Development of a First-in-Class Small-Molecule Inhibitor of the C-Terminal Hsp90 Dimerization.

Heat shock proteins 90 (Hsp90) are promising therapeutic targets due to their involvement in stabilizing several aberrantly expressed oncoproteins. In cancerous cells, Hsp90 expression is elevated, thereby exerting antiapoptotic effects, which is essential for the malignant transformation and tumor progression. Most of the Hsp90 inhibitors (Hsp90i) under investigation target the ATP binding site in the N-terminal domain of Hsp90. However, adverse effects, including induction of the prosurvival resistance mechanism (heat shock response or HSR) and associated dose-limiting toxicity, have so far precluded their clinical approval. In contrast, modulators that interfere with the C-terminal domain (CTD) of Hsp90 do not inflict HSR. Since the CTD dimerization of Hsp90 is essential for its chaperone activity, interfering with the dimerization process by small-molecule protein-protein interaction inhibitors is a promising strategy for anticancer drug research. We have developed a first-in-class small-molecule inhibitor (5b) targeting the Hsp90 CTD dimerization interface, based on a tripyrimidonamide scaffold through structure-based molecular design, chemical synthesis, binding mode model prediction, assessment of the biochemical affinity, and efficacy against therapy-resistant leukemia cells. 5b reduces xenotransplantation of leukemia cells in zebrafish models and induces apoptosis in BCR-ABL1+ (T315I) tyrosine kinase inhibitor-resistant leukemia cells, without inducing HSR.

Cryptotanshinone from Salvia miltiorrhiza Roots Reduces Cytokeratin CK1/10 Expression in Keratinocytes by Activation of Peptidyl-prolyl-cis-trans-isomerase FKBP1A.

Cryptotanshinone (CTS) (1 µM) from the roots of Salvia miltiorrhiza exerts a strong influence on the terminal differentiation of human keratinocytes (HaCaT cell line, primary natural human keratinocytes) and downregulates the expression of differentiation-specific cytokeratins CK1 and CK10 on protein and gene level. Other differentiation specific proteins as involucrin, filaggrin, loricrin, and transglutaminase were not affected to a higher extent. CTS (1 µM) did not influence the cell viability and the proliferation of keratinocytes. Using a combination of drug affinity response target stability assay in combination with a proteomic approach and multivariate statistics for target elucidation, peptidyl-prolyl-cis-trans-isomerase FKBP1A (known target of inhibitors such as tacrolimus or rapamycin) was addressed as potential molecular target of CTS. The interaction of CTS with FKBP1A was additionally shown by thermal shift and enzymatic activity assays. Interestingly, CTS served as an activator of FKBP1A, which led to a reduced activity of the TGFß receptor pathway and therefore to a diminished CK1 and CK10 expression. The combination of the FKBP1A activator CTS with the inhibitor tacrolimus neutralized the effects of both compounds. From these data, a potential dermatological use of CTS and CTS-containing plant extracts (e.g., hydroalcoholic extract from the roots of S. miltiorrhiza) for keratinopathic ichthyosis, a disease characterized by overexpression of CK1 and CK10, is discussed. This study displays an experimental strategy for combining phytochemical aspects on active natural products with systematic identification of molecular targets on gene, protein, and cell level.

Targeting HSP90 dimerization via the C terminus is effective in imatinib-resistant CML and lacks the heat shock response.

Heat shock protein 90 (HSP90) stabilizes many client proteins, including the BCR-ABL1 oncoprotein. BCR-ABL1 is the hallmark of chronic myeloid leukemia (CML) in which treatment-free remission (TFR) is limited, with clinical and economic consequences. Thus, there is an urgent need for novel therapeutics that synergize with current treatment approaches. Several inhibitors targeting the N-terminal domain of HSP90 are under investigation, but side effects such as induction of the heat shock response (HSR) and toxicity have so far precluded their US Food and Drug Administration approval. We have developed a novel inhibitor (aminoxyrone [AX]) of HSP90 function by targeting HSP90 dimerization via the C-terminal domain. This was achieved by structure-based molecular design, chemical synthesis, and functional preclinical in vitro and in vivo validation using CML cell lines and patient-derived CML cells. AX is a promising potential candidate that induces apoptosis in the leukemic stem cell fraction (CD34+CD38-) as well as the leukemic bulk (CD34+CD38+) of primary CML and in tyrosine kinase inhibitor (TKI)-resistant cells. Furthermore, BCR-ABL1 oncoprotein and related pro-oncogenic cellular responses are downregulated, and targeting the HSP90 C terminus by AX does not induce the HSR in vitro and in vivo. We also probed the potential of AX in other therapy-refractory leukemias. Therefore, AX is the first peptidomimetic C-terminal HSP90 inhibitor with the potential to increase TFR in TKI-sensitive and refractory CML patients and also offers a novel therapeutic option for patients with other types of therapy-refractory leukemia because of its low toxicity profile and lack of HSR.

Design and biological testing of peptidic dimerization inhibitors of human Hsp90 that target the C-terminal domain.

BACKGROUND: Small molecules targeting the dimerization interface of the C-terminal domain of Hsp90, a validated target for cancer treatment, have yet to be identified. METHODS: Three peptides were designed with the aim to inhibit the dimerization of Hsp90. Computational and biophysical methods examined the α-helical structure for the three peptides. Based on the Autodisplay technology, a novel flow cytometer dimerization assay was developed to test inhibition of Hsp90 dimerization. Microscale thermophoresis was used to determine the K(D) of the peptides towards the C-terminal domain of Hsp90. RESULTS: MD simulations and CD spectroscopy indicated an α-helical structure for two of the three peptides. By flow cytometer analysis, IC(50) values of 2.08 µM for peptide H2 and 8.96 µM for peptide H3 were determined. Dimer formation of the C-terminal dimerization domain was analyzed by microscale thermophoresis, and a K(D) of 1.29 nM was determined. Furthermore, microscale thermophoresis studies demonstrated a high affinity binding of H2 and H3 to the C-terminal domain, with a K(D) of 1.02 µM and 1.46 µM, respectively. CONCLUSIONS: These results revealed the first peptidic inhibitors of Hsp90 dimerization targeting the C-terminal domain. Furthermore, it has been shown that these peptides bind to the C-terminal domain with a low micromolar affinity. GENERAL SIGNIFICANCE: These results can be used to design and screen for small molecules that inhibit the dimerization of the C-terminal domain of Hsp90, which could open a new route for cancer therapy.

Naphthol AS-E Phosphate Inhibits the Activity of the Transcription Factor Myb by Blocking the Interaction with the KIX Domain of the Coactivator p300.

The transcription factor c-Myb is highly expressed in hematopoietic progenitor cells and controls the transcription of genes important for lineage determination, cell proliferation, and differentiation. Deregulation of c-Myb has been implicated in the development of leukemia and certain other types of human cancer. c-Myb activity is highly dependent on the interaction of the c-Myb with the KIX domain of the coactivator p300, making the disruption of this interaction a reasonable strategy for the development of Myb inhibitors. Here, we have used bacterial Autodisplay to develop an in vitro binding assay that mimics the interaction of Myb and the KIX domain of p300. We have used this binding assay to investigate the potential of Naphthol AS-E phosphate, a compound known to bind to the KIX domain, to disrupt the interaction between Myb and p300. Our data show that Naphthol AS-E phosphate interferes with the Myb-KIX interaction in vitro and inhibits Myb activity in vivo. By using several human leukemia cell lines, we demonstrate that Naphthol AS-E phosphate suppresses the expression of Myb target genes and induces myeloid differentiation and apoptosis. Our work identifies Naphthol AS-E phosphate as the first low molecular weight compound that inhibits Myb activity by disrupting its interaction with p300, and suggests that inhibition of the Myb-KIX interaction might be a useful strategy for the treatment of leukemia and other tumors caused by deregulated c-Myb.

First structure of protein kinase CK2 catalytic subunit with an effective CK2ß-competitive ligand.

The constitutively active Ser/Thr kinase CK2 (casein kinase 2) is used by tumor cells to acquire apoptosis resistance. CK2 exists as a heterotetrameric holoenzyme with two catalytic chains (CK2α) attached to a dimer of noncatalytic subunits (CK2ß). A druggable cavity at the CK2ß interface of CK2α allows the design of small molecules disturbing the CK2α/CK2ß interaction and thus affecting activity, stability, and substrate specificity. We describe here the first structure of CK2α with an effective CK2ß-competitive compound, namely, a 13-meric cyclic peptide derived from the C-terminal CK2ß segment. Some well-ordered water molecules not visible in CK2 holoenzyme structures were detected at the interface. Driven mainly by enthalpy, the peptide binds with submicromolar affinity to CK2α, stimulates its catalytic activity, and reduces effectively the CK2α/CK2ß affinity. The results provide a thermodynamic and structural rationalization of the peptide's CK2ß-competitive functionality and pave thus the way to a peptidomimetic drug addressing the CK2α/CK2ß interaction.