Background Exposure to cigarette smoke stimulates the proliferation of human pulmonary artery smooth muscle cells (HPASMCs) in vivo and in vitro. However, the molecular mechanism remains unclear. This study aimed at investigating the role of signaling pathways involving protein kinase C alpha (PKCα) and cyclin D1 in the cigarette smoke extract (CSE)-induced HPASMCs proliferation.Methods Synchronized HPASMCs were treated with different concentrations of CSE. Cell proliferation was evaluated by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyttetrazolium bromide (MTT) assay and cell counting. Cell cycle was analyzed by flow cytometry with propidium iodide staining. Activation of PKCα was measured by detecting the expression of PKCαprotein in the cytosolic and membrane fractions using West blotting analysis. Small interfering RNA (siRNA) was used to knockdown PKCα and cyclin D1. The cyclin D1 mRNA was assessed by real-time RT-PCR. The PKCα and cyclin D1protein levels were detected by West blotting.Results Low concentrations of CSE (1%-10%) stimulated proliferation of HPASMCs, with its maximal effect at 5%.CSE (5%) led to PKCα activation. Inhibition of PKCα activity using G(o) 6976 or siRNA-mediated knockdown of PKCα significantly attenuated CSE-induced cell proliferation and G1/S transition. Cyclin D1, one of key regulators of G1/S transition, was found to be upregulated by 5% CSE at both the mRNA and protein levels. CSE-stimulated cellproliferation and G1/S transition was abolished by cyclin D1 siRNA. Moreover, G(o) 6976 or PKCα siRNA significantlysuppressed CSE-induced upregulation of cyclin D1 at both the mRNA and protein levels.Conclusion PKCα-cyclin D1 pathway at least partially mediates the CSE-induced proliferation in HPASMCs.