In this study, we have shown that SCFAs can induce either robust PGE2 production alone or in synergy with LPS in human monocytes and PBMC. Since LPS stimulation did not affect the mRNA level of GPR43 and GPR41, the synergy is likely from 2 independent signaling pathways. This effect on PGE2 production is specific, since other lipid mediators including PGI2, LTB4 and TXB2, were not affected by SCFAs. Furthermore, this effect is receptor-mediated since it is sensitive to PTX.
Interestingly, we showed that SCFAs induced robust calcium flux in human neutrophils, but not in human monocytes. Our finding is consistent with the previous report that propionic acid induced calcium mobilization in human neutrophils, but not in monocytes, platelets and lymphocytes[12
]. It is likely that SCFAs activate neutrophils and monocytes for the immune response through different signaling pathways that would be consistent with the different temporal roles of these cells in an inflammatory response. SCFAs induce calcium flux and chemotaxis in neutrophils which arrive early in an inflammatory response, while they regulate production of PGE2
, cytokines and chemokines in monocytes which are also present in the inflammatory site but mature into macrophages during extravasation and remain at the site of inflammation until resolution.
This in vitro
effect of SCFAs on human monocytes may have relevance to human diseases that target the intestinal tract. Among various prostanoids, PGE2
, in particular, seems to play a critical role in inflammatory bowel disease (IBD) via
the EP4 receptor, one of the four PGE2
]. Among 8 prostanoid receptor-deficient mice, only EP4-deficient mice developed severe colitis with 3% dextran sodium sulphate (DSS) treatment. It is suggested that EP4 maintains intestinal homeostasis by maintaining mucosal integrity and downregulating the immune response[32
]. Therefore, it is possible that SCFAs induce production of PGE2
and play a protective role via
the GPR43-expressing cells of the colon. Furthermore, we demonstrate that PGE2
production induced by SCFAs was inhibited by a COX-1 inhibitor, but not a COX-2 inhibitor. This is consistent with the notion that in DSS-induced colitis, the significant reduction in PGE2
resulted from decreased expression of COX-1, but not COX-2[33
In human monocytes, SCFAs specifically inhibited constitutive MCP-1 production and LPS-induced IL-10 production out of the total 16 cytokines and chemokines examined. The activities of SCFAs in our human monocyte cultures were all confirmed in human PBMC, which contain both monocytes and lymphocytes. We made the additional observation in these PBMC cultures that SCFAs would inhibit LPS-induced TNF-α and IFN-γ production.
This unique effect of SCFAs on the production of cytokines and chemokines in monocytes and PBMC is relevant to the design of new therapies for colitis. Intestinal inflammation present in inflammatory bowel disease is driven by the production of cytokines, chemokines and growth factors that draw in immune cells to the mucosa. It has been shown that MCP-1, IL-10, IFN-γ and TNF-α were significantly upregulated in experimental colitis models[34,35
]. Furthermore, the increased production of SCFAs from dietary fiber supplementation or probiotics administration inhibited the production of proinflammatory mediators and recovered damaged colonic mucosa in colitic animals[36,37
]. These experiments show that SCFAs can inhibit multiple inflammatory mediators which may control intestinal inflammation. Clinical trial evidence for this use of SCFAs is still not widely available or accepted as a mainstream therapy.
We tried to design a model system in vivo
to duplicate our in vitro
findings. We present the results of a model system using the intraplantar space in the rat paw, a space from which there was limited diffusion of the injected SCFAs. We had previously tried injecting SCFAs into the pleural cavity and into the peritoneal cavity to induce an inflammatory response with and without suboptimal inflammatory stimuli such as LPS. However, SCFAs are extremely labile with a very high diffusion rate and require high concentrations to achieve their effect in vivo
. Maintenance of a sufficient concentration of SCFAs at a local site or tissue space (such as the colon) has been a major challenge to obtain clear cytokine profiles and in vivo
efficacy. Therefore, better pharmacological tools, such as GPR43 specific small molecules[38
], are needed to confirm our in vitro
findings and further elucidate their biology. We were able to utilize the intraplantar space of the paw to illustrate the ability of SCFAs to synergize with LPS and induce mediator production but there was significant variability in this response.
The present findings of antiinflammatory activities of SCFAs in immune cells expand the database on these fatty acids as lipid mediators and may help explain the known beneficial effects of SCFAs in the colon and on colitis. Future studies are needed in knockout mice and with specific small molecules (both agonists and antagonists) to prove the exact molecular target for these effects of SCFAs.