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BACKGROUND: Previous studies have demonstrated that brain-derived neurotrophic factor (BDNF) promotes neural differentiation. However, the mechanisms involved in cell cycle-related protein regulation, which highly correlates to neural proliferation and apoptosis, remain poorly understood.OBJECTIVE: To investigate the effects of various concentrations of BDNF on cycle-related protein mRNA expression in induce-differentiated SH-SY5Y cells in vitro prior to and following G_2 phase,and to analyze the neuroprotective effects of BDNF.DESIGN, TIME AND SETTING: A comparison, observational study, based on cell biology, was performed at the Department of Biochemistry, Medical College of Tongji University, from March 2005 to October 2006.MATERIALS: SH-SY5Y cells were provided by Shanghai Institute of Cytology, Chinese Academy of Science; BDNF by Alomone Labs, Israel; all-trans retinoic acid (ATRA) by Sigma-Aldrich, USA.METHODS: SH-SY5Y cells were randomly divided into three groups: blank control [cells were treated in Insulin-Transferrin-Selenium (ITS) solution for 7 days], ATRA (cells were treated with ITS solution containing 10 μmol/L ATRA for 7 days), and BDNF (cells were treated identical to the ATRA group for 5 days, and then respectively treated in ITS solution containing 1, 10, and 100 μg/L BDNF for 2 days). The experiment was repeated three times for each group.MAIN OUTCOME MEASURES: mRNA expression levels of cyclin A1, B1, B2, cyclin-dependent kinase 1, and 5 were detected using quantitative real-time RT-PCR; percentage of cells in G_1, S, and G_2 phases were detected using fluorescence-activated cell sorting.RESULTS: mRNA expression levels of cyclin A1 in the high-dose BDNF group was significantly less than the ATRA group (P