MicroRNA 21 Is a Homeostatic Regulator of Macrophage Polarization and Prevents Prostaglandin E2-Mediated M2 Generation

Abstract

Macrophages dictate both initiation and resolution of inflammation. During acute inflammation classically activated macrophages (M1) predominate, and during the resolution phase alternative macrophages (M2) are dominant. The molecular mechanisms involved in macrophage polarization are understudied. MicroRNAs are differentially expressed in M1 and M2 macrophages that influence macrophage polarization. We identified a role of miR-21 in macrophage polarization, and found that cross-talk between miR-21 and the lipid mediator prostaglandin E2 (PGE2) is a determining factor in macrophage polarization. miR-21 inhibition impairs expression of M2 signature genes but not M1 genes. PGE2 and its downstream effectors PKA and Epac inhibit miR-21 expression and enhance expression of M2 genes, and this effect is more pronounced in miR-21-/- cells. Among potential targets involved in macrophage polarization, we found that STAT3 and SOCS1 were enhanced in miR-21-/- cells and further enhanced by PGE2. We found that STAT3 was a direct target of miR-21 in macrophages. Silencing the STAT3 gene abolished PGE2-mediated expression of M2 genes in miR-21-/- macrophages. These data shed light on the molecular brakes involved in homeostatic macrophage polarization and suggest new therapeutic strategies to prevent inflammatory responses.

MiRNA-Mediated Macrophage Polarization and its Potential Role in the Regulation of Inflammatory Response

“…In contrast to the findings by Caescu et al (42), Wang et al (43) reported that miR-21 promoted M1 polarization in peritoneal macrophages and inhibited M2 polarization. In miR-21-KO macrophages, expression levels of M1 markers ( Tnfa, Il12p40, Il1b, and Il6 ) were lower while M2 markers ( Il10, Arg1, Retnla, and Chi3l3 ) were increased remarkably compared to levels in wild type (WT) macrophages (43).…”

“…In miR-21-KO macrophages, expression levels of M1 markers ( Tnfa, Il12p40, Il1b, and Il6 ) were lower while M2 markers ( Il10, Arg1, Retnla, and Chi3l3 ) were increased remarkably compared to levels in wild type (WT) macrophages (43). Treatment of WT macrophages with prostaglandin E2 (PGE2), an endogenous lipid mediator produced in inflammation and infection, inhibited miR-21 expression (43). Further studies demonstrated that PGE2 decreased the expression of M1 markers and enhanced the expression of M2 markers (43).…”

“…Treatment of WT macrophages with prostaglandin E2 (PGE2), an endogenous lipid mediator produced in inflammation and infection, inhibited miR-21 expression (43). Further studies demonstrated that PGE2 decreased the expression of M1 markers and enhanced the expression of M2 markers (43). These data suggest that miR-21 acts as an endogenous brake to block PGE2-mediated generation of M2 macrophages.…”

MicroRNA Regulation of Atherosclerosis

“…Atherosclerosis progression is associated with the predominance of an M1 macrophage phenotype in the plaque, whereas plaques undergoing regression are enriched in M2 macrophages 92 . A growing list of miRNAs is implicated in regulating the balance between the M1 and M2 phenotypes, including miR-let7a 98 , miR-19a 99 , miR-21 100 , miR-27a 101 , miR-33 42 , miR-124 102 , miR-125a 103 , miR-146a 104 , miR-155 105 , miR-214 106 , miR-223 107 . However, few of these have been investigated in the context of atherosclerosis.…”

SOCS1 is a negative regulator of metabolic reprogramming during sepsis

“…Macrophages from Socs1 Δmyel mice were induced by injection with 2 ml 3% thioglycollate, and these were harvested 4 days after injection, as previously described (84). Macrophages were stimulated with 100 ng/ml LPS (Sigma-Aldrich), and 24 hours after stimulation, these were harvested for analysis by ChIP assay.…”