Non-alcoholic fatty liver disease pathophysiology
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Vamsikrishna Gunnam M.B.B.S [2]
Overview
The exact pathogenesis of NAFLD is not fully understood, but is believed due to interaction of multiple factors such as obesity, Insulin resistance, and metabolic syndrome. Pathogenesis of non-alcoholic liver disease can be best explained by 2 hit hypothesis. The first hit is steatosis. The second hit is controversial and is likely cause changes that leads from hepatic steatosis to hepatic inflammation and fibrosis by way of lipid peroxidation.
Pathophysiology
The exact pathogenesis of NAFLD is not fully understood, but is believed due to interaction of multiple factors.
2 hit hypothesis
Pathogenesis of non-alcoholic liver disease can be summarized by 2 hit hypothesis. According to 2 hit hypothesis:
- The first hit results in increased fat accumulation especially triglycerides within the hepatocyte and increases the risk of liver injury.
- On the second hit inflammatory cytokines causes mitochondrial dysfunction and oxidative stress which in turn lead to steatohepatitis and/or fibrosis.[1]
Free fatty acids
- Free fatty acids (FFA) play very crucial role in damaging the liver indirectly by either undergoing β-oxidation or are esterified with glycerol to form triglycerides, leading to hepatic fat accumulation.
- By upregulating TNF-alpha expression via lysosomal pathway, free fatty acids make the liver susceptible to oxidative stress.[2]
- Oxidative stress inhibits the replication process in the mature hepatocytes.
- Inhibition of hepatocyte replication results in the proliferation of progenitor cell population which can also differentiates into hepatocyte-like cells.
- Progenitor cells along with hepatocyte-like cells are responsible for fibrosis and carcinogenesis in non alcoholic fatty liver.[1]
Endotoxins
- Obese patients who underwent jejuno-ileal bypass surgery has the risk of developing bacterial endotoxins in the portal circulation due to small intestinal deformity.[3]
- Increase in small bowel bacterial overgrowth due to decreased gastric motility.
- Bacterial toxins released by this bacteria overgrowth stimulate an elevation of intra-hepatic levels of pro-inflammatory cytokines, such as tumor necrosis factor-alpha.
- Expression of TNF-alpha begins the cascade of events making liver susceptible for free radical injury.
Adiponectin
- Adiponectin is an anti-atherogenic, insulin sensitizing cytokine whose secretion is decreased in obesity.
- There is an inverse relationship between circulating concentrations of adiponectin and tumor necrosis factor.[4]
- Any conditions that cause low production of adiponectin ( consuming high amounts of poly unsaturated fatty acids) results in production of TNF alpha.[5]
Adenosine
- Alteration of purinergic metabolism is another important pathway responsible for development of non-alcoholic liver disease.
- Adenosine receptor A2A is a major factor in the pathogenesis of cirrhosis.[6]
- CD39 is the dominant vascular ectonucleotidase in the liver that hydrolyzes extracellular ATP and ADP to AMP which can then be converted to adenosine via ecto-5’-nucleotidase/CD73.
- Alterations in purinergic signaling induced by altered CD39 mutation have major impacts upon hepatic metabolism, repair mechanisms, regeneration and associated immune responses.
- Adenosine forms a supportive link in the cell’s cascade healing response to inflammation.
- Adenosine suppresses inflammation by enhancing fibrosis.
- CD39 deletion shifts the local population of cytokines to produce TNF alpha.[7][8][6][9]
Fibroblast Growth Factor 21
- Fibroblast growth factor 21 (FGF21) is an important metabolic regulator of glucose and lipid metabolism.
- FGF21 moderates or induces the hepatic response to a fasting state by gluconeogenesis, fatty acid oxidation, and ketogenesis.
- Moreover, it is a crucial component of the hepatic lipid oxidation machinery, as proliferator-activated receptor activation.
- FGF21 is responsible for normal blood glucose, insulin, and lipid levels in normal individuals.
- Low levels of FGF21 are closely associated with the obesity, insulin resistance, type two diabetes mellitus and hyperlipidemia.
Associated Conditions
- Most patients have associated features of the metabolic syndrome including obesity, diabetes mellitus type 2, hyperlipidemia (hypertriglyceridemia), and hypertension
- Patients may suffer from complications of obesity such as obstructive sleep apnea , orthopedic complications, and polycystic ovary syndrome.
Microscopic Pathology
On microscopic histopathological analysis characteristic findings of non-alcoholic liver disease include:
- Macrovesicular steatosis
- Predominant lobular inflammation in form of spotty necrosis in cases where steatosis is associated with inflammation.
- Ballooning degeneration (hallmark of steatohepatitis)
- Characterized by cellular swelling, rarefaction of the hepatocytic cytoplasm and clumped strands of intermediate filaments.
- Mallory-Denk bodies (MDB)
- Fibrosis
- Perivenular and pericellular (peri-sinusoidal) fibrosis.
References
- ↑ 1.0 1.1 Dowman JK, Tomlinson JW, Newsome PN (2010). "Pathogenesis of non-alcoholic fatty liver disease". QJM. 103 (2): 71–83. doi:10.1093/qjmed/hcp158. PMC 2810391. PMID 19914930.
- ↑ Feldstein AE, Werneburg NW, Canbay A, Guicciardi ME, Bronk SF, Rydzewski R, Burgart LJ, Gores GJ (2004). "Free fatty acids promote hepatic lipotoxicity by stimulating TNF-alpha expression via a lysosomal pathway". Hepatology. 40 (1): 185–94. doi:10.1002/hep.20283. PMID 15239102.
- ↑ Hocking et al. Jejunoileal bypass for morbid obesity. Late follow-up in 100 cases. NEJM 1983;308(17):995-999
- ↑ "Adiponectin Resistance Exacerbates Insulin Resistance in Insulin Receptor Transgenic/Knockout Mice | Diabetes".
- ↑ "The Pathogenesis of Nonalcoholic Fatty Liver Disease: Interplay between Diet, Gut Microbiota, and Genetic Background".
- ↑ 6.0 6.1 Chan ES, Montesinos MC, Fernandez P, Desai A, Delano DL, Yee H, Reiss AB, et al. adenosine A(2A) receptors play a role in the pathogenesis of hepatic cirrhosis. Br J Pharmacol 2006;148:1144-1155.
- ↑ Kunzli BM et al. Upregulation of CD39/NTPDases and P2 receptors in human pancreatic disease. AJP-Gastrointest Liver Physiol 2007;292:223-230
- ↑ Montesinos MC et al. Wound healing is accelerated by agonists of adenosine A2 (G alpha s-linked) receptors. J. Exp. Med.1997;186:1615–162010-11)
- ↑ Kunzli BM et al. Disordered Pancreatic Inflammatory Responses and Inhibition of Fibrosis in CD39-null mice. Gastroenterology. 2008 January ; 134(1): 292–305.