Specific reduction in osteopontin synthesis by antisense RNA inhibits the tumorigenicity of transformed Rat1 fibroblasts.

Osteopontin (OPN) is a secreted phosphoglycoprotein abundant in secretory luminal epithelia (Brown et al., 1992) and in bone (Reinholt et al., 1990). It contains a functional gly-arg-gly-asp-ser (GRGDS) integrin binding domain (Oldberg et al., 1986), promotes the adhesion of a variety of cell types (Somerman et al., 1989; Brown et al., 1992) and is a ligand for the vitronectin binding integrin alpha v beta 3 (Miyauchi et al., 1991). Elevated expression of OPN correlates with tumorigenic transformation in a great variety of stromal and epithelial cell lines (Senger et al., 1980, 1983, 1989; Craig et al., 1988; Chambers et al., 1992; Chang & Prince, 1993). The protein is also present in excess in the blood of patients with metastatic disease (Senger et al., 1988). To find whether OPN contributes significantly to the tumorigenic phenotype, we expressed antisense mRNA to OPN in high OPN producing malignant B77-Rat1 fibroblasts. This caused a reduction in their OPN secretion and reduced their ability to form both lung tumors in nude mice after intravenous injection, and colonies in soft agar. Antisense transfectants also showed increased spreading on vitronectin. These observations suggest that OPN overproduction is advantageous to the metastatic phenotype, possibly by altering adhesion via, or signal transduction from, vitronectin receptors.