The first small-molecule inhibitor of 14-3-3s: modulating the master regulator.

TLDR: The elucidation of 14-3-3 PPI stabilizers from a chemical library screen that are specific for a given 143-3 protein–protein interaction has also been reported; this potentially expands the repertoire of these tool compounds to every 14- 3-3 Protein–protein interactions that have already been used successfully to study the role of 13-3–3 proteins in plant physiology and cancer, respectively.

Abstract: 14-3-3 proteins are a class of eukaryotic adapter proteins that are implicated in a wide variety of cellular processes like regulation of cell cycle, apoptosis, signal transduction, growth and development. The seven isoforms of 14-3-3 proteins in humans have been shown to be involved in many diseases such as cancer and neurodegeneration. The astonishing number and variety of physiological processes mediated by 14-3-3 proteins results from their interaction with several hundred protein partners in the cell, including well-studied key players like the tumour suppressor p53, the protein kinases of the Raf family and the Cdc25 dual-specificity phosphatases. [4] In general, 14-3-3 proteins act as adapter proteins. Their binding can, for example, lead to a change in the enzymatic activity status or the subcellular localisation of their partner proteins. Additionally, due to their dimeric nature, the 14-3-3 proteins can conjoin different proteins, thereby contributing to the assembly of larger signalling complexes. [4] Although their involvement in many severe disorders such as cancer and neurodegeneration has been shown, the exact mechanistic and biological details are still largely unexplored. The use of genetic methods like gene knockout for the analysis of this class of proteins is limited by their high functional isoform redundancy and the fact that they are absolutely essential for eukaryotic organisms. As an alternative, chemicalbiology approaches might enable a deeper insight into the biological roles of 14-3-3 proteins. Only small-molecule stabilizers of 14-3-3 PPIs were known; the most prominent examples are the complex natural products fusicoccin and cotylenin— two naturally occurring modulators of 14-3-3 protein–protein interactions that have already been used successfully to study the role of 14-3-3 proteins in plant physiology and cancer, respectively. [5, 6] Recently, the elucidation of 14-3-3 PPI stabilizers from a chemical library screen that are specific for a given 143-3 protein–protein interaction has also been reported; this potentially expands the repertoire of these tool compounds to every 14-3-3 protein–target complex of interest. [7] Nevertheless, before the report of Wu et al. presenting the molecule 2-5 (Figure 1), reverse-acting small-molecule inhibitors of 14-3-3 PPIs were missing. [3] This molecule was identified by employing the small-molecule-microarray (SMM) technique. Based on the amino acid sequence, RFRpSYPP, that has previously been shown as optimal for 14-3-3 binding, a library of small-molecule/peptide hybrids was synthesised in which either 50 diverse amines were coupled to the N-terminal peptide sequence (RFRpS) or 243 diverse acid building blocks were coupled to the C-terminal peptide sequence (pSYPP). [8] The resulting 293 single hybrids were spotted onto a glass slide, and the binding of fluorescently labelled GST-14-3-3 (isoform sigma) was measured. Five compounds showed robust 14-3-3 binding. Subsequent titration revealed a Kd of 0.25 mm for the control peptide (biotin-GG-RLSHpSLPG), and Kd values of between 0.6 and 1.03 mm for the five “hit” compounds. In an orthogonal, fluorescence-polarisation (FP) competition assay, the whole library was again tested for 14-3-3 binding, and the five “hit” compounds from the SMM were verified; IC50 values comparable to the microarray-measured Kd values were yielded. Of these five compounds, three featured a substitution of the N-terminal peptide part and two bore substitutions of the C-terminal peptide part. The six possible combinations of these fragments were then tested in the FP competition assay and gave rise to molecules with IC50 values between 2.6 and 3.6 mm. Identification, synthesis and initial biochemical evaluation with the microarray and FP-binding assay was followed by several more physiologically relevant studies. First, a pull-down experiment with lysates from the human cancer cell line A549 and bead-immobilized GST-14-3-3s was performed. The specific eluate was then probed with antibodies against the known 14-3-3 partners C-RAF and p53 or with antibodies that recognise phosphorylated 14-3-3 binding motifs. In comparable studies, several hundred 14-3-3 interaction partners, for example from HeLa or HEK293 cells, could successfully be identified. [9] Whereas GST-14-3-3s pulled down C-RAF, p53 and a sig