BRD4-targeting PROTACs Synergize With Chemotherapeutics Against Osteosarcoma Cell Lines.

BACKGROUND/AIM: Osteosarcoma at an advanced stage has a poor outcome, and novel targeted therapies are needed, especially for metastatic disease. Bromodomain inhibitors (BETi) are epigenetic modulators that broadly impair the expression of oncogenic proteins and exert antitumor effects. BETi can be combined with chemotherapeutics to increase therapeutic responses with superior effects in the form of proteolysis targeting chimeras (PROTACs) that degrade proteins of interest (POI) in multiple cycles. This work aimed to investigate the efficacy of BETi, such as JQ1, dBET57, and MZ1 PROTACs in combination with cytotoxic drugs against osteosarcoma cell lines. MATERIALS AND METHODS: Chemosensitivity of the osteosarcoma cell lines HOS, Saos-2, MG-63, and G292 were tested with BET-directed agents alone or in combination with cytotoxic drugs comprising cisplatin, doxorubicin, topotecan, and gemcitabine using cell viability assays. RESULTS: The BET degraders exhibited highest toxicity to HOS cells and showed synergistic activity in combination with the chemotherapeutics, except for the degrader - topotecan/gemcitabine combinations. Highest synergy between BET agents and chemotherapeutics were found for the more chemoresistant Saos-2 cells and potentiation of toxicity in MG-63 cells for the BET agents - doxorubicin combinations and the MZ1-topotecan pair. HOS and Saos-2 cell lines had reduced protein expression of AXL, BCL-X, e-cadherin, CAIX, EpCAM, ErbB2, and vimentin in response to JQ1, MZ1, and BET57. CONCLUSION: The study suggests that the application of novel BET PROTACs in combination with chemotherapeutics could represent a new therapeutic option to improve the therapy of osteosarcomas. First orally available PROTACs have reached clinical trials.

BET inhibition rescues acinar-ductal-metaplasia and ciliogenesis and ameliorates chronic pancreatitis-driven changes in mice with loss of the polarity protein Par3.

BACKGROUND AND AIMS: The apical-basal polarity of pancreatic acinar cells is essential for maintaining tissue architecture. However, the mechanisms by which polarity proteins regulate acinar pancreas injury and regeneration are poorly understood. METHODS: Cerulein-induced pancreatitis was induced in mice with conditional deletion of the polarity protein Par3 in the pancreas. The impact of Par3 loss on pancreas injury and regeneration was assessed by histological analyses and transcriptional profiling by RNA sequencing. Mice were pretreated with the bromodomain and extraterminal domain (BET) inhibitor JQ1 before co-treatment with cerulein to determine the effect of BET inhibition on pancreas injury and regeneration. RESULTS: Initially, we show that Par3 is increased in acinar-ductal metaplasia (ADM) lesions present in human and mouse chronic pancreatitis specimens. While Par3 loss disrupts tight junctions, Par3 is dispensable for pancreatogenesis. However, with aging, Par3 loss results in low-grade inflammation, acinar degeneration, and pancreatic lipomatosis. Par3 loss exacerbates acute pancreatitis-induced injury and chronic pancreatitis-induced acinar cell loss, promotes pancreatic lipomatosis, and prevents regeneration. Par3 loss also results in suppression of chronic pancreatitis-induced ADM and primary ciliogenesis. Notably, targeting BET proteins attenuates chronic pancreatitis-induced loss of primary cilia and promotes ADM in mice lacking pancreatic Par3. Targeting BET proteins also attenuates cerulein-induced acinar cell loss and enhances recovery of acinar cell mass and body weight of mice lacking pancreatic Par3. CONCLUSIONS: Combined, this study demonstrates how Par3 restrains chronic pancreatitis-induced changes in the pancreas and identifies a potential role for BET inhibitors to attenuate pancreas injury and facilitate regeneration.

The Prognostic Role of BRD4 Expression in High-Grade Serous Ovarian Cancer.

BACKGROUND: Bromodomain and extra-terminal (BET) domain proteins that bind to acetylated lysine residues of histones serve as the "readers" of DNA acetylation. BRD4 is the most thoroughly studied member of the BET family and regulates the expression of key oncogenes. BRD4 gene amplification has been identified in ovarian cancer (~18-19%) according to The Cancer Genome Atlas (TCGA) analysis. BET inhibitors are novel small molecules that displace BET proteins from acetylated histones and are currently tested in Phase I/II trials. We here aim to explore the prognostic role of the BRD4 gene and protein expression in the ascitic fluid of patients with advanced FIGO III/IV high-grade serous ovarian carcinoma (HGSC). METHODS: Ascitic fluid was obtained from 28 patients with advanced stage (FIGO III/IV) HGSC through diagnostic/therapeutic paracentesis or laparoscopy before the initiation of chemotherapy. An amount of ~200 mL of ascitic fluid was collected from each patient and peripheral blood mononuclear cells (PBMCs) were isolated. Each sample was evaluated for BRD4 and GAPDH gene expression through RT-qPCR and BRD4 protein levels through enzyme-linked immunosorbent assay (ELISA). The study protocol was approved by the Institutional Review Board of Alexandra University Hospital and the Committee on Ethics and Good Practice (CEGP) of the National and Kapodistrian University of Athens (NKUA). RESULTS: Low BRD4 gene expression was associated with worse prognosis at 12 months compared to intermediate/high expression (95% CI; 1.75-30.49; p = 0.008). The same association was observed at 24 months although this association was not statistically significant (95% CI; 0.96-9.2; p = 0.065). Progression-free survival was shorter in patients with low BRD4 gene expression at 12 months (5.6 months; 95% CI; 2.6-8.6) compared to intermediate/high expression (9.8 months; 95% CI; 8.3-11.3) (95% CI; 1.2-16.5; p = 0.03). The same association was confirmed at 24 months (6.9 months vs. 13.1 months) (95% CI; 1.1-8.6; p = 0.048). There was a trend for worse prognosis in patients with high BRD4 protein levels versus intermediate/low BRD4 protein expression both at 12 months (9.8 months vs. 7.6 months; p = 0.3) and at 24 months (14.2 months vs. 16.6 months; p = 0.56) although not statistically significant. Again, there was a trend for shorter PFS in patients with high BRD4 protein expression although not statistically significant both at 12 months (p = 0.29) and at 24 months (p = 0.47). CONCLUSIONS: There are contradictory data in the literature over the prognostic role of BRD4 gene expression in solid tumors. In our study, intermediate/high BRD4 gene expression was associated with a favorable prognosis in terms of overall survival and progression-free survival compared to low BRD4 gene expression.

BET inhibition sensitizes innate checkpoint inhibitor resistant melanoma to anti-CTLA-4 treatment.

Approximately 50% of melanoma patients fail to respond to immune checkpoint blockade (ICB), and acquired resistance hampers long-term survival in about half of initially responding patients. Whether targeting BET reader proteins, implicated in epigenetic dysregulation, can enhance ICB response rates and durability, remains to be determined. Here we show elevated BET proteins correlate with poor survival and ICB responses in melanoma patients. The BET inhibitor IBET151, combined with anti-CTLA-4, overcame innate ICB resistance however, sequential BET inhibition failed against acquired resistance in mouse models. Combination treatment response in the innate resistance model induced changes in tumor-infiltrating immune cells, reducing myeloid-derived suppressor cells (MDSCs). CD4+ and CD8+ T cells showed decreased expression of inhibitory receptors, with reduced TIM3, LAG3, and BTLA checkpoint expression. In human PBMCs in vitro, BET inhibition reduced expression of immune checkpoints in CD4+ and CD8+ T cells, restoring effector cytokines and downregulating the transcriptional driver TOX. BET proteins in melanoma may play an oncogenic role by inducing immune suppression and driving T cell dysfunction. The study demonstrates an effective combination for innately unresponsive melanoma patients to checkpoint inhibitor immunotherapy, yet highlights BET inhibitors' limitations in an acquired resistance context.

Bromodomain Protein-directed Agents and MYC in Small Cell Lung Cancer.

Small cell lung cancer (SCLC) has a dismal prognosis. In addition to the inactivation of the tumor suppressors TP53 and RB1, tumor-promoting MYC and paralogs are frequently overexpressed in this neuroendocrine carcinoma. SCLC exhibits high resistance to second-line chemotherapy and all attempts of novel drugs and targeted therapy have failed so far to achieve superior survival. MYC and paralogs have key roles in the oncogenic process, orchestrating proliferation, apoptosis, differentiation, and metabolism. In SCLC, MYC-L and MYC regulate the neuroendocrine dedifferentiation of SCLC cells from Type A (ASCL1 expression) to the other SCLC subtypes. Targeting MYC to suppress tumor growth is difficult due to the lack of suitable binding pockets and the most advanced miniprotein inhibitor Omomyc exhibits limited efficacy. MYC may be targeted indirectly via the bromodomain (BET) protein BRD4, which activates MYC transcription, by specific BET inhibitors that reduce the expression of this oncogenic driver. Here, novel BET-directed Proteolysis Targeting Chimeras (PROTACs) are discussed that show high antiproliferative activity in SCLC. Particularly, ARV-825, targeting specifically BRD4, exhibits superior cytotoxic effects on SCLC cell lines and may become a valuable adjunct to SCLC combination chemotherapy.

A phenotypic screening approach to target p60AmotL2-expressing invasive cancer cells.

BACKGROUND: Tumor cells have the ability to invade and form small clusters that protrude into adjacent tissues, a phenomenon that is frequently observed at the periphery of a tumor as it expands into healthy tissues. The presence of these clusters is linked to poor prognosis and has proven challenging to treat using conventional therapies. We previously reported that p60AmotL2 expression is localized to invasive colon and breast cancer cells. In vitro, p60AmotL2 promotes epithelial cell invasion by negatively impacting E-cadherin/AmotL2-related mechanotransduction. METHODS: Using epithelial cells transfected with inducible p60AmotL2, we employed a phenotypic drug screening approach to find compounds that specifically target invasive cells. The phenotypic screen was performed by treating cells for 72 h with a library of compounds with known antitumor activities in a dose-dependent manner. After assessing cell viability using CellTiter-Glo, drug sensitivity scores for each compound were calculated. Candidate hit compounds with a higher drug sensitivity score for p60AmotL2-expressing cells were then validated on lung and colon cell models, both in 2D and in 3D, and on colon cancer patient-derived organoids. Nascent RNA sequencing was performed after BET inhibition to analyse BET-dependent pathways in p60AmotL2-expressing cells. RESULTS: We identified 60 compounds that selectively targeted p60AmotL2-expressing cells. Intriguingly, these compounds were classified into two major categories: Epidermal Growth Factor Receptor (EGFR) inhibitors and Bromodomain and Extra-Terminal motif (BET) inhibitors. The latter consistently demonstrated antitumor activity in human cancer cell models, as well as in organoids derived from colon cancer patients. BET inhibition led to a shift towards the upregulation of pro-apoptotic pathways specifically in p60AmotL2-expressing cells. CONCLUSIONS: BET inhibitors specifically target p60AmotL2-expressing invasive cancer cells, likely by exploiting differences in chromatin accessibility, leading to cell death. Additionally, our findings support the use of this phenotypic strategy to discover novel compounds that can exploit vulnerabilities and specifically target invasive cancer cells.