Total Synthesis and Target Identification of the Curcusone Diterpenes.

The curcusone natural products are complex diterpenes featuring a characteristic [6-7-5] tricyclic carbon skeleton similar to the daphnane and tigliane diterpenes. Among them, curcusones A-D demonstrated potent anticancer activity against a broad spectrum of human cancer cell lines. Prior to this study, no total synthesis of the curcusones was achieved and their anticancer mode of action remained unknown. Herein, we report our synthetic and chemoproteomics studies of the curcusone diterpenes which culminate in the first total synthesis of several curcusone natural products and identification of BRCA1-associated ATM activator 1 (BRAT1) as a cellular target. Our efficient synthesis is highly convergent, builds upon cheap and abundant starting materials, features a thermal [3,3]-sigmatropic rearrangement and a novel FeCl3-promoted cascade reaction to rapidly construct the critical cycloheptadienone core of the curcusones, and led us to complete the first total synthesis of curcusones A and B in only 9 steps, C and D in 10 steps, and dimericursone A in 12 steps. The chemical synthesis of dimericursone A from curcusones C and D provided direct evidence to support the proposed Diels-Alder dimerization and cheletropic elimination biosynthetic pathway. Using an alkyne-tagged probe molecule, BRAT1, an important but previously "undruggable" oncoprotein, was identified as a key cellular target via chemoproteomics. We further demonstrate for the first time that BRAT1 can be inhibited by curcusone D, resulting in impaired DNA damage response, reduced cancer cell migration, potentiated activity of the DNA damaging drug etoposide, and other phenotypes similar to BRAT1 knockdown.

Chemoproteomics-Enabled De Novo Discovery of Photoswitchable Carboxylesterase Inhibitors for Optically Controlled Drug Metabolism.

Herein, we report arylazopyrazole ureas and sulfones as a novel class of photoswitchable serine hydrolase inhibitors and present a chemoproteomic platform for rapid discovery of optically controlled serine hydrolase targets in complex proteomes. Specifically, we identify highly potent and selective photoswitchable inhibitors of the drug-metabolizing enzymes carboxylesterases 1 and 2 and demonstrate their pharmacological application by optically controlling the metabolism of the immunosuppressant drug mycophenolate mofetil. Collectively, this proof-of-concept study provides a first example of photopharmacological tools to optically control drug metabolism by modulating the activity of a metabolizing enzyme. Our arylazopyrazole ureas and sulfones offer synthetically accessible scaffolds that can be expanded to identify specific photoswitchable inhibitors for other serine hydrolases, including lipases, peptidases, and proteases. Our chemoproteomic platform can be applied to other photoswitches and scaffolds to achieve optical control over diverse protein classes.

Combined Omics Approach Identifies Gambogic Acid and Related Xanthones as Covalent Inhibitors of the Serine Palmitoyltransferase Complex.

In this study, we identify the natural product gambogic acid as well as structurally related synthetic xanthones as first-in-class covalent inhibitors of the de novo sphingolipid biosynthesis. We apply chemoproteomics to determine that gambogic acid binds to the regulatory small subunit B of the serine palmitoyltransferase complex (SPTSSB). We then test structurally related synthetic xanthones to identify 18 as an equally potent but more selective binder of SPTSSB and show that 18 reduces sphingolipid levels in situ and in vivo. Finally, using various biological methods, we demonstrate that 18 induces cellular responses characteristic for diminished sphingosine-1-phosphate (S1P) signaling. This study demonstrates that SPTSSB may become a viable therapeutic target in various diseases with pathological S1P signaling. Furthermore, we believe that our compound will become a valuable tool for studying the sphingolipid metabolism and serve as a blueprint for the development of a new generation of sphingolipid biosynthesis inhibitors.

Small-molecule targeted recruitment of a nuclease to cleave an oncogenic RNA in a mouse model of metastatic cancer.

As the area of small molecules interacting with RNA advances, general routes to provide bioactive compounds are needed as ligands can bind RNA avidly to sites that will not affect function. Small-molecule targeted RNA degradation will thus provide a general route to affect RNA biology. A non-oligonucleotide-containing compound was designed from sequence to target the precursor to oncogenic microRNA-21 (pre-miR-21) for enzymatic destruction with selectivity that can exceed that for protein-targeted medicines. The compound specifically binds the target and contains a heterocycle that recruits and activates a ribonuclease to pre-miR-21 to substoichiometrically effect its cleavage and subsequently impede metastasis of breast cancer to lung in a mouse model. Transcriptomic and proteomic analyses demonstrate that the compound is potent and selective, specifically modulating oncogenic pathways. Thus, small molecules can be designed from sequence to have all of the functional repertoire of oligonucleotides, including inducing enzymatic degradation, and to selectively and potently modulate RNA function in vivo.

A Designed Small Molecule Inhibitor of a Non-Coding RNA Sensitizes HER2 Negative Cancers to Herceptin.

A small molecule (1) with overlapping affinity for two microRNA (miRNA) precursors was used to inform design of a dimeric compound (2) selective for one of the miRNAs. In particular, 2 selectively targets the microRNA(miR)-515 hairpin precursor to inhibit production of miR-515 that represses sphingosine kinase 1 (SK1), a key enzyme in the biosynthesis of sphingosine 1-phosphate (S1P). Application of 2 to breast cancer cells enhanced SK1 and S1P levels, triggering a migratory phenotype. Knockout of SK1, forced overexpression of miR-515, and application of a small molecule SK1 inhibitor all ablated 2's effect on phenotype, consistent with its designed mode of action. Target profiling studies via Chem-CLIP showed that 2 bound selectively to the miR-515 hairpin precursor in cells. Global neoprotein synthesis upon addition of 2 to MCF-7 breast cancer cells demonstrated 2's selectivity and upregulation of cancer-associated proteins regulated by S1P. The most upregulated protein was human epidermal growth factor receptor 2 (ERBB2/HER2), which is regulated by the SK1/S1P pathway and is normally not expressed in MCF-7 cells. Like triple negative breast cancer (TNBC) cells, the lack of HER2 renders them insusceptible to Herceptin and its antibody-drug conjugate Kadcyla. In addition to proteomics, an RNA-seq study supports that 2 has limited off target effects and other studies support that 2 is more selective than an oligonucleotide. We therefore hypothesized that 2 could sensitize MCF-7 cells to anti-HER2 therapies. Indeed, application of 2 sensitized cells to Herceptin. These results were confirmed in two other cell lines that express miR-515 and are HER2-, the hepatocellular carcinoma cell line HepG2 and the TNBC line MDA-MB-231. Importantly, normal breast epithelial cells (MCF-10A) that do not express miR-515 are not affected by 2. These observations suggest a precision medicine approach to sensitize HER2- cancers to approved anticancer medicines. This study has implications for broadening the therapeutic utility of known targeted cancer therapeutics by using a secondary targeted approach to render otherwise insensitive cells, sensitive to a targeted therapeutic.

Total Synthesis, Biological Evaluation, and Target Identification of Rare Abies Sesquiterpenoids.

Abiespiroside A (1), beshanzuenone C (2), and beshanzuenone D (3) belong to the Abies sesquiterpenoid family. Beshanzuenones C (2) and D (3) are isolated from the critically endangered Chinese fir tree species Abies beshanzuensis and demonstrated weak inhibiting activity against protein tyrosine phosphatase 1B (PTP1B). We describe herein the first total syntheses of these Abies sesquiterpenoids relying on the sustainable and inexpensive chiral pool molecule (+)-carvone. The syntheses feature a palladium-catalyzed hydrocarbonylative lactonization to install the 6,6-fused bicyclic ring system and a Dreiding-Schmidt reaction to build the oxaspirolactone moiety of these target molecules. Our chemical total syntheses of these Abies sesquiterpenoids have enabled (i) the validation of beshanzuenone C's weak PTP1B inhibiting potency, (ii) identification of new synthetic analogs with promising and selective protein tyrosine phosphatase SHP2 inhibiting potency, and (iii) preparation of azide-tagged probe molecules for target identification via a chemoproteomic approach. The latter has resulted in the identification and evaluation of DNA polymerase epsilon subunit 3 (POLE3) as one of the novel cellular targets of these Abies sesquiterpenoids and their analogs. More importantly, via POLE3 inactivation by probe molecule 29 and knockdown experiment, we further demonstrated that targeting POLE3 with small molecules may be a novel strategy for chemosensitization to DNA damaging drugs such as etoposide in cancer.

Cysteine-reactive probes and their use in chemical proteomics.

Proteomic profiling using bioorthogonal chemical probes that selectively react with certain amino acids is now a widely used method in life sciences to investigate enzymatic activities, study posttranslational modifications and discover novel covalent inhibitors. Over the past two decades, researchers have developed selective probes for several different amino acids, including lysine, serine, cysteine, threonine, tyrosine, aspartate and glutamate. Among these amino acids, cysteines are particularly interesting due to their highly diverse and complex biochemical role in our cells. In this feature article, we focus on the chemical probes and methods used to study cysteines in complex proteomes.

Strained Cyclic Disulfides Enable Cellular Uptake by Reacting with the Transferrin Receptor.

In this study, we demonstrate that appendage of a single asparagusic acid residue (AspA tag) is sufficient to ensure efficient cellular uptake and intracellular distribution of fully unprotected peptides. We apply this new delivery method to induce apoptotic response in cancer cells using long (up to 20mer) BH3 domain peptides. Moreover, to understand the molecular mechanism of the cellular uptake, we perform chemical proteomics experiments and identify the direct molecular targets of the asparagusic acid tag. Our findings document covalent bond formation between the asparagusic acid moiety and the cysteines 556 and 558 on the surface of the transferrin receptor resulting in subsequent endocytic uptake of the payload. We believe that the small size, low cellular toxicity and the efficient transferrin receptor-mediated uptake render the AspA tag highly attractive for various life science applications.

Divergent synthesis and identification of the cellular targets of deoxyelephantopins.

Herbal extracts containing sesquiterpene lactones have been extensively used in traditional medicine and are known to be rich in α,ß-unsaturated functionalities that can covalently engage target proteins. Here we report synthetic methodologies to access analogues of deoxyelephantopin, a sesquiterpene lactone with anticancer properties. Using alkyne-tagged cellular probes and quantitative proteomics analysis, we identified several cellular targets of deoxyelephantopin. We further demonstrate that deoxyelephantopin antagonizes PPARγ activity in situ via covalent engagement of a cysteine residue in the zinc-finger motif of this nuclear receptor.

Chemoproteomics-Enabled Discovery of a Potent and Selective Inhibitor of the DNA Repair Protein MGMT.

We present a novel chemical scaffold for cysteine-reactive covalent inhibitors. Chloromethyl triazoles (CMTs) are readily accessed in only two chemical steps, thus enabling the rapid optimization of the pharmacological properties of these inhibitors. We demonstrate the tunability of the CMTs towards a specific biological target by synthesizing AA-CW236 as the first potent non-pseudosubstrate inhibitor of the O(6) -alkylguanine DNA methyltransferase (MGMT), a protein of major clinical significance for the treatment of several severe cancer forms. Using quantitative proteomics profiling techniques, we show that AA-CW236 exhibits a high degree of selectivity towards MGMT. Finally, we validate the effectiveness of our MGMT inhibitor in combination with the DNA alkylating drug temozolomide in breast and colon cancer cells by fluorescence imaging and a cell-viability assay. Our results may open a new avenue towards the development of a clinically approved MGMT inhibitor.

Cysteine-specific Chemical Proteomics: From Target Identification to Drug Discovery.

Our laboratory focuses on chemical proteomics-enabled discovery of new cysteine-reactive small molecules with intriguing biomedical activities as well as identification and detailed characterization of their proteomic targets. In this overview article, we summarize our progress since 2013 in this research field. We have developed a novel mass spectrometry-based chemoproteomic method that allows detection and monitoring of up to ~3000 reactive cysteines in any cellular proteome. This is achieved via strategic use of two clickable, cysteine-reactive chemical probes with complementary substrate selectivity profiles, iodoacetamide and ethynyl benziodoxolone. Using this method, we have been able to identify the direct biological targets of curcumin, a diarylheptanoid natural product with anticancer activity, and deoxyelephantopin, a highly cytotoxic natural sesquiterpene lactone. Furthermore, we have developed chloromethyl triazoles (CMTs) as a novel chemical scaffold for cysteine-reactive inhibitors that can be accessed from commercially available substrates in only two chemical steps. From a small collection of chloromethyl triazoles, we have identified compound AA-CW236 as the first non-pseudosubstrate inhibitor of MGMT, a DNA repair protein that renders several devastating cancer forms resistant to chemotherapy.