Pro-adhesive and Chemotactic Activities of Thrombospondin-1 for Breast Carcinoma Cells Are Mediated by α3β1 Integrin and Regulated by Insulin-like Growth Factor-1 and CD98

Thrombospondin-1 (TSP1) is a matricellular protein that displays both pro- and anti-adhesive activities. Binding to sulfated glycoconjugates mediates most high affinity binding of soluble TSP1 to MDA-MB-435 cells, but attachment and spreading of these cells on immobilized TSP1 is primarily β 1 integrin-dependent. The integrin α 3 β 1 is the major mediator of breast carcinoma cell adhesion and chemotaxis to TSP1. This integrin is partially active in MDA-MB-435 cells but is mostly inactive in MDA-MB-231 and MCF-7 cells, which require β 1 integrin activation to induce spreading on TSP1. Integrin-mediated cell spreading on TSP1 is accompanied by extension of filopodia containing β 1 integrins. TSP1 binding activity of the α 3 β 1 integrin is not stimulated by CD47-binding peptides from TSP1 or by protein kinase C activation, which activate α v β 3 integrin function in the same cells. In MDA-MB-231 but not MDA-MB-435 cells, this integrin is activated by pertussis toxin, whereas serum, insulin, insulin-like growth factor-1, and ligation of CD98 increase activity of this integrin in both cell lines. Serum stimulation is accompanied by increased surface expression of CD98, whereas insulin-like growth factor-1 does not increase CD98 expression. Thus, the pro-adhesive activity of TSP1 for breast carcinoma cells is controlled by several signals that regulate activity of the α 3 β 1 integrin.