Karen Bedard and Karl-Heinz Krause
Karen Bedard and Karl-Heinz Krause
Karen Bedard and Karl-Heinz Krause
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280 KAREN BEDARD AND KARL-HEINZ KRAUSE<br />
hepatocytes. Already in 1981, an NADPH oxidase-like activity<br />
had been described in membrane preparations from<br />
hepatocytes; as opposed to membranes from phagocytes,<br />
the hepatic system appeared to be able to use either<br />
NADH or NADPH (727). Histochemical localization<br />
placed the NADPH oxidase-like activity within the bilecanalicular<br />
plasma membranes (617). ROS generation by<br />
hepatocytes in response to capsaicin (521), tamoxifen<br />
(522), CD95 (Fas) lig<strong>and</strong> (CD95L) (739), <strong>and</strong> TGF-� (371)<br />
is thought to be NOX mediated. In total liver mRNA,<br />
relatively large amounts of NOX2 (143, 454) <strong>and</strong> trace<br />
amounts of NOX4 (143) have been described. Little information<br />
is currently available regarding the NOX isoforms<br />
present specifically in hepatocytes. PCR analysis suggested<br />
that rat hepatocytes expressed mRNAs of NOX1,<br />
NOX2, NOX4, DUOX1, <strong>and</strong> DUOX2 (739). However, these<br />
results were obtained by PCR exclusively, <strong>and</strong> the large<br />
set of NOX isoforms identified in hepatic stellate cells<br />
(739) is not in line with other studies (63).<br />
The main function attributed to NOX-derived ROS in<br />
hepatocytes is apoptosis (521, 739). Several arguments<br />
point towards a role of NOX-derived ROS in CD95L-dependent<br />
hepatocyte apoptosis: 1) p47 phox is phosphorylated<br />
in response to CD95L, 2) NADPH oxidase inhibitors<br />
prevent ROS generation <strong>and</strong> reduce apoptosis, <strong>and</strong> 3)<br />
antisense RNA against p47 phox prevents CD95L-mediated<br />
ROS generation <strong>and</strong> reduces apoptosis (739). Hepatocyte<br />
ROS generation in response to TGF-� is also thought to<br />
lead to apoptosis through NOX activation (371). Thus it<br />
appears that a p47 phox -containing NADPH oxidase (possibly<br />
NOX1 or NOX2) mediates hepatocyte apoptosis.<br />
There is, however, a difference between the results<br />
obtained in vitro in isolated hepatocytes <strong>and</strong> those obtained<br />
from in vivo studies. In one study, treatment of rats<br />
with a peroxisome proliferator compound led to an enhanced<br />
hepatocyte proliferation through NOX-dependent<br />
hepatic ROS generation (767). The different outcome of<br />
hepatocyte exposure to ROS (proliferation vs. cell death)<br />
might be explained by the different source <strong>and</strong>/or quantity<br />
of ROS. While TGF-� <strong>and</strong> CD95L activate an NADPH<br />
oxidase within hepatocytes, the peroxisome proliferator<br />
compound is thought to generate ROS in Kupffer cells.<br />
Thus ROS generated by hepatocytes themselves might<br />
lead to apoptosis, while exposure to ROS, generated by<br />
neighboring cells, might rather cause hepatocyte proliferation.<br />
There may also be species-specific factors involved,<br />
as the phosphorylation events in response to peroxisome<br />
proliferator agents that regulate kinase <strong>and</strong> phosphatase<br />
activity do not occur in human hepatic-derived cells (694).<br />
In addition to hepatocytes, the liver consists of many<br />
nonparenchymal cells types, including endothelial cells,<br />
pit cells, Kupffer cells, <strong>and</strong> hepatic stellate cells (also<br />
known as Ito cells, hepatic pericytes, perisinusoidal cells,<br />
<strong>and</strong> lipocytes).<br />
B) HEPATIC STELLATE CELLS. The many names of hepatic<br />
stellate cells (HSC) reflect their many proposed functions,<br />
which include regulation of microvascular blood flow as<br />
well as storage of fat <strong>and</strong> vitamin A. In response to<br />
hepatocyte injury, HSCs transform from star-shaped vitamin<br />
A-rich to myoblast-like, vitamin A-deficient cells <strong>and</strong><br />
are involved in both the secretion <strong>and</strong> degradation of<br />
extracellular matrix (805, 837).<br />
HSCs generate ROS in response to various stimuli (9,<br />
63, 793, 838), particularly TGF-� (177). HSCs express<br />
p22 phox<br />
Physiol Rev VOL 87 JANUARY 2007 www.prv.org<br />
(46), but which NOX isoform is expressed is less<br />
clear. Both NOX1 (mRNA) <strong>and</strong> NOX2 (mRNA <strong>and</strong> protein)<br />
are found in HSC primary culture <strong>and</strong> cell lines (9,<br />
63). However, in freshly isolated human HSCs, the situation<br />
is different: neither isoform is found in healthy controls,<br />
while NOX1, but not NOX2, is found in samples<br />
from patients with hepatic fibrosis (63). Interestingly,<br />
p47 phox -deficient mice are protected from hepatic fibrosis<br />
in a bile duct ligation model of hepatic injury, suggesting<br />
that a p47 phox -containing NOX complex is involved (63).<br />
Taken together, these data are compatible with p47 phox<br />
acting as the organizer subunit for NOX1 in HSCs, although<br />
this model presently remains speculative. Proposed<br />
functions of NOX-derived ROS in HSCs include the<br />
induction of cell proliferation (9, 793, 838) <strong>and</strong> the production<br />
of collagen (177, 838). Consequently, a role for<br />
NOX enzymes in hepatic fibrosis, characterized by HSC<br />
proliferation <strong>and</strong> accumulation of extracellular matrix<br />
proteins, has been suggested (9, 63).<br />
C) KUPFFER CELLS. Kupffer cells are specialized tissue<br />
macrophages within the liver. ROS generation in Kupffer<br />
cells occurs in response to stimuli such as LPS, peroxisome<br />
proliferator compounds (767), as well as to the<br />
yeast cell wall extract zymosan (86). ROS generation by<br />
Kupffer cells also occurs under pathological conditions<br />
such as hypoxia-reoxygenation (768) <strong>and</strong> in response to<br />
ethanol (358).<br />
Being tissue macrophages, Kupffer cells express the<br />
phagocyte NADPH oxidase NOX2 <strong>and</strong> its subunits (379,<br />
767). LPS-stimulated ROS generation is absent in p47 phox -<br />
deficient Kupffer cells (767), demonstrating the importance<br />
of a p47 phox -containing NOX enzyme, presumably<br />
NOX2, in this cell type. NOX2-derived ROS in Kupffer<br />
cells most likely have several functions. They have been<br />
suggested to play an important role in clearing pathogens<br />
derived from the gut (500). They are also involved in the<br />
regulation of gene expression, particularly the expression<br />
of the CD95 lig<strong>and</strong> (890) <strong>and</strong> NF�B-dependent production<br />
of TNF-� (753). Finally, they have been suggested to be<br />
essential mediators in antigen presentation by Kupffer<br />
cells (568).<br />
D) LIVER CIRRHOSIS AND ALCOHOLIC LIVER DISEASE. The involvement<br />
of oxidative stress in alcohol-induced liver<br />
disease was suggested 40 years ago (212) <strong>and</strong> has been<br />
largely confirmed since (36). Exposure to alcohol leads to<br />
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