Clotting factor XII (Hageman factor) contains epidermal growth factor (EGF)-homologous domains and is reported to be a potent mitogen for human hepatoma (HepG2) cells. In this study, we tested whether factor XII exhibits growth factor activity on several other EGF-sensitive target cells, including fetal hepatocytes, endothelial cells, alveolar type II cells, and aortic smooth muscle cells. We found that factor XII significantly enhanced [3H]thymidine incorporation in aortic smooth muscle cells (SMCs) and all other cells tested. Tyrphostin, a growth factor receptor/tyrosine kinase antagonist, inhibited both EGF-and factor XIIinduced responses. However, differences in the levels of magnitude of DNA synthesis, the observed synergism between EGF and factor XII, and the differential sensitivity to tyrphostin suggest that the EGF receptor and the factor XII receptor may be nonidentical. The factor XlI-induced mitogenic response was achieved at concentrations that were 1/10th the physiologic range for the circulating factor and was reduced by popcorn inhibitor, a specific factor XII protease inhibitor. Treatment of aortic SMCs with factor XII, as well as activated factor XII, resulted in a rapid and transient activation of a mitogen-activated/extracellular signalregulated protein kinase with peak activity/tyrosine phosphorylation observed at 5 to 10 min of exposure. Taken together, these data (i) confirm that clotting factor XII functions as a mitogenic growth factor and (ii) demonstrate that factor XII activates a signal transduction pathway, which includes a mitogen-activated protein kinase.

The structure of coagulation factor XII (Hageman factor), inferred from its DNA sequence, includes two epidermal growth factor (EGF)-homologous domains in its amino-terminal region. This suggests that factor XII may exhibit EGF-like activities. Reciprocal antigenic crossreactivity between factor XII and EGF was shown by exposing purified human factor XII or mouse EGF to anti-mouse EGF or anti-human factor XII. Western blot analysis showed that anti-mouse EGF recognized intact factor XII at 80 kDa. Together, these results suggest that the EGF-homologous domains are accessible for anti-EGF binding in native factor XII. To determine whether factor XII has mitogenic activity, HepG2 or L cells (104 cells per well) were grown in serum-free medium in the presence or absence of factor XII or kaolin-activated factor XII (factor XIIa). Both factors XII and XIIa (6.0 jug/ml) enhanced cell proliferation by --2-fold (P < 0.001 and P < 0.005, respectively). In contrast, L cells, which are not EGF target cells, were not affected by either factor XII or factor XIIa. Various doses of factor XII enhanced cell proliferation, PH~thymidine incorporation, and [3H]leucine incorporation in HepG2 cells cultured under the same conditions. These data indicate that factor XII, like EGF, is a mitogen for HepG2 cells and suggest a possible autocrine role in the liver.Factor XII is a procoagulant, found in normal plasma, that participates directly or indirectly in activation of the intrinsic clotting pathway, factor VII, fibrinolysis, and the reninangiotensin and kallikrein-kinin systems (1-3). The structure of factor XII, inferred from its DNA and amino acid sequences, includes two epidermal growth factor (EGF)-homologous domains in the amino-terminal region (4-7). This suggests that factor XII may mimic EGF and act as a growth factor, Hepatocytes and hepatoma cells are target cells for EGF and hepatocytes synthesize factor XII, raising the possibility that factor XII may act as an autocrine or paracrine hepatocyte growth factor, perhaps in a feedback process, to regulate liver growth or productive activity under physiologic conditions. EGF is a well-characterized mitogen that stimulates the proliferation of numerous cell types in vitro and of epithelial cells in vivo. The EGF receptor is detectable on a large variety of cell types, mostly epithelial. Available evidence indicates that the EGF receptor mediates the biological signals of EGF and possibly those of two EGF-like growth factors-namely, transforming growth factor a and the vaccinia virus growth factor (8), which have amino acid sequences homologous to EGF (9). Since the putative tertiary structure of factor XII includes domains that appear to be homologous to EGF (4-7), we examined whether factor XII might have growth factor activity as well.Growth factors can elicit numerous responses in target cells. Among these are (i) early cellular responses, such as increased protein synthesis; (ii) later cellular responses, such as increased DNA synthesis; and (iii) the definitive ce...

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