Transcriptional Repression of C4 Complement by Hepatitis C Virus Proteins

Abstract

The fourth component of human complement (C4) plays an important role in innate immune function. C4 activity has been observed to be significantly lower in patients with chronic hepatitis C virus (HCV) infections, although the mechanism remains unknown. In this study, we have examined the mechanisms of C4 regulation by HCV. Liver biopsy specimens from patients with chronic HCV infections displayed significantly lower C4 mRNA levels than liver tissue samples from patients with unrelated liver disease. Further, C4 mRNA levels of the two isoforms (C4A and C4B) were significantly reduced in hepatocytes transfected with RNA from HCV genotype 1a or 2a. Subsequently, a significant C4 regulatory role of HCV core or NS5A upon C4 promoter activity was observed. HCV core or NS5A transgenic mice displayed a reduction in C4 mRNA. Gamma interferon (IFN-␥)-induced C4 promoter activation was also impaired in the presence of HCV proteins. We further demonstrated that HCV core reduced the expression of upstream stimulating factor 1 (USF-1), a transcription factor important for basal C4 expression. On the other hand, the expression of interferon regulatory factor 1 (IRF-1), which is important for IFN-␥-induced C4 expression, was inhibited by hepatocytes expressing HCV NS5A. These results underscore the roles of HCV proteins in innate immune regulation in establishing a chronic infection.The complement system is a set of biochemical pathways that helps to clear pathogens from an organism as part of the innate and acquired immunity programs. Activation of the complement system triggers a wide range of cellular responses ranging from apoptosis to opsonization (27). The complement system plays a critical role in the pathogenesis of a variety of chronic human diseases. Viruses have been shown to attenuate complement activation. Recently, the nonstructural protein (NS1) from flaviviruses (dengue fever virus, West Nile virus, and yellow fever virus) has been reported to attenuate complement activation by interacting with several complement components (1,8). Results from another study suggest that the complement system is also involved in the pathogenesis of a variety of liver disorders, including liver injury and repair, fibrosis, viral hepatitis, alcoholic liver disease, and liver ischemia/reperfusion injury (24). The fourth component of the complement system, C4, plays a vital role in mounting a proper immune response against infection (35). Chronic hepatitis C virus (HCV) infection is a leading cause of progressive liver disease. The mechanisms responsible for HCV persistence are not well understood, although the interactions between HCV and host cells appear to play an important role. Dumestre-Perard et al. (11) reported in a study of a large cohort that in patients with chronic HCV infections, the blood level of specific complement components is depleted, and C4 activity is significantly lower. A decrease in specific C4 activity was reported among relapsers compared with sustained responders, before and during therapy, suggesting it...

Advancing the High Throughput Identification of Liver Fibrosis Protein Signatures Using Multiplexed Ion Mobility Spectrometry

“…Though likely not specific as markers of fibrosis progression, these signatures still reflect the gross alterations visible in this biofluid as a direct result of liver injury. Of specific relevance to HCV infection, however, is complement component 4A (C4A) which exhibited lower abundance in the SP and FP patient groups, consistent with recent papers that have shown it has significantly lower activity in HCV infected patients due to transcriptional repression by HCV proteins (24,25).…”

“…All five proteins were detected with significant differential abundance in our analysis, and observed with the same expression trends. Additionally, markers such as C4A (24,25), LGALS3BP (31), HPX (32), A2M (32) have also been reported previously, and overlap with the current findings, as well as a vast majority of the serum proteins observed with significant differential abundance in a nonalcoholic fatty liver disease study (33). Due to the high sensitivity of the LC-IMS-MS measurements, we were able to detect significance of many proteins simultaneously that have only been observed as subsets in prior manuscripts.…”

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“…We have previously observed that C4 synthesis is mediated by USF-1 and IRF-1 transcription factors (24), and thus we examined whether cell-to-cell interaction affects the status of these transcription factors for C4 synthesis. IRF-1 was expressed at a modest level in untreated cells, while a dramatic recovery of IRF-1 and USF-1 was observed after coculture of IL-2-activated NK cells and hepatoma cells (Fig.…”

“…Hepatocytes in the liver are the primary host for HCV replication and are the main source for complement synthesis. We have previously identified C4 downregulation in hepatoma cells expressing HCV core or NS5A protein (24). NK cells play an important role in controlling viral hepatitis, liver fibrosis, and liver tumorigenesis and also contribute to the pathogenesis of liver injury and inflammation (25).…”

Promotion of Cancer Stem-Like Cell Properties in Hepatitis C Virus-Infected Hepatocytes

“…Specimens were collected from subjects with their written consent, and the human studies protocol (number 10592) was approved by the Saint Louis University Internal Review. RNA was prepared from liver specimens by using TRIzol (Invitrogen) as previously described (15). Commercially available control liver RNAs (Clontech, CA, and Lonza, NJ) were used for comparison.…”