A University of Colorado Boulder-led team has
discovered two prime targets of the Hepatitis B
virus in liver cells, findings that could lead to
treatment of liver disease in some of the 400
million people worldwide currently infected with the
virus.
CU-Boulder Professor Ding Xue, who led the studies,
said scientists have been looking for cellular
targets of the Hepatitis B virus, or HBV, for more
than three decades. Infections from HBV promote
hepatitis (inflammation of the liver), cirrhosis
(scarring of the liver) and liver cancer and can be
transmitted through blood and bodily fluids,
unprotected sex, unsterile needles and from infected
mother to offspring during birth.
Xue said scientists have known for some time that
HBV encodes a pathogenic, tumor-promoting protein
known as HBx, but how it works has remained largely
unknown. In two new studies, Xue and his colleagues
showed that the “host targets” of HBx in human cells
are two small cell proteins known as Bcl-2 and
Bcl-xL, both of which are well-known cell death
inhibitors but which have not previously been
implicated in HBV infection.
HBx uses a particular “motif,” a small string of
protein building blocks known as amino acids that
resemble those seen in some cell death-causing
proteins, to interact with the Bcl-2 and Bcl-xL
targets and stimulate an elevation of calcium in the
host cell. The calcium elevation then triggers both
viral HBV replication and cell death, said Xue.
When the researchers introduced gene mutations into
the motif, HBx binding to the Bcl-2 and Bcl-xL
proteins and viral replication were prevented.
Similarly, when either Bcl-2 or Bcl-xL proteins were
“knocked out” or weakened in human liver cells, HBx
was less able to cause an increase in calcium and
viral replication inside the infected cells.
“Our most important findings are the identification
of the motif itself and the two HBx host targets,”
said Xue of CU-Boulder’s molecular, cellular and
developmental biology department. “Now we can start
thinking about new drug targets to treat HBV.”
Two papers on the subject led by Xue were published
online Oct. 22 in the Proceedings of the National
Academy of Sciences. In addition to major CU-Boulder
co-authors Xin Geng, Brian Harry, Qinghua Zhou, Yan
Qin and Amy Palmer, a group led by Professor
Ning-Shao Xia from the National Institute of
Diagnostics and Vaccine Development in Infectious
Diseases at Xiamen University in China collaborated
on one of the studies.
The World Health Organization estimated in July that
about 600,000 people die annually from acute or
chronic HBV infection, which is most predominant in
Asia and Africa.
In one of the PNAS studies, the authors used a tiny
roundworm known as C. elegans, a widely used animal
model in biomedical research, to identify HBx host
targets within the cell. Xue and his team showed
that HBx can induce cell death in C. elegans through
a protein known as CED-9, mimicking an early stage
of liver infection by HBV.
Previous work had shown CED-9 in C. elegans is a
homolog of the human Bcl-2 protein -- a different
protein in a different animal that has a similar
function. Despite the stark differences between
roundworms and humans, scientists estimate 35
percent of C. elegans genes have human homologs.
“Our results suggest that C. elegans can serve as a
good animal model for identifying crucial host
factors and cell signaling pathways and aid in
development of strategies to treat HBV-induced liver
disorders,” said Xue. “I think the use of C. elegans
will galvanize the field of HBV study, which has
been in search of a good animal model for three
decades.”
Simple animal models like fruit flies and roundworms
have been critical for understanding fundamental
biological processes such as aging, cell death and
the regulation of gene expression. “Many would not
have considered using C. elegans as a model to study
HBV, but the genetic ‘tools’ of C. elegans are ideal
for the identification of viral host targets, even
though C. elegans is not a native host for the
virus,” said CU’s Harry. Harry is pursuing both a
Ph.D. degree in MCD Biology at CU-Boulder and an
M.D. at the CU School of Medicine on the Anschutz
Medical Campus in Aurora through the Medical
Scientist Training Program.
“Both studies show that if you create two mutations
in this small HBx motif, it takes away its ability
to bind to Bcl-2 family proteins. This wipes out
viral replication and host cell death caused by HBx
expression,” said Harry.
Xue said there currently is no effective treatment
for chronic HBV carriers, although some people with
chronic HBV are treated with interferon and
anti-viral drugs. But such treatments are either
unavailable or too expensive in developing countries
where most of the HBV infections are occurring.
“That’s why these new findings could have profound
clinical and pharmaceutical implications for the
treatment of HBV patients,” he said.
Harry said the Hepatitis B vaccine, which was
developed in 1982, is administered around the world
and has been shown to work well in preventing new
infections. “The problem is that once you are
infected, there is no effective way to remove the
virus from the body,” he said. “When the virus
replicates in liver cells, it causes cycles of
cellular damage, inflammation and tissue
regeneration, resulting in the accumulation of
genetic mutations and liver cancer.”
HBV is 50 to 100 times more infectious than the HIV
virus, according to WHO officials. In China and
other parts of Asia, most people acquire HBV during
childhood and 8 to 10 percent of the adult
population is chronically infected. “Because of
this, understanding how HBV and HBx cause
pathogenesis can have dramatic clinical impact,”
said Xue.
Funding for the two PNAS studies was provided by the
National Institutes of Health grants F30 NS070596
and R01 GM059083, GM079097, GM088241 and GM084027.
Additional funding came from the China National
Science Foundation and the Burroughs Wellcome Fund.
For more information on HBV visit
http://www.who.int/mediacentre/factsheets/fs204/en/
.For more information on CU-Boulder’s MCD Biology
department visit:
http://mcdb.colorado.edu/
MDN)
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