A selective Seoul-Fluor-based bioprobe, SfBP, for vaccinia H1-related phosphatase--a dual-specific protein tyrosine phosphatase

Polyphyllin D Shows Anticancer Effect through a Selective Inhibition of Src Homology Region 2-Containing Protein Tyrosine Phosphatase-2 (SHP2)

Natural products have continued to offer tremendous opportunities for drug development, as they have long been used in traditional medicinal systems. SHP2 has served as an anticancer target. To identify novel SHP2 inhibitors with potential anticancer activity, we screened a library containing 658 natural products. Polyphyllin D was found to selectively inhibit SHP2 over SHP1, whereas two other identified compounds (echinocystic acid and oleanolic acid) demonstrated dual SHP1 and SHP2 inhibition. In a cell-based assay, polyphyllin D exhibited cytotoxicity in Jurkat cells, an acute lymphoma leukemia cell line, whereas the other two compounds were ineffective. Polyphyllin D also decreased the level of phosphorylated extracellular signal-regulated kinase (p-ERK), a proliferation marker in Jurkat cells. Furthermore, knockdown of protein tyrosine phosphatase (PTP)N6 (SHP1) or PTPN11 (SHP2) decreased p-ERK levels. However, concurrent knockdown of PTPN6 and PTPN11 in Jurkat cells recovered p-ERK levels. These results demonstrated that polyphyllin D has potential anticancer activity, which can be attributed to its selective inhibition of SHP2 over SHP1.

Ultrasound-promoted regioselective synthesis of chalcogeno-indolizines by a stepwise 1,3-dipolar cycloaddition

A series of new organochalcogen derivatives of indolizines was synthesized in moderate to excellent yields from pyridinium salts and chalcogeno-alkynes. The reaction can be carried out under thermal conditions or by sonochemical processes in short reaction times. The stepwise cycloaddition reaction forming chalcogeno-indolizines is regioselective and extends to a broad range of functional groups. Furthermore, novel chalcogeno-alkynes are reported and the first derivatives of teluro-indolizine are described. The influence of selenium functionalization on the photophysical properties of indolizines is also described, in which the compounds showed absorption in the UV-Vis region around 360 nm and emission in the blue-to-green region. Relatively low fluorescence quantum yield (ϕ_fl) values were calculated, in agreement with the chalcogen effect on other heterocycles.

Interrogating Protein Phosphatases with Chemical Activity Probes

Protein phosphatases, while long overlooked, have recently become appreciated as drivers of both normal- and disease-associated signaling events. As a result, the spotlight is now turning torwards this enzyme family and efforts geared towards the development of modern chemical tools for studying these enzymes are well underway. This Minireview focuses on the evolution of chemical activity probes, both optical and covalent, for the study of protein phosphatases. Small-molecule probes, global monitoring of phosphatase activity through the use of covalent modifiers, and targeted fluorescence-based activity probes are discussed. We conclude with an overview of open questions in the field and highlight the potential impact of chemical tools for studying protein phosphatases.

Identification of sennoside A as a novel inhibitor of the slingshot (SSH) family proteins related to cancer metastasis

Phospho-cofilin (p-cofilin), which has a phosphate group on Ser-3, is involved in actin polymerization. Its dephosphorylated form promotes filopodia formation and cell migration by enhancing actin depolymerization. Protein phosphatase slingshot homologs (SSHs), known as dual-specificity phosphatases, catalyze hydrolytic removal of the Ser-3 phosphate group from phospho-cofilin. Aberrant SSH activity results in cancer metastasis, implicating SSHs as potential therapeutic targets for cancer metastasis. In this study, we screened 658 natural products purified from traditional oriental medicinal plants to identify three potent SSH inhibitors with submicromolar or single-digit micromolar K_i values: gossypol, hypericin, and sennoside A. The three compounds were purified from cottonseed, Saint John's wort, and rhubarb, respectively. Sennoside A markedly increased cofilin phosphorylation in pancreatic cancer cells, leading to impaired actin dynamics in pancreatic cancer cells with or without EGF stimulation and reduced motility and invasiveness in vitro and in vivo. Collaboratively, these results demonstrate that sennoside A is a novel inhibitor of SSHs and suggest that it may be valuable in the development of pharmaceutical drugs for treating cancer metastasis.

Structural Insight into the Critical Role of the N-Terminal Region in the Catalytic Activity of Dual-Specificity Phosphatase 26

Human dual-specificity phosphatase 26 (DUSP26) is a novel target for anticancer therapy because its dephosphorylation of the p53 tumor suppressor regulates the apoptosis of cancer cells. DUSP26 inhibition results in neuroblastoma cell cytotoxicity through p53-mediated apoptosis. Despite the previous structural studies of DUSP26 catalytic domain (residues 61-211, DUSP26-C), the high-resolution structure of its catalytically active form has not been resolved. In this study, we determined the crystal structure of a catalytically active form of DUSP26 (residues 39-211, DUSP26-N) with an additional N-terminal region at 2.0 Å resolution. Unlike the C-terminal domain-swapped dimeric structure of DUSP26-C, the DUSP26-N (C152S) monomer adopts a fold-back conformation of the C-terminal α8-helix and has an additional α1-helix in the N-terminal region. Consistent with the canonically active conformation of its protein tyrosine phosphate-binding loop (PTP loop) observed in the structure, the phosphatase assay results demonstrated that DUSP26-N has significantly higher catalytic activity than DUSP26-C. Furthermore, size exclusion chromatography-multiangle laser scattering (SEC-MALS) measurements showed that DUSP26-N (C152S) exists as a monomer in solution. Notably, the crystal structure of DUSP26-N (C152S) revealed that the N-terminal region of DUSP26-N (C152S) serves a scaffolding role by positioning the surrounding α7-α8 loop for interaction with the PTP-loop through formation of an extensive hydrogen bond network, which seems to be critical in making the PTP-loop conformation competent for phosphatase activity. Our study provides the first high-resolution structure of a catalytically active form of DUSP26, which will contribute to the structure-based rational design of novel DUSP26-targeting anticancer therapeutics.

Unique photophysical properties of 9-styryl-1,2-dihydropyrrolo[3,4-β]indolizin-3-one and its efficient synthesis via direct C-H activation

A styryl Seoul-Fluor (SF) skeleton was rationally designed by introducing an olefin unit at the C-9 of 1,2-dihydropyrrolo[3,4-β]indolizin-3-one via regioselective direct C-H activation. We synthesized a series of styryl SF analogues, maintaining unique photophysical properties that can be systematically controlled by the electronic characteristics of the substituents, with an average bathochromic shift of 39 nm in emission maxima.

A pyrazolo[1,5-a]pyridine-fused pyrimidine based novel fluorophore and its bioapplication to probing lipid droplets

Pyrimidine-containing novel organic fluorophores were discovered and successively applied to monitor the lipid droplets in live cellular systems.

Discovery, understanding, and bioapplication of organic fluorophore: a case study with an indolizine-based novel fluorophore, Seoul-Fluor

Owing to its high sensitivity and great applicability, the fluorescence phenomenon has been considered as an inevitable research tool in the modern scientific fields of chemistry, biology, materials science, biomedical science, and their interfaces. Many strategies have been pursued to understand and manipulate the photophysical properties of fluorescent materials, but the scientific community has been focused on the repeated application of existing organic fluorophores or the identification of unique fluorescence properties in a trial-and-error basis without systematic studies. Moreover, recent studies are emphasizing the necessity of deeper understanding about the structure-photophysical property relationship of organic fluorophores for the development of better fluorescent probes. Herein, we provide an overview of a novel fluorescent molecular framework, Seoul-Fluor, which can be rationally engineered to furnish a wide variety of fluorophores in terms of the photophysical properties. Seoul-Fluor is built on an indolizine-based fluorescent platform with three different positions to introduce various substituents: R(1) and R(2) substituents for electronic perturbation; R(3) substituent as a functional handle for bioconjugation. Over the past decade, we have demonstrated that the Seoul-Fluor system has (i) tunable and predictable emission wavelength covering a full visible-color range; (ii) controllable quantum yield via photoinduced electron transfer phenomenon; and (iii) environment-sensitive fluorogenic properties that can be modified through intramolecular charge transfer processes. We convincingly demonstrated the prediction of photophysical properties, that is, emission wavelength and quantum yield, through the construction of a systematic set of analogues and the subsequent analysis of their photophysical properties without the highly sophisticated theoretical support. Guided by quantifiable parameters such as the Hammett substituent constants or energy levels of the molecular orbitals, this unique organic fluorophore can serve as a versatile molecular platform for the development of novel fluorescent switchable biosensors and fluorogenic bioprobes. In this Account, we will discuss the discovery and recent progress made on Seoul-Fluor, the rational design of Seoul-Fluor-based bioprobes, and their practical applications to specific biological processes that are facilitated by systematic studies of the structure-photophysical property relationships.

Construction of benzo-fused indolizines, pyrrolo[1,2-a]quinolines via alkyne–carbonyl metathesis

The strategic use of a sequential Sonogashira coupling/intramolecular alkyne–carbonyl metathesis process for the synthesis of a pyridine ring from 1-(2-haloaryl)-1H-pyrrole-2-carbaldehydes allowed ready access to diverse novel benzo-fused indolizines, pyrrolo[1,2-a]quinolines, in good to excellent yields.

Rational perturbation of the fluorescence quantum yield in emission-tunable and predictable fluorophores (Seoul-Fluors) by a facile synthetic method involving C-H activation

Fluorescence imaging enables the uniquely sensitive observation of functional‐ and molecular‐recognition events in living cells. However, only a limited range of biological processes have been subjected to imaging because of the lack of a design strategy and difficulties in the synthesis of biosensors. Herein, we report a facile synthesis of emission‐tunable and predictable Seoul‐Fluors, 9‐aryl‐1,2‐dihydrolopyrrolo[3,4‐b]indolizin‐3‐ones, with various R¹ and R² substituents by coinage‐metal‐catalyzed intramolecular 1,3‐dipolar cycloaddition and subsequent palladium‐mediated C—H activation. We also showed that the quantum yields of Seoul‐Fluors are controlled by the electronic nature of the substituents, which influences the extent of photoinduced electron transfer. On the basis of this understanding, we demonstrated our design strategy by the development of a Seoul‐Fluor‐based chemosensor 20 for reactive oxygen species that was not accessible by a previous synthetic route.