Department of Life Sciences

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Browsing Department of Life Sciences by Author "Saha, Abhik"

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    Crosstalk between Proteasomal and Lysosomal Protein Degradation Pathway in EBV-Induced B-cell Lymphomas
    Saha, Abhik
    Epstein-Barr virus (EBV) nuclear oncoprotein EBNA3C plays an important role during B-cell transformation and subsequent development of several B-cell lymphomas specifically in those who are from immuno-compromised background. EBNA3C manipulates several important cellular pathways including ubiquitin-proteasome machinery and deregulate multiple cellular oncoproteins and tumor suppressor proteins. Studies have revealed that EBNA3C is ubiquitinated at its N-terminal domain and can interact with 20S proteasome machinery. However, In vitro, the viral protein is extremely stable in EBV transformed growing B-lymphocytes. EBNA3C can also bypass autophagy-lysosomal mediated protein degradation and subsequent antigen presentation for T-cell recognition. Moreover, recently, our group demonstrated that in response to metabolic stress, EBNA3C elevates the basal level of autophagy through epigenetic alternation and transcriptional activation of several autophagy related genes (ATGs). This also serves as a prerequisite for B-cell survival under growth limiting conditions. The crosstalk between EBNA3C mediated proteasomal and autophagy-lysosomal machinery further prompted us to investigate the underlying proteolytic mechanism governing EBNA3C’s turn-over. We demonstrate that proteasomal inhibition accelerates EBNA3C degradation both in EBV transformed B-lymphocytes and ectopic-expression systems. Interestingly, in presence of proteasomal inhibitors, two EBNA3 family oncoproteins–EBNA3A and EBNA3C were degraded, but not the viral tumor suppressor protein EBNA3B. EBNA3C degradation induced by proteasomal inhibition is partially blocked when autophagy-lysosomal pathway is inhibited. In response to proteasomal inhibition, EBNA3C is predominantly K63-linked polyubiquitinated and colocalized with the autophagy-lysosomal fraction in the cytoplasm and participated within p62-LC3B complex, thus facilitating autophagy-mediated degradation. We further describe that the degradation signal is located at the first 50 residues of the N-terminal domain of EBNA3C. The colony formation ability of this important viral oncoprotein is also reduced when proteasome is blocked. In addition, proteasomal inhibition induces apoptotic cell death and accelerates transcriptional activation of both latent and lytic gene expression which further induces viral reactivation from EBV transformed B-lymphocytes. Overall, this study provides rationale to use proteasome inhibitors as potential therapeutic strategy against multiple EBV associated B-cell lymphomas.
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    Understanding the Role of Epstein Barr virus essential Nuclear antigen EBNA3C in regulating unfolding protein response UPR Autophagy network
    Saha, Abhik
    In rapidly proliferating cancer cells, endoplasmic reticulum (ER) tends to be overloaded with unfolded and misfolded proteins due to high metabolic demand. With the limited protein folding capacity of ER, cancer cells suffer from Unfolded Protein Response (UPR) stress and subsequently elevate autophagy, in which the protein-aggregates are degraded and provide free energy in form of amino acids allowing cancer cells for an uninterrupted proliferation. Consequently, several anticancer molecules targeting UPR-autophagy network are currently under clinical trials. Epstein-Barr virus (EBV) is associated with a number of B-cell lymphomas. EBV can readily transform quiescent B-lymphocytes into continuously proliferating lymphoblastoid cell lines (LCLs), providing an excellent model system for studying EBV associated B-cell lymphomagenesis. EBNA3C, one of the essential viral oncoproteins for B-cell transformation, manipulates multiple cell pathways – most strikingly cell-cycle/apoptosis and ubiquitin-targeted protein-degradation machineries. Interestingly, several components of these pathways were previously shown to be intricately connected with the UPR-autophagy network in the development of human malignancies. In our study, we have shown that EBNA3C transcriptionally upregulates several autophagy genes (ATG3, ATG5, and ATG7), particularly involved in autophagosome formation, under nutrient deprived conditions. EBNA3C recruits several histone activation epigenetic marks to regulate autophagy gene transcription. Moreover, EBNA3C specifically elevates PERK-eIF2α-ATF4 UPR signaling cascade in normal growth conditions, whereas in the presence of UPR-inducers it blocks further UPR activation, indicating EBNA3C promotes an adaptive condition for cell survival. Overall, our study provides a new role of an essential EBV oncoprotein in regulating autophagy-UPR cascade and offers novel targets for potential therapeutic expansion against multiple EBV induced B-cell lymphomas.