THE MITOGENIC EFFECTS OF THE B-BETA CHAIN OF FIBRINOGEN ARE MEDIATED THROUGH CELL-SURFACE CALRETICULIN

We have previously shown that soluble partially degraded fibrin(ogen) remains in solution after fibrin clot formation and is a potent fibrob last mitogen (Gray, A, J,, Bishop, J, E,, Beeves, J, T,, Mecham, R.P,, and Laurent, G.J. (1995) Am, J. Cell Mot Biol. 12, 684-690), Mitogeni c sites within the fibrin(ogen) molecule are located on the Aa! and B beta chains of the protein (Gray, A. J,, Bishop, J, E,, Reeves, J, T,, and Laurent, G. J, (1993) J. Cell Sci. 104, 409-413), However, recept or pathways through which mitogenic effects are mediated are unknown, The present study sought to determine the nature of fibrin (ogen) rece ptors expressed on human fibroblasts which interact with the fibrinoge n B beta chain, Receptor complexes were isolated from I-125-surface-la beled fibroblasts and purified on a fibrinogen B beta chain affinity c olumn. Subsequent high performance liquid chromatography and SDS-polya crylamide gel electrophoresis analysis indicated fibrinogen B beta cha in bound specifically to a 60-kDa surface protein. Sequence analysis o f the amino terminus of this protein indicated 100% homology to human calreticulin. Immunoprecipitation experiments employing a polyclonal a nti-calreticulin antibody provided further evidence that the 60-kDa pr otein isolated in this study was calreticulin. Further, polyclonal ant ibodies to human calreticulin significantly inhibited the mitogenic ac tivity of fibrinogen B beta chain on human fibroblasts. The present st udy has shown that cell surface calreticulin binds to the B beta chain of fibrinogen mediating its mitogenic activity.