Upon binding to thalidomide and other immunomodulatory drugs, the E3 ligase substrate receptor cereblon (CRBN) promotes proteosomal destruction by engaging the DDB1-CUL4A-Roc1-RBX1 E3 ubiquitin ligase in human cells but not in mouse cells, suggesting that sequence variations in CRBN may cause its inactivation. Therapeutically, CRBN engagers have the potential for broad applications in cancer and immune therapy by specifically reducing protein expression through targeted ubiquitin-mediated degradation. To examine the effects of defined sequence changes on CRBN's activity, we performed a comprehensive study using complementary theoretical, biophysical, and biological assays aimed at understanding CRBN's nonprimate sequence variations. With a series of recombinant thalidomide-binding domain (TBD) proteins, we show that CRBN sequence variants retain their drug-binding properties to both classical immunomodulatory drugs and dBET1, a chemical compound and targeting ligand designed to degrade bromodomain-containing 4 (BRD4) via a CRBN-dependent mechanism. We further show that dBET1 stimulates CRBN's E3 ubiquitin-conjugating function and degrades BRD4 in both mouse and human cells. This insight paves the way for studies of CRBN-dependent proteasome-targeting molecules in nonprimate models and provides a new understanding of CRBN's substrate-recruiting function.

Resistance to Androgen receptor (AR) antagonists is a significant problem in the treatment of Castration resistant prostate cancers (CRPCs). Identification of the mechanisms by which CRPCs evade Androgen Deprivation Therapies (ADT) is critical to develop novel therapeutics. We uncovered that CRPCs rely on BRD4-HOXB13 epigenetic reprogramming for androgen-independent cell proliferation. Mechanistically, BRD4, a member of the BET bromodomain family epigenetically promotes HOXB13 expression. Consistently, genetic disruption or pharmacological suppression of HOXB13 mRNA and protein expression by the novel dual activity BET bromodomain-kinase inhibitors directly correlates with rapid induction of apoptosis, potent inhibition of tumor cell proliferation, inhibits cell migration and suppresses CRPC growth. Integrative analysis revealed that the BRD4-HOXB13 transcriptome comprises a proliferative gene network implicated in cell cycle progression, nucleotide metabolism and chromatin assembly. Notably, while the core HOXB13 target genes responsive to BET inhibitors (HOTBIN10) are overexpressed in metastatic cases, in ADT treated CRPC cell lines and patient derived circulating tumor cells (CTCs) they are insensitive to AR depletion or blockade. Among the HOTBIN10 genes, AURKB and MELK expression correlate with HOXB13 expression in CTCs of mCRPC patients who did not respond to Abiraterone (ABR), suggesting that AURKB inhibitors could be used additionally against high-risk HOXB13 positive metastatic PCs. Combined, our study demonstrates that BRD4-HOXB13-HOTBIN10 regulatory circuit maintains the malignant state of CRPCs and identifies a core pro-proliferative network driving ADT resistance that is targetable with potent dual activity bromodomain-kinase inhibitors.

Reversible protein phosphorylation regulates virtually all aspects of life in the cell. As a result, dysregulation of protein kinases, the enzymes responsible for transferring phosphate groups from ATP to proteins, are often the cause or consequence of many human diseases including cancer. Almost three dozen protein kinase inhibitors (PKIs) have been approved for clinical applications since 1995, the vast majority of them for the treatment of cancer. According to the NCI, there are more than 100 types of cancer. However, FDA-approved PKIs only target 14 of them. Importantly, of the more than 500 protein kinases encoded by the human genome, only 22 are targets for currently approved PKIs, suggesting that the reservoir of PKIs still has room to grow significantly. In this short review we will discuss the most recent advances, challenges, and alternatives to currently adopted strategies in this burgeoning field.

Inhibition of the bromodomain containing protein 9 (BRD9) by small molecules is an attractive strategy to target mutated SWI/SNF chromatin-remodeling complexes in cancer. However, reported BRD9 inhibitors also inhibit the closely related bromodomain-containing protein 7 (BRD7), which has different biological functions. The structural basis for differential potency and selectivity of BRD9 inhibitors is largely unknown because of the lack of structural information on BRD7. Here, we biochemically and structurally characterized diverse inhibitors with varying degrees of potency and selectivity for BRD9 over BRD7. Novel cocrystal structures of BRD7 liganded with new and previously reported inhibitors of five different chemical scaffolds were determined alongside BRD9 and BRD4. We also report the discovery of first-in-class dual bromodomain−kinase inhibitors outside the bromodomain and extraterminal family targeting BRD7 and BRD9. Combined, the data provide a new framework for the development of BRD7/9 inhibitors with improved selectivity or additional polypharmacologic properties.