Involvement of thrombospondin in the adherence of human breast-adenocarcinoma cells: a possible role in the metastatic process.

The attachment of cancer cells to adhesive molecules, such as laminin (LN) and fibronectin (FN) in the extracellular matrix is a critical step in tumor invasion and metastasis. Recent data have suggested a potential role for thrombospondin (TSP), a 420-kDa cyto-adhesive glycoprotein, in the growth and spread of breast cancer. In this study, we have measured the ability of the human breast adenocarcinoma cell line, MDA-MB-231, to synthesize TSP and to use this molecule as an adhesion factor. The level of TSP in cells and secreted into the culture medium were determined by an enzyme-linked immunosorbent assay (ELISA). At pre-confluence, MDA-MB-231 cells were shown to produce a high level of TSP, most of which was retained within the cells. In comparison, FN was almost entirely secreted into the culture medium. An increased secretion of TSP was however measured at low cell density, suggesting that TSP might be required for cell/substratum or cell/cell interactions. As shown by flow cytometry, the cells expressed membrane-bound TSP as well as unoccupied TSP receptors. 125I-TSP bound saturably to 1.2 x 10(6) sites per cell with an apparent dissociation constant of 23 nM. The binding was inhibited by an excess of unlabeled TSP and by heparin, suggesting that the receptor could be a heparan-sulfate proteoglycan or a sulfatide. TSP promoted attachment but not spreading of MDA-MB-231 cells which attached and spread on FN and LN substrates. These results suggest that endogenously synthesized TSP may have a role in the cyto-adherence of tumor cells during the spread of breast cancer.