Wnt-5a and G-protein signaling are required for collagen-induced DDR1 receptor activation and normal mammary cell adhesion

The collagen-induced phosphorylation of discoidin domain receptor I (DDRI) in Wnt-5a-expressing HB2 mammary cells was effectively inhibited by pertussis toxin, but not by cholera toxin or antibodies blocking beta(1) integrins. Moreover, pertussis toxin reduced adhesion of the cells to collagen by approximately 50%, and antibodies against beta(1) integrins had a similar effect that was in fact additive to that of pertussis toxin. Cholera toxin had accordingly no such effect on adhesion. By comparison, pertussis toxin did not influence adhesion of Wnt-5a-antisense HB2 cells or MCF-7 mammary tumor cells, neither of which express Wnt-5a or exhibit activation of DDRI. In accordance with these results, direct mastoparan-induced activation of G-proteins in Wnt-Sa-deficient MCF-7 cells enabled collagen-induced phosphorylation of DDRI and enhanced their adhesion. The inactive analogue mastoparan-17 had no such effects on MCF-7 cells nor did active mastoparan affect adhesion of Wnt-5a-expressing HB2 cells. A possible explanation for how DDRI, a receptor tyrosine kinase (RTK), potentiates mammary cell adhesion comes from our observations that pertussis toxin also inhibited the recruitment of the cytoskeletal regulator phosphatidylinositol 3-kinase (PI3K) to DDRI as well as its phosphorylation/activation. In accordance with that, the PI3K inhibitor wortmannin significantly impaired adhesion of normal Wnt-5a-expressing HB2 cells but had little effect on adhesion of Wnt-5a-antisense HB2 cells. Thus, a G(i/o)-protein signaling pathway mediates the effect of Wnt-5a expression by enabling collagen-induced activation of DDRI, which, in parallel with 0, integrins, regulates adhesion of mammary cells. (C) 2002 Wiley-Liss. Inc.