Protease‐activated receptor (PAR)‐1 and PAR‐2 participate in the cell growth of alveolar capillary endothelium in primary lung adenocarcinomas

BACKGROUND Cell growth can be induced via elicitation of protease‐activated receptors (PAR) with serine proteases such as thrombin and trypsin. METHODS To understand whether PAR are involved in tumor vessel formation in the neoplastic cell‐bearing alveolar walls, immunohistochemical and reverse transcriptase‐polymerase chain reaction analyses were performed using the lung tissues from 16 patients with primary lung adenocarinomas. RESULTS In microdissected tumor alveolar walls, the expressions of PAR‐1 and PAR‐2 mRNA were increased by 10‐fold ( P < 0.05) and 16‐fold ( P < 0.01), respectively, as compared with normal alveolar walls. Confocal microscopy revealed that tumor capillary endothelial cells in alveolar walls lost thrombomodulin expression. Instead, the expression of PAR‐2 often became obvious at the normal border. Both PAR‐1 and PAR‐2 were expressed in the microvessel endothelial cells in tumors. Trypsin mRNA was expressed in 7 of the 16 cancer cell‐bearing tissue specimens in contrast to 1 of the 14 normal alveolar walls. Immunohistochemically, trypsin was positive in the neoplastic cells from 10 patients and in lung adenocarcinoma cell lines (A549, HLC‐1, LC‐2, and PC‐14). An in vitro assay showed a significant increase in idoxuridine (IdU) or bromodeoxyuridine uptake in human pulmonary artery endothelial cells and human umbilical cord vein endothelial cells after treatments with α‐thrombin or activating peptides; SFLLRN for PAR‐1 and SLIGKV for PAR‐2, respectively. CONCLUSIONS Thus, proliferation of alveolar capillary endothelial cells is initialized in part by PAR activation with serum thrombin and neoplastic cell‐released trypsin. These results suggest a synergistic effect of PAR with vascular endothelial growth factor in alveolar angiogenesis. Cancer 2003;97:703–13. © 2003 American Cancer Society. DOI 10.1002/cncr.11087