Cathepsin G Recruits Osteoclast Precursors via Proteolytic Activation of Protease-Activated Receptor-1

Metastatic breast cancer shows extreme tropism for the bone microenvironment, leading to the establishment of osteolytic metastases. Perpetuation of tumor-induced osteolysis requires a continuous supply of osteoclast precursors migrating into the bone microenvironment that can subsequently differentiate into mature osteoclasts and resorb bone. Thus, identification and subsequent targeting of chemoattractants of osteoclast precursors that are up-regulated at the tumor-bone interface represents a potential avenue to interrupt osteolysis. We report that cathepsin G, a serine protease, plays a vital role in the bone microenvironment by modulating tumor-stromal interaction in a manner that favors tumor establishment and regulates chemotaxis of monocytes, a subset of which has the potential to differentiate into osteoclasts. Our data show that cathepsin G-induced chemotaxis of monocytes is mediated by proteolytic activation of protease-activated receptor-1 (PAR-1). Attenuation of PAR-1 activation abrogates cathepsin G-mediated induction of monocyte chemotaxis. We also show that in vivo inhibition of cathepsin G reduces the number of CD11b(+) osteoclast precursors and mature osteoclasts at the tumor-bone interface. Together, these data suggest that therapeutic targeting of both PAR-1 signaling in osteoclast precursors as well as cathepsin G at the tumor-bone interface has the potential to reduce osteolysis by inhibiting the recruitment, differentiation, and activation of osteoclast precursors. [Cancer Res 2009;69(7):3188-95]