Caspase-1 triggers smooth muscle cell proliferation in hypoxic pulmonary hypertension

Background: Pulmonary hypertension is a serious complication to chronic lung diseases, often with alveolar hypoxia. Mechanisms for hypoxia-induced pulmonary hypertension are suggested to be initial vasoconstriction, followed by inflammation, proliferation of smooth muscle cells (SMCs) and fibrosis in pulmonary arteries. Elevated levels of Interleukin (IL)-18, IL-1β and IL-6 are found in patients with pulmonary hypertension. IL-18 and IL-1β are proinflammatory cytokines activated by the enzyme caspase-1. We have documented that caspase-1 deficient mice have reduced hypoxia-induced pulmonary hypertension and reduced muscularization in pulmonary arteries compared to wild-type (WT) mice. Objective: To study mechanisms of SMC proliferation in hypoxic pulmonary hypertension initiated by the enzyme caspase-1. Methods: Pulmonary arteries from WT and caspase-1-/- mice were harvested and grown “ex vivo”. Human SMCs were exposed to hypoxia and treated with caspase-1 inhibitor. Proliferation of SMCs was measured by cell count, BrdU incorporation, Ki67 and cyclin D1 mRNA. Results: Caspase-1 abrogated arteries showed reduced SMC proliferation, together with lowered levels of IL-18, IL-1β and IL-6. Supply of IL-18 or IL-1β rescued SMC proliferation in caspase-1 deficient arteries, and the level of IL-6 protein was restored. Hypoxic stimulation of human SMCs showed increased BrdU incorporation and Ki67 protein levels, indicating hypoxia-induced cell proliferation. Cell count, BrdU incorporation and cyclin D1 mRNA levels were reduced by adding a caspase-1 inhibitor. Conclusions: The enzyme caspase-1 regulates smooth muscle cell proliferation through IL-18/IL-1β and IL-6, being novel targets in pulmonary hypertension.