The microenvironment encompasses all components of a tumor other than the cancer cells themselves. It is highly heterogenous, comprising a cellular component that includes immune cells, fibroblasts, adipocytes, and endothelial cells, and a non-cellular component, which is a meshwork of polymeric proteins and accessory molecules, termed the extracellular matrix (ECM). The ECM provides both a biochemical and biomechanical context within which cancer cells exist. Cancer progression is dependent on the ability of cancer cells to traverse the ECM barrier, access the circulation and establish distal metastases. Communication between cancer cells and the microenvironment is therefore an important aspect of tumor progression. Significant progress has been made in identifying the molecular mechanisms that enable cancer cells to subvert the immune component of the microenvironment to facilitate tumor growth and spread. While much less is known about how the tumor cells adapt to changes in the ECM nor indeed how they influence ECM structure and composition, the importance of the ECM to cancer progression is now well established. Plasticity refers to the ability of cancer cells to modify their physiological characteristics, permitting them to survive hostile microenvironments and resist therapy. Examples include the acquisition of stemness characteristics and the epithelial-mesenchymal and mesenchymal-epithelial transitions. There is emerging evidence that the biochemical and biomechanical properties of the ECM influence cancer cell plasticity and vice versa. Outstanding challenges for the field remain the identification of the cellular mechanisms by which cancer cells establish tumor-promoting ECM characteristics and delineating the key molecular mechanisms underlying ECM-induced cancer cell plasticity. Here we summarize the current state of understanding about the relationships between cancer cells and the main stromal cell types of the microenvironment that determine ECM characteristics, and the key molecular pathways that govern this three-way interaction to regulate cancer cell plasticity. We postulate that a comprehensive understanding of this dynamic system will be required to fully exploit opportunities for targeting the ECM regulators of cancer cell plasticity.

CXCR4 is a G protein-coupled receptor of considerable biological significance, and among its numerous functions, it is suggested to play a critical role in cancer metastasis. We have investigated the expression and function of CXCR4 in a range of breast cancer cell lines covering a spectrum of invasive phenotypes and found that, while surface levels of CXCR4 were uniform across the entire panel, only highly invasive cells that are metastatic in immunocompromised mice expressed functional receptors. CXCL12/SDF-1 induced cellular responses such as calcium mobilization, actin polymerization, and chemotaxis in metastatic cells, whereas noninvasive cells were unresponsive. Moreover, CXCL12 activated multiple signaling pathways downstream of G proteins in highly invasive cells but failed to activate any of the examined kinase cascades in noninvasive cell lines. This blockade in nonmetastatic cell lines seems to be due to the inability of G protein A and B subunits to form a heterotrimeric complex with CXCR4. GA and GB were able to bind to CXCR4 independently in all cell lines, but the association of G protein AB; heterotrimers with the receptor, a prerequisite for signal transduction downstream from G protein-coupled receptors, was only observed in the highly invasive cell lines. Our findings show, for the first time, that CXCR4 function is subject to complex and potentially tightly controlled regulation in breast cancer cells via differential G protein-receptor complex formation, and this regulation may play a role in the transition from nonmetastatic to malignant tumors. (Cancer Res 2006; 66(8): 4117-24)

In the multimolecular environment in tissues and organs, crosstalk between growth factor and G protein-coupled receptors is likely to play an important role in both normal and pathological responses. In this report, we demonstrate transactivation of the chemokine receptor CXCR4 by the growth factor insulin-like growth factor (IGF) The G protein-coupled receptor (GPCR) 2 CXCR4 is the receptor for the chemokine CXCL12. Both molecules are essential for life, with genetic deletion in mice of either CXCR4 or CXCL12 resulting in a lethal phenotype (1-3). Activation of CXCR4 by CXCL12 has been implicated in the homeostasis and activation of the immune system, and influences a range of other biological systems under both normal and pathological conditions (4 -6). These include angiogenesis (7-9), cell survival (10, 11), and more recently, tumor growth and metastasis (12)(13)(14). Indeed, it has recently been shown that CXCR4 is expressed in breast cancer tissues and cell lines, and that CXCL12 is expressed in several target organs of breast cancer metastasis (13). Additionally, treatment of mice with neutralizing Abs against CXCR4 inhibits metastasis in a mouse model of breast cancer, as does RNAi-mediated knockdown of CXCR4 on orthotopically transplanted breast carcinoma cells (12, 13). These data point to an important role for CXCR4 in cancer.-The cellular signal transduction pathways induced by CXCL12 have been well characterized in leukocytes. Interaction of CXCL12 with CXCR4 leads to the release of the G protein subunits G i ␣ and G␤␥ from intracellular domains of CXCR4. These subunits then bind and activate downstream enzyme systems including phospholipase C, which leads to a transient increase in the level of intracellular Ca 2ϩ , and phosphoinositide 3-kinase (PI3K), which results in activation of AKT and subsequently, cell migration (15-17). In contrast, the role of CXCR4, including characterization of signal transduction mechanisms in cell types other than leukocytes is less well established despite the fact that CXCR4 is expressed in most tissues and organs.Cross-talk between GPCRs and growth factor receptor-tyrosine kinase (RTKs) induced signaling pathways has become increasingly well documented in different cellular systems. For example, EGFR is tyrosine-phosphorylated in response to CCL11, a ligand for the GPCR CCR3, leading to MAP kinase activation and IL-8 production in bronchial epithelial cells (18). In rat aortic vascular smooth muscle cells, both PDGFR and EGFR are phosphorylated by sphingosine 1-phosphate (S1P), a lipid mediator that is a ligand for the S1PR family of GPCRs, leading to activation of effectors downstream of PDGFR and EGFR including Shc, and the p85 regulatory subunit of the class IA PI3K (19). In contrast, examples of transactivation of GPCRs by RTKs are less abundant, although recently it has been shown that IGF-1 stimulated phosphorylation of CCR5 in MCF-7 cells. Chemotaxis induced by IGF-1 was inhibited by a neutralizing anti-CCL5 antibody, which indicates that transactivation of ...