Flavopiridol Mitigates the Progression of Monocrotaline-Induced Pulmonary Hypertension in Rats by Targeting Cyclin-dependent Kinase 9

Abstract Purpose: To investigate the role of cyclin-dependent kinase 9 (CDK9) and the therapeutic potential of CDK9 inhibitor (flavopiridol) in monocrotaline (MCT)-induced pulmonary hypertension.Methods: In vivo experiments, pulmonary hypertension rats were established by a single intraperitoneal injection of MCT (60 mg/kg) for 2 weeks and treated with flavopiridol (5 mg/kg, i.p., twice a week) or vehicle for 2 weeks. In vitro experiments, human pulmonary artery smooth muscle cells (HPASMCs) were treated with flavopiridol (0.025-1μM) or vehicle under hypoxic condition. Hemodynamic recording, right ventricle and lung histology, isolation of pulmonary arterial tissues were performed. The expressions of CDK9, RNA polymerase II, c-Myc, Mcl-1 and survivin were determined by qRT-PCR and western blotting, proliferation and apoptosis of PASMCs were also assayed.Results: CDK9 was upregulated in both rat pulmonary arterial tissues and HPASMCs. Upregulation of CDK9 increased the phosphorylation of the C-terminal domain (CTD) of RNA polymerase II (RNA pol II) on serine-2, promoting the expression of prosurvival and antiapoptotic proteins (c-Myc, Mcl-1 and survivin). Furthermore, treatment with flavopiridol (5 mg/kg) significantly alleviated pulmonary artery remodeling and partially reversed the progression of monocrotaline-induced pulmonary hypertension. Consistently, flavopiridol (0.5 μM) treatment decreased the proliferation and induced the apoptosis of cultured HPASMCs under hypoxic conditions. As a result of CDK9 inhibition and subsequent inhibition of RNA pol II CTD phosphorylation at serine 2, flavopiridol decreased c-Myc, Mcl-1 and survivin expressions in isolated pulmonary small arteries, leading to cell growth inhibition and apoptosis. Conclusion: Flavopiridol mitigates the progression of monocrotaline-induced pulmonary hypertension in rats by targeting cyclin-dependent kinase 9.