Osteopontin induces increased invasiveness and plasminogen activator expression of human mammary epithelial cells

Osteopontin (OPN) has been associated with enhanced malignancy in breast ca ncer, but its functional role in this disease is poorly understood. To stud y the effect of OPN on cellular invasiveness, basal OPN expression was firs t assessed in members of a progression series of human mammary epithelial c ell lines (21PT: immortalized, non-tumorigenic; 21NT: weakly tumorigenic; 2 1MT-1: tumorigenic, weakly metastatic; MDA-MB-435 cells: tumorigenic, highl y metastatic). The two lines which expressed lowest basal levels of OPN (21 PT, 21NT) were then examined for up-regulation of invasive behavior in resp onse to exogenous or transfected (endogenous) OPN. Both 21PT and 21NT showe d increased invasiveness through Matrigel when human recombinant (hr)OPN wa s added to the lower chamber of trans wells. Both also showed a cell migrat ion response to hrOPN. Populations of 21PT and 21NT cells stably transfecte d with an OPN-expression vector showed higher levels of cell invasiness tha n control vector transfectants. Examination of transfectants for mRNA of a number of secreted proteases showed that only urokinase-type plasminogen ac tivator (uPA) expression was closely associated with OPN expression and cel lular invasiveness, Treatment of the parental 21PT and 21NT cells with exog enous hrOPN resulted in increased uPA mRNA expression and increased urokina se activity of the conditioned media. Both increased cell migration and ind uction of uPA expression are thus potential mechanisms of increased invasin ess of breast epithelial cells in response to OPN.