Phenolic 1,3‐diketones attenuate lipopolysaccharide‐induced inflammatory response by an alternative magnesium‐mediated mechanism

Abstract

BACKGROUND AND PURPOSEToll-like receptor 4 (TLR4) plays a key role in the induction of inflammatory responses both in peripheral organs and the CNS. Curcumin exerts anti-inflammatory functions by interfering with LPS-induced dimerization of TLR4-myeloid differentiation protein-2 (MD-2) complex and suppressing pro-inflammatory mediator release. However, the inhibitory mechanism of curcumin remains to be defined. EXPERIMENTAL APPROACHBinding of bis-demethoxycurcumin (GG6) and its cyclized pyrazole analogue (GG9), lacking the 1,3-dicarbonyl function, to TLR4-MD-2 was determined using molecular docking simulations. The effects of these compounds on cytokine release and NF-κB activation were examined by ELISA and fluorescence staining in LPS-stimulated primary microglia. Interference with TLR4 dimerization was assessed by immunoprecipitation in Ba/F3 cells. KEY RESULTSBoth curcumin analogues bound to the hydrophobic region of the MD-2 pocket. However, only curcumin and GG6, both possessing the 1,3-diketone moiety, inhibited LPS-induced TLR4 dimerization, activation of NF-κB and secretion of proinflammatory cytokines in primary microglia. Consistent with the ability of 1,3-diketones to coordinate divalent metal ions, LPS stimulation in a low magnesium environment decreased pro-inflammatory cytokine release and NF-κB p65 nuclear translocation in microglia and decreased TLR4-MD-2 dimerization in Ba/F3 cells. Curcumin and GG6 also significantly reduced cytokine output in contrast to the pyrazole analogue GG9. CONCLUSIONS AND IMPLICATIONSThese results indicate that phenolic 1,3-diketones, with a structural motif able to coordinate magnesium ions, can modulate LPSmediated TLR4-MD-2 signalling. Taken together, these studies identify a previously uncharacterized mechanism involving magnesium, underlying the inflammatory responses to LPS. Abbreviations

The Emerging Role of Curcumin in the Modulation of TLR-4 Signaling Pathway: Focus on Neuroprotective and Anti-Rheumatic Properties

“…In 1999, Jobin et al found that CUR has powerful anti-inflammatory effects because it suppressed the activation of NF-κB induced by various pro-inflammatory stimuli, through inhibition of IKKβ kinase activity or DNA binding of p65 [139,140]. Interesting studies analyzed more deeply this mechanism showing that curcumin contains α, β-unsaturated carbonyl group, and this group inhibits TLR-4 activation by interfering with receptor dimerization [140] an important step for the activation of downstream signaling pathways of this receptor [141,142]. According to Youn et al, the target of curcumin is TLR-4, but not the downstream components of TRIF pathway [141].…”

Natural products against renin-angiotensin system for antifibrosis therapy

“…Curcumin is a phenolic compound extracted from the rhizome of turmeric (Curcuma longa) that is usually used in Asia as an additive, spice and pigment [67][68][69][70][71][72]. Curcumin treatment suppressed the growth of colon cancer cells through retarding cell proliferation via inhibiting the Wnt/β-catenin pathway rather than by promoting apoptosis in mice [73].…”

Bisdemethoxycurcumin and Its Cyclized Pyrazole Analogue Differentially Disrupt Lipopolysaccharide Signalling in Human Monocyte-Derived Macrophages

“…This compound can modulate multiple signalling molecules such as transcription factors, enzymes, and secondary messengers, thereby controlling expression of a variety of genes, and is potentially effective in disease conditions associated with impaired regulation of such signalling pathways [ 7 ]. In particular, curcumin reduces LPS-induced activation and release of inflammatory cytokines by microglia and macrophages [ 8 – 13 ]. We and others demonstrated that curcumin and related compounds prevent activation of nuclear factor- κ B (NF- κ B), a transcriptional factor that regulates many genes involved in the initiation of inflammatory responses [ 13 , 14 ].…”

“…In particular, curcumin reduces LPS-induced activation and release of inflammatory cytokines by microglia and macrophages [ 8 – 13 ]. We and others demonstrated that curcumin and related compounds prevent activation of nuclear factor- κ B (NF- κ B), a transcriptional factor that regulates many genes involved in the initiation of inflammatory responses [ 13 , 14 ]. This polyphenol also affects the immunophenotype of LPS-activated human THP-1 macrophages, especially the expression of M1 markers that are modulated through the NF- κ B pathway [ 10 , 15 ].…”

“…LPS signalling is mediated mainly by TLR4-MD-2 activation and dimerization [ 20 ]. Previous studies demonstrated its inhibition by curcumin and its structurally related bisdemethoxycurcumin (GG6), both carrying the hepta-1,4,6-trien-3-one scaffold, whereas the GG6-based pyrazole analogue (GG9) was inactive [ 13 ].…”