AH is a condition with features similar to systemic inflammatory response syndrome: fever, tender enlarged liver, leukocytosis, and increased hepatic blood flow. Initiating events include expression of gut-derived lipopolysaccharides (LPS), interaction of LPS with TLR4 receptors, activation of inflammatory signaling pathways, cytokine release, and Kupffer cells (Figure ).
Overview of the pathway of alcoholic injury resulting in the direct production of oxidative stress and the LPS-TLR pathway that culminates in production of cytokines and other inflammatory processes that result in hepatitis of the liver.
LPS, a component of the outer membrane of gram-negative bacteria interacts with immune cells and triggers inflammatory reactions with release of cytokines. Chronic alcohol exposure increases gut permeability and facilitates translocation of endotoxins from the intestinal lumen to the portal circulation (Figure ). Increased levels of endotoxins and increase in gut permeability have been shown in patients with alcoholic liver disease[4
]. Further, pretreatment with antibiotics or lactobacillus in animal models decreases the LPS-endotoxin and the severity of liver injury.[5
Link between gut derived bacterial lipopolysacaride and Toll like receptor-4 stimulation within the liver.
Toll-like receptors (TLRs), important components of the innate immune system, function as pattern recognition receptors which recognize and bind proteins and toxins released by pathogens. There are many TLRs, but in alcoholic liver disease TLR4 is most relevant[7
]. TLR4 signals activate early growth response 1 (EGR1, a transcription factor), nuclear factor-kappaB (NF-κB), and TLR4 adaptor (toll-interleukin-1-receptor-domain-containg adapter-inducing interferon-beta or TRIF)[9
]. CD14 and TLR4 deficient mice are protected from alcoholic liver injury[10
]. Interestingly, TLR4 is expressed by a number of other cells including hepatocytes, hepatic stellate cells, and sinusoidal epithelial cells which may further contribute to ALD[7
]. Thus, TLR4 up regulation, in response to endotoxins, prompts Kupffer cells to release large amounts of TNF-α and NF-κB.
TLR4 acts through a pathway common to other TLRs, myeloid differentiation factor 88 (MyD88) pathway, as well as a specific and unique pathway to TLR4, TRIF signaling (the MyD88 independent pathway)[12
]. In the MyD88 pathway, interleukin 1 receptor associated kinase (IRAK) and possibly TNF receptor associated kinase 6 (TRAF6) are recruited to the TLR4 complex by MyD88 and eventually express TNF-α and apoptosis transcription factor AP-1. The TRIF pathway (My88-independent) activates IRF-3, NF-κB and eventually IFN-β and TNF-α production[13
]. IRF-3 may also bind to the promoter region and up regulate transcription of TNF-α[13
NF-κB is a transcription factor that is translocated to the nucleus in response to stress signals and binds to the promoter region of pro-inflammatory genes[14
]. Chronic alcohol intake primes the liver through continued NF-κB activation and increased TNF-α production in response to LPS, and murine models show increased binding of NF-κB to the DNA[15
]. Monocytes from chronic alcoholic patients show increased NF-κB activation in comparison to controls[16
]. Additionally, monocytes cultured from chronic alcoholics show increased amounts of TNF in response to LPS stimulation[17
]. Studies have shown that in rats chronically fed ethanol, injected LPS results in a much higher plasma level of TNF-α compared to controls[18
Other pathways which are stimulated in this process are: (1) the signal transducer and activator transcription factor (STAT) pathway activated by interleukin-6 (IL-6) and interferons-LPS causes STAT3 to induce IL-10 production during acute alcohol exposure in monocytes[20
]; (2) LPS stimulation of MAPK family members which include JNK (c-jun-N-terminal kinase), ERK (extracellular receptor activated kinases), and p38 culminates in NF-κB production and mRNA stabilization[21
]; and (3) Changes in immune function accompany LPS-induced hepatic inflammation- CD4+ and CD8+ T lymphocyte infiltrates are found in 40% of patients with ALD[22
]. It is thought that abnormal proteins may leak into the portal system and collect in germinal centers where the antigens are presented to CD4+ T-cells resulting in antibody creation against the proteins. Hepatic stellate cells can also present such antigens to CD4+ cells, and dendrites that scavenge dead hepatocytes may present antigens to CD8+ cells, thus providing numerous possible pathways by which oxidative stress can stimulate the immune system through both humoral and cellular mechanisms.
All these pathways result in recruitment of inflammatory cells (polymorphs and mononuclear cells) and cause necroinflammation. This is followed by Kupffer cell activation, hepatocyte ballooning, and apoptosis.