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

Abstract

Osteopontin (OPN) has been associated with enhanced malignancy in breast cancer, but its functional role in this disease is poorly understood. To study the e ect of OPN on cellular invasiveness, basal OPN expression was ®rst assessed in members of a progression series of human mammary epithelial cell lines (21PT: immortalized, non-tumorigenic; 21NT: weakly tumorigenic; 21MT-1: tumorigenic, weakly metastatic; MDA-MB-435 cells: tumorigenic, highly metastatic). The two lines which expressed lowest basal levels of OPN (21PT, 21NT) were then examined for up-regulation of invasive behavior in response to exogenous or transfected (endogenous) OPN. Both 21PT and 21NT showed increased invasiveness through Matrigel when human recombinant (hr)OPN was added to the lower chamber of transwells. Both also showed a cell migration response to hrOPN. Populations of 21PT and 21NT cells stably transfected with an OPN-expression vector showed higher levels of cell invasiness than control vector transfectants. Examination of transfectants for mRNA of a number of secreted proteases showed that only urokinase-type plasminogen activator (uPA) expression was closely associated with OPN expression and cellular invasiveness. Treatment of the parental 21PT and 21NT cells with exogenous hrOPN resulted in increased uPA mRNA expression and increased urokinase activity of the conditioned media. Both increased cell migration and induction of uPA expression are thus potential mechanisms of increased invasiness of breast epithelial cells in response to OPN.

Role of osteopontin in tumour progression

“…Invasiveness is clearly related to migration, but not only do cells need to be motile to invade, they also need to degrade the extracellular matrix. Several studies suggest that OPN increases invasiveness by inducing proteinases, particularly uPA (Tuck et al, 1999;Das et al, 2003). Finally, recent experiments suggest that OPN acts in concert with several growth factors, including HGF (Medico et al, 2001) and EGF (Tuck et al, 2003), to induce malignant properties.…”

Metastasis: cell-autonomous mechanisms versus contributions by the tumor microenvironment

“…Furthermore, HGF/SF specifically induces the expression of OPN and its association with the hyaluronan-CD44-ERM complex at the leading edge of metastatic cells [83,84]. OPN is a protein of the ECM with a wide variety of cell surface binding proteins and has been shown to promote tumor cell invasiveness [85]. The functional cooperation of c-Met, CD44 and OPN thus may be critical for metastasis [83].…”

Osteopontin expression correlates with adhesive and metastatic potential in metastasis-inducing DNA-transfected rat mammary cell lines

“…Although the six Met-DNAs produced different elevated levels of osteopontin mRNA in the resultant pools of transfectants, there was no obvious correlation between these levels and the number of animals bearing metastases, suggesting that osteopontin may be acting indirectly (Chen et al, 1997). Moreover, although the mechanism by which osteopontin overexpression induces a metastatic phenotype in the Rama 37 system is unknown, there is evidence to link osteopontin with many of the cellular events that are involved in metastasis, such as avoidance of the immune system, cellular migration (Liaw et al, 1995;Singh et al, 1995;Tuck et al, 2000), invasion (Tuck, 1999) and adhesion (Liaw et al, 1994;Barry et al, 2000).…”