['To verify the suppressive effect of berberine on the proliferation of the human pulmonary giant cell carcinoma cell line PG and to demonstrate the mechanisms behind the antitumoral effects of berberine.', 'The proliferative effects of PG cells were detected by 3-(4,5-dimethylthiazol-2-yl)-2,5- diphenyltetrazolium bromide colorimetry. The cell cycle was examined by flow cytometry. The expression level of cyclin D1 was detected by RT-PCR. The activities of the activating protein-1 (AP-1) and NF-kappaB signaling pathways related to cyclin D1 were examined by luciferase assay. The cytoplasmic level of c-Jun was detected by Western blot analysis. An electrophoretic mobility shift assay was used to examine the binding of transcription factors to the cyclin D1 gene (CCND1) AP-1 motif.', 'The results showed that the proliferation of PG cells treated with different concentrations (10, 20, and 40 microg/mL) of berberine for 24 and 48 h was suppressed significantly compared to the control group. After treatment with berberine, the proportion of PG cells at the G0/G1 phase increased, while cells at the S and G2/M phases decreased. Berberine could inhibit the expression of cyclin D1 in PG cells. Berberine inhibited the activity of the AP-1 signaling pathway, but had no significant effect on the NF-kappaB signaling pathway. Berberine suppressed the expression of c-Jun and decreased the binding of transcription factors to the CCND1 AP-1 motif.'] Does berberine inhibit cyclin D1 expression via suppressed binding of AP-1 transcription factors to CCND1 AP-1 motif?