Phorbol ester-induced differentiation of U937 cells enhances attachment to fibronectin and distinctly modulates the α5β1 and α4β1 fibronectin receptors

Abstract

Monocyte interaction with fibronectin (Fn) involves specific cell surface receptors and results in cell attachment and differentiation. We have studied the regulation of these receptors using the promonocytic cell line U937 and its PMA-induced differentiation as a model. We recently reported that U937 cells interact with two sites in Fn, RGD and CS-1, via two independent receptors (O.C. Ferreira, A. Garcia-Pardo, and C. Bianco (1990)J. Exp. Med. 171, 351). In this study we have determined the effects of PMA on the interaction of U937 cells with both sites in Fn. PMA-U937 cells showed an enhanced attachment to Fn and to an RGD-containing 80-kDa Fn fragment. This enhancement paralleled a two- to threefold increase in the surface expression of the RGD-dependent receptor α5β1. An anti-α5β1 mAb completely inhibited cell adhesion to Fn and to the 80-kDa fragment. α5β1 receptors from untreated and PMA-treated U937 cells were isolated on 80-kDa-Sepharose columns and shown to contain a similar complex of 152125-kDa proteins, although proteins from PMA-treated cells had slightly faster mobility on SDS-gels. In contrast, the total number of PMA-U937 cells adhering to a 38-kDa Fn fragment (containing the CS-1 site) was lower when compared to that of untreated cells. This decrease was accompanied by a 50% loss of cell surface α4β1, the specific receptor for CS-1. Our results indicate that differentiation of U937 cells enhances adhesion to Fn primarily by up-regulating the α5β1 Fn receptor. PMA also induces a down-regulation of α4β1, suggesting that these two integrins play different roles during monocyte differentiation.