[Study on the anti-sepsis mechanism of ursolic acid by targeting myeloid differentiation protein-2].

Objective: To explore the mechanism of ursolic acid in treating sepsis using myeloid differentiation protein-2 (MD-2) as the research carrier.

Methods: The affinity of ursolic acid and MD-2 was determined by biofilm interferometry technique, and the bonding mode between ursolic acid and MD-2 was tested with the aid of molecular docking technique. Raw 264.7 cells were cultured in RPMI 1640 medium and subcultured was conducted when the cell density reached 80%-90%. The second-generation cells were used for in the experiment. The effects of 8, 40 and 100 mg/L ursolic acid on cell viability were assessed by methyl thiazolyl tetrazolium (MTT) method. Cells were divided into blank group, lipopolysaccharide (LPS) group (LPS 100 μg/L) and ursolic acid group (100 μg/L LPS treatment after addition of 8, 40 or 100 mg/L ursolic acid). The effect of ursolic acid on the release of cytokines nitric oxide (NO), tumor necrosis factor-α (TNF-α) and interleukins (IL-6, IL-1β) were evaluated by enzyme-linked immunosorbent assay (ELISA). The influence of ursolic acid on the mRNA expressions of TNF-α, IL-6, IL-1β, inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2) were detected by reverse transcription-polymerase chain reaction (RT-PCR). The implication of ursolic acid on the protein expressions of LPS-Toll-like receptor 4 (TLR4)/MD-2-nuclear factor-κB (NF-κB) pathway were tested by Western blotting.

Results: Ursolic acid could bind to the hydrophobic cavity of MD-2 through hydrophobic bond with the amino acid residues of the protein. Therefore, ursolic acid showed high affinity with MD-2 [dissociation constant (KD) = 1.43×10]. The cell viability were decreased slightly, with the concentration of ursolic acid increasing, and the cell viability of 8, 40 and 100 mg/L ursolic acid were 96.01%, 94.32% and 92.12%, respectively, and there was no significant difference compared with the blank group (100%). Compared with the blank group, the cytokine level of the LPS group was significantly increased. The level of cytokines were significantly reduced by the treatment of 8, 40 and 100 mg/L ursolic acid, and the higher the concentration, the more obvious effect [compared between 100 mg/L ursolic acid group and LPS group: IL-1β (μmol/L): 38.018±0.675 vs. 111.324±1.262, IL-6 (μmol/L): 35.052±1.664 vs. 115.255±5.392, TNF-α (μmol/L): 39.078±2.741 vs. 119.035±4.269, NO (μmol/L): 40.885±2.372 vs. 123.405±1.291, all P < 0.01]. Compared with the blank group, the mRNA expressions of TNF-α, IL-6, IL-1β, iNOS and COX-2 in the LPS group were significantly increased, and the protein expressions of MD-2, myeloid differentiation factor 88 (MyD88), phosphorylation NF-κB p65 (p-NF-κB p65) and iNOS in the LPS-TLR4/MD-2-NF-κB pathway were significantly up-regulated. Compared with the LPS group, the mRNA expressions of TNF-α, IL-6, IL-1β, iNOS and COX-2 were significantly reduced by the treatment of 100 mg/L ursolic acid bound with MD-2 protein [TNF-α (2): 4.659±0.821 vs. 8.652±0.787, IL-6 (2): 4.296±0.802 vs. 11.132±1.615, IL-1β (2): 4.482±1.224 vs. 11.758±1.324, iNOS (2): 1.785±0.529 vs. 4.249±0.811, COX-2 (2): 5.591±1.586 vs. 16.953±1.651, all P < 0.01], and the proteins expressions of MD-2, MyD88, p-NF-κB p65 and iNOS in the LPS-TLR4/MD-2-NF-κB pathway were significantly down-regulated (MD-2/β-actin: 0.191±0.038 vs. 0.704±0.049, MyD88/β-actin: 0.470±0.042 vs. 0.875±0.058, p-NF-κB p65/β-actin: 0.178±0.012 vs. 0.571±0.012, iNOS/β-actin: 0.247±0.035 vs. 0.549±0.033, all P < 0.01). However, there was no difference in protein expression of NF-κB p65 among the three groups.

Conclusions: Ursolic acid inhibits the release and expression of cytokines and mediators and regulates LPS-TLR4/MD-2-NF-κB signaling pathway by blocking MD-2 protein, and thus plays an anti-sepsis role.