Objective: Adhesive interactions between tumor cell surface receptors and endothelial cell adhesion molecules are thought to contribute to tumor cell arrest and extravasation during hematogenous metastasis. Recent reports suggest that melanoma cell integrin alpha4beta1 (very late antigen-4, VLA-4) interaction with the inducible cell adhesion molecule, vascular cell adhesion molecule-1 (VCAM-1), is critical for tumor cell arrest. However, no information is available regarding microvascular VCAM-1 expression during spontaneous melanoma metastasis. The objectives of this study were to evaluate VCAM-1 expression in pulmonary and extrapulmonary vascular beds during melanoma progression, and to determine whether there is an organ-specific profile for VCAM-1 expression which corresponds with the clinical pattern of melanoma metastasis.
Methods: The dual-radiolabeled monoclonal antibody (mAb) technique for quantification of VCAM-1 in different vascular beds was applied to a physiological model of melanoma (B16-BL6) metastasis. Measurements of VCAM-1 were obtained when primary tumors reached 5 mm in size, and every 7 days following removal of the primary lesion. Histological examinations were performed, and mice were placed into 2 groups, based on the presence (+colonies) or absence (-colonies) of pulmonary metastases. VCAM-1 measurements obtained from several organ systems were then compared between these 2 groups of mice. Localization of VCAM-1 was achieved through immunohistochemical staining of tissues. Plasma collected from each experimental animal, as well as melanoma-conditioned media, was assayed to determine levels of the cytokines tumor necrosis factor-alpha (TNF-alpha) and interleukin-1alpha (IL-1alpha).
Results: Data collected from the dual-radiolabeled mAb technique indicate that 3 weeks following removal of the primary lesion, there is a tendency for VCAM-1 expression to increase in cardiac, hepatic, and cerebral vascular beds. Four weeks following primary resection, when pulmonary metastatic burden was maximal, VCAM-1 was significantly upregulated in each of these vascular beds. Results obtained from the lung indicate that VCAM-1 remains unchanged in pulmonary vessels at all time points examined. Immunohistochemical staining of heart and brain support the radiolabeled mAb measurements, and reveals that these organs exhibit an inflammatory phenotype in mice with heavy pulmonary tumor burden. Furthermore, 25% of these mice had histological evidence of melanoma metastases in heart and brain. Transplantation of liver fragments from mice with advanced pulmonary metastases into subcutaneous tissue of donor mice resulted in the formation of melanotic outgrowths. Plasma levels of the cytokines TNF-alpha and IL-1alpha were negligible in both groups of mice.
Conclusions: Our results indicate that upregulation of VCAM-1 is not a prerequisite for the formation of pulmonary metastases during spontaneous melanoma metastases. However, once lung metastases become well established, organ-specific increases in VCAM-1 expression become apparent. Furthermore, these organ-specific increments in VCAM-1 expression correspond with documented clinical patterns of melanoma metastasis. The enhanced expression of VCAM-1 is independent of systemic levels of TNF-alpha and IL-1alpha, but may be the result of melanoma-induced alterations at the local level, as we found evidence of melanoma cell occupation in heart, brain, and liver in pulmonary metastases-bearing mice.