Cirrhosis pathophysiology

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor(s)-in-Chief:

Overview

Cirrhosis occurs due to long term liver injury which causes an imbalance between matrix production and degradation. Early disruption of the normal hepatic matrix results in its replacement by scar tissue, which in turn has deleterious effects on cell function.

Pathophysiology

The pathogenesis of cirrhosis is as follows:

• When an injured tissue is replaced by a collagenous scar, it is termed as fibrosis. The development of fibrosis requires several months, or even years, of ongoing injury.
• The pathological hallmark of cirrhosis is the development of scar tissue that leads to replacement of normal liver parenchyma, leading to blockade of portal blood flow and disturbance of normal liver function.
• When fibrosis of the liver reaches an advanced stage where distortion of the hepatic vasculature also occurs, it is termed as cirrhosis of the liver. If the damage progresses, panlobular cirrhosis may result.
• The cellular mechanisms responsible for cirrhosis are similar regardless of the type of initial insult and site of injury within the liver lobule.
• Viral hepatitis involves the periportal region, whereas involvement in alcoholic liver disease is largely pericentral.
• Cirrhosis involves the following steps:
  • Inflammation
  • Hepatic stellate cell activation
  • Angiogenesis
  • Fibrogenesis
• Kupffer cells are hepatic macrophages responsible for hepatic stellate cell activation during injury.
• The stellate cell, (also known as the perisinusoidal cell or Ito cell) is a cell type that normally stores vitamin A and plays a pivotal role in the development of cirrhosis.
• Hepatic stellate cells (HSC) are usually located in the subendothelial space of Disse and become activated to a myofibroblast-like phenotype in areas of liver injury. This contractile cell (known as a myofibroblast) obstructs blood flow in the circulation.
• The stellate cell secretes TGF-β₁, which leads to a fibrotic response and proliferation of connective tissue.
• Connective tissue proliferation leads to the formation of extracellular matrix around hepatocytes and is composed of collagens (especially type I, III, IV), glycoprotein and proteoglycans.
• Collagen and non collagenous matrix proteins responsible for fibrosis are produced by the activated Hepatic Stellate Cells (HSC).
• Hepatocyte damage causes the release of lipid peroxidases from injured cell membranes leading to necrosis of parenchymal cells.
• Activated HSC produce numerous cytokines and their receptors, such as PDGF and TGF-f31 which are responsible for fibrogenesis.
• The matrix formed due to HSC activation is deposited in the space of Disse and leads to loss of fenestrations of endothelial cells, which is a process called capillarization.
• Stellate cell activation leads to disturbance of the balance between matrix metalloproteinases and the naturally occurring inhibitors (TIMP 1 and 2). This is followed by matrix breakdown and replacement by connective tissue-secreted matrix.
• Matrix metalloproteinase (MMP) are calcium dependent enzymes that specifically degrade collagen and non collagenous substrate.
• MMP-2 and stromyelysin-1 are produced by stellate cells.
• MMP-2 degrades collagen and stromelysin-1 degrades proteoglycan and glycoprotein.
• Cirrhosis leads to hepatic microvascular changes characterised by:
  • Formation of intra hepatic shunts (due to angiogenesis and loss of parenchymal cells) 
  • Hepatic endothelial dysfunction
• Sinusoidal endothelial cells are also important contributors of early fibrosis. Endothelial cells from a normal liver produces collagen, laminin and fibronectin.
• The endothelial dysfunction is characterised by
  • Insufficient release of vasodilators, such as nitric oxide due to oxidative stress
  • Increased production of vasoconstrictors (mainly adrenergic stimulation and activation of endothelins and RAAS)
• The liver responds to injury with new blood vessel formation. Mediators involved in angiogenesis include:
  • Platelet derived growth factor (PDGF)
  • Vascular endothelial growth factor (VEGF)
  • Nitric oxide
  • Carbon monoxide
• Angiogenesis in cirrhosis results in the production of immature and permeable VEGF induced neo-vessels that further exacerbate liver injury.
• Fibrosis eventually leads to formation of septae that grossly distort the liver architecture which includes both the liver parenchyma and the vasculature.
• A cirrhotic liver compromises hepatic sinusoidal exchange by shunting arterial and portal blood directly into the central veins (hepatic outflow).
• Vascularized fibrous septa connect central veins with portal tracts leading to islands of hepatocytes surrounded by fibrous bands without central veins.
• These mechanisms simultaneously occurring in the liver lead to fibrous tissue band (septa) and regenerative hepatocyte nodule formation, which eventually replace the entire liver architecture, leading to decreased blood flow throughout.
• The formation of fibrotic bands is accompanied by regenerative nodule formation in the hepatic parenchyma.
• Advancement of cirrhosis may lead to parenchymal dysfunction and development of portal hypertension.
• The pathological hallmark of cirrhosis is the development of scar tissue that replaces normal parenchyma, leading to blockade of portal blood flow and disturbance of normal liver function.
• Due to portal hypertension, the spleen becomes congested, which leads to hypersplenism and increased platelet sequestration.
• Pathogenesis of cirrhosis based upon the underlying cause is as follows:
  • Alcoholic liver disease: Alcohol seems to injure the liver by blocking the normal metabolism of protein, fats, and carbohydrates. Patients may also have concurrent alcoholic hepatitis with fever, hepatomegaly, jaundice, and anorexia. Liver damage due to alcoholic hepatitis may progress to cirrhosis.
  • Chronic hepatitis C: Infection with the hepatitis C virus causes inflammation of and low grade damage to the liver that over several decades can lead to cirrhosis.
  • Non-alcoholic steatohepatitis (NASH): In NASH, fat builds up in the liver and eventually causes scar tissue. This type of hepatitis appears to be associated with diabetes, protein malnutrition, obesity, coronary artery disease, and treatment with corticosteroid medications.
  • Primary sclerosing cholangitis: PSC is a progressive cholestatic disorder presenting with pruritus, steatorrhea, fat soluble vitamin deficiencies, and metabolic bone disease.
    • There is a strong association with inflammatory bowel disease (IBD), especially ulcerative colitis.
  • Autoimmune hepatitis: Immunologic damage to the liver leads to inflammation, scarring and cirrhosis.

• Portal hypertension may result from a combination of the following:
  • Structural disturbances associated with advanced liver disease account for 70% of total hepatic vascular resistance.
  •  Functional abnormalities such as endothelial dysfunction and increased hepatic vascular tone account for 30% of total hepatic vascular resistance.

Pathogenesis of Cirrhosis due to Alcohol:

Pathophysiology of cirrhosis due to alcohol

• Mechanisms of alcohol-induced liver damage include:
  • Impairment of:
    • Protein synthesis
    • Secretion
    • Glycosylation
• Ethanol intake leads to elevated accumulation of intracellular triglycerides by:
  • Lipoprotein secretion
  • Decreased fatty acid oxidation
  • Increased fatty acid uptake
• Alcohol is converted by Alcohol dehydrogenase to acetaldehyde.
• Due to the high reactivity of acetaldehyde, it forms acetaldehyde-protein adducts which cause damage to cells by:
  • Trafficking of hepatic proteins
  • Interrupting microtubule formation
  • Interfering with enzyme activities
• Reactive oxygen species begin to form as a result of hepatocyte damage that activate Kupffer cells.
• Kupffer cell activation leads to the production of profibrogenic cytokines which in turn, stimulates stellate cells.
• Stellate cell activation leads to connective tissue formation due to deposition extracellular matrix and collagen.
• Portal triads develop connections with central veins due to connective tissue formation in pericentral and periportal zones, leading to the formation of regenerative nodules.
• Shrinkage of the liver occurs over years due to repeated insults that lead to:
  • Loss of hepatocytes
  • Increased production and deposition of collagen and regenerative nodule formation in a background of fibrosis

Pathophysiology of Portal Hypertension

Increased resistance

• Portal hypertension is related to elevation of portal vasculature resistance.
• Increased resistance in portal system may be due to both intra-hepatic and also portosystemic collateral resistance.
  • Intra-hepatic resistance
    • The main factor responsible for intra-hepatic resistance is hepatic vascular compliance, which is greatly decreased in various liver diseases, such as liver fibrosis or cirrhosis.
    • Portal hypertension occurs when compliance is decreased and blood flow is increased in liver.
    • Pre-hepatic and post-hepatic portal hypertension arise due to some secondary obstruction before or after liver vasculature, respectively.
    • Schistosomiasis causes both pre-sinusoidal and sinusoidal pathologies. The granulomas compress the pre-sinusoidal veins. In late stages, sinusoidal resistance may also be increased.
    • Alcoholic hepatitis causes both sinusoidal and post-sinusoidal pathologies.
    • Hepatic vascular endothelium synthesizes and secretes both vasodilators (e.g., nitric oxide, prostacyclins) and vasoconstrictors (e.g., endothelin and prostanoids).
    • Increased resistance due to the elevation of vascular tone may be caused by excess of vasoconstrictors or lack of vasodilators.
    • It is postulated that in cirrhotic liver the nitric oxide level is lower and the response to endothelin response in myofibrils is higher than normal liver.
  • Portosystemic collateral resistance
    • Collateral blood circulation develops as a consequence of portal hypertension which is the main contributor to esophageal varices.
    • The main purpose of the collaterals is to decompress and bypass portal blood flow.
    • However, portosystemic collaterals may not lead to a complete decompression.
    • Portosystemic collateraling occurs between the short gastric, coronary veins, and the esophageal azygos and the intercostal veins; the superior, the middle, and the inferior hemorrhoidal veins; the paraumbilical venous plexus, the venous system of abdominal organs juxtaposed with the retroperitoneum and abdominal wall; the left renal vein, the splanchnic, the adrenal, and the spermatic veins.

Hyperdynamic circulation in portal hypertension

• Peripheral vasodilatation is the basis for decreased systemic vascular resistance and mean arterial pressure, plasma volume expansion, elevated splanchnic blood flow, and elevated cardiac index.
• Systemic vasodilation
  • Three main mechanisms which contribute to the peripheral vasodilation are as following:
    • Increased vasodilators production in systemic circulation
    • Increased vasodilators production in local endothelium
    • Decreased vascular response to local vasoconstrictors
• Plasma volume
  • There are several events which contribute to the hyperdynamic circulation such as:
    • Initial vasodilatation, induced by systemic and local endothelial factors
    • Subsequent plasma volume expansion

Genetics

• Certain TERT (Telomerase reverese transcriptase) gene variants resulting in reduced telomerase activity have been found to be a risk factor for sporadic cirrhosis
• An uncharacterized nucleolar protein, NOL11, has a role in the pathogenesis of North American Indian childhood cirrhosis
• Loss of interaction between the C-terminus of Utp4/cirhin and other SSU processome proteins may cause North American Indian childhood cirrhosis
• Genes involved in the pathogenesis of cirrhosis and portal hypertension include the following:

Gene	OMIM number	Chromosome	Function	Gene expression in portal hypertension	Notes
Deoxyguanosine kinase (DGUOK)	601465	2p13.1	DNA replication	Point mutation	Mutation leads to: Liver failure Neurologic abnormalities Hypoglycemia Increased lactate in body fluids Homozygous missense mutation leads to: portal hypertension
Adenosine deaminase (ADA)	608958	20q13.12	Irreversible deamination of adenosine and deoxyadenosine in the purine catabolic pathway	Reduced	Some roles in modulating tissue response to IL-13 The main effects of IL-13 are: Inflammation Chemokine elaboration Fibrosis
Phospholipase A2 (PL2G10)	603603	16p13.12	Catalyzing the release of fatty acids from phospholipids	Reduced	Identifier of PL2G10 expression: Arachidonic acid (AA) Prostaglandins (PG) Leukotrienes (LT)
Cytochrome P450, family 4, subfamily F, polypeptide 3 (CYP4F3)	601270	19p13.12	Catalyzing the omega-hydroxylation of leukotriene B4 (LTB4)	Increased	-
Glutathione peroxidase 3 (GPX3)	138321	5q33.1	Reduction of glutathione which reduce: Hydrogen peroxide Organic hydroperoxide Lipid peroxides	Increased	Protects various organs against oxidative stress: Liver Kidney Breast
Leukotriene B4 (LTB4)	601531	14q12	Include: Increasing intra-cellular calcium Elevation of inositol 3-phosphate (IP3) Inhibition of adenylyl cyclase	Mutated	Increase blood flow to target tissue (esp. heart) about 4 times more.
Prostaglandin E receptor 2 (PTGER2)	176804	14q22.1	Various biological activities in diverse tissues	Reduced	-
Endothelin (EDN1)	131240	6p24.1	Vasoconstriction	Increased	The most powerful vasoconstrictor known
Endothelin receptor type A (EDNRA)	131243	4q31.22-q31.23	Vasoconstriction through binding to endothelin	Reduced	Directly related to hypertension in patients
Natriuretic peptide receptor 3 (NPR3)	108962	5p13.3	Maintenance of: Blood pressure Extracellular fluid volume	Increased	Released from heart muscle in response to increase in wall tension. ANP can modulate blood pressure by binding to NPR3
Cluster of differentiation 44 (CD44)	107269	11p13	Lymphocyte activation Lymph node homing	Reduced	Related to fibroblast growth factor (FGF) Increased expression during collateral arteriogenesis
Transforming growth factor (TGF)-β	190180	19q13.2	Tissue transformation Apoptosis regulation	Reduced	Hyper-expressed in African-American hypertensive patients
Ectonucleoside triphosphate diphosphohydrolase 4 (ENTPD4)	607577	8p21.3	Increasing phosphatase activity in intracellular membrane-bound nucleosides	Reduced	-
ATP-binding cassette, subfamily C, member 1 (ABCC1)	158343	16p13.11	Multi-drug resistance in small cell lung cancer	Reduced	-

Pathology

• There are four stages of Cirrhosis as it progresses:
  • Chronic nonsuppurative destructive cholangitis - inflammation and necrosis of portal tracts with lymphocyte infiltration leading to the destruction of the bile ducts.
  • Development of biliary stasis and fibrosis
• Periportal fibrosis progresses to bridging fibrosis
• Increased proliferation of smaller bile ductules leading to regenerative nodule formation.