The liver is one of the major metabolic organs in the body, playing a critical role
in the metabolism of carbohydrates, proteins, amino acids, lipids, drugs and xenobiotics.
As a result, the liver often becomes the target of drug and xenobiotics-induced damage.
Currently, the most common cause for acute liver failure in the United States and
multiple European countries is drug-induced liver injury (DILI). This is one of the
leading causes for halting drug development and removing drugs from the consumer market.
In Asia, the incidence of liver injury induced by using herbal or traditional medicine
is also on the rise. In addition to DILI, alcohol abuse and alcoholic liver disease
(ALD), is another major health problem worldwide that claimed more than 3 million
lives in 2012. Liver cirrhosis was the 12th leading cause of death in the United States
in 2015, 49.5% of which was estimated to be attributed to ALD according to a report
published by the National Institute on Alcohol Abuse and Alcoholism (NIAAA).
1
The pathogenesis of ALD begins with alcoholic steatosis to alcoholic hepatitis (AH),
liver fibrosis, cirrhosis and finally hepatocellular carcinoma (HCC).
2-4
Unfortunately, there are no available treatments for ALD.
In this special issue, multiple review articles and research papers were gathered
to provide a cumulative understanding on the current understanding of hepatic drug
metabolism, DILI and steatosis, steatotic ischemia-reperfusion injury and ALD as well
as potential serum biomarkers and role of long non-coding RNA (lncRNA) in HCC.
The liver is a central organ that is enriched with drug metabolizing enzymes, which
play a critical role in the metabolism, elimination and detoxification of drugs and
xenobiotics, including alcohol. These metabolism enzymes include phase I enzymes (mainly
cytochrome P450 (CYP) oxidases), phase II reactions (conjugation enzymes) and phase
III excretion (transporters). As a result, drugs and xenobiotics will be added with
reactive and polar groups, which convert lipophilic drugs into hydrophilic products
to be more readily excreted. These reactions help the body to detoxify the drugs/xenobiotics,
although some metabolic intermediates have also caused toxic effects.
Among all the known human CYP enzymes, the CYP3A family enzymes are responsible for
metabolizing almost half of the currently prescribed drugs. The review paper from
Dr. Chen’s group
5
elegantly summarizes CYP3A family enzymes and their substrate recognition and modulation
by small molecules. The review article goes into in depth discussion on the structural
perspectives of CYP3A, along with a detailed comparison of CYP3A to other human CYP
enzymes. The information in this review paper may help to design specific CYP3A inhibitors
that may affect drug-drug interactions and other important biological processes. In
addition to CYP-mediated phase I metabolism, phase II enzyme also plays important
roles in biotransformation of drugs/xenobiotics. There are many phase II metabolizing
enzymes including uridine diphosphate (UDP)-glucuronosyltransferases (UGTs), sulfotransferases
(SULTs), N-acetyl-transferases (NATs) and glutathione S-transferases. Dr. Xie’s group
6
from the University of Pittsburgh provided novel insights on the role of nuclear hepatocyte
nuclear factor 4alpha (HNF4α) in the regulation of cholesterol sulfotransferase 2B1b
(SULT2B1b) and their impact on hepatic glucose homeostasis. HNF4α is a nuclear receptor
transcription factor that binds to deoxyribonucleic acid (DNA) as a homodimer, which
regulates the group of hepatic CYP genes and glucose, as well as lipid metabolism
genes. There are 44 cytosolic SULTs; and the SULT2 family enzymes primarily add sulfonation
on neutral steroids (SULT2A) and sterols (SULT2B). There are two isoforms of SULT2B1
hydroxysteroid sulfotransferase, SULT2B1b catalyzes the sulfonation of 3β-hydroxysteroid
hormones and cholesterol, whereas SULT2B1a catalyzes pregnenolone sulfonation. Obesity
and the transition from fasting to the fed state increased hepatic expression of SULT2B1b
which then inhibits HNF4α, thereby decreasing glucose production and gluconeogenic
gene expression. Interestingly, activation of HNF4α increases the expression of SULT2b1B,
suggesting a negative feedback loop between SULT2B1b and HNF4α in the regulation of
glucose homeostasis. Therefore, in addition to regulating drug metabolism, phase III
enzymes, such as SULT2B1b, also plays an important role in regulating energy homeostasis,
particularly gluconeogenesis.
Acetaminophen (APAP) overdose can cause severe hepatotoxicity and is one of the top
causes of acute liver failure in the United States.
7
It is well known that therapeutic dose of APAP is safe as majority (85–90%) of APAP
undergoes glucuronidation and sulfation and secretes to bile and plasm from the liver,
whereas a small portion (15–10%) of APAP is metabolized by CYP2E1 to generate a reactive
metabolite, N-acetyl-p-benzoquinone imine (NAPQI).
8
NAPQI further conjugates with liver glutathione (GSH) to be detoxified. However, overdose
of APAP leads to the generation of excessive NAPQI that depletes hepatocellular GSH
and further covalently binds to intracellular proteins to form APAP-adducts (APAP-AD).
Some of the APAP-AD are on the mitochondria that leads to mitochondrial damage and
subsequent necrosis and liver injury. It has been well accepted that mitochondrial
damage is the key event resulting in APAP-induced necrosis and liver injury.
9
The liver is a unique organ of which damaged hepatocytes can be replaced by proliferating
hepatocytes via liver regeneration. In the review article entitled “Mitochondrial
damage and biogenesis in acetaminophen-induced liver injury”, Jaeschke et al.
10
highlights recent progress on targeting mitochondrial and mitochondrial biogenesis
for ameliorating APAP-induced liver injury. Activation of c-Jun N-terminal kinase
(JNK) especially mitochondrial JNK translocation is associated with APAP-induced hepatocyte
necrosis. Post-treatment with 4-methylpyrazole (4 MP) did not affect oxidative metabolism
and protein adduct formation; however, it did significantly attenuate APAP-induced
mitochondrial translocation of JNK and liver injury. Mitochondrial biogenesis is regulated
at the transcription level by several transcription factors, including the nuclear
respiratory factor (NRF) 1 & 2, and mitochondrial transcription factor A (Tfam), and
transcription coactivator peroxisome proliferator-activated receptor-gamma coactivator
1 alpha (PGC1α). Increased PGC1α by SRT1720 or recombinant fibroblast growth factor
21 (FGF21) attenuates APAP-induced liver injury by increasing mitochondrial biogenesis
and liver regeneration. These data clearly indicate that targeting mitochondrial biogenesis
may be a promising approach for treating APAP-induced liver injury.
In addition to regulating cell death, mitochondria also play critical roles in regulating
lipid metabolism, as mitochondria are the major sites for fatty acid beta-oxidation.
Since many drugs can cause mitochondrial dysfunction, it is not surprising that drugs
including APAP, amiodarone, ibuprofen, linezolid, etc., all can lead to hepatic steatosis.
However, some drugs can also induce steatosis independent of mitochondrial damage.
Dr. Fromenty’s review article
11
in this issue summarizes the role and mechanisms of drug induced steatosis.
There are two types of DILI, the first is intrinsic DILI, DILI in this category is
predictable and caused by a known source, usually an overdose. (e.g., APAP). The second
type of DILI is idiosyncratic, which is considered unpredictable and can happen at
what is considered a therapeutic dose. In the review article entitled “Hepatic macrophages
in drug-induced liver injury”, Shan and Ju
12
present the most current knowledge on the role of hepatic macrophages in DILI. Liver
macrophages contain the resident macrophages (also known as Kupffer cells) and the
infiltrating monocyte-derived macrophages. Increasing evidence suggests that hepatic
macrophages play important roles in both intrinsic DILI and idiosyncratic DILI. Accumulating
evidence suggests that Kupffer cells and infiltrating macrophages may promote liver
regeneration to ameliorate DILI.
In Asian countries, the incidence of acute liver injury and liver failure due to the
consumption of herbal teas or traditional medicine is on the rise and is becoming
one of the major causes of acute live failure. Pyrrolizidine alkaloids (PAs) are the
ester derivatives of necine base and necic acid, which are commonly found in more
than 6000 plants which is found in high amounts in the Chinese medicine Tusanqi (Gynura
segetum). Notably, hundreds of people in China have been developing hepatic sinusoidal
obstruction syndrome (HSOS) after ingesting Tusanqi. PAs are predominantly metabolized
in the liver by CYP3A, generating reactive metabolites dehydropyrrolizidine alkaloids
(DHPAs), which is further hydrolyzed to dehydroretronecine (DHR) when it binds to
cellular GSH to form GSH-conjugates. Once GSH is depleted, DHPAs and DHR bind to proteins
to form pyrrole-protein adducts to initiate the hepatotoxicity likely by inducing
mitochondrial fragmentation in hepatocytes.
13
In addition to damaging the parenchymal cells, PAs can also damage the non-parenchyma
cells, such as the hepatic sinusoidal endothelial cells (HSECs). In a review article
in this issue, Xu and her colleagues
14
have highlighted the current understanding of the hepatotoxicity induced by PAs.
ALD is a major chronic liver disease worldwide. One of the early major pathological
features of ALD is hepatic steatosis, which is characterized by the accumulation of
lipid droplets (LDs) in hepatocytes. LDs are intracellular organelles that store intracellular
triglycerides (TG) and cholesteryl esters (CE), which are generally thought to be
originated from endoplasmic reticulum (ER). The hydrophobic mixture of TG and CE within
the ER lumen protrudes out of ER to form LD with the ER-derived delimiting phospholipid
monolayer. Chronic alcohol consumption induces the accumulation of hepatic LDs, which
may involve multiple mechanisms including increased uptake of circulating free fatty
acids, increased de novo lipogenesis, decreased mitochondria beta-oxidation, and very
low-density lipoprotein (VLDL) secretion as well as deceased autophagic degradation
of LDs via impaired lipophagy.
15
Decreased lipophagy is likely due to the decreased hepatic lysosome numbers as chronic
alcohol consumption may impair transcription factor EB (TFEB), a master regulator
of lysosomal biogenesis.
16
In addition to lysosome-mediated LD catabolism, alcohol may also impair adipose triglyceride
lipase (ATGL) and hormone-sensitive lipase (HSL)-mediated lipolysis in hepatocytes
resulting in the accumulation of LDs. In a review article of this issue, Dr. Schulze
and Ding
17
have summarized the dynamic changes of LD and the role of lysosome in the regulation
of LD homeostasis after alcohol. Besides the autophagy-lysosomal pathway, ubiquitin
proteasome system (UPS) is another cellular catabolism pathway in mammalian cells.
Interestingly, both the autophagy and UPS pathways may connect with each other as
decreased UPS activity can activate autophagy and vice versa, decreased autophagy
may also lead to compensatory activation of autophagy.
18,19
Dr. Donohue et al.
20
have excellently discussed how alcohol consumption can impair both autophagy and UPS
in the liver resulting in the accumulation of LDs and Mallory-Denk bodies and liver
injury in the review article entitled “Lysosome and proteasome dysfunction in alcohol-induced
liver injury”.
The composition of dietary fat is known to play important roles in the pathogenesis
of ALD. The review from Zirnheld et al.
21
highlights recent advances regarding the role of dietary fat, distinct fatty acids,
and bioactive fatty acid metabolites in ALD. Both deleterious and beneficial effects
of distinct dietary polyunsaturated fatty acids (PUFAs) in ALD have been reported
in clinical ALD animal models. Dietary enrichment in the n-6 PUFA and linoleic acid
resulted in exacerbation of liver injury in experimental ALD. In contrast, little
is known of the effects of other PUFAs, specifically n-3 PUFAs on ALD. Future research
is required to determine the role of dietary PUFAs and their bioactive metabolites
in clinical ALD. Interestingly, different types of dietary fat can also alter the
gut microbiome, which can influence individual susceptibility to ALD, and affect disease
severity. Dr. Feng’s group
22
comprehensively reviewed the role and mechanisms of gut microbiome in the pathogenesis
of ALD. Microbiome dysbiosis can induce short chain fatty acid changes, alter bile
acid metabolism, impair intestinal barrier function, and increase the release of bacterial
and fungal products resulting in inflammation; all of which contribute to the pathogenesis
of ALD. Strategies to target the microbiome, including dietary nutrient interference,
herbal medicine, antibiotics, anti-fungal agents, probiotics, engineered bacterial
therapy, fecal transplantation, oral hygiene and bacteriophage have become a potential
strategy for the prevention and treatment of ALD.
For end-stage of liver diseases, including DILI and ALD, liver transplantation remains
the most effective treatment option. However, the shortage of liver organ donors has
led to more aggressive acceptance and usage of liver grafts from extended criteria
donors, including using steatotic livers. The review from Sun et al.
23
highlights the role of necroptosis in ischemia/reperfusion injury of steatotic livers.
Necroptosis is a regulated non-apoptotic cell death that is mediated by the receptor-interacting
protein1 (RIP1), RIP3 and its downstream molecule mixed lineage kinase domain-like
protein (MLKL). Necroptosis is morphologically similar to necrosis, which is activated
when caspase-8 is inhibited leading to RIP1 and RIP3 heterodimerization resulting
in the assembly of the necrosome. The necrosome then activates MLKL by increasing
the phosphorylation of MLKL resulting in MLKL plasma membrane translocation and oligomerization,
and the eventual plasma membrane rupture and necrosis. Both alcoholic and non-alcoholic
fatty liver induce the expression and activation of necroptosis proteins including
RIP3 and MLKL, and MLKL knockout mice were resistant to ischemia-reperfusion injury
when these mice were fed either with a normal chow diet or a high-fat diet.
24
Since pharmacological inhibitors for RIP1 and RIP3 are available, targeting RIP1-RIP3-MLKL-mediated
necroptosis may be promising to extend the use of steatotic liver for liver transplantation
by improving ischemia-reperfusion liver injury.
As discussed above, some of the late stage of ALD patients may eventually develop
HCC. HCC is currently the fifth most common malignant cancer worldwide, without successful
treatment. Liver transplantation is still one of the most efficient therapies for
small unresectable HCC with cirrhosis. In an original research paper of this special
issue, Zeng et al.
25
investigated the prognostic potential of preoperative fibrinogen levels in HCC patients
receiving liver transplantation. It was found that a preoperative serum fibrinogen
level above 2.68 g/L was associated with tumor recurrence in patients after liver
transplantation. It is possible that the pretransplant plasma fibrinogen levels may
be used for patient selection who would more benefit from liver transplantation. However,
more future works are needed to confirm this because it was only from a single-centered
retrospective study. LncRNAs are a type of RNAs longer than 200 bp that are not translated
into proteins but have been implicated in regulation of gene transcription through
the recruitment of chromatin-modifying enzymes. In a study of this issue, Deng et
al.
26
identified RP11-307C12.11 using The Cancer Genome Atlas (TCGA) database and investigated
its role in vitro cultured liver tumor cell lines and in vivo tumor xenograft models.
They found that overexpression of RP11-307C12.11 promoted HCC cell growth both in
vitro and in vivo. Mechanistically, RP11-307C12.11 increased expression of CCND1 and
PDK1 likely by affecting microRNA (miR)-138 expression.
In summary, articles in this special issue summarize and discuss the current knowledge
on drug metabolism enzymes that are important for both DILI and ALD. It also provides
an update on the role of mitochondrial damage and innate immunity in DILI and how
DILI leads to hepatic steatosis. Several novel aspects on the pathogenesis of ALD
are mentioned, including LD biogenesis and catabolism, autophagy and UPS, dietary
factors, microbiota and steatotic hepatic ischemia and reperfusion injury. Finally,
potential clinical markers and treatment targets for HCC are also discussed in this
issue. Due to the page limitation, genetic factors, which are also important for both
DILI and ALD, are not covered in this special issue which may be an interesting topic
for future issues.