Combined use of subclinical hydroxyurea and CHK1 inhibitor effectively controls melanoma and lung cancer progression, with reduced normal tissue toxicity compared to gemcitabine

Drugs such as gemcitabine that increase replication stress are effective chemotherapeutics in a range of cancer settings. These drugs effectively block replication and promote DNA damage, triggering a cell cycle checkpoint response through the ATR–CHK1 pathway. Inhibiting this signalling pathway sensitises cells to killing by replication stress‐inducing drugs. Here, we investigated the effect of low‐level replication stress induced by low concentrations (> 0.2 mm) of the reversible ribonucleotide reductase inhibitor hydroxyurea (HU), which slows S‐phase progression but has little effect on cell viability or proliferation. We demonstrate that HU effectively synergises with CHK1, but not ATR inhibition, in > 70% of melanoma and non‐small‐cell lung cancer cells assessed, resulting in apoptosis and complete loss of proliferative potential in vitro and in vivo. Normal fibroblasts and haemopoietic cells retain viability and proliferative potential following exposure to CHK1 inhibitor plus low doses of HU, but normal cells exposed to CHK1 inhibitor combined with submicromolar concentrations of gemcitabine exhibited complete loss of proliferative potential. The effects of gemcitabine on normal tissue correlate with irreversible ATR–CHK1 pathway activation, whereas low doses of HU reversibly activate CHK1 independently of ATR. The combined use of CHK1 inhibitor and subclinical HU also triggered an inflammatory response involving the recruitment of macrophages in vivo. These data indicate that combining CHK1 inhibitor with subclinical HU is superior to combination with gemcitabine, as it provides equal anticancer efficacy but with reduced normal tissue toxicity. These data suggest a significant proportion of melanoma and lung cancer patients could benefit from treatment with this drug combination.

Endogenous Replication Stress Marks Melanomas Sensitive to CHEK1 Inhibitors In Vivo

Purpose: Checkpoint kinase 1 inhibitors (CHEK1i) have single-agent activity in vitro and in vivo Here, we have investigated the molecular basis of this activity.Experimental Design: We have assessed a panel of melanoma cell lines for their sensitivity to the CHEK1i GNE-323 and GDC-0575 in vitro and in vivo The effects of these compounds on responses to DNA replication stress were analyzed in the hypersensitive cell lines.Results: A subset of melanoma cell lines is hypersensitive to CHEK1i-induced cell death in vitro, and the drug effectively inhibits tumor growth in vivo In the hypersensitive cell lines, GNE-323 triggers cell death without cells entering mitosis. CHEK1i treatment triggers strong RPA2 hyperphosphorylation and increased DNA damage in only hypersensitive cells. The increased replication stress was associated with a defective S-phase cell-cycle checkpoint. The number and intensity of pRPA2 Ser4/8 foci in untreated tumors appeared to be a marker of elevated replication stress correlated with sensitivity to CHEK1i.Conclusions: CHEK1i have single-agent activity in a subset of melanomas with elevated endogenous replication stress. CHEK1i treatment strongly increased this replication stress and DNA damage, and this correlated with increased cell death. The level of endogenous replication is marked by the pRPA2Ser4/8 foci in the untreated tumors, and may be a useful marker of replication stress in vivoClin Cancer Res; 24(12); 2901-12. ©2018 AACR.

Genome-wide gain-of-function screen for genes that induce epithelial-to-mesenchymal transition in breast cancer

Epithelial to mesenchymal transition (EMT) is a developmental program that has been implicated in progression, metastasis and therapeutic resistance of some carcinomas. To identify genes whose overexpression drives EMT, we screened a lentiviral expression library of 17000 human open reading frames (ORFs) using high-content imaging to quantitate cytoplasmic vimentin. Hits capable of increasing vimentin in the mammary carcinoma-derived cell line MDA-MB-468 were confirmed in the non-tumorigenic breast-epithelial cell line MCF10A. When overexpressed in this model, they increased the rate of cell invasion through Matrigel™, induced mesenchymal marker expression and reduced expression of the epithelial marker E-cadherin. In gene-expression datasets derived from breast cancer patients, the expression of several novel genes correlated with expression of known EMT marker genes, indicating their in vivo relevance. As EMT-associated properties are thought to contribute in several ways to cancer progression, genes identified in this study may represent novel targets for anti-cancer therapy.

Cell Cycle Phase-Specific Drug Resistance as an Escape Mechanism of Melanoma Cells

The tumor microenvironment is characterized by cancer cell subpopulations with heterogeneous cell cycle profiles. For example, hypoxic tumor zones contain clusters of cancer cells that arrest in G1 phase. It is conceivable that neoplastic cells exhibit differential drug sensitivity based on their residence in specific cell cycle phases. In this study, we used two-dimensional and organotypic melanoma culture models in combination with fluorescent cell cycle indicators to investigate the effects of cell cycle phases on clinically used drugs. We demonstrate that G1-arrested melanoma cells, irrespective of the underlying cause mediating G1 arrest, are resistant to apoptosis induced by the proteasome inhibitor bortezomib or the alkylating agent temozolomide. In contrast, G1-arrested cells were more sensitive to mitogen-activated protein kinase pathway inhibitor-induced cell death. Of clinical relevance, pretreatment of melanoma cells with a mitogen-activated protein kinase pathway inhibitor, which induced G1 arrest, resulted in resistance to temozolomide or bortezomib. On the other hand, pretreatment with temozolomide, which induced G2 arrest, did not result in resistance to mitogen-activated protein kinase pathway inhibitors. In summary, we established a model to study the effects of the cell cycle on drug sensitivity. Cell cycle phase-specific drug resistance is an escape mechanism of melanoma cells that has implications on the choice and timing of drug combination therapies.

Abstract 1781: G1 phase melanoma cells escape proteasome inhibitor cytotoxicity

Utilising the fluorescent ubiquitination-based cell cycle indicator (FUCCI), which facilitates real-time cell cycle tracking, we have demonstrated in vitro and in vivo that melanomas are composed of differentially cycling tumour cells in a subcompartment-specific distribution. Further, we have shown that targeting the endoplasmic reticulum with fenretinide (synthetic retinoid) or bortezomib (26S proteasome inhibitor) induces cell cycle arrest and apoptosis of metastatic melanoma cells in vitro and in vivo.

This study aims to investigate the effect of ER stress-inducing agents on the dynamics of cell division and cell death of individual melanoma cells within the complex tumor microenvironment, and to develop combination strategies that increase the efficacy of ER stress-inducing agents for melanoma therapy.

FUCCI-melanoma cells were grown as 3D spheroids and implanted into a collagen matrix to mimic tumor architecture and microenvironment. Utilising the F-XBP1ΔDBD-venus reporter construct, which labels the cytoplasm in response to ER stress, we found that bortezomib induced ER stress, delayed cell cycle progression, and combination with fenretinide increased cell death in 2D and 3D culture. Flow cytometry and confocal microscopy indicated that treatment of FUCCI-melanoma cells with bortezomib induced G2 accumulation in 2D and 3D culture over the course of 24 h. In contrast, by 72 h the majority of cells were in G1 phase. Analysis of 2D and 3D real-time cell cycle imaging movies revealed that bortezomib induced both G1- and G2 arrest, but preferentially killed G2-phase cells. Consequently, pretreatment with temozolomide or fenretinide, leading to G2-arrest, sensitised melanoma cells to bortezomib cytotoxicity. In contrast, pretreatment with MEK inhibitors, leading to G1 arrest, inhibited bortezomib cytotoxicity in all melanoma cells, as did selective BRAF inhibitors in BRAF mutant melanoma cells.

Our data suggest that bortezomib combined with fenretinide or temozolamide is a strategy worth exploring for the treatment of BRAF-inhibitor insensitive or resistant melanoma. Importantly, melanoma cells arrested in G1 are protected from bortezomib cytotoxicity, which excludes MAPK pathway inhibitors as combination partners.

Citation Format: Sheena M. Daignault, David S. Hill, Kimberley A. Beaumont, Andrea Anfosso, Penny E. Lovat, Wolfgang Weninger, Nikolas K. Haass. G1 phase melanoma cells escape proteasome inhibitor cytotoxicity. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 1781. doi:10.1158/1538-7445.AM2015-1781

Abstract 1420: MITF regulates proliferative subpopulation tumor architecture and modifies invasion and characteristics of the epithelial to mesenchymal transition within melanoma

Melanoma is the leading cause of skin cancer-related death. Survival rates are high if the disease is diagnosed early, but drop precipitously at later stages.

Small molecule inhibitor therapy has given robust responses in the clinic, but relapse is almost certain. This relapse may be due, in part, to tumor heterogeneity. Not only are there multiple cell types within a tumor, but cancer cells themselves can exhibit various phenotypes. This can be due to genotype variation or nutrient availability, and result in populations with different proliferative and invasive capabilities. As these cells display various behaviors, they could also respond to therapies uniquely. Understanding the molecular signature influencing different sub-populations is therefore crucial to design the most effective therapeutic regimen.

The fluorescence ubiquitination cell cycle indicator (FUCCI) system, which delineates phases of the cell cycle by visual means, was employed to better understand melanoma tumor heterogeneity. Using this model, it was found that tumor xenografts grown in mice produce two cohorts. One that contained distinct clusters of either arrested or proliferating cells, and another that displayed a homogenous dispersion of proliferating cells throughout the breadth of the tumor. These cohorts were subsequently discovered to display either low or high levels of microphthalmia-associated transcription factor (MITF) expression, respectively. Additionally, loss of MITF by shRNA treatment resulted in conversion of the ability of melanoma cells to give rise to a homogenous xenograft, to instead produce a clustered tumor phenotype. Furthermore, in a 3D in vitro tumor spheroid model, MITF expression was predominantly found in the periphery of the spheroid, which corresponds with the region of highly proliferative cells. Forced over-expression of MITF within these spheroids results in loss of the distinct proliferative ring, and instead a homogenous growth pattern. Not only do spheroids express MITF around the perimeter, but also markers of the Epithelial to Mesenchymal Transition (EMT). These markers, such as Vimentin and Slug, also switch to become expressed homogenously upon high MITF expression. Surprisingly, the increased levels of EMT marker expression by MITF do not correlate to increased migration, and these spheroids in fact show reduced invasion into collagen. We are currently exploring what other means of cancer cell migration could trump an enhanced EMT phenotype and slow invasion in our model. These data outline how tumor heterogeneity, including proliferative and invasive potential, is tightly intertwined with MITF expression, making it an important marker for therapy design.

Citation Format: Crystal A. Tonnessen, Kimberley A. Beaumont, David S. Hill, Sheena M. Daignault, Andrea Anfosso, Russell J. Jurek, Wolfgang Weninger, Nikolas K. Haass. MITF regulates proliferative subpopulation tumor architecture and modifies invasion and characteristics of the epithelial to mesenchymal transition within melanoma. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 1420. doi:10.1158/1538-7445.AM2015-1420