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Méndez-Sánchez et al. Hepatoma Res 2020;6:5 I http://dx.doi.org/10.20517/2394-5079.2019.29 Page 7 of 13

[32]
proteins (mainly claudins) with the consequent dysfunction of the AJ, establishing a leaky gut . Rodent
studies have also demonstrated that alcohol-associated intestinal permeability is favored by a reduction
in the intestinal hypoxia-induced factor 1-alpha (HIF-1α) activity, a condition reversed by probiotic
Lactobacillus rhamnosus GG supplementation [33,34] . Moreover, ALD patients show a decreased bacterial
diversity associated with an increase of endotoxin-producing Enterobacteriaceae and Proteobacteriaceae
and a reduction in taxa that produce short-chain fatty acids such as Lachnospiraceae, Bacteroidaceae,
and Ruminococcaceae [35-37] . Interestingly, a reduced expression of lectins Reg3β and regenerating islet-
derived protein 3 gamma (Reg3γ) is another important characteristic commonly seen in ALD, associated
[38]
with bacterial overgrowth and translocation . All these factors will induce endotoxins formation such as
lipopolysaccharides (LPS), peptidoglycans, and bacterial DNA. This favors intestinal inflammation and the
activation of the TNF-α receptor I signaling in intestinal epithelial cells associated with increased intestinal
permeability of endotoxins to the liver, boosting systemic inflammation via recognition of specific toll-
like receptors (TLRs) [39,40] , as discussed below in more detail. Moreover, commensal fungi such as Candida
spp., Saccharomyces cerevisiae, and Malassezia spp. will develop tolerance from the host immune system
[32]
during chronic alcohol consumption, fomenting an increase in these fungal species . Interestingly,
studies in ALD patients have also shown higher systemic endotoxemia levels in subjects with an increased
alcohol consumption regardless of the stage of liver disease, demonstrating that alcohol consumption is an
independent factor for systemic endotoxemia [41,42] .

MECHANISMS INVOLVED IN HCC DEVELOPMENT
In the liver, Kupffer cells and bone-marrow derived macrophages will recognize small sequences of
molecules formally called pathogen-associated molecular patterns (PAMPs) from endotoxins coming
from enterohepatic circulation via Toll-like receptor-4 (TLR4). The upregulation of TLR4 will promote
binding with its ligand, myeloid differentiation primary response 88, resulting in the activation of c-Jun
N-terminal kinase, the inhibitor of nuclear factor kappa-B kinase 2, and mitogen-activated protein kinase
(MAPK) p38, with the consequent activation of the nuclear factor kappa-B (NF-κβ) pathway. This favors
the release of TNF-α, IFN-γ, prostaglandin-2, chemokine C-C motif ligand, IL-1α, IL-1β, IL-6, ROS, and
[43]
nitric oxide, perpetuating liver inflammation . NF-κβ can also induce the antiapoptotic genes (TRAF-1 and
[4]
TRAF-2) with important carcinogenic effects . Increased TNF-α production has been shown to deregulate
TJ, causing disruption of the intestinal barrier. Interestingly, high levels of TNF-α and IL-6 have been
found in duodenal biopsies of alcohol-dependent subjects, which tend to confirm data obtained in animal
[28]
models . In another study carried out in 52 subjects diagnosed with alcohol dependence according to
the DSM-IV criteria, a biochemical panel measuring LPS, TNFα, IL-6, IL-10, and high C reactive protein
[44]
sensitivity showed an important elevation of these biochemical markers . On the other hand, IL-37 has
been associated with anti-inflammatory effects via IL-18Rα and IL-1R8 expression. In liver samples of ASH
subjects, IL-37 expression was substantially reduced when compared to non-alcoholic fatty liver disease
[45]
subjects . An in vivo system in wild-type mice suggested that hepatic IL-37 expression was suppressed by
ethanol through the administration of human recombinant IL-37 followed by oral gavage of an ethanol
[45]
shot in those animals . This is important since HCC clinical specimens have shown that decreased
expression of IL-37 is negatively correlated with tumor size and positively associated with better overall
[46]
survival and disease-free survival via the induction of tumor-infiltrating CD571 natural killer cells .

In the liver, TLR4 can also be expressed in hepatic stellate cells (HSCs), endothelial cells, and
[48]
[47]
hepatocytes . In HSCs, this molecule is involved in the upregulation of hepatocytogen epiregulin ,
[49]
an epidermoid growth factor with a potent mitogenic effect on hepatocytes . In conjunction with the
antiapoptotic effect of NF-κβ, it significantly promotes the hepatocarcinogenesis process. Knock-out mice
studies with TLR-4 deficiency and intestinal sterilization with non-absorbable antibiotics have found a
reduction in steatosis, oxidative stress, and liver inflammation with a consequent decrease in HCC risk
development [50,51] , although the risk for liver injury increased, probably due to a deficiency in the innate