In groups of friends that have become set in their ways, it often takes the influence of an outsider to shake things up. In a similar way, signals from the environment are required in colon cancer to stimulate cells to become invasive. In a new study, Ancy Leroy and colleagues now provide evidence that gut bacteria are one such outside influence that contributes to triggering invasiveness in colon tumour cells.

Leroy and co-workers found that incubating human colon cancer cells with Listeria monocytogenes, a gut pathogen, increased their ability to invade a collagen gel. This increased invasiveness was due to the production of a soluble factor, as the same effect was produced by incubating cells with filtered supernatant obtained from bacterial cultures. The ability of bacteria to produce a pro-invasive factor is not restricted to L. monocytogenes, as incubation of colon cancer cells with conditioned media from cultures of Escherichia coli, Salmonella typhimurium and bacteria that were isolated from colon tumour biopsies also had a stimulatory effect on invasiveness.

To identify the pro-invasive factor that is producd by L. monocytogenes, Leroy and colleagues isolated fractions of the growth-culture supernatant that retained the ability to stimulate invasiveness. This led to the identification of a 13-mer peptide with pro-invasive properties that corresponded to a region of the bovine β-casein protein — a principal component of milk products. The authors found that a bacterial metalloprotease, Mp1, is required for the production of this pro-invasive factor from larger β-casein-derived peptides. In addition to this bacterial enzyme, the production of the pro-invasive peptide was also shown to be dependent on a serine protease that is associated with the type I collagen gel used for the invasion assay.

To provide evidence that all the factors that are required for this process are present in vivo, the authors incubated tumour biopsies from patients with colon cancer with a 33-mer β-casein peptide containing the smaller pro-invasive factor. In the absence of a collagen gel or experimentally added bacteria, the colon cancer tissue was able to generate the 13-mer peptide. Addition of an antibiotic to the sample, however, abolished the production of the pro-invasive factor. This indicates that both the host serine protease and the bacteria that are required for the cleavage of β-casein peptides are present in the guts of patients with colon cancer.

Leroy and colleagues also studied the mechanism by which the pro-invasive peptide stimulates increased motility in colon tumour cells, and found that expression of a dominant-negative form of CDC42 or a constitutively active RHOA protein abolished invasion in response to the β-casein 13-mer. This indicates that the pro-invasive peptide that is produced by the combined efforts of the bacterial and host enzymes stimulates increased motility by modulating the activities of RHO-family GTPases.

The link between gut microflora and the development of colon cancer has been established for some time, but the precise roles of microorganisms in this disease are poorly understood. This study indicates a mechanism by which gut bacteria trigger invasiveness of tumour cells, and provides a new insight into the way that bacteria and dietary factors influence the progression of human colon cancer.