Phospholipase D confers rapamycin resistance in human breast cancer cells

PLD1 is a key player in cancer stemness and chemoresistance: Therapeutic targeting of cross-talk between the PI3K/Akt and Wnt/β-catenin pathways

The development of chemoresistance is a major challenge in the treatment of several types of cancers in clinical settings. Stemness and chemoresistance are the chief causes of poor clinical outcomes. In this context, we hypothesized that understanding the signaling pathways responsible for chemoresistance in cancers is crucial for the development of novel targeted therapies to overcome drug resistance. Among the aberrantly activated pathways, the PI3K-Akt/Wnt/β-catenin signaling pathway is clinically implicated in malignancies such as colorectal cancer (CRC) and glioblastoma multiforme (GBM). Aberrant dysregulation of phospholipase D (PLD) has been implicated in several malignancies, and oncogenic activation of this pathway facilitates tumor proliferation, stemness, and chemoresistance. Crosstalk involving the PLD and Wnt/β-catenin pathways promotes the progression of CRC and GBM and reduces the sensitivity of cancer cells to standard therapies. Notably, both pathways are tightly regulated and connected at multiple levels by upstream and downstream effectors. Thus, gaining deeper insights into the interactions between these pathways would help researchers discover unique therapeutic targets for the management of drug-resistant cancers. Here, we review the molecular mechanisms by which PLD signaling stimulates stemness and chemoresistance in CRC and GBM. Thus, the current review aims to address the importance of PLD as a central player coordinating cross-talk between the PI3K/Akt and Wnt/β-catenin pathways and proposes the possibility of targeting these pathways to improve cancer therapy and overcome drug resistance.

Raf/MEK/ERK Signaling Pathway Is Involved in the Inhibition of Glioma Cell Proliferation and Invasion in the Ketogenic Microenvironment

OBJECTIVE

A high-fat, low-carbohydrate ketogenic diet has been used to treat malignant glioma, in which the Raf/MEK/ERK signaling pathway is overactivated. However, whether the Raf/MEK/ERK signaling pathway is involved in the therapeutic effect of ketone bodies remains unknown. In this study, we investigated the effects of a major ketone body, 3-hydroxybutyric acid (3-HBA), on the proliferation and metastasis of malignant glioblastoma cells and the underlying mechanism.

METHODS

Two human malignant glioblastoma cell lines (U87 and U251) were treated with different concentrations of 3-HBA with or without the Raf inhibitor PAF C-16 for 24 h. Cell proliferation, cell cycle, cell invasion, and phospholipase D1 (PLD1) activity were determined. Protein and gene expression levels of Raf/MEK/ERK signaling pathway members were examined.

RESULTS

3-HBA significantly decreased cell proliferation, invasion, and intracellular PLD1 activity in both U87 and U251 glioblastoma cell lines. 3-HBA treatment significantly increased the proportion of cells in the G1 phase and decreased the proportion of cells in S phase in U87 cells. In the U251 line, the proportion of treated cells in S phase was increased and proportion of cells in G2 was decreased. 3-HBA treatment also significantly decreased the protein expression levels of Raf, MEK, p-MEK, ERK, p-ERK, and PLD1 while increasing p53 expression; an effect that was similar to treatment with the Raf inhibitor. Co-treatment of 3-HBA with the Raf inhibitor further enhanced the effects of the 3-HBA in both cell lines.

CONCLUSION

We confirmed that a ketogenic microenvironment can inhibit glioma cell proliferation and invasion by downregulating the expression of PLD1 through the Raf/MEK/ERK signaling pathway.

Regulation of mTOR by phosphatidic acid

mTORC1, the mammalian target of rapamycin complex 1, is a key regulator of cellular physiology. The lipid metabolite phosphatidic acid (PA) binds to and activates mTORC1 in response to nutrients and growth factors. We review structural findings and propose a model for PA activation of mTORC1. PA binds to a highly conserved sequence in the α4 helix of the FK506 binding protein 12 (FKBP12)/rapamycin-binding (FRB) domain of mTOR. It is proposed that PA binding to two adjacent positively charged amino acids breaks and shortens the C-terminal region of helix α4. This has profound consequences for both substrate binding and the catalytic activity of mTORC1.

Phospholipase D and cancer metastasis: A focus on exosomes

In mammals, phospholipase D (PLD) enzymes involve 6 isoforms, of which only three have established lipase activity to produce the signaling lipid phosphatidic acid (PA). This phospholipase activity has been postulated to contribute to cancer progression for over three decades now, but the exact mechanisms involved have yet to be uncovered. Indeed, using various models, an altered PLD activity has been proposed altogether to increase cell survival rate, promote angiogenesis, boost rapamycin resistance, and favor metastasis. Although for some part, the molecular pathways by which this increase in PA is pro-oncogenic are partially known, the pleiotropic functions of PA make it quite difficult to distinguish which among these simple signaling pathways is responsible for each of these PLD facets. In this review, we will describe an additional potential contribution of PA generated by PLD1 and PLD2 in the biogenesis, secretion, and uptake of exosomes. Those extracellular vesicles are now viewed as membrane vehicles that carry informative molecules able to modify the fate of receiving cells at distance from the original tumor to favor homing of metastasis. The perspectives for a better understanding of these complex role of PLDs will be discussed.

Targeting mTOR in the Context of Diet and Whole-body Metabolism

The mechanistic target of the rapamycin (mTOR) signaling pathway is the central regulator of cell growth and proliferation by integrating growth factor and nutrient availability. Under healthy physiological conditions, this process is tightly coordinated and essential to maintain whole-body homeostasis. Not surprisingly, dysregulated mTOR signaling underpins several diseases with increasing incidence worldwide, including obesity, diabetes, and cancer. Consequently, there is significant clinical interest in developing therapeutic strategies that effectively target this pathway. The transition of mTOR inhibitors from the bench to bedside, however, has largely been marked with challenges and shortcomings, such as the development of therapy resistance and adverse side effects in patients. In this review, we discuss the current status of first-, second-, and third-generation mTOR inhibitors as a cancer therapy in both preclinical and clinical settings, with a particular emphasis on the mechanisms of drug resistance. We focus especially on the emerging role of diet as an important environmental determinant of therapy response, and posit a conceptual framework that links nutrient availability and whole-body metabolic states such as obesity with many of the previously defined processes that drive resistance to mTOR-targeted therapies. Given the role of mTOR as a central integrator of cell metabolism and function, we propose that modulating nutrient inputs through dietary interventions may influence the signaling dynamics of this pathway and compensatory nodes. In doing so, new opportunities for exploiting diet/drug synergies are highlighted that may unlock the therapeutic potential of mTOR inhibitors as a cancer treatment.

Phosphatidylcholine-Derived Lipid Mediators: The Crosstalk Between Cancer Cells and Immune Cells

To become resistant, cancer cells need to activate and maintain molecular defense mechanisms that depend on an energy trade-off between resistance and essential functions. Metabolic reprogramming has been shown to fuel cell growth and contribute to cancer drug resistance. Recently, changes in lipid metabolism have emerged as an important driver of resistance to anticancer agents. In this review, we highlight the role of choline metabolism with a focus on the phosphatidylcholine cycle in the regulation of resistance to therapy. We analyze the contribution of phosphatidylcholine and its metabolites to intracellular processes of cancer cells, both as the major cell membrane constituents and source of energy. We further extended our discussion about the role of phosphatidylcholine-derived lipid mediators in cellular communication between cancer and immune cells within the tumor microenvironment, as well as their pivotal role in the immune regulation of therapeutic failure. Changes in phosphatidylcholine metabolism are part of an adaptive program activated in response to stress conditions that contribute to cancer therapy resistance and open therapeutic opportunities for treating drug-resistant cancers.

Upregulated phospholipase D2 expression and activity is related to the metastatic properties of melanoma

The incidence rates of melanoma have increased steadily in recent decades and nearly 25% of the patients diagnosed with early-stage melanoma will eventually develop metastasis, for which there is currently no fully effective treatment. The link between phospholipases and tumors has been studied extensively, particularly in breast and colon cancers. With the aim of finding new biomarkers and therapeutic options for melanoma, the expression of different phospholipases was assessed in 17 distinct cell lines in the present study, demonstrating that phospholipase D2 (PLD2) is upregulated in metastatic melanoma as compared to normal skin melanocytes. These results were corroborated by immunofluorescence and lipase activity assays. Upregulation of PLD2 expression and increased lipase activity were observed in metastatic melanoma relative to normal skin melanocytes. So far, the implication of PLD2 activity in melanoma malignancies has remained elusive. To the best of our knowledge, the present study was the first to demonstrate that the overexpression of PLD2 enhances lipase activity, and its effect to increase the proliferation, migration and invasion capacity of melanoma cells was assessed with XTT and Transwell assays. In addition, silencing of PLD2 in melanoma cells reduced the metastatic potential of these cells. The present study provided evidence that PLD2 is involved in melanoma malignancy and in particular, in its metastatic potential, and established a basis for future studies evaluating PLD2 blockade as a therapeutic strategy to manage this condition.

Aldolase A and Phospholipase D1 Synergistically Resist Alkylating Agents and Radiation in Lung Cancer

Exposure to alkylating agents and radiation may cause damage and apoptosis in cancer cells. Meanwhile, this exposure involves resistance and leads to metabolic reprogramming to benefit cancer cells. At present, the detailed mechanism is still unclear. Based on the profiles of several transcriptomes, we found that the activity of phospholipase D (PLD) and the production of specific metabolites are related to these events. Comparing several particular inhibitors, we determined that phospholipase D1 (PLD1) plays a dominant role over other PLD members. Using the existing metabolomics platform, we demonstrated that lysophosphatidylethanolamine (LPE) and lysophosphatidylcholine (LPC) are the most critical metabolites, and are highly dependent on aldolase A (ALDOA). We further demonstrated that ALDOA could modulate total PLD enzyme activity and phosphatidic acid products. Particularly after exposure to alkylating agents and radiation, the proliferation of lung cancer cells, autophagy, and DNA repair capabilities are enhanced. The above phenotypes are closely related to the performance of the ALDOA/PLD1 axis. Moreover, we found that ALDOA inhibited PLD2 activity and enzyme function through direct protein–protein interaction (PPI) with PLD2 to enhance PLD1 and additional carcinogenic features. Most importantly, the combination of ALDOA and PLD1 can be used as an independent prognostic factor and is correlated with several clinical parameters in lung cancer. These findings indicate that, based on the PPI status between ALDOA and PLD2, a combination of radiation and/or alkylating agents with regulating ALDOA-PLD1 may be considered as a new lung cancer treatment option.

Pathophysiological Integration of Metabolic Reprogramming in Breast Cancer

Simple Summary

Tumors exhibit metabolic changes that differentiate them from the normal tissues from which they derive. These metabolic changes favor tumor growth, are primarily induced by cancer cells, and produce metabolic and functional changes in the surrounding stromal cells. There is a close functional connection between the metabolic changes in tumor cells and those that appear in the surrounding stroma. A better understanding of intratumoral metabolic interactions may help identify new vulnerabilities that will facilitate new, more individualized treatment strategies against cancer. We review the metabolic changes described in tumor and stromal cells and their functional changes and then consider, in depth, the metabolic interactions between the cells of the two compartments. Although these changes are generic, we illustrate them mainly with reference to examples in breast cancer.

Abstract

Metabolic changes that facilitate tumor growth are one of the hallmarks of cancer. The triggers of these metabolic changes are located in the tumor parenchymal cells, where oncogenic mutations induce an imperative need to proliferate and cause tumor initiation and progression. Cancer cells undergo significant metabolic reorganization during disease progression that is tailored to their energy demands and fluctuating environmental conditions. Oxidative stress plays an essential role as a trigger under such conditions. These metabolic changes are the consequence of the interaction between tumor cells and stromal myofibroblasts. The metabolic changes in tumor cells include protein anabolism and the synthesis of cell membranes and nucleic acids, which all facilitate cell proliferation. They are linked to catabolism and autophagy in stromal myofibroblasts, causing the release of nutrients for the cells of the tumor parenchyma. Metabolic changes lead to an interstitium deficient in nutrients, such as glucose and amino acids, and acidification by lactic acid. Together with hypoxia, they produce functional changes in other cells of the tumor stroma, such as many immune subpopulations and endothelial cells, which lead to tumor growth. Thus, immune cells favor tissue growth through changes in immunosuppression. This review considers some of the metabolic changes described in breast cancer.

Selectivity of mTOR-Phosphatidic Acid Interactions Is Driven by Acyl Chain Structure and Cholesterol

The need to gain insights into the molecular details of peripheral membrane proteins’ specificity towards phosphatidic acid (PA) is undeniable. The variety of PA species classified in terms of acyl chain length and saturation translates into a complicated, enigmatic network of functional effects that exert a critical influence on cell physiology. As a consequence, numerous studies on the importance of phosphatidic acid in human diseases have been conducted in recent years. One of the key proteins in this context is mTOR, considered to be the most important cellular sensor of essential nutrients while regulating cell proliferation, and which also appears to require PA to build stable and active complexes. Here, we investigated the specific recognition of three physiologically important PA species by the mTOR FRB domain in the presence or absence of cholesterol in targeted membranes. Using a broad range of methods based on model lipid membrane systems, we elucidated how the length and saturation of PA acyl chains influence specific binding of the mTOR FRB domain to the membrane. We also discovered that cholesterol exerts a strong modulatory effect on PA-FRB recognition. Our data provide insight into the molecular details of some physiological effects reported previously and reveal novel mechanisms of fine-tuning the signaling cascades dependent on PA.

AMPK-mTOR Signaling and Cellular Adaptations in Hypoxia

Cellular energy is primarily provided by the oxidative degradation of nutrients coupled with mitochondrial respiration, in which oxygen participates in the mitochondrial electron transport chain to enable electron flow through the chain complex (I-IV), leading to ATP production. Therefore, oxygen supply is an indispensable chapter in intracellular bioenergetics. In mammals, oxygen is delivered by the bloodstream. Accordingly, the decrease in cellular oxygen level (hypoxia) is accompanied by nutrient starvation, thereby integrating hypoxic signaling and nutrient signaling at the cellular level. Importantly, hypoxia profoundly affects cellular metabolism and many relevant physiological reactions induce cellular adaptations of hypoxia-inducible gene expression, metabolism, reactive oxygen species, and autophagy. Here, we introduce the current knowledge of hypoxia signaling with two-well known cellular energy and nutrient sensing pathways, AMP-activated protein kinase (AMPK) and mechanistic target of rapamycin complex 1 (mTORC1). Additionally, the molecular crosstalk between hypoxic signaling and AMPK/mTOR pathways in various hypoxic cellular adaptions is discussed.

Multiomics integrative analysis reveals antagonistic roles of CBX2 and CBX7 in metabolic reprogramming of breast cancer

Striking similarity exists between metabolic changes associated with embryogenesis and tumorigenesis. Chromobox proteins‐CBX2/4/6/7/8, core components of canonical polycomb repressor complex 1, play essential roles in embryonic development and aberrantly expressed in breast cancer. Understanding how altered CBX expression relates to metabolic reprogramming in breast cancer may reveal vulnerabilities of therapeutic pertinence. Using transcriptomic and metabolomic data from breast cancer patients (N > 3000 combined), we performed pathway‐based analysis and identified outstanding roles of CBX2 and CBX7 in positive and negative regulation of glucose metabolism, respectively. Genetic ablation experiments validated the contrasting roles of two isoforms in cancer metabolism and cell growth. Furthermore, we provide evidence for the role of mammalian target of rapamycin complex 1 signaling in mediating contrary effects of CBX2 and CBX7 on breast cancer metabolism. Underpinning the biological significance of metabolic roles, CBX2 and CBX7 were found to be the most up‐ and downregulated isoforms, respectively, in breast tumors compared with normal tissues. Moreover, CBX2 and CBX7 expression (not other isoforms) correlated strongly, but oppositely, with breast tumor subtype aggressiveness and the proliferation markers. Consistently, genomic data also showed higher amplification frequency of CBX2, not CBX7, in breast tumors. Highlighting the clinical significance of findings, disease‐specific survival and drug sensitivity analysis revealed that CBX2 and CBX7 predicted patient outcome and sensitivity to FDA‐approved/investigational drugs. In summary, this work identifies novel cross talk between CBX2/7 and breast tumor metabolism, and the results presented may have implications in strategies targeting breast cancer.

MYC-binding lncRNA EPIC1 promotes AKT-mTORC1 signaling and rapamycin resistance in breast and ovarian cancer

AKT-mTORC1 (mammalian target of rapamycin complex 1) signaling pathway plays a critical role in tumorigenesis and can be targeted by rapamycin. However, the underlying mechanism of how long noncoding RNA (lncRNAs) regulate the AKT-mTORC1 pathway remains unclear. EPIC1 (epigenetically-induced lncRNA 1) is a Myc-binding lncRNA, which has been previously demonstrated to be overexpressed in multiple cancer types. In a pathway analysis including 4962 cancer patients, we observed that lncRNA EPIC1 expression was positively correlated with the AKT-mTORC1 signaling pathway in more than 10 cancer types, including breast and ovarian cancers. RNA-seq analysis of breast and ovarian cancer cells demonstrated that EPIC1-knockdown led to the downregulation of genes in the AKT-mTORC1 signaling pathway. In MCF-7, OVCAR4, and A2780cis cell lines, EPIC1 knockdown and overexpression, respectively, inhibited and activated phosphorylated AKT and the downstream phosphorylation levels of 4EBP1 and S6K. Further knockdown of Myc abolished the EPIC1's regulation of AKT-mTORC1 signaling; suggested that the regulation of phosphorylation level of AKT, 4EBP1, and S6K by EPIC1 depended on the expression of Myc. Moreover, EPIC1 overexpressed MCF-7, A2780cis, and OVCAR4 cells treated with rapamycin showed a significant decreasing in rapamycin mediated inhibition of p-S6K and p-S6 comparing with the control group. In addition, Colony Formation assay and MTT assay indicated that EPIC1 overexpression led to rapamycin resistance in breast and ovarian cancer cell lines. Our results demonstrated the lncRNA EPIC1 expression activated the AKT-mTORC1 signaling pathway through Myc and led to rapamycin resistance in breast and ovarian cancer.

SCD1 activity promotes cell migration via a PLD-mTOR pathway in the MDA-MB-231 triple-negative breast cancer cell line

BACKGROUND

Breast cancer is the most common cancer in women. Despite high survival rates in Western countries, treatments are less effective in metastatic cases and triple-negative breast cancer (TNBC) patient survival is the shortest across breast cancer subtypes. High expression levels of stearoyl-CoA desaturase-1 (SCD1) have been reported in breast cancer. The SCD1 enzyme catalyzes the formation of oleic acid (OA), a lipid stimulating the migration of metastatic breast cancer cells. Phospholipase activity is also implicated in breast cancer metastasis, notably phospholipase D (PLD).

METHODS

Kaplan-Meier survival plots generated from gene expression databases were used to analyze the involvement of SCD1 and PLD in several cancer subtypes. SCD1 enzymatic activity was modulated with a pharmaceutical inhibitor or by OA treatment (to mimic SCD1 over-activity) in three breast cancer cell lines: TNBC-derived MDA-MB-231 cells as well as non-TNBC MCF-7 and T47D cells. Cell morphology and migration properties were characterized by various complementary methods.

RESULTS

Our survival analyses suggest that SCD1 and PLD2 expression in the primary tumor are both associated to metastasis-related morbid outcomes in breast cancer patients. We show that modulation of SCD1 activity is associated with the modification of TNBC cell migration properties, including changes in speed, direction and cell morphology. Cell migration properties are regulated by SCD1 activity through a PLD-mTOR/p70S6K signaling pathway. These effects are not observed in non-TNBC cell lines.

CONCLUSION

Our results establish a key role for the lipid desaturase SCD1 and delineate an OA-PLD-mTOR/p70S6K signaling pathway in TNBC-derived MDA-MB-231 cell migration.

Mammalian phospholipase D: Function, and therapeutics

Despite being discovered over 60 years ago, the precise role of phospholipase D (PLD) is still being elucidated. PLD enzymes catalyze the hydrolysis of the phosphodiester bond of glycerophospholipids producing phosphatidic acid and the free headgroup. PLD family members are found in organisms ranging from viruses, and bacteria to plants, and mammals. They display a range of substrate specificities, are regulated by a diverse range of molecules, and have been implicated in a broad range of cellular processes including receptor signaling, cytoskeletal regulation and membrane trafficking. Recent technological advances including: the development of PLD knockout mice, isoform-specific antibodies, and specific inhibitors are finally permitting a thorough analysis of the in vivo role of mammalian PLDs. These studies are facilitating increased recognition of PLD's role in disease states including cancers and Alzheimer's disease, offering potential as a target for therapeutic intervention.

Structure Based Multitargeted Molecular Docking Analysis of Selected Furanocoumarins against Breast Cancer

Breast cancer is one of the biggest global dilemmas and its current therapy is to target the hormone receptors by the use of partial agonists/antagonists. Potent drugs for breast cancer treatment are Tamoxifen, Trastuzumab, Paclitaxel, etc. which show adverse effects and resistance in patients. The aim of the study has been on certain phytochemicals which has potent actions on ERα, PR, EGFR and mTOR inhibition. The current study is performed by the use of molecular docking as protein-ligand interactions play a vital role in drug design. The 3D structures of ERα, PR, EGFR and mTOR were obtained from the protein data bank and docked with 23 3D PubChem structures of furanocoumarin compounds using FlexX. Drug-likeness property was checked by applying the Lipinski's rule of five on the furanocoumarins to evaluate anti-breast cancer activity. Antagonist and inhibition assay of ERα, EGFR and mTOR respectively has been performed using appropriate in-vitro techniques. The results confirm that Xanthotoxol has the best docking score for breast cancer followed by Bergapten, Angelicin, Psoralen and Isoimperatorin. Further, the in-vitro results also validate the molecular docking analysis. This study suggests that the selected furanocoumarins can be further investigated and evaluated for breast cancer treatment and management strategies.

Metabolic crosstalk in the breast cancer microenvironment

During tumorigenesis, breast tumour cells undergo metabolic reprogramming, which generally includes enhanced glycolysis, tricarboxylic acid cycle activity, glutaminolysis and fatty acid biosynthesis. However, the extension and functional importance of these metabolic alterations may diverge not only according to breast cancer subtypes, but also depending on the interaction of cancer cells with the complex surrounding microenvironment. This microenvironment comprises a variety of non-cancerous cells, such as immune cells (e.g. macrophages, lymphocytes, natural killer cells), fibroblasts, adipocytes and endothelial cells, together with extracellular matrix components and soluble factors, which influence cancer progression and are predictive of clinical outcome. The continuous interaction between cancer and stromal cells results in metabolic competition and symbiosis, with oncogenic-driven metabolic reprogramming of cancer cells shaping the metabolism of neighbouring cells and vice versa. This review addresses current knowledge on this metabolic crosstalk within the breast tumour microenvironment (TME). Improved understanding of how metabolism in the TME modulates cancer development and evasion of tumour-suppressive mechanisms may provide clues for novel anticancer therapeutics directed to metabolic targets.

Rapamycin Protects Spiral Ganglion Neurons from Gentamicin-Induced Degeneration In Vitro

Gentamicin, one of the most widely used aminoglycoside antibiotics, is known to have toxic effects on the inner ear. Taken up by cochlear hair cells and spiral ganglion neurons (SGNs), gentamicin induces the accumulation of reactive oxygen species (ROS) and initiates apoptosis or programmed cell death, resulting in a permanent and irreversible hearing loss. Since the survival of SGNs is specially required for cochlear implant, new procedures that prevent SGN cell loss are crucial to the success of cochlear implantation. ROS modulates the activity of the mammalian target of rapamycin (mTOR) signaling pathway, which mediates apoptosis or autophagy in cells of different organs. However, whether mTOR signaling plays an essential role in the inner ear and whether it is involved in the ototoxic side effects of gentamicin remain unclear. In the present study, we found that gentamicin induced apoptosis and cell loss of SGNs in vivo and significantly decreased the density of SGN and outgrowth of neurites in cultured SGN explants. The phosphorylation levels of ribosomal S6 kinase and elongation factor 4E binding protein 1, two critical kinases in the mTOR complex 1 (mTORC1) signaling pathway, were modulated by gentamicin application in the cochlea. Meanwhile, rapamycin, a specific inhibitor of mTORC1, was co-applied with gentamicin to verify the role of mTOR signaling. We observed that the density of SGN and outgrowth of neurites were significantly increased by rapamycin treatment. Our finding suggests that mTORC1 is hyperactivated in the gentamicin-induced degeneration of SGNs, and rapamycin promoted SGN survival and outgrowth of neurites.

Lack of effective translational regulation of PLD expression and exosome biogenesis in triple-negative breast cancer cells

Triple-negative breast cancer (TNBC) is an aggressive subtype of breast cancer that is difficult to treat since cells lack the three receptors (ES, PR, or HER) that the most effective treatments target. We have used a well-established TNBC cell line (MDA-MB-231) from which we found evidence in support for a phospholipase D (PLD)-mediated tumor growth and metastasis: high levels of expression of PLD, as well as the absence of inhibitory miRs (such as miR-203) and 3'-mRNA PARN deadenylase activity in these cells. Such findings are not present in a luminal B cell line, MCF-7, and we propose a new miR•PARN•PLD node that is not uniform across breast cancer molecular subtypes and as such TNBC could be pharmacologically targeted differentially. We review the participation of PLD and phosphatidic acid (PA), its enzymatic product, as new "players" in breast cancer biology, with the aspects of regulation of the tumor microenvironment, macrophage polarization, regulation of PLD transcripts by specific miRs and deadenylases, and PLD-regulated exosome biogenesis. A new signaling miR•PARN•PLD node could serve as new biomarkers for TNBC abnormal signaling and metastatic disease staging, potentially before metastases are able to be visualized using conventional imaging.

Rapalog resistance is associated with mesenchymal-type changes in Tsc2-null cells

Tuberous Sclerosis Complex (TSC) and Lymphangioleiomyomatosis (LAM) are caused by inactivating mutations in TSC1 or TSC2, leading to mTORC1 hyperactivation. The mTORC1 inhibitors rapamycin and analogs (rapalogs) are approved for treating of TSC and LAM. Due to their cytostatic and not cytocidal action, discontinuation of treatment leads to tumor regrowth and decline in pulmonary function. Therefore, life-long rapalog treatment is proposed for the control of TSC and LAM lesions, which increases the chances for the development of acquired drug resistance. Understanding the signaling perturbations leading to rapalog resistance is critical for the development of better therapeutic strategies. We developed the first Tsc2-null rapamycin-resistant cell line, ELT3-245, which is highly tumorigenic in mice, and refractory to rapamycin treatment. In vitro ELT3-245 cells exhibit enhanced anchorage-independent cell survival, resistance to anoikis, and loss of epithelial markers. A key alteration in ELT3-245 is increased β-catenin signaling. We propose that a subset of cells in TSC and LAM lesions have additional signaling aberrations, thus possess the potential to become resistant to rapalogs. Alternatively, when challenged with rapalogs TSC-null cells are reprogrammed to express mesenchymal-like markers. These signaling changes could be further exploited to induce clinically-relevant long-term remissions.

Phospholipase D and the Mitogen Phosphatidic Acid in Human Disease: Inhibitors of PLD at the Crossroads of Phospholipid Biology and Cancer

Lipids are key building blocks of biological membranes and are involved in complex signaling processes such as metabolism, proliferation, migration, and apoptosis. Extracellular signaling by growth factors, stress, and nutrients is transmitted through receptors that activate lipid-modifying enzymes such as the phospholipases, sphingosine kinase, or phosphoinositide 3-kinase, which then modify phospholipids, sphingolipids, and phosphoinositides. One such important enzyme is phospholipase D (PLD), which cleaves phosphatidylcholine to yield phosphatidic acid and choline. PLD isoforms have dual role in cells. The first involves maintaining cell membrane integrity and cell signaling, including cell proliferation, migration, cytoskeletal alterations, and invasion through the PLD product PA, and the second involves protein-protein interactions with a variety of binding partners. Increased evidence of elevated PLD expression and activity linked to many pathological conditions, including cancer, neurological and inflammatory diseases, and infection, has motivated the development of dual- and isoform-specific PLD inhibitors. Many of these inhibitors are reported to be efficacious and safe in cells and mouse disease models, suggesting the potential for PLD inhibitors as therapeutics for cancer and other diseases. Current knowledge and ongoing research of PLD signaling networks will help to evolve inhibitors with increased efficacy and safety for clinical studies.

HPV-16 E7 expression up-regulates phospholipase D activity and promotes rapamycin resistance in a pRB-dependent manner

Background

Human Papillomavirus (HPV) infection is the main risk factor for the development and progression of cervical cancer. HPV-16 E6 and E7 expression is essential for induction and maintenance of the transformed phenotype. These oncoproteins interfere with the function of several intracellular proteins, including those controlling the PI3K/AKT/mTOR pathway in which Phospolipase D (PLD) and Phosphatidic acid (PA) play a critical role.

Methods

PLD activity was measured in primary human keratinocytes transduced with retroviruses expressing HPV-16 E6, E7 or E7 mutants. The cytostatic effect of rapamycin, a well-known mTOR inhibitor with potential clinical applications, was evaluated in monolayer and organotypic cultures.

Results

HPV-16 E7 expression in primary human keratinocytes leads to an increase in PLD expression and activity. Moreover, this activation is dependent on the ability of HPV-16 E7 to induce retinoblastoma protein (pRb) degradation. We also show that cells expressing HPV-16 E7 or silenced for pRb acquire resistance to the antiproliferative effect of rapamycin.

Conclusion

This is the first indication that HPV oncoproteins can affect PLD activity. Since PA can interfere with the ability of rapamycin to bind mTOR, the use of combined strategies to target mTOR and PLD activity might be considered to treat HPV-related malignancies.

Electronic supplementary material

The online version of this article (10.1186/s12885-018-4392-8) contains supplementary material, which is available to authorized users.

A Unique Homeostatic Signaling Pathway Links Synaptic Inactivity to Postsynaptic mTORC1

mTORC1-dependent translational control plays a key role in several enduring forms of synaptic plasticity such as long term potentiation (LTP) and mGluR-dependent long term depression. Recent evidence demonstrates an additional role in regulating synaptic homeostasis in response to inactivity, where dendritic mTORC1 serves to modulate presynaptic function via retrograde signaling. Presently, it is unclear whether LTP and homeostatic plasticity use a common route to mTORC1-dependent signaling or whether each engage mTORC1 through distinct pathways. Here, we report a unique signaling pathway that specifically couples homeostatic signaling to postsynaptic mTORC1 after loss of excitatory synaptic input. We find that AMPAR blockade, but not LTP-inducing stimulation, induces phospholipase D (PLD)-dependent synthesis of the lipid second messenger phosphatidic acid (PA) in rat cultured hippocampal neurons of either sex. Pharmacological blockade of PLD1/2 or pharmacogenetic disruption of PA interactions with mTOR eliminates mTORC1 signaling and presynaptic compensation driven by AMPAR blockade, but does not alter mTORC1 activation or functional changes during chemical LTP (cLTP). Overexpression of PLD1, but not PLD2, recapitulates both functional synaptic changes as well as signature cellular adaptations associated with homeostatic plasticity. Finally, transient application of exogenous PA is sufficient to drive rapid presynaptic compensation requiring mTORC1-dependent translation of BDNF in the postsynaptic compartment. These results thus define a unique homeostatic signaling pathway coupling mTORC1 activation to changes in excitatory synaptic drive. Our results further imply that more than one canonical mTORC1 activation pathway may be relevant for the design of novel therapeutic approaches against neurodevelopmental disorders associated with mTORC1 dysregulation.SIGNIFICANCE STATEMENT Homeostatic and Hebbian forms of synaptic plasticity are thought to play complementary roles in regulating neural circuit function, but we know little about how these forms of plasticity are distinguished at the single neuron level. Here, we define a signaling pathway that uniquely links mTORC1 with homeostatic signaling in neurons.

A Phosphatidic Acid (PA) conveyor system of continuous intracellular transport from cell membrane to nucleus maintains EGF receptor homeostasis

The intracellular concentration of the mitogen phosphatidic acid (PA) must be maintained at low levels until the need arises for cell proliferation. How temporal and spatial trafficking of PA affects its target proteins in the different cellular compartments is not fully understood. We report that in cancer cells, PA cycles back and forth from the cellular membrane to the nucleus, affecting the function of epidermal growth factor (EGF), in a process that involves PPARα/LXRα signaling. Upon binding to its ligand, EGF receptor (EGFR)-initiated activation of phospholipase D (PLD) causes a spike in intracellular PA production that forms vesicles transporting EGFR from early endosomes (EEA1 marker) and prolonged internalization in late endosomes and Golgi (RCAS marker). Cells incubated with fluorescent-labeled PA (NBD-PA) show PA in “diffuse” locations throughout the cytoplasm, punctae (small, <0.1 μm) vesicles) and large (>0.5 μm) vesicles that co-localize with EGFR. We also report that PPARα/LXRα form heterodimers that bind to new Responsive Elements (RE) in the EGFR promoter. Nuclear PA enhances EGFR expression, a role compatible with the mitogenic ability of the phospholipid. Newly made EGFR is packaged into PA recycling vesicles (Rab11 marker) and transported back to the cytoplasm and plasma membrane. However, a PLD+PA combination impedes binding of PPARα/LXRα to the EGFR promoter. Thus, if PA levels inside the nucleus reach a certain threshold (>100 nM) PA outcompetes the nuclear receptors and transcription is inhibited. This new signaling function of PLD-PA targeting EGFR trafficking and biphasically modulating its transcription, could explain cell proliferation initiation and its maintenance in cancer cells.

Proliferative and metastatic roles for Phospholipase D in mouse models of cancer

Phospholipase D (PLD) activity has been proposed to facilitate multiple steps in cancer progression including growth, metabolism, angiogenesis, and mobility. The canonical enzymes PLD1 and PLD2 enact their diverse effects through hydrolyzing the membrane lipid phosphatidylcholine to generate the second messenger and signaling lipid phosphatidic acid (PA). However, the widespread expression of PLD1 and PLD2 in normal tissues and the additional distinct enzymatic mechanisms through which PA can be generated have produced uncertainty regarding the optimal settings in which PLD inhibition might ameliorate cancer. Recent studies in mouse model systems have demonstrated that inhibition or elimination of PLD activity reduces tumor growth and metastasis. One mechanism proposed for this outcome involves proliferative signaling mediated by receptor tyrosine kinases (RTK) and G protein-coupled receptors (GPCR), which is attenuated when downstream PLD signal propagation is suppressed. The reduced proliferative signaling has been reported to be compounded by dysfunctional energetic metabolism in the tumor cells under conditions of nutrient deprivation. Moreover, cancer cells lacking PLD activity display inefficiencies across multiple steps of the metastatic cascade, limiting the tumor's lethal spread. Using PLD isoform knockout mice, recent studies have reported on the net effects of inhibition and ablation in multiple cancer models through examining the role of PLD in the non-tumor cells comprising the stroma and microenvironment. The promising results of such in vivo studies, combined with the apparent low toxicity of highly-specific and potent inhibitors, highlights PLD as an attractive target for therapeutic inhibition in cancer. We discuss here the array of anti-tumor effects produced by PLD inhibition and ablation in cancer models with a focus on animal studies.

Protein Translation and Signaling in Human Eosinophils

We have recently reported that, unlike IL-5 and GM-CSF, IL-3 induces increased translation of a subset of mRNAs. In addition, we have demonstrated that Pin1 controls the activity of mRNA binding proteins, leading to enhanced mRNA stability, GM-CSF protein production and prolonged eosinophil (EOS) survival. In this review, discussion will include an overview of cap-dependent protein translation and its regulation by intracellular signaling pathways. We will address the more general process of mRNA post-transcriptional regulation, especially regarding mRNA binding proteins, which are critical effectors of protein translation. Furthermore, we will focus on (1) the roles of IL-3-driven sustained signaling on enhanced protein translation in EOS, (2) the mechanisms regulating mRNA binding proteins activity in EOS, and (3) the potential targeting of IL-3 signaling and the signaling leading to mRNA binding activity changes to identify therapeutic targets to treat EOS-associated diseases.

Rapamycin enhances the antiproliferative effect of transforming growth factor-β on MCF-7 human breast cancer cells

Transforming growth factor-β (TGF-β), a well-known cytokine with pleiotropic biological functions, has an important role in the regulation of cellular proliferation. Rapamycin has specific antagonistic activity on the function of the mammalian target of the rapamycin signaling pathway. The cooperation of TGF-β and rapamycin on the proliferation of Michigan Cancer Foundation (MCF)-7 human breast cancer cells is unclear. The present study demonstrated that TGF-β had a growth-arresting effect on MCF-7 cancer cells. TGF-β stimulation resulted in the upregulation of several cyclin-dependent kinase inhibitors, including p14^ARF, p15^INK4b, p16I^NK4a and p21^WAF1/CIP1. The present study also demonstrated that rapamycin enhances the antiproliferative effect of TGF-β. The combination of rapamycin and TGF-β induced apoptosis of MCF-7 tumor cells. These findings advance the current understanding of the biological effects of TGF-β and rapamycin.

The intricate regulation and complex functions of the Class III phosphoinositide 3-kinase Vps34

The Class III phosphoinositide 3-kinase Vps34 (vacuolar protein sorting 34) plays important roles in endocytic trafficking, macroautophagy, phagocytosis, cytokinesis and nutrient sensing. Recent studies have provided exciting new insights into the structure and regulation of this lipid kinase, and new cellular functions for Vps34 have emerged. This review critically examines the wealth of new data on this important enzyme, and attempts to integrate these findings with current models of Vps34 signalling.

Antitumor Synergism and Enhanced Survival with a Tumor Vasculature-Targeted Enzyme Prodrug System, Rapamycin, and Cyclophosphamide

Mutant cystathionine gamma-lyase was targeted to phosphatidylserine exposed on tumor vasculature through fusion with Annexin A1 or Annexin A5. Cystathionine gamma-lyase E58N, R118L, and E338N mutations impart nonnative methionine gamma-lyase activity, resulting in tumor-localized generation of highly toxic methylselenol upon systemic administration of nontoxic selenomethionine. The described therapeutic system circumvents systemic toxicity issues using a novel drug delivery/generation approach and avoids the administration of nonnative proteins and/or DNA required with other enzyme prodrug systems. The enzyme fusion exhibits strong and stable in vitro binding with dissociation constants in the nanomolar range for both human and mouse breast cancer cells and in a cell model of tumor vascular endothelium. Daily administration of the therapy suppressed growth of highly aggressive triple-negative murine 4T1 mammary tumors in immunocompetent BALB/cJ mice and MDA-MB-231 tumors in SCID mice. Treatment did not result in the occurrence of negative side effects or the elicitation of neutralizing antibodies. On the basis of the vasculature-targeted nature of the therapy, combinations with rapamycin and cyclophosphamide were evaluated. Rapamycin, an mTOR inhibitor, reduces the prosurvival signaling of cells in a hypoxic environment potentially exacerbated by a vasculature-targeted therapy. IHC revealed, unsurprisingly, a significant hypoxic response (increase in hypoxia-inducible factor 1 α subunit, HIF1A) in the enzyme prodrug-treated tumors and a dramatic reduction of HIF1A upon rapamycin treatment. Cyclophosphamide, an immunomodulator at low doses, was combined with the enzyme prodrug therapy and rapamycin; this combination synergistically reduced tumor volumes, inhibited metastatic progression, and enhanced survival. Mol Cancer Ther; 16(9); 1855-65. ©2017 AACR.

Role of Akt-independent mTORC1 and GSK3β signaling in sublethal NMDA-induced injury and the recovery of neuronal electrophysiology and survival

Glutamate-induced excitotoxicity, mediated by overstimulation of N-methyl-D-aspartate (NMDA) receptors, is a mechanism that causes secondary damage to neurons. The early phase of injury causes loss of dendritic spines and changes to synaptic activity. The phosphatidylinositol-4,5-bisphosphate 3-kinase/Akt/ mammalian target of rapamycin (PI3K/Akt/mTOR) pathway has been implicated in the modulation and regulation of synaptic strength, activity, maturation, and axonal regeneration. The present study focuses on the physiology and survival of neurons following manipulation of Akt and several downstream targets, such as GSK3β, FOXO1, and mTORC1, prior to NMDA-induced injury. Our analysis reveals that exposure to sublethal levels of NMDA does not alter phosphorylation of Akt, S6, and GSK3β at two and twenty four hours following injury. Electrophysiological recordings show that NMDA-induced injury causes a significant decrease in spontaneous excitatory postsynaptic currents at both two and twenty four hours, and this phenotype can be prevented by inhibiting mTORC1 or GSK3β, but not Akt. Additionally, inhibition of mTORC1 or GSK3β promotes neuronal survival following NMDA-induced injury. Thus, NMDA-induced excitotoxicity involves a mechanism that requires the permissive activity of mTORC1 and GSK3β, demonstrating the importance of these kinases in the neuronal response to injury.

Targeting phospholipase D in cancer, infection and neurodegenerative disorders

Phospholipase D (PLD) enzymes are one source of receptor-generated phosphatidic acid (PtdOH),which may subsequently be metabolized to diacylglycerol (DAG) and lysophosphatidic acid. There are other pathways that lead to PtdOH generation, but differences in pathways and in the acyl composition of the products seem to provide some specificity.

Both direct and indirect inhibitors of PLD activity have been identified despite a long-held suspicion that this pathway was undruggable. The identification of raloxifene and halopemide as direct inhibitors was followed by the systematic development of isoenzyme-preferring compounds that have been used to further differentiate the functions of PLD1 and PLD2.

PLD2 in host cells has been associated with viral entry processes and innate immune response pathways such that inhibition blocks efficient infection. This PLD2 pathway has been linked to autophagy via AKT kinases.

As a potential target in antiretroviral therapy, PLD1 works through the CAD enzyme (which contains carbamoyl aspartate synthase, aspartate transcarbamylase and dihydro-orotase domains) to modulate pyrimidine biosynthesis.

PLD activity and expression have been shown to be upregulated in several types of human cancers, in which PLD enzymes function downstream of a variety of known oncogenes. Inhibition of PtdOH production has a marked effect on tumorigenesis and malignant invasion.

PLD1, PLD2 and PLD3 have each been suggested to have a role in Alzheimer disease and other neurodegenerative conditions, but a mechanism has not yet emerged to explain the roles of these proteins in central nervous system pathophysiology.

Lipid second messengers such as phosphatidic acid (PtdOH) have a role in a wide range of pathological processes, and phospholipase D (PLD) enzymes are one of the major sources of signal-activated PtdOH generation. In this Review, Brown, Thomas and Lindsley discuss the development of PLD inhibitors, with a focus on isoform-specific inhibitors, and their potential applications in the treatment of cancer, neurodegeneration and infection.

Lipid second messengers have essential roles in cellular function and contribute to the molecular mechanisms that underlie inflammation, malignant transformation, invasiveness, neurodegenerative disorders, and infectious and other pathophysiological processes. The phospholipase D (PLD) isoenzymes PLD1 and PLD2 are one of the major sources of signal-activated phosphatidic acid (PtdOH) generation downstream of a variety of cell-surface receptors, including G protein-coupled receptors (GPCRs), receptor tyrosine kinases (RTKs) and integrins. Recent advances in the development of isoenzyme-selective PLD inhibitors and in molecular genetics have suggested that PLD isoenzymes in mammalian cells and pathogenic organisms may be valuable targets for the treatment of several human diseases. Isoenzyme-selective inhibitors have revealed complex inter-relationships between PtdOH biosynthetic pathways and the role of PtdOH in pathophysiology. PLD enzymes were once thought to be undruggable owing to the ubiquitous nature of PtdOH in cell signalling and concerns that inhibitors would be too toxic for use in humans. However, recent promising discoveries suggest that small-molecule isoenzyme-selective inhibitors may provide novel compounds for a unique approach to the treatment of cancers, neurodegenerative disorders and other afflictions of the central nervous system, and potentially serve as broad-spectrum antiviral and antimicrobial therapeutics.

Targeting Phospholipid Metabolism in Cancer

All cancers tested so far display abnormal choline and ethanolamine phospholipid metabolism, which has been detected with numerous magnetic resonance spectroscopy (MRS) approaches in cells, animal models of cancer, as well as the tumors of cancer patients. Since the discovery of this metabolic hallmark of cancer, many studies have been performed to elucidate the molecular origins of deregulated choline metabolism, to identify targets for cancer treatment, and to develop MRS approaches that detect choline and ethanolamine compounds for clinical use in diagnosis and treatment monitoring. Several enzymes in choline, and recently also ethanolamine, phospholipid metabolism have been identified, and their evaluation has shown that they are involved in carcinogenesis and tumor progression. Several already established enzymes as well as a number of emerging enzymes in phospholipid metabolism can be used as treatment targets for anticancer therapy, either alone or in combination with other chemotherapeutic approaches. This review summarizes the current knowledge of established and relatively novel targets in phospholipid metabolism of cancer, covering choline kinase α, phosphatidylcholine-specific phospholipase D1, phosphatidylcholine-specific phospholipase C, sphingomyelinases, choline transporters, glycerophosphodiesterases, phosphatidylethanolamine N-methyltransferase, and ethanolamine kinase. These enzymes are discussed in terms of their roles in oncogenic transformation, tumor progression, and crucial cancer cell properties such as fast proliferation, migration, and invasion. Their potential as treatment targets are evaluated based on the current literature.

5-Aminoimidazole-4-carboxamide-1-β-4-ribofuranoside (AICAR) enhances the efficacy of rapamycin in human cancer cells

mTOR - the mammalian/mechanistic target of rapamycin - has been implicated as a key signaling node for promoting survival of cancer cells. However, clinical trials that have targeted mTOR with rapamycin or rapamycin analogs have had minimal impact. In spite of the high specificity of rapamycin for mTOR, the doses needed to suppress key mTOR substrates have proved toxic. We report here that rapamycin when combined with AICAR - a compound that activates AMP-activated protein kinase makes rapamycin cytotoxic rather than cytostatic at doses that are tolerated clinically. AICAR by itself is able to suppress mTOR complex 1 (mTORC1), but also stimulates a feedback activation of mTORC2, which activates the survival kinase Akt. However, AICAR also suppresses production of phosphatidic acid (PA), which interacts with mTOR in a manner that is competitive with rapamycin. The reduced level of PA sensitizes mTORC2 to rapamycin at tolerable nano-molar doses leading reduced Akt phosphorylation and apoptosis. This study reveals how the use of AICAR enhances the efficacy of rapamycin such that rapamycin at low nano-molar doses can suppress mTORC2 and induce apoptosis in human cancer cells at doses that are clinically tolerable.

The transcription factors Slug (SNAI2) and Snail (SNAI1) regulate phospholipase D (PLD) promoter in opposite ways towards cancer cell invasion

Slug (SNAI2) and Snail (SNAI1) are master regulatory transcription factors for organogenesis and wound healing, and they are involved in the epithelial to mesenchymal transition (EMT) of cancer cells. We found that the activity of phospholipase D isoform 2 (PLD2) is highly increased in cancers with larger size and poor prognosis (MDA-MB-231 versus MCF-7 cells), so we determined if Snail or Slug were responsible for PLD2 gene transcription regulation. Unexpectedly, we found that PLD2 expression was positively regulated by Slug but negatively regulated by Snail. The differential effects are amplified in breast cancer cells over normal cells and with MDA-MB-231 more robustly than MCF-7. Slug putatively binds to the PLD2 promoter and transactivates it, which is negated when Slug and Snail compete with each other. Meanwhile, PLD2 has a negative effect on Snail expression and a positive effect on Slug, thus closing a feedback loop between the lipase and the transcription factors. Further, PA, the product of PLD2 enzymatic reaction, has profound effects on its own and it further regulates the transcription factors. Thus, we show for the first time that the overexpressed PLD2 in human breast tumors is regulated by Slug and Snail transcription factors. The newly uncovered feedback loops in highly invasive cancer cells have important implications in the process of EMT.

The Enigma of Rapamycin Dosage

The mTOR pathway is a critical regulator of cell growth, proliferation, metabolism, and survival. Dysregulation of mTOR signaling has been observed in most cancers and, thus, the mTOR pathway has been extensively studied for therapeutic intervention. Rapamycin is a natural product that inhibits mTOR with high specificity. However, its efficacy varies by dose in several contexts. First, different doses of rapamycin are needed to suppress mTOR in different cell lines; second, different doses of rapamycin are needed to suppress the phosphorylation of different mTOR substrates; and third, there is a differential sensitivity of the two mTOR complexes mTORC1 and mTORC2 to rapamycin. Intriguingly, the enigmatic properties of rapamycin dosage can be explained in large part by the competition between rapamycin and phosphatidic acid (PA) for mTOR. Rapamycin and PA have opposite effects on mTOR whereby rapamycin destabilizes and PA stabilizes both mTOR complexes. In this review, we discuss the properties of rapamycin dosage in the context of anticancer therapeutics.

Cytoplasmic TRAF4 contributes to the activation of p70s6k signaling pathway in breast cancer

Tumor necrosis factor receptor associated factor 4 (TRAF4) is an important adaptor protein that plays a significant role in several signaling pathways. By studying the relationship between TRAF4 and 70 kDa ribosomal protein S6 kinase (p70s6k) in vivo, we demonstrated that cytoplasmic TRAF4 was correlated with the activation of p70s6k in breast cancer. Moreover, we found that cytoplasmic TRAF4 expression in breast cancer patients was significantly associated with a poor prognosis. To determine the exact mechanism, we analyzed the interaction between TRAF4 and p70s6k and identified the Zinc fingers domain of TRAF4 was responsible for their interaction in MCF7 cells. Furthermore, we found that activation of p70s6k/S6 signaling pathway by TRAF4 requires the mammalian target of rapamycin (mTOR) activity; TRAF4 acted as a sensitizer. Tumor necrosis factor receptor associated factor 2 (TRAF2), as a binding partner of TRAF4, could also promoted activation of p70s6k signaling via upregulating cytoplasm expression of TRAF4 and played a critical role in TNFa-induced activation of p70s6k/S6 pathway. Finally, we demonstrated p70s6k/S6 signaling pathway played an important role in the promoting function of TRAF4 on cell proliferation. In summary, our work suggests a new direction for understanding the oncogenic function of TRAF4 in breast cancer.

A Repertoire of MicroRNAs Regulates Cancer Cell Starvation by Targeting Phospholipase D in a Feedback Loop That Operates Maximally in Cancer Cells

We report a negative feedback loop between the signaling protein phospholipase D (PLD), phosphatidic acid (PA), and a specific set of microRNAs (miRNAs) during nutrient starvation of breast cancer cells. We show that PLD expression is increased in four breast cancer cell lines and that hypoxia, cell overcrowding, and nutrient starvation for 3 to 6 h increase expression even further. However, after prolonged (>12-h) starvation, PLD levels return to basal or lower levels. The mechanism for this is as follows. First, during initial starvation, an elevated PA (the product of PLD enzymatic activity) activates mTOR and S6K, known to inhibit apoptosis, and enhances cell migration especially in post-epithelial-to-mesenchymal transition (post-EMT) cancer cells. Second, continued PA production in later starvation induces expression of PLD-targeting microRNA 203 (miR-203), miR-887, miR-3619-5p, and miR-182, which reduce PLD translation. We provide direct evidence for a feedback loop, whereby PLD induction upon starvation leads to PA, which induces expression of miRNAs, which in turn inhibits PLD2 translation. The physiological relevance for breast cancer cells is that as PA can activate cell invasion, then, due to the negative feedback, it can deprive mTOR and S6K of their natural activator. It can further prevent inhibition of apoptosis and allow cells to survive nutrient deprivation, which normal cells cannot do.

Halofuginone Synergistically Enhances Anti-Proliferation of Rapamycin in T Cells and Reduces Cytotoxicity of Cyclosporine in Cultured Renal Tubular Epithelial Cells

Both rapamycin (RAPA) and cyclosporin A (CsA) are commonly used for immunosuppression, however their adverse side effects limit their application. Thus, it is of interest to develop novel means to enhance or preserve the immunosuppressive activity of RAPA or CsA while reducing their toxicity. Halofuginone (HF) has been recently tested as a potential immunosuppressant. This study investigated the interaction of HF with RAPA or with CsA in cell cultures. Cell proliferation in cultures was determined using methylthiazol tetrazolium assay, and cell apoptosis assessed by flow cytometric analysis and Western blot. The drug-drug interaction was determined according to Loewe’s equation or Bliss independence. Here, we showed that addition of HF to anti-CD 3 antibody-stimulated splenocyte cultures induced synergistic suppression of T cell proliferation in the presence of RAPA, indicated by an interaction index (γ) value of < 1.0 between HF and RAPA, but not in those with CsA. The synergistic interaction of RAPA with HF in the suppression of T cell proliferation was also seen in a mixed lymphocyte reaction and Jurkat T cell growth, and was positively correlated with an increase in cell apoptosis, but not with proline depletion. In cultured kidney tubular epithelial cells, HF attenuated the cytotoxicity of CsA. In conclusion, these data indicate that HF synergistically enhances anti-T cell proliferation of RAPA and reduces the nephrotoxicity of CsA in vitro, suggesting the potential use of HF for enhancing anti-T cell proliferation of RAPA and reducing CsA-mediated nephrotoxicity.

Phospholipase D signaling pathways and phosphatidic acid as therapeutic targets in cancer

Phospholipase D is a ubiquitous class of enzymes that generates phosphatidic acid as an intracellular signaling species. The phospholipase D superfamily plays a central role in a variety of functions in prokaryotes, viruses, yeast, fungi, plants, and eukaryotic species. In mammalian cells, the pathways modulating catalytic activity involve a variety of cellular signaling components, including G protein-coupled receptors, receptor tyrosine kinases, polyphosphatidylinositol lipids, Ras/Rho/ADP-ribosylation factor GTPases, and conventional isoforms of protein kinase C, among others. Recent findings have shown that phosphatidic acid generated by phospholipase D plays roles in numerous essential cellular functions, such as vesicular trafficking, exocytosis, autophagy, regulation of cellular metabolism, and tumorigenesis. Many of these cellular events are modulated by the actions of phosphatidic acid, and identification of two targets (mammalian target of rapamycin and Akt kinase) has especially highlighted a role for phospholipase D in the regulation of cellular metabolism. Phospholipase D is a regulator of intercellular signaling and metabolic pathways, particularly in cells that are under stress conditions. This review provides a comprehensive overview of the regulation of phospholipase D activity and its modulation of cellular signaling pathways and functions.

The phospholipid code: a key component of dying cell recognition, tumor progression and host-microbe interactions

A significant effort is made by the cell to maintain certain phospholipids at specific sites. It is well described that proteins involved in intracellular signaling can be targeted to the plasma membrane and organelles through phospholipid-binding domains. Thus, the accumulation of a specific combination of phospholipids, denoted here as the 'phospholipid code', is key in initiating cellular processes. Interestingly, a variety of extracellular proteins and pathogen-derived proteins can also recognize or modify phospholipids to facilitate the recognition of dying cells, tumorigenesis and host-microbe interactions. In this article, we discuss the importance of the phospholipid code in a range of physiological and pathological processes.

Diacylglycerol kinase-ζ regulates mTORC1 and lipogenic metabolism in cancer cells through SREBP-1

Diacylglycerol kinases (DGKs) transform diacylglycerol (DAG) into phosphatidic acid (PA), balancing the levels of these key metabolic and signaling lipids. We previously showed that PA derived from the DGKζ isoform promotes mammalian target of rapamycin complex 1 (mTORC1) activation. This function might be crucial for the growth and survival of cancer cells, especially for those resistant to the allosteric mTOR inhibitor rapamycin. How this positive function of DGKζ coordinates with DAG metabolism and signaling is unknown. In this study, we used a rapamycin-resistant colon cancer cell line as a model to address the role of DGKζ in tumor cells. We found that DGKζ predominated over other PA sources such as DGKα or phospholipase D to activate mTORC1, and that its activity was a component of the rapamycin-induced feedback loops. We show that the DGKζ DAG-consuming function is central to cell homeostasis, as DAG negatively regulates levels of the lipogenic transcription factor SREBP-1. Our findings suggest a model in which simultaneous regulation of DAG and PA levels by DGKζ is integrated with mTOR function to maintain tumor cell homeostasis; we provide new evidence of the crosstalk between mTOR and lipid metabolism that will be advantageous in the design of drug therapies.

Apoptotic effects of high-dose rapamycin occur in S-phase of the cell cycle

Mutations in genes encoding regulators of mTOR, the mammalian target of rapamycin, commonly provide survival signals in cancer cells. Rapamycin and analogs of rapamycin have been used with limited success in clinical trials to target mTOR-dependent survival signals in a variety of human cancers. Suppression of mTOR predominantly causes G1 cell cycle arrest, which likely contributes to the ineffectiveness of rapamycin-based therapeutic strategies. While rapamycin causes the accumulation of cells in G1, its effect in other cell cycle phases remains largely unexplored. We report here that when synchronized MDA-MB-231 breast cancer cells are allowed to progress into S-phase from G1, rapamycin activates the apoptotic machinery with a concomitant increase in cell death. In Calu-1 lung cancer cells, rapamycin induced a feedback increase in Akt phosphorylation at Ser473 in S-phase that mitigated rapamycin-induced apoptosis. However, sensitivity to rapamycin in S-phase could be reestablished if Akt phosphorylation was suppressed. We recently reported that glutamine (Gln) deprivation causes K-Ras mutant cancer cells to aberrantly arrest primarily in S-phase. Consistent with observed sensitivity of S-phase cells to rapamycin, interfering with Gln utilization sensitized both MDA-MB-231 and Calu-1 K-Ras mutant cancer cells to the apoptotic effect of rapamycin. Importantly, rapamycin induced substantially higher levels of cell death upon Gln depletion than that observed in cancer cells that were allowed to progress through S-phase after being synchronized in G1. We postulate that exploiting metabolic vulnerabilities in cancer cells such as S-phase arrest observed with K-Ras-driven cancer cells deprived of Gln, could be of great therapeutic potential.

Rapamycin-induced G1 cell cycle arrest employs both TGF-β and Rb pathways

The mammalian target of rapamycin complex 1 (mTORC1) is a critical regulator of G1 cell cycle progression. Two key substrates of mTORC1 are ribosomal subunit S6 kinase (S6K) and eukaryotic initiation factor 4E (eIF4E) binding protein-1 (4E-BP1). We reported previously that simultaneous knockdown of S6K and eIF4E causes a transforming growth factor-β (TGF-β)-dependent G1 cell cycle arrest in MDA-MB-231 human breast cancer cells. Rapamycin inhibits the phosphorylation of S6K at nano-molar concentrations in MDA-MB-231 cells; however, micro-molar concentrations of rapamycin are required to inhibit phosphorylation of 4E-BP1 - the phosphorylation of which liberates eIF4E to initiate translation. Micro-molar doses of rapamycin are required for complete G1 cell cycle arrest - indicating that 4E-BP1 is a critical target of mTOR for promoting cell cycle progression. Data are provided demonstrating that G1 cell cycle arrest induced by rapamycin is due to up-regulation of TGF-β signaling and down-regulation of Rb phosphorylation via phosphorylation of the mTORC1 substrates S6K and 4E-BP1 respectively. These findings enhance the current understanding of the cytostatic effects of mTORC1 suppression with therapeutic implications.