Jacky Y. Suen

Short Hydrophobic Peptides with Cyclic Constraints Are Potent Glucagon-like Peptide-1 Receptor (GLP-1R) Agonists.

Cyclic constraints are incorporated into an 11-residue analogue of the N-terminus of glucagon-like peptide-1 (GLP-1) to investigate effects of structure on agonist activity. Cyclization through linking side chains of residues 2 and 5 or 5 and 9 produced agonists at nM concentrations in a cAMP assay. 2D NMR and CD spectra revealed an N-terminal β-turn and a C-terminal helix that differentially influenced affinity and agonist potency. These structures can inform development of small molecule agonists of the GLP-1 receptor to treat type 2 diabetes.

Structure, function and pathophysiology of protease activated receptors.

Discovered in the 1990s, protease activated receptors(1) (PARs) are membrane-spanning cell surface proteins that belong to the G protein coupled receptor (GPCR) family. A defining feature of these receptors is their irreversible activation by proteases; mainly serine. Proteolytic agonists remove the PAR extracellular amino terminal pro-domain to expose a new amino terminus, or tethered ligand, that binds intramolecularly to induce intracellular signal transduction via a number of molecular pathways that regulate a variety of cellular responses. By these mechanisms PARs function as cell surface sensors of extracellular and cell surface associated proteases, contributing extensively to regulation of homeostasis, as well as to dysfunctional responses required for progression of a number of diseases. This review examines common and distinguishing structural features of PARs, mechanisms of receptor activation, trafficking and signal termination, and discusses the physiological and pathological roles of these receptors and emerging approaches for modulating PAR-mediated signaling in disease.

Biased signalling and proteinase‐activated receptors (PARs): targeting inflammatory disease

Although it has been known since the 1960s that trypsin and chymotrypsin can mimic hormone action in tissues, it took until the 1990s to discover that serine proteinases can regulate cells by cleaving and activating a unique four‐member family of GPCRs known as proteinase‐activated receptors (PARs). PAR activation involves the proteolytic exposure of its N‐terminal receptor sequence that folds back to function as a ‘tethered’ receptor‐activating ligand (TL). A key N‐terminal arginine in each of PARs 1 to 4 has been singled out as a target for cleavage by thrombin (PARs 1, 3 and 4), trypsin (PARs 2 and 4) or other proteases to unmask the TL that activates signalling via Gq, Gi or G12/13. Similarly, synthetic receptor‐activating peptides, corresponding to the exposed ‘TL sequences’ (e.g. SFLLRN—, for PAR1 or SLIGRL— for PAR2) can, like proteinase activation, also drive signalling via Gq, Gi and G12/13, without requiring receptor cleavage. Recent data show, however, that distinct proteinase‐revealed ‘non‐canonical’ PAR tethered‐ligand sequences and PAR‐activating agonist and antagonist peptide analogues can induce ‘biased’ PAR signalling, for example, via G12/13‐MAPKinase instead of Gq‐calcium. This overview summarizes implications of this ‘biased’ signalling by PAR agonists and antagonists for the recognized roles the PARs play in inflammatory settings.

Modulating human proteinase activated receptor 2 with a novel antagonist (GB88) and agonist (GB110)

BACKGROUND AND PURPOSE Many cells express proteinase activated receptor 2 (PAR2) on their plasma membrane. PAR2 is activated by proteolytic enzymes, such as trypsin and tryptase that cleave the receptor N‐terminus, inititating signalling to intracellular G proteins. Studies on PAR2 have relied heavily upon activating effects of proteases and peptide agonists that lack stability and bioavailability in vivo.

Novel agonists and antagonists for human protease activated receptor 2.

Human protease activated receptor 2 (PAR2) is a G protein-coupled receptor that is associated with inflammatory diseases and cancers. PAR2 is activated by serine proteases that cleave its N-terminus and by synthetic peptides corresponding to the new N-terminus. Peptide agonists are widely used to characterize physiological roles for PAR2 but typically have low potency (e.g., SLIGKV-NH(2), SLIGRL-NH(2)), uncertain target selectivity, and poor bioavailability, limiting their usefulness for specifically interrogating PAR2 in vivo. Structure-activity relationships were used to derive new PAR2 agonists and antagonists containing nonpeptidic moieties. Agonist GB110 (19, EC(50) 0.28 μM) selectively induced PAR2-, but not PAR1-, mediated intracellular Ca(2+) release in HT29 human colorectal carcinoma cells. Antagonist GB83 (36, IC(50) 2 μM) is the first compound at micromolar concentrations to reversibly inhibit PAR2 activation by both proteases and other PAR2 agonists (e.g., trypsin, 2f-furoyl-LIGRLO-NH(2), 19). The new compounds are selective for PAR2 over PAR1, serum stable, and suitable for modulating PAR2 in disease models.

An antagonist of human protease activated receptor-2 attenuates PAR2 signaling, macrophage activation, mast cell degranulation, and collagen-induced arthritis in rats

Multiple serine proteases exert proinflammatory actions by signaling through protease‐activated receptor‐2 (PAR2) on the cell surface. Although inhibitors of individual proteases are anti‐inflammatory, we sought to discover whether the first potent antagonist of their common target PAR2 might be beneficial in treating chronic arthritis‐like inflammatory disease. Using a fluorescence assay, a novel compound, GB88, was shown to antagonize PAR2‐induced intracellular Ca2+ release in human monocyte‐derived macrophages, being 1000 times more potent than a control compound, ENMD‐1068 (IC50 1.6±0.5 μM vs. 1.2±0.4 mM, respectively). In Wistar rats, GB88 was orally bioavailable (F=55%, Tmax 4 h, Cmax 1.7 μM, 10 mg/kg). GB88 inhibited the acute paw edema induced in Wistar rats by intraplantar λ‐carrageenan or PAR2 agonists 2‐furoyl‐LIGRLO‐NH2 or mast cell β‐tryptase, without inhibiting proteolytic activity of tryptase in vitro. In the chronic collagen‐induced model of arthritis in rats, GB88 (10 mg/kg) was disease modifying and ameliorated pathological and histopathological changes (edema, pannus formation, synovial hyperplasia, collagen degradation, macrophage invasion, mast cell degranulation) compared to untreated arthritic controls. The results suggest that an orally active PAR2 antagonist is effective in treating chronic arthritis in rats through inhibiting macrophage infiltration, mast cell degranulation, and β‐tryptase‐PAR2 signaling in joint inflammation.—Lohman, R.‐J., Cotterell, A. J., Barry, G. D., Liu, L., Suen, J. Y., Vesey, D. A., Fairlie, D. P. An antagonist of human protease activated receptor‐2 attenuates PAR2 signaling, macrophage activation, mast cell degranulation, and collagen‐induced arthritis in rats. FASEB J. 26, 2877–2887 (2012). www.fasebj.org

C5aR and C3aR antagonists each inhibit diet-induced obesity, metabolic dysfunction, and adipocyte and macrophage signaling

Mammalian survival depends on metabolizing nutrients, storing energy, and combating infection. Complement activation in blood triggers energy‐depleting immune responses to fight infections. Here we identify surprising energy‐conserving roles for complement proteins C5a and C3a and their receptors, C5aR and C3aR, roles that are contraindicated in complement biology. Rats fed a high‐carbohydrate high‐fat diet developed obesity, visceral adiposity, adipose inflammation, glucose/insulin intolerance, and cardiovascular dysfunction that correlated with increased plasma C3a, adipose C5aR, and C3aR. These in vivo changes were dramatically attenuated by receptor‐selective antagonists of either C5aR (5 mg/kg/d p.o.) or C3aR (30 mg/kg/d p.o.), which both reduced proinflammatory adipokines and altered expression of inflammatory genes in adipose tissue. In vitro C5a and C3a (100 nM) exhibited novel insulin‐like effects on 3T3‐L1 adipocytes, promoting energy conservation by increasing glucose and fatty acid uptake while inhibiting cAMP signaling and lipolysis, and induced PGE2 release from macrophages, effects all blocked by each respective antagonist (10 μM). These studies reveal important new links between complement signaling and metabolism, highlight new complement functions on adipocytes and in adipose tissue, demonstrate how aberrant immune responses may exacerbate obesity and metabolic dysfunction, and show that targeting C3aR or C5aR with antagonists is a new strategy for treating metabolic dysfunction.—Lim, J., Iyer, A., Suen, J. Y., Seow, V., Reid, R. C., Brown, L., Fairlie, D. P. C5aR and C3aR antagonists each inhibit diet‐induced obesity, metabolic dysfunction, and adipocyte and macrophage signaling. FASEB J. 27, 822–831 (2013). www.fasebj.org

Antagonism of protease-activated receptor 2 protects against experimental colitis.

Many trypsin-like serine proteases such as β-tryptase are involved in the pathogenesis of colitis and inflammatory bowel diseases. Inhibitors of individual proteases show limited efficacy in treating such conditions, but also probably disrupt digestive and defensive functions of proteases. Here, we investigate whether masking their common target, protease-activated receptor 2 (PAR2), is an effective therapeutic strategy for treating acute and chronic experimental colitis in rats. A novel PAR2 antagonist (5-isoxazoyl-Cha-Ile-spiro[indene-1,4′-piperidine]; GB88) was evaluated for the blockade of intracellular calcium release in colonocytes and anti-inflammatory activity in acute (PAR2 agonist-induced) versus chronic [2,4,6-trinitrobenzenesulfonic acid (TNBS)-induced] models of colitis in Wistar rats. Disease progression (disease activity index, weight loss, and mortality) and postmortem colonic histopathology (inflammation, bowel wall thickness, and myeloperoxidase) were measured. PAR2 and tryptase colocalization were investigated by using immunohistochemistry. GB88 was a more potent antagonist of PAR2 activation in colonocytes than another reported compound, N1-3-methylbutyryl-N4-6-aminohexanoyl-piperazine (ENMD-1068) (IC50 8 μM versus 5 mM). Acute colonic inflammation induced in rats by the PAR2 agonist SLIGRL-NH2 was inhibited by oral administration of GB88 (10 mg/kg) with markedly reduced edema, mucin depletion, PAR2 receptor internalization, and mastocytosis. Chronic TNBS-induced colitis in rats was ameliorated by GB88 (10 mg/kg/day p.o.), which reduced mortality and pathology (including colon obstruction, ulceration, wall thickness, and myeloperoxidase release) more effectively than the clinically used drug sulfasalazine (100 mg/kg/day p.o.). These disease-modifying properties for the PAR2 antagonist in both acute and chronic experimental colitis strongly support a pathogenic role for PAR2 and PAR2-activating proteases and therapeutic potential for PAR2 antagonism in inflammatory diseases of the colon.

Diet-induced obesity, adipose inflammation, and metabolic dysfunction correlating with PAR2 expression are attenuated by PAR2 antagonism

Excessive uptake of fatty acids and glucose by adipose tissue triggers adipocyte dysfunction and infiltration of immune cells. Altered metabolic homeostasis in adipose tissue promotes insulin resistance, type 2 diabetes, hypertension, and cardiovascular disease. Inflammatory and metabolic processes are mediated by certain proteolytic enzymes that share a common cellular target, protease‐activated receptor 2 (PAR2). This study showed that human and rat obesity correlated in vivo with increased expression of PAR2 in adipose tissue, primarily in stromal vascular cells (SVCs) including macrophages. PAR2 was expressed more than other PARs on human macrophages and was increased by dietary fatty acids (palmitic, stearic, and myristic). A novel PAR2 antagonist, GB88 (5‐isoxazoyl‐Cha‐Ile‐spiroindene‐1,4‐piperidine), given orally at 10 mg/kg/d (wk 8–16) reduced body weight by ~10% in obese rats fed a high‐carbohydrate high‐fat (HCHF) diet for 16 wk, and strongly attenuated adiposity, adipose tissue inflammation, infiltrated macrophages and mast cells, insulin resistance, and cardiac fibrosis and remodeling; while reversing liver and pancreatic dysfunction and normalizing secretion of PAR2‐directed glucose‐stimulated insulin secretion in MIN6 β cells. In summary, PAR2 is a new biomarker for obesity, and its expression is stimulated by dietary fatty acids; PAR2 is a substantial contributor to inflammatory and metabolic dysfunction; and a PAR2 antagonist inhibits diet‐induced obesity and inflammatory, metabolic, and cardiovascular dysfunction.—Lim, J., Iyer A., Liu, L., Suen J. Y., Lohman R.‐J., Seow V., Yau M.‐K., Brown, L., Fairlie, D. P., Diet‐induced obesity, adipose inflammation, and metabolic dysfunction correlating with PAR2 expression are attenuated by PAR2 antagonism. FASEB J. 27, 4757–4767 (2013). www.fasebj.org

Profiling Gene Expression Induced by Protease-Activated Receptor 2 (PAR2) Activation in Human Kidney Cells

Protease-Activated Receptor-2 (PAR2) has been implicated through genetic knockout mice with cytokine regulation and arthritis development. Many studies have associated PAR2 with inflammatory conditions (arthritis, airways inflammation, IBD) and key events in tumor progression (angiogenesis, metastasis), but they have relied heavily on the use of single agonists to identify physiological roles for PAR2. However such probes are now known not to be highly selective for PAR2, and thus precisely what PAR2 does and what mechanisms of downstream regulation are truly affected remain obscure. Effects of PAR2 activation on gene expression in Human Embryonic Kidney cells (HEK293), a commonly studied cell line in PAR2 research, were investigated here by comparing 19,000 human genes for intersecting up- or down-regulation by both trypsin (an endogenous protease that activates PAR2) and a PAR2 activating hexapeptide (2f-LIGRLO-NH2). Among 2,500 human genes regulated similarly by both agonists, there were clear associations between PAR2 activation and cellular metabolism (1,000 genes), the cell cycle, the MAPK pathway, HDAC and sirtuin enzymes, inflammatory cytokines, and anti-complement function. PAR-2 activation up-regulated four genes more than 5 fold (DUSP6, WWOX, AREG, SERPINB2) and down-regulated another six genes more than 3 fold (TXNIP, RARG, ITGB4, CTSD, MSC and TM4SF15). Both PAR2 and PAR1 activation resulted in up-regulated expression of several genes (CD44, FOSL1, TNFRSF12A, RAB3A, COPEB, CORO1C, THBS1, SDC4) known to be important in cancer. This is the first widespread profiling of specific activation of PAR2 and provides a valuable platform for better understanding key mechanistic roles of PAR2 in human physiology. Results clearly support the development of both antagonists and agonists of human PAR2 as potential disease modifying therapeutic agents.