Discovery of New Molecular Entities Able to Strongly Interfere with Hsp90 C-Terminal Domain Received: 11 May 2017 Stefania Terracciano1, Alessandra Russo1, Maria G
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www.nature.com/scientificreports OPEN Discovery of new molecular entities able to strongly interfere with Hsp90 C-terminal domain Received: 11 May 2017 Stefania Terracciano1, Alessandra Russo1, Maria G. Chini1, Maria C. Vaccaro1, Accepted: 19 October 2017 Marianna Potenza1, Antonio Vassallo2, Rafaele Riccio1, Giuseppe Bifulco1 & Ines Bruno1 Published: xx xx xxxx Heat shock protein 90 (Hsp90) is an ATP dependent molecular chaperone deeply involved in the complex network of cellular signaling governing some key functions, such as cell proliferation and survival, invasion and angiogenesis. Over the past years the N-terminal protein domain has been fully investigated as attractive strategy against cancer, but despite the many eforts lavished in the feld, none of the N-terminal binders (termed “classical inhibitors”), currently in clinical trials, have yet successfully reached the market, because of the detrimental heat shock response (HSR) that showed to induce; thus, recently, the selective inhibition of Hsp90 C-terminal domain has powerfully emerged as a more promising alternative strategy for anti-cancer therapy, not eliciting this cell rescue cascade. However, the structural complexity of the target protein and, mostly, the lack of a co-crystal structure of C-terminal domain-ligand, essential to drive the identifcation of new hits, represent the largest hurdles in the development of new selective C-terminal inhibitors. Continuing our investigations on the identifcation of new anticancer drug candidates, by using an orthogonal screening approach, here we describe two new potent C-terminal inhibitors able to induce cancer cell death and a considerable down- regulation of Hsp90 client oncoproteins, without triggering the undesired heat shock response. Heat shock proteins (Hsps), Hsp27, Hsp70 and Hsp90 are powerful anti-apoptotic proteins involved in vital mechanisms of cancerous cells, such as proliferation, diferentiation, metastasis and invasiveness1,2. Te ampli- fed expression of Hsps is a common feature in human cancers and is associated with increased tumor growth, metastatic potential of tumor cells and resistance to chemotherapy3. As a consequence, the inhibition of Hsps might provide a broad and efective strategy in cancer therapy. Among these molecular chaperones, Hsp90 is a key protein that plays a central role in the folding and maturation of many factors, including important signaling proteins with high relevance to human cancer pathways4. Many Hsp90 clients are oncogenes that drive a wide range of malignant transformations in which cells have ofen become “addicted” to Hsp90’s functions5–7. Over the past years, Hsp90 has been deeply investigated, from both industry and academic research institutes, as new potential target for cancer and Hsp90 inhibition has, thus, become an attractive therapeutic concept to develop clinically viable antitumor agents (see http://clinicaltrilas.gov). Despite the many progress made in the discovery and development of Hsp90 inhibitors, and the presence of several N-terminal binders (termed “classical inhibi- tors”) currently in clinical trials in several tumor types, none of these molecules have yet successfully reached the market8–12. These disappointing results may be associated with the N-terminal modulators’ inherent toxicity (that limits their clinical applicable dosages) and with the strong induction of heat shock response (HSR)3,13–15, a well-defned compensatory mechanism leading to an increased expression of heat shock proteins, and responsi- ble for N-domain inhibitors resistance16,17. In contrast to these modulators, molecules that interfere with Hsp90 C-terminus have been shown to not produce the deleterious HSR emerging, thus, as a promising alternative and a more efective therapeutic anti-cancer strategy18–22. So far, for this less-targeted C-terminal domain only a few inhibitors have been disclosed, including both natural products and their synthetic derivatives18–22, that interact with the molecular chaperone at non-overlapping sites (due to their ability to bind Hsp90 in its distinct conformational states)4,23,24. Indeed, Hsp90 is a large and conformationally dynamic protein that is known to undergo conformational changes associated with remarkable rearrangements in its structure, and, for this reason, 1Department of Pharmacy, University of Salerno, via Giovanni Paolo II, 132, 84084, Fisciano, Italy. 2Department of Science, University of Basilicata, Viale dell’Ateneo Lucano n.10, 85100, Potenza, Italy. Correspondence and requests for materials should be addressed to I.B. (email: [email protected]) SCIENTIFIC REPORTS | (2018) 8:1709 | DOI:10.1038/s41598-017-14902-y 1 www.nature.com/scientificreports/ it represents a challenging target for structural analysis. In addition to the structural complexity of this protein, the absence of crystal structures of C-terminal Hsp90-inhibitor complexes, represents the main drawback for progress in the feld. Despite the above-mentioned difculties and the lack of a convincing grasp regarding the exact structural requirements for C-terminal domain interactions, recently we reported the identifcation of new potent dihydro- pyrimidinone based Hsp90 inhibitors that target the C-terminal binding pocket25–28. Results and Discussion In order to continue our research program aimed at expanding the number of Hsp90 C- terminal inhibitors, we decided to utilize the surface plasmon resonance (SPR)25–29 assay for screening a collection of low molecular weight synthetically accessible compounds, selected in order to explore the chemical space encoded by diferent scafolds. In more details, a set of forty-eight commercially available small molecules (Table S1, Supplementary Material), endowed with diferent structural features (Fig. S1, Supplementary Material) was subjected to SPR screening on recombinant Hsp90α for testing their ability to bind to the immobilized protein. Based on this assay, sixteen compounds with novel chemical scafolds Fig. 1 have been identifed as high afnity leads for the Hsp90 chaperone with low KD values (see surface plasmon resonance analyses in material and methods section). In light of these results, we decided to evaluate their potential anti-proliferative activity against Jurkat (human T-lymphocyte) and U937 (human monocyte form histiocytic lymphoma) cell lines, that were exposed to increas- ing concentrations of compounds 1, 4–5, 7–15, 20–22, 26 or novobiocin, a well characterized Hsp90 C-terminal 30 inhibitor . Cells viability was evaluated at 24 or 48 h by MTT assay and IC50 values are reported in Tables S2 and S3 (Supplementary Material), respectively. Data showed the best results for 7 and 10 that afected the cell viability in both leukemia-derived cell lines with the lowest IC50 values, so they were selected for a deeper biolog- ical evaluation. Notably these compounds do not afect the proliferation of non-tumor cell line PHA-stimulated human peripheral blood mononuclear cells (PBMC). In fact, the non-viable cells percentage afer 24 or 48 h of treatment with compound 7 (25 µM) (about 6 ± 1.8%, 8 ± 1.5%, respectively), or with with compound 10 (25 µM) (about 7 ± 1.3%, 9 ± 1.8%, respectively), was similar to the value observed in DMSO treated control cells (about 5 ± 0.9%). Since Jurkat (human leukemic T lymphocyte) cells were about two-fold more susceptible than U937 (human monocytic cell line from histiocytic lymphoma) cells, they were used for additional analysis with the aim of char- acterizing the mechanism of the observed anti-proliferative efect. In order to discriminate if 7 and 10 afected cell cycle progression and/or inducing cell death, the cells were incubated for 24 or 48 h with a concentration of these two compounds close to their IC50 values or two and four times higher. Novobiocin was used at the same conditions with concentrations close to its IC50 value (200 µM) or at lower doses (50 or 100 μM). Cell cycle distribution analysis showed that 7 and 10 moderately prevented the cycle progression by arresting the cells in G0/G1 or S phase, respectively, afer 24 h (Fig. 2A,B) without any signifcant increase of subG0/G1 cell fraction, indicative of apoptotic/necrosis cell death (Fig. 2D,E). Tese dose-dependent cytostatic efects of 7 and 10 were retained even afer 48 h (Fig. 2A,B). Conversely, the hypodiploid cells (subG0/G1 DNA content) increased at 10%, afer 48 h cell treatment, with 7 or 10 used at high concentrations (100 µM), as shown in Fig. 2D,E. Te novobiocin cell treatment triggered diferent responses with respect to 7 and 10, indeed, when it was used at low concentration (50 and 100 µM) for 24 or 48 h, it did not afect cell cycle progression. On the other hand, the exposure to high dose of novobiocin (200 µM), caused cell accumulation in G0/G1, or S phase (Fig. 2C) with a 14% or 30% of hypodiploid cells afer 24 h or 48 h, respectively (Fig. 2F). Tese data indicate that 7 and 10 induced cytostatic efect afer 24 h-exposure and cytostatic/cytotoxic efect afer 48 h-exposure on leukemic cells at low doses, compared to novobiocin. In addition we analyzed the cell cycle cyclin-dependent kinases, Cyclin A, Cyclin D, CDK2, and CDK4 fol- lowing the treatment with compounds 7 and 10. Cyclin D and CDK4 regulates the G0/G1 phase cycle, and their expression levels were slightly down-regulated in Jurkat cells with compound 7 treatment (see Fig. 3A). Similarly compound 10 caused the decrease of Cyclin A and CDK2, involved in normal S-phase progression. In order to further characterize the most efective compounds and to provide evidence of a bona fde inhibi- tion of Hsp90′s activity, the levels of some representative oncogenic Hsp90-client proteins from Jurkat cell lysates, treated with the selected compounds (7 and 10), were verifed through western blot analysis. As shown in Fig. 3B incubation with 7 and 10 induced a signifcant degradation (50–60%) of Hsp90-dependent client proteins Raf-1, p-Akt, p-Erk and p53 in a concentration-dependent way, without afecting actin levels, which is not dependent upon Hsp90; a minimal reduction of Hsc70 (15–20%) was observed.