Identification of novel non-phosphorylated ligands, which bind selectively to the SH2 domain of Grb7

Grb7 knockout mice develop normally but litters born to knockout females fail to thrive

BACKGROUND

Growth factor receptor-bound 7 (Grb7) is an adaptor protein involved in signal transduction downstream of multiple receptor tyrosine kinases, including ERBB, FGFR, and PDGFR pathways. Experimental studies have implicated Grb7 in regulating cell proliferation, survival, migration, and invasion through its large repertoire of protein-protein interactions.

RESULTS

Here, we describe the generation and characterization of a Grb7 knockout mouse. These mice are viable and fertile. A lacZ knock-in reporter was used to visualize Grb7 promoter activity patterns in adult tissues, indicating widespread Grb7 expression in glandular epithelium, the central nervous system, and other tissues. The sole defect observed in these animals was a failure of Grb7 knockout females to successfully raise pups to weaning age, a phenotype that was independent of both paternal and pup genotypes.

CONCLUSIONS

These data suggest a regulatory role for Grb7 in mammary lactational physiology.

GRB7 plays a promoting role in the progression of gastric cancer

BACKGROUND

Gastric cancer is a clinically common tumor, showing an upward trend of both incidence and mortality. GRB7 has been identified as a vital regulator in tumor progression. This study aims to uncover the biological function of GRB7 in gastric cancer process.

METHODS

immunohistochemical (IHC) staining using a tissue microarray (TMA), quantitative reverse transcription PCR (qRT-PCR) and Western blotting were performed to detect the expression of genes. Furthermore, gastric cancer cell lines AGS and MGC-803 were transfected with short hairpin RNAs against GRB7. The biological function of GRB7 in gastric cancer cells were examined by CCK-8, flow cytometry, wound healing and Transwell assays. Then, in vivo tumor formation assay was conducted to explore the effects of GRB7 on tumor growth. Finally, expression levels of proteins related to cell functions were determined by Western blotting. Coimmunoprecipitation (CoIP) assay was performed to assess the protein-protein interaction.

RESULTS

GRB7 was up-regulated in gastric cancer tissues and cell lines, and its expression was inversely proportional to survival of gastric cancer patients. Moreover, GRB7 knockdown inhibited proliferative, migratory abilities, as well as promoted cell apoptosis in gastric cancer cells. Further study suggested that GRB7 silencing could suppress gastric cancer tumor growth in vivo. Furthermore, our study uncovered an important interaction between GRB7 and MyD88. Silencing MyD88 was observed to alleviate the malignant phenotypes promoted by GRB7 in gastric cancer cells.

CONCLUSIONS

Together, this study provided evidence that GRB7 may be an effective molecular targets for the treatment of gastric cancer.

The Src-Family Kinases SRC and BLK Contribute to the CLDN6-Adhesion Signaling

Cell adhesion molecules, including integrins, cadherins, and claudins (CLDNs), are known to activate Src-family kinases (SFKs) that organize a variety of physiological and pathological processes; however, the underlying molecular basis remains unclear. Here, we identify the SFK members that are coupled with the CLDN6-adhesion signaling. Among SFK subtypes, BLK, FGR, HCK, and SRC were highly expressed in F9 cells and concentrated with CLDN6 along cell borders during epithelial differentiation. Immunoprecipitation assay showed that BLK and SRC, but not FGR or HCK, form a complex with CLDN6 via the C-terminal cytoplasmic domain. We also demonstrated, by pull-down assay, that recombinant BLK and SRC proteins directly bind to the C-terminal cytoplasmic domain of CLDN6 (CLDN6C). Unexpectedly, both recombinant SFK proteins recognized the CLDN6C peptide in a phosphotyrosine-independent manner. Furthermore, by comparing phenotypes of F9:Cldn6:Blk-/- and F9:Cldn6:Src-/- cells with those of wild-type F9 and F9:Cldn6 cells, we revealed that BLK and SRC are essential for CLDN6-triggered cellular events, namely epithelial differentiation and the expression of retinoid acid receptor target genes. These results indicate that selective SFK members appear to participate in the CLDN-adhesion signaling.

Targeting Protein-Protein Interfaces with Peptides: The Contribution of Chemical Combinatorial Peptide Library Approaches

Protein-protein interfaces play fundamental roles in the molecular mechanisms underlying pathophysiological pathways and are important targets for the design of compounds of therapeutic interest. However, the identification of binding sites on protein surfaces and the development of modulators of protein-protein interactions still represent a major challenge due to their highly dynamic and extensive interfacial areas. Over the years, multiple strategies including structural, computational, and combinatorial approaches have been developed to characterize PPI and to date, several successful examples of small molecules, antibodies, peptides, and aptamers able to modulate these interfaces have been determined. Notably, peptides are a particularly useful tool for inhibiting PPIs due to their exquisite potency, specificity, and selectivity. Here, after an overview of PPIs and of the commonly used approaches to identify and characterize them, we describe and evaluate the impact of chemical peptide libraries in medicinal chemistry with a special focus on the results achieved through recent applications of this methodology. Finally, we also discuss the role that this methodology can have in the framework of the opportunities, and challenges that the application of new predictive approaches based on artificial intelligence is generating in structural biology.

Up-regulated GRB7 protein in gastric cancer cells correlates with clinical properties and increases proliferation and stem cell properties

Introduction

It has been reported that GRB7 is closely related to a variety of human solid tumors, but its role in gastric cancer has not been reported yet. The purpose of this study was to investigate the expression level and intracellular effects of GRB7 in human gastric cancer.

Methods

Real-time fluorescent quantitative PCR and Western blot were used to detect the expression of GRB7 in gastric cancer cell lines. The immunohistochemical staining and SPSS analysis verified the GRB7 protein expression. Stable gastric cancer cell lines, MTT experiments, clone formation experiments, cell cycle flow cytometry experiments, sphere formation experiments and lateral subpopulation cell sorting experiments were conducted to investigate the role of GRB7 in gastric cancer cells.

Results

We found that the expression of GRB7 in gastric cancer cell lines was higher than that of the corresponding normal gastric epithelial cells, and correspondingly higher in gastric cancer tissues than its paired adjacent tissues. GRB7 protein was expressed more highly in cancer tissues than in adjacent tissues. GRB7 protein expression levels were positively correlated with the clinical stage of gastric cancer patients, and negatively correlated with the survival prognosis of patients. GSEA analysis of GRB7 mRNA levels in gastric cancer tissues and normal gastric epithelial tissues from public databases showed that GRB7 may affect cell proliferation and related processes of intracellular stem cells. GRB7 can promote the proliferation of gastric cancer cells and is positively related to the self-renewal ability of gastric cancer stem cells.

Discussion

This study shows that GRB7 molecules highly expressed in gastric cancer tissues can promote the proliferation of gastric cancer cells and increase the proportion of gastric cancer stem cells, so it is expected to become a diagnostic molecule or potential therapeutic target for gastric cancer.

Using Surface Plasmon Resonance to Study SH2 Domain-Peptide Interactions

Biosensor measurement using surface plasmon resonance enables precise evaluation of peptide-protein interactions. It is a sensitive technique that provides kinetic and affinity data with very little sample and without the need for analyte labels. Here, we describe its application for the analysis of peptide interactions with the Grb7-SH2 domain prepared with a GST-tag for tethering to the chip surface. This has been successfully and reliably used for direct comparison of a range of peptides under different solution conditions as well as direct comparison of peptides flowed over different related SH2 domains in real time. We have used the BIAcore system and describe both the methodology for data collection and analysis, with principles also applicable to other biosensor platforms.

Peptide therapeutics in the management of metastatic cancers

Cancer remains a leading health concern threatening lives of millions of patients worldwide. Peptide-based drugs provide a valuable alternative to chemotherapeutics as they are highly specific, cheap, less toxic and easier to synthesize compared to other drugs. In this review, we have discussed various modes in which peptides are being used to curb cancer. Our review highlights specially the various anti-metastatic peptide-based agents developed by targeting a plethora of cellular factors. Herein we have given a special focus on integrins as targets for peptide drugs, as these molecules play key roles in metastatic progression. The review also discusses use of peptides as anti-cancer vaccines and their efficiency as drug-delivery tools. We hope this work will give the reader a clear idea of the mechanisms of peptide-based anti-cancer therapeutics and encourage the development of superior drugs in the future.

Enhancing the Bioactivity of Bicyclic Peptides Targeted to Grb7-SH2 by Restoring Cell Permeability

The development of peptide inhibitors against intracellular targets depends upon the dual challenge of achieving a high affinity and specificity for the target and maintaining cellular permeability for biological activity. Previous efforts to develop bicyclic peptides targeted to the Grb7 signalling protein implicated in HER2+ve cancer progression have resulted in improved affinity. However, these same peptides demonstrated a lowered activity due to their decreased ability to penetrate cell membranes. Here, we report the testing of a new series of bicyclic G7 peptides designed to possess improved bioactivity. We discovered that the incorporation of two amino acids (Phe-Pro, Phe-Trp or Phe-Arg) within the bicyclic peptide framework maintains an enhanced binding affinity for the Grb7-SH2 domain compared to that of the first-generation monocyclic peptide G7-18NATE. Structure determination using X-ray crystallography revealed that the mode of binding by the expanded bicyclic G7 peptide is analogous to that of G7-18NATE. Interestingly, while the bicyclic peptide containing Phe-Trp did not display the highest affinity for Grb7-SH2 in the series, it was the most potent inhibitor of HER2+ve SKBR3 breast cancer cell migration when coupled to Penetratin. Together, this demonstrates that peptide flexibility as well as the amino acid tryptophan can play important roles in the uptake of peptides into the cell.

Peptide-Based Strategies for Targeted Tumor Treatment and Imaging

Cancer is one of the leading causes of death worldwide. The development of cancer-specific diagnostic agents and anticancer toxins would improve patient survival. The current and standard types of medical care for cancer patients, including surgery, radiotherapy, and chemotherapy, are not able to treat all cancers. A new treatment strategy utilizing tumor targeting peptides to selectively deliver drugs or applicable active agents to solid tumors is becoming a promising approach. In this review, we discuss the different tumor-homing peptides discovered through combinatorial library screening, as well as native active peptides. The different structure–function relationship data that have been used to improve the peptide’s activity and conjugation strategies are highlighted.

Upregulated GRB7 promotes proliferation and tumorigenesis of Bladder Cancer via Phospho-AKT Pathway

Growth factor receptor-bound protein 7 (GRB7) has been found closely related to the occurrence and development of various tumors, but its function in bladder cancer has not yet been elucidated. The study is aiming at investigating the expression and function of GRB7 in bladder cancer. The Cancer Genome Atlas (TCGA) database was selected to analyze mRNA levels of GRB7 in bladder cancer. RT-qPCR and Western blot were conducted to detect the expression of GRB7 in normal bladder epithelial cells, seven bladder cancer cell lines and eight pairs of malignant/nonmalignant bladder tissues. The role of GRB7 in tumor proliferation and tumorigenesis was explored by establishing stable cells, in vitro cell experiments and in vivo xenograft models. The molecular regulation mechanism of GRB7 in bladder cancer was investigated by treatment with AKT inhibitor. GRB7 mRNA was upregulated in bladder cancer samples compared with that in normal tissue samples. Overexpressing GRB7 significantly promoted the proliferation and tumorigenesis of bladder cancer. However, silencing GRB7 played the retarding part. GRB7 promoted G1/S transition by activating the AKT pathway. Our results indicate that GRB7 plays an important role in promoting proliferation and tumorigenesis of bladder cancer.

Phosphotyrosine isosteres: past, present and future

Tyrosine phosphorylation is a critical component of signal transduction for multicellular organisms, particularly for pathways that regulate cell proliferation and differentiation. While tyrosine kinase inhibitors have become FDA-approved drugs, inhibitors of the other important components of these signaling pathways have been harder to develop. Specifically, direct phosphotyrosine (pTyr) isosteres have been aggressively pursued as inhibitors of Src homology 2 (SH2) domains and protein tyrosine phosphatases (PTPs). Medicinal chemists have produced many classes of peptide and small molecule inhibitors that mimic pTyr. However, balancing affinity with selectivity and cell penetration has made this an extremely difficult space for developing successful clinical candidates. This review will provide a comprehensive picture of the field of pTyr isosteres, from early beginnings to the current state and trajectory. We will also highlight the major protein targets of these medicinal chemistry efforts, the major classes of peptide and small molecule inhibitors that have been developed, and the handful of compounds which have been tested in clinical trials.

Evaluation of Cyclic Peptide Inhibitors of the Grb7 Breast Cancer Target: Small Change in Cargo Results in Large Change in Cellular Activity

Grb7 is an adapter protein, overexpressed in HER2+ve breast and other cancers, and identified as a therapeutic target. Grb7 promotes both proliferative and migratory cellular pathways through interaction of its SH2 domain with upstream binding partners including HER2, SHC, and FAK. Here we present the evaluation of a series of monocyclic and bicyclic peptide inhibitors that have been developed to specifically and potently target the Grb7 SH2-domain. All peptides tested were found to inhibit signaling in both ERK and AKT pathways in SKBR-3 and MDA-MB-231 cell lines. Proliferation, migration, and invasion assays revealed, however, that the second-generation bicyclic peptides were not more bioactive than the first generation G7-18NATE peptide, despite their higher in vitro affinity for the target. This was found not to be due to steric hindrance by the cell-permeability tag, as ascertained by ITC, but to differences in the ability of the bicyclic peptides to interact with and penetrate cellular membranes, as determined using SPR and mass spectrometry. These studies reveal that just small differences to amino acid composition can greatly impact the effectiveness of peptide inhibitors to their intracellular target and demonstrate that G7-18NATE remains the most effective peptide inhibitor of Grb7 developed to date.

Grb7, a Critical Mediator of EGFR/ErbB Signaling, in Cancer Development and as a Potential Therapeutic Target

The partner of activated epidermal growth factor receptor (EGFR), growth factor receptor bound protein-7 (Grb7), a functionally multidomain adaptor protein, has been demonstrated to be a pivotal regulator for varied physiological and pathological processes by interacting with phospho-tyrosine-related signaling molecules to affect the transmission through a number of signaling pathways. In particular, critical roles of Grb7 in erythroblastic leukemia viral oncogene homolog (ERBB) family-mediated cancer development and malignancy have been intensively evaluated. The overexpression of Grb7 or the coamplification/cooverexpression of Grb7 and members of the ERBB family play essential roles in advanced human cancers and are associated with decreased survival and recurrence of cancers, emphasizing Grb7's value as a prognostic marker and a therapeutic target. Peptide inhibitors of Grb7 are being tested in preclinical trials for their possible therapeutic effects. Here, we review the molecular, functional, and clinical aspects of Grb7 in ERBB family-mediated cancer development and malignancy with the aim to reveal alternative and effective therapeutic strategies.

Insight into the Selectivity of the G7-18NATE Inhibitor Peptide for the Grb7-SH2 Domain Target

Growth factor receptor bound protein 7 (Grb7) is an adaptor protein with established roles in the progression of both breast and pancreatic cancers. Through its C-terminal SH2 domain, Grb7 binds to phosphorylated tyrosine kinases to promote proliferative and migratory signaling. Here, we investigated the molecular basis for the specificity of a Grb7 SH2-domain targeted peptide inhibitor. We identified that arginine 462 in the BC loop is unique to Grb7 compared to Grb2, another SH2 domain bearing protein that shares the same consensus binding motif as Grb7. Using surface plasmon resonance we demonstrated that Grb7-SH2 binding to G7-18NATE is reduced 3.3-fold when the arginine is mutated to the corresponding Grb2 amino acid. The reverse mutation in Grb2-SH2 (serine to arginine), however, was insufficient to restore binding of G7-18NATE to Grb2-SH2. Further, using a microarray, we confirmed that G7-18NATE is specific for Grb7 over a panel of 79 SH2 domains, and identified that leucine at the βD6 position may also be a requirement for Grb7-SH2 binding. This study provides insight into the specificity defining features of Grb7 for the inhibitor molecule G7-18NATE, that will assist in the development of improved Grb7 targeted inhibitors.

Shortened Penetratin Cell-Penetrating Peptide Is Insufficient for Cytosolic Delivery of a Grb7 Targeting Peptide

Delivery across the cell membrane is of critical importance for the development of therapeutics targeting intracellular proteins. The use of cell-penetrating peptides (CPPs), such as Penetratin (P16), has facilitated the delivery of otherwise cell-impermeable molecules allowing them to carry out their biological function. A truncated form of Penetratin (RRMKWKK) has been previously described as the minimal Penetratin sequence that is required for translocation across the cell membrane. Here, we performed a detailed comparison of cellular uptake by Penetratin (P16) and the truncated Penetratin peptide (P7), including their ability to deliver G7-18NATE, a cyclic peptide targeting the cytosolic cancer target Grb7-adapter protein into cells. We identified that both P16 and P7 were internalized by cells to comparable levels; however, only P16 was effective in delivering G7-18NATE to produce a biological response. Live-cell imaging of fluorescein isothiocyanate-labeled peptides suggested that while P7 may be taken up into cells, it does not gain access to the cytosolic compartment. Thus, this study has identified that the P7 peptide is a poor CPP for the delivery of G7-18NATE and may also be insufficient for the intracellular delivery of other bioactive molecules.

Unexpected involvement of staple leads to redesign of selective bicyclic peptide inhibitor of Grb7

The design of potent and specific peptide inhibitors to therapeutic targets is of enormous utility for both proof-of-concept studies and for the development of potential new therapeutics. Grb7 is a key signaling molecule in the progression of HER2 positive and triple negative breast cancers. Here we report the crystal structure of a stapled bicyclic peptide inhibitor G7-B1 in complex with the Grb7-SH2 domain. This revealed an unexpected binding mode of the peptide, in which the staple forms an alternative contact with the surface of the target protein. Based on this structural information, we designed a new series of bicyclic G7 peptides that progressively constrain the starting peptide, to arrive at the G7-B4 peptide that binds with an approximately 2-fold enhanced affinity to the Grb7-SH2 domain (K_D = 0.83 μM) compared to G7-B1 and shows low affinity binding to Grb2-, Grb10- and Grb14-SH2 domains (K_D > 100 μM). Furthermore, we determined the structure of the G7-B4 bicyclic peptide in complex with the Grb7-SH2 domain, both before and after ring closing metathesis to show that the closed staple is essential to the target interaction. The G7-B4 peptide represents an advance in the development of Grb7 inhibitors and is a classical example of structure aided inhibitor development.

Design and testing of bicyclic inhibitors of Grb7--are two cycles better than one?

Grb7 is an adapter protein involved in the propagation of signals in cancer cell migration and proliferation, and is thus a target for the development of novel anti-cancer agents. An 11-residue thioether-cyclized peptide known as G7-18NATE has previously been developed, that inhibits Grb7 via specific interactions with its SH2 domain with micromolar affinity. Here we explore whether the peptide binding is enhanced by the addition of a second linkage designed to restrain the peptide in its bound conformation and thus reduce the entropic loss upon binding. The use of an O-ally ser covalent linkage between residue positions 1 and 8 successfully enhanced the affinity, and ITC showed that the entropic loss was reduced. A peptide with thioether-cyclization exchanged for an amide linkage showed reduce affinity, though the formation of a disulfide bond between positions 1 and 8 in this peptide enhanced its binding. This study paves the way for improving the G7-18NATE scaffold for second generation inhibitors of Grb7.

Targeting SH2 domains in breast cancer

Breast cancer is among the most commonly diagnosed cancer types in women worldwide and is the second leading cause of cancer-related disease in the USA. SH2 domains recruit signaling proteins to phosphotyrosine residues on aberrantly activated growth factor and cytokine receptors and contribute to cancer cell cycling, metastasis, angiogenesis and so on. Herein we review phosphopeptide mimetic and small-molecule approaches targeting the SH2 domains of Grb2, Grb7 and STAT3 that inhibit their targets and reduce proliferation in in vitro breast cancer models. Only STAT3 inhibitors have been evaluated in in vivo models and have led to tumor reduction. Taken together, these studies suggest that targeting SH2 domains is an important approach to the treatment of breast cancer.

Preparation of crystals for characterizing the Grb7 SH2 domain before and after complex formation with a bicyclic peptide antagonist

Human growth factor receptor-bound protein 7 (Grb7) is an adapter protein involved in cell growth, migration and proliferation. It is now recognized that Grb7 is an emerging therapeutic target in specific cancer subtypes. Recently, the discovery of a bicyclic peptide inhibitor that targets the Grb7 SH2 domain, named G7-B1, was reported. In an attempt to probe the foundation of its interaction with Grb7, the crystallization and preliminary data collection of both the apo and G7-B1-bound forms of the Grb7 SH2 domain are reported here. Diffraction-quality crystals were obtained using the hanging-drop vapour-diffusion method. After several rounds of microseeding, crystals of the apo Grb7 SH2 domain were obtained that diffracted to 1.8 Å resolution, while those of the G7-B1-Grb7 SH2 domain complex diffracted to 2.2 Å resolution. The apo Grb7 SH2 domain crystallized in the trigonal space group P63, whereas the G7-B1-Grb7 SH2 domain complex crystallized in the monoclinic space group P21. The experimental aspects of crystallization, crystal optimization and data collection and the preliminary data are reported.

Receptor protein-tyrosine phosphatase α regulates focal adhesion kinase phosphorylation and ErbB2 oncoprotein-mediated mammary epithelial cell motility

We investigated the role of protein-tyrosine phosphatase α (PTPα) in regulating signaling by the ErbB2 oncoprotein in mammary epithelial cells. Using this model, we demonstrated that activation of ErbB2 led to the transient inactivation of PTPα, suggesting that attenuation of PTPα activity may contribute to enhanced ErbB2 signaling. Furthermore, RNAi-induced suppression of PTPα led to increased cell migration in an ErbB2-dependent manner. The ability of ErbB2 to increase cell motility in the absence of PTPα was characterized by prolonged interaction of GRB7 with ErbB2 and increased association of ErbB2 with a β1-integrin-rich complex, which depended on GRB7-SH2 domain interactions. Finally, suppression of PTPα resulted in increased phosphorylation of focal adhesion kinase on Tyr-407, which induced the recruitment of vinculin and the formation of a novel focal adhesion kinase complex in response to ErbB2 activation in mammary epithelial cells. Collectively, these results reveal a new role for PTPα in the regulation of motility of mammary epithelial cells in response to ErbB2 activation.

Cyclic peptides containing tryptophan and arginine as Src kinase inhibitors

A number of cyclic and linear peptides containing various combinations of amino acids were evaluated for their Src kinase inhibitory potency. Among all the peptides, cyclic decapeptide C[RW]5 containing alternative arginine (R) and tryptophan (W) residues was found to be the most potent Src kinase inhibitor. C[RW]5 showed higher inhibitory activity (IC50=2.8 μM) than C[KW]5, L(KW)5, C[RW]4, and C[RW]3 with IC50 values of 46.9, 69.1, 21.5, and 25.0 μM, respectively, as determined in a fluorescence intensity-based assay. Thus, the cyclic nature, the presence of arginine, ring size, and the number of amino acids in the structure of the peptide were found to be critical in Src kinase inhibitory potency. The IC50 value of C[RW]5 was found to be 0.8 μM in a radioactive assay using [γ-(32)P]-ATP and polyE4Y as the substrate. C[RW]5 was a noncompetitive Src kinase inhibitor, showing approximately fourfold more selectivity towards Src than Abl.

The HER2 amplicon in breast cancer: Topoisomerase IIA and beyond

HER2 gene amplification is observed in about 15% of breast cancers. The subgroup of HER2-positive breast cancers appears to be heterogeneous and presents complex patterns of gene amplification at the locus on chromosome 17q12-21. The molecular variations within the chromosome 17q amplicon and their clinical implications remain largely unknown. Besides the well-known TOP2A gene encoding Topoisomerase IIA, other genes might also be amplified and could play functional roles in breast cancer development and progression. This review will focus on the current knowledge concerning the HER2 amplicon heterogeneity, its clinical and biological impact and the pitfalls associated with the evaluation of gene amplifications at this locus, with particular attention to TOP2A and the link between TOP2A and anthracycline benefit. In addition it will discuss the clinical and biological implications of the amplification of ten other genes at this locus (MED1, STARD3, GRB7, THRA, RARA, IGFPB4, CCR7, KRT20, KRT19 and GAST) in breast cancer.

Emerging strategies to access peptide macrocycles from genetically encoded polypeptides

Macrocyclic peptides have emerged as attractive molecular scaffolds for the development of chemical probes and therapeutics. In this synopsis, we highlight contemporary strategies to access peptide macrocycles from ribosomally produced polypeptides. Challenges that have been tackled in this area involve orchestrating the desired macrocyclization process in the presence of unprotected polypeptide precursors and expanding the functional space encompassed by these molecules beyond that of canonical amino acid structures. Applications of these methodologies for the discovery of bioactive molecules are also discussed.

Potential of phage-displayed peptide library technology to identify functional targeting peptides

Combinatorial peptide library technology is a valuable resource for drug discovery and development. Several peptide drugs developed through phage-displayed peptide library technology are presently in clinical trials and the authors envision that phage-displayed peptide library technology will assist in the discovery and development of many more. This review attempts to compile and summarize recent literature on targeting peptides developed through peptide library technology, with special emphasis on novel peptides with targeting capacity evaluated in vivo.

Targeting GRB7/ERK/FOXM1 signaling pathway impairs aggressiveness of ovarian cancer cells

Ovarian cancer is a highly lethal disease with poor prognosis and especially in high-grade tumor. Emerging evidence has reported that aberrant upregulation and activation of GRB7, ERK as well as FOXM1 are closely associated with aggresivenesss of human cancers. However, the interplay between these factors in the pathogenesis of human cancers still remains unclear. In this study, we found that GRB7 (P<0.0001), ERK phosphorylation (P<0.0001) and FOXM1 (P = 0.001) were frequently increased and associated with high-grade tumors, as well as a high tendency in association with advanced stage ovarian cancer by immunohistochemical analysis. Intriguingly, the expressions of GRB7 (P<0.0001), ERK phosphorylation (P<0.001) and FOXM1 (P<0.001) showed a significant stepwise increase pattern along Grade 1 to Grade 3 ovarian cancers. Biochemical studies using western blot analysis demonstrated that enforced expression or knockdown of GRB7 showed GRB7 could elevate the levels of ERK phosphorylation and FOXM1, whereas enforced expression of FOXM1 could not alter levels of GRB7 and ERK phosphorylation. But inhibition of ERK signaling by U0126 or PD98059 could reduce the level of FOXM1 in GRB7-overexpressing ovarian cancer cells, suggesting that GRB7, ERK and FOXM1 are regulated orderly. Moreover, inhibition of ERK activity by U0126 or PD98059, or decreased FOXM1 expression by Thiostrepton significantly inhibited cell migration/invasion, tumor growth in vitro and in vivo. Collectively, our findings confer that targeting GRB7/ERK/FOXM1 signaling cascade may be a promising molecular therapeutic choice in combating ovarian cancer.

The discovery of phenylbenzamide derivatives as Grb7-based antitumor agents

Grb7 is a non-catalytic protein, the overexpression of which has been associated with the proliferative and migratory potentials of cancer cells. Virtual screening strategies involving a shape-based similarity search, molecular docking, and 2D-similarity searches complemented by experimental binding studies (Thermofluor and isothermal titration calorimetry) resulted in the identification of nine novel phenylbenzamide-based antagonists of the Grb7 SH2 domain. Moderate binding affinities were observed, ranging from K(d)=32.3 μM for lead phenylbenzamide NSC 104999 (1) to K(d)=1.1 μM for a structurally related compound, NSC 57148 (2). Deconvolution of the affinity data into its components revealed differences in lead binding, from being entropy based (lead 1) to enthalpically driven (NSC 100874 (3), NSC 55158 (4), and compound 2). Finally, the lead compound 1 was found to decrease the growth of MDA-MB-468 breast cancer cells, with an IC(50) value of 39.9 μM. It is expected that these structures will serve as novel leads in the development of Grb7-based anticancer therapeutics.

Interaction of the non-phosphorylated peptide G7-18NATE with Grb7-SH2 domain requires phosphate for enhanced affinity and specificity

Src-homology (SH2) domains are an attractive target for the inhibition of specific signalling pathways but pose the challenge of developing a truly specific inhibitor. The G7-18NATE cyclic peptide is reported to specifically inhibit the growth factor receptor bound protein 7 (Grb7) adapter protein, implicated in the progression of several cancer types, via interactions with its SH2 domain. G7-18NATE effectively inhibits the interaction of Grb7 with ErbB3 and focal adhesion kinase in cell lysates and, with the addition of a cell permeability sequence, inhibits the growth and migration of a number of breast cancer cell lines. It is thus a promising lead in the development of therapeutics targeted to Grb7. Here we investigate the degree to which G7-18NATE is specific for the Grb7-SH2 domain compared with closely related SH2 domains including those of Grb10, Grb14, and Grb2 using surface plasmon resonance. We demonstrate that G7-18NATE binds with micromolar binding affinity to Grb7-SH2 domain (K(D)  = 4-6 μm) compared with 50-200 times lower affinity for Grb10, Grb14, and Grb2 but that this specificity depends critically on the presence of phosphate in millimolar concentrations. Other differences in buffer composition, including use of Tris or 2-(N-Morpholino)ethanesulfonic acid or varying the pH, do not impact on the interaction. This suggests that under cellular conditions, G7-18NATE binds with highest affinity to Grb7. In addition, our findings demonstrate that the basis of specificity of G7-18NATE binding to the Grb7-SH2 domain is via other than intrinsic structural features of the protein, representing an unexpected mode of molecular recognition.

Selective thioether macrocyclization of peptides having the N-terminal 2-chloroacetyl group and competing two or three cysteine residues in translation

The mode of thioether macrocyclization of peptides containing an N-terminal 2-chloroacetyl group and two or three competing cysteine residues at downstream positions has been extensively studied, leading to a strategy for designated formation of overlapping-bicyclic peptides or dumbbell-type bicyclic peptides.

Novel nonphosphorylated peptides with conserved sequences selectively bind to Grb7 SH2 domain with affinity comparable to its phosphorylated ligand

The Grb7 (growth factor receptor-bound 7) protein, a member of the Grb7 protein family, is found to be highly expressed in such metastatic tumors as breast cancer, esophageal cancer, liver cancer, etc. The src-homology 2 (SH2) domain in the C-terminus is reported to be mainly involved in Grb7 signaling pathways. Using the random peptide library, we identified a series of Grb7 SH2 domain-binding nonphosphorylated peptides in the yeast two-hybrid system. These peptides have a conserved GIPT/K/N sequence at the N-terminus and G/WD/IP at the C-terminus, and the region between the N-and C-terminus contains fifteen amino acids enriched with serines, threonines and prolines. The association between the nonphosphorylated peptides and the Grb7 SH2 domain occurred in vitro and ex vivo. When competing for binding to the Grb7 SH2 domain in a complex, one synthesized nonphosphorylated ligand, containing the twenty-two amino acid-motif sequence, showed at least comparable affinity to the phosphorylated ligand of ErbB3 in vitro, and its overexpression inhibited the proliferation of SK-BR-3 cells. Such nonphosphorylated peptides may be useful for rational design of drugs targeted against cancers that express high levels of Grb7 protein.

Charging of tRNAs using ribozymes and selection of cyclic peptides containing thioethers

In vitro selection methods represent a powerful approach toward identifying high-affinity peptide ligands from highly diverse peptide libraries against a desired target. We herein describe a method for the display and selection of cyclic thioether peptide libraries. Reprogramming the initiation event from fMet to an N-chloroacetyl-amino acid by utilizing flexizyme to rapidly and efficiently prepare the aa-tRNA can be effectively used to initiate translation, upon which the thiol group of an inserted cysteine at the C terminus of the designed library spontaneously reacts to yield a nonreducible cyclic thioether peptide readily compatible with any in vitro display methods. Thus, cyclic peptides already in a nonreducible stable form can be selected directly against the target of interest.

GRB7 is required for triple-negative breast cancer cell invasion and survival

Triple-negative breast cancer (TNBC) is a heterogeneous disease that is usually associated with poor prognosis, and frequently associated with the basal-like breast cancer gene expression profile. There are no targeted therapeutic modalities for this disease, and no useful biomarkers. High GRB7 RNA expression levels are associated with an elevated risk of recurrence in patients with operable TNBC treated with standard adjuvant anthracycline and taxane therapy. To determine whether GRB7 is involved in the pathobiology of TNBC, we evaluated the biological effects of GRB7 inhibition in a panel of triple-negative cell lines-MDA-MB-468, MDA-MB-231, HCC70, and T4-2. We found GRB7 inhibition reduced cell motility and invasion of these cell lines and promoted cell death by apoptosis in 3D culture. These data suggest that GRB7 itself, or GRB7-dependent pathways, may prove to be important therapeutic targets in this disease.

Structural basis of binding by cyclic nonphosphorylated peptide antagonists of Grb7 implicated in breast cancer progression

Growth-receptor-bound protein (Grb)7 is an adapter protein aberrantly overexpressed, along with the erbB-2 receptor in breast cancer and in other cancers. Normally recruited to focal adhesions with a role in cell migration, it is associated with erbB-2 in cancer cells and is found to exacerbate cancer progression via stimulation of cell migration and proliferation. The G7-18NATE peptide (sequence: WFEGYDNTFPC cyclized via a thioether bond) is a nonphosphorylated peptide that was developed for the specific inhibition of Grb7 by blocking its SH2 domain. Cell-permeable versions of G7-18NATE are effective in the reduction of migration and proliferation in Grb7-overexpressing cells. It thus represents a promising starting point for the development of a therapeutic against Grb7. Here, we report the crystal structure of the G7-18NATE peptide in complex with the Grb7-SH2 domain, revealing the structural basis for its interaction. We also report further rounds of phage display that have identified G7-18NATE analogues with micromolar affinity for Grb7-SH2. These peptides retained amino acids F2, G4, and F9, as well as the YDN motif that the structural biology study showed to be the main residues in contact with the Grb7-SH2 domain. Isothermal titration calorimetry measurements reveal similar and better binding affinity of these peptides compared with G7-18NATE. Together, this study facilitates the optimization of second-generation inhibitors of Grb7.

Solution structure of tensin2 SH2 domain and its phosphotyrosine-independent interaction with DLC-1

Background

Src homology 2 (SH2) domain is a conserved module involved in various biological processes. Tensin family member was reported to be involved in tumor suppression by interacting with DLC-1 (deleted-in-liver-cancer-1) via its SH2 domain. We explore here the important questions that what the structure of tensin2 SH2 domain is, and how it binds to DLC-1, which might reveal a novel binding mode.

Principal Findings

Tensin2 SH2 domain adopts a conserved SH2 fold that mainly consists of five β-strands flanked by two α-helices. Most SH2 domains recognize phosphorylated ligands specifically. However, tensin2 SH2 domain was identified to interact with nonphosphorylated ligand (DLC-1) as well as phosphorylated ligand.

Conclusions

We determined the solution structure of tensin2 SH2 domain using NMR spectroscopy, and revealed the interactions between tensin2 SH2 domain and its ligands in a phosphotyrosine-independent manner.

Preparation and crystallization of the Grb7 SH2 domain in complex with the G7-18NATE nonphosphorylated cyclic inhibitor peptide

Grb7 is an adapter protein that is involved in signalling pathways that mediate eukaryotic cell proliferation and migration. Its overexpression in several cancer types has implicated it in cancer progression and led to the development of the G7-18NATE cyclic peptide inhibitor. Here, the preparation of crystals of G7-18NATE in complex with its Grb7 SH2 domain target is reported. Crystals of the complex were grown by the hanging-drop vapour-diffusion method using PEG 3350 as the precipitant at room temperature. X-ray diffraction data were collected from crystals to 2.4 Å resolution using synchrotron X-ray radiation at 100 K. The diffraction was consistent with space group P2(1), with unit-cell parameters a=52.7, b=79.1, c=54.7 Å, α=γ=90.0, β=104.4°. The structure of the G7-18NATE peptide in complex with its target will facilitate the rational development of Grb7-targeted cancer therapeutics.

The intertwining of structure and function: proposed helix-swapping of the SH2 domain of Grb7, a regulatory protein implicated in cancer progression and inflammation

Grb7 is a multidomain intracellular signaling protein that links activated tyrosine kinases with downstream signaling targets. Best known for its regulatory role in cell migration and tumor metastasis, Grb7 also regulates inflammation by coupling NF-kappaB-inducing kinase with erbB/EGFR family receptors. The "adaptor" role of Grb7 in these processes depends upon binding to membrane-associated tyrosine kinases through its C-terminal SH2 domain. The Grb7-SH2 domain shares structural and functional similarity with the SH2 domain of Grb2, a constituent of the MAP kinase pathway. Both domains show unusual affinity for cyclic (beta-turn) ligands. The Grb2-SH2 domain also shows distinctive self-association behavior, forming intertwined ("swapped") dimers. While Grb7 and its SH2 domain are each known to dimerize, the mechanisms and functional significance of this self-association are incompletely understood. Additional residues in the Grb7-SH2 domain effectively lengthen its "EF loop" and render the domain a good candidate for swapped dimerization, through exchange of a C-terminal helix. We propose the existence of a swapped dimeric form of the Grb7-SH2 domain and offer a structural model derived through novel application of nuclear magnetic resonance-derived restraints.

Benzopyrazine derivatives: A novel class of growth factor receptor bound protein 7 antagonists

Growth factor receptor bound protein 7 (Grb7) is an adapter protein that functions as a downstream effector of growth factor mediated signal transduction. Over-expression of Grb7 has been implicated in a variety of cancers such as breast, blood, pancreatic, esophageal, and gastric carcinomas. Inhibition of Grb7 has been shown to reduce the migratory and proliferative potential of these cancers, making it an attractive therapeutic target. Starting with a known peptide antagonist, the present work reports the application of a succession of computational ligand design tools comprising a ligand shape based similarity search, molecular docking and a 2D-similarity search to identify small molecular antagonists of the Grb7-SH2 domain from the NCI chemical database. Binding to the Grb7-SH2 domain was then experimentally tested using melting point shift assays and isothermal titration calorimetry. Overall, a total of 11 benzopyrazine based small molecular antagonists were identified with affinity for the Grb7-SH2 domain. Representative compounds tested using ITC were revealed to possess moderate binding affinity in the low micromolar range. Finally, the lead compound (NSC642056) was found to reduce the growth of a Grb7-expressing breast cancer cell line with an IC(50) of 86μM. It is expected that the identified antagonists will be useful additions to further explore the function of Grb7 and for the development of inhibitors with therapeutic potential.

Grb7 upregulation is a molecular adaptation to HER2 signaling inhibition due to removal of Akt-mediated gene repression

The efficacy of anti-HER2 therapeutics, such as lapatinib and trastuzumab, is limited by primary and acquired resistance. Cellular adaptations that allow breast cancer cell to survive prolonged HER2 inhibition include de-repression of the transcription factor FOXO3A with consequent estrogen receptor activation, and/or increased HER3 signaling. Here, we used low-density arrays, quantitative PCR, and western blotting to determine how HER2 signaling inhibition with lapatinib or PI3K inhibitors affects the expression of genes involved in breast cancer metastatic spread and overall prognosis. Retroviral transgenesis was used to express constitutively active forms of Akt in the HER2⁺ breast cancer cell line SKBR3, and Grb7 in MCF7 cells. Specific gene silencing was obtained by siRNAs transfection. A murine BT474 xenograft cancer model was used to assess the effect of lapatinib on gene expression in vivo. We found that lapatinib induces upregulation of Grb7, an adaptor protein involved in receptor tyrosine kinase signaling and promoting cell survival and cell migration. Grb7 upregulation induced by lapatinib was found to occur in cancer cells in vitro and in vivo. We demonstrate that Grb7 upregulation is recreated by PI3K inhibitors while being prevented by constitutively active Akt. Thus, Grb7 is repressed by PI3K signaling and lapatinib-mediated Akt inhibition is responsible for Grb7 de-repression. Finally, we show that Grb7 removal by RNA-interference reduces breast cancer cell viability and increases the activity of lapatinib. In conclusion, Grb7 upregulation is a potentially adverse consequence of HER2 signaling inhibition. Preventing Grb7 accumulation and/or its interaction with receptor tyrosine kinases may increase the benefit of HER2-targeting drugs.

17q12-21 - the pursuit of targeted therapy in breast cancer

PURPOSE

Identification of HER2/neu, and the subsequent development of targeted therapy for patients who over-express it, has revolutionized their management. Research has since focused on the area of chromosome 17 in which HER2/neu is located in order to identify other genes in the vicinity. The aims of this review are, firstly, to discuss current thinking in relation to the role of these genes in the pathogenesis of breast cancer and, secondly, to examine how this evidence may be assimilated such that new forms of targeted therapy can be developed.

EXPERIMENTAL DESIGN

This review discusses the evidence in relation to 4 genes located at the HER2/neu amplicon, namely TOP2A, GRB7, STARD3 and RARA.

RESULTS

TOP2A has aroused particular interest as over-expression of its protein has been shown to correlate, both with amplification of HER2/neu, and with response to anthracycline-based chemotherapeutic agents in breast cancer. GRB7 is included on Oncotype DXtm, and has recently been implicated in gastric and oesophageal cancer. STARD3 and RARA also hold clinical relevance, the former having been shown to function in steroidogenesis and therefore implicated in hormone-receptor-positive breast cancer. Finally, RARA may be the key to unlocking the problem of resistance to all-trans retinoic acid (ATRA) in breast cancer sufferers; this treatment has previously been demonstrated to induce remission in over 80% of patients with acute promyelocytic leukaemia (APML).

CONCLUSION

These genes hold potential as therapeutic targets, and warrant further investigation as we move towards our goal of individually tailored therapeutic strategies in breast cancer.

Growth factor receptor-bound protein-7 (Grb7) as a prognostic marker and therapeutic target in breast cancer

BACKGROUND

Growth factor receptor-bound protein-7 (Grb7) is an adapter-type signaling protein recruited to various tyrosine kinases, including HER2/neu. Grb7-specific inhibitors are in early development. As with other targeted therapies, response to therapy might be associated with target expression.

MATERIALS AND METHODS

Tissue microarrays containing 638 primary breast cancer specimens with 15-year patient follow-up were employed to assess Grb7 expression using our Automated QUantitative Analysis method; cytokeratin defines pixels as breast cancer (tumor mask) within the histospot, and Grb7 expression within the mask is measured with Cy5-conjugated antibodies.

RESULTS

High Grb7 expression was strongly associated with decreased survival in the entire cohort and in the node-positive subset (P = 0.0034 and P = 0.0019, respectively). On multivariable analysis, it remained an independent prognostic marker (P = 0.01). High Grb7 was strongly associated with high HER2/neu, and coexpression of these molecules was associated with worse prognosis than HER2/neu overexpression alone.

CONCLUSIONS

High Grb7 defines a subset of breast cancer patients with decreased survival, indicating that Grb7 might be a valuable prognostic marker and drug target. Coexpression with HER2/neu indicates that cotargeting these molecules might be an effective approach for treating HER2/neu-positive breast cancers. Future studies using Grb7-targeting agents should include assessment of Grb7 levels.

Insulin receptor kinase-independent signaling via tyrosine phosphorylation of phosphatase PHLPP1

Most insulin responses correlate well with insulin receptor (IR) Tyr kinase activation; however, critical exceptions to this concept have been presented. Specific IR mutants and stimulatory IR antibodies demonstrate a lack of correlation between IR kinase activity and specific insulin responses in numerous independent studies. IR conformation changes in response to insulin observed with various IR antibodies define an IR kinase-independent signal that alters the C-terminus. IR-related receptors in lower eukaryotes that lack a Tyr kinase point to an alternative mechanism of IR signaling earlier in evolution. However, the implied IR kinase-independent signaling mechanism remained obscure at the molecular level. Here we begin to define the molecular basis of an IR-dependent but IR kinase-independent insulin signal that is equally transmitted by a kinase-inactive mutant IR. This insulin signal results in Tyr phosphorylation and catalytic activation of phosphatase PHLPP1 via a PI 3-kinase-independent, wortmannin-insensitive signaling pathway. Dimerized SH2B1/PSM is a critical activator of the IR kinase and the resulting established insulin signal. In contrast it is an inhibitor of the IR kinase-independent insulin signal and disruption of SH2B1/PSM dimer binding to IR potentiates this signal. Dephosphorylation of Akt2 by PHLPP1 provides an alternative, SH2B1/PSM-regulated insulin-signaling pathway from IR to Akt2 of opposite polarity and distinct from the established PI 3-kinase-dependent signaling pathway via IRS proteins. In combination, both pathways should allow the opposing regulation of Akt2 activity at two phosphorylation sites to specifically define the insulin signal in the background of interfering Akt-regulating signals, such as those controlling cell proliferation and survival.

Tyrosine phosphorylation of growth factor receptor-bound protein-7 by focal adhesion kinase in the regulation of cell migration, proliferation, and tumorigenesis

We have previously reported that growth factor receptor-bound protein-7 (Grb7), an Src-homology 2 (SH2)-containing adaptor protein, enables interaction with focal adhesion kinase (FAK) to regulate cell migration in response to integrin activation. To further elucidate the signaling events mediated by FAK*Grb7 complexes in promoting cell migration and other cellular functions, we firstly examined the phosphorylated tyrosine site(s) of Grb7 by FAK using an in vivo mutagenesis. We found that FAK was capable of phosphorylating at least 2 of 12 tyrosine residues within Grb7, Tyr-188 and Tyr-338. Moreover, mutations converting the identified Tyr to Phe inhibited integrin-dependent cell migration as well as impaired cell proliferation but not survival compared with the wild-type control. Interestingly, the above inhibitory effects caused by the tyrosine phosphorylation-deficient mutants are probably attributed to their down-regulation of phospho-Tyr-397 of FAK, thereby implying a mechanism by competing with wild-type Grb7 for binding to FAK. Consequently, these tyrosine phosphorylation-deficient mutants evidently altered the phospho-Tyr-118 of paxillin and phosphorylation of ERK1/2 but less on phospho-Ser-473 of AKT, implying their involvement in the FAK*Grb7-mediated cellular functions. Additionally, we also illustrated that the formation of FAK*Grb7 complexes and Grb7 phosphorylation by FAK in an integrin-dependent manner were essential for cell migration, proliferation and anchorage-independent growth in A431 epidermal carcinoma cells, indicating the importance of FAK*Grb7 complexes in tumorigenesis. Our data provide a better understanding on the signal transduction event for FAK*Grb7-mediated cellular functions as well as to shed light on a potential therapeutic in cancers.

Identification of novel peptides specifically binding to endometriosis by screening phage-displaying peptide libraries

OBJECTIVE

To search for novel peptides and common binding motif that specifically bind to endometriosis.

DESIGN

Prospective study.

SETTING

Department of Biological Science and Technology in national university.

PATIENT(S)

Specimens were divided into [1] ectopic endometrium (n = 10); [2] eutopic endometrium (n = 10).

INTERVENTION(S)

Peptides specifically binding to endometriosis are screened from a phage-displaying peptide library (Ph.D.-12) by using whole-cell screening technique after an adsorption elution amplification procedure.

MAIN OUTCOME MEASURE(S)

Combinatorial peptide libraries were used to identify small molecules that bind with high affinity to receptor molecules and mimic the interaction with natural ligands. Few pans of positive phage clones with significantly positive signals were identified by ELISA and analyzed by DNA sequencing.

RESULT(S)

During the biopanning processes, the recovered phage number (10(6) pfu/mL) in parts 1, 2, 3, 4, and 5 of the study were 9, 33, 82, 142, and 169. Nine phages consistently had residue Arg, whereas six clones had a consensus motif of Arg-X-Arg-X-X-X-X-Arg. The biotin-labeled peptide bound to endometriosis cells in a dose-dependent manner, yet the control peptide revealed lesser binding activity.

CONCLUSION(S)

The novel motif is associated with higher affinity of endometriosis, which might be useful in endometriosis targeting and as potential antiendometriosis therapies. We provide one potential approach for novel therapies toward endometriosis.

Genes, chromosomes and the development of testicular germ cell tumors of adolescents and adults

Testicular germ cell tumors (TGCTs) of adults and adolescents are thought to be derived from primordial germ cells or gonocytes. TGCTs develop postpuberty from precursor lesions known as intratubular germ cell neoplasia undifferentiated. The tumors can be divided into two groups based on their histology and clinical behavior; seminomas resemble primordial germ cells or gonocytes and nonseminomas resemble embryonic or extraembryonic tissues at various stages of differentiation. The most undifferentiated form of nonseminoma, embryonal carcinoma, resembles embryonic stem cells in terms of morphology and expression profiling, both mRNAs and microRNAs. Evidence supports both environmental factors and genetic predisposition underlying the development of TGCTs. Various models of development have been proposed and are discussed. In TGCTs, gain of material from the short arm of chromosome 12 is invariable: genes from this region include the proto-oncogene KRAS, which has activating mutations in approximately 10% of tumors or is frequently overexpressed. A number of different approaches to increase the understanding of the development and progression of TGCTs have highlighted the involvement of KIT, RAS/RAF/MAPK, STAT, and PI3K/AKT signaling. We review the role of these signaling pathways in this process and the potential influence of environmental factors in the development of TGCTs.

Ribosomal synthesis of nonstandard peptidesThis paper is one of a selection of papers published in this Special Issue, entitled CSBMCB — Systems and Chemical Biology, and has undergone the Journal's usual peer review process

It is well known that standard peptides, which comprise proteinogenic amino acids, can act as specific chemical probes to target proteins with high affinity. Despite this fact, a number of peptide drug leads have been abandoned because of their poor cell permeability and protease instability. On the other hand, nonstandard peptides isolated as natural products often exhibit remarkable pharmaco-behavior and stability in vivo. Although it is likely that numerous nonstandard therapeutic peptides capable of recognizing various targets could have been synthesized, enzymes for nonribosomal peptide syntheses are complex; therefore, it is difficult to engineer such modular enzymes to build nonstandard peptide libraries. Here we describe an emerging technology for the synthesis of nonstandard peptides that employs an integrated system of reconstituted cell-free translation and flexizymes. We summarize the historical background of this technology and discuss its current and future applications to the synthesis of nonstandard peptides and drug discovery.