A Peptide-Based Ligand-Directed Chemistry Enables Protein Functionalization.

The ligand-directed (LD) chemistry provides powerful tools for site-specific modification of proteins. We utilized a peptide with an appended methionine (Met) as a ligand; then, the Met thioether was modified into sulfonium which enabled a proximity induced group transfer onto protein cysteine in the vicinity upon peptide-target binding. The sulfonium warhead could be easily constructed with unprotected peptides, and the transferable group scope was conducted on model protein PDZ and its ligand peptides. In addition, a living cell labeling was successfully achieved.

Solid phase diversity-oriented lysine modification of cyclic peptides.

Herein, we report a novel strategy for diversity-oriented lysine modification of cyclic peptides based on the orthogonal alkylation of the lysine residues. All steps can be achieved in the solid phase with satisfying conversions. Notably, we demonstrated that the tether modification could help to improve the cellular uptake of peptides.

Selective Covalent Targeting of Anti-apoptotic BFL-1 by a Sulfonium-Tethered Peptide.

Anti-apoptotic B cell lymphoma 2 (BCL-2) family proteins are proven targets for human cancers. Targeting the BH3-binding pockets of these anti-apoptotic proteins could reactivate apoptosis in BCL-2-depedent cancers. BFL-1 is a BCL-2 family protein overexpressed in various chemoresistant cancers. A unique cysteine at the binding interface of the BH3 and BFL-1 was previously proven to be an intriguing targeting site to irreversibly inhibit BFL-1 functions with stabilized cyclic peptide bearing a covalent warhead. Recently, we developed a sulfonium-tethered peptide cyclization strategy to construct peptide ligands that could selectively and efficiently react with the cysteine(s) of target proteins near the interacting interface. Using this method, we constructed a BFL-1 peptide inhibitor, B4-MC, that could selectively conjugate with BFL-1 both in vitro and in cell. B4-MC showed good cellular uptake, colocalized with BFL-1 on mitochondria, and showed obvious growth inhibition of BFL-1 over-expressed cancer cell lines.

Intramolecular methionine alkylation constructs sulfonium tethered peptides for protein conjugation.

Continuous efforts have been invested in the selective modification of proteins. Herein, we first report the construction of sulfonium tethered cyclic peptides via an intramolecular cyclization by an aliphatic halide. This cyclization could enhance the stability and cellular uptake of peptides. Furthermore, the sulfonium center could be recognized by cysteine in the vicinity of the protein-peptide interacting interface and form a peptide-protein conjugate.

A Sulfonium Triggered Thiol-yne Reaction for Cysteine Modification.

We report a facile thiol-yne type reaction triggered by the sulfonium center. After facile propargylation of thiolethers, the resulting sulfonium could undergo facile addition with thiols in aqueous media at ambient temperature. Further applying this reaction in unprotected peptides bearing neighboring methionine and cysteine could enable a facile intramolecular addition to construct cyclic peptides with better stability, good glutathione resistance, and increased cellular uptakes. Also, the propargylated sulfonium may be used as robust and versatile probes to target cysteines containing biomolecules.

A sulfonium tethered peptide ligand rapidly and selectively modifies protein cysteine in vicinity.

Significant efforts have been invested to develop site-specific protein modification methodologies in the past two decades. In most cases, a reactive moiety was installed onto ligands with the sole purpose of reacting with specific residues in proteins. Herein, we report a unique peptide macrocyclization method via the bis-alkylation between methionine and cysteine to generate cyclic peptides with significantly enhanced stability and cellular uptake. Notably, when the cyclized peptide ligand selectively recognizes its protein target with a proximate cysteine, a rapid nucleophilic substitution could occur between the protein Cys and the sulfonium center on the peptide to form a conjugate. The conjugation reaction is rapid, facile and selective, triggered solely by proximity. The high target specificity is further proved in cell lysate and hints at its further application in activity based protein profiling. This method enhances the peptide's biophysical properties and generates a selective ligand-directed reactive site for protein modification and fulfills multiple purposes by one modification. This proof-of-concept study reveals its potential for further broad biological applications.

Stabilized Peptide HDAC Inhibitors Derived from HDAC1 Substrate H3K56 for the Treatment of Cancer Stem-Like Cells In Vivo.

FDA-approved HDAC inhibitors exhibit dose-limiting adverse effects; thus, we sought to improve the therapeutic windows for this class of drugs. In this report, we describe a new class of peptide-based HDAC inhibitors derived from the HDAC1-specific substrate H3K56 with improved nonspecific toxicity compared with traditional small-molecular inhibitors. We showed that our designed peptides exerted superior antiproliferation effects on cancer stem-like cells with minimal toxicity to normal cells compared with the small-molecular inhibitor SAHA, which showed nonspecific toxicity to normal and cancer cells. These peptide inhibitors also inactivated cellular HDAC1 and HDAC6 and disrupted the formation of the HDAC1, LSD1, and CoREST complex. In ovarian teratocarcinoma (PA-1) and testicular embryonic carcinoma (NTERA-2) cell xenograft animal models (5 mice/group, 50 mg/kg, every other day, intraperitoneal injection), these peptides inhibited tumor growth by 80% to 90% with negligible organ (heart, liver, spleen, lung, kidney, brain) lesions. These results represent the first attempt to design chemically stabilized peptide inhibitors to investigate HDAC inhibition in cancer stem-like cells. These novel peptide inhibitors have significantly enhanced therapeutic window and offer promising opportunities for cancer therapy. SIGNIFICANCE: Selective antiproliferative effects of stabilized peptide HDAC inhibitors toward cancer stem-like cells provide a therapeutic alternative that avoids high nonspecific toxicity of current drugs.Graphical Abstract: http://cancerres.aacrjournals.org/content/canres/79/8/1769/F1.large.jpg.

Autophagy inducing cyclic peptides constructed by methionine alkylation.

Peptides that induced autophagy at micromolar concentrations with improved proteolytic resistance properties were generated using the facile methionine bis-alkylation method. Notably, a short bicyclic peptide 7f was proven to be the most potent one among the designed peptides in regards to autophagy inducing activity. This study facilitated the development of a peptide-based autophagy inducer and demonstrated the potential applications of the methionine alkylation-based macrocyclization method for the diversity-oriented generation of peptide-based autophagy inducers.

The Development of Epigenetics and Related Inhibitors for Targeted Drug Design in Cancer Therapy.

Epigenetics process is the heritable change in gene function that does not involve changes in the DNA sequence. Until now, several types of epigenetic mechanisms have been characterized, including DNA methylation, histone modification (acetylation, methylation, etc.), nucleosome remodeling, and noncoding RNAs. With the biological investigations of these modifiers, some of them are identified as promoters in the process of various diseases, such as cancer, cardiovascular disease and virus infection. Epigenetic changes may serve as potential "first hits" for tumorigenesis. Hence, targeting epigenetic modifiers is being considered as a promising way for disease treatment. To date, six agents in two epigenetic target classes (DNMT and HDAC) have been approved by the US Food and Drug Administration (FDA). Most of these drugs are applied in leukemia, lymphoma therapy, or are combined with other drugs for the treatment of solid tumor. Due to the rapid development of epigenetics and epigenetics targeted drugs, it is becoming an emerging area in targeted drug design.

Constructing Thioether/Vinyl Sulfide-tethered Helical Peptides Via Photo-induced Thiol-ene/yne Hydrothiolation.

Here, we describe a detailed protocol for the preparation of thioether-tethered peptides using on-resin intramolecular/intermolecular thiol-ene hydrothiolation. In addition, this protocol describes the preparation of vinyl-sulfide-tethered peptides using in-solution intramolecular thiol-yne hydrothiolation between amino acids that possess alkene/alkyne side chains and cysteine residues at i, i+4 positions. Linear peptides were synthesized using a standard Fmoc-based solid-phase peptide synthesis (SPPS). Thiol-ene hydrothiolation is carried out using either an intramolecular thio-ene reaction or an intermolecular thio-ene reaction, depending on the peptide length. In this research, an intramolecular thio-ene reaction is carried out in the case of shorter peptides using on-resin deprotection of the trityl groups of cysteine residues following the complete synthesis of the linear peptide. The resin is then set to UV irradiation using photoinitiator 4-methoxyacetophenone (MAP) and 2-hydroxy-1-[4-(2-hydroxyethoxy)-phenyl]-2-methyl-1-propanone (MMP). The intermolecular thiol-ene reaction is carried out by dissolving Fmoc-Cys-OH in an N,N-dimethylformamide (DMF) solvent. This is then reacted with the peptide using the alkene-bearing residue on resin. After that, the macrolactamization is carried out using benzotriazole-1-yl-oxytripyrrolidinophosphonium hexafluorophosphate (PyBop), 1-hydroxybenzotriazole (HoBt), and 4-Methylmorpholine (NMM) as activation reagents on the resin. Following the macrolactamization, the peptide synthesis is continued using standard SPPS. In the case of the thio-yne hydrothiolation, the linear peptide is cleaved from the resin, dried, and subsequently dissolved in degassed DMF. This is then irradiated using UV light with photoinitiator 2,2-dimethoxy-2-phenylacetophenone (DMPA). Following the reaction, DMF is evaporated and the crude residue is precipitated and purified using high-performance liquid chromatography (HPLC). These methods could function to simplify the generation of thioether-tethered cyclic peptides due to the use of the thio-ene/yne click chemistry that possesses superior functional group tolerance and good yield. The introduction of thioether bonds into peptides takes advantage of the nucleophilic nature of cysteine residues and is redox-inert relative to disulfide bonds.

Stabilized ß-Hairpin Peptide Inhibits Insulin Degrading Enzyme.

Insulin-degrading enzyme (IDE) plays a critical role in both the proteolytic degradation and inactivation of insulin. The exploration of novel IDE inhibitors could aid in the study of novel therapeutics for type-2 diabetes. Herein, we report a hypothesized stabilized ß-hairpin peptide that can efficiently inhibit the enzymatic activity of IDE. The resulting stabilized peptide B35 is demonstrated to activate the AKT phosphorylation pathway in skeletal muscle cells and is shown to slow insulin degradation. An 80 mg kg-1 intraperitoneal (i.p.) injection of the stabilized ß-hairpin peptide B35 is demonstrated to improve glucose tolerance during an oral glucose tolerance test in obese mouse model. We note that this stabilized peptide exhibited negligible cytotoxicity in both in vitro and in vivo assays, even at high concentrations (300 µM). This study suggests that IDE peptide inhibitors could function as potentially meaningful candidates for the development of type-2 diabetes therapeutics.

Reversible stapling of unprotected peptides via chemoselective methionine bis-alkylation/dealkylation.

We have developed a general peptide macrocyclization strategy that involves a facile and chemoselective methionine bis-alkylation/dealkylation process. This method provides a straightforward and easy approach to generate cyclic peptides with tolerances of all amino acids (including Cys), variable loop sizes, and different linkers. The Met bis-alkylation we apply in this strategy yields two additional on-tether positive charges that could assist in the cellular uptake of the peptides. Notably, the bis-alkylated peptide could be reduced to release the original peptide both in vitro and within cellular environments. This strategy provides an intriguing and facile traceless post-peptide-synthesis modification with enhanced cellular uptakes. Peptides constructed with this method could be utilized to zero in on various protein targets or to achieve other goals, such as drug delivery.