MCAM functions as a receptor for laminin alpha 4, a matrix molecule that is broadly expressed within the vascular wall. Accordingly, MCAM is highly expressed by cells that are components of the blood vessel wall, including vascular endothelial cells, smooth muscle cells and pericytes. Its function is still poorly understood, but evidence points to it being part of the endothelial junction associated with the actin cytoskeleton. A member of the Immunoglobulin superfamily, it consists of five Ig domains, a transmembrane domain, and a cytoplasmic region. It is expressed on chicken embryonic spleen and thymus, activated human T cells, endothelial progenitors such as angioblasts and mesenchymal stem cells, and strongly expressed on blood vessel endothelium and smooth muscle. Two isoforms exist which differ in the length of their cytoplasmic domain. Activation of these isoforms seems to produce functional differences as well. Natural killer cells transfected with MCAM-1 demonstrate decreased rolling velocity and increased cell adhesion to an endothelial cell monolayer and increased microvilli formation while cells transfected with MCAM-s showed no change in adhesion characteristics. Since these characteristics are important in leukocyte extravasation, MCAM-1 may be an important part of the inflammatory response. CD146 has been demonstrated to appear on a small subset of T and B lymphocytes in the peripheral blood of healthy individuals. The CD146+ T cells display an immunophenotype consistent with effector memory cells and have a distinct gene profile from the CD146- T cells. CD146 T cells have been shown by Dagur and colleagues to produce IL-17. CD146 has been seen as a marker for mesenchymal stem cells isolated from multiple adult and fetal organs, and its expression may be linked to multipotency; mesenchymal stem cells with greater differentiation potential express higher levels of CD146 on the cell surface.
Relevance in cancer
MCAM inhibits breast cancer progression. Normal melanocytes do not express MCAM and the expression of MCAM is first found in nevi and melanoma cells. MCAM expression is positively correlated to melanoma progression at which the expression of MCAM is highest in metastatic melanoma cells. The significance of MCAM upregulation is evident in melanoma cells cultured in 3D skin reconstruct in which MCAM facilitates the migration of melanoma into the dermis. Without the expression of MCAM melanoma cells are controlled by keratinocytes in the epidermis that inhibit penetrance beyond the basement membrane. The control by keratinocytes are only achieved by E-cadherin expression on the surface of melanoma cells. Melanoma cells with functional E-cadherin on the surface can only exclusively grow in the epidermis as keratinocytes frequently downregulate the expression of MCAM on melanoma cells.
