Modification of integrin-mediated cell attachment to substrata by serine proteinases in the presence and absence of divalent cations

The sensitivity to serine proteinases of cellular proteins involved in cell-matrix adhesion was investigated using C32 melanoma cells. Cells dissociated from monolayers by the metal chelator ethylenediaminetetraacetic acid were incubated with proteolytic enzymes, and then attachment was quantified by standard cell adhesion assays. The effect of proteinases was found to depend on the presence of Ca²⁺ in the incubations. Incubation with 100 nM trypsin or chymotrypsin for 1-2 h (37°C) in the absence of Ca²⁺ reduced cell attachment to vitronectin (Vn), fibrinogen (Fb), laminin, and fibronectin by approximately 80, 80, 40, and 30%, respectively. Viability studies indicated that such treatment with proteinases was not cytotoxic. Inclusion of 0.1 mM CaCl₂ in the incubations prevented the loss in attachment to all substrata. In the case of Kb, proteinase treatment in the presence of Ca²⁺ had an additional effect; it improved cell attachment to this substratum by about 50%. C32 cells have been shown to express the integrin α_vβ₃ (Vn receptor) which mediates attachment to Vn and Fb in a GRGDS-sensitive manner. Attachment of C32 cells to Vn and Fb prior to proteinase treatment and after proteinase treatment in the presence of Ca²⁺ was 90% inhibited by the addition of GRGDS peptide to the attachment assays. These results suggest that the adhesion observed both before and after proteinase treatment was mediated by this integrin. Analysis of the Vn receptor from proteinase-treated cells by immunoblotting of cell extracts and by SDS gel electrophoresis of immunoprecipitated receptor revealed no detectable change in either the α_v or β₃ subunit that correlated with loss in attachment. Similarly proteinase treatment in the presence of Ca²⁺ did not produce detectable alterations in the subunits which might correlate with the improved attachment to Fb. Consistent with these results, an enzyme-linked immunoassay to quantify cell surface receptors revealed little difference in the amount of Vn receptor on cells treated with proteinase in the presence or absence of Ca²⁺. Degradation of the α_v subunit was demonstrated, however, at proteinase concentrations higher than those required to affect cell attachment. Thus, treatment of cells with serine proteinases can affect integrin-mediated attachment to matrix proteins in a manner moderated by Ca²⁺, but the alterations in attachment do not appear to be accompanied by detectable proteolytic modification of the integrin.