Thrombin generation increases in several pathological conditions, including cancer, thromboembolism, diabetes and myeloproliferative syndromes. During tumor development, thrombin levels increase along with several other molecules, including cytokines and angiogenic factors. Under such conditions, it is reasonable to predict that thrombin may recognize new low-affinity substrates that usually are not recognized under low-expression levels conditions. In the present study, we hypothesized that fibroblast growth factor (FGF)-2 may be cleaved by thrombin and that such action may lead to an impairment of its biological activity. The evidence collected in the present study indicates that FGF-2-induced proliferation and chemotaxis/invasion of SK-MEL-110 human melanoma cells were significantly reduced when FGF-2 was pre-incubated with active thrombin. The inhibition of proliferation was not influenced by heparin. Phe-Pro-Arg-chloromethyl ketone, a specific inhibitor of the enzymatic activity of thrombin, abolished the thrombin-induced observed effects. Accordingly, both FGF-2-binding to cell membranes as well as FGF-2-induced extracellular signal-regulated kinase phosphorylation were decreased in the presence of thrombin. Finally, HPLC analyses demonstrated that FGF-2 is cleaved by thrombin at the peptide bond between residues Arg42 and Ile43 of the mature human FGF-2 sequence. The apparent k(cat)/K(m) of FGF-2 hydrolysis was 1.1 x 10(4) M(-1) x s(-1), which is comparable to other known low-affinity thrombin substrates. Taken together, these results demonstrate that thrombin digests FGF-2 at the site Arg42-Ile43 and impairs FGF-2 activity in vitro, indicating that FGF-2 is a novel thrombin substrate.