Jinho Seo

Lysosome-targeting chimeras (LYTACs) harness the cell's lysosomal degradation machinery to break down extracellular and membrane proteins. Previous methods used a synthetic glycopeptide containing multiple serine-O-mannose-6-phosphate (poly-M6Pn), which presented challenges such as synthetic complexity and potential immunogenicity associated with poly-M6Pn. This study introduced a LYTAC formulation, LYTAC^gyM6pG, which uses glyco-engineered yeast-derived mannose-6-phosphate glycans (gyM6pGs) for lysosomal transport, overcoming synthetic complexities and immunogenic risks. The gyM6pGs used in LYTAC^gyM6pG are human-compatible (identical to the structures found in humans) and are efficiently produced through yeast fermentation, followed by the preparation of cell wall glycans and their in vitro modifications. We employed copper-free click chemistry (azide and dibenzocyclooctyne reactions) for the robust conjugation of gyM6pGs to a nanobody targeting the immune checkpoint protein PD-L1, thereby streamlining the assembly of LYTAC^gyM6pG. We demonstrated that LYTAC^gyM6pG effectively degraded endogenous and recombinant PD-L1 proteins on the cell surface by directing them to the lysosome via the cation-independent mannose-6-phosphate receptor pathway. Furthermore, LYTAC^gyM6pG significantly enhanced T cell-mediated cytotoxicity against cancer cells, surpassing the efficacy of nanobodies alone. The successful application of gyM6pGs in the development of LYTAC^gyM6pG highlights the potential for a more viable and scalable therapeutic production of LYTACs, paving the way for broader therapeutic applications, including cancer treatment.

Seobin Kim, Eun-Woo Lee, Doo-Byoung Oh & Jinho Seo. BMB Reports -0001;0:. https://doi.org/10.5483/BMBRep.2023-0174

Bak is a critical executor of apoptosis belonging to the Bcl-2 protein family. Bak contains a hydrophobic groove where the BH3 domain of proapoptotic Bcl-2 family members can be accommodated, which initiates its activation. Once activated, Bak undergoes a conformational change to oligomerize, which leads to mitochondrial destabilization and the release of cytochrome c into the cytosol and eventual apoptotic cell death. In this study, we investigated the molecular aspects and functional consequences of the interaction between Bak and peroxisomal testis-specific 1 (Pxt1), a noncanonical BH3-only protein exclusively expressed in the testis. Together with various biochemical approaches, this interaction was verified and analyzed at the atomic level by determining the crystal structure of the Bak–Pxt1 BH3 complex. In-depth biochemical and cellular analyses demonstrated that Pxt1 functions as a Bak-activating proapoptotic factor, and its BH3 domain, which mediates direct intermolecular interaction with Bak, plays a critical role in triggering apoptosis. Therefore, this study provides a molecular basis for the Pxt1-mediated novel pathway for the activation of apoptosis and expands our understanding of the cell death signaling coordinated by diverse BH3 domain-containing proteins.

A lysosome, an acidic membrane-bound organelle, contains hydrolytic enzymes to digest macromolecules for recycling. Many lysosomal enzymes (LEs) traffic to the lysosome through the mannose-6-phosphate (M6P)-dependent pathway. Some mannose residues of high-mannose type N-glycans on LEs can be phosphorylated in the Golgi apparatus through two-step enzyme reactions. The consequent M6P moiety is recognized by M6P receptors (MPRs) on the trans-Golgi network membrane and delivered through the endo-lysosomal pathway. On the other hand, secreted LEs containing M6P glycans can be recaptured by MPRs on the plasma membrane and targeted to the lysosome. Enzyme replacement therapy (ERT) for lysosomal storage diseases exploits this M6P-MPR-dependent endocytosis to deliver recombinant enzymes to lysosomes. This review discusses various engineering and application technologies using M6P’s lysosomal targeting. Glyco-engineering for increasing M6P contents developed ‘Bio-better’ ERT enzymes with enhanced therapeutic efficacy. M6P-decorated peptides, proteins, liposomes, and nanoparticles have been developed for drug delivery and subcellular imaging. A recently developed lysosome-targeting chimera uses an M6P-based bifunctional binder to degrade specific extracellular and membrane proteins. The success and efficiency of M6P-based lysosomal targeting will boost further technological developments with new applications in the biomedical field.

Significance: Phosphatidylethanolamine (PE)-linked arachidonic acid (AA) and adrenic acid (AdA) are well-known substrates for lipid peroxidation, which are indispensable for ferroptosis, an iron-dependent regulated necrosis. However, how cells differentially regulate the intracellular pools of AA and AdA is not fully understood. Here, elongation of very long-chain fatty acid protein 5 (ELOVL5) and fatty acid desaturase 1 (FADS1) are differentially expressed in gastric cancer cells, discriminating the cellular susceptibility to ferroptosis. Biochemical and lipidomics analyses support the hypothesis that ELOVL5 and FADS1 are required to maintain intracellular levels of AA and AdA and promote ferroptosis. Our study highlights the biosynthesis of AA and AdA by ELOVL5 and FADS1 as a critical checkpoint in the ferroptosis pathway.

Eun Bi Cha, Keun Koo Shin, Jinho Seo & Doo-Byoung Oh. BMB Reports 2020;53:442-7. https://doi.org/10.5483/BMBRep.2020.53.8.024

Significance: A major yet perplexing question in the field of necroptosis is the role and involvement of necroptosis in cancer cells. Many cancer cells have protective mechanisms against necroptosis, but the underlying mechanism remains elusive. We report findings of cross-talk and a regulatory pathway that exist between MYC, a potent oncogene, and RIPK3, a pivotal factor in necroptosis. We find that MYC pathway is downregulated upon necroptotic, while MYC inhibits TNF-α–induced necroptosis. The inhibitory effect of MYC on necroptosis is unexpected because no transcriptional activity by MYC is required. Mechanistically, a direct interaction between MYC and RIPK3 takes place in the cytosol, preventing RIPK1–RIPK3 complex formation. Finally, MYC depletion enhances antitumor activity of necroptosis-inducing agents in a xenograft model.

Tumorigenesis can be induced by various stresses that cause aberrant DNA mutations and unhindered cell proliferation. Under such conditions, normal cells autonomously induce defense mechanisms, thereby stimulating tumor suppressor activation. ARF, encoded by the CDKN2a locus, is one of the most frequently mutated or deleted tumor suppressors in human cancer. The safeguard roles of ARF in tumorigenesis are mainly mediated via the MDM2-p53 axis, which plays a prominent role in tumor suppression. Under normal conditions, low p53 expression is stringently regulated by its target gene, MDM2 E3 ligase, which induces p53 degradation in a ubiquitin-proteasome-dependent manner. Oncogenic signals induced by MYC, RAS, and E2Fs trap MDM2 in the inhibited state by inducing ARF expression as a safeguard measure, thereby activating the tumor-suppressive function of p53. In addition to the MDM2-p53 axis, ARF can also interact with diverse proteins and regulate various cellular functions, such as cellular senescence, apoptosis, and anoikis, in a p53-independent manner. As the evidence indicating ARF as a key tumor suppressor has been accumulated, there is growing evidence that ARF is sophisticatedly fine-tuned by the diverse factors through transcriptional and post-translational regulatory mechanisms. In this review, we mainly focused on how cancer cells employ transcriptional and post-translational regulatory mechanisms to manipulate ARF activities to circumvent the tumor-suppressive function of ARF. We further discussed the clinical implications of ARF in human cancer.