Background Airway smooth muscle （ASM） is suspected to be a determining factor in the structural change of asthma. However, the role of protein kinase C α （PKCα） and cyclin D1 involved in the dysfunction of ASM leading to asthmatic symptoms is not clear. In this study, the central role of PKCα and cyclin D1 in ASM proliferation in asthmatic rats was explored. Methods Thirty-six pathogen-free male Brown Norway （BN） rats were randomly divided into 2 groups： control groups （group N1, N2 and N3） and asthmatic groups （group A1, A2, and A3）. Groups A1, A2 and A3 were challenged with ovalbumin （OA） for 2 weeks, 4 weeks and 8 weeks respectively. Control animals were exposed to an aerosolized sterile phosphate buffered saline （PBS）. The ASM mass and nucleus numbers were studied to estimate the degree of airway remodeling by the hematoxylin-eosin staining method. PKCα and cyclin D1 expression in the ASM cells was detected by reverse transcription polymerase chain reaction （RT-PCR） and immunohistochemistry. The relation between PKCα and cyclin D1 was assessed by linear regression analysis. PKC agonist phorbol 12-myristate 13-acetate （PMA）, PKC inhibitor Ro31-8220 and an antisense oligonucleotide against cyclin D1 （ASOND） were used to treat ASM cells （ASMCs） obtained from the 2 weeks asthmatic rats. The cyclin D1 protein expression level was detected by Western blotting.Results Compared with the control group, the PKCα and cyclin D1 mRNA levels were increased in the asthmatic group. Similar to RT-PCR results, immunohistochemistry analysis for PKCα and cyclin D1 expression revealed an increased production in ASMCs after allergen treatment for 2, 4 and 8 weeks compared with the respective control groups. No difference in expression of PKCα and cyclin D1 in ASM were found in the 2, 4 or 8 weeks asthmatic rats. There were significant positive correlations between PKCα and cyclin D1 expression, both transcriptionally （r=0.944, P 〈0.01） and translationally （r=0.826, P �
