Mutant KRas is a significant driver of human oncogenesis and confers resistance to therapy, underscoring the need to develop approaches that disable mutant KRas-driven tumors. Because targeting KRas directly has proven difficult, identifying vulnerabilities specific for mutant KRas tumors is an important alternative approach. Here we show that glycogen synthase kinase 3 (GSK3) is required for the in vitro and in vivo growth and survival of human mutant KRas-dependent tumors but is dispensable for mutant KRas-independent tumors. Further, inhibiting phosphorylation of GSK3 substrates c-Myc on T58 and β-catenin on S33/S37/T41 and their subsequent upregulation contribute to the antitumor activity of GSK3 inhibition. Importantly, GSK3 blockade inhibits the in vivo growth of G12D, G12V, and G12C mutant KRas primary and metastatic patient-derived xenografts from pancreatic cancer patients who progressed on chemo- and radiation therapies. This discovery opens new avenues to target mutant KRas-dependent cancers.
Synergistic action of kinase and BET bromodomain inhibitors in cell killing has been reported for a variety of cancers. Using the chemical scaffold of the JAK2 inhibitor TG101348 we developed and characterized single agents which potently and simultaneously inhibit BRD4 and a specific set of oncogenic tyrosine kinases including JAK2, FLT3, RET, and ROS1. Lead compounds showed on-target inhibition in several blood cancer cell lines and were highly efficacious at inhibiting the growth of hematopoietic progenitor cells from myeloproliferative neoplasm (MPN) patients. Screening across 931 cancer cell lines revealed differential growth inhibitory potential with highest activity against bone and blood cancers, and greatly enhanced activity over the single BET inhibitor JQ1. Gene-drug sensitivity analyses and drug combination studies indicate synergism of BRD4 and kinase inhibition as a plausible reason for the superior potency in cell killing. Combined, our findings indicate promising potential of these agents as novel chemical probes and cancer therapeutics.
The discovery that aberrant activity of Janus kinase 2 (JAK2) is a driver of myeloproliferative neoplasms (MPNs) has led to significant efforts to develop small molecule inhibitors for this patient population. Ruxolitinib and fedratinib have been approved for use in MPN patients, while baricitinib, an achiral analogue of ruxolitinib, has been approved for rheumatoid arthritis. However, structural information on the interaction of these therapeutics with JAK2 remains unknown. Here, we describe a new methodology for the large-scale production of JAK2 from mammalian cells, which enabled us to determine the first crystal structures of JAK2 bound to these drugs and derivatives thereof. Along with biochemical and cellular data, the results provide a comprehensive view of the shape complementarity required for chiral and achiral inhibitors to achieve highest activity, which may facilitate the development of more effective JAK2 inhibitors as therapeutics.
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.
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