Thymidine kinase (TK), encoded by EBV (Epstein-Barr virus), is an attractive target for antiviral therapy and provides a novel approach to the treatment of EBV-associated malignancies. Despite the extensive use of nucleoside analogues for the treatment of viral infections and cancer, the structure-function relationship of EBV TK has been addressed rarely. In the absence of any structural information, we sought to identify and elucidate the functional roles of amino acids in the nucleoside-binding site using site-directed mutagenesis. Through alignment with other human herpesviral TK protein sequences, we predicted that certain conserved regions comprise the nucleoside-binding site of EBV TK and, through site-directed mutagenesis, showed significant changes in activity and binding affinity for thymidine of site 3 (-DRH-) and 4 (-VFP-) mutants. For site 3, only mutants D392E (Asp 392 → Glu) and R393H retain activity, indicating that a negative charge is important for Asp 392 and a positive charge is required for Arg 393 . The increased binding affinities of these two mutants for 3 -deoxy-2 ,3 -didehydrothymidine suggest that the two residues are also important for substrate selection. Interestingly, the changed metal-ion usage pattern of D392E reveals that Asp 392 plays multiple roles in this region. His 394 cannot be compensated by other amino acids, also indicating a crucial role. In site 4, the F402Y mutant retains full activity; however, F402S retains only 60 % relative activity. Strikingly, when Phe 402 is substituted with serine residue, the original preferred pyrimidine substrates, such as 3 -azido-3 -deoxythymidine, iododeoxyuridine and β-L-5-iododioxolane uracil (L-form substrate), have decreased competitiveness with thymidine, suggesting that Phe 402 plays a crucial role in substrate specificity and that the aromatic ring is important for function.

Desmogleins (Dsg2) are the major components of desmosomes. Dsg2 has five extracellular tandem cadherin domains (EC1-EC5) for cell-cell interaction. We had previously confirmed the Dsg2 antibody and its epitope (named KC21) derived from EC2 domain suppressing epithelial-mesenchymal transition and invasion in human cancer cell lines. Here, we screened six peptide fragments derived from EC2 domain and found that KR20, the parental peptide of KC21, was the most potent one on suppressing endothelial colony-forming cell (ECFC) tube-like structure formation. KC21 peptide also attenuated migration but did not disrupt viability and proliferation of ECFCs, consistent with the function to inhibit VEGF-mediated activation of p38 MAPK but not AKT and ERK. Animal studies showed that KC21 peptides suppressed capillary growth in Matrigel implant assay and inhibited oxygen-induced retinal neovascularization. The effects were comparable to bevacizumab (Bev). In conclusion, KC21 peptide is an angiogenic inhibitor potentially useful for treating angiogenesis-related diseases. Electronic supplementary material The online version of this article (10.1007/s12265-019-09865-6) contains supplementary material, which is available to authorized users.

AbstractMetastasis is the major cause of cancer death. Metastatic cancer cells that have intravasated into the circulatory system and formed clusters of circulating tumor cells (CTCs) are particularly associated with colonization of distant organs and poor prognosis. However, the key factors required for tumor cell dissemination and CTC clustering remain elusive. We found that high expression of Desmoglein2 (DSG2), a cell adhesion molecule involved in desmosome mediated intercellular adhesion, promoted tumor growth, increased the prevalence of CTC clusters and facilitated distant organ colonization. The dynamic regulation of DSG2 by hypoxia was key to this process as downregulation of DSG2 in hypoxic regions of primary tumors led to elevated epithelial-mesenchymal transition (EMT) gene expression allowing cells to detach from the primary tumor and undergo intravasation. Subsequent derepression of DSG2 after intravasation and release of hypoxic stress allowed CTCs to colonize distant organs. This dynamic regulation of DSG2 was mediated by Hypoxia-Induced Factor1α (HIF1α). In contrast to its more widely observed function to promote expression of hypoxia-inducible genes, HIF1α repressed DSG2 by recruitment of the Polycomb Repressive Complex 2 components, EZH2 and SUZ12, to the DSG2 promoter in hypoxic cells. Consistent with our experimental data, DSG2 expression level correlated with poor prognosis and recurrence risk in breast cancer patients. Together, these results demonstrated the importance of DSG2 expression in metastasis and revealed a new mechanism by which hypoxia drives metastasis.Significant StatementDuring metastasis, a hypoxic microenvironment is a major force driving primary tumor cells to disseminate into the circulatory system. The majority of these circulating tumor cells (CTCs) cannot survive when traveling alone. However, collective movement as CTC clusters enables them to avoid immune surveillance and increases the probability that they will successfully metastasize to distal organs. Dynamic down regulation of DSG2 in hypoxic tumor tissue and reactivation of expression in CTCs allowed high rates of tumor cell dissemination, CTC cluster formation and metastasis that are characteristics of the most aggressive breast cancers. The findings highlight both the potential, and potential pitfalls, of using DSG2 as a therapeutic target.