Apelin induces microglia polarization to pro-inflammatory state

Aims: Patients with proliferative diabetic retinopathy show an increased level of apelin in their vitreous, which is the endogenous ligand for the G protein-coupled receptor APJ. Microglial activation is involved in the development of diabetic retinopathy. Thereby their different functional properties are dependent on the state of polarization. However to what extend apelin might be involved in this process of microglial polarization and by which mechanism the apelin-APJ signaling is mediated in the cell remain unclear.

Methods: Primary murine microglial BV2 cells were cultured and incubated with recombinant apelin-13 for 30 min to 6h. The APJ-expression on the cells was assessed by immunofluorescence (IF) staining and effects of the recombinant apelin-13 on microglial polarization were investigated using IF and real time PCR. Western Blots were performed to determine the APJ signaling pathway.

Results: APJ was expressed on BV2 cells and translocated into cytoplasma. Apelin induced a polarization towards the M1 phenotype with an upregulation of the cytokines TNF-α (41.8%), IL-6 (22.1%), IL-1ß (148%), MCP-1 (93.6%) and MIP-1α (92.9%). In contrast, the IF intensity of the M2 marker IL-10 was decreased by 40.8% after apelin administration. Quantitative real time PCR demonstrated that the expression of M1 phenotype cytokines IL-1ß, MCP-1 and MIP-1α was increased by 10.4 ± 1.4, 10.8 ± 3.9 and 3.7 ± 0.8 folds, respectively. Western blots showed an increased Erk phosphorylation.

Conclusion: We demonstrate that apelin induced microglial polarization towards the pro-inflammatory M1 state. Our results suggest that APJ signaling is mediated via Erk phosphorylation making Erk a potentially target to inhibit M1 polarization.