SPECIFIC REDUCTION IN OSTEOPONTIN SYNTHESIS BY ANTISENSE RNA INHIBITSTHE TUMORIGENICITY OF TRANSFORMED RAT1 FIBROBLASTS

Osteopontin (OPN) is a secreted phosphoglycoprotein abundant in secret ory luminal epithelia (Brown et al., 1992) and in bone (Reinholt et al ., 1990). It contains a functional gly-arg-gly-asp-ser (GRGDS) integri n binding domain (Oldberg ct al., 1986), promotes the adhesion of a va riety of cell types (Somerman et al., 1989; Brown et al., 1999) and is a ligand for the vitronectin binding integrin alpha v beta 3 (Miyauch i et al., 1991). Elevated expression of OPN correlates with tumorigeni c transformation in a great variety of stromal and epithelial cell lin es (Senger et al., 1980, 1983, 1989; Craig et al., 1988; Chambers et a l., 1992; Chang and Prince, 1993). The protein is also present in exce ss in the blood of patients with metastatic disease (Senger et al., 19 88). To find whether OPN contributes significantly to the tumorigenic phenotype, we expressed antisense mRNA to OPN in high OPN producing ma lignant B77-Rat1 fibroblasts. This caused a reduction in their OPN sec retion and reduced their ability to form both lung tumors in nude mice after intravenous injection, and colonies in soft agar. Antisense tra nsfectants also showed increased spreading on vitronectin. These obser vations suggest that OPN overproduction is advantageous to the metasta tic phenotype, possibly by altering adhesion via, or signal transducti on from, vitronectin receptors.