L8: Molecular and Cellular Mechanism of Fibrosis

Learning Objectives

• Define tissue fibrosis

• Understand the role of cytokines and growth factors in tissue fibrosis

• Understand epithelial-to-mesenchymal transition and its role in tissue fibrosis

• Understand the cellular and molecular mechanisms of tissue fibrosis

• Understand how fibrosis impact on tissue function

Definitions

Extracellular Matrix (ECM)

• collagen, laminin, fibronectin and proteoglycan in distinct proportion in diff organs

• function

  • bind to cell receptors → scaffold for tissue formation and growth

  • intercellular signalling

  • bind to growth factor

  • cell migration and proliferation

• changes qualitatively and quantitatively

  • cellular functions of resident cells in any tissue

  • structural and functional integrity of tissue

• fibronectin

  • cell adhesion, migration, growth and differentiation

  • 1st matrix protein deposited in tubulu-interstitium

  • ✔︎ scaffoild: allow deposition of other matrix protein in tubulo-interstitium

  • soluble fibronectin

    • indyce MCP-1 secretion in tubular epithelial clls

    • chemoattractnat of myofibroblasts

  • polymerisation of soluble fibronectin → insoluble → accumulation

• collagen I

  • fibrillar collagen

  • collagen accumulation → tissue stiffening due to

    • increased TGF-ß1

    • impaired collagen degradation

    • formation of specific covalent intermolecular cross-links → collagen fibres resistant to MMP degradation

  • require firbonectin for polymerisation

• decorin

  • interact with

    • membrane receptor → affect cell behaviour

    • collagen fibres → regulate ECM assembly

  • form complex with TGF-ß1 → inhibit TGF-ß1 in ECM

Tissue fibrosis

• a wound-healing response that fails to resolve

• due to imbalance between synthesis and degradation of ECM

• characterised by accumulation of ECM components and qualitative and quantitative changes in ECM components

• pathophysiologic events

  • chemotaxis of inflammatory cells

  • increased permeabiloty and cell adhesion of microvascular cells

  • activation of resident and infiltrating cells

  • inflammation

Mediators of tissue fibrosis

Transforming growth factor TGF-ß1 (TGF-ß1)

• released by monocytes and macrophages

• pathway

  • Smad-Dependent Pathway: TGF-β1 binds to its receptor, activating Smad2/3, which promotes myofibroblast differentiation, ECM synthesis, and epithelial-to-mesenchymal transition (EMT).

  • Smad-Independent Pathways: Involves MAPK cascades (JNK/p38/ERK), PI3K/AKT, and Wnt/β-catenin, enhancing fibroblast proliferation and survival.

ERK Signaling

• member of MAPK family (mitogen-activated protein kinase)

• subfamily

  • Jun N-terminal kinase (JNK)

  • p38 MAPK

• important for macrophage to regulate cytokine production (e.g. IL-6)

Epithelial-to-mesenchymal transition

• Step 1

  • dissociation of tight junctions

  • loss of microvilli

• step 2

  • loss of apical-basal polarity

  • induced by

    • notch signalling

    • NF-κB signalling

    • TGF-ß1

    • HA fragments

• step 3

  • cytokeratin reorganisation

  • increased proliferation

  • increased motility

  • mesenchymal markers

    • vimentin

    • fibronectin

    • collagen I

    • α-smooth muscle actin

    • fibroblast specific protein-1

    • SNAIL

Cellular mechnaism of tissue fibrosis

Macrophages

• release of growth factors and TGF-ß (esp by M2) → fibrosis

• contribute to the inflammatory response → chemotaxis of inflammatory cells

• activation of resident and infiltrating cells

• remodel ECM by affecting its composition and influencing fibroblast activity

Fibroblasts and myofibroblasts

• resting or quiescent fibroblast

  • normal ECM components

• wound healing-associated activated fibroblast (NAF)

  • activation via stimuli (reversible)

    • stress

    • growth factors

    • hypoxia or ROS

    • cytokines

  • remodel ECM

    • collagens

    • differential crosslinking of ECM

    • fibronectin production

  • secretory phenotype: TGFß, IL-6, CCL5, TNF, INFγ…

  • high contractility

• cancer-associated / fibrosis-associated fibroblasts

  • epigenetic and irreversible change from NAF

  • produce lactate metabolites: self activation

  • high proliferation

  • ECM molecules

    • tenascin

    • periostin

    • SPARC

    • collagen

    • EDA-FN

  • enhanced secretory phenotype: NF-κΒ, IL-8, CXCL7…

Pericytes

• found in the vicinity of blood vessels

  • contribute to the stability and integrity of the microvascular structure

• response to injury or inflammation: migrate and differentiate into myofibroblasts