ABSTRACT
Targeted protein degradation (TPD), employing proteolysis-targeting chimeras (PROTACs) composed of ligands for both a target protein and ubiquitin ligase (E3) to redirect the ubiquitin-proteasome system (UPS) to the target protein, has emerged as a promising strategy in drug discovery. However, despite the vast number of E3 ligases, the repertoire of E3 ligands utilized in PROTACs remains limited. Here, we report the discovery of a small-molecule degron with a phenylpropionic acid skeleton, derived from a known ligand of S-phase kinase-interacting protein 2 (Skp2), an E3 ligase. We used this degron to design PROTACs inducing proteasomal degradation of HaloTag-fused proteins, and identified key structural relationships. Surprisingly, our mechanistic studies excluded the involvement of Skp2, suggesting that this degron recruits other protein(s) within the UPS.
Subject(s)
S-Phase Kinase-Associated Proteins , Small Molecule Libraries , Humans , S-Phase Kinase-Associated Proteins/metabolism , S-Phase Kinase-Associated Proteins/antagonists & inhibitors , Small Molecule Libraries/chemistry , Small Molecule Libraries/pharmacology , Small Molecule Libraries/chemical synthesis , Proteolysis/drug effects , Phenylpropionates/chemistry , Phenylpropionates/pharmacology , Structure-Activity Relationship , Proteasome Endopeptidase Complex/metabolism , Molecular Structure , Ligands , HEK293 Cells , DegronsABSTRACT
BACKGROUND: Xenotransplantation is an appealing alternative to human allotransplantation because of a worldwide shortage of organs. One of the obstacles for xenografts is cellular rejection by the innate immune system, comprised of NK cells, monocytes, and macrophages. In this study the inhibitory function of HLA-G1, a MHC Ib molecule, on macrophage-mediated cytotoxicity was examined. Furthermore, this study also evaluates the suppressive effect of cytokine production by macrophages. METHODS: The expression of inhibitory receptors that interact with HLA-G1, immunoglobulin-like transcript 2 (ILT2), ILT4 and KIR2DL4 (CD158d) on in vitro generated macrophages were examined by flow cytometry. Complementary DNA (cDNA) of HLA-G1, HLA-E and human ß2-microglobulin (hß2m) were prepared and transfected into swine endothelial cells (SECs). The expression of the transgenic genes was evaluated by flow cytometry, and macrophage-mediated SEC cytolysis was assessed using the macrophages. RESULTS: In vitro generated macrophages expressed not only ILT2 and ILT4 but CD158d as well. The transgenic HLA-G1 on SECs indicated significant suppression in macrophage-mediated cytotoxicity, which was equivalent to that of transgenic HLA-E. Furthermore, the results on real time PCR and ELISA revealed that transgenic HLA-G1 induces the anti-inflammatory cytokines, such as IL-10 and TGF-ß, and suppresses iNOS mRNA expression, indicating that transgenic HLA-G1 has suppressive effects in a broad range of transplant rejection. CONCLUSION: These results indicate that generating HLA-G1 transgenic pigs can protect porcine grafts from macrophage-mediated cytotoxicity.
