1.1.1. ALD and NAFLD
In the industrial city, increasing alcohol abuse and excessive eating are responsible for alcoholic liver disease (ALD) and non-alcoholic liver disease (NAFLD), respectively; both diseases have hepatic steatosis in common and range from a mild condition to more aggressive forms of the disease called alcoholic steatohepatitis (ASH) and non-alcoholic steatohepatitis (NASH), where steatosis, hepatocyte apoptosis, inflammation and fibrogenesis coexist [
5,
6].
Hepatic steatosis represents the first step of these pathologies and is characterized by the accumulation of lipids, especially triglycerides, in the cytoplasm of hepatocytes, yet in the absence of inflammation, cell death or hepatic fibrosis. In this regard, two forms of steatosis can be distinguished: a macrovesicular form, where there is an arrangement of fat in a single large droplet that dislodges the nucleus in a peripheral location of the hepatocyte; and a microvesicular form, where fat collects in small droplets that do not dislodge the nucleus of the cells. These two aspects often reflect different stages of steatosis, and indeed clinical cases in which the two forms are associated are not uncommon [
7,
8].
Many hypotheses have been proposed to explain the pathogenetic mechanisms of ALD and NAFLD.
In ALD, as the liver is the main organ responsible for ethanol metabolism, ethanol and its metabolites (acetaldehyde-acetate, fatty acid ethanol esters (FAEE), ethanol-protein adducts) may exert a direct cytotoxic effect as hepatotoxins [
9].
Hepatic ethanol metabolism involves oxidative and non-oxidative pathways. The main steps of the oxidative pathway are mediated by alcohol dehydrogenase (ADH) and acetaldehyde dehydrogenase (ALDH). ADH and ALDH convert ethanol to acetaldehyde and acetaldehyde to acetate, respectively [
10]. The end products of these reactions are acetaldehyde, acetate, and high levels of NADH. Acetaldehyde injures the liver by directly inducing inflammation, extracellular matrix remodeling, and fibrogenesis [
11]. In addition, it covalently binds to proteins and DNA, leading to lipid peroxidation and the subsequent production of toxic metabolites (e.g., malondialdehyde, 4-hydroxy-2,3-trans-nonenal), which are commonly analyzed under oxidative stress conditions [
12,
13]. Finally, acetaldehyde stimulates transforming growth factor (TGF)-β signaling in HSCs, which acquire a pro-fibrogenic and pro-inflammatory profile [
14].
Changes in the NADH/NAD
+ ratio can affect biochemical reactions in the mitochondria and gene expression in the nucleus. In particular, the re-oxidation of NADH requires additional oxygen in the mitochondria. To this end, hepatocytes absorb more than their normal share of oxygen from the arterial blood, but not enough to supply all regions of the liver adequately. Therefore, alcohol consumption causes significant hypoxia in perivenous hepatocytes, which are the first to show evidence of chronic alcohol-induced damage [
15].
Cytochrome P450, in particular cytochrome P450 2E1 (CYP2E1), is upregulated in chronic alcohol abuse and assists ADH in the conversion of alcohol to acetaldehyde [
16] (
Figure 1). Reactive oxygen species (ROS), such as hydrogen peroxide and superoxide ions, generated by CYP2E1 are responsible for the pro-inflammatory profile of alcohol-induced liver injury by:
(1) activation of redox-sensitive transcription factors (e.g., nuclear factor kappa B (NF-κB), nuclear factor erythroid 2-related factor 2 (Nrf2), activating transcription factor 4 (ATF4), etc.) [
17,
18,
19];
(2) recruitment of neutrophils and other immune cells [
20];
(3) increasing levels of circulating inflammatory cytokines (e.g., tumor necrosis factor-α (TNF-α) and interleukin-1β (IL-1β)) [
21];
(4) exacerbated lipid peroxidation associated with alcoholic liver injury [
22].
Finally, the last player in the inflammatory and fibrogenic phenomena is catalase, a peroxisomal enzyme involved in the regulation of non-oxidative alcohol metabolism, the product by which FAEE are responsible for alcoholic steatosis and useful as biomarkers of chronic alcohol consumption [
16] (
Figure 1). Obesity, type 2 diabetes, hyperlipidemia and other conditions associated with insulin resistance are commonly present in patients with metabolic syndrome (MS). A large body of evidence suggests that NAFLD is associated with MS. The latter is a clinical entity characterized by a cluster of metabolic changes, including glucose intolerance, dyslipidemia, and hypertension [
23,
24,
25]. Approximately 90% of NAFLD patients have one or more diagnostic criteria for MS, allowing NAFLD to be defined as the "hepatic representation" of MS [
26].
We can distinguish two types of NAFLD: primary (associated with MS) and secondary (associated with other metabolic or iatrogenic conditions, distinct from MS). The pathophysiology of primary NAFLD is not fully understood.
Since 1998, the two-hit pathogenetic model of liver injury proposed by Day and James has been widely accepted [
27]. The first hit leads to the initial accumulation of triglycerides within the hepatocyte, caused by increased hepatic lipid uptake. These metabolic imbalances correlate with insulin resistance, a characteristic feature of MS. Such an event is, among others, bidirectional to steatosis: on one hand, insulin resistance promotes the progression of NAFLD, but on the other hand, NAFLD triggers the development of insulin resistance [
28]. Later, the condition of steatosis would thus predispose the liver to the second event (second hit), after which inflammation, fibrosis and liver damage would develop. The factors determining the second hit included several biomolecules capable of interfering with the insulin-induced signal transduction mechanism, including inflammatory cytokines (e.g., TNF-α, IL-6, IL-1β, hormones produced by adipose tissue such as adipokines, leptins, and adiponectins), oxidative stress and lipid peroxidation markers, fatty acids, and even ceramides.
In fact, inhibition of ceramide synthesis relieves NAFLD and fibrosis. Moreover, increased hepatic steatosis is associated with ceramide-rich hepatic lipids [
29,
30].
In addition, insulin resistance results in increased hyperglycemia-induced endoplasmic reticulum (ER) stress, peripheral lipolysis, hepatic fatty acid (FA) uptake, and hepatic synthesis of de novo lipids [
31,
32,
33]. Therefore, the oxidation and disposal of fatty acids results in a defect compared to their accumulation and neo-synthesis, leading to an accumulation of fats within hepatocytes with important consequences, such as dysregulated ketogenesis [
34,
35]. In addition, fatty acids negatively affect the intracellular insulin signaling mechanism and cause hepatic insulin resistance through multiple pathways mediating activation of protein kinase C (PKC-3), c-Jun N-terminal kinase (JNK), I-kappaB (I-κB) kinase β, and NF-κB [
36,
37,
38].
Furthermore, insulin resistance has been found to be associated in increased mitochondrial fatty acid oxidation. This process, both mitochondrial and peroxisomal, can produce ROS which are hepatotoxic and contribute to the development of oxidative stress. In addition, fatty acids and their metabolites are ligands of PPARα (peroxisomal proliferator-activated receptor-α), a transcription factor that regulates the expression of several genes encoding enzymes involved in fatty acid oxidation at the mitochondrial, peroxisomal and microsomal levels. In support of this, deletion of PPARα in hepatocytes impaired fatty acid catabolism, resulting in hepatic lipid accumulation in steatosis models [
39,
40].
Regarding the role of inflammatory cytokines in the pathogenesis of NAFLD, they could cause hepatic and systemic insulin resistance, as well as promote liver injury, apoptosis, neutrophil chemotaxis, and HSC activation [
41,
42,
43]. Studies have reported several cytokines involved in the development and progression of NAFLD, such as IL-1β, IL-6, TNF-α, C-reactive protein (CRP), and NOD-like receptor protein 3 (NLRP3) inflammasome activation. Some of these inflammatory mediators can be used as biomarkers to assess the severity and prognosis of NAFLD [
44,
45] (
Figure 1).
In addition, fatty acids accumulated in hepatocytes can stimulate cytokine production via the NF-κB-dependent pathway [
46]. An additional source of pro-inflammatory cytokines is the infiltrating macrophages in the adipose tissue of obese subjects; cytokines are increased in addition to promoting the onset of a state of insulin resistance. They appear to reduce the production of certain peptides produced by visceral adipose tissue, such as leptins, resistins and adiponectins, which are involved in the pathogenesis of NAFLD [
47].
1.1.2. ASH and NASH
Alcoholic steatohepatitis (ASH) and non-alcoholic steatohepatitis (NASH) are two of the most common causes of chronic liver disease worldwide. ASH is caused by chronic alcohol consumption, while NASH is associated with poor dietary habits and obesity, and is associated with insulin resistance and type 2 diabetes [
48]. Although ASH and NASH differ in their etiology, both diseases are characterized by excessive fat deposition and lipotoxicity, leading to inflammation, hepatocellular swelling, and fibrogenesis [
49].
Between 90 and 100% of individuals who consume more than 40 g of alcohol per day will eventually develop alcoholic fatty liver. Approximately 10-35% of these individuals develop ASH, which is characterized by a severe inflammatory state of the liver, hepatocyte swelling, neutrophil infiltration, and/or hepatic fibrosis [
50]. In approximately 8-20% of patients with ASH, the risk of developing hepatocellular carcinoma (HCC) increases by approximately 2% per year [
51]. Factors involved in the pathophysiology of ASH include hepatic steatosis, oxidative stress, acetaldehyde-mediated toxicity, and cytokine- and chemokine-induced inflammation, but hepatic inflammation is the critical prerequisite for the development of liver fibrosis, cirrhosis, and HCC [
52].
Recent studies indicate that 5-20% of patients with NAFLD develop NASH during their clinical course, 10-20% develop higher grade fibrosis and <5% progress to cirrhosis [
53]. Thus, ASH and NASH share a common pathogenesis and are mediated by different mechanisms [
54]. The "two-hit" theory suggests that oxidative stress and cytokines lead to the progression of necroinflammation and ultimately to fibrosis and cirrhosis [
55]. Indeed, hepatic macrophages and Kupffer cells are able to activate HSCs through the production of IL-6 and TNF-β, while HSCs in turn produce pro-inflammatory cytokines, such as TNF-α and IL-8 [
56]. Fat-derived products can also cause inflammation and injury in the liver by activating the inflammasome and inducing the production of IL-1 and IL-18. Fat products can also cause inflammation and injury in the liver by activating the NLRP3 inflammasome, which induces the production of IL-1β and IL-18; evidence for the critical role of NLRP3 activation is the marked protection from disease in knockout mice following acute or chronic alcohol administration [
57]. In addition, oxidative stress strains antioxidant defenses, which become inadequate. This results in liver injury through direct cellular damage and NF-κB-mediated cellular signaling [
58]. Several studies also show that oxidative stress and the increase in ER stress-mediated Nrf2 are induced, which appears to be some common features of ASH and NASH (
Figure 1); accordingly, altered markers of ER stress are recorded in numerous experimental preclinical models whose diets are administered alcohol or HFD [
59].
As previously reported, the mechanisms that drive disease progression also induce steatosis. Therefore, steatosis could be considered as an "adaptive" response of the liver to stress, and continuous insult leads to ASH and NASH, in which we find not only steatosis but also inflammation and fibrosis [
61,
62].
In addition to oxidative stress and cytokines production, insulin resistance and hyperglycemia are active players in the progression of the pathology by directly inducing fibrosis or by regulating connective tissue synthesis [
63,
64].
Unfortunately, although significant progress has been made in identifying the key players that mediate the transition from steatosis to steatohepatitis, treatment options for ASH and NASH are limited [
65]. Exercise and dietary interventions remain the main recommendations for patients with NAFLD and NASH. However, it is difficult for patients to adhere to such lifestyle changes for a variety of social, psychological, physical, genetic and epigenetic reasons; therefore, pharmacotherapy is essential [
66,
67].
Figure 1.
Role of alcohol consumption and inadequate diet on liver disease.
Figure 1.
Role of alcohol consumption and inadequate diet on liver disease.
ADH, catalase and CYP2E1 contribute to oxidative metabolism of ethanol to generate acetaldehyde. Alcohol metabolism, high fat and high carbohydrate diet induce NAFLD and ALD by FAEE accumulating in lipid droplets. Accumulation of ROS resulting from ethanol or β-oxidation leads to formation of lipid peroxides and ER stress, which induce NF-kB activation and consequent inflammatory state at the base of ASH and NASH.