Inhibitory effect of phloretin on inflammation and oxidative stress in IL-1β induced orbital fibroblasts in Graves orbitopathy and its mechanism

Objective

To investigate the inhibitory effect of phloretin on inflammation and oxidative stress in interleukin (IL)-1β induced orbital fibroblasts (OFs) from Graves orbitopathy (GO) patients and its mechanism.

Methods

The orbital fat and connective tissue from 6 eyes of 6 patients diagnosed as inactive GO who underwent orbital decompression in Henan Eye Hospital from January 2019 to December 2020 were collected.Primary OFs were isolated and passaged by explant culture and were identified by cell immunofluorescence assay.OFs were divided into control group, IL-1β induced group, and groups of various phloretin concentrations (25, 50, 75, 100 and 200 μmol/L). The viability of OFs after 24- and 48-hour treatment of the various phloretin concentrations was determined by cell counting kit-8 (CCK-8). OFs were induced by IL-1β to simulate an inflammatory environment of GO in vitro.Intracellular reactive oxygen species (ROS) levels of the normal control group, IL-1β induced group, 50 μmol/L phloretin group and 100 μmol/L phloretin group were detected by fluorescent probe (H₂DCF-DA). The concentrations of pro-inflammatory cytokines IL-6, IL-8 and monocyte chemoattractant protein-1 (MCP-1) in cell culture supernatant of the normal control group, IL-1β induced group and phloretin treated groups (25, 50, 75, and 100 μmol/L) were examined by enzyme-linked immunosorbent assay (ELISA). The expressions of heme oxygenase-1 (HO-1), nuclear factor erythroid 2-related factor (Nrf2) proteins, as well as P38, extracelluar regulated protein kinase (ERK) and c-Jun N-terminal kinase (JNK) proteins as well as their phosphorylated proteins in the MAPK signal pathway of the normal control group, IL-1β induced group and 100 μmol/L phloretin group, were detected by Western blot.The purpose and methods of the study were explained to the patients and their family members.Written informed consent was obtained.The study protocol was approved by the Ethics Committee of Henan Provincial People’s Hospital (No.HNEECKY-2020[07]).

Results

For cultured OFs, the mesenchymal origin was confirmed by positive expression of vimentin and fibroblasts were identified by negative expression of desmin, S-100 and cytokeratin-18.CCK-8 showed that there was no significant difference in absorbance value after 24- and 48-hour treatment between 25 μmol/L, 50 μmol/L, 75 μmol/L and 100 μmol/L phloretin groups and control group (all at P>0.05). The ROS levels of 50 μmol/L and 100 μmol/L phloretin groups were 21.95±1.71 and 10.01±1.03, respectively, which were significantly lower than 39.27±4.01 of IL-1β induced group (both at P<0.01). ELISA showed that IL-6 concentrations in 25 μmol/L, 50 μmol/L, 75 μmol/L and 100 μmol/L phloretin groups were (4 544.25±572.98), (1 000.25±133.96), (724.25±98.63), (519.50±118.02)pg/ml, respectively, which were all significantly lower than (7 581.75±565.93)pg/ml in IL-1β induced group (all at P<0.01). IL-8 concentrations in 50 μmol/L, 75 μmol/L and 100 μmol/L phloretin groups were (3 679.50±676.76), (2 143.75±616.20), (1 174.75±284.18)pg/ml, respectively, which were all significantly lower than (8 411.00±939.67)pg/ml in IL-1β induced group (all at P<0.01). The concentrations of MCP-1 in 50 μmol/L, 75 μmol/L and 100 μmol/L phloretin groups were (3 783.25±610.24), (1 565.75±457.89), (745.75±227.01)pg/ml, respectively, which were all significantly lower than (5 533.00±602.87)pg/ml in IL-1β induced group (all at P<0.01). The relative expression levels of HO-1 and Nrf2 were significantly higher and the relative expression levels of p-P38, p-ERK, and p-JNK were significantly lower in 100 μmol/L phloretin group than IL-1β induced group (all at P<0.01).

Conclusions

Phloretin reduces the oxidative stress level of IL-1β induced OFs from GO patients and inhibits the production of pro-inflammatory cytokines.The mechanism is related to the activation of Nrf2/HO-1 and the inhibition of the MAPK signal pathway.

Phloretin; Graves orbitopathy; Fibroblasts, orbital; Oxidative stress; Inflammation