Interleukin-1 β Stimulates the Proliferation of Cultured Airway Smooth Muscle Cells via Platelet-derived Growth Factor

Bronchoalveolar lavage fluid from symptomatic asthmatics contains elevated levels of several proinflammatory interleukins including interleukin-1β (IL-1β). Biologic activities of IL-1β are considered to be critical in the inflammatory process. Since these characteristics include mitogenic properties, we investigated the effect of IL-1β on the proliferation of airway smooth muscle (ASM) cells isolated from guinea pig tracheas. Primary tissue culture of ASM cells was maintained in media containing 0%, 1%, or 10% fetal bovine serum (FBS). Cultures were exposed up to 6 days to a human recombinant IL-1β (20, 40, or 100 pg/ml) in the presence or absence of indomethacin. The proliferation of ASM cells was assessed with two techniques: a direct counting of cells with a hemocytometer and/or with a [3H]-thymidine incorporation, an established marker of DNA synthesis. The evaluation was done daily, up to the sixth day after exposure of cells to different doses of IL-1β. We found that the exposure of ASM cells to human recombinant IL-1β significantly (P < 0.01) increased the number of tracheal myocytes as well as the [3H]-thymidine incorporation into ASM cells. These changes were dependent upon the dose of IL-1β and the concentration of FBS in the cultured medium. The most active proliferation of ASM cells was observed in medium containing 1% FBS, indomethacin (1 µg/ml), and IL-1β (100 pg/ml). The presence of indomethacin in the culture medium was essential to demonstrate the proliferative effect of IL-1β. Pretreatment with specific polyclonal antibodies against platelet-derived growth factor (PDGF-BB homodimer) completely inhibited the IL-1β-induced increase in [3H]-thymidine incorporation by ASM cells. In contrast, the presence of human recombinant PDGF-BB (1.5 ng/ml) in the incubation medium further potentiated the ASM proliferation induced by IL-1β. This potentiation of proliferation was also dose-dependent: PDGF-BB (1.5 and 2.5 ng/ml) alone potentiated (P < 0.01) proliferation of ASM cells. It can be concluded that proinflammatory cytokine, such as IL-1β which is produced in asthma, stimulates proliferation of ASM cells through the PDGF-dependent mechanism. This effect may be critically important in an increase of ASM cell mass observed in asthma.