Combination therapy with the CDK7 inhibitor and the tyrosine kinase inhibitor exerts synergistic anticancer effects against MYCN-amplified neuroblastoma.

Patients with neuroblastoma due to MYCN oncogene amplification and consequent N-Myc oncoprotein overexpression have very poor prognosis. The cyclin-dependent kinase 7 (CDK7)/super-enhancer inhibitor THZ1 suppresses MYCN gene transcription, reduces neuroblastoma cell proliferation, but does not cause significant cell death. The protein kinase phosphatase 1 nuclear targeting subunit (PNUTS) has recently been shown to interact with c-Myc protein and suppresses c-Myc protein degradation. Here we screened the U.S. Food and Drug Administration-Approved Oncology Drugs Set V from the National Cancer Institute, and identified tyrosine kinase inhibitors (TKIs), including ponatinib and lapatinib, as the Approved Oncology Drugs exerting the best synergistic anticancer effects with THZ1 in MYCN-amplified neuroblastoma cells. Combination therapy with THZ1 and ponatinib or lapatinib synergistically induced neuroblastoma cell apoptosis, while having little effects in normal nonmalignant cells. Differential gene expression analysis identified PNUTS as one of the genes most synergistically reduced by the combination therapy. Reverse transcription polymerase chain reaction and immunoblot analyses confirmed that THZ1 and the TKIs synergistically downregulated PNUTS mRNA and protein expression and reduced N-Myc protein but not N-Myc mRNA expression. In addition, PNUTS knockdown resulted in decreased N-Myc protein but not mRNA expression and decreased MYCN-amplified neuroblastoma cell proliferation and survival. As CDK7 inhibitors are currently under clinical evaluation in patients, our data suggest the addition of the TKI ponatinib or lapatinib in CDK7 inhibitor clinical trials in patients.

A novel combination therapy targeting ubiquitin-specific protease 5 in MYCN-driven neuroblastoma.

Histone deacetylase (HDAC) inhibitors are effective in MYCN-driven cancers, because of a unique need for HDAC recruitment by the MYCN oncogenic signal. However, HDAC inhibitors are much more effective in combination with other anti-cancer agents. To identify novel compounds which act synergistically with HDAC inhibitor, such as suberanoyl hydroxamic acid (SAHA), we performed a cell-based, high-throughput drug screen of 10,560 small molecule compounds from a drug-like diversity library and identified a small molecule compound (SE486-11) which synergistically enhanced the cytotoxic effects of SAHA. Effects of drug combinations on cell viability, proliferation, apoptosis and colony forming were assessed in a panel of neuroblastoma cell lines. Treatment with SAHA and SE486-11 increased MYCN ubiquitination and degradation, and markedly inhibited tumorigenesis in neuroblastoma xenografts, and, MYCN transgenic zebrafish and mice. The combination reduced ubiquitin-specific protease 5 (USP5) levels and increased unanchored polyubiquitin chains. Overexpression of USP5 rescued neuroblastoma cells from the cytopathic effects of the combination and reduced unanchored polyubiquitin, suggesting USP5 is a therapeutic target of the combination. SAHA and SE486-11 directly bound to USP5 and the drug combination exhibited a 100-fold higher binding to USP5 than individual drugs alone in microscale thermophoresis assays. MYCN bound to the USP5 promoter and induced USP5 gene expression suggesting that USP5 and MYCN expression created a forward positive feedback loop in neuroblastoma cells. Thus, USP5 acts as an oncogenic cofactor with MYCN in neuroblastoma and the novel combination of HDAC inhibitor with SE486-11 represents a novel therapeutic approach for the treatment of MYCN-driven neuroblastoma.

Withaferin A activates TRIM16 for its anti-cancer activity in melanoma.

Although selective BRAF inhibitors and novel immunotherapies have improved short-term treatment responses in metastatic melanoma patients, acquired resistance to these therapeutics still represent a major challenge in clinical practice. In this study, we evaluated the efficacy of Withaferin A (WFA), derived from the medicinal plant Withania Somnifera, as a novel therapeutic agent for the treatment of melanoma. WFA showed selective toxicity to melanoma cells compared to non-malignant cells. WFA induced apoptosis, significantly reduced cell proliferation and inhibited migration of melanoma cells. We identified that repression of the tumour suppressor TRIM16 diminished WFA cytotoxicity, suggesting that TRIM16 was in part responsible for the cytotoxic effects of WFA in melanoma cells. Together our data indicates that WFA has potent cytopathic effects on melanoma cells through TRIM16, suggesting a potential therapeutic application of WFA in the disease.

Drugging MYCN Oncogenic Signaling through the MYCN-PA2G4 Binding Interface.

MYCN is a major driver for the childhood cancer, neuroblastoma, however, there are no inhibitors of this target. Enhanced MYCN protein stability is a key component of MYCN oncogenesis and is maintained by multiple feedforward expression loops involving MYCN transactivation target genes. Here, we reveal the oncogenic role of a novel MYCN target and binding protein, proliferation-associated 2AG4 (PA2G4). Chromatin immunoprecipitation studies demonstrated that MYCN occupies the PA2G4 gene promoter, stimulating transcription. Direct binding of PA2G4 to MYCN protein blocked proteolysis of MYCN and enhanced colony formation in a MYCN-dependent manner. Using molecular modeling, surface plasmon resonance, and mutagenesis studies, we mapped the MYCN-PA2G4 interaction site to a 14 amino acid MYCN sequence and a surface crevice of PA2G4. Competitive chemical inhibition of the MYCN-PA2G4 protein-protein interface had potent inhibitory effects on neuroblastoma tumorigenesis in vivo. Treated tumors showed reduced levels of both MYCN and PA2G4. Our findings demonstrate a critical role for PA2G4 as a cofactor in MYCN-driven neuroblastoma and highlight competitive inhibition of the PA2G4-MYCN protein binding as a novel therapeutic strategy in the disease. SIGNIFICANCE: Competitive chemical inhibition of the PA2G4-MYCN protein interface provides a basis for drug design of small molecules targeting MYC and MYCN-binding partners in malignancies driven by MYC family oncoproteins.