Osteopontin expression in primary sarcomas of the pulmonary artery

Primary tumors of the great vessels (aorta, pulmonal artery, and inferior v ena cava) are rare and represent in most cases vascular leiomyosarcomas. Fu rthermore, there also exists a group of sarcomas arising from the intima, k nown as intimal sarcomas, associated with early metastasis and a very poor prognosis. Osteopontin (OPN) is an extracellular matrix protein that binds to av integrins, thereby promoting cell attachment, chemotaxis, and signal transduction. The reported association of OPN with malignancy and metastasi s prompted us to examine the expression of this protein in seven sarcomas o f the pulmonary artery. Strong OPN-specific staining could be detected in t umor cells and the adjacent extracellular matrix. Using a double labeling p rocedure, proliferating cells showed a strong positive reaction with antibo dies against OPN. In addition, this protein could be demonstrated in the cy toplasm of macrophages. CD44, a putative receptor of OPN, was expressed on the cellular surface of tumor-associated lymphocytes. The expression of OPN in macrophages and tumor cells indicates that this molecule could possibly mediate cellular adhesion of both cell types in pulmonary sarcomas. The de tection in the extracellular matrix shows that OPN is actively secreted and may interact with the corresponding receptor, CD44, on the surface of lymp hocytes. Although the function of OPN is not yet fully understood, our data indicate that strong expression of this molecule in poorly differentiated sarcomas could play a role in the progression of malignancy and metastasis as described previously for carcinomas.