MoiRNAiFold: a novel tool for complex in silico RNA design.

Novel tools for in silico design of RNA constructs such as riboregulators are required in order to reduce time and cost to production for the development of diagnostic and therapeutic advances. Here, we present MoiRNAiFold, a versatile and user-friendly tool for de novo synthetic RNA design. MoiRNAiFold is based on Constraint Programming and it includes novel variable types, heuristics and restart strategies for Large Neighborhood Search. Moreover, this software can handle dozens of design constraints and quality measures and improves features for RNA regulation control of gene expression, such as Translation Efficiency calculation. We demonstrate that MoiRNAiFold outperforms any previous software in benchmarking structural RNA puzzles from EteRNA. Importantly, with regard to biologically relevant RNA designs, we focus on RNA riboregulators, demonstrating that the designed RNA sequences are functional both in vitro and in vivo. Overall, we have generated a powerful tool for de novo complex RNA design that we make freely available as a web server (https://moiraibiodesign.com/design/).

P-glycoprotein (ABCB1) activity decreases raltegravir disposition in primary CD4+P-gphigh cells and correlates with HIV-1 viral load.

OBJECTIVES: To evaluate the role of P-glycoprotein (P-gp) and multidrug-resistant-protein 1 (MRP1) on raltegravir intracellular drug disposition in CD4+ T cells, investigate the effect of HIV-1 infection on P-gp expression and correlate HIV-1 viraemia with P-gp activity in primary CD4+ T cell subsets. METHODS: The cellular accumulation ratio of [(3)H]raltegravir was quantified in CD4+ T cell lines overexpressing either P-gp (CEM-P-gp) or MRP1 (CEM-MRP1) and in primary CD3+CD4+ T cells with high (P-gp(high)) and low P-gp activity (P-gp(low)); inhibition of efflux transporters was confirmed by the intracellular retention of calcein-AM. The correlation of P-gp activity with HIV-1 viraemia was assessed in naive and memory T cell subsets from 21 HIV-1-infected treatment-naive subjects. RESULTS: [(3)H]Raltegravir cellular accumulation ratio decreased in CEM-P-gp cells (P < 0.0001). XR9051 (a P-gp inhibitor) and HIV-1 PIs reversed this phenomenon. Primary CD4+P-gp(high) cells accumulated less raltegravir (38.4% ±â€Š9.6%) than P-gp(low) cells, whereas XR9051 also reversed this effect. In vitro HIV-1 infection of PBMCs and stimulation of CD4+ T cells increased P-gp mRNA and P-gp activity, respectively, while primary CD4+P-gp(high) T cells sustained a higher HIV-1 replication than P-gp(low) cells. A significant correlation between HIV-1 viraemia and P-gp activity was found in different CD4+ T cell subsets, particularly memory CD4+ T cells (r = 0.792, P < 0.0001). CONCLUSIONS: Raltegravir is a substrate of P-gp in CD4+ T cells. Primary CD4+P-gp(high) T cells eliminate intracellular raltegravir more readily than P-gp(low) cells and HIV-1 viraemia correlates with P-gp overall activity. Specific CD4+P-gp(high) T cell subsets could facilitate the persistence of viral replication in vivo and ultimately promote the appearance of drug resistance.

A non-infectious cell-based phenotypic assay for the assessment of HIV-1 susceptibility to protease inhibitors.

OBJECTIVES: HIV-1 genotyping is widely accepted as a diagnostic tool to optimize therapy changes in patients whose antiretroviral regimen is failing. Phenotyping can substantially complement the information obtained from genotyping, especially in the presence of complex mutational patterns. However, drug susceptibility tests are laborious and require biosafety facilities. We describe the molecular mechanism of a non-infectious HIV-1 protease phenotypic assay in eukaryotic cells and validate its applicability as a tool for monitoring drug resistance. METHODS: A cloning vector containing the fusion protein green fluorescent protein-HIV-1 protease (GFP-PR) was modified to facilitate the insertion of HIV-1 protease from infected subjects. Real-time quantitative PCR and western blot analysis were used to establish the molecular mechanism of the new phenotypic assay. The method was validated by analysing HIV-1 protease from 46 clinical isolates. Statistical comparisons were made between values obtained using our assay and those reported from alternative standardized phenotypic assays. RESULTS: The capacity of HIV-1 protease to cleave cellular translation factors, such as the eukaryotic translation initiation factor 4 (eIF4GI) and the poly(A)-binding protein (PABP), led to cyclical accumulation of GFP that varied with the dose of protease inhibitors. Validation and comparison revealed a significant correlation with the Virco TYPE HIV-1 test (P < 0.0001, Spearman's ρ = 0.60), the Antivirogram test (P = 0.0001, Spearman's ρ = 0.60) and the Stanford HIVdb (P < 0.0001, Spearman's ρ = 0.69). CONCLUSIONS: This cell-based non-infectious phenotypic method with a well-understood molecular mechanism was highly reliable and comparable to other widely used assays. The method can be used for both phenotyping of HIV-1 viral isolates resistant to protease inhibitors and screening of new protease inhibitors.

Convergent organization of aberrant MYB complex controls oncogenic gene expression in acute myeloid leukemia.

Dysregulated gene expression contributes to most prevalent features in human cancers. Here, we show that most subtypes of acute myeloid leukemia (AML) depend on the aberrant assembly of MYB transcriptional co-activator complex. By rapid and selective peptidomimetic interference with the binding of CBP/P300 to MYB, but not CREB or MLL1, we find that the leukemic functions of MYB are mediated by CBP/P300 co-activation of a distinct set of transcription factor complexes. These MYB complexes assemble aberrantly with LYL1, E2A, C/EBP family members, LMO2, and SATB1. They are organized convergently in genetically diverse subtypes of AML and are at least in part associated with inappropriate transcription factor co-expression. Peptidomimetic remodeling of oncogenic MYB complexes is accompanied by specific proteolysis and dynamic redistribution of CBP/P300 with alternative transcription factors such as RUNX1 to induce myeloid differentiation and apoptosis. Thus, aberrant assembly and sequestration of MYB:CBP/P300 complexes provide a unifying mechanism of oncogenic gene expression in AML. This work establishes a compelling strategy for their pharmacologic reprogramming and therapeutic targeting for diverse leukemias and possibly other human cancers caused by dysregulated gene control.

Transport of lamivudine [(-)-beta-L-2',3'-dideoxy-3'-thiacytidine] and high-affinity interaction of nucleoside reverse transcriptase inhibitors with human organic cation transporters 1, 2, and 3.

Nucleoside reverse transcriptase inhibitors (NRTIs) need to enter cells to act against the HIV-1. Human organic cation transporters (hOCT1-3) are expressed and active in CD4+ T cells, the main target of HIV-1, and have been associated with antiviral uptake in different tissues. In this study, we examined whether NRTIs interact and are substrates of hOCT in cells stably expressing these transporters. Using [(3)H]N-methyl-4-phenylpyridinium, we found a high-affinity interaction among abacavir [[(1S,4R)-4-[2-amino-6-(cyclopropylamino)purin-9-yl]-cyclopent-2-enyl]methanol sulfate] (ABC); <0.08 nM], azidothymidine [3'-azido-3'-deoxythymidine (AZT); <0.4 nM], tenofovir disoproxil fumarate (<1.0 nM), and emtricitabine (<2.5 nM) and hOCTs. Using a wide range of concentrations of lamivudine [(-)-beta-L-2',3'-dideoxy-3'-thiacyitidine (3TC)], we determined two different binding sites for hOCTs: a high-affinity site (K(d1) = 12.3-15.4 pM) and a low-affinity site (K(d2) = 1.9-3.4 mM). Measuring direct uptake of [(3)H]3TC and inhibition with hOCT substrates, we identified 3TC as a novel substrate for hOCT1, 2, and 3, with hOCT1 as the most efficient transporter (K(m) = 1.25 +/- 0.1 mM; V(max) = 10.40 +/- 0.32 nmol/mg protein/min; V(max)/K(m) = 8.32 +/- 0.40 microl/mg protein/min). In drug-drug interaction experiments, we analyzed cis-inhibition of [(3)H]3TC uptake by ABC and AZT and found that 40 to 50% was inhibited at low concentrations of the drugs (K(i) = 22-500 pM). These data reveal that NRTIs experience a high-affinity interaction with hOCTs, suggesting a putative role for these drugs as modulators of hOCT activity. Finally, 3TC is a novel substrate for hOCTs and the inhibition of its uptake at low concentrations of ABC and AZT could have implications for the pharmacokinetics of 3TC.

PI3K Inhibition Activates SGK1 via a Feedback Loop to Promote Chromatin-Based Regulation of ER-Dependent Gene Expression.

The PI3K pathway integrates extracellular stimuli to phosphorylate effectors such as AKT and serum-and-glucocorticoid-regulated kinase (SGK1). We have previously reported that the PI3K pathway regulates estrogen receptor (ER)-dependent transcription in breast cancer through the phosphorylation of the lysine methyltransferase KMT2D by AKT. Here, we show that PI3Kα inhibition, via a negative-feedback loop, activates SGK1 to promote chromatin-based regulation of ER-dependent transcription. PI3K/AKT inhibitors activate ER, which promotes SGK1 transcription through direct binding to its promoter. Elevated SGK1, in turn, phosphorylates KMT2D, suppressing its function, leading to a loss of methylation of lysine 4 on histone H3 (H3K4) and a repressive chromatin state at ER loci to attenuate ER activity. Thus, SGK1 regulates the chromatin landscape and ER-dependent transcription via the direct phosphorylation of KMT2D. These findings reveal an ER-SGK1-KMT2D signaling circuit aimed to attenuate ER response through a role for SGK1 to program chromatin and ER transcriptional output.

Small-molecule targeting of MUSASHI RNA-binding activity in acute myeloid leukemia.

The MUSASHI (MSI) family of RNA binding proteins (MSI1 and MSI2) contribute to a wide spectrum of cancers including acute myeloid leukemia. We find that the small molecule Ro 08-2750 (Ro) binds directly and selectively to MSI2 and competes for its RNA binding in biochemical assays. Ro treatment in mouse and human myeloid leukemia cells results in an increase in differentiation and apoptosis, inhibition of known MSI-targets, and a shared global gene expression signature similar to shRNA depletion of MSI2. Ro demonstrates in vivo inhibition of c-MYC and reduces disease burden in a murine AML leukemia model. Thus, we identify a small molecule that targets MSI's oncogenic activity. Our study provides a framework for targeting RNA binding proteins in cancer.

Expression and functionality of anti-human immunodeficiency virus and anticancer drug uptake transporters in immune cells.

Almost all drugs used in anti-human immunodeficiency virus (HIV)-1 and anticancer therapies require membrane proteins to get into the cell to develop their proper activity. Nevertheless, little is known regarding the expression and activity of specific carriers involved in the uptake of these drugs in immune cells. Here, we assessed the mRNA levels, protein expression profile, and activity of the gene families SLC28 (coding for concentrative nucleoside transporters, hCNT1-3), SLC29 (equilibrative nucleoside transporters, hENT1-2), and SLC22 (organic cation transporters, hOCT1-3 and hOCTN1-2). Both hENTs and hCNT2 were abundant in primary lymphocytes, with a preferential activity of hENT1. A significant up-regulation in hENTs expression (100-fold) and activity (30-fold) was seen under stimulation of primary T lymphocytes. In contrast, monocytes, monocyte-derived macrophages (MDMs), and immature monocyte-derived dendritic cells predominantly expressed hCNT3, a functional transporter in MDMs. Finally, in immune cells, hOCTs showed a more heterogeneous expression profile and a lower activity than human nucleoside transporters (hNTs), although up-regulation of hOCTs also occurred upon lymphocyte activation. Overall, the expression and activity of most of the studied transporters emphasize their relevance in relation to anti-HIV and anticancer therapies. The identification of the transporter involved in each specific drug uptake in immune cells could help to optimize pharmacological therapeutic responses.

Formation of Toxic Oligomeric Assemblies of RNA-binding Protein: Musashi in Alzheimer's disease.

Alzheimer's disease (AD) is the most common neurodegenerative disorder associated with structural and functional alterations of brain cells causing progressive deterioration of memory and other cognitive functions. Recent studies demonstrate that several neurodegenerative diseases, including AD exhibit RNA-binding proteins (RBPs) pathologies, including TAR DNA -binding protein (TDP-43), fused in sarcoma (FUS), superoxide dismutase (SOD1) and T-interacting antigen-1 (TIA-1), highlighting the role of RBPs in neurodegeneration. One such group of RBPs, Musashi proteins comprised of MSI1 and MSI2, has been long studied in neurogenesis and cancer biology. Herein, we have investigated the aggregation properties of MSI1 and MSI2 by in vitro assays, their expression and accumulation as well as their possible interactions with other cellular proteins, such as tau in AD pathology. We have performed atomic force microscopy, Western blot, and immunoprecipitation to demonstrate the aggregation properties of recombinant Musashi proteins. Furthermore, we have studied cortical brain sections from AD (N = 4) and age-matched non-demented subjects (N = 4) by Western blot and immunofluorescence microscopy to investigate MSI1 and MSI2 levels and their localization in human brain tissues. Musashi proteins showed in vitro aggregation properties by forming oligomers. We have observed an increase in Musashi proteins levels in AD brain tissues as compared with age-matched non-demented subjects. Moreover, Musashi proteins are observed to form oligomers in the diseased brain tissues. Interestingly, the co-immunofluorescence study has revealed a change in fluorescence pattern of oligomeric Musashi proteins and tau with a high association in the perinuclear area of the cells suggesting changes in function of Musashi proteins. Our data have demonstrated for the first time that MSI1 and MSI2 are present in an oligomeric state in AD brains compared to the age-matched non-demented subjects and that these large assemblies co-localize with tau contributing to the neurodegenerative pathogenesis.

Peptidomimetic blockade of MYB in acute myeloid leukemia.

Aberrant gene expression is a hallmark of acute leukemias. MYB-driven transcriptional coactivation with CREB-binding protein (CBP)/P300 is required for acute lymphoblastic and myeloid leukemias, including refractory MLL-rearranged leukemias. Using structure-guided molecular design, we developed a peptidomimetic inhibitor MYBMIM that interferes with the assembly of the molecular MYB:CBP/P300 complex and rapidly accumulates in the nuclei of AML cells. Treatment of AML cells with MYBMIM led to the dissociation of the MYB:CBP/P300 complex in cells, its displacement from oncogenic enhancers enriched for MYB binding sites, and downregulation of MYB-dependent gene expression, including of MYC and BCL2 oncogenes. AML cells underwent mitochondrial apoptosis in response to MYBMIM, which was partially rescued by ectopic expression of BCL2. MYBMIM impeded leukemia growth and extended survival of immunodeficient mice engrafted with primary patient-derived MLL-rearranged leukemia cells. These findings elucidate the dependence of human AML on aberrant transcriptional coactivation, and establish a pharmacologic approach for its therapeutic blockade.

The N6-methyladenosine (m6A)-forming enzyme METTL3 controls myeloid differentiation of normal hematopoietic and leukemia cells.

N6-methyladenosine (m6A) is an abundant nucleotide modification in mRNA that is required for the differentiation of mouse embryonic stem cells. However, it remains unknown whether the m6A modification controls the differentiation of normal and/or malignant myeloid hematopoietic cells. Here we show that shRNA-mediated depletion of the m6A-forming enzyme METTL3 in human hematopoietic stem/progenitor cells (HSPCs) promotes cell differentiation, coupled with reduced cell proliferation. Conversely, overexpression of wild-type METTL3, but not of a catalytically inactive form of METTL3, inhibits cell differentiation and increases cell growth. METTL3 mRNA and protein are expressed more abundantly in acute myeloid leukemia (AML) cells than in healthy HSPCs or other types of tumor cells. Furthermore, METTL3 depletion in human myeloid leukemia cell lines induces cell differentiation and apoptosis and delays leukemia progression in recipient mice in vivo. Single-nucleotide-resolution mapping of m6A coupled with ribosome profiling reveals that m6A promotes the translation of c-MYC, BCL2 and PTEN mRNAs in the human acute myeloid leukemia MOLM-13 cell line. Moreover, loss of METTL3 leads to increased levels of phosphorylated AKT, which contributes to the differentiation-promoting effects of METTL3 depletion. Overall, these results provide a rationale for the therapeutic targeting of METTL3 in myeloid leukemia.

Functional screen of MSI2 interactors identifies an essential role for SYNCRIP in myeloid leukemia stem cells.

The identity of the RNA-binding proteins (RBPs) that govern cancer stem cells remains poorly characterized. The MSI2 RBP is a central regulator of translation of cancer stem cell programs. Through proteomic analysis of the MSI2-interacting RBP network and functional shRNA screening, we identified 24 genes required for in vivo leukemia. Syncrip was the most differentially required gene between normal and myeloid leukemia cells. SYNCRIP depletion increased apoptosis and differentiation while delaying leukemogenesis. Gene expression profiling of SYNCRIP-depleted cells demonstrated a loss of the MLL and HOXA9 leukemia stem cell program. SYNCRIP and MSI2 interact indirectly though shared mRNA targets. SYNCRIP maintains HOXA9 translation, and MSI2 or HOXA9 overexpression rescued the effects of SYNCRIP depletion. Altogether, our data identify SYNCRIP as a new RBP that controls the myeloid leukemia stem cell program. We propose that targeting these RBP complexes might provide a novel therapeutic strategy in leukemia.

3'-Azido-2',3'-dideoxythymidine (zidovudine) uptake mechanisms in T lymphocytes.

The nucleoside reverse transcriptase inhibitors (NRTIs) make up a family of antiretroviral drugs widely used in the treatment of HIV-1 infection. Human concentrative nucleoside transporters and equilibrative nucleoside transporters, encoded by SLC28 and SLC29 gene families, respectively, are known to be involved in the uptake of a variety of nucleoside analogues used in anticancer treatment. We therefore examined whether SLC28- and SLC29-encoded proteins contribute to the entry of these NRTls into the human leukaemic T-cell line Molt-4. Cis-inhibition experiments demonstrated that nucleoside transporters have a negligible role in antiviral drug uptake. Moreover, the previously identified 3'-azido-2',3'-dideoxythymidine (zidovudine; AZT) carriers, organic anion transporters (organic anion transporter [hOATs], members of the SLC22 gene family) have not been detected in T cells, either functionally or at the mRNA level, thus ruling out a role for hOATs in antiviral drug uptake in these cells. Nevertheless, the data provided here argue against the hypothesis of simple diffusion across the plasma membrane as the unique mechanism of AZT uptake. Actually, this pyrimidine derivative seems to have a temperature-sensitive route of entrance, a finding that, along with the evidence that, AZT inhibits its own uptake and its transport into phytohaemagglutinin-stimulated peripheral blood mononuclear cells is upregulated, strongly support the idea that AZT uptake into T-cells is associated with a mediated and regulated, transport mechanism.

Role of Human Organic Cation Transporter 1 (hOCT1) Polymorphisms in Lamivudine (3TC) Uptake and Drug-Drug Interactions.

Lamivudine (3TC), a drug used in the treatment of HIV infection, needs to cross the plasma membrane to exert its therapeutic action. Human Organic cation transporter 1 (hOCT1), encoded by the SLC22A1 gene, is the transporter responsible for its uptake into target cells. As SLC22A1 is a highly polymorphic gene, the aim of this study was to determine how SNPs in the OCT1-encoding gene affected 3TC internalization and its interaction with other co-administered drugs. HEK293 cells stably transfected with either the wild type form or the polymorphic variants of hOCT1 were used to perform kinetic and drug-drug interaction studies. Protein co-immunoprecipitation was used to assess the impact of selected polymorphic cysteines on the oligomerization of the transporter. Results showed that 3TC transport efficiency was reduced in all polymorphic variants tested (R61C, C88R, S189L, M420del, and G465R). This was not caused by lack of oligomerization in case of variants located at the transporter extracellular loop (R61C and C88R). Drug-drug interaction measurements showed that co-administered drugs [abacavir (ABC), zidovudine (AZT), emtricitabine (FTC), tenofovir diproxil fumarate (TDF), efavirenz (EFV) and raltegravir (RAL)], differently inhibited 3TC uptake depending upon the polymorphic variant analyzed. These data highlight the need for accurate analysis of drug transporter polymorphic variants of clinical relevance, because polymorphisms can impact on substrate (3TC) translocation but even more importantly they can differentially affect drug-drug interactions at the transporter level.

Transformation of the intestinal epithelium by the MSI2 RNA-binding protein.

The MSI2 RNA-binding protein is a potent oncogene playing key roles in haematopoietic stem cell homeostasis and malignant haematopoiesis. Here we demonstrate that MSI2 is expressed in the intestinal stem cell compartment, that its expression is elevated in colorectal adenocarcinomas, and that MSI2 loss-of-function abrogates colorectal cancer cell growth. MSI2 gain-of-function in the intestinal epithelium in a drug-inducible mouse model is sufficient to phenocopy many of the morphological and molecular consequences of acute loss of the APC tumour suppressor in the intestinal epithelium in a Wnt-independent manner. Transcriptome-wide RNA-binding analysis indicates that MSI2 acts as a pleiotropic inhibitor of known intestinal tumour suppressors including Lrig1, Bmpr1a, Cdkn1a and Pten. Finally, we demonstrate that inhibition of the PDK-AKT-mTORC1 axis rescues oncogenic consequences of MSI2 induction. Taken together, our findings identify MSI2 as a central component in an unappreciated oncogenic pathway promoting intestinal transformation.

Musashi2 sustains the mixed-lineage leukemia-driven stem cell regulatory program.

Leukemia stem cells (LSCs) are found in most aggressive myeloid diseases and contribute to therapeutic resistance. Leukemia cells exhibit a dysregulated developmental program as the result of genetic and epigenetic alterations. Overexpression of the RNA-binding protein Musashi2 (MSI2) has been previously shown to predict poor survival in leukemia. Here, we demonstrated that conditional deletion of Msi2 in the hematopoietic compartment results in delayed leukemogenesis, reduced disease burden, and a loss of LSC function in a murine leukemia model. Gene expression profiling of these Msi2-deficient animals revealed a loss of the hematopoietic/leukemic stem cell self-renewal program and an increase in the differentiation program. In acute myeloid leukemia patients, the presence of a gene signature that was similar to that observed in Msi2-deficent murine LSCs correlated with improved survival. We determined that MSI2 directly maintains the mixed-lineage leukemia (MLL) self-renewal program by interacting with and retaining efficient translation of Hoxa9, Myc, and Ikzf2 mRNAs. Moreover, depletion of MLL target Ikzf2 in LSCs reduced colony formation, decreased proliferation, and increased apoptosis. Our data provide evidence that MSI2 controls efficient translation of the oncogenic LSC self-renewal program and suggest MSI2 as a potential therapeutic target for myeloid leukemia.

The Msi Family of RNA-Binding Proteins Function Redundantly as Intestinal Oncoproteins.

Members of the Msi family of RNA-binding proteins have recently emerged as potent oncoproteins in a range of malignancies. MSI2 is highly expressed in hematopoietic cancers, where it is required for disease maintenance. In contrast to the hematopoietic system, colorectal cancers can express both Msi family members, MSI1 and MSI2. Here, we demonstrate that, in the intestinal epithelium, Msi1 and Msi2 have analogous oncogenic effects. Further, comparison of Msi1/2-induced gene expression programs and transcriptome-wide analyses of Msi1/2-RNA-binding targets reveal significant functional overlap, including induction of the PDK-Akt-mTORC1 axis. Ultimately, we demonstrate that concomitant loss of function of both MSI family members is sufficient to abrogate the growth of human colorectal cancer cells, and Msi gene deletion inhibits tumorigenesis in several mouse models of intestinal cancer. Our findings demonstrate that MSI1 and MSI2 act as functionally redundant oncoproteins required for the ontogeny of intestinal cancers.

PI3K inhibition results in enhanced estrogen receptor function and dependence in hormone receptor-positive breast cancer.

Activating mutations of PIK3CA are the most frequent genomic alterations in estrogen receptor (ER)-positive breast tumors, and selective phosphatidylinositol 3-kinase α (PI3Kα) inhibitors are in clinical development. The activity of these agents, however, is not homogeneous, and only a fraction of patients bearing PIK3CA-mutant ER-positive tumors benefit from single-agent administration. Searching for mechanisms of resistance, we observed that suppression of PI3K signaling results in induction of ER-dependent transcriptional activity, as demonstrated by changes in expression of genes containing ER-binding sites and increased occupancy by the ER of promoter regions of up-regulated genes. Furthermore, expression of ESR1 mRNA and ER protein were also increased upon PI3K inhibition. These changes in gene expression were confirmed in vivo in xenografts and patient-derived models and in tumors from patients undergoing treatment with the PI3Kα inhibitor BYL719. The observed effects on transcription were enhanced by the addition of estradiol and suppressed by the anti-ER therapies fulvestrant and tamoxifen. Fulvestrant markedly sensitized ER-positive tumors to PI3Kα inhibition, resulting in major tumor regressions in vivo. We propose that increased ER transcriptional activity may be a reactive mechanism that limits the activity of PI3K inhibitors and that combined PI3K and ER inhibition is a rational approach to target these tumors.