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Objective To investigate the effects of angiotensin Ⅱ (Ang Ⅱ) on the expression of TLR4 and its role in lipopolysaccharide (LPS)-induced NF-κB activation and CD40 expression in rat peritoneal mesothelial cells (RPMCs). Methods RPMCs were harvested from Spragne-Dawley rat peritoneal cavity and maintained under defined in vitro condition. The cells were treated with Ang Ⅱ at different concentrations (10-9, 10-8, 10-7, 10-6 mol/L) and exposed to Ang Ⅱ (10-7 mol/L) for different times (1, 2, 4, 8, 12, 24, 48 h for mRNA and 6, 12, 24, 36, 48 h for protein, respectively). Meanwhile, the influence of AT1 receptor antagonist (AT1R, losartan, 10-5 mol/L) and AT2 receptor blocker (AT2R, PD123177, 10-5 mol/L) on the TLR4 induced by Ang Ⅱ was observed. After synchronization for 24 hours, the cells were randomly assigned to four groups: the control group, the Ang Ⅱ (10-7 tool/L) group, the LPS (1 mg/L) group, the Ang Ⅱ (10-7 mol/L) plus LPS (1 mg/L) group, which were used to investigate the effects of Ang Ⅱ on the NF-κB activation and CD40 expression induced by LPS. The mRNA expression of TLR4 and CD40 was measured by RT-PCR and the protein abundance of TLR4, NF-κB p65, phospho-p65, IKBα and phospho-IκBα were analyzed by Western blot. Immunofluorescence was performed to determine the subcellular localization of p65 subunit of NF-κB. Results (1) Treatment of RPMCs with Ang Ⅱ resulted in a concentration-dependent increase in the expression of TLR4. Ang Ⅱ at 10-9, 10-8, 10-7 and 10-6 mol/L increased TLR4 mRNA expression by 70.5%, 89.5%, 102.9%, and 121.9%, respectively and protein expression by 12.1%, 27.7%, 51.2%, and 41.6%, respectively (P<0.01). Treatment of RPMCs with 10-7 mol/L Ang Ⅱ resulted in a time-dependent increase in the expression of TLR4, with the peak of mRNA expression at 8 and 12 h (P<0.01) and the protein expression at 12 and 24 h (P<0.01). (2) Losartan antagonized Ang Ⅱ-stimulated expression of TLR4 by 33.5% (P<0.05), PD123177 had no such effect (P0.05). (3) Treatment of RPMCs with LPS (1 mg/L) for 60 rain significantly increased the ratio of phospho-IκBα to IκBα by 362.6% (P< 0.01) , phospho-p65 to p65 by 67.4% (P<0.05), and LPS (1 mg/L) for 4 h significantly increased the expression of CD40 mRNA by 299.9% (P<0.01) compared to the control group. In comparison to the LPS (1 mg/L) group, preincubation of RPMCs with AngⅡ (10-7 mol/L) for 24 h then treated with LPS (1 mg/L) for 60 rain significantly increased the ratio of phospho-IκBα to IκBα by 49.1% (P<0.01), phospho-p65 to p65 by 29.3%(P<0.05), and LPS (1 mg/L) for 4 h significantly increased the expression of CD40 mRNA by 56.8%(P<0.01). (4) The p65 subunit of NF-κB was dominantly distributed in the cytoplasm in the control and Ang Ⅱ group. Following exposure to LPS for 60 min, p65 subunit labeling was upregulated and translocated into the nuclei. A significantly increased nuclear staining of p65 in ceils treated with Ang Ⅱ plus LPS were observed. Conclusions Ang Ⅱ induces the expression of TLR4 in dose- and time-dependent manner in RPMCs, resulting in enhanced NF-κB signaling and induction of CD40 expression, Locally produced Ang Ⅱ in the peritoneum may play an amplified role in LPS-induced peritoneal inflammation.