Objective The statin family of cholesterol-lowering drugs has been shown to induce tumor-specific apoptosis by inhibiting the rate-limiting enzyme of the mevalonate (MVA) pathway, HMG-CoA reductase (HMGCR). Accumulating evidence suggests that statin use may delay prostate cancer (PCa) progression in a subset of patients; however, the determinants of statin drug sensitivity in PCa remain unclear. Our goal was to identify molecular features of statin-sensitive PCa and opportunities to potentiate statin-induced PCa cell death. Methods Deregulation of HMGCR expression in PCa was evaluated by immunohistochemistry. The response of PCa cell lines to fluvastatin-mediated HMGCR inhibition was assessed using cell viability and apoptosis assays. Activation of the sterol-regulated feedback loop of the MVA pathway, which was hypothesized to modulate statin sensitivity in PCa, was also evaluated. Inhibition of this statin-induced feedback loop was performed using RNA interference or small molecule inhibitors. The achievable levels of fluvastatin in mouse prostate tissue were measured using liquid chromatography–mass spectrometry. Results High HMGCR expression in PCa was associated with poor prognosis; however, not all PCa cell lines underwent apoptosis in response to treatment with physiologically-achievable concentrations of fluvastatin. Rather, most cell lines initiated a feedback response mediated by sterol regulatory element-binding protein 2 (SREBP2), which led to the further upregulation of HMGCR and other lipid metabolism genes. Overcoming this feedback mechanism by knocking down or inhibiting SREBP2 potentiated fluvastatin-induced PCa cell death. Notably, we demonstrated that this feedback loop is pharmacologically-actionable, as the drug dipyridamole can be used to block fluvastatin-induced SREBP activation and augment apoptosis in statin-insensitive PCa cells. Conclusion Our study implicates statin-induced SREBP2 activation as a PCa vulnerability that can be exploited for therapeutic purposes using clinically-approved agents.

The mevalonate (MVA) pathway is often dysregulated or overexpressed in many cancers suggesting tumor dependency on this classic metabolic pathway. Statins, which target the rate-limiting enzyme of this pathway, 3-hydroxy-3-methylglutaryl-CoA reductase (HMGCR), are promising agents currently being evaluated in clinical trials for anti-cancer efficacy. To uncover novel targets that potentiate statin-induced apoptosis when knocked down, we carried out a pooled genome-wide short hairpin RNA (shRNA) screen. Genes of the MVA pathway were amongst the top-scoring targets, including sterol regulatory element binding transcription factor 2 (SREBP2), 3-hydroxy-3-methylglutaryl-coenzyme A synthase 1 (HMGCS1) and geranylgeranyl diphosphate synthase 1 (GGPS1). Each gene was independently validated and shown to significantly sensitize A549 cells to statin-induced apoptosis when knocked down. SREBP2 knockdown in lung and breast cancer cells completely abrogated the fluvastatin-induced upregulation of sterol-responsive genes HMGCR and HMGCS1. Knockdown of SREBP2 alone did not affect three-dimensional growth of lung and breast cancer cells, yet in combination with fluvastatin cell growth was disrupted. Taken together, these results show that directly targeting multiple levels of the MVA pathway, including blocking the sterol-feedback loop initiated by statin treatment, is an effective and targetable anti-tumor strategy.

Yeasts are known to have versatile metabolic traits, while how these metabolic traits have evolved has not been elucidated systematically. We performed integrative evolution analysis to investigate how genomic evolution determines trait generation by reconstructing genome-scale metabolic models (GEMs) for 332 yeasts. These GEMs could comprehensively characterize trait diversity and predict enzyme functionality, thereby signifying that sequence-level evolution has shaped reaction networks towards new metabolic functions. Strikingly, using GEMs, we can mechanistically map different evolutionary events, e.g. horizontal gene transfer and gene duplication, onto relevant subpathways to explain metabolic plasticity. This demonstrates that gene family expansion and enzyme promiscuity are prominent mechanisms for metabolic trait gains, while GEM simulations reveal that additional factors, such as gene loss from distant pathways, contribute to trait losses. Furthermore, our analysis could pinpoint to specific genes and pathways that have been under positive selection and relevant for the formulation of complex metabolic traits, i.e. thermotolerance and the Crabtree effect. Our findings illustrate how multidimensional evolution in both metabolic network structure and individual enzymes drives phenotypic variations.

The statin family of drugs target the mevalonate pathway and have been used for decades in the control of hypercholesterolemia, however recent evidence suggests these approved agents may also be useful as anti-cancer therapeutics (see our recent Nature Review Cancer article1). For example, statins can trigger tumor-specific apoptosis and two independent pre-op clinical trials in breast cancer, evaluating cholesterol-lowering doses of fluvastatin and atorvastatin, resulted in breast tumor shrinkage due to decreased growth and increased apoptosis. Our hypothesis is that statins have utility as anti-breast cancer agents. To maximize efficacy and speak to personalized medicine, our objectives are to develop biomarkers to distinguish which patients will benefit from the addition of statins to their treatment regimen and how best to use statins in combination with other agents to augment anti-tumor efficacy. To identify biomarkers of statin sensitivity we evaluated fluvastatin activity across a panel of BCa cell lines and showed that the basal, estrogen receptor-negative subtype were significantly sensitive to statin-induced apoptosis. As this included the difficult-to-treat triple negative BCa (TNBCa) we have extended this work and further evaluated a panel of TNBCa cell lines for statin sensitivity. From these results we are identifying features associated with robust apoptosis in response to statin exposure. To identify how best to use statins, we conducted two unbiased screens and have shown that blocking the restorative feedback response to statin exposure potentiates statin-induced apoptosis. This is reversible with exogenous mevalonate, reinforcing that this is an on-target effect. We also identified another approved agent, dipyridamole, as able to potentiate the anti-cancer activity of statins. Mechanistically we have shown that dipyridamole blocks the feedback response to statin exposure. We have extended these studies and shown that the combination of statins and dipyridamole is effective against TNBCa both in vitro and in vivo. Thus, we provide essential pre-clinical data to support the further evaluation of statins and dipyridamole in BCa. Reference: Mullen, P.J. et al. The interplay between cell signalling and the mevalonate pathway in cancer. Nat Rev Cancer. 16, 718-731 (2016). Citation Format: Jenna van Leeuwen, Aleksandra Pandyra, Carolyn Goard, Peter J. Mullen, Rosemary Yu, Linda Z. Penn. Targeting the metabolic mevalonate pathway with statins as anti-breast cancer agents [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 3543. doi:10.1158/1538-7445.AM2017-3543

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Background:Eukaryotic organisms, like the model yeast S. cerevisiae, have linear chromosomes that facilitate organization and protection of nuclear DNA. A recent work described a stepwise break/repair method that enabled fusion of the sixteen chromosomes of S. cerevisiae into a single large chromosome. Construction of this strain resulted in the removal of 30 of 32 telomeres, over 300kb of subtelomeric DNA, and 107 subtelomeric ORFs. Despite these changes, characterization of the single chromosome strain uncovered modest phenotypes compared to a reference strain.Results:This study further characterized the single chromosome strain and found that it exhibited a longer lag phase, increased doubling time, and lower final biomass concentration compared with a reference strain when grown on YPD. These phenotypes were amplified when ethanol was added to the medium or used as the sole carbon source. RNAseq analysis showed poor induction of genes involved in diauxic shift, ethanol metabolism, and fatty-acid ß-oxidation during growth on ethanol compared to the reference strain. Enzyme-constrained metabolic modeling identified decreased flux through the enzymes that are encoded by these poorly induced genes as a likely cause of diminished biomass accumulation. The diminished growth on ethanol for the single chromosome strain was rescued by nicotinamide, an inhibitor of sirtuin family deacetylases, which have been shown to silence gene expression in heterochromatic regions. Conclusions:Our results indicate that sirtuin-mediated silencing in the single chromosome strain interferes with growth on non-fermentable carbon sources. We propose that the removal of subtelomeric DNA that would otherwise be bound by sirtuins leads to silencing at other loci in the single chromosome strain. Further, we hypothesize that the poorly induced genes in the single chromosome strain during ethanol growth could be silenced by sirtuins in wildtype S. cerevisiae during growth on glucose.

Recent comprehensive breast cancer studies examining mutations and genomic alterations have determined that deregulation of MYC and the PI3K pathway occur frequently during breast cancer progression and may be useful targets for therapy. As a result, there have been large efforts to develop PI3K, AKT and mTOR inhibitors (PAM inhibitors) for clinical use, however clinical trial data demonstrates that many patients treated with PAM inhibitors develop resistant disease. An alternative strategy would be to target Myc, though a lack of effective and specific inhibitors makes this difficult. To identify the core vulnerabilities in these cancers we developed an in vivo xenograft model of triple-negative breast cancer driven by deregulated PI3K signaling and MYC. We hypothesize that determining how these pathways co-operate to transform normal human breast cells into breast carcinomas will reveal a tumor progression signature and highlight new therapeutic opportunities. We developed our model using the spontaneously immortalized, basal, triple-negative MCF10A cell line. By expressing the hotspot PIK3caH1047R protein alone in MCF10A cells (MCF10.H) in addition to MYC (MCF10.HM), we can model normal/early breast cancer and invasive ductal carcinoma respectively. This is the first in vivo human model of breast cancer dependent on MYC for transformation. When injected into female NOD-SCID mice, MCF10A.H cells form organized acinar ducts embedded in extracellular matrix. MCF10A.H ducts form with hollow lumen and a single layer of myoepithelial cells, recapitulating normal human breast histology. Alternatively, MCF10A.HM cells grow as high-grade carcinomas indicative of invasive disease. MCF10A.H benign growths and MCF10A.HM tumors remain basal-like and triple-negative by immunohistochemistry. Importantly, MCF10A.HM tumors are sensitive to MYC repression and therefore may be a suitable model to evaluate direct and indirect anti-MYC therapies. Having relevant human xenograft samples representing both normal and IDC tissue, we performed RNA-seq to identify a MYC-signature driving breast cancer transformation. Our current work will involve targeting the resulting MYC-driven pathways identified by RNA-seq to therapeutically target MYC in breast cancer. Citation Format: Corey Lourenco, Manpreet Kalkat, Dharmesh Dingar, Jason De Melo, Rosemary Yu, Linda Penn. MYC-dependent transformation model of triple-negative breast cancer in vivo [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 2558. doi:10.1158/1538-7445.AM2017-2558