Investigating anti-inflammatory effects of common dietary phenolic acids and their structurally related synthetic analogues in immune cells

Atli, Nursabah (2022) Investigating anti-inflammatory effects of common dietary phenolic acids and their structurally related synthetic analogues in immune cells. Doctoral thesis, University of East Anglia.

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A diet rich in flavonoids and phenolic acids (PAs) is inversely correlated with chronic age-related disorders including cardiovascular disease (CVD) and neurological disorders. A central underlying cause of these disorders is chronic inflammation, orchestrated by a wide array of pro-inflammatory mediators including adhesion molecules, cytokines, and chemokines. In CVD, atherosclerosis develops, which is characterised in the early stage by endothelial activation and transendothelial migration of monocytes and macrophages. In neurological disorders, microglial activation takes the centre stage. Flavonoids and PAs are thought to exert their health effects by downregulating pro-inflammatory biomarkers and/or upregulating anti-inflammatory mediators. Flavonoids have been extensively studied for their effects on pro- and anti-inflammatory mediators at supraphysiological concentrations. However, they are poorly absorbed in their native form and instead are metabolised into PAs in the body. These PA metabolites are more bioavailable and stay longer in the circulation. This study investigated the anti-inflammatory effects of common PAs and related synthetic analogues at physiological concentrations (1-10 μM) on different type of immune cells, including monocytes, macrophages, neutrophils, and microglial cells as well as their effect on monocyte and neutrophil transendothelial migration.

Seven common diet-derived PA (ferulic acid (FA), isoferulic acid (IFA), caffeic acid (CA), protocatechuic acid (PCA), vanillic acid (VA), isovanillic acid (IVA) and 4-hydroxybenzoic acid (4HBA)), and 33 related non-dietary derived analogues, including 7 glycinated analogues, 12 alkyl esters and 14 structurally related, non-natural PAs were investigated for their effects on LPS-induced TNF-α in THP-1 monocytes. None of the diet-derived or glycinated phenolic acids had any effect on TNF-α secretion. FA-hexyl ester (FA-Hex-Es) and FA-propyl ester (FA-Pro-Es) dose-dependently inhibited LPS-induced TNF-α secretion but did not affect TNF-α mRNA levels suggesting involvement of a post-translational mechanism. FA-Hex-Es and FA-Pro-Es upregulated haem oxygenase-1 and NADPH quinone oxidoreductase-1 expression, potentially via the Nrf2 pathway.

In microglial cells, three diet-derived PAs (FA, IFA and PCA) and their synthetic esters were studied for their effects on LPS-induced TNF-α and IL-6 secretion. None of the diet-derived PA affected secretion of either cytokine. In contrast, FA-Hex-Es and IFA ethyl ester (IFA-Et-Es) showed a trend in reducing TNF-α and suppressed IL-6 protein and gene expression, most likely through inhibition of the NF-κB signalling pathway.

In a transendothelial migration assay, the 7 diet-derived PA and 14 related analogues were screened at 1 μM for their effects on neutrophil and monocyte migration in response to fMLP. None of the PA inhibited neutrophil migration. However, PCA and 3-iodobenzoic acid (3IBA) significantly inhibited monocyte migration. The mechanism of action was then investigated. PA did not inhibit MCP-1 secretion from HL60-differentiated neutrophils but 2IBA and 3IBA had a moderate inhibitory effect on MCP-1 mediated THP-1 monocyte chemotaxis, although this did not reach statistical significance. Higher concentrations of PA could possibly have resulted in significant effects.

These studies suggest that diet-derived PA and their related non-esterified analogues may have poor absorption or uptake by immune cells. Higher concentrations may help to reveal their mechanisms of action. Synthetic esters of FA and IFA with longer alkyl groups exert anti-inflammatory properties in monocytes and microglial cells and could be used as potential therapeutics or nutraceuticals with anti-inflammatory properties. Ultimately, these findings could help future study design of new analogues of metabolites with enhanced uptake and more immunomodulatory effects in vitro and in vivo.

Item Type: Thesis (Doctoral)
Faculty \ School: Faculty of Science > School of Pharmacy
Depositing User: Chris White
Date Deposited: 20 Jun 2022 10:13
Last Modified: 20 Jun 2022 10:13

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