Elaboration of Non-naturally Occurring Helical Tripeptides as p53-MDM2/MDMX Interaction Inhibitors.

Protein-protein interactions (PPIs) are often mediated by helical, strand and/or coil secondary structures at the interface regions. We previously showed that non-naturally occurring, stable helical trimers of bicyclic ß-amino acids (Abh) with all-trans amide bonds can block the p53-MDM2/MDMX α-helix-helix interaction, which plays a role in regulating p53 function. Here, we conducted docking and molecular dynamics calculations to guide the structural optimization of our reported compounds, focusing on modifications of the C-terminal/N-terminal residues. We confirmed that the modified peptides directly bind to MDM2 by means of thermal shift assay, isothermal titration calorimetry, and enzyme-linked immunosorbent assay (ELISA) experiments. Biological activity assay in human osteosarcoma cell line SJSA-1, which has wild-type p53 and amplification of the Mdm2 gene, indicated that these peptides are membrane-permeable p53-MDM2/MDMX interaction antagonists that can rescue p53 function in the cells.

Non-naturally Occurring Helical Molecules Can Interfere with p53-MDM2 and p53-MDMX Protein-Protein Interactions.

We have discovered that ß-amino acid homooligomers with cis- or trans-amide conformation can fold themselves into highly ordered helices. Moreover, unlike α-amino acid peptides, which are significantly stabilized by intramolecular hydrogen bonding, these helical structures are autogenous conformations that are stable without the aid of hydrogen bonding and irrespective of solvent (protic/aprotic/halogenated) or temperature. A structural overlap comparison of helical cis/trans bicyclic ß-proline homooligomers with typical α-helix structure of α-amino acid peptides reveals clear differences of pitch and diameter per turn. Bridgehead substituents of the present homooligomers point outwards from the helical surface. We were interested to know whether such non-naturally occurring divergent helical molecules could mimic α-helix structures. In this study, we show that bicyclic ß-proline oligomer derivatives inhibit p53-MDM2 and p53-MDMX protein-protein interactions, exhibiting MDM2-antagonistic and MDMX-antagonistic activities.

PHLDA1, another PHLDA family protein that inhibits Akt.

The PHLDA family (pleckstrin homology-like domain family) of genes consists of 3 members: PHLDA1, 2, and 3. Both PHLDA3 and PHLDA2 are phosphatidylinositol (PIP) binding proteins and function as repressors of Akt. They have tumor suppressive functions, mainly through Akt inhibition. Several reports suggest that PHLDA1 also has a tumor suppressive function; however, the precise molecular functions of PHLDA1 remain to be elucidated. Through a comprehensive screen for p53 target genes, we identified PHLDA1 as a novel p53 target, and we show that PHLDA1 has the ability to repress Akt in a manner similar to that of PHLDA3 and PHLDA2. PHLDA1 has a so-called split PH domain in which the PH domain is divided into an N-terminal (ß sheets 1-3) and a C-terminal (ß sheets 4-7 and an α-helix) portions. We show that the PH domain of PHLDA1 is responsible for its localization to the plasma membrane and binding to phosphatidylinositol. We also show that the function of the PH domain is essential for Akt repression. In addition, PHLDA1 expression analysis suggests that PHLDA1 has a tumor suppressive function in breast and ovarian cancers.

Metabolic intervention by low carbohydrate diet suppresses the onset and progression of neuroendocrine tumors.

Insulin signaling often plays a role in the regulation of cancer, including tumor initiation, progression, and response to treatment. In addition, the insulin-regulated PI3K-Akt-mTOR pathway plays an important role in the regulation of islet cell proliferation, and this pathway is hyperactivated in human non-functional pancreatic neuroendocrine tumors (PanNETs). We, therefore, investigated the effect of a very low carbohydrate diet (ketogenic diet) on a mouse model that develops non-functional PanNETs to ask how reduced PI3K-Akt-mTOR signaling might affect the development and progression of non-functional PanNET. We found that this dietary intervention resulted in lower PI3K-Akt-mTOR signaling in islet cells and a significant reduction in PanNET formation and progression. We also found that this treatment had a significant effect on the suppression of pituitary NET development. Furthermore, we found that non-functional PanNET patients with lower blood glucose levels tend to have a better prognosis than patients with higher blood glucose levels. This preclinical study shows that a dietary intervention that results in lower serum insulin levels leads to lower insulin signals within the neuroendocrine cells and has a striking suppressive effect on the development and progression of both pancreatic and pituitary NETs.

PHLDA3 Is an Important Downstream Mediator of p53 in Squamous Cell Carcinogenesis.

Squamous cell carcinomas (SCCs) are one of the most frequent solid cancer types in humans and are derived from stratified epithelial cells found in various organs. SCCs derived from various organs share common important properties, including genomic abnormalities in the tumor suppressor gene p53. There is a carcinogen-induced mouse model of SCC that produces benign papilloma, some of which progress to advanced carcinoma and metastatic SCCs. These SCCs undergo key genetic alterations that are conserved between humans and mice, including alterations in the genomic p53 sequence, and are therefore an ideal system to study the mechanisms of SCC tumorigenesis. Using this SCC model, we show that the PHLDA3 gene, a p53-target gene encoding a protein kinase B repressor, is involved in the suppression of benign and metastatic tumor development. Loss of PHLDA3 induces an epithelial‒mesenchymal transition and can complement p53 loss in the formation of metastatic tumors. We also show that in human patients with SCC, low PHLDA3 expression is associated with a poorer prognosis. Collectively, this study identifies PHLDA3 as an important downstream molecule of p53 involved in SCC development and progression.

Overall Shape Constraint of Alternating α/ß-Hybrid Peptides Containing Bicyclic ß-Proline.

Our NMR, IR/Raman, CD spectroscopic, and X-ray crystallographic studies, as well as accelerated molecular dynamics simulations, showed that alternating hybrid α/ß-peptides containing a bicyclic ß-proline surrogate form unique extended curved folds, regardless of the peptide length and solvent environment. It is suggested that extended ß/PPII structures are preferred in the insulating α-alanine moieties between the rigid bicyclic ß-proline structures. These hybrid peptides inhibit p53-MDM2 and p53-MDMX protein-protein interactions.