Primary tumours influence the environment in the lungs before metastasis. However, the mechanism of metastasis is not well understood. Here, we show that the inflammatory chemoattractants S100A8 and S100A9, whose expression is induced by distant primary tumours, attract Mac 1 (macrophage antigen 1)(+)-myeloid cells in the premetastatic lung. In addition, tumour cells use this mechanism, through activation of the mitogen-activated protein kinase (MAPK) p38, to acquire migration activity with pseudopodia for invasion (invadopodia). The expression of S100A8 and S100A9 was eliminated in lung Mac 1(+)-myeloid cells and endothelial cells deprived of soluble factors, such as vascular endothelial growth factor A (VEGF-A), tumour necrosis factor alpha (TNFalpha) and transforming growth factor beta (TGFbeta) both in vitro and in vivo. Neutralizing anti-S100A8 and anti-S100A9 antibodies blocked the morphological changes and migration of tumour cells and Mac 1(+)-myeloid cells. Thus, the S100A8 and S100A9 pathway may be common to both myeloid cell recruitment and tumour-cell invasion.

A large number of macrophages and haematopoietic progenitor cells accumulate in pre-metastatic lungs in which chemoattractants, such as S100A8 and S100A9, are produced by distant primary tumours serving as metastatic soil. The exact mechanism by which these chemoattractants elicit cell accumulation is not known. Here, we show that serum amyloid A (SAA) 3, which is induced in pre-metastatic lungs by S100A8 and S100A9, has a role in the accumulation of myeloid cells and acts as a positive-feedback regulator for chemoattractant secretion. We also show that in lung endothelial cells and macrophages, Toll-like receptor (TLR) 4 acts as a functional receptor for SAA3 in the pre-metastatic phase. In our study, SAA3 stimulated NF-kappaB signalling in a TLR4-dependent manner and facilitated metastasis. This inflammation-like state accelerated the migration of primary tumour cells to lung tissues, but this was suppressed by the inhibition of either TLR4 or SAA3. Thus, blocking SAA3-TLR4 function in the pre-metastatic phase could prove to be an effective strategy for the prevention of pulmonary metastasis.

Activation of proMMP‐2 by MT1‐MMP is considered to be a critical event in cancer cell invasion. In the activation step, TIMP‐2 bound to MT1‐MMP on the cell surface acts as a receptor for proMMP‐2. Subsequently, adjacent TIMP‐2‐free MT1‐MMP activates the proMMP‐2 in the ternary complex. In this study, we demonstrate that MT1‐MMP forms a homophilic complex through the hemopexin‐like (PEX) domain that acts as a mechanism to keep MT1‐MMP molecules close together to facilitate proMMP‐2 activation. Deletion of the PEX domain in MT1‐MMP, or swapping the domain with the one derived from MT4‐MMP, abolished the ability to activate proMMP‐2 on the cell surface without affecting the proteolytic activities. In addition, expression of the mutant MT1‐MMP lacking the catalytic domain (MT1PEX‐F) efficiently inhibited complex formation of the full‐length enzymes and activation of pro MMP‐2. Furthermore, expression of MT1PEX‐F inhibited proMMP‐2 activation and Matrigel invasion activity of invasive human fibrosarcoma HT1080 cells. These findings elucidate a new function of the PEX domain: regulating MT1‐MMP activity on the cell surface, which accelerates cellular invasiveness in the tissue.

Growth factors and their receptors are involved in the regulation of cell proliferation and also play a key role in oncogenesis. In this study, a novel putative kinase receptor gene, termed eph, has been identified and characterized by molecular cloning. Its primary structure is similar to that of tyrosine kinase receptors thus far cloned and includes a cysteine-rich region in the extracellular domain. However, other features of the sequence distinguish the eph gene product from known receptors with tyrosine kinase activity. Thus the eph protein may define a new class of these molecules. The eph gene is overexpressed in several human carcinomas, suggesting that this gene may be involved in the neoplastic process of some tumors.