Tumor microenvironment: paracrine and autocrine growth mechanisms and metastasis to specific sites

A history of exploring cancer in context

The concept that progression of cancer is regulated by interactions of cancer cells with their microenvironment was postulated by Stephen Paget over a century ago. Contemporary tumour microenvironment (TME) research focuses on the identification of tumour-interacting microenvironmental constituents, such as resident or infiltrating non-tumour cells, soluble factors and extracellular matrix components, and the large variety of mechanisms by which these constituents regulate and shape the malignant phenotype of tumour cells. In this Timeline article, we review the developmental phases of the TME paradigm since its initial description. While illuminating controversies, we discuss the importance of interactions between various microenvironmental components and tumour cells and provide an overview and assessment of therapeutic opportunities and modalities by which the TME can be targeted.

Metastasis suppressor genes: signal transduction, cross-talk and the potential for modulating the behavior of metastatic cells

In the past decade, research from various disciplines has stimulated a re-evaluation of our ideas of how cancers metastasize. Two important findings have been fundamental to this re-evaluation: that cancer cells are subject to growth regulation at the secondary site and that a specific class of proteins suppresses the metastatic phenotype. These proteins are encoded by metastasis suppressor genes, which are operationally defined as genes that suppress in vivo metastasis without inhibiting primary tumor growth when transfected into metastatic cell lines and injected into experimental animals. Recent biochemical studies have shown that certain metastasis suppressor proteins participate in highly conserved signal transduction cascades that mediate cellular responses to growth factors, cytokines and cellular stresses. Further elucidation of the biochemical foundations of these pathways coupled with strong in vivo studies should give us insight into the mechanisms of cancer metastasis, and may hold important implications for the future of cancer staging and therapy, using both existing and novel modalities.

Enhanced invasion and liver colonization by lung carcinoma cells overexpressing the type 1 insulin-like growth factor receptor

The receptor for the type 1 insulin-like growth factor (IGF-1R) and its ligands IGF-1 and IGF-2 play important roles in the maintenance of the malignant phenotype. In previous studies with two sublines of the Lewis lung carcinoma (H-59 and M-27, expressing high and low levels of IGF-1R, respectively) we have shown that receptor levels in these tumor cells correlated with metastasis to the liver. In the present study we asked whether the metastatic properties can be modulated by increasing receptor levels. M-27 carcinoma cells were transfected with a plasmid vector expressing a full-length human IGF-1R cDNA. Expression of the human receptor in the stable transfectants was confirmed by RT-PCR and immunoprecipitation analysis. These cells had an enhanced proliferative response to IGF-1 and hepatocyte-conditioned medium and an increased clonogenic potential in semisolid medium. Moreover, they acquired an invasive potential as measured in the reconstituted basement membrane (Matrigel) invasion assay. When inoculated via the splenic/portal route in vivo, these cells but not mock-transfected cells gave rise to multiple tumor nodules. The results suggest that IGF-1R can modulate several cellular functions which impact on the metastatic phenotype including invasion and liver colonization.