(19)F NMR monitoring of the eukaryotic 20S proteasome chymotrypsin-like activity: an investigative tool for studying allosteric regulation

(R)-α-Trifluoromethylalanine as a 19 F NMR Probe for the Monitoring of Protease Digestion of Peptides

Fluorinated non-natural amino acids are useful tools for improving the bioavailability of peptides but can also serve as fluorinated probes in ¹⁹ F NMR-based enzymatic assays. We report herein that the use of the non-natural α-quaternarized (R)-α-trifluoromethylalanine ((R)-α-TfmAla) provides convenient and accurate monitoring of trypsin proteolytic activity and increases resistance towards pepsin degradation.

Impaired mRNA splicing and proteostasis in preadipocytes in obesity-related metabolic disease

Preadipocytes are crucial for healthy adipose tissue expansion. Preadipocyte differentiation is altered in obese individuals, which has been proposed to contribute to obesity-associated metabolic disturbances. Here, we aimed at identifying the pathogenic processes underlying impaired adipocyte differentiation in obese individuals with insulin resistance (IR)/type 2 diabetes (T2D). We report that down-regulation of a key member of the major spliceosome, PRFP8/PRP8, as observed in IR/T2D preadipocytes from subcutaneous (SC) fat, prevented adipogenesis by altering both the expression and splicing patterns of adipogenic transcription factors and lipid droplet-related proteins, while adipocyte differentiation was restored upon recovery of PRFP8/PRP8 normal levels. Adipocyte differentiation was also compromised under conditions of endoplasmic reticulum (ER)-associated protein degradation (ERAD) hyperactivation, as occurs in SC and omental (OM) preadipocytes in IR/T2D obesity. Thus, targeting mRNA splicing and ER proteostasis in preadipocytes could improve adipose tissue function and thus contribute to metabolic health in obese individuals.

Fluorine NMR functional screening: from purified enzymes to human intact living cells

The substrate- or cofactor-based fluorine NMR screening, also known as n-FABS (n fluorine atoms for biochemical screening), represents a powerful method for performing a direct functional assay in the search of inhibitors or enhancers of an enzymatic reaction. Although it suffers from the intrinsic low sensitivity compared to other biophysical techniques usually applied in functional assays, it has some distinctive features that makes it appealing for tackling complex chemical and biological systems. Its strengths are represented by the easy set-up, robustness, flexibility, lack of signal interference and rich information content resulting in the identification of bona fide inhibitors and reliable determination of their inhibitory strength. The versatility of the n-FABS allows its application to either purified enzymes, cell lysates or intact living cells. The principles, along with theoretical, technical and practical aspects, of the methodology are discussed. Furthermore, several applications of the technique to pharmaceutical projects are presented.

Development and Application of a Sensitive Peptide Reporter to Discover 20S Proteasome Stimulators

To attenuate an overabundance of cellular protein, it has been hypothesized that the 20S core particle (20S CP) of the proteasome can be chemically stimulated to degrade proteins into nontoxic peptides more quickly. Screening for small molecule 20S CP stimulators is typically performed with a reporter peptide composed of four amino acids and a coumarin group that is released upon proteasome-mediated hydrolysis to generate a fluorescent signal. Screening with this small reporter can lead to false negatives because the reporter peptide is rapidly turned-over without stimulation. To improve the screening for 20S CP stimulators, we have developed a peptide FRET reporter nearly four times more sensitive to stimulation but still amenable for high throughput screening. Through application of our FRET reporter, we have discovered two 20S CP gate-opening stimulators and also a molecule that elicits its mechanism of action through an interaction with a 20S CP active site.

Strategic Incorporation of Fluorine into Taxoid Anticancer Agents for Medicinal Chemistry and Chemical Biology Studies

This account exemplifies our recent progress on the strategic incorporation of fluorine and organofluorine groups to taxoid anticancer agents and their tumor-targeted drug delivery systems (TTDDSs) for medicinal chemistry and chemical biology studies. Novel 3'-difluorovinyltaxoids were strategically designed to block the metabolism by cytochrome P-450, synthesized, and evaluated for their cytotoxicity against drug-sensitive and multidrug-resistant (MDR) human cancer cell lines. 3'-Difluorovinyltaxoids exhibited impressive activities against these cancer cell lines. More significantly, a representative 3'-difluorovinyltaxoid exhibited 230-33,000 times higher potency than conventional anticancer drugs against cancer stem cell-enriched HCT-116 cell line. Studies on the mechanism of action (MOA) of these fluorotaxoids were performed by tubulin polymerization assay, morphology analysis by electron microscopy (EM) and protein binding assays. Novel ¹⁹F NMR probes, BLT-F₂ and BLT-S-F₆, were designed by strategically incorporating fluorine, CF₃ and CF₃O groups into tumor-targeting drug conjugates. These ¹⁹F-probes were designed and synthesized to investigate the mechanism of linker cleavage and factors that influence their plasma and metabolic stability by real-time ¹⁹F NMR analysis. Time-resolved ¹⁹F NMR study on probe BLT-F₂ revealed a stepwise mechanism for the release of a fluorotaxoid, which might not be detected by other analytical methods. Probe BLT-S-F₆ were very useful to study the stability and reactivity of the drug delivery system in human blood plasma by ¹⁹F NMR. The clean analysis of the linker stability and reactivity of drug conjugates in blood plasma by HPLC and ¹H NMR is very challenging, but the use of ¹⁹F NMR and suitable ¹⁹F probes can provide a practical solution to this problem.

Fluorine nuclear magnetic resonance-based assay in living mammalian cells

Nuclear magnetic resonance (NMR)-based screening has been recognized as a powerful approach for the identification and characterization of molecules interacting with pharmaceutical targets. Indeed, several NMR methods have been developed and successfully applied to many drug discovery projects. Whereas most of these approaches have targeted isolated biomolecular receptors, very few cases are reported with the screening performed in intact cells and cell extracts. Here we report the first successful application of the fluorine NMR-based assay n-FABS (n-fluorine atoms for biochemical screening) in living mammalian cells expressing the membrane protein fatty acid amide hydrolase (FAAH). This method allows the identification of both weak and potent inhibitors and the measurement of their potency in a physiological environment.

Molecular characterization of the boron adducts of the proteasome inhibitor bortezomib with epigallocatechin-3-gallate and related polyphenols

The green tea polyphenol epigallocatechin-3-gallate (EGCG) was reported to effectively antagonize the ability of Bortezomib (BZM) to induce apoptosis in cancer cells. This interaction was attributed to the formation of a covalent adduct between a phenolic moiety of EGCG with the boronic acid group of Bortezomib. However, the structural details of this boron adduct and the molecular factors that contribute to its formation and its ability to inhibit Bortezomib's activity remain unclear. This paper describes the use of NMR spectroscopy and cell assays to characterize the structures and properties of the boron adducts of EGCG and related polyphenols. The observed boron adducts included both boronate and borate derivatives, and their structural characteristics were correlated with cell-based evaluation of the ability of EGCG and other phenols to antagonize the anticancer activity of Bortezomib. The enhanced stability of the BZM/EGCG adduct was attributed to electronic and steric reasons, and a newly identified intramolecular interaction of the boron atom of BZM with the adjacent amide bond. The reported approach provides a useful method for determining the potential ability of polyphenols to form undesired adducts with boron-based drugs and interfere with their actions.

Chaperones and chaperone-substrate complexes: Dynamic playgrounds for NMR spectroscopists

The majority of proteins depend on a well-defined three-dimensional structure to obtain their functionality. In the cellular environment, the process of protein folding is guided by molecular chaperones to avoid misfolding, aggregation, and the generation of toxic species. To this end, living cells contain complex networks of molecular chaperones, which interact with substrate polypeptides by a multitude of different functionalities: transport them towards a target location, help them fold, unfold misfolded species, resolve aggregates, or deliver them towards a proteolysis machinery. Despite the availability of high-resolution crystal structures of many important chaperones in their substrate-free apo forms, structural information about how substrates are bound by chaperones and how they are protected from misfolding and aggregation is very sparse. This lack of information arises from the highly dynamic nature of chaperone-substrate complexes, which so far has largely hindered their crystallization. This highly dynamic nature makes chaperone-substrate complexes good targets for NMR spectroscopy. Here, we review the results achieved by NMR spectroscopy to understand chaperone function in general and details of chaperone-substrate interactions in particular. We assess the information content and applicability of different NMR techniques for the characterization of chaperones and chaperone-substrate complexes. Finally, we highlight three recent studies, which have provided structural descriptions of chaperone-substrate complexes at atomic resolution.

Design, Synthesis and Application of Fluorine-Labeled Taxoids as 19F NMR Probes for the Metabolic Stability Assessment of Tumor-Targeted Drug Delivery Systems

Novel tumor-targeting drug conjugates, BLT-F₂ (1) and BLT-S-F₆ (2), bearing a fluorotaxoid as the warhead, a mechanism-based self-immolative disulfide linker, and biotin as the tumor-targeting module, were designed and synthesized as ¹⁹F NMR probes. Fluorine atoms and CF₃ groups were strategically incorporated into the conjugates to investigate the mechanism of linker cleavage and factors that influence their plasma and metabolic stability by real-time monitoring with ¹⁹F NMR. Time-resolved ¹⁹F NMR study on probe 1 disclosed a stepwise mechanism for release of a fluorotaxoid, which might not have been detected by other analytical methods. Probe 2 was designed to bear two CF₃ groups in the taxoid moiety as "3-FAB" reporters for enhanced sensitivity and a polyethylene glycol oligomer insert to improve solubility. The clean analysis of the linker stability and reactivity of drug conjugates in blood plasma or cell culture media by HPLC and ¹H NMR is troublesome, due to the overlap of key signals/peaks with background arising from highly complex ingredients in biological systems. Accordingly, the use of ¹⁹F NMR would provide a practical solution to this problem. In fact, our "3-FAB" probe 2 was proven to be highly useful to investigate the stability and reactivity of the self-immolative disulfide linker system in human blood plasma by ¹⁹F NMR. It has also been revealed that the use of polysorbate 80 as excipient for the formulation of probe 2 dramatically increases the stability of the disulfide linker system. This finding further indicates that the tumor-targeting drug conjugates with polysorbate 80/EtOH/saline formulation for in vivo studies would have high stability in blood plasma, while the drug release in cancer cells proceeds smoothly.

Computational19F NMR. 2. Organic compounds

DFT calculations allow to model and predict¹⁹F NMR chemical shift in a wide variety of organofluorine molecules.