This document discusses how hepatitis C virus (HCV) infection may lead to hepatic steatosis (fatty liver) by impairing the secretion of very low density lipoproteins (VLDL) from the liver. The author hypothesizes that HCV infection results in impaired VLDL secretion, leading to intracellular accumulation of triglycerides in the liver and steatosis. Experiments show that HCV infection decreases the activity of microsomal triglyceride transfer protein and cellular lipases, which are required for triglyceride mobilization and VLDL assembly, thereby impairing the secretion of newly synthesized and preformed apolipoprotein B-100, a major component of VLDL. This impaired V

1. Impact of Hepatitis C Infection on Very Low Density Lipoprotein Secretion by the Liver
Mahra Nourbakhsh, Donna Douglas, Norman M Kneteman
Department of Surgery, University of Alberta, Edmonton, Canada
Hepatitis C virus (HCV) infection is a major health concern as 170 million people
are chronically infected with HCV world-wide. Acute HCV infection progresses to chronic
hepatitis in 70%-80% of infected patients, characterized by steatosis, progressive liver
fibrosis, cirrhosis and hepatocellular carcinoma. Most of the fatal complications of HCV are
related to the expansion of fibrosis and the factors that accelerate the rate of fibrosis worsen
the prognosis of chronic hepatitis C (CHC) infection. Over the last decade, it has become
apparent that HCV infection is significantly associated with liver steatosis. HCV-associated
steatosis is clinically relevant because it correlates with more rapid disease progression and
reduced response to antiviral therapy.
The mechanism of HCV-associated steatosis is not completely understood, although, hepatic
steatosis will develop whenever hepatic production/sequestration of triglyceride (TG)
exceeds TG secretion or metabolism. In CHC, patients have abnormalities in serum lipids.
Specifically they exhibit hypobetalipoproteinemia characterized by low serum level of Very
Low Density Lipoproteins (VLDL) and their lipoprotein derivatives. VLDL is exclusively
secreted by liver and its assembly and secretion is the major pathway for the export of lipids
(mostly TG) by the liver. As the liver is the main repository for HCV infection, it is tempting
to speculate a direct role of HCV on the regulation of lipid export from the thus contributing
to steatosis.
Hypothesis: HCV infection can result in impaired VLDL secretion, leading to hepatic
intracellular accumulation of triglycerides and hence steatosis.
VLDL Secretion by the Liver: In
the liver, VLDLs are formed in the
luminal space within the endoplasmic
reticulum (ER), whereas the lipids (mostly
TGs) that are required for their formation
are synthesized on the cytosolic side of the
ER membrane (Fig-1). Apolipoprotein
B100 (ApoB) is the major structural
protein associated with VLDL. In the
presence of sufficient lipid, mainly TG,
ApoB is efficiently assembled into
secretion-competent VLDL particles.
There is substantial evidence that the bulk
of TG secreted with VLDL arises from the
lipolysis of intracellular cytosolic TG
stores, followed by re-esterification and
finally lipid transfer to nascent ApoB in a process that requires microsomal transfer protein
(MTP), the cytosolic TG stores are mobilized for the lipidation of nascent ApoB. MTP is
also thought to play a role in the formation of luminal TG droplet which maybe the direct TG
source for the ApoB lipidation. Poorly lipidated ApoB is directed to the proteosome for
degradation. Therefore, lipolysis and re-esterification of stored TG, MTP activity, ApoB
synthesis and lipidation represent key steps in the hepatic export of VLDL-bound TG.
Objective: To determine if impaired secretion of VLDL is a possible mechanism
contributing to HCV associated steatosis
Aims: To determine if HCV infection alters ApoB100 synthesis, secretion or degradation
by modulation of MTP and/or cellular lipolytic activity required for TG mobilization.
Introduction and background
Fig-1: JFH-1 (Japanese Fulminant Hepatitis)-1 HCV
genotype 2a strain replicates efficiently in Human
Hepatocellular Carcinoma (Huh7.5).
Fig-2: JFH-1 Infection specifically impairs mass secretion of
ApoB-100 (not Albumin).
Fig-3: JFH-1 Infection impairs secretion of newly
synthesizedApoB-100.
Fig-4: * Cellular lysates from
JFH-1 infected culture have
significantly reduced MTP
activity in vitro(P<0.05, t-test).
Fig-5: * Cellular lysates from
JFH-1 infected culture have
significantly reduced cellular
lipase activity (P<0.05, t-test).
Fig-6: Relative to human
hepatocytes Huh7.5 cells
have impaired secretion of
TG stores despite having
appreciable amounts of
intracellular TG stores
present even 8h after
withdrawal of exogenously
supplied OA.
Methods and Results
Conclusions:
Taken from Wang et. al., JBC 2007
282(45): p.33218-26.
Significantly decreased MTP and cellular lipolytic activities may contribute to
observed decreased secretion of preformed and newly synthesized ApoB-100.
Impaired ApoB-100 secretion by HCV infection may be a mechanism contributing to
HCV-associated steatosis.
Future Directions:
1- Determine if observed decrease in the secretion of ApoB-100 is a significantly
result with JFH-1 infected cultures.
2-generate Huh7.5 cells having stable expression of Triacylglycerol Hydrolase (TGH).
Note: TGH has shown to be absent from all Hepatoma derived cell line and its expression has been shown to increase the mobilization of
TG stores for VLDL assembly/secretion. We will evaluate TGH as a potential target for disruption by HCV.
3- It is known that HCV-associated steatosis is more prevalent with HCV genotype 3
infected CHC patients. A specific role for the HCV core protein has been established. I
have generated a chimeric JFH-1 virus in which the core coding region has been
replaced with the core coding region obtained from a HCV 3a patient. This chimeric
virus has similar infection characteristics as the origin JFH-1 strain (data not shown).
We will determine if HCV core protein from HCV-3 has a distinct role in the
modulation of VLDL synthesis and secretion.