Hepatitis B virus X protein reduces starvation-induced cell death through activation of autophagy and inhibition of mitochondrial apoptotic pathway.
The hepatitis B virus X protein (HBx) has been implicated in the development of hepatocellular carcinoma (HCC) associated with chronic infection. As a multifunctional protein, HBx regulates numerous cellular pathways, including autophagy. Although autophagy has been shown to participate in viral DNA replication and envelopment, it remains unclear whether HBx-activated autophagy affects host cell death, which is relevant to both viral pathogenicity and the development of HCC. Here, we showed that enforced expression of HBx can inhibit starvation-induced cell death in hepatic (L02 and Chang) or hepatoma (HepG2 and BEL-7404) cell lines. Starvation-induced cell death was greatly increased in HBX-expressing cell lines treated either with the autophagy inhibitor 3-methyladenine (3-MA) or with an siRNA directed against an autophagy gene, beclin 1. In contrast, treatment of cells with the apoptosis inhibitor Z-Vad-fmk significantly reduced cell death. Our results demonstrate that HBx-mediated cell survival during starvation is dependent on autophagy. We then further investigated the mechanisms of cell death inhibition by HBx. We found that HBx inhibited the activation of caspase-3, an execution caspase, blocked the release of mitochondrial apoptogenic factors, such as cytochrome c and apoptosis-inducing factor (AIF), and inhibited the activation of caspase-9 during starvation. These results demonstrate that HBx reduces cell death through inhibition of mitochondrial apoptotic pathways. Moreover, increased cell viability was also observed in HepG2.2.15 cells that replicate HBV and in cells transfected with HBV genomic DNA. Our findings demonstrate that HBx promotes cell survival during nutrient deprivation through inhibition of apoptosis and activation of autophagy. This highlights an important potential role of autophagy in HBV-infected hepatocytes growing under nutrient-deficient conditions.
State Key Laboratory of Molecular Biology, Institute of Biochemistry and Cell Biology, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, Shanghai, PR China.
Yi
Y
Mao
Liang
L
Da
Hong
H
Tang
Jiali
J
Yang
Yinrui
Y
Lei
Pierre
P
Tiollais
Tsaiping
T
Li
Mujun
M
Zhao
D017209:Apoptosis; D001343:Autophagy; D053148:Caspase 3 / Q000096:biosynthesis; D002470:Cell Survival; D056945:Hep G2 Cells; D022781:Hepatocytes / Q000502:physiology* / Q000821:virology; D006801:Humans; D008928:Mitochondria / Q000502:physiology*; D015534:Trans-Activators / Q000235:genetics / Q000378:metabolism*; D054334:Viral Regulatory and Accessory Proteins
D015534:Trans-Activators; D054334:Viral Regulatory and Accessory Proteins; C057628:hepatitis B virus X protein; D053148:Caspase 3