Chen, Jing

Chronic hepatitis B (CHB), a major global public health burden, is characterized by the presence of high circulating levels of non-infectious subviral particles (SVPs). These SVPs are mainly composed of small hepatitis B surface antigen (SHBs) and are associated with a dysfunctional HBV-specific immune response. Previous studies exploring the cellular mechanisms and molecular pathways by which SVPs are secreted from hepatocytes have utilized either non-hepatic cells or whole-genome HBV-integrated hepatoma cell lines in which SHBs can also be secreted as a component of the virion envelope. In this study, we provide a detailed examination of SHBs trafficking using biochemical approaches coupled with high-resolution confocal microscopy and live-cell imaging via a novel fluorophore-tagged SHBs probe in both HepG2 and primary human hepatocytes stably expressing SHBs. Our results provide evidence that SHBs utilize a modified ER-Golgi secretory pathway with transit from the ER to the multivesicular body and lysosomal/late endosomal compartments. Importantly, we identified the autophagy adapters ATG5 and LC3B, and the ER-phagy protein atlastin 3 (ATL3), central to mediating SHBs secretion. These findings suggest that SHBs is trafficked through a non-canonical path for secretion and provides new potential therapeutic targets for the treatment of CHB.

It is well established that many tumor types possess defective autophagic pathways. Several studies have reported that the transmembrane, autophagic lipid scramblase ATG9B is altered in multiple cancers, suggesting that this dysregulation could contribute to oncogenesis. Therefore, the goal of this study was to define the cellular distribution of ATG9B in two different tumor cell types and to provide insights into its cellular function. Surprisingly, we found that ATG9B shows a modest association with autophagic structures and exhibits a unique and prominent localization to mitochondria, in contrast to its related form ATG9A. Upon expression of tagged ATG9B forms, this mitochondrial distribution was accompanied by aberrant changes in mitochondrial morphology as well as a reduction in the mitochondrial membrane potential and the release of mtDNA. Few indicators for ATG9B-dependent mitophagy were noted. Instead, ATG9B overexpression led to pronounced apoptotic cell death as assessed by a variety of indicators. Further, we find that the N-terminal sequence of ATG9B acts as a mitochondrial targeting domain and that expression of this peptide alone can induce apoptotic cell death. These findings provide new insights into a putative cellular localization and function for ATG9B.