Activation of sterol regulatory element-binding protein 1c and fatty acid synthase transcription by hepatitis C virus non-structural protein 2

Abstract

Transcriptional factor sterol regulatory element-binding protein 1c activates the transcription of lipogenic genes, including fatty acid synthase (FAS). Hepatitis C virus (HCV) infection is often associated with lipid accumulation within the liver, known as steatosis in the clinic. The molecular mechanisms of HCV-associated steatosis are not well characterized. Here, we showed that HCV non-structural protein 2 (NS2) activated SREBP-1c transcription in human hepatic Huh-7 cells as measured by using a human SREBP-1c promoter-luciferase reporter plasmid. We further showed that sterol regulatory element (SRE) and liver X receptor element (LXRE) in the SREBP-1c promoter were involved in SREBP-1c activation by HCV NS2. Furthermore, expression of HCV NS2 resulted in the upregulation of FAS transcription. We also showed that FAS upregulation by HCV NS2 was SREBP-1-dependent since deleting the SRE sequence in a FAS promoter and expressing a dominant-negative SREBP-1 abrogated FAS promoter upregulation by HCV NS2. Taken together, our results suggest that HCV NS2 can upregulate the transcription of SREBP-1c and FAS, and thus is probably a contributing factor for HCV-associated steatosis.Sterol regulatory element-binding protein 1c (SREBP-1c) is a member of the basic helix-loop-helix leucine zipper family of transcription factors (Horton et al., 2002;Eberle et al., 2004). SREBP-1c expression is regulated at the transcription level (Desvergne et al., 2006). For instance, liver X receptor (LXR) activates SREBP-1c transcription by binding to the LXR element (LXRE) sequences in the SREBP-1c promoter (Yoshikawa et al., 2001;Tarling et al., 2004;Dif et al., 2006). SREBP-1c can also regulate its own transcription in a positive feed-back loop through binding to the sterol regulatory elements (SREs) in the SREBP-1c promoter (Amemiya-Kudo et al., 2000). The newly synthesized, precursor SREBP-1c protein is bound to the endoplasmic reticulum (ER). Following proteolytic digestions, the amino-terminal domain is released and transported to the nucleus as an active transcriptional factor (Brown et al., 2000). The processed, mature SREBP-1 proteins are modified by phosphorylation by protein kinases, such as mitogen-activated protein kinase (MAPK), protein kinase A (PKA) and glycogen synthase kinase-3b (Kotzka et al., 1998;Roth et al., 2000;Lu & Shyy, 2006;Punga et al., 2006). The impact of phosphorylation on the transcriptional activity of SREBP-1 is less clear. For instance, phosphorylation by MAPK appears to enhance the transcriptional activity of SREBP-1, whereas PKA phosphorylation suppresses the function of SREBP-1 (Kotzka et al., 1998Lu & Shyy, 2006). A major function of SREBP-1c is to activate genes involved in the synthesis of fatty acid and their incorporation into triglycerides and phospholipids (Horton et al., 2002;Eberle et al., 2004). As such, abnormal higher levels of SREBP-1c will result in lipid accumulation in the liver and cause steatosis (Ferre & Foufelle, 2007). Fatty acid synthase (FAS) is a well established target gen...

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“…Furthermore, FAS gene is highly expressed in HCV-infected chimpanzees (37). Recent studies have shown that HCV infection enhances the proteolytic processing of SREBPs in hepatic cells (38) and HCV NS2 protein can up-regulate the transcription of SREBP-1c and FAS (39). Nevertheless, the molecular mechanisms underlying lipo- genic signaling by either HCV or cellular factors are not fully understood.…”

Liver X receptor α-mediated regulation of lipogenesis by core and NS5A proteins contributes to HCV-induced liver steatosis and HCV replication

“…Although the LXRa-mediated lipogenesis observed in replicon-containing cells could be partially induced by the expression of HCV NS5A and core proteins, other HCV proteins, such as NS2 and NS4B, may contribute to higher LXRa levels, and increased SREBP-1c and FAS expression, as previously indicated. 15,16,49 Future studies should determine whether different HCV proteins regulate lipid metabolism through different mechanisms.…”

“…[11][12][13] Other HCV proteins, such as NS2, NS4B, and NS5A, have also been shown to be able to modulate lipogenic gene expression. [14][15][16] However, the precise functions of HCV proteins in the development of fatty liver remain unknown.…”

Hepatitis c virus and host cell lipids: An intimate connection

“…Accordingly, it is detectable in infected cells as a basal and a hyperphosphorylated form with apparent molecular weights of 56 and 58 kDa, respectively. 63 NS5A binds to the viral RNA and to numerous host factors and colocalizes with core in close upon expression of individual viral proteins such as core, NS2 or NS4B, [91][92][93][94] and its inhibition by 25-hydroxycholesterol causes a decrease in fatty acid biosynthesis and a subsequent block of HCV replication. 95 In agreement with this, the expression of fatty acid synthase (FASN) and other genes related to the synthesis and transport of fatty acids is upregulated in infected cells.…”