One person in the US dies from the metastatic consequences of melanoma every hour. Currently, no effective treatment exists to prevent metastasis of melanoma cells or to inhibit metastatic tumors growing at this site. This is due, in part, to a lack of therapeutic approaches designed to inhibit melanoma metastasis to the lung. Cancer metastasis requires that tumor cells detach from a primary site and invade the surrounding stroma, survive immune defenses and turbulence of the blood circulation after intravasating into the circulatory system, extravasate through the endothelial lining of blood vessels and finally form a new colony in the surrounding tissue. A better understanding of the underlying mechanism of cancer metastasis holds the promise to design effective interventions for metastatic cancer. The goal of this study is to investigate mechanisms involved in an important step in the metastatic process---the interactions of melanoma cells with leukocytes within the tumor microenvironment that results in tumor cell adhesion to the vascular endothelium (EC) and subsequent extravasation in the microcirculation.;Polymorphonuclear neutrophils (PMNs) can facilitate melanoma cell adhesion to the EC through the binding of beta2 integrins on PMNs and intercellular adhesion molecule-1 (ICAM-1) on melanoma cells as well as on the EC. This process is regulated by the hydrodynamic shear rate, which is inversely proportional to the convection-driven cell-cell contact time (or adhesion on-rate kinetics), rather than by the shear stress, which is proportional to the force exerted on formed bonds (or adhesion off-rate kinetics). This shear-rate-dependence may be ascribed to two possible mechanisms which have been shown all to be affected by shear rate: first, heterotypic aggregation between melanoma cells and PMNs, a critical step for PMN-facilitated melanoma adhesion to the EC; second, the up-regulation of beta2 integrins on PMNs in response to chemokines endogenously-produced within the tumor microenvironment.;Molecular mechanisms involved in initiating the communications between melanoma cells, PMNs, and the ECs could be responsible for melanoma adhesion and extravasation, especially those involving inflammatory cytokine communications under microcirculatory flow conditions. Results of studies have indicated that melanoma cells induce PMNs to secrete inflammatory IL-8 through activation of NF-kappaB. Melanoma-derived IL-8 is also required, as a primary factor among many other tumor-released factors, for induction of endogenous IL-8 in PMNs. PMN-derived chemokines in response to melanoma cells can act through autocrine and/or paracrine mechanisms to enhance PMN inflammatory activities. The functional significance of IL-8, through the CXC chemokine receptors 1 and 2 (CXCR1 and CXCR2) on PMNs, is that it facilitates the ability of PMNs to enhance melanoma cells to form shear-resisted arrest on the EC and subsequent extravasation within the circulation. A potential role of B-Raf, the gene that is mutated at the highest frequency in malignant melanomas has also been studied. Results have indicated that targeting mutant V600EB-Raf reduces melanoma metastasis through reductions in active melanoma cell extravasation through the EC. Mechanistically, reduced melanoma extravasation following inhibition of mutant V600EB-Raf is due to the disruption of downstream NF-kappaB signaling that regulates the constitutive IL-8 production and ICAM-1 expression, which are essential for PMN-facilitated melanoma extravasation under flow conditions.;Identification and characterization of molecular targets related to inflammation and cancer remain a high priority in cancer research. Results from this study provide new evidence for the complex roles of hydrodynamic forces, inflammatory factors such as chemokines within the tumor microenvironment, PMN-melanoma adhesion and oncogenes in the recruitment of metastatic cancer cells to the EC, which provide insights for the development of experimental therapeutic approaches to prevent melanoma metastasis.
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