The thrombin inhibitor Argatroban reduces breast cancer malignancy and metastasis via osteopontin-dependent and osteopontin-independent mechanisms

Osteopontin (OPN) has been clinically and experimentally associated with breast cancer metastasis. Proteolytic cleavage of OPN by thrombin has been reported to increase its biologic activity. The purpose of this study was to determine if inhibition of thrombin could reduce the malignancy-promoting effects of OPN on breast cancer cell behavior in vitro and in vivo. MDA-MB-468 human breast cancer cells were stably transfected to overexpress OPN (468-OPN) or a control vector (468-CON) and compared for functional differences in malignant/metastatic behavior in response to treatment with the thrombin-specific inhibitor Argatroban. Western blot analysis revealed that both 468-CON and 468-OPN cells produce thrombin and the thrombin-related protein tissue factor, and express very low levels of thrombin receptor (PAR-1). In vitro assays demonstrated that Argatroban treatment (25 μg/ml) of 468-OPN cells resulted in decreased cell growth, colony-forming ability, adhesion, and migration relative to untreated controls (P < 0.05), but did not have a significant effect on 468-CON cells. Following mammary fat pad injection, treatment with Argatroban (9 mg/kg/day) increased the in vivo tumor latency of both 468-CON and 468-OPN cells, and reduced primary tumor growth of 468-OPN cells (relative to untreated controls; P < 0.05). Furthermore, Argatroban treatment significantly decreased lymphatic metastasis of both 468-CON (P < 0.04) and 468-OPN (P < 0.01) cells relative to untreated controls. These novel findings indicate that inhibition of thrombin can reduce malignant and metastatic behavior of MDA-MB-468 breast cancer cells using both OPN-dependent and OPN-independent mechanisms, and suggest that thrombin inhibitors such as Argatroban may hold potential as therapeutic agents to combat breast cancer progression.