Cigarette smoke could induce pulmonary smooth muscle cells (PASMCs) proliferation. Although our previous study had implied the involvement of protein kinase C alpha (PKC alpha), the molecular mechanism underlying PKC alpha pathway in this process is still unknown. In this study, rat PASMCs were stimulated by cigarette smoke extract (CSE) or PMA (a special activator to PKC alpha). Two percent CSE and PMA significantly enhanced cyclin D1 expression and cells proliferation. But cyclin D1-specific siRNA successfully inhibited DNA synthesis in CSE-treated or PMA-treated cells. On the other hand, PKC alpha-specific siRNA significantly suppressed cyclin D1 expression in CSE-treated cells. Moreover, PKC alpha-specific siRNA resulted in a cell-cycle arrest in G0/G1 and decreased cells number significantly. We conclude that CSE induced rat PASMCs proliferation at least partly via PKC alpha-mediated cyclin D1 expression. J. Cell. Biochem. 112: 2082-2088, 2011. (C) 2011 Wiley-Liss, Inc.