Acidosis promotes invasiveness of breast cancer cells through ROS-AKT-NF-κB pathway

Tumour acidosis: from the passenger to the driver's seat

The high metabolic demand of cancer cells leads to an accumulation of H⁺ ions in the tumour microenvironment. The disorganized tumour vasculature prevents an efficient wash-out of H⁺ ions released into the extracellular medium but also favours the development of tumour hypoxic regions associated with a shift towards glycolytic metabolism. Under hypoxia, the final balance of glycolysis, including breakdown of generated ATP, is the production of lactate and a stoichiometric amount of H⁺ ions. Another major source of H⁺ ions results from hydration of CO₂ produced in the more oxidative tumour areas. All of these events occur at high rates in tumours to fulfil bioenergetic and biosynthetic needs. This Review summarizes the current understanding of how H⁺-generating metabolic processes segregate within tumours according to the distance from blood vessels and inversely how ambient acidosis influences tumour metabolism, reducing glycolysis while promoting mitochondrial activity. The Review also presents novel insights supporting the participation of acidosis in cancer progression via stimulation of autophagy and immunosuppression. Finally, recent advances in the different therapeutic modalities aiming to either block pH-regulatory systems or exploit acidosis will be discussed.

Depletion of carbonic anhydrase IX abrogates hypoxia-induced overexpression of stanniocalcin-1 in triple negative breast cancer cells

Carbonic anhydrase IX (CAIX) is a pH-regulating enzyme that plays a key role in maintaining an alkaline intracellular pH under hypoxic conditions. It is overexpressed in a variety of solid cancers, including breast cancer (BC), and has been implicated in the migration, invasion and stemness of breast cancer cells. Therefore, CAIX recently emerged as a novel therapeutic target for the treatment of BC. To gain an insight into the mechanism of action of CAIX inhibitors, we investigated the impact of CAIX knock-down on the transcriptional response to hypoxia in 2 BC cell lines - MCF7 and MDA-MB-231, by performing a global gene expression analysis. This showed that CAIX knock-down had a relatively minor effect on the global transcriptional response to hypoxia, however it blocked hypoxia-induced upregulation of stanniocalcin-1 (STC1), a secreted glycoprotein that has been shown to promote tumor progression and metastasis in BC. Kaplan-Meier survival analysis showed that high STC1 expression is significantly associated with poor survival in patients with basal-type breast cancer but not luminal A and HER2+ subtypes. Moreover, the association was particularly high in a subgroup of basal-type BC patients with TP53 mutations thus revealing a putative cooperation of STC1 with mutated TP53 in generating highly aggressive BC subgroup. Taken together, these findings show that CAIX inhibitors at least partially act through blocking STC1 induction in BC cells and reveal a subgroup of BC patients, who potentially would benefit most from the treatment with CAIX inhibitors.

Cancer-associated mesenchymal stroma fosters the stemness of osteosarcoma cells in response to intratumoral acidosis via NF-κB activation

The role of mesenchymal stem cells (MSC) in osteosarcoma (OS), the most common primary tumor of bone, has not been extensively elucidated. We have recently shown that OS is characterized by interstitial acidosis, a microenvironmental condition that is similar to a wound setting, in which mesenchymal reactive cells are activated to release mitogenic and chemotactic factors. We therefore intended to test the hypothesis that, in OS, acid-activated MSC influence tumor cell behavior. Conditioned media or co-culture with normal MSC previously incubated with short-term acidosis (pH 6.8 for 10 hr, H⁺ -MSC) enhanced OS clonogenicity and invasion. This effect was mediated by NF-κB pathway activation. In fact, deep-sequencing analysis, confirmed by Real-Time PCR and ELISA, demonstrated that H⁺ -MSC differentially induced a tissue remodeling phenotype with increased expression of RelA, RelB and NF-κB1, and downstream, of CSF2/GM-CSF, CSF3/G-CSF and BMP2 colony-promoting factors, and of chemokines (CCL5, CXCL5 and CXCL1), and cytokines (IL6 and IL8), with an increased expression of CXCR4. An increased expression of IL6 and IL8 were found only in normal stromal cells, but not in OS cells, and this was confirmed in tumor-associated stromal cells isolated from OS tissue. Finally, H⁺ -MSC conditioned medium differentially promoted OS stemness (sarcosphere number, stem-associated gene expression), and chemoresistance also via IL6 secretion. Our data support the hypothesis that the acidic OS microenvironment is a key factor for MSC activation, in turn promoting the secretion of paracrine factors that influence tumor behavior, a mechanism that holds the potential for future therapeutic interventions aimed to target OS.

LncRNA AK023948 is a positive regulator of AKT

Despite the overwhelming number of human long non-coding RNAs (lncRNAs) reported so far, little is known about their physiological functions for the majority of them. The present study uses a CRISPR/Cas9-based synergistic activation mediator (SAM) system to identify potential lncRNAs capable of regulating AKT activity. Among lncRNAs identified from this screen, we demonstrate that AK023948 is a positive regulator for AKT. Knockout of AK023948 suppresses, whereas rescue with AK023948 restores the AKT activity. Mechanistically, AK023948 functionally interacts with DHX9 and p85. Importantly, AK023948 is required for the interaction between DHX9 and p85 to hence the p85 stability and promote AKT activity. Finally, AK023948 is upregulated in breast cancer; interrogation of TCGA data set indicates that upregulation of DHX9 in breast cancer is associated with poor survival. Together, this study demonstrates two previously uncharacterized factors AK023948 and DHX9 as important players in the AKT pathway, and that their upregulation may contribute to breast tumour progression.

The function of many human long non-coding RNAs (lncRNAs) is still undetermined. Here, the authors setup a gain of function CRISPR-based screen and identify a lncRNA that positively regulates AKT activity by interacting with the RNA helicase DHX9 resulting in stabilization of PI3K regulatory subunit p85.

Therapeutic implications of tumor interstitial acidification

Interstitial acidification is a hallmark of solid tumor tissues resulting from the combination of different factors, including cellular buffering systems, defective tissue perfusion and high rates of cellular metabolism. Besides contributing to tumor pathogenesis and promoting tumor progression, tumor acidosis constitutes an important intrinsic and extrinsic mechanism modulating therapy sensitivity and drug resistance. In fact, pharmacological properties of anticancer drugs can be affected not only by tissue structure and organization but also by the distribution of the interstitial tumor pH. The acidic tumor environment is believed to create a chemical barrier that limits the effects and activity of many anticancer drugs. In this review article we will discuss the general protumorigenic effects of acidosis, the role of tumor acidosis in the modulation of therapeutic efficacy and potential strategies to overcome pH-dependent therapy-resistance.

Crosstalk of ROS/RNS and autophagy in silibinin-induced apoptosis of MCF-7 human breast cancer cells in vitro

Reactive oxygen species (ROS) and reactive nitrogen species (RNS) play important roles in regulating cell survival and death. Silibinin is a natural polyphenolic flavonoid isolated from milk thistle with anti-tumor activities, but it was found to induce cytoprotective ROS/RNS in human breast cancer MCF-7 cells. Furthermore, treatment with silibinin down-regulates ERα expression in MCF-7 cells, and inducing both autophagy and apoptosis. In this study we explored the relationship between ER-associated pathways and RNS/ROS in MCF-7 cells. We also investigated the molecular mechanisms underlying the reciprocal regulation between ROS/RNS levels and autophagy in the death signaling pathways in silibinin-treated MCF-7 cells. Silibinin (100-300 μmol/L) dose-dependently increased ROS/RNS generation in MCF-7 cells (with high expression of ERα and low expression of ERβ) and MDA-MB-231 cells (with low expression of ERα and high expression of ERβ). Scavenging ROS/RNS significantly enhanced silibinin-induced death of MCF-7 cells, but not MDA-MB231 cells. Pharmacological activation or blockade of ERα in MCF-7 cells significantly enhanced or decreased, respectively, silibinin-induced ROS/RNS generation, whereas activation or block of ERβ had no effect. In silibinin-treated MCF-7 cells, exposure to the ROS/RNS donators decreased the autophagic levels, whereas inhibition of autophagy with 3-MA significantly increased ROS/RNS levels. We further showed that increases in ROS/RNS generation, ERα activation or autophagy down-regulation had protective roles in silibinin-treated MCF-7 cells. Under a condition of ERα activation, scavenging ROS/RNS or stimulating autophagy enhanced the cytotoxicity of silibinin. These results demonstrate the existence of two conflicting pathways in silibinin-induced death of MCF-7 cells: one involves the down-regulation of ERα and thereby augmenting the pro-apoptotic autophagy downstream, leading to cell death; the other involves the up-regulation of pro-survival ROS/RNS; and that the generation of ROS/RNS and autophagy form a negative feedback loop whose balance is regulated by ERα.

GSH-dependent antioxidant defense contributes to the acclimation of colon cancer cells to acidic microenvironment

Due to increased glycolysis and poor local perfusion, solid tumors are usually immersed in an acidic microenvironment. While extracellular acidosis is cytotoxic, cancer cells eventually become acclimated to it. While previous studies have addressed the acute effect of acidosis on cancer cells, little is known about how cancer cells survive chronic acidosis. In this study we exposed colorectal cancer (CRC) cells (HCT15, HCT116 and LoVo) to acidic pH (pH 6.5) continuously for over three months and obtained CRC cells that become acclimated to acidic pH, designated as CRC-acidosis-acclimated or CRC-AA. We unexpectedly found that while acute exposure to low pH resulted in an increase in the level of intracellular reactive oxygen species (ROS), CRC-AA cells exhibited a significantly reduced level of ROS when compared to ancestor cells. CRC-AA cells were found to maintain a higher level of reduced glutathione, via the upregulation of CD44 and glutathione reductase (GSR), among others, than their ancestor cells. Importantly, CRC-AA cells were more sensitive to agents that deplete GSH. Moreover, downregulation of GSR by RNA interference was more deleterious to CRC-AA cells than to control cells. Together, our results demonstrate a critical role of glutathione-dependent antioxidant defense in acclimation of CRC cells to acidic extracellular pH.

Inflammatory stimuli promote growth and invasion of pancreatic cancer cells through NF-κB pathway dependent repression of PP2Ac

Previous studies have indicated that inflammatory stimulation represses protein phosphatase 2A (PP2A), a well-known tumor suppressor. However, whether PP2A repression participates in pancreatic cancer progression has not been verified. We used lipopolysaccharide (LPS) and macrophage-conditioned medium (MCM) to establish in vitro inflammation models, and investigated whether inflammatory stimuli affect pancreatic cancer cell growth and invasion PP2A catalytic subunit (PP2Ac)-dependently. Via nude mouse models of orthotopic tumor xenografts and dibutyltin dichloride (DBTC)-induced chronic pancreatitis, we evaluated the effect of an inflammatory microenvironment on PP2Ac expression in vivo. We cloned the PP2Acα and PP2Acβ isoform promoters to investigate the PP2Ac transcriptional regulation mechanisms. MCM accelerated pancreatic cancer cell growth; MCM and LPS promoted cell invasion. DBTC promoted xenograft growth and metastasis, induced tumor-associated macrophage infiltration, promoted angiogenesis, activated the nuclear factor-κB (NF-κB) pathway, and repressed PP2Ac expression. In vitro, LPS and MCM downregulated PP2Ac mRNA and protein. PP2Acα overexpression attenuated JNK, ERK, PKC, and IKK phosphorylation, and impaired LPS/MCM-stimulated cell invasion and MCM-promoted cell growth. LPS and MCM activated the NF-κB pathway in vitro. LPS and MCM induced IKK and IκB phosphorylation, leading to p65/RelA nuclear translocation and transcriptional activation. Overexpression of the dominant negative forms of IKKα attenuated LPS and MCM downregulation of PP2Ac, suggesting inflammatory stimuli repress PP2Ac expression NF-κB pathway-dependently. Luciferase reporter gene assay verified that LPS and MCM downregulated PP2Ac transcription through an NF-κB-dependent pathway. Our study presents a new mechanism in inflammation-driven cancer progression through NF-κB pathway-dependent PP2Ac repression.

ERK-mediated NF-κB activation through ASIC1 in response to acidosis

Acidic microenvironment is a common feature of solid tumors. We have previously shown that neuron specific acid-sensing ion channel 1 (ASIC1) is expressed in breast cancer, and it is responsible for acidosis-induced cellular signaling through AKT, leading to nuclear factor-κB (NF-κB) activation, and cell invasion and metastasis. However, AKT is frequently activated in cancer. Thus, a key question is whether ASIC1-mediated cell signaling still takes place in the cancer cells carrying constitutively active AKT. In the present study, we show that among four prostate cancer cell lines tested, 22Rv1 cells express the highest level of phosphorylated AKT that is not impacted by acidosis. However, acidosis can still induce NF-κB activation during which extracellular signal-regulated kinase (ERK) serves as an alternative pathway for ASIC-mediated cell signaling. Inhibition of ERK by chemical inhibitors or small interfering RNAs suppresses the acidosis-induced NF-κB activity through regulation of the inhibitory subunit IκBα phosphorylation. Furthermore, suppression of ASIC1-mediated generation of reactive oxygen species (ROS) by ROS scavengers, such as glutathione or N-acetyl-cysteine causes a decrease in ERK phosphorylation and degradation of IκBα. Finally, ASIC1 is upregulated in a subset of prostate cancer cases and ASIC1 knockout by CRISPR/Cas9 significantly suppresses cell invasion, and castration resistance both in vitro and in vivo. Together, these results support the significance of ASIC1-ROS-ERK-IκBα-NF-κB axis in prostate tumorigenesis, especially in the constitutively active AKT background.

Acidic tumor microenvironment abrogates the efficacy of mTORC1 inhibitors

Background

Blocking the mechanistic target of rapamycin complex-1 (mTORC1) with chemical inhibitors such as rapamycin has shown limited clinical efficacy in cancer. The tumor microenvironment is characterized by an acidic pH which interferes with cancer therapies. The consequences of acidity on the anti-cancer efficacy of mTORC1 inhibitors have not been characterized and are thus the focus of our study.

Methods

Cancer cell lines were treated with rapamycin in acidic or physiological conditions and cell proliferation was investigated. The effect of acidity on mTORC1 activity was determined by Western blot. The anticancer efficacy of rapamycin in combination with sodium bicarbonate to increase the intratumoral pH was tested in two different mouse models and compared to rapamycin treatment alone. Histological analysis was performed on tumor samples to evaluate proliferation, apoptosis and necrosis.

Results

Exposing cancer cells to acidic pH in vitro significantly reduced the anti-proliferative effect of rapamycin. At the molecular level, acidity significantly decreased mTORC1 activity, suggesting that cancer cell proliferation is independent of mTORC1 in acidic conditions. In contrast, the activation of mitogen-activated protein kinase (MAPK) or AKT were not affected by acidity, and blocking MAPK or AKT with a chemical inhibitor maintained an anti-proliferative effect at low pH. In tumor mouse models, the use of sodium bicarbonate increased mTORC1 activity in cancer cells and potentiated the anti-cancer efficacy of rapamycin. Combining sodium bicarbonate with rapamycin resulted in increased tumor necrosis, increased cancer cell apoptosis and decreased cancer cell proliferation as compared to single treatment.

Conclusions

Taken together, these results emphasize the inefficacy of mTORC1 inhibitors in acidic conditions. They further highlight the potential of combining sodium bicarbonate with mTORC1 inhibitors to improve their anti-tumoral efficacy.

Transcriptome- and proteome-oriented identification of dysregulated eIF4G, STAT3, and Hippo pathways altered by PIK3CA H1047R in HER2/ER-positive breast cancer

PURPOSE

Phosphatidylinositol 3-kinase (PI3K)/AKT pathway aberrations are common in human breast cancer. Furthermore, PIK3CA mutations are commonly associated with resistance to anti-epidermal growth factor receptor 2 (HER2) or anti-estrogen receptor (ER) agents in HER2 or ER positive (HER2⁺/ER⁺) breast cancer. Hence, deciphering the underlying mechanisms of PIK3CA mutations in HER2⁺/ER⁺ breast cancer would provide novel insights into elucidating resistance to anti-HER2/ER therapies.

METHODS

In this study, we systematically investigated the biological consequences of PIK3CA ^H1047R in HER2⁺/ER⁺ breast cancer by uniquely incorporating mRNA transcriptomic data from The Cancer Genome Atlas and proteomic data from reverse-phase protein arrays.

RESULTS

Our integrative bioinformatics analyses revealed that several important pathways such as STAT3 and VEGF/hypoxia were selectively altered by PIK3CA ^H1047R in HER2⁺/ER⁺ breast cancer. Protein differential expression analysis indicated that an elevated eIF4G might promote tumor angiogenesis and growth via regulation of the hypoxia-activated switch in HER2⁺ PIK3CA ^H1047R breast cancer. We observed hypo-phosphorylation of EGFR in HER2⁺ PIK3CA ^H1047R breast cancer versus HER2⁺PIK3CA^wild-type (PIK3CA ^WT). In addition, ER and PIK3CA ^H1047R might cooperate to activate STAT3, MAPK, AKT, and Hippo pathways in ER⁺ PIK3CA ^H1047R breast cancer. A higher YAP_pS127 level was observed in ER⁺ PIK3CA ^H1047R patients than that in an ER⁺ PIK3CA ^WT subgroup. By examining breast cancer cell lines having both microarray gene expression and drug treatment data from the Genomics of Drug Sensitivity in Cancer and the Stand Up to Cancer datasets, we found that the elevated YAP1 mRNA expression was associated with the resistance of BCL-2 family inhibitors, but with the sensitivity to MEK/MAPK inhibitors in breast cancer cells.

CONCLUSIONS

In summary, these findings shed light on the functional consequences of PIK3CA ^H1047R-driven breast tumorigenesis and resistance to the existing therapeutic agents in HER2⁺/ER⁺ breast cancer.

Regulation of breast tumorigenesis through acid sensors

The low extracellular pH in the microenvironment has been shown to promote tumor growth and metastasis; however, the underlying mechanism is poorly understood. Particularly, little is known how the tumor cell senses the acidic signal to activate the acidosis-mediated signaling. In this study, we show that breast cancer cells express acid-sensing ion channel 1 (ASIC1), a proton-gated cation channel primarily expressed in the nervous system. RNA interference, knockout and rescue experiments demonstrate a critical role for ASIC1 in acidosis-induced reactive oxidative species and NF-κB activation, two key events for tumorigenesis. Mechanistically, ASIC1 is required for acidosis-mediated signaling through calcium influx. We show that as a cytoplasmic membrane protein, ASIC1 is also associated with mitochondria, suggesting that ASIC1 may regulate mitochondrial calcium influx. Importantly, interrogation of the Cancer Genome Atlas breast invasive carcinoma data set indicates that alterations of ASIC1 alone or combined with other 4 ASIC genes are significantly correlated with poor patient survival. Furthermore, ASIC1 inhibitors cause a significant reduction of tumor growth and tumor load. Together, these results suggest that ASIC1 contributes to breast cancer pathogenesis in response to acidic tumor microenvironments, and ASIC1 may serve as a prognostic marker and a therapeutic target for breast cancer.

Nuclear HDAC6 inhibits invasion by suppressing NF-κB/MMP2 and is inversely correlated with metastasis of non-small cell lung cancer

Histone deacetylase 6 (HDAC6) is a unique member of the histone deacetylase family. Although HDAC6 is mainly localized in the cytoplasm, it can regulate the activities of the transcription factors in the nucleus. However, a correlation of intracellular distribution of HDAC6 with tumor progression is lacking. In this study, we found that a low frequency of nuclear HDAC6-positive cells in tumors was associated with distant metastasis and a worse overall survival in 134 patients with non-small cell lung cancer (NSCLC). Ectopic expression of wild-type HDAC6 promoted migration and invasion of A549 and H661 cells. However, the enforced expression of nuclear export signal-deleted HDAC6 inhibited the invasion but not the migration of both cell lines. The inhibitory effect of nuclear HDAC6 on invasion was mediated by the deacetylation of the p65 subunit of nuclear factor-κB, which decreased its DNA-binding activity to the MMP2 promoter, leading to the downregulation of MMP2 expression. Our findings indicated that the loss of nuclear HDAC6 may be a potential biomarker for predicting metastasis in patients with NSCLC.

Roles of PTEN with DNA Repair in Parkinson's Disease

Oxidative stress is considered to play key roles in aging and pathogenesis of many neurodegenerative diseases such as Parkinson’s disease, which could bring DNA damage by cells. The DNA damage may lead to the cell apoptosis, which could contribute to the degeneration of neuronal tissues. Recent evidence suggests that PTEN (phosphatase and tensin homolog on chromosome 10) may be involved in the pathophysiology of the neurodegenerative disorders. Since PTEN expression appears to be one dominant determinant of the neuronal cell death, PTEN should be a potential molecular target of novel therapeutic strategies against Parkinson’s disease. In addition, defects in DNA damage response and DNA repair are often associated with modulation of hormone signaling pathways. Especially, many observations imply a role for estrogen in a regulation of the DNA repair action. In the present review, we have attempted to summarize the function of DNA repair molecules at a viewpoint of the PTEN signaling pathway and the hormone related functional modulation of cells, providing a broad interpretation on the molecular mechanisms for treatment of Parkinson’s disease. Particular attention will be paid to the mechanisms proposed to explain the health effects of food ingredients against Parkinson’s disease related to reduce oxidative stress for an efficient therapeutic intervention.

Extracellular acidity strengthens mesenchymal stem cells to promote melanoma progression

Mesenchymal stem cells (MSC) participate to tumor stroma development and several evidence suggests that they play a role in facilitating cancer progression. Because melanoma often shows extracellular pH low enough to influence host cell as tumor cell behavior, the aim of this study is to elucidate whether acidity affects cross talk between MSC and melanoma cells to disclose new liaisons promoting melanoma progression, and to offer new therapeutic opportunities. We found that MSC grown in a low pH medium (LpH-MSC) stimulate melanoma xenografts more than MSC grown in a standard pH medium. LpH-MSC express a higher level of TGFβ that is instrumental of epithelial-to-mesenchymal transition (EMT)-like phenotype induction in melanoma cells. LpH-MSC profile also shows a switching to an oxidative phosphorylation metabolism that was accompanied by a forced glycolytic pathway of melanoma cells grown in LpH-MSC-conditioned medium. Metformin, an inhibitor of mitochondrial respiratory chain was able to reconvert oxidative metabolism and abrogate TGFβ expression in LpH-MSC. In addition, esomeprazole, a proton pump inhibitor activated in acidosis, blocked TGFβ expression in LpH-MSC through the downregulation of IkB. Both agents, metformin and esomeprazole, inhibited EMT profile in melanoma cells grown in LpH-MSC medium, and reduced glycolytic markers. Thus, acidosis of tumor microenvironment potentiates the pro-tumoral activity of MSC and orchestrates for a new potential symbiosis, which could be target to limit melanoma progression.