Interaction of the Hepatitis B Spliced Protein with Cathepsin B Promotes Hepatoma Cell Migration and Invasion

Abstract

C hronic hepatitis B virus (HBV) infection has been proven to be one of the most important risk factors for the development of hepatocellular carcinoma (HCC) (3, 9). However, the pathogenesis of cancer in HBV infection is still not fully understood, and it appears that multiple factors and cellular signaling pathways are involved in hepatocarcinogenesis (28). Integration of HBV genome into host DNA can lead to alterations in cellular gene function or generate chromosomal instability (1, 10, 43). Expression of some oncogenic HBV proteins such as HBx and truncated Pre-S2/S has been shown to have a direct effect on malignant transformation of the liver (42) and to promote metastasis of the malignant cells, and there is thus a very high mortality rate for HCC patients (18). Another HBV protein, encoded by a singly spliced 2.2-kb HBV DNA and referred to as the hepatitis B spliced protein (HBSP), is found to be expressed in liver biopsy tissues from four out of five chronic patients but not from two hepatitis C virus-infected patient samples and one normal liver sample (39). Our prior study also showed that a 2.2-kb splice variant was present in all tumor and peritumor samples from 12 HCC patients studied (20). Exogenous expression of HBSP in transfected Huh-7 cells can induce cell apoptosis (40). However, the cytopathic effect of the HBSP was unclear, and its role in progression, invasion, and metastasis of HBV-related HCC has not yet been elucidated.We previously showed that cathepsin B (CTSB) was one of the major intracellular interacting partners of HBSP, and it was identified using a yeast two-hybrid screening assay (8). This led us to investigate whether the HBSP could interact with CTSB in the context of hepatoma cells to affect cell migration, invasion, and metastasis, all of which can be modulated by CTSB acting directly and indirectly on extracellular matrix (ECM) component remodeling and degradation (26,47). CTSB is a lysosomal cysteine protease that plays an important role in physiological protein turnover and processing (19,41). In nonmalignant cells, CTSB is mainly stored in the lysosome, whereas in malignant cells, CTSB redistributes into exocytic vesicles at the cell periphery, leading to its secretion and association with binding partners on the tumor cell surface (22,33). CTSB can cleave and activate a wide variety of substrates in proteolytic pathways that increase neoplastic progression. It has been shown that activation of urokinase-type plasminogen activator (uPA) and matrix metalloproteinases (MMPs) by CTSB not only enhances ECM degradation and cancer cell motility and invasion (14) but also induces in vitro angiogenesis (2). In addition to interacting with other proteases, CTSB may regulate the activity of kinase signaling networks, cell surface receptors, and signaling molecules such as chemokines, cytokines and growth factors (25). However, such interactions are usually bidirectional because kinases can also regulate many proteases through phosphorylation while proteases can control the actio...

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“…In fact increased ratio of the splicing variant relative to wild-type virus in the blood correlates with liver fibrosis and HCC [134-135]. Cell culture studies suggested that HBSP promotes hepatoma cell proliferation and migration [136]. Alternatively, pathogenicity of the splicing variant could be attributed to increased expression of core and HBx proteins as well as DNA replication [133, 137], as a consequence of the large deletion which prevents the transcription of 2.4-kb and 2.1-kb RNAs.…”

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“…35, 36 Chen et al reported over-expression of hepatitis B spliced protein in HCC cells with increased cell invasion and motility. 37 Recently, Yang et al reported the HBV infection status to be strongly associated with an elevated activity of the TGF-β-miR-34a-CCL22 pathway which renders an immune-subversive microenvironment to favor vascular dissemination of HCC cells. 38 These studies support our clinical findings that the viral load is an independent risk factor of tumor recurrence.…”