NK4 exhibits two distinct biological actions: antagonistic inhibition of hepatocyte growth factor (HGF) through binding to the Met/HGF receptor, and antiangiogenic action through binding to perlecan. Here, the anti-tumor effect of NK4 on malignant pleural mesothelioma was investigated. Of the 7 human malignant mesothelioma cell lines (ACC-Meso-1, ACC-Meso-4, EHMES-1, EHMES-10, H28, H2052 and JMN-1B), only EHMES-10 cells formed subcutaneous tumors when implanted into mice. For EHMES-10 cells, HGF facilitated invasion of the cells in collagen gel, whereas NK4 and neutralizing anti-HGF antibody suppressed the HGF-induced invasion. In addition, NK4 but not anti-HGF antibody suppressed proliferation of EHMES-10 cells in collagen, suggesting that the suppression by NK4 was independent of the HGF-Met pathway. In the subcutaneous tumor model, recombinant adenovirus-mediated intratumoral expression of NK4 inhibited tumor growth, while the invasive characteristic of tumor cells was not observed. Analysis of Met receptor tyrosine phosphorylation, proliferation, apoptosis and blood vessels in the tumor tissues indicated that the inhibitory effect of NK4 expression might be primarily caused by the inhibition of tumor angiogenesis. In all the 7 mesothelioma lines, HGF stimulated Met tyrosine phosphorylation, and this was associated with enhanced cell migration. HGF-dependent Met activation and migration were inhibited by NK4. Since malignant pleural mesothelioma represents an aggressive neoplasm characterized by extensive invasive growth, suppression of invasive growth has therapeutic value. Thus, the simultaneous inhibition of the HGF-Met pathway and angiogenesis by NK4 for treatment of malignant pleural mesothelioma is significant, particularly to attenuate migration and invasive growth.