Selective Targeting of Bromodomains of the Bromodomain-PHD Fingers Family Impairs Osteoclast Differentiation.

Julia Meier ,David Heidenreich,Elliott D Bayle,Mark J Birnbaum,Sung-Yong Hwang,Paul V Fish,Clarence Yapp,Hanna Witwicka,Vita Fedele,Brian S Gerstenberger,Susanne Müller,Oleg Fedorov,Stefan Knapp,Dafydd R Owen,Bernard Haendler,Paul R Odgren,Jean-Philippe Lambert,Danette L Daniels,Catherine Rogers,C Tallant ,Anne‐Claude Gingras ,Francesca M Buffa ,Ruud Gpm Van Stiphout ,P Savitsky ,U Oppermann ,P.e Brennan ,Niall Igoe

doi:10.1021/acschembio.7b00481

Abstract

Histone acetyltransferases of the MYST family are recruited to chromatin by BRPF scaffolding proteins. We explored functional consequences and the therapeutic potential of inhibitors targeting acetyl-lysine dependent protein interaction domains (bromodomains) present in BRPF1–3 in bone maintenance. We report three potent and selective inhibitors: one (PFI-4) with high selectivity for the BRPF1B isoform and two pan-BRPF bromodomain inhibitors (OF-1, NI-57). The developed inhibitors displaced BRPF bromodomains from chromatin and did not inhibit cell growth and proliferation. Intriguingly, the inhibitors impaired RANKL-induced differentiation of primary murine bone marrow cells and human primary monocytes into bone resorbing osteoclasts by specifically repressing transcriptional programs required for osteoclastogenesis. The data suggest a key role of BRPF in regulating gene expression during osteoclastogenesis, and the excellent druggability of these bromodomains may lead to new treatment strategies for patients suffering from bone loss or osteolytic malignant bone lesions.

Highlights

Articles impaired RANKL-induced differentiation of primary murine bone marrow cells and human primary monocytes into bone resorbing osteoclasts by repressing transcriptional programs required for osteoclastogenesis
While to date most efforts have focused on BET inhibitor development, recent publications have demonstrated that non-BET bromodomains can be selectively targeted.[10−18] A first inhibitor specific for the BRPF1B bromodomain has been recently disclosed,[19] and inhibitors that showed dual activity for the bromodomains present in BRPF1 and TIF1α have been developed by our laboratory and others.[20,21]
MOZ is frequently translocated in acute myeloid leukemia (AML), and it is required for hematopoietic stem cell maintenance.[27]

Summary

■ RESULTS AND DISCUSSION

The human BRPF family (BRPF1, BRPF2, and BRPF3) shares a conserved domain architecture[22,23] and a high degree of sequence homology within their bromodomains (Figure 1A,B). Estimated IC50 values were 0.07 ± 0.0034 and 0.24 ± 0.039 μM for NI-57 and PFI-4, respectively We verified these data using FRAP (fluorescence recovery after photobleaching) assays.[33] As expected from our selectivity screening data, PFI-4 led only to the dissociation of the bromodomain of BRPF1b, but not any of the full-length family members from histone H3.3 (Supporting Information Figure 1B). The dominant negative form BRPF1A exhibited significantly decreased RNA expression levels, whereas the active acetyl-lysine binding isoform BRPF1B was not affected OF-1 was the only inhibitor to completely suppress the fusion into multinucleated “osteoclast-like” cells This suggests that during osteoclastogenesis, other BRPF family members may, at least in part, functionally replace BRPF1B, which was the only BRPF family member inhibited by PFI-4 at the concentrations tested (Figure 7B). BRPF is widely expressed in a variety of tissue types, and the developed probes will help to elucidate further functions of these interesting epigenetic modulators

■ METHODS

■ ACKNOWLEDGMENTS

■ REFERENCES