That miR-21 controls LPS actions has been shown. Sheedy et al have shown that miR-21 inhibits the production of IL-6 and enhances IL-10 levels by controlling both NFκB p65 levels and the proinflammatory molecule tumor suppressor programmed cell death 4, an inhibitor of IL-10 production. These findings are not in perfect agreement with ours, since we found that miR-21 -/- elicited macrophages show enhanced IL-6 and IL-10 levels, whereas Sheedy et al. show that miR-21 mimic enhances IL-6 and decrease IL-10 levels. The reason for this discrepancy is uncertain, but there are several differences in the respective experiments. Sheedy et al studied miR-21 effects in a macrophage cell line and bone marrow derived macrophages; whereas, we performed our experiments in thioglycollate-elicited macrophages, which exhibit an inflammatory phenotype. Also, we investigated the role of miR-21 in basal/homeostatic expression of these cytokines, and Sheedy et al determined the production of IL-16 and IL-10 in LPS-stimulated cells. Whether thioglycollate-elicited miR-21 deficient macrophages respond to LPS in the same manner as cell lines or bone marrow macrophages remains to be determined. Our results are in agreement with Shi et al. who showed in a model of colitis that M1-like cytokines such as TNF-αand MIP-2 are decreased in miR-21 deficient mice. Clearly, the elucidation of such cell-specific functions of miR-21 should help in the development of effective miR-based therapeutic strategies. Here, we investigated the role of the cAMP inducer PGE2 in the expression of miR-21 in macrophages. Our data show that PGE2 challenge decreased miR-21 expression, and incubation of macrophages with the downstream effectors PKA or the Epac agonist also decreased miR-21 levels. Whether the cAMP/PKA/Epac axis influences global microRNA expression and their effects in macrophage biology remains to be determined. These data lead us to speculate that miR-21 acts as a brake on PGE2 effects, and PGE2-mediated miR-21 inhibition is part of an inhibitory loop ASP1517 involved in macrophage M2 polarization by the PGE2/PKA/Epac axis. The molecular mechanisms involved in miR-21 and PGE2-induced M2 macrophages were studied. Initially, we tested whether miR-21 inhibition influenced EP 1–4 mRNA expression, but we did not observe any differences in the expression of these receptors between WT and miR-21 -/- cells or WT cells treated with miR-21 mimic. We also did not observe any change in the expression of the CREB. We next investigated which transcription factors and effectors were involved in enhanced expression of M2 genes. While STAT6 is activated by IL-4 and IL-13, and has been suggested to be the master regulator of M2 differentiation, STAT1 and NFκB are thought to be essential for M1 generation. We did not observe a change in the expression of STAT6 and phosphorylation in miR-21-/- macrophages, which led us to study the expression of other STAT proteins in miR-21 deficient cells, and to ask whether PGE2 further influenced the expression of these proteins.