Mitogenic effect of fibrinogen on hematopoietic cells: involvement of two distinct specific receptors, MFR and ICAM-1

In addition to its well-known functions in blood clotting and cell adhesion, fibrinogen has been reported to be a mitogen for lymphoid cell lines and for human hematopoietic progenitors. Two specific receptors, the mitogenic fibrinogen receptor (MFR) and intercellular adhesion molecule-1 (ICAM-1/CD54), have been identified as possible candidates for the mediation of the mitogenic effect of fibrinogen. However, it has been questioned whether the MFR and ICAM-1 are truly distinct molecules. Using an antiserum specific for the MFR, we demonstrate that the MFR is a cell surface molecule clearly distinct from ICAM-1. Both receptors can be expressed separately or coexpressed on different cell types. Moreover, they are regulated differently: ICAM-1 is calcium-dependent whereas the MFR is not and the MFR is down-regulated by fibrinogen whereas ICAM-1 is not. The inhibition by an anti-MFR serum of the mitogenic effect of fibrinogen confirms the mitogenic function of the MFR.