by ecancer reporter Clare Sansom

Matrix metalloproteinases (MMPs) are a family of 24 proteins that degrade the proteins that make up the extra-cellular matrix by cleaving their peptide bonds.

Some members of this family have other functions, including the precise control of cellular activity through the proteolysis of growth factors and their receptors, adhesion molecules and chemokines.

These proteins were originally thought to promote tumour growth and metastasis and some MMP inhibitors were developed as experimental anti-cancer drugs, although none have been successful in clinical trials.

It is now understood that some matrix metalloproteinases can also have anti-proliferative effects.

In particular, matrix metalloproteinase-8 (MMP-8), which is expressed in many different cell types and cleaves collagen fibrils, has been recognised as playing a role in suppressing tumour growth and metastasis.

It is likely that the protective role played by MMP-8 in cancer and some inflammatory conditions arises from its ability to influence the immune system through cleaving proteins that modulate this system such as chemokines.

A group of researchers led by Dylan Edwards of the University of East Anglia, Norwich, UK has now explored the effect of MMP-8 on networks of cytokines and chemokines in breast cancer cells.

Firstly, Edwards and his colleagues transfected MMP-8 into the human breast cancer cell line MCF-7 and examined the surrounding media 24 hours later for expression levels of interleukins and other immune mediators.

They found that expression of interleukin-6 (IL-6) increased by 20% and expression of interleukin-8 (IL-8) by 65% in cells that had been transfected with wild type MMP-8, but not at all in cells transfected with an inactive mutant form of the protein or with an empty vector.

Similar results were observed with other breast cancer cell lines, but not with normal breast epithelial cells.

All cell lines tested, including the normal cells, expressed and released into the medium comparable levels of both wild type and mutant MMP-8 mRNA transcript and protein, and the expressed wild type protein was fully functional.

These results indicated that catalytically active MMP-8 appears to affect the interleukins IL-6 and IL-8 primarily through controlling their gene expression, and that this occurs only in breast tumour cells.

Breast cancer cells from the highly metastatic MDA-MB-231 cell line were then transfected with wild type or mutant MMP-8 or with empty vector, and observed for up to three months with or without the broad spectrum antibiotic zeotin.

Initially, there was little difference between the growth patterns of cells transfected with any of the vectors.

Over a longer period, however, significantly fewer colonies were formed by the cells transfected with wild type MMP-8, indicating that expression of this protein is detrimental to long-term tumour cell growth.

Expression levels of wild type but not mutant protein decreased gradually to almost zero during the three months in culture, although there was little change in mRNA levels; levels of the two cytokines induced by functional MMP-8, IL-6 and IL-8, also returned to the baseline over this period.

Rare clones of MDA-MB-231 cells were observed that maintained both expression of wild type MMP-8 and secretion of IL-6 and IL-8 over the entire time course of the experiment.

Furthermore, long-term cytokine expression in these clones appeared to be no longer linked to expression of functional MMP-8.

Both IL-6 and IL-8 are pro-inflammatory cytokines that have been conventionally assumed to promote malignancy.

However, it is likely that the activation of these cytokines by MMP-8 also acts as a signal to recruit other components of the innate immune system to the tumour site and to restrict its growth.

MMP-8 is clearly involved in the orchestration of an inflammatory response to breast cancer development, and these results suggest that IL-6 and IL-8 are important in the mechanism of that orchestration.

These proteins act together to influence breast cancer growth in a complex and ambiguous way, and understanding the mechanism through which they do may eventually lead to novel treatments for this disease.

Reference

Thirkettle, S., Decock, J., Arnold, H., Pennington, C.J., Jaworski, D.M. and Edwards, D.R. (2013). Matrix metalloproteinase-8 (collagenase-2) induces the expression of interleukins-6 and -8 in breast cancer cells. J. Biol. Chem., published online ahead of print 30 April 2013. doi: 10.1074/jbc.M113.464230