OSTEOPONTIN IS NOT A MARKER FOR PROLIFERATING HUMAN VASCULAR SMOOTH-MUSCLE CELLS

Osteopontin (OF) is a secreted glycoprotein that contains the Arg-Gly- Asp (RGD) cell-binding sequence that binds calcium and is chemotactic and adhesive for rat vascular smooth muscle cells (VSMCs). OP gene exp ression is upregulated in cultured rat VSMCs in vitro and after balloo n carotid injury in vivo, suggesting that OP may be a marker for proli ferating VSMCs in vivo and in vitro. Our in situ hybridization studies of human atherosclerotic coronary vessels, however, have shown OP mRN A expression in plaque macrophages but not in VSMCs. The current study investigated OP mRNA expression in cultured human VSMCs and macrophag es and in an organ culture model of neointima formation in human saphe nous vein. OP mRNA expression was not detected by Northern blot analys is of total RNA from subconfluent or confluent cultures of human VSMCs of any passage maintained in normal growth medium or after stimulatio n with TGF beta(1) (20 ng/mL), angiotensin II (1 mu mol/L), or basic f ibroblast growth factor (10 ng/mL) but was just detectable after stimu lation with activated vitamin D-3 (1 mu mol/L). In contrast, cultured human macrophages expressed high levels of OP mRNA that were not depen dent on lipid loading. OP mRNA was detected in isolated foci in all la yers of saphenous veins maintained in organ culture for 14 days, inclu ding <2% of neointimal cells, a distribution that paralleled that of t issue macrophages. These results suggest that OP gene expression is no t a marker for proliferation of human VSMCs in vitro and highlight a f undamental difference in the biology of human and rodent VSMCs.