In
multiple myeloma, malignant plasma cells are localized in marrow and rarely circulate in peripheral blood. To investigate the role of adhesion
proteins in this process, we determined the expression and function of adhesion molecules on cell lines derived from patients with myeloma. The U266, ARH-77, IM-9, and HS-
Sultan cell lines strongly expressed beta 1 and alpha 4
integrins (89% to 98% positive), confirming that
VLA-4 is the principal
integrin on these cell lines. The U266 and IM-9 cell lines also expressed
alpha 3 integrin on 15% to 20% cells. In contrast, all lines lacked cell surface alpha 2, alpha 5, and
alpha 6 integrin expression (< 5% positive). These cell lines adhered to
fibronectin (20% to 40% specific binding), without significant binding to either
collagen or
laminin. Adhesion of these cell lines to
fibronectin was partially blocked with either anti-beta 1
integrin monoclonal antibody (MoAb) (75% inhibition), anti-alpha 4
integrin MoAb (75% inhibition), or
RGD peptide (50% inhibition), but was unaffected by anti-alpha v beta 3 or anti-alpha IIb beta 3 MoAbs. Moreover, the combination of anti-beta 1 plus
RGD peptide or anti-alpha 4 plus
RGD peptide inhibited binding to
fibronectin by 80% and 95%, respectively. Finally, pretreatment and coculture of the IM-9 cell line with
interleukin-6 (IL-6) resulted in a 52% decrease in specific binding to
fibronectin (30% +/- 6% to 15% +/- 6%; P = .001), associated with a decrease in the number of cells expressing
VLA-4 and a decrease in intensity of
VLA-4 expression. These data suggest that myeloma cells adhere to
fibronectin through
VLA-4 as well as through RGD-dependent mechanisms, and that this binding can be downregulated by
IL-6. Future studies of binding of both myeloma cell lines and freshly isolated
tumor cells to
extracellular matrix proteins and to marrow stroma may enhance our understanding of localization and trafficking of cells within the bone marrow microenvironment.