Fatty Acids Modulate Toll-like Receptor 4 Activation through Regulation of Receptor Dimerization and Recruitment into Lipid Rafts in a Reactive Oxygen Species-dependent Manner

The saturated fatty acids acylated on Lipid A of lipopolysaccharide (LPS) or bacterial lipoproteins play critical roles in ligand recognition and receptor activation for Toll-like Receptor 4 (TLR4) and TLR2. The results from our previous studies demonstrated that saturated and polyunsaturated fatty acids reciprocally modulate the activation of TLR4. However, the underlying mechanism has not been understood. Here, we report for the first time that the saturated fatty acid lauric acid induced dimerization and recruitment of TLR4 into lipid rafts, however, dimerization was not observed in non-lipid raft fractions. Similarly, LPS and lauric acid enhanced the association of TLR4 with MD-2 and downstream adaptor molecules, TRIF and MyD88, into lipid rafts leading to the activation of downstream signaling pathways and target gene expression. However, docosahexaenoic acid (DHA), an n-3 polyunsaturated fatty acid, inhibited LPS- or lauric acid-induced dimerization and recruitment of TLR4 into lipid raft fractions. Together, these results demonstrate that lauric acid and DHA reciprocally modulate TLR4 activation by regulation of the dimerization and recruitment of TLR4 into lipid rafts. In addition, we showed that TLR4 recruitment to lipid rafts and dimerization were coupled events mediated at least in part by NADPH oxidase-dependent reactive oxygen species generation. These results provide a new insight in understanding the mechanism by which fatty acids differentially modulate TLR4-mediated signaling pathway and consequent inflammatory responses which are implicated in the development and progression of many chronic diseases.Toll-like receptors (TLRs)³ are one of the key pattern recognition receptor families that play a critical role in inducing innate and adaptive immune responses in mammals by recognizing conserved pathogen-associated molecular pattern of invading microbes. So far, at least thirteen TLRs have been identified in mammalian species (1, 2).Lipopolysaccharide (LPS) from Gram-negative bacteria is the ligand for the TLR4 complex (3), whereas, TLR2 can recognize lipoproteins/lipopeptides of Gram-positive bacteria and mycoplasma (1, 2). LPS forms a complex with LPS-binding protein in serum leading to the conversion of oligomeric micelles of LPS to monomers, which are delivered to CD14. Monomeric LPS is known to bind TLR4/MD-2/CD14 complex (4). Lipid A, which possesses most of the biological activities of LPS, is acylated with hydroxy saturated fatty acids. The 3-hydroxyl groups of these saturated fatty acids are further 3-Ο-acylated by saturated fatty acids. Removal of these Ο-acylated saturated fatty acids from Lipid A not only results in complete loss of endotoxic activity, but also makes Lipid A act as an antagonist against the native Lipid A (5, 6). One or more Lipid As containing unsaturated fatty acids are known to be non-toxic and act as an antagonist against endotoxin (7, 8). In addition, deacylated lipoproteins are unable to activate TLR2 and to induce cytokine expression in monocytes (9). Together, these results suggest that saturated fatty acids acylated on Lipid A or bacterial lipoproteins play critical roles in ligand recognition and receptor activation for TLR4 and TLR2. Indeed, it is suggested that the rapid interaction of bacterial lipopeptides with plasma membrane of macrophages occurs via insertion of their acylated saturated fatty acids as determined by electron energy loss spectroscopy and freeze-fracture techniques (10, 11). TLR2 can form a heterodimer with TLR1 or TLR6, which can discriminate the molecular structure of triacyl or diacyl lipopeptides (12–14). So far there is no evidence that microbial ligands for other TLRs are acylated by saturated fatty acids.Results from our previous studies demonstrated that saturated fatty acids activate TLR4 and polyunsaturated fatty acids (PUFA) inhibit both saturated fatty acid- and LPS-induced activation of TLR4 (15, 16). In addition, the saturated fatty acid lauric acid potentiates, but the n-3 PUFA docosahexaenoic acid (DHA) inhibits lipopeptide (TLR2 agonist)-induced TLR2 activation (17). Together, these results suggest that both TLR2 and TLR4 signaling pathways and target gene expression are reciprocally modulated by saturated and polyunsaturated fatty acids. However, the mechanism for this modulation by fatty acids is not understood.TLR4 is recruited to lipid raft factions after cells are treated with LPS and subsequently induces tumor necrosis factor-α expression in RAW264.7 cells (18). This process occurs in an ROS-dependent manner because inhibition of NADPH oxidase suppresses TLR4 recruitment to lipid rafts (19). Methyl-β-dextrin, a lipid raft inhibitor, significantly inhibits the LPS-induced expression of cytokine (19), suggesting that lipid rafts are essential for TLR4-mediated signal transduction and target gene expression. Lipid rafts are a collection of lipid membrane microdomains characterized by insolubility in non-ionic detergents. Lipid rafts serve as a platform where receptor-mediated signal transduction is initiated (20). Lipid rafts have a special lipid composition that is rich in cholesterol, sphingomyelin, and glycolipids (21). The polar lipids in detergent-resistant membrane contain predominantly saturated fatty acyl residues with underrepresented PUFAs (22–24), suggesting that saturated fatty acyl chains favor lipid raft association. On the other hand, n-3 PUFAs displace signaling proteins from lipid rafts by altering lipid composition, and the displacement leads to the suppression of T-cell receptor-mediated signaling (25). It is now well documented that TLRs form homo- or hetero-oligomers (1, 2). TLR4 homodimerization is the initial step of the receptor activation. Results from our previous studies suggest that the molecular target by which saturated fatty acids and n-3 PUFAs reciprocally modulate TLR4 activation is the receptor complex itself or the event leading to the receptor activation instead of the downstream signaling components (15, 16). Therefore, we determined whether the reciprocal modulation of TLR4 activation is mediated by regulation of the dimerization and recruitment of TLR4 into lipid rafts, and if these processes occur in an ROS-dependent manner.     

TLR2 Mediates Gap Junctional Intercellular Communication through Connexin-43 in Intestinal Epithelial Barrier Injury

Gap junctional intercellular communication (GJIC) coordinates cellular functions essential for sustaining tissue homeostasis; yet its regulation in the intestine is not well understood. Here, we identify a novel physiological link between Toll-like receptor (TLR) 2 and GJIC through modulation of Connexin-43 (Cx43) during acute and chronic inflammatory injury of the intestinal epithelial cell (IEC) barrier. Data from in vitro studies reveal that TLR2 activation modulates Cx43 synthesis and increases GJIC via Cx43 during IEC injury. The ulcerative colitis-associated TLR2-R753Q mutant targets Cx43 for increased proteasomal degradation, impairing TLR2-mediated GJIC during intestinal epithelial wounding. In vivo studies using mucosal RNA interference show that TLR2-mediated mucosal healing depends functionally on intestinal epithelial Cx43 during acute inflammatory stress-induced damage. Mice deficient in TLR2 exhibit IEC-specific alterations in Cx43, whereas administration of a TLR2 agonist protects GJIC by blocking accumulation of Cx43 and its hyperphosphorylation at Ser³⁶⁸ to prevent spontaneous chronic colitis in MDR1α-deficient mice. Finally, adding the TLR2 agonist to three-dimensional intestinal mucosa-like cultures of human biopsies preserves intestinal epithelial Cx43 integrity and polarization ex vivo. In conclusion, Cx43 plays an important role in innate immune control of commensal-mediated intestinal epithelial wound repair.The intestinal epithelial cell (IEC)³ barrier provides the front line of mucosal host defense in the intestine. The IEC barrier confers anatomic integrity and immunologic protection of the intestinal mucosal surface. Because the IEC barrier constantly faces diverse populations of lumenal microbes and other potential threats, it must exert a highly defined process of continuous discrimination: excluding harmful antigens while allowing host-beneficial substances to permeate (1, 2). Para- and intercellular transit of molecules is modulated by a complex network of closely arranged tight (TJ) and gap junctions (GJ) between juxtaposed IEC. Gap junctional intercellular communication (GJIC) is an essential, but not well understood, mechanism for cellular and tissue homeostasis that coordinates cell-cell passage of ions and small metabolites (<1 kDa). Thus, GJIC regulates cell proliferation, migration, and differentiation (3). GJ channels are formed by hexameric connexins at the plasma membrane. Cx43 is the major connexin and represents a key target in GJIC regulation (4). It is differentially phosphorylated at a dozen or more residues throughout its life cycle (5–9). Alteration of GJIC caused by changes in Cx43 has been proposed to be involved in the pathophysiology of diverse IEC barrier diseases, including inflammatory bowel diseases, necrotizing enterocolitis, cancer, and enteric infection (10–12). However, immune mediators that allow protective GJIC via Cx43 to sustain IEC barrier function during mucosal damage have not yet been identified.Toll-like receptor 2 (TLR2), a member of the TLR family that is constitutively expressed in IEC (13–15), recognizes conserved molecular patterns associated with both Gram-negative and -positive bacteria (16). We have previously shown that commensal-mediated TLR2 helps to maintain functional TJ barrier integrity of the intestinal epithelial layer. TLR2 enhances transepithelial resistance of the IEC barrier by apical redistribution of ZO-1 via protein kinase Cα/δ (17). Treatment with the TLR2 ligand PCSK protects ZO-1-associated IEC barrier integrity and decreases intestinal permeability in acute colitis (18). Previous studies in other cell types have demonstrated that the second PDZ domain of ZO-1 interacts with the carboxyl terminus of Cx43 (19, 20). ZO-1 binds to Cx43 preferentially during the G₀ phase, enhancing assembly and stabilization of GJIC (21, 22). Like TLR2, Cx43 and ZO-1 reside in caveolin-1-associated lipid raft microdomains (23–25). We therefore hypothesized that the binding between ZO-1 and Cx43 may allow TLR2 to control IEC barrier function by GJIC.In this study, we identified a new physiological mechanism of innate immune host defense in the injured intestine. Our findings indicated that Cx43 serves as an important component of the protective innate immune response of the intestinal epithelium. TLR2-induced GJIC via Cx43 appears to control IEC barrier function and restitution during acute and chronic inflammatory damage, enhancing mucosal homeostasis between commensals and host. UC-associated TLR2 mutant results in impaired GJIC by a proteasomal-dependent increase in Cx43 turnover.     

Analysis of Free Energy Signals Arising from Nucleotide Hybridization Between rRNA and mRNA Sequences during Translation in Eubacteria

A decoding algorithm is tested that mechanistically models the progressive alignments that arise as the mRNA moves past the rRNA tail during translation elongation. Each of these alignments provides an opportunity for hybridization between the single-stranded, -terminal nucleotides of the 16S rRNA and the spatially accessible window of mRNA sequence, from which a free energy value can be calculated. Using this algorithm we show that a periodic, energetic pattern of frequency 1/3 is revealed. This periodic signal exists in the majority of coding regions of eubacterial genes, but not in the non-coding regions encoding the 16S and 23S rRNAs. Signal analysis reveals that the population of coding regions of each bacterial species has a mean phase that is correlated in a statistically significant way with species () content. These results suggest that the periodic signal could function as a synchronization signal for the maintenance of reading frame and that codon usage provides a mechanism for manipulation of signal phase.[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32]     

Splitting the BLOSUM Score into Numbers of Biological Significance

Mathematical tools developed in the context of Shannon information theory were used to analyze the meaning of the BLOSUM score, which was split into three components termed as the BLOSUM spectrum (or BLOSpectrum). These relate respectively to the sequence convergence (the stochastic similarity of the two protein sequences), to the background frequency divergence (typicality of the amino acid probability distribution in each sequence), and to the target frequency divergence (compliance of the amino acid variations between the two sequences to the protein model implicit in the BLOCKS database). This treatment sharpens the protein sequence comparison, providing a rationale for the biological significance of the obtained score, and helps to identify weakly related sequences. Moreover, the BLOSpectrum can guide the choice of the most appropriate scoring matrix, tailoring it to the evolutionary divergence associated with the two sequences, or indicate if a compositionally adjusted matrix could perform better.[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29]     

Role of Partitioning-defective 1/Microtubule Affinity-regulating Kinases in the Morphogenetic Activity of Helicobacter pylori CagA

Helicobacter pylori CagA plays a key role in gastric carcinogenesis. Upon delivery into gastric epithelial cells, CagA binds and deregulates SHP-2 phosphatase, a bona fide oncoprotein, thereby causing sustained ERK activation and impaired focal adhesions. CagA also binds and inhibits PAR1b/MARK2, one of the four members of the PAR1 family of kinases, to elicit epithelial polarity defect. In nonpolarized gastric epithelial cells, CagA induces the hummingbird phenotype, an extremely elongated cell shape characterized by a rear retraction defect. This morphological change is dependent on CagA-deregulated SHP-2 and is thus thought to reflect the oncogenic potential of CagA. In this study, we investigated the role of the PAR1 family of kinases in the hummingbird phenotype. We found that CagA binds not only PAR1b but also other PAR1 isoforms, with order of strength as follows: PAR1b > PAR1d ≥ PAR1a > PAR1c. Binding of CagA with PAR1 isoforms inhibits the kinase activity. This abolishes the ability of PAR1 to destabilize microtubules and thereby promotes disassembly of focal adhesions, which contributes to the hummingbird phenotype. Consistently, PAR1 knockdown potentiates induction of the hummingbird phenotype by CagA. The morphogenetic activity of CagA was also found to be augmented through inhibition of non-muscle myosin II. Because myosin II is functionally associated with PAR1, perturbation of PAR1-regulated myosin II by CagA may underlie the defect of rear retraction in the hummingbird phenotype. Our findings reveal that CagA systemically inhibits PAR1 family kinases and indicate that malfunctioning of microtubules and myosin II by CagA-mediated PAR1 inhibition cooperates with deregulated SHP-2 in the morphogenetic activity of CagA.Infection with Helicobacter pylori strains bearing cagA (cytotoxin-associated gene A)-positive strains is the strongest risk factor for the development of gastric carcinoma, the second leading cause of cancer-related death worldwide (1–3). The cagA gene is located within a 40-kb DNA fragment, termed the cag pathogenicity island, which is specifically present in the genome of cagA-positive H. pylori strains (4–6). In addition to cagA, there are ∼30 genes in the cag pathogenicity island, many of which encode a bacterial type IV secretion system that delivers the cagA-encoded CagA protein into gastric epithelial cells (7–10). Upon delivery into gastric epithelial cells, CagA is localized to the plasma membrane, where it undergoes tyrosine phosphorylation at the C-terminal Glu-Pro-Ile-Tyr-Ala motifs by Src family kinases or c-Abl kinase (11–14). The C-terminal Glu-Pro-Ile-Tyr-Ala-containing region of CagA is noted for the structural diversity among distinct H. pylori isolates. Oncogenic potential of CagA has recently been confirmed by a study showing that systemic expression of CagA in mice induces gastrointestinal and hematological malignancies (15).When expressed in gastric epithelial cells, CagA induces morphological transformation termed the hummingbird phenotype, which is characterized by the development of one or two long and thin protrusions resembling the beak of the hummingbird. It has been thought that the hummingbird phenotype is related to the oncogenic action of CagA (7, 16–19). Pathophysiological relevance for the hummingbird phenotype in gastric carcinogenesis has recently been provided by the observation that infection with H. pylori carrying CagA with greater ability to induce the hummingbird phenotype is more closely associated with gastric carcinoma (20–23). Elevated motility of hummingbird cells (cells showing the hummingbird phenotype) may also contribute to invasion and metastasis of gastric carcinoma.In host cells, CagA interacts with the SHP-2 phosphatase, C-terminal Src kinase, and Crk adaptor in a tyrosine phosphorylation-dependent manner (16, 24, 25) and also associates with Grb2 adaptor and c-Met in a phosphorylation-independent manner (26, 27). Among these CagA targets, much attention has been focused on SHP-2 because the phosphatase has been recognized as a bona fide oncoprotein, gain-of-function mutations of which are found in various human malignancies (17, 18, 28). Stable interaction of CagA with SHP-2 requires CagA dimerization, which is mediated by a 16-amino acid CagA-multimerization (CM)² sequence present in the C-terminal region of CagA (29). Upon complex formation, CagA aberrantly activates SHP-2 and thereby elicits sustained ERK MAP kinase activation that promotes mitogenesis (30). Also, CagA-activated SHP-2 dephosphorylates and inhibits focal adhesion kinase (FAK), causing impaired focal adhesions. It has been shown previously that both aberrant ERK activation and FAK inhibition by CagA-deregulated SHP-2 are involved in induction of the hummingbird phenotype (31).Partitioning-defective 1 (PAR1)/microtubule affinity-regulating kinase (MARK) is an evolutionally conserved serine/threonine kinase originally isolated in C. elegans (32–34). Mammalian cells possess four structurally related PAR1 isoforms, PAR1a/MARK3, PAR1b/MARK2, PAR1c/MARK1, and PAR1d/MARK4 (35–37). Among these, PAR1a, PAR1b, and PAR1c are expressed in a variety of cells, whereas PAR1d is predominantly expressed in neural cells (35, 37). These PAR1 isoforms phosphorylate microtubule-associated proteins (MAPs) and thereby destabilize microtubules (35, 38), allowing asymmetric distribution of molecules that are involved in the establishment and maintenance of cell polarity.In polarized epithelial cells, CagA disrupts the tight junctions and causes loss of apical-basolateral polarity (39, 40). This CagA activity involves the interaction of CagA with PAR1b/MARK2 (19, 41). CagA directly binds to the kinase domain of PAR1b in a tyrosine phosphorylation-independent manner and inhibits the kinase activity. Notably, CagA binds to PAR1b via the CM sequence (19). Because PAR1b is present as a dimer in cells (42), CagA may passively homodimerize upon complex formation with the PAR1 dimer via the CM sequence, and this PAR1-directed CagA dimer would form a stable complex with SHP-2 through its two SH2 domains.Because of the critical role of CagA in gastric carcinogenesis (7, 16–19), it is important to elucidate the molecular basis underlying the morphogenetic activity of CagA. In this study, we investigated the role of PAR1 isoforms in induction of the hummingbird phenotype by CagA, and we obtained evidence that CagA-mediated inhibition of PAR1 kinases contributes to the development of the morphological change by perturbing microtubules and non-muscle myosin II.