Overcoming acquired resistance to HSP90 inhibition by targeting JAK-STAT signalling in triple-negative breast cancer.

BACKGROUND: Due to the lack of effective therapies and poor prognosis in TNBC (triple-negative breast cancer) patients, there is a strong need to develop effective novel targeted therapies for this subtype of breast cancer. Inhibition of heat shock protein 90 (HSP90), a conserved molecular chaperone that is involved in the regulation of oncogenic client proteins, has shown to be a promising therapeutic approach for TNBC. However, both intrinsic and acquired resistance to HSP90 inhibitors (HSP90i) limits their effectiveness in cancer patients. METHODS: We developed models of acquired resistance to HSP90i by prolonged exposure of TNBC cells to HSP90i (ganetespib) in vitro. Whole transcriptome profiling and a 328-compound bioactive small molecule screen were performed on these cells to identify the molecular basis of acquired resistance to HSP90i and potential therapeutic approaches to overcome resistance. RESULTS: Among a panel of seven TNBC cell lines, the most sensitive cell line (Hs578T) to HSP90i was selected as an in vitro model to investigate acquired resistance to HSP90i. Two independent HSP90i-resistant clones were successfully isolated which both showed absence of client proteins degradation, apoptosis induction and G2/M cell cycle arrest after treatment with HSP90i. Gene expression profiling and pathway enrichment analysis demonstrate significant activation of the survival JAK-STAT signalling pathway in both HSP90i-resistant clones, possibly through IL6 autocrine signalling. A bioactive small molecule screen also demonstrated that the HSP90i-resistant clones showed selective sensitivity to JAK2 inhibition. Inhibition of JAK and HSP90 caused higher induction of apoptosis, despite prior acquired resistance to HSP90i. CONCLUSIONS: Acquired resistance to HSP90i in TNBC cells is associated with an upregulated JAK-STAT signalling pathway. A combined inhibition of the JAK-STAT signalling pathway and HSP90 could overcome this resistance. The benefits of the combined therapy could be explored further for the development of effective targeted therapy in TNBC patients.

PARP Inhibition Combined With Thoracic Irradiation Exacerbates Esophageal and Skin Toxicity in C57BL6 Mice.

PURPOSE: Poly (ADP-ribose) polymerase (PARP) inhibitors have been shown to enhance the radiosensitivity of cancer cells in vitro in a replication-dependent manner. Their in vivo radiosensitizing effects have also been demonstrated in preclinical tumor models. However, whether PARP inhibition can enhance the response to radiation therapy in normal tissues has been largely neglected. We hypothesized that PARP inhibition might also potentiate the response of replicating normal tissues to radiation therapy. In this study, we examined the normal tissue response in mice treated with PARP inhibitors (BMN673 or AZD2281) in combination with thoracic irradiation. METHODS AND MATERIALS: The antitumor effects of fractionated irradiation (5 Gy × 4) in combination with BMN673 were evaluated in nude mice bearing established Calu-6 human lung cancer xenografts. The normal tissue response was evaluated in C57BL6 mice that were treated with BMN673 or AZD2281 combined with fractionated irradiation, 5 Gy × 4, delivered to the whole thorax. Body weight and histology of the esophagus and skin in the field of irradiation were examined. The DNA damage response in the esophagus and skin was assessed by γH2AX immunohistochemistry. RESULTS: While PARP inhibition enhanced irradiation-induced tumor growth inhibition in nude mice, it was also associated with significant body weight loss and increased damage to the esophagus and skin within the field of irradiation in C57BL6 mice. PARP inhibition compromised the repair of irradiation-induced DNA damage in the esophagus and skin. CONCLUSIONS: Although PARP inhibition enhanced the antitumor response to fractionated irradiation, it also enhanced the irradiation response in replicating normal tissues. Therefore, our study suggests that additional caution may be warranted in the clinical development of combination therapies using PARP inhibitors and radiation therapy, in particular where the field of irradiation includes the esophagus.

Metalloprotease SPRTN/DVC1 Orchestrates Replication-Coupled DNA-Protein Crosslink Repair.

The cytotoxicity of DNA-protein crosslinks (DPCs) is largely ascribed to their ability to block the progression of DNA replication. DPCs frequently occur in cells, either as a consequence of metabolism or exogenous agents, but the mechanism of DPC repair is not completely understood. Here, we characterize SPRTN as a specialized DNA-dependent and DNA replication-coupled metalloprotease for DPC repair. SPRTN cleaves various DNA binding substrates during S-phase progression and thus protects proliferative cells from DPC toxicity. Ruijs-Aalfs syndrome (RJALS) patient cells with monogenic and biallelic mutations in SPRTN are hypersensitive to DPC-inducing agents due to a defect in DNA replication fork progression and the inability to eliminate DPCs. We propose that SPRTN protease represents a specialized DNA replication-coupled DPC repair pathway essential for DNA replication progression and genome stability. Defective SPRTN-dependent clearance of DPCs is the molecular mechanism underlying RJALS, and DPCs are contributing to accelerated aging and cancer.

Phenotypic consequences of somatic mutations in the ataxia-telangiectasia mutated gene in non-small cell lung cancer.

Mutations in the Ataxia-telangiectasia mutated (ATM) gene are frequently found in human cancers, including non-small cell lung cancer (NSCLC). Loss of ATM function confers sensitivity to ionising radiation (IR) and topoisomerase inhibitors and may thus define a subset of cancer patients that could get increased benefit from these therapies. In this study, we evaluated the phenotypic consequences of ATM missense changes reported in seven NSCLC cell lines with regard to radiosensitivity and functionality of ATM signalling. Our data demonstrate that only 2/7 NSCLC cell lines (H1395 and H23) harbouring ATM missense mutations show a functional impairment of ATM signalling following IR-exposure. In these two cell lines, the missense mutations caused a significant reduction in ATM protein levels, impairment of ATM signalling and marked radiosensitivity. Of note, only cell lines with homozygous mutations in the ATM gene showed significant impairment of ATM function. Based on these observations, we developed an immunohistochemistry-based assay to identify patients with loss or reduction of ATM protein expression in a clinical setting. In a set of 137 NSCLC and 154 colorectal cancer specimens we identified tumoral loss of ATM protein expression in 9.5% and 3.9% of cases, respectively, demonstrating the potential utility of this method.

Inhibiting WEE1 Selectively Kills Histone H3K36me3-Deficient Cancers by dNTP Starvation.

Histone H3K36 trimethylation (H3K36me3) is frequently lost in multiple cancer types, identifying it as an important therapeutic target. Here we identify a synthetic lethal interaction in which H3K36me3-deficient cancers are acutely sensitive to WEE1 inhibition. We show that RRM2, a ribonucleotide reductase subunit, is the target of this synthetic lethal interaction. RRM2 is regulated by two pathways here: first, H3K36me3 facilitates RRM2 expression through transcription initiation factor recruitment; second, WEE1 inhibition degrades RRM2 through untimely CDK activation. Therefore, WEE1 inhibition in H3K36me3-deficient cells results in RRM2 reduction, critical dNTP depletion, S-phase arrest, and apoptosis. Accordingly, this synthetic lethality is suppressed by increasing RRM2 expression or inhibiting RRM2 degradation. Finally, we demonstrate that WEE1 inhibitor AZD1775 regresses H3K36me3-deficient tumor xenografts.