Hepatitis B virus X protein induces cell death by causing loss of mitochondrial membrane potential

The hepatitis B virus X protein (HBx) has been implicated in the carcinogenicity of this virus as a causative factor by means of its transactivation function in development of hepatocellular carcinoma. However, we and others have recently reported that HBx is located in mitochondria and causes subsequent cell death (Takada, S., Shirakata, Y., Kaneniwa, N., and Koike, K. (1999) Oncogene 18, 6965-6973; Rahmani, Z., Huh, K. W., Lasher, R., and Siddiqui, A. ( 2000) J. Virol. 74, 2840-2846). In this study, we, therefore, examined the mechanism of HBx-related cell death. Using enhanced green fluorescent protein (EGFP) fusion constructs of HBx, the region required for its mitochondrial localization was mapped to amino acids (aa) 68-117, which is essential for cell death but inactive for transactivation function. In vitro binding analysis supported the notion that the recombinant HBx associates with isolated mitochondria through the region of aa 68-117 without causing redistribution of cytochrome c and apoptosis-inducing factor (AIF). A cytochemical analysis revealed that mitochondrial membrane potential was decreased by HBx association with mitochondria, suggesting that HBx induces dysfunction of permeability transition pore (PTP) complex. Furthermore, PTP inhibitors, reactive oxygen species (ROS) scavengers and Bcl-xL, which are known to stabilize mitochondrial membrane potential, prevented HBx-induced cell death. Collectively, the present results suggest that location of HBx in mitochondria of hepatitis B virus-infected cells causes loss of mitochondrial membrane potential and subsequently induces mitochondria-dependent cell death.