylinositol 3‐kinase. Delta opioid receptor (δOR) is a candidate receptor for the 13mer peptide since naloxone, an δOR antagonist, blocks both δOR serine phosphorylation and 13mer peptide‐mediated invasion.