The conversion by thrombin of soluble plasma fibrinogen to an insoluble fibrin matrix is central to hemostasis and subsequent wound healing. Fibroblasts adhere to and rapidly grow into fibrin clots, resulting in collagen deposition and, ultimately, scar formation. Although a number of soluble mediators have been implicated in this process, a role for fibrin(ogen) itself has not been described. The present study further investigated the nature of mitogenic activity remaining in solution after in vitro fibrin clot formation. Liquid expressed from a fibrin clot (clot supernatant) elicited a mitogenic response of up to 83 +/- 4.7% above media control. Upon addition of a polyclonal fibrinogen antibody, this activity was reduced by 50%. The remaining activity was attributed to the presence of thrombin and was neutralized by the addition of a specific thrombin inhibitor. Fibrinogen cleavage products were separated by molecular sieve chromatography and the mitogenic potential of each fraction assessed. A peak of activity was observed in fractions containing proteins with apparent molecular weights of 200 to 300 kD. Enhanced chemiluminescence Western blotting of these fractions established the presence of several fibrin(ogen)-derived protein bands. It is therefore proposed that thrombin cleavage of fibrinogen, in addition to producing fibrin, generates high-molecular-weight soluble cleavage products that may play an important role during normal wound healing and in the pathogenesis of disease states associated with vascular leakage and fibrosis.

1. The ability of airway epithelial cells to produce insulin-like growth factor I may be important in the pathogenesis of subepithelial fibrosis observed in the airways of patients with asthma. We determined whether human airway epithelial cells are capable of producing polypeptide mediators that could induce fibroblast proliferative activity, in particular insulin-like growth factor I. 2. We examined 12 primary cultures of human airway epithelial cells grown to confluence on collagen gel-coated dishes. Using a colorimetric assay based on the uptake and subsequent release of Methylene Blue, increased proliferation of human fetal lung fibroblasts was detected in conditioned media from airway epithelial cells. The median stimulation of fibroblast proliferation was 49.9% (range 25.6-113.3%) above control values (observed at 1:2 dilution of media). 3. A neutralizing antiserum to insulin-like growth factor I partly inhibited fibroblast proliferation induced by epithelial cell conditioned media by 52.2% (49.9-109%; n = 5). 4. Radioimmunoassay for insulin-like growth factor I in conditioned media demonstrated a median concentration of 54.1 ng/ml (32.4-96.8 ng/ml). 5. Insulin-like growth factor I mRNA was detected in epithelial cell monolayers by Northern blot analysis using an insulin-like growth factor I cDNA probe. 6. The insulin-like growth factor I gene is expressed in cultured human airway epithelial cells, which also secrete insulin-like growth factor I protein. Insulin-like growth factor I also accounts for the major mitogenic activity for fibroblasts of cultured human epithelial cell conditioned media. Insulin-like growth factor I may function in a paracrine manner to modulate fibroblast behaviour and may be involved in airway processes, such as those occurring in asthma.

We have previously shown that soluble partially degraded fibrin(ogen) remains in solution after fibrin clot formation and is a potent fibroblast mitogen (Gray, A. J., Bishop, J. E., Reeves, J. T., Mecham, R. P., and Laurent, G. J.(1995) Am. J. Cell Mol. Biol. 12, 684-690). Mitogenic sites within the fibrin(ogen) molecule are located on the Aα and Bβ 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, receptor pathways [Abstract] through which mitogenic effects are mediated are unknown. The present study sought to determine the nature of fibrin (ogen) receptors expressed on human fibroblasts which interact with the fibrinogen Bβ chain. Receptor complexes were isolated from 125I-surface-labeled fibroblasts and purified on a fibrinogen Bβ chain affinity column. Subsequent high performance liquid chromatography and SDS-polyacrylamide gel electrophoresis analysis indicated fibrinogen Bβ chain bound specifically to a 60-kDa surface protein. Sequence analysis of the amino terminus of this protein indicated 100% homology to human calreticulin. Immunoprecipitation experiments employing a polyclonal anti-calreticulin antibody provided further evidence that the 60-kDa protein isolated in this study was calreticulin. Further, polyclonal antibodies to human calreticulin significantly inhibited the mitogenic activity of fibrinogen Bβ chain on human fibroblasts. The present study has shown that cell surface calreticulin binds to the Bβ chain of fibrinogen mediating its mitogenic activity.

Pulmonary fibrosis commonly develops in systemic sclerosis. We assessed the role of thrombin in promoting fibroblast proliferation in the lungs in this disorder. Bronchoalveolar lavage fluid (BALF) thrombin concentrations were higher in ten patients with systemic sclerosis than in 12 healthy controls (14.6 vs 3.6 nmol/L, p<0.02), but values in patients with cryptogenic fibrosing alveolitis (n=10) or sarcoidosis (n=10) were not increased. BALF from all patients induced fibroblast proliferation. This proliferation was attenuated by thrombin inhibitors for BALF from systemic sclerosis patients only. We suggest thrombin contributes to lung fibroblast proliferation in this disorder.