Calpain activation by hepatitis C virus proteins inhibits the extrinsic apoptotic signaling pathway

Abstract

An unresolved question regarding the physiopathology of hepatitis C virus (HCV) infection is the remarkable efficiency with which host defenses are neutralized to establish chronic infection. Modulation of an apoptotic response is one strategy used by viruses to escape immune surveillance. We previously showed that HCV proteins down-regulate expression of BH3-only Bcl2 interacting domain (Bid) in hepatocytes of HCV transgenic mice. As a consequence, cells acquire resistance to Fas-mediated apoptosis, which in turn leads to increased persistence of experimental viral infections in vivo. This mechanism might participate in the establishment of chronic infections and the resulting pathologies, including hepatocellular carcinoma. We now report that Bid is also down-regulated in patients in the context of noncirrhotic HCV-linked tumorigenesis and in the HCV RNA replicon system. We show that the nonstructural HCV viral protein NS5A is sufficient to activate a calpain cysteine protease, leading to degradation of Bid. Moreover, pharmacological inhibitors of calpains restore both the physiological levels of Bid and the sensitivity of cells toward a death receptor-mediated apoptotic signal. Finally, human HCV-related tumors and hepatocytes from HCV transgenic mice that display low Bid expression contain activated calpains. Conclusion: Calpains activated by HCV proteins degrade Bid and thus dampen apoptotic signaling. These results suggest that inhibiting calpains could lead to an improved efficiency of immune-mediated elimination of HCV-infected cells. (HEPATOLOGY 2009;50:1370-1379.)P ersistent infection with hepatitis C virus (HCV) is among the most common infectious causes of chronic liver disease. The majority of patients fail to clear the virus and become chronic carriers, with a persistent presence of detectable virus in the serum. 1 Patients with chronic hepatitis C are at risk for hepatic fibrosis, frequently culminating in hepatic cirrhosis and hepatocellular carcinoma (HCC). 2 FL-N/35 transgenic mice, with hepatocyte-targeted expression of the entire open reading frame of the genotype 1b HCV, are at risk for steatosis and hepatocellular adenoma and carcinoma. 3 We previously showed that the FL-N/35 hepatocytes are resistant to apoptosis induced by the Fas/CD95 death receptor stimulation. The lack of sensitivity to apoptotic stimulation was related to decreased expression of BH3-only Bcl2 interacting domain (Bid), a BH3-only member of the Bcl-2 family of apoptosis regulators. 4 Importantly, Bid-deficient hepatocytes are refractory to T lymphocyte-induced cell death, suggesting that apoptosis contributes to HCV persistence and, as a consequence, to liver pathologies characteristic of chronic HCV infection. 4 Here, we report that Bid is also down-regulated in the context of the subgenomic and genome-length HCV replicons and in a subset of HCV-linked human HCC.

BAD, a Proapoptotic Member of the BCL2 Family, Is a Potential Therapeutic Target in Hepatocellular Carcinoma

“…Our findings are consistent with the emerging view that the BH3-only proteins are key regulators of apoptosis, whose expression is often altered in tumor cells (10). Recently, BID, another member of the subfamily of BH3-only proteins, was shown to be downregulated in a subset of HCC occurring in the context of viral hepatitis (12). Based on our results, we suggest that the alteration of the expression of selected BH3-only proteins is an important event in a subset of HCC.…”

“…In particular, little is known about the variations in the expression of the proapoptotic members of the BH3-only family of proteins, although they have recently emerged as an important barrier against malignant transformation (10). A recent report indicates that BID, a member of the BH3-only subfamily, is downregulated in a subset of HCC appearing in the context of viral hepatitis (12). Whether the expression of other members of the BCL2 family of proteins is similarly altered in HCC remains unclear.…”

Hepatitis C virus (HCV) protein expression enhances hepatic fibrosis in HCV transgenic mice exposed to a fibrogenic agent

“…In this model, the HCV proteins are expressed within hepatocytes at levels close to those observed in human infection, without infectious virus production or an antiviral immune response. These mice develop lesions similar to those observed during natural infection, including hepatic steatosis and hepatocellular carcinoma [2][3][4][5][6].…”

Genome of brown tide virus (AaV), the little giant of the Megaviridae, elucidates NCLDV genome expansion and host–virus coevolution

“…In other systems (e.g., Hepatitis C) activation of calpains have been shown to inhibit the host's extrinsic apoptotic signaling pathway, which is necessary for successful infection. (Simonin et al, 2009). Consequently, the possession of such a protease in the AaV genome might allow for avoidance of the host's virus exclusion strategy.…”