OSTEOPONTIN MESSENGER-RNA IS EXPRESSED BY SMOOTH MUSCLE-DERIVED FOAM CELLS IN HUMAN ATHEROSCLEROTIC LESIONS OF THE AORTA

Osteopontin is a phosphorylated, sialic acid-rich, noncollagenous bone matrix protein containing the Arg-Gly-Asp-Ser amino acid sequence res ponsible for cell adhesion. The protein strongly binds to hydroxyapati te and play an important role in calcification. Expression of osteopon tin mRNA was analyzed in human aortic atherosclerotic lesion by Northe rn blot hybridization, as well as by in situ hybridization. The expres sion of osteopontin mRNA was detected in 24 out of 25 samples of aorta obtained from 17 autopsy cases, but not in one normal aortic sample. The magnitude of expression was proportional to the stage of atheroscl erosis. In situ hybridization revealed that the cells expressing osteo pontin mRNA were detected in the wall surrounding atheroma and closely associated with calcification. They were morphologically identified a s foam cells and immunohistologically positive with HHF35, appearing t o be derived from smooth muscle cells. These findings have suggested t hat smooth muscle cell-derived foam cells express osteopontin mRNA and play an important role in calcification of the atherosclerotic lesion s.