The anti-tumor efficacy of CDK4/6 inhibition is enhanced by the combination with PI3K/AKT/mTOR inhibitors through impairment of glucose metabolism in TNBC cells

Sequential or Concomitant Inhibition of Cyclin-Dependent Kinase 4/6 Before mTOR Pathway in Hormone-Positive HER2 Negative Breast Cancer: Biological Insights and Clinical Implications

About 75% of all breast cancers are hormone receptor-positive (HR+). However, the efficacy of endocrine therapy is limited due to the high rate of either pre-existing or acquired resistance. In this work we reconstructed the pathways around estrogen receptor (ER), mTOR, and cyclin D in order to compare the effects of CDK4/6 and PI3K/AKT/mTOR inhibitors. A positive feedback loop links mTOR and ER that support each other. We subsequently considered whether a combined or sequential inhibition of CDK4/6 and PI3K/AKT/mTOR could ensure better results. Studies indicate that inhibition of CDK4/6 activates mTOR as an escape mechanism to ensure cell proliferation. In literature, the little evidence dealing with this topic suggests that pre-treatment with mTOR pathway inhibitors could prevent or delay the onset of CDK4/6 inhibitor resistance. Additional studies are needed in order to find biomarkers that can identify patients who will develop this resistance and in whom the sensitivity to CDK4/6 inhibitors can be restored.

Glucose Metabolic Reprogramming of ER Breast Cancer in Acquired Resistance to the CDK4/6 Inhibitor Palbociclib

The majority of breast cancers express the estrogen receptor (ER) and are dependent on estrogen for their growth and survival. Endocrine therapy (ET) is the standard of care for these tumors. However, a superior outcome is achieved in a subset of ER positive (ER⁺)/human epidermal growth factor receptor 2 negative (HER2⁻) metastatic breast cancer patients when ET is administrated in combination with a cyclin-dependent kinases 4 and 6 (CDK4/6) inhibitor, such as palbociclib. Moreover, CDK4/6 inhibitors are currently being tested in ER⁺/HER2⁺ breast cancer and reported encouraging results. Despite the clinical advances of a combinatorial therapy using ET plus CDK4/6 inhibitors, potential limitations (i.e., resistance) could emerge and the metabolic adaptations underlying such resistance warrant further elucidation. Here we investigate the glucose-dependent catabolism in a series of isogenic ER⁺ breast cancer cell lines sensitive to palbociclib and in their derivatives with acquired resistance to the drug. Importantly, ER⁺/HER2⁻ and ER⁺/HER2⁺ cell lines show a different degree of glucose dependency. While ER⁺/HER2⁻ breast cancer cells are characterized by enhanced aerobic glycolysis at the time of palbociclib sensitivity, ER⁺/HER2⁺ cells enhance their glycolytic catabolism at resistance. This metabolic phenotype was shown to have prognostic value and was targeted with multiple approaches offering a series of potential scenarios that could be of clinical relevance.

Targeting Glucose Transporters for Breast Cancer Therapy: The Effect of Natural and Synthetic Compounds

Reprogramming of cellular energy metabolism is widely accepted to be a cancer hallmark. The deviant energetic metabolism of cancer cells-known as the Warburg effect-consists in much higher rates of glucose uptake and glycolytic oxidation coupled with the production of lactic acid, even in the presence of oxygen. Consequently, cancer cells have higher glucose needs and thus display a higher sensitivity to glucose deprivation-induced death than normal cells. So, inhibitors of glucose uptake are potential therapeutic targets in cancer. Breast cancer is the most commonly diagnosed cancer and a leading cause of cancer death in women worldwide. Overexpression of facilitative glucose transporters (GLUT), mainly GLUT1, in breast cancer cells is firmly established, and the consequences of GLUT inhibition and/or knockout are under investigation. Herein we review the compounds, both of natural and synthetic origin, found to interfere with uptake of glucose by breast cancer cells, and the consequences of interference with that mechanism on breast cancer cell biology. We will also present data where the interaction with GLUT is exploited in order to increase the efficiency or selectivity of anticancer agents, in breast cancer cells.

Short- and Long-Term Effects of CDK4/6 Inhibition on Early-Stage Breast Cancer

CDK4/6 inhibitors are used in the treatment of advanced estrogen receptor (ER)(+) breast cancer. Their efficacy in ER(-) and early-stage breast cancer is currently under investigation. Here, we show that palbociclib, a CDK4/6 inhibitor, can inhibit both progression of ductal carcinoma in situ (DCIS) and growth of invasive disease in both an ER(-) basal breast cancer model (MCFDCIS) and an ER(+) luminal model (MCF7 intraductal injection). In MCFDCIS cells, palbociclib repressed cell-cycle gene expression, inhibited proliferation, induced senescence, and normalized tumorspheres formed in Matrigel while the formation of acini by normal mammary epithelial cells (MCF10A) was not affected. Palbociclib treatment of mice with MCFDCIS tumors inhibited their malignant progression and reduced proliferation of invasive lesions. Transcriptomic analysis of the tumor and stromal cell compartments showed that cell cycle and senescence genes, and MUC16, an ovarian cancer biomarker gene, were repressed during treatment. Knockdown of MUC16 in MCFDCIS cells inhibited proliferation of invasive lesions but not progression of DCIS. After cessation of palbociclib treatment genes associated with differentiation, for example, P63, inflammation, IFNγ response, and antigen processing and presentation remained suppressed in the tumor and surrounding stroma. We conclude that palbociclib can prevent progression of DCIS and is antiproliferative in ER(-) invasive disease mediated in part via MUC16. Lasting effects of CDK4/6 inhibition after drug withdrawal on differentiation and the immune response could impact the approach to treatment of early-stage ER(-) breast cancer.

Multiple effects of CDK4/6 inhibition in cancer: From cell cycle arrest to immunomodulation

Dysregulation of the cell cycle is a hallmark of cancer that leads to aberrant cellular proliferation. CDK4/6 are cyclin-dependent kinases activated in response to proliferative signaling, which induce RB hyper-phosphorylation and hence activation of E2F transcription factors, thus promoting cell cycle progression through the S phase. Pharmacologic inhibition of CDK4/6 by palbociclib, ribociclib, or abemaciclib has been showing promising activity in multiple cancers with the best results achieved in combination with other agents. Indeed, CDK4/6 inhibitors are currently approved in combination with endocrine therapy for the treatment of estrogen receptor-positive, human epidermal growth factor receptor 2-negative advanced or metastatic breast cancer. Moreover, a number of clinical trials are currently underway to test the efficacy of combining CDK4/6 inhibitors with different drugs not only in breast but also in other types of cancer. Beyond the inhibition of cell proliferation, CDK4/6 inhibitors have recently revealed new effects on cancer cells and on tumor microenvironment. In particular, it has been reported that these agents induce a senescent-like phenotype, impact on cell metabolism and exert both immunomodulatory and immunogenic effects. Here we describe recent data on the anti-tumor effects of CDK4/6 inhibitors as single agents or in combined therapies, focusing in particular on their metabolic and immunomodulatory activities.

Pre-treatment with the CDK4/6 inhibitor palbociclib improves the efficacy of paclitaxel in TNBC cells

Triple Negative Breast Cancer (TNBC) is a challenging disease due to the lack of druggable targets; therefore, chemotherapy remains the standard of care and the identification of new targets is a high clinical priority. Alterations in the components of the cell cycle machinery have been frequently reported in cancer; given the success obtained with the CDK4/6 inhibitor palbocicib in ER-positive BC, we explored the potential of combining this drug with chemotherapy in Rb-positive TNBC cell models. The simultaneous combination of palbociclib with paclitaxel exerted an antagonistic effect; by contrast, the sequential treatment inhibited cell proliferation and increased cell death more efficaciously than single treatments. By down-regulating the E2F target c-myc, palbociclib reduced HIF-1α and GLUT-1 expression, and hence glucose uptake and consumption both under normoxic and hypoxic conditions. Importantly, these inhibitory effects on glucose metabolism were enhanced by palbociclib/paclitaxel sequential combination; the superior efficacy of such combination was ascribed to the ability of paclitaxel to inhibit palbociclib-mediated induction of AKT and to further down-regulate the Rb/E2F/c-myc signaling. Our results suggest that the efficacy of standard chemotherapy can be significantly improved by a pre-treatment with palbociclib, thus offering a better therapeutic option for Rb-proficient TNBC.

Cyclin-Dependent Kinases 4/6 Inhibitors in Breast Cancer: Current Status, Resistance, and Combination Strategies

Dysregulated activation of the cyclin-dependent kinases (CDKs) 4/6, leading to uncontrolled cell division, is hallmark of cancers. Further study of the cell cycle will advance the cancer treatment. As powerful and effective drugs, inhibitors of CDK 4/6 have been widely used in clinical practice for several malignancies, particularly against breast cancers driven by the estrogen receptor (ER). Three CDK4/6 inhibitors, including palbociclib (PD0332991), ribociclib (LEE011) and abemaciclib (LY2835219), have been approved by the US Food and Drug Administration (FDA) for the treatment of hormone receptor-positive, human epidermal growth factor receptor 2-negative advanced or metastatic breast cancer. However, CDK4/6 inhibitors act downstream of many mitogenic signaling pathways, and this has implications for resistance. It is worth to note that the mechanisms of resistance are not very clear. Up to now, a small number of preclinical and clinical studies have explored potential mechanisms of CDK4/6 inhibitors resistance in breast cancer. On this basis, rational and effective combination therapy is under development. Here we review the current knowledge about the mechanisms and efficacy of CDK4/6 inhibitors, and summarize data on resistance mechanisms to make future combination therapies more accurate and reasonable.

A Systematic Review Exploring the Anticancer Activity and Mechanisms of Glucomannan

Glucomannan, long recognized as the active ingredient of the traditional Chinese medicinal herb Konjac glucomannan, is a naturally occurring polysaccharide existing in certain plant species and fungi. Due to its special property to also serve as a dietary supplement, glucomannan has been widely applied in clinic to lower body weight and circulation cholesterol level and to treat constipation, diabetes, and arterial sclerosis. Besides the regulatory role engaged with gastroenterological and metabolic syndrome, recently, its therapeutic effect and the underlying mechanisms in treating cancerous diseases have been appreciated by mounting researches. The present review aims to emphasize the multifaceted aspects of how glucomannan exerts its anti-tumor function.

Multi-targeted kinase inhibition alleviates mTOR inhibitor resistance in triple-negative breast cancer

Purpose

Owing to its genetic heterogeneity and acquired resistance, triple-negative breast cancer (TNBC) is not responsive to single-targeted therapy, causing disproportional cancer-related death worldwide. Combined targeted therapy strategies to block interactive oncogenic signaling networks are being explored for effective treatment of the refractory TNBC subtype.

Methods

A broad kinase inhibitor screen was applied to profile the proliferative responses of TNBC cells, revealing resistance of TNBC cells to inhibition of the mammalian target of rapamycin (mTOR). A systematic drug combination screen was subsequently performed to identify that AEE788, an inhibitor targeting multiple receptor tyrosine kinases (RTKs) EGFR/HER2 and VEGFR, synergizes with selective mTOR inhibitor rapamycin as well as its analogs (rapalogs) temsirolimus and everolimus to inhibit TNBC cell proliferation.

Results

The combination treatment with AEE788 and rapalog effectively inhibits phosphorylation of mTOR and 4EBP1, relieves mTOR inhibition-mediated upregulation of cyclin D1, and maintains suppression of AKT and ERK signaling, thereby sensitizing TNBC cells to the rapalogs. siRNA validation of cheminformatics-based predicted AEE788 targets has further revealed the mTOR interactive RPS6K members (RPS6KA3, RPS6KA6, RPS6KB1, and RPS6KL1) as synthetic lethal targets for rapalog combination treatment.

Conclusions

mTOR signaling is highly activated in TNBC tumors. As single rapalog treatment is insufficient to block mTOR signaling in rapalog-resistant TNBC cells, our results thus provide a potential multi-kinase inhibitor combinatorial strategy to overcome mTOR-targeted therapy resistance in TNBC cells.

Electronic supplementary material

The online version of this article (10.1007/s10549-019-05380-z) contains supplementary material, which is available to authorized users.

Palbociclib, a selective CDK4/6 inhibitor, restricts cell survival and epithelial-mesenchymal transition in Panc-1 and MiaPaCa-2 pancreatic cancer cells

The mortality rate of pancreatic cancer has close parallels to its incidence rate because of limited therapeutics and lack of effective prognosis. Despite various novel chemotherapeutics combinations, the 5-year survival rate is still under 5%. In the current study, we aimed to modulate the aberrantly activated PI3K/AKT pathway and epithelial-mesenchymal transition (EMT) signaling with the treatment of CDK4/6 inhibitor PD-0332991 (palbociclib) in Panc-1 and MiaPaCa-2 pancreatic cancer cells. It was found that PD-0332991 effectively reduced cell viability and proliferation dose-dependently within 24 hours. In addition, PD-0332991 induced cell cycle arrest at the G1 phase by downregulation of aberrant expression of CDK4/6 through the dephosphorylation of Rb in each cell lines. Although PD-0332991 treatment increased epithelial markers and decreased mesenchymal markers, the nuclear translocation of β-catenin was not prevented by PD-0332991 treatment, especially in MiaPaCa-2 cells. Effects of PD-0332991 on the regulation of PI3K/AKT signaling and its downstream targets such as GSK-3 were cell type-dependent. Although the activity of AKT was inhibited in both cell lines, the phosphorylation of GSK-3β at Ser9 increased only in Panc-1. In conclusion, PD-0332991 induced cell cycle arrest and reduced the cell viability of Panc-1 and MiaPaCa-2 cells. However, PD-0332991 differentially affects the regulation of the PI3K/AKT pathway and EMT process in cells due to its distinct influence on Rb and GSK-3/β-catenin signaling. Understanding the effect of PD-0332991 on the aberrantly activated signaling axis may put forward a new therapeutic strategy to reduce the cell viability and metastatic process of pancreatic cancer.

The Proliferative and Apoptotic Landscape of Basal-like Breast Cancer

Basal-like breast cancer (BLBC) is an aggressive molecular subtype that represents up to 15% of breast cancers. It occurs in younger patients, and typically shows rapid development of locoregional and distant metastasis, resulting in a relatively high mortality rate. Its defining features are that it is positive for basal cytokeratins and, epidermal growth factor receptor and/or c-Kit. Problematically, it is typically negative for the estrogen receptor and human epidermal growth factor receptor 2 (HER2), which means that it is unsuitable for either hormone therapy or targeted HER2 therapy. As a result, there are few therapeutic options for BLBC, and a major priority is to define molecular subgroups of BLBC that could be targeted therapeutically. In this review, we focus on the highly proliferative and anti-apoptotic phenotype of BLBC with the goal of defining potential therapeutic avenues, which could take advantage of these aspects of tumor development.

Synergistic anti-cancer activity of CDK4/6 inhibitor palbociclib and dual mTOR kinase inhibitor MLN0128 in pRb-expressing ER-negative breast cancer

PURPOSE

Palbociclib is an approved cyclin-dependent kinase (CDK) 4/6 inhibitor for treatment of patients with ER-positive and HER2-negative breast cancers. While Retinoblastoma protein (pRb), a major substrate of CDK4/6, is a potential target in triple negative breast cancer (TNBC), the usefulness of CDK4/6 inhibitors in this cancer has not been established. This preclinical study investigated the combination effects of palbociclib and the dual mammalian target of rapamycin (mTOR) kinase inhibitor MLN0128 in estrogen receptor (ER)-negative breast cancer in vitro and in vivo.

METHODS

The combined effects of two drugs on three TNBC cell lines (MB231, MB468, and CAL148) and an ER-negative and HER2-positive cell line (MB453) were investigated by MTT assay and colony formation analysis. Cell cycle measurements were examined as well as changes in expression of molecules related to G1/S transition and the mTOR pathway. Importantly, a pRb-expressing TNBC patient-derived xenograft (PDX) model was used to assess the effects of the combination in vivo.

RESULTS

A combination of palbociclib and MLN0128 synergistically inhibited the proliferation of pRb-expressing cell lines and induced G1 cell cycle arrest. Western blot analysis revealed that CDK4/6-pRb and mTOR pathways were inhibited by these treatments. In pRb-expressing TNBC PDX, the combination treatment drastically suppressed tumor growth compared to either the control or single drug treatments. In addition, the combination treatment significantly reduced the number of Ki67-positive cells.

CONCLUSIONS

We revealed that palbociclib and MLN0128 had synergistic anti-cancer activity in both pRb + ER-negative cell lines and a TNBC PDX model. Our results indicate that such combination therapy is worthy of further investigation in a clinical setting.

Therapeutic Vulnerabilities in FLT3-Mutant AML Unmasked by Palbociclib

While significant progress has been made in the treatment of acute myeloid leukemia (AML), not all patients can be cured. Mutated in about 1/3 of de novo AML, the FLT3 receptor tyrosine kinase is an attractive target for drug development, activating mutations of the FLT3 map to the juxtamembrane domain (internal tandem duplications, ITD) or the tyrosine kinase domain (TKD), most frequently at codon D835. While small molecule tyrosine kinase inhibitors (TKI) effectively target ITD mutant forms, those on the TKD are not responsive. Moreover, FLT3 inhibition fails to induce a persistent response in patients due to mutational resistance. More potent compounds with broader inhibitory effects on multiple FLT3 mutations are highly desirable. We describe a critical role of CDK6 in the survival of FLT3⁺ AML cells as palbociclib induced apoptosis not only in FLT3–ITD⁺ cells but also in FLT3–D835Y⁺ cells. Antineoplastic effects were also seen in primary patient-derived cells and in a xenograft model, where therapy effectively suppressed tumor formation in vivo at clinically relevant concentrations. In cells with FLT3–ITD or -TKD mutations, the CDK6 protein not only affects cell cycle progression but also transcriptionally regulates oncogenic kinases mediating intrinsic drug resistance, including AURORA and AKT—a feature not shared by its homolog CDK4. While AKT and AURORA kinase inhibitors have significant therapeutic potential in AML, single agent activity has not been proven overly effective. We describe synergistic combination effects when applying these drugs together with palbociclib which could be readily translated to patients with AML bearing FLT3–ITD or –TKD mutations. Targeting synergistically acting vulnerabilities, with CDK6 being the common denominator, may represent a promising strategy to improve AML patient responses and to reduce the incidence of selection of resistance-inducing mutations.

MYC Amplification as a Potential Mechanism of Primary Resistance to Crizotinib in ALK-Rearranged Non-Small Cell Lung Cancer: A Brief Report

INTRODUCTION

Translocations of the anaplastic lymphoma kinase (ALK) can be effectively targeted in advanced non-small cell lung cancer by ALK-TKI inhibitors including Crizotinib. However, the development of acquired resistance often limits the duration of these therapies. While several mechanisms of secondary resistance have been already identified, little is known about molecular determinants of primary resistance. In our brief report we investigated the tumor molecular profile of a patient who failed to respond to Crizotinib.

METHODS

Fluorescence in situ hybridization (FISH) and next-generation sequencing (NGS) were run on tumor specimen as well as search and characterization of circulating tumor cells (CTCs) in the blood. Confirmation of clinical findings was achieved using a translational cell-line in vitro model.

RESULTS

We identified the amplification of MYC as a potential new mechanism of primary resistance to ALK inhibition. Human EML4-ALK rearranged cells infected with a lentiviral vector carrying full-length human MYC cDNA were treated in vitro with crizotinib and alectinib. Overexpression of MYC overexpression was associated with a reduced sensitivity to both ALK-inhibitors. MYC-overexpressing clones displayed also increased levels of both cyclin D and E and their growth was reduced by using Cdk4/6 inhibitors such as Palbociclib.

CONCLUSIONS

We postulate that the MYC gene may be implicated in the mechanism of primary resistance to ALK inhibitors. We also suggest potential MYC-directed inhibition strategies to overcome primary resistance in advanced ALK-rearranged NSCLC.