Nonphosphorylated peptide ligands for the Grb2 Src homology 2 domain

Critical intracellular signals in normal and malignant cells are transmitted by the adaptor protein Grb2 by means of its Src homology 2 (SH2) domain, which binds to phosphotyrosyl (pTyr) residues generated by the activation of tyrosine kinases. To understand this important control point and to design inhibitors, previous investigations have focused on the molecular mechanisms by which the Grb2 SH2 domain selectively binds pTyr containing peptides. In the current study, we demonstrate that the Grb2 SH2 domain can also bind in a pTyr independent manner. Using phage display, an 11-amino acid cyclic peptide, G1, has been identified that binds to the Grb2 SH2 domain but not the src SH2 domain. Synthetic G1 peptide blocks Grb2 SH2 domain association (IC50 10-25 microM) with a 9-amino acid pTyr-containing peptide derived from the SHC protein (pTyr317). These data and amino acid substitution analysis indicate that G1 interacts in the phosphopeptide binding site. G1 peptide requires a YXN sequence similar to that found in natural pTyr-containing ligands, and phosphorylation of the tyrosine increases G1 inhibitory activity. G1 also requires an internal disulfide bond to maintain the active binding conformation. Since the G1 peptide does not contain pTyr, it defines a new type of SH2 domain binding motif that may advance the design of Grb2 antagonists.

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

Grb7 is an adapter-type signaling protein, which is recruited via its SH2 domain to a variety of receptor tyrosine kinases (RTKs), including ErbB2 and ErbB3. It is overexpressed in breast, esophageal, and gastric cancers, and may contribute to the invasive potential of cancer cells. Molecular interactions involving Grb7 therefore provide attractive targets for therapeutic intervention. We have utilized phage display random peptide libraries as a source of small peptide ligands to the SH2 domain of Grb7. Screening these libraries against purified Grb7 SH2 resulted in the identification of Grb7-binding peptide phage clones that contained a non-phosphorylated Tyr-X-Asn (YXN) motif. The tyrosine-phosphorylated form of this motif is characteristic of Grb7 SH2 domain binding sites identified in RTKs and other signaling proteins such as Shc. Peptides that are non-phosphorylated have greater potential in the development of therapeutics because of the instability of a phosphate group in vivo. Using a biased library approach with this conserved YXN motif, we identified seven different peptide phage clones, which bind specifically to the SH2 domain of Grb7. These peptides did not bind to the SH2 domain of Grb2 (which also selects for Asn at pY(+2)) or Grb14, a closely related family member. The cyclic structure of the peptides was required to bind to the Grb7 SH2 domain. Importantly, the synthetic Grb7-binding peptide G7-18 in cell lysates was able to specifically inhibit the association of Grb7 with the ErbB family of RTKs, in particular ErbB3, in a dose-dependent manner. These peptides will be useful in the development of targeted molecular therapeutics for cancers overexpressing Grb7 and in the development of Grb7-specific inhibitors to gain a complete understanding of the physiological role of Grb7.

Overexpression and purification of a fimbria-associated adhesin of Streptococcus parasanguis

A Streptococcus parasanguis adhesin that blocks the attachment of S. parasanguis FW213 to saliva-coated hydroxyapatite (SCHA) has been purified. Previous work demonstrated that the attachment of FW213 to SCHA is mediated by fimbriae and that one component associated with fimbriae is a 36-kDa protein (FimA) that reacts with antifimbria serum in Western blots (immunoblots) and is not present in afimbriated mutants. To obtain amounts of FimA sufficient for adhesion blocking assays, we cloned the gene coding for FimA into an Escherichia coli T7 overexpression system. The resulting strain produced large amounts of FimA, as much as 50% of the total cell protein. FimA was purified by elution from sodium dodecyl sulfate-polyacrylamide gels, and its native conformation was reestablished by sodium dodecyl sulfate removal, resolubilization in guanidine hydrochloride, and 50-fold dilution. Some refolded FimA aggregated into dimers and trimers. Preincubation of SCHA with 100 micrograms of purified, renatured FimA per ml blocked 85% of the binding of FW213. The FimA-SCHA complex was quite stable and could be washed continuously for at least 2 h with only a slight loss of FimA blocking activity. When FimA was added to preformed bacterium-SCHA complexes, it displaced 40% of the bacteria already bound to SCHA. The results suggest that FimA is an adhesin with a high substrate affinity and may prove useful in the development of a therapeutic agent for the prevention of plaque formation and endocarditis initiated by the sanguis streptococci.

Identification by monoclonal antibody of a major (28 kDa) surface membrane antigen of Schistosoma mansoni

Monoclonal antibody M.1 was generated from mice immunized with membrane enriched extracts of mechanically transformed schistosomula. M.1 bound to the surface membranes of cercariae and young (0-24 h post-transformation) schistosomula but did not bind to older schistosomula or cultured worms. M.1 immunoprecipitated an antigen of approximate molecular weight 27-28 kDa from schistosomula. Experiments using metabolic labeling showed that the antigen was actively synthesized by developing schistosomula. Further M.1 immunoprecipitated a similar 27-28 kDa antigen from membrane-enriched extracts of miracidia, lung and adult worms as well as from schistosomula.

Phage-displayed random peptide libraries in mice: toxicity after serial panning

PURPOSE

In vivo screening of phage-displayed random peptide libraries (RPLs) has been used to identify peptide ligands to targets found on endothelial cells of blood vessels supplying specific tissues such as brain, kidney, and tumor tissue. Peptides that bind specifically to blood vessels supplying tumor tissue have been conjugated to cytotoxic agents and used to successfully eradicate tumors in a mouse model. With the ultimate goal of developing similar methods for treating human cancer, we describe an in vivo RPL screening process that, unlike previous in vivo experiments, does not harm the animal being screened.

METHODS

RPLs were administered to FVB, BalbC, and tumor-bearing MRL/MpJ-fas(LPR) mice in a variety of dosing formats. Tumor nodules were excised 10 min following infusion and phage were amplified from the specimens. Phage were reinjected into the same animal within 48 h. This process was repeated twice for a total of three in vivo screens of mouse tumor tissue within the same animal. Mice were observed for systemic side effects, histopathologic damage, and presence of phage in organs. Peptide sequences were determined from several third-pan phage clones.

RESULTS

Overall there was minimal toxicity from administration of single or repeat doses of RPLs. Amino acid consensus sequences were identified and some of the sequences were similar to those of peptide ligands that bind matrix metalloproteinases.

CONCLUSIONS

Serial administration of an RPL is well tolerated and serial panning in individual mice leading to consensus sequence motifs is possible. Based on these preclinical data the Food and Drug Administration has approved the implementation of human clinical trials with this technique.

Design and synthesis of paclitaxel conjugated with an ErbB2-recognizing peptide, EC-1

The selective delivery of therapeutic agents to receptors overexpressed in cancer cells without harming the rest of the body is a major challenge in clinical oncology today. In this study, we report the design and synthesis of paclitaxel (PTX) conjugated with an erbB2-recognizing peptide (EC-1). The cyclic peptide EC-1 specifically binds to the extracellular domain of ErbB2 and selectively inhibits proliferation of breast cancer cells overexpressing ErbB2. PTX is a potent antitumor agent commonly used in the treatment of advanced metastatic breast cancer, yet patients have to suffer some side effects caused by its systemic toxicity. The aim of our conjugate is to specifically deliver antitumor agent PTX to breast cancer cells that overexpress oncogenic ErbB2 with the purpose to reduce toxicity and enhance selective killing of cancer cells. In this study, a concise and efficient synthetic route for the preparation of the PTX-EC-1 conjugate has been developed in 6% overall yield. This synthetic approach provides a general method for conjugating a highly functionalized and disulfide-bridge containing cyclopeptide to Taxol or other antitumor agents.