Interactions of promonocytic U937 cells with proteins of the extracellular matrix.

Monocyte interaction with proteins of the extracellular matrix (ECM) is regulated by expression of specific cell-surface receptors. 12-O-tetradecanoyl phorbol-13-acetate (TPA) has been shown to induce the promonocytic cell line U937 to a more differentiated monocyte-like state. In this study we have analysed the attachment of U937 cells to ECM proteins and the effects of treatment with TPA on this process. Non-induced U937 cells attach to fibronectin- and Matrigel-coated surfaces without TPA stimulation, but TPA further increases adherence to these substrates as measured by an enhanced binding and by the lower concentration of proteins needed in the substrate to achieve 50% of maximal cell adhesion. Attachment to type I collagen was seen only with activated U937 cells, whereas no measurable attachment to bovine serum albumin, vitronectin, and type IV collagen was detected. TPA-activated U937 cells showed a two-fold increase in the expression of the RGD-dependent integrin receptors alpha 3 and alpha 5, and a reduction in the expression of alpha 4, another fibronectin-specific receptor, whereas the common beta 1 chain was unchanged. Attachment of U937 cells to fibronectin was primarily mediated by the alpha 3 and alpha 5 integrins, as revealed by the ability of GRGDS peptides to inhibit attachment, whereas the CS-1 peptide, containing the alpha 4 binding site, was largely ineffective in blocking attachment.