Inflammation Mediators

REPORT: Dr M Morgan MB BS BSc July 2025

Pro-inflammatory Mediators and Pathways in Human Metabolism

Inflammation underpins host defence yet fuels many chronic diseases. Below is an integrated review of every major **pro-inflammatory mediator**—small molecules, lipids, peptides, proteins and reactive species—and the canonical **signalling pathways** that orchestrate their production and action in human metabolism.

Contents

1. Introduction
2. Cytokines and Chemokines
3. Lipid-Derived Eicosanoids
4. Vaso-active Peptides and Amines
5. Complement-Derived Anaphylatoxins
6. Reactive Species and Nitrogen/Oxygen Metabolites
7. Acute-Phase Proteins
8. Intracellular Signalling Pathways
9. Crosstalk and Resolution Failure
10. Therapeutic Implications
11. Conclusion

1. Introduction

Pro-inflammatory mediators are molecules whose **net effect is to initiate, amplify or sustain inflammatory responses**. They derive from diverse metabolic routes yet converge on overlapping signalling hubs, notably NF-κB, MAPKs and NLRP3. Chronic over-activation disturbs metabolic homoeostasis and promotes arthritis, atherosclerosis, neurodegeneration and cancer[1][2].

2. Cytokines and Chemokines

2.1 Interleukin-1 Family

• **IL-1β** is processed by caspase-1 following NLRP3 inflammasome activation; it induces fever, COX-2, and neutrophil recruitment[3][4].
• **IL-1α** acts as an alarmin released from necrotic cells and initiates sterile inflammation[5].

2.2 Tumour Necrosis Factor-α (TNF-α)

A trimeric cytokine cleaved by TACE. TNF-α binds TNFR1/TNFR2 to activate NF-κB, MAPKs and cell-death cascades; excess production drives rheumatoid arthritis and IBD[1][6].

2.3 Interleukin-6 (IL-6)

Signals via gp130-JAK–STAT3 (classic) or soluble receptor (trans-signalling). IL-6 bridges innate and adaptive immunity, induces CRP synthesis and skews metabolic fuel usage[7][8].

2.4 Interferon-γ (IFN-γ)

Primes macrophages, inhibits IL-10/STAT3 feedback, and synergises with TLR4 to heighten inflammasome activity[9][10].

2.5 Chemokines (CXCL8/IL-8, CCL2/MCP-1, CCL3/4/5, CXCL17)

These small proteins direct leukocyte trafficking. CXCL8 attracts neutrophils and promotes angiogenesis[11][12]; CCL2 recruits monocytes and is elevated in obesity-linked inflammation[13][14]; CXCL17 drives early neutrophil influx[15].

3. Lipid-Derived Eicosanoids

Released arachidonic acid is metabolised by COX, LOX or CYP enzymes.

• **Prostaglandin E2 (PGE₂)**: via EP1–EP4 receptors it induces vasodilatation, pain, fever and immunomodulation; tumour cells exploit PGE₂ to evade immunity[16][17].
• **Prostaglandin I2 and D2** possess context-dependent roles but can augment vasopermeability.
• **Thromboxane A₂**: platelet aggregator and vasoconstrictor.
• **Leukotriene B4 (LTB₄)**: potent neutrophil chemoattractant; drives postoperative pain and insulin resistance in obesity[18][19].
• **Cysteinyl-leukotrienes (LTC₄, LTD₄, LTE₄)**: bronchoconstrictors central to asthma.

4. Vaso-active Peptides and Amines

4.1 Histamine

Stored in mast-cell granules. Binding to H₁R on endothelium causes rapid vasodilatation, pruritus and leukocyte chemotaxis; H₂R modulates cytokine profiles[20][21].

4.2 Bradykinin

Generated via kallikrein–kinin cascade; increases vascular permeability and pain through B₂ (constitutive) and B₁ (inducible) receptors, with excessive levels noted in COVID-19 and lupus[22][23].

5. Complement-Derived Anaphylatoxins

**C3a** and **C5a** are cleavage products that bind C3aR/C5aR1 to trigger chemotaxis, ROS burst and cytokine release. C5a up-regulates PD-L1, dampening T-cell responses and fuelling sepsis pathology[24][25].

6. Reactive Species and Nitrogen/Oxygen Metabolites

6.1 Nitric Oxide (NO)

iNOS-derived NO promotes vasodilatation but, in excess, forms peroxynitrite causing DNA damage[26][27].

6.2 Reactive Oxygen Species (ROS)

NADPH-oxidase and mitochondria generate superoxide and hydrogen peroxide that activate NF-κB, oxidise lipids and open endothelial junctions[28][29].

7. Acute-Phase Proteins

Hepatic CRP, serum amyloid A and ferritin rise within hours of IL-6 stimulation; they opsonise microbes and modulate complement, serving as biomarkers of systemic inflammation[30][31].

8. Intracellular Signalling Pathways

8.1 NF-κB

Canonical activation by TLRs, TNFR or IL-1R recruits MyD88/TRAF6 → IKK complex → IκB degradation → nuclear translocation of p65/p50 to transcribe cytokine, chemokine and iNOS genes[32][33].

8.2 MAPKs (p38, JNK, ERK)

p38 MAPK phosphorylates transcription factors stabilising TNF-α and COX-2 mRNA; blockade attenuates arthritis and colitis[34][35].

8.3 NLRP3 Inflammasome

Priming (NF-κB-dependent) plus activation signals (K⁺ efflux, ROS, mtDNA) promote NLRP3–ASC–caspase-1 assembly, processing pro-IL-1β/IL-18 and inducing pyroptosis[36][37].

8.4 Complement and Kinin Crosstalk

C5a enhances bradykinin release through mast-cell proteases, amplifying vascular leakage[24][38].

9. Crosstalk and Resolution Failure

Sustained NF-κB activity suppresses apoptosis of inflammatory cells, whereas PGE₂ skews towards Th2/Th17 and recruits M2-like macrophages[17][39]. Dysregulated ROS inhibit mitophagy, perpetuating NLRP3 activation[40][28].

10. Therapeutic Implications

• **NSAIDs / COX-2 inhibitors** dampen PGE₂ but risk GI and cardiovascular toxicity; selective EP4 antagonists show promise in cancer immunotherapy[17].
• **Anti-cytokine biologics** (anti-TNF, IL-1 blockers) revolutionise RA and autoinflammatory diseases[1][41].
• **NLRP3 inhibitors** (e.g., MCC950) mitigate IL-1β-driven pathologies but require safety validation[37].
• **Complement inhibitors** (eculizumab) neutralise C5a-mediated damage in paroxysmal nocturnal haemoglobinuria and are under trial for sepsis[25].
• **p38 MAPK blockers** showed preclinical success yet dose-limiting hepatotoxicity in trials; targeting downstream MK2 is an alternative[35][42].

11. Conclusion

Human metabolism yields a rich repertoire of pro-inflammatory mediators—cytokines, eicosanoids, vaso-active peptides, complement fragments and reactive species—whose production is tightly choreographed by NF-κB, MAPK and inflammasome pathways. **Balanced activation is essential for host defence; dysregulation seeds chronic metabolic, autoimmune and malignant diseases.** Understanding these networks not only clarifies disease mechanisms but also unveils nodes for precision anti-inflammatory therapy.

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