Interleukin-4 inhibits mitogen-induced proliferation of human airway smooth muscle cells in culture.

The increase in the amount of airway smooth muscle in the bronchial wall associated with asthma is partly due to hyperplasia. It is therefore important to determine which factors regulate growth and especially proliferation. In this study, we describe the effect of interleukin-4 (IL-4), a mast cell- and T lymphocyte-derived cytokine, on human airway smooth muscle proliferation as determined by [3H]thymidine uptake in the presence of fetal bovine serum (FBS), platelet-derived growth factor, basic fibroblast growth factor, and thrombin. IL-4 (5, 15, 50, and 150 ng/ml) significantly decreased 10% FBS-induced proliferation by 50, 73, 43, and 46%, respectively. The proliferative responses to platelet-derived growth factor (20 and 40 ng/ml), basic fibroblast growth factor (30 ng/ml), and thrombin (1 and 10 U/ml) were significantly reduced by 19, 21, 37, 36, and 57% respectively in the presence of 50 ng/ml of IL-4. We investigated the effect of IL-4 and other known inhibitors of smooth muscle proliferation, namely PGE2, heparin, and forskolin, on intracellular cAMP concentrations. IL-4 (50 ng/ml) and heparin (100 U/ml) did not alter intracellular cAMP levels when cells were treated with 1 or 10% FBS. PGE2 (1 microM) and forskolin (10 microM) significantly increased cAMP concentration above the control value in nonproliferating cells (1% FBS treated) by 7- and 37-fold, respectively. The effect of IL-4 (50 ng/ml), PGE2 (1 microM), and forskolin (10 microM) on cyclin D1 protein expression in 10% FBS-stimulated human airway smooth muscle cells was also examined. PGE2 and forskolin did not significantly inhibit cyclin D1 expression. However, IL-4 decreased cyclin D1 expression by 21%. These results provide evidence that IL-4 decreases human airway smooth muscle cell proliferation via a mechanism that is cAMP independent and mediated, in part, by a decrease in cyclin D1 protein expression.