A dipalmitoyl peptide that binds SH3 domain, disturbs intracellular signal transduction, and inhibits tumor growth in vivo

Grb2-SH3 ligand inhibits the growth of HER2+ cancer cells and has antitumor effects in human cancer xenografts alone and in combination with docetaxel

HER2 represents an important signaling pathway in breast and other cancers. Herceptin has demonstrated clinical activity, but resistance is common. Thus, new therapeutic approaches to the HER2 pathway are needed. Grb2 is an adaptor protein involved in Ras-dependent signaling induced by HER2 receptors. A specific Grb2-SH3 ligand, designed and synthesized in our laboratory, called peptidimer-c, inhibited colony formation in HER2 overexpressing SKBr3 cancer cells. Combined treatment of peptidimer-c with docetaxel further inhibited both colony formation and tumor cell survival compared to docetaxel treatment alone. Efficacy of this combined treatment was correlated with a reduction in the phosphorylation of MAPK and AKT. Finally, peptidimer-c reduced the growth of a HER2(+) human breast cancer (BK111) xenograft in nude mice and potentiated the antitumor effect of docetaxel in a HER2+ hormone-independent human prostate adenocarcinoma (PAC120 HID28) xenograft. These results validate Grb2 as a new target for the HER2 pathway.

A novel six-transmembrane protein hhole functions as a suppressor in MAPK signaling pathways

Src homology 3 (SH3) domains mediate intracellular protein-protein interactions through the recognition of proline-rich sequence motifs on cellular proteins. Such protein-protein interactions can activate the protein kinase cascade that mediates MAPK signaling pathway. The human hole gene, hhole, is a 319-amino acid six-transmembrane protein with proline-rich C-terminal motifs and N-terminal ERK binding domains (D-domains). The hhole protein is highly conserved in evolution across different species from elegent, mouse to human. Northern blot analysis indicates that hhole is expressed in heart, liver, skeletal muscle, and pancreas at adult stages and in most of the examined embryonic tissues, especially at a higher level in heart. Using a GFP-labeled hhole protein, we demonstrate that hhole is localized in plasma membrane or proximal region of the membrane. Overexpression of hhole in COS-7 cells strongly inhibited the transcriptional activities of AP-1 and SRE while deletion of the C-terminal proline-rich motifs or the N-terminal ERK binding D-domain motifs reduced the repressive activity of the gene. These results suggest that the hhole protein may interact with SH3-domain proteins or ERKs to mediate signaling pathways/networks that lead to the suppression of AP-1 and SRE.

A Miniprotein Scaffold Used to Assemble the Polyproline II Binding Epitope Recognized by SH3 Domains

SH3 domains are molecular-recognition modules that function by interacting with proteins containing sequences in polyproline II (PPII) conformation. The main limitation in designing short-ligand peptides to interact with these domains is the preservation of this helical arrangement, for which a high content of proline is needed. We have overcome this limitation by using a protein scaffold provided by the avian pancreatic polypeptide (APP), a natural hormone of 36 amino acid residues. The APP protein contains a PPII stretch packed against an alpha-helix. We have designed a structure in which some residues of the APP PPII helix are replaced by a sequence motif, named RP1, which interacts with the SH3 domain of the Abelson tyrosine kinase (Abl-SH3). This design, which we call APP-RP1, is folded and, as shown by circular dichroism, has a structural content similar to that of natural APP (APP-WT). The stability of both miniproteins has been compared by unfolding experiments; the designed APP-RP1 is almost 20 deg. C more stable than the wild-type and has a higher Gibbs energy function. This increase in stability has an entropic origin. Isothermal titration calorimetry and fluorescence spectroscopy show that the thermodynamics of the binding of the APP-RP1 molecule to Abl-SH3 is comparable to that of the shorter RP1 peptide. Furthermore, the mutation by Tyr of two proline residues in APP-RP1, which are essential for the binding of some linear peptides to Abl-SH3, demonstrates the effectiveness of the scaffold in enhancing the variability in the design of high-affinity and high-specificity ligands for any SH3 domain. The application of this strategy may help in the design of ligands for other polyproline-recognition domains such as WW, PX or EVH1, and even for the in vivo application of these miniproteins.

Thermodynamic Dissection of the Binding Energetics of Proline-rich Peptides to the Abl-SH3 Domain: Implications for Rational Ligand Design

The inhibition of the interactions between SH3 domains and their targets is emerging as a promising therapeutic strategy. To date, rational design of potent ligands for these domains has been hindered by the lack of understanding of the origins of the binding energy. We present here a complete thermodynamic analysis of the binding energetics of the p41 proline-rich decapeptide (APSYSPPPPP) to the SH3 domain of the c-Abl oncogene. Isothermal titration calorimetry experiments have revealed a thermodynamic signature for this interaction (very favourable enthalpic contributions opposed by an unfavourable binding entropy) inconsistent with the highly hydrophobic nature of the p41 ligand and the Abl-SH3 binding site. Our structural and thermodynamic analyses have led us to the conclusion, having once ruled out any possible ionization events or conformational changes coupled to the association, that the establishment of a complex hydrogen-bond network mediated by water molecules buried at the binding interface is responsible for the observed thermodynamic behaviour. The origin of the binding energetics for proline-rich ligands to the Abl-SH3 domain is further investigated by a comparative calorimetric analysis of a set of p41-related ligands. The striking effects upon the enthalpic and entropic contributions provoked by conservative substitutions at solvent-exposed positions in the ligand confirm the complexity of the interaction. The implications of these results for rational ligand design are discussed.