INTEGRIN ALPHA-3-BETA-1 PARTICIPATES IN THE PHAGOCYTOSIS OF EXTRACELLULAR-MATRIX MOLECULES BY HUMAN BREAST-CANCER CELLS

The mechanisms and receptors involved in phagocytosis by nonhematopoie tic cells are not well understood. The involvement of the alpha 3 beta 1 integrin in phagocytosis of the extracellular matrix by human breas t cancer cells was studied. The possible role of this integrin was sug gested since alpha 3 and beta 1 but not alpha 2 subunits are concentra ted at membrane sites where local degradation of fluorescently labeled gelatin occurs. Strikingly, anti-alpha 3 integrin monoclonal antibodi es (mAbs) stimulate the phagocytosis of fluorescently labeled gelatin films, gelatin beads, and Matrigel films in a quantitative phagocytosi s assay. Stimulation of the gelatin uptake by the anti-alpha 3 mAb is dose responsive, saturable, and time dependent. Antibodies against oth er integrin subunits have a lower stimulatory effect (anti-beta 1) or no significant effect (anti-alpha 2, -alpha 5, -alpha 6, and -alpha v) on gelatin phagocytosis. The synthetic HGD-6 human laminin peptide th at binds specifically the alpha 3 beta 1 integrin, but not the scrambl ed HSGD-6 control peptide, also markedly stimulates gelatin uptake in a dose-responsive way. Furthermore, the stimulatory effects of the HGD -6 peptide and the anti-alpha 3 mAb are additive, suggesting that they might promote phagocytosis in different ways. Other laminin (YIGSR, I KVAV) and fibronectin (GRGDS) peptides have no effect on gelatin phago cytosis, Immunofluorescence shows that the alpha 3 and the beta 1, but not the alpha 2 integrin subunit, concentrate into patches on the cel l surface after treatment with their respective mAbs. And, both gelati n and the alpha 3 beta 1 but not the alpha 2 beta 1 integrin are coint ernalized and routed to acidic vesicles such as lysosomes. In conclusi on, we demonstrate that human breast cancer cells locally degrade and phagocytose the extracellular matrix and show for the first time that the alpha 3 beta 1 integrin participates in this phagocytosis. We hypo thesize that the anti-alpha 3 antibodies and the laminin peptide HGD-6 activate the alpha 3 beta 1 integrin, which results in a downstream s ignaling cascade stimulating phagocytosis.