Arrest-defective-1 Protein, an Acetyltransferase, Does Not Alter Stability of Hypoxia-inducible Factor (HIF)-1α and Is Not Induced by Hypoxia or HIF

The role of N-acetyltransferases in cancers

Cancer is a major global health problem that disrupts the balance of normal cellular growth and behavior. Mounting evidence has shown that epigenetic modification, specifically N-terminal acetylation, play a crucial role in the regulation of cell growth and function. Acetylation is a co- or post-translational modification to regulate important cellular progresses such as cell proliferation, cell cycle progress, and energy metabolism. Recently, N-acetyltransferases (NATs), enzymes responsible for acetylation, regulate signal transduction pathway in various cancers including hepatocellular carcinoma, breast cancer, lung cancer, colorectal cancer and prostate cancer. In this review, we clarify the regulatory role of NATs in cancer progression, such as cell proliferation, metastasis, cell apoptosis, autophagy, cell cycle arrest and energy metabolism. Furthermore, the mechanism of NATs on cancer remains to be further studied, and few drugs have been developed. This provides us with a new idea that targeting acetylation, especially NAT-mediated acetylation, may be an attractive way for inhibiting cancer progression.

Targeting of the ELR+CXCL/CXCR1/2 Pathway Is a Relevant Strategy for the Treatment of Paediatric Medulloblastomas

Medulloblastoma (MB) is the most common and aggressive paediatric brain tumour. Although the cure rate can be as high as 70%, current treatments (surgery, radio- and chemotherapy) excessively affect the patients' quality of life. Relapses cannot be controlled by conventional or targeted treatments and are usually fatal. The strong heterogeneity of the disease (four subgroups and several subtypes) is related to innate or acquired resistance to reference treatments. Therefore, more efficient and less-toxic therapies are needed. Here, we demonstrated the efficacy of a novel inhibitor (C29) of CXCR1/2 receptors for ELR+CXCL cytokines for the treatment of childhood MB. The correlation between ELR+CXCL/CXCR1/2 expression and patient survival was determined using the R2: Genomics Analysis and Visualization platform. In vitro efficacy of C29 was evaluated by its ability to inhibit proliferation, migration, invasion, and pseudo-vessel formation of MB cell lines sensitive or resistant to radiotherapy. The growth of experimental MB obtained by MB spheroids on organotypic mouse cerebellar slices was also assayed. ELR+CXCL/CXCR1/2 levels correlated with shorter survival. C29 inhibited proliferation, clone formation, CXCL8/CXCR1/2-dependent migration, invasion, and pseudo-vessel formation by sensitive and radioresistant MB cells. C29 reduced experimental growth of MB in the ex vivo organotypic mouse model and crossed the blood-brain barrier. Targeting CXCR1/2 represents a promising therapeutic strategy for the treatment of paediatric MB in first-line treatment or after relapse following conventional therapy.

Precise and fast control of the dissolved oxygen level for tumor-on-chip

In vitro cell cultures are most often performed in unphysiological hyperoxia since the oxygen partial pressure of conventional incubators is set at 141 mmHg (18.6%, close to ambient air oxygen 20.1%). This value is higher than human tissue oxygen levels, as the in vivo oxygen partial pressures range from 104 mmHg (lung alveoli) to 8 mmHg (skin epidermis). Importantly, under pathological conditions such as cancer, cells can experience oxygen pressure lower than the healthy tissue. Although hypoxic incubators can regulate gas oxygen, they do not take into account the dissolved oxygen concentration in the cell culture medium. In the context of organ on chip and micro-physiological system development, we present here a new system, called Oxalis (OXygen ALImentation System) that allows fine control of the dissolved oxygen level in the cell culture medium. Oxalis regulates simultaneously the gas composition and the inlet reservoir pressure by modulating the pneumatic valve opening. This dual regulation allows both the pressure driven liquid flowrate and the level of oxygen dissolved in the chip to be controlled independently. Oxalis offers unprecedented features such as an oxygen equilibration time lower than 3 minutes and an accuracy of 3 mmHg. These performances can be reached for chip perfusion flow as low as 1 μL min^-1. This low flow rate allows the shear stress experienced by the cells in the chip to be accurately controlled. In addition, the system enables modulation of the pH in the cell culture medium through the modulation of CO₂. The fine control and monitoring of both O₂ and pH pave the way for new precise investigations on physiological and pathological biological processes. Using Oxalis in the context of tumor-on-chip, we demonstrate the capacity of the system to recapitulate hypoxia-induced gene expression, offering an innovative strategy for future studies on the role of hypoxia in malignant progression and drug resistance.

'Warburg effect' controls tumor growth, bacterial, viral infections and immunity - Genetic deconstruction and therapeutic perspectives

The evolutionary pressure for life transitioning from extended periods of hypoxia to an increasingly oxygenated atmosphere initiated drastic selections for a variety of biochemical pathways supporting the robust life currently present on the planet. First, we discuss how fermentative glycolysis, a primitive metabolic pathway present at the emergence of life, is instrumental for the rapid growth of cancer, regenerating tissues, immune cells but also bacteria and viruses during infections. The 'Warburg effect', activated via Myc and HIF-1 in response to growth factors and hypoxia, is an essential metabolic and energetic pathway which satisfies nutritional and energetic demands required for rapid genome replication. Second, we present the key role of lactic acid, the end-product of fermentative glycolysis able to move across cell membranes in both directions via monocarboxylate transporting proteins (i.e. MCT1/4) contributing to cell-pH homeostasis but also to the complex immune response via acidosis of the tumour microenvironment. Importantly lactate is recycled in multiple organs as a major metabolic precursor of gluconeogenesis and energy source protecting cells and animals from harsh nutritional or oxygen restrictions. Third, we revisit the Warburg effect via CRISPR-Cas9 disruption of glucose-6-phosphate isomerase (GPI-KO) or lactate dehydrogenases (LDHA/B-DKO) in two aggressive tumours (melanoma B16-F10, human adenocarcinoma LS174T). Full suppression of lactic acid production reduces but does not suppress tumour growth due to reactivation of OXPHOS. In contrast, disruption of the lactic acid transporters MCT1/4 suppressed glycolysis, mTORC1, and tumour growth as a result of intracellular acidosis. Finally, we briefly discuss the current clinical developments of an MCT1 specific drug AZ3965, and the recent progress for a specific in vivo MCT4 inhibitor, two drugs of very high potential for future cancer clinical applications.

Genetic Disruption of the γ-Glutamylcysteine Ligase in PDAC Cells Induces Ferroptosis-Independent Cell Death In Vitro without Affecting In Vivo Tumor Growth

Simple Summary

The newly described form of iron-dependent cell death, called ferroptosis, has emerged as a powerful strategy for eradicating cancer cells. This is of particular importance for pancreatic ductal adenocarcinoma (PDAC), which has been shown to be one of the most aggressive tumors, with a five-year overall survival of less than 8%. The aim of the present study is to identify the most potent and selective target for the induction of ferroptosis in PDAC cells. The results presented here are of great importance not only for the development of novel and more effective anti-cancer therapeutics, but also anticipate potential resistant mechanisms that cancer cells might deploy. This way, ferroptosis-based therapeutics may be a step ahead of highly adaptable cancer cells.

Abstract

The conceptualization of a novel type of cell death, called ferroptosis, opens new avenues for the development of more efficient anti-cancer therapeutics. In this context, a full understanding of the ferroptotic pathways, the players involved, their precise role, and dispensability is prerequisite. Here, we focused on the importance of glutathione (GSH) for ferroptosis prevention in pancreatic ductal adenocarcinoma (PDAC) cells. We genetically deleted a unique, rate-limiting enzyme for GSH biosynthesis, γ-glutamylcysteine ligase (GCL), which plays a key role in tumor cell proliferation and survival. Surprisingly, although glutathione peroxidase 4 (GPx4) has been described as a guardian of ferroptosis, depletion of its substrate (GSH) led preferentially to apoptotic cell death, while classical ferroptotic markers (lipid hydroperoxides) have not been observed. Furthermore, the sensitivity of PDAC cells to the pharmacological/genetic inhibition of GPx4 revealed GSH dispensability in this context. To the best of our knowledge, this is the first time that the complete dissection of the xCT-GSH-GPx4 axis in PDAC cells has been investigated in great detail. Collectively, our results revealed the necessary role of GSH in the overall redox homeostasis of PDAC cells, as well as the dispensability of this redox-active molecule for a specific, antioxidant branch dedicated to ferroptosis prevention.

Oxygen-sensing mechanisms in development and tissue repair

Under normoxia, hypoxia inducible factor (HIF) α subunits are hydroxylated by PHDs (prolyl hydroxylase domain proteins) and subsequently undergo polyubiquitylation and degradation. Normal embryogenesis occurs under hypoxia, which suppresses PHD activities and allows HIFα to stabilize and regulate development. In this Primer, we explain molecular mechanisms of the oxygen-sensing pathway, summarize HIF-regulated downstream events, discuss loss-of-function phenotypes primarily in mouse development, and highlight clinical relevance to angiogenesis and tissue repair.

Hydroxylation of the Acetyltransferase NAA10 Trp38 Is Not an Enzyme-Switch in Human Cells

NAA10 is a major N-terminal acetyltransferase (NAT) that catalyzes the cotranslational N-terminal (Nt-) acetylation of 40% of the human proteome. Several reports of lysine acetyltransferase (KAT) activity by NAA10 exist, but others have not been able to find any NAA10-derived KAT activity, the latter of which is supported by structural studies. The KAT activity of NAA10 towards hypoxia-inducible factor 1α (HIF-1α) was recently found to depend on the hydroxylation at Trp38 of NAA10 by factor inhibiting HIF-1α (FIH). In contrast, we could not detect hydroxylation of Trp38 of NAA10 in several human cell lines and found no evidence that NAA10 interacts with or is regulated by FIH. Our data suggest that NAA10 Trp38 hydroxylation is not a switch in human cells and that it alters its catalytic activity from a NAT to a KAT.

A Cystine-Cysteine Intercellular Shuttle Prevents Ferroptosis in xCTKO Pancreatic Ductal Adenocarcinoma Cells

Simple Summary

The xCT transporter of oxidized form of cysteine has been recognized as fundamental for cellular amino acid and redox homeostasis. Increasing number of data suggests that xCT inhibition-induced ferroptosis has great potential for development of novel anti-cancer therapeutics for pancreatic cancer patients. The aim of this study was to investigate potential resistance mechanisms that cancer cells with genetically disrupted xCT (xCT^KO) may exploit in order to develop resistance to ferroptosis. Our data clearly showed that shuttle of reduced cysteine between cancer xCT^KO and neighboring cells provide protection of the former. Importantly, this shuttle seems to be fueled by the import and reduction of oxidized cysteine by xCT-proficient feeder layer. In summary, two important findings are: (1) supply of the reduced cysteine has to be taken in consideration when xCT-based ferroptosis inducers are used, and (2) systemic inhibition of xCT could be potential approach in overcoming this resistant mechanism.

Abstract

In our previous study, we showed that a cystine transporter (xCT) plays a pivotal role in ferroptosis of pancreatic ductal adenocarcinoma (PDAC) cells in vitro. However, in vivo xCT^KO cells grew normally indicating that a mechanism exists to drastically suppress the ferroptotic phenotype. We hypothesized that plasma and neighboring cells within the tumor mass provide a source of cysteine to confer full ferroptosis resistance to xCT^KO PDAC cells. To evaluate this hypothesis, we (co-) cultured xCT^KO PDAC cells with different xCT-proficient cells or with their conditioned media. Our data unequivocally showed that the presence of a cysteine/cystine shuttle between neighboring cells is the mechanism that provides redox and nutrient balance, and thus ferroptotic resistance in xCT^KO cells. Interestingly, although a glutathione shuttle between cells represents a good alternative hypothesis as a “rescue-mechanism”, our data clearly demonstrated that the xCT^KO phenotype is suppressed even with conditioned media from cells lacking the glutathione biosynthesis enzyme. Furthermore, we demonstrated that prevention of lipid hydroperoxide accumulation in vivo is mediated by import of cysteine into xCT^KO cells via several genetically and pharmacologically identified transporters (ASCT1, ASCT2, LAT1, SNATs). Collectively, these data highlight the importance of the tumor environment in the ferroptosis sensitivity of cancer cells.

Evidences of a Direct Relationship between Cellular Fuel Supply and Ciliogenesis Regulated by Hypoxic VDAC1-ΔC

Metabolic flexibility is the ability of a cell to adapt its metabolism to changes in its surrounding environment. Such adaptability, combined with apoptosis resistance provides cancer cells with a survival advantage. Mitochondrial voltage-dependent anion channel 1 (VDAC1) has been defined as a metabolic checkpoint at the crossroad of these two processes. Here, we show that the hypoxia-induced cleaved form of VDAC1 (VDAC1-ΔC) is implicated in both the up-regulation of glycolysis and the mitochondrial respiration. We demonstrate that VDAC1-ΔC, due to the loss of the putative phosphorylation site at serine 215, concomitantly with the loss of interaction with tubulin and microtubules, reprograms the cell to utilize more metabolites, favoring cell growth in hypoxic microenvironment. We further found that VDAC1-ΔC represses ciliogenesis and thus participates in ciliopathy, a group of genetic disorders involving dysfunctional primary cilium. Cancer, although not representing a ciliopathy, is tightly linked to cilia. Moreover, we highlight, for the first time, a direct relationship between the cilium and cancer cell metabolism. Our study provides the first new comprehensive molecular-level model centered on VDAC1-ΔC integrating metabolic flexibility, ciliogenesis, and enhanced survival in a hypoxic microenvironment.

VEGFC negatively regulates the growth and aggressiveness of medulloblastoma cells

Medulloblastoma (MB), the most common brain pediatric tumor, is a pathology composed of four molecular subgroups. Despite a multimodal treatment, 30% of the patients eventually relapse, with the fatal appearance of metastases within 5 years. The major actors of metastatic dissemination are the lymphatic vessel growth factor, VEGFC, and its receptors/co-receptors. Here, we show that VEGFC is inversely correlated to cell aggressiveness. Indeed, VEGFC decreases MB cell proliferation and migration, and their ability to form pseudo-vessel in vitro. Irradiation resistant-cells, which present high levels of VEGFC, lose the ability to migrate and to form vessel-like structures. Thus, irradiation reduces MB cell aggressiveness via a VEGFC-dependent process. Cells intrinsically or ectopically overexpressing VEGFC and irradiation-resistant cells form smaller experimental tumors in nude mice. Opposite to the common dogma, our results give strong arguments in favor of VEGFC as a negative regulator of MB growth.

Manon Penco-Campillo, Yannick Comoglio et al. show that VEGFC decreases the proliferation and migration of medulloblastoma cells, as well as their ability to form pseudo vessels. Cells expressing high levels of VEGFC also form smaller tumors when subcutaneously injected into the flank of nude mice, thus highlighting a negative regulatory role for VEGFC on tumor growth.

Genetic Ablation of the Cystine Transporter xCT in PDAC Cells Inhibits mTORC1, Growth, Survival, and Tumor Formation via Nutrient and Oxidative Stresses

Although chemoresistance remains a primary challenge in the treatment of pancreatic ductal adenocarcinoma (PDAC), exploiting oxidative stress might offer novel therapeutic clues. Here we explored the potential of targeting cystine/glutamate exchanger (SLC7A11/xCT), which contributes to the maintenance of intracellular glutathione (GSH). Genomic disruption of xCT via CRISPR-Cas9 was achieved in two PDAC cell lines, MiaPaCa-2 and Capan-2, and xCT-KO clones were cultivated in the presence of N-acetylcysteine. Although several cystine/cysteine transporters have been identified, our findings demonstrate that, in vitro, xCT plays the major role in intracellular cysteine balance and GSH biosynthesis. As a consequence, both xCT-KO cell lines exhibited amino acid stress with activation of GCN2 and subsequent induction of ATF4, inhibition of mTORC1, proliferation arrest, and cell death. Tumor xenograft growth was delayed but not suppressed in xCT-KO cells, which indicated both the key role of xCT and also the presence of additional mechanisms for cysteine homeostasis in vivo. Moreover, rapid depletion of intracellular GSH in xCT-KO cells led to accumulation of lipid peroxides and cell swelling. These two hallmarks of ferroptotic cell death were prevented by vitamin E or iron chelation. Finally, in vitro pharmacologic inhibition of xCT by low concentrations of erastin phenocopied xCT-KO and potentiated the cytotoxic effects of both gemcitabine and cisplatin in PDAC cell lines. In conclusion, our findings strongly support that inhibition of xCT, by its dual induction of nutritional and oxidative cellular stresses, has great potential as an anticancer strategy. SIGNIFICANCE: The cystine/glutamate exchanger xCT is essential for amino acid and redox homeostasis and its inhibition has potential for anticancer therapy by inducing ferroptosis.

Co-translational, Post-translational, and Non-catalytic Roles of N-Terminal Acetyltransferases

Recent studies of N-terminal acetylation have identified new N-terminal acetyltransferases (NATs) and expanded the known functions of these enzymes beyond their roles as ribosome-associated co-translational modifiers. For instance, the identification of Golgi- and chloroplast-associated NATs shows that acetylation of N termini also happens post-translationally. In addition, we now appreciate that some NATs are highly specific; for example, a dedicated NAT responsible for post-translational N-terminal acetylation of actin was recently revealed. Other studies have extended NAT function beyond Nt acetylation, including functions as lysine acetyltransferases (KATs) and non-catalytic roles. Finally, emerging studies emphasize the physiological relevance of N-terminal acetylation, including roles in calorie-restriction-induced longevity and pathological α-synuclein aggregation in Parkinson's disease. Combined, the NATs rise as multifunctional proteins, and N-terminal acetylation is gaining recognition as a major cellular regulator.

VEGFC acts as a double-edged sword in renal cell carcinoma aggressiveness

Hypoxic zones are common features of metastatic tumors. Due to inactivation of the von Hippel-Lindau gene (VHL), renal cell carcinomas (RCC) show constitutive stabilization of the alpha subunit of the hypoxia-inducible factor (HIF). Thus, RCC represents a model of chronic hypoxia. Development of the lymphatic network is dependent on vascular endothelial growth factor C (VEGFC) and lies at the front line of metastatic spreading. Here, we addressed the role of VEGFC in RCC aggressiveness and the regulation of its expression in hypoxia. Methods: Transcriptional and post transcriptional regulation of VEGFC expression was evaluated by qPCR and with reporter genes. The involvement of HIF was evaluated using a siRNA approach. Experimental RCC were performed with immuno-competent/deficient mice using human and mouse cells knocked-out for the VEGFC gene by a CRISPR/Cas9 method. The VEGFC axis was analyzed with an online available data base (TCGA) and using an independent cohort of patients. Results: Hypoxia induced VEGFC protein expression but down-regulated VEGFC gene transcription and mRNA stability. Increased proliferation, migration, over-activation of the AKT signaling pathway and enhanced expression of mesenchymal markers characterized VEGFC-/- cells. VEGFC-/- cells did not form tumors in immuno-deficient mice but developed aggressive tumors in immuno-competent mice. These tumors showed down-regulation of markers of activated lymphocytes and M1 macrophages, and up-regulation of M2 macrophages markers and programmed death ligand 1 (PDL1). Over-expression of lymphangiogenic genes including VEGFC was linked to increased disease-free and overall survival in patients with non-metastatic tumors, whereas its over-expression correlated with decreased progression-free and overall survival of metastatic patients. Conclusion: Our study revisited the admitted dogma linking VEGFC to tumor aggressiveness. We conclude that targeting VEGFC for therapy must be considered with caution.

FIH permits NAA10 to catalyze the oxygen-dependent lysyl-acetylation of HIF-1α

The N-terminal acetyltransferase A (NatA) complex, which is composed of NAA10 and NAA15, catalyzes N-terminal acetylation of many proteins in a co-translational manner. Structurally, the catalytic subunit NAA10 was believed to have no activity toward an internal lysine residue because the gate of its catalytic pocket is too narrow. However, several studies have demonstrated that the monomeric NAA10 can acetylate the internal lysine residues of several substrates including hypoxia-inducible factor 1α (HIF-1α). How NAA10 acetylates lysine residues has been an unsolved question. We here found that human FIH (factor inhibiting HIF) hydroxylates human NAA10 at W38 oxygen-dependently and this permits NAA10 to express the lysyl-acetyltransferase activity. The hydroxylated W38 forms a new hydrogen-bond with A67 and widens the gate at the catalytic pocket, which allows the entrance of a lysine residue to the site. Since the FIH-dependent hydroxylation of NAA10 occurs oxygen-dependently, NAA10 acetylates HIF-1α under normoxia but does not under hypoxia. Consequently, the acetylation promotes the pVHL binding to HIF-1α, and in turn HIF-1α is destructed via the ubiquitin-proteasome system. This study provides a novel oxygen-sensing process that determines the substrate specificity of NAA10 depending on an ambient oxygen tension.

ARD1/NAA10 in hepatocellular carcinoma: pathways and clinical implications

Hepatocellular carcinoma (HCC), a representative example of a malignancy with a poor prognosis, is characterized by high mortality because it is typically in an advanced stage at diagnosis and leaves very little hepatic functional reserve. Despite advances in medical and surgical techniques, there is no omnipotent tool that can diagnose HCC early and then cure it medically or surgically. Several recent studies have shown that a variety of pathways are involved in the development, growth, and even metastasis of HCC. Among a variety of cytokines or molecules, some investigators have suggested that arrest-defective 1 (ARD1), an acetyltransferase, plays a key role in the development of malignancies. Although ARD1 is thought to be centrally involved in the cell cycle, cell migration, apoptosis, differentiation, and proliferation, the role of ARD1 and its potential mechanistic involvement in HCC remain unclear. Here, we review the present literature on ARD1. First, we provide an overview of the essential structure, functions, and molecular mechanisms or pathways of ARD1 in HCC. Next, we discuss potential clinical implications and perspectives. We hope that, by providing new insights into ARD1, this review will help to guide the next steps in the development of markers for the early detection and prognosis of HCC.

A protein that is highly expressed in cancer with extensive blood vessel development may provide a potential biomarker for early-stage liver cancer. Liver cancer is often not diagnosed until it is advanced and is also hard to be cured despite of advances in treatment, meaning patients often die from the disease. No tools for early detection or prognosis prediction exist, and scientists are keen to find useful biomarker molecules. Young-Hwa Chung at the University of Ulsan College of Medicine, Asan Medical Center, Seoul, and co-workers in South Korea reviewed recent research into one possible cancer-related protein, arrest-defective 1 (ARD1), known to be highly expressed in certain cancers and possibly associated with poor prognosis. While ARD1 appears to regulate pathways critical to cancer progression and promote cancer cell invasiveness, further in-depth investigations are needed to clarify its specific role in liver cancer.

Genetic disruption of the pHi-regulating proteins Na+/H+ exchanger 1 (SLC9A1) and carbonic anhydrase 9 severely reduces growth of colon cancer cells

Hypoxia and extracellular acidosis are pathophysiological hallmarks of aggressive solid tumors. Regulation of intracellular pH (pH_i) is essential for the maintenance of tumor cell metabolism and proliferation in this microenvironment and key proteins involved in pH_i regulation are of interest for therapeutic development. Carbonic anhydrase 9 (CA9) is one of the most robustly regulated proteins by the hypoxia inducible factor (HIF) and contributes to pH_i regulation. Here, we have investigated for the first time, the role of CA9 via complete genomic knockout (ko) and compared its impact on tumor cell physiology with the essential pH_i regulator Na⁺/H⁺ exchanger 1 (NHE1). Initially, we established NHE1-ko LS174 cells with inducible CA9 knockdown. While increased sensitivity to acidosis for cell survival in 2-dimensions was not observed, clonogenic proliferation and 3-dimensional spheroid growth in particular were greatly reduced. To avoid potential confounding variables with use of tetracycline-inducible CA9 knockdown, we established CA9-ko and NHE1/CA9-dko cells. NHE1-ko abolished recovery from NH₄Cl pre-pulse cellular acid loading while both NHE1 and CA9 knockout reduced resting pH_i. NHE1-ko significantly reduced tumor cell proliferation both in normoxia and hypoxia while CA9-ko dramatically reduced growth in hypoxic conditions. Tumor xenografts revealed substantial reductions in tumor growth for both NHE1-ko and CA9-ko. A notable induction of CA12 occurred in NHE1/CA9-dko tumors indicating a potential means to compensate for loss of pH regulating proteins to maintain growth. Overall, these genomic knockout results strengthen the pursuit of targeting tumor cell pH regulation as an effective anti-cancer strategy.

The glutamine transporter ASCT2 (SLC1A5) promotes tumor growth independently of the amino acid transporter LAT1 (SLC7A5)

The transporters for glutamine and essential amino acids, ASCT2 (solute carrier family 1 member 5, SLC1A5) and LAT1 (solute carrier family 7 member 5, SLC7A5), respectively, are overexpressed in aggressive cancers and have been identified as cancer-promoting targets. Moreover, previous work has suggested that glutamine influx via ASCT2 triggers essential amino acids entry via the LAT1 exchanger, thus activating mechanistic target of rapamycin complex 1 (mTORC1) and stimulating growth. Here, to further investigate whether these two transporters are functionally coupled, we compared the respective knockout (KO) of either LAT1 or ASCT2 in colon (LS174T) and lung (A549) adenocarcinoma cell lines. Although ASCT2^KO significantly reduced glutamine import (>60% reduction), no impact on leucine uptake was observed in both cell lines. Although an in vitro growth-reduction phenotype was observed in A549-ASCT2^KO cells only, we found that genetic disruption of ASCT2 strongly decreased tumor growth in both cell lines. However, in sharp contrast to LAT1^KO cells, ASCT2^KO cells displayed no amino acid (AA) stress response (GCN2/EIF2a/ATF4) or altered mTORC1 activity (S6K1/S6). We therefore conclude that ASCT2^KO reduces tumor growth by limiting AA import, but that this effect is independent of LAT1 activity. These data were further supported by in vitro cell proliferation experiments performed in the absence of glutamine. Together these results confirm and extend ASCT2's pro-tumoral role and indicate that the proposed functional coupling model of ASCT2 and LAT1 is not universal across different cancer types.

HDAC6 regulates sensitivity to cell death in response to stress and post-stress recovery

Histone deacetylase 6 (HDAC6) plays an important role in stress responses such as misfolded protein-induced aggresomes, autophagy, and stress granules. However, precisely how HDAC6 manages response during and after cellular stress remains largely unknown. This study aimed to investigate the effect of HDAC6 on various stress and post-stress recovery responses. We showed that HIF-1α protein levels were reduced in HDAC6 knockout (KO) MEFs compared to wild-type (WT) MEFs in hypoxia. Furthermore, under hypoxia, HIF-1α levels were also reduced following rescue with either a catalytically inactive or a ubiqiutin-binding mutant HDAC6. HDAC6 deacetylated and upregulated the stability of HIF-1α, leading to activation of HIF-1α function under hypoxia. Notably, both the deacetylase and ubiquitin-binding activities of HDAC6 contributed to HIF-1α stabilization, but only deacetylase activity was required for HIF-1α transcriptional activity. Suppression of HDAC6 enhanced the interaction between HIF-1α and HSP70 under hypoxic conditions. In addition to hypoxia, depletion of HDAC6 caused hypersensitivity to cell death during oxidative stress and post-stress recovery. However, HDAC6 depletion had no effect on cell death in response to heat shock or ionizing radiation. Overall, our data suggest that HDAC6 may serve as a critical stress regulator in response to different cellular stresses.

Resistance to sunitinib in renal clear cell carcinoma results from sequestration in lysosomes and inhibition of the autophagic flux

Metastatic renal cell carcinomas (mRCC) are highly vascularized tumors that are a paradigm for the treatment with antiangiogenesis drugs targeting the vascular endothelial growth factor (VEGF) pathway. The available drugs increase the time to progression but are not curative and the patients eventually relapse. In this study we have focused our attention on the molecular mechanisms leading to resistance to sunitinib, the first line treatment of mRCC. Because of the anarchic vascularization of tumors the core of mRCC tumors receives only suboptimal concentrations of the drug. To mimic this in vivo situation, which is encountered in a neoadjuvant setting, we exposed sunitinib-sensitive mRCC cells to concentrations of sunitinib below the concentration of the drug that gives 50% inhibition of cell proliferation (IC50). At these concentrations, sunitinib accumulated in lysosomes, which downregulated the activity of the lysosomal protease CTSB (cathepsin B) and led to incomplete autophagic flux. Amino acid deprivation initiates autophagy enhanced sunitinib resistance through the amplification of autolysosome formation. Sunitinib stimulated the expression of ABCB1 (ATP-binding cassette, sub-family B [MDR/TAP], member 1), which participates in the accumulation of the drug in autolysosomes and favor its cellular efflux. Inhibition of this transporter by elacridar or the permeabilization of lysosome membranes with Leu-Leu-O-methyl (LLOM) resensitized mRCC cells that were resistant to concentrations of sunitinib superior to the IC50. Proteasome inhibitors also induced the death of resistant cells suggesting that the ubiquitin-proteasome system compensates inhibition of autophagy to maintain a cellular homeostasis. Based on our results we propose a new therapeutic approach combining sunitinib with molecules that prevent lysosomal accumulation or inhibit the proteasome.

AMP-activated protein kinase is dispensable for maintaining ATP levels and for survival following inhibition of glycolysis, but promotes tumour engraftment of Ras-transformed fibroblasts

Lactic acid generated by highly glycolytic tumours is exported by the MonoCarboxylate Transporters, MCT1 and MCT4, to maintain pHi and energy homeostasis. We report that MCT1 inhibition combined with Mct4 gene disruption severely reduced glycolysis and tumour growth without affecting ATP levels. Because of the key role of the 5′-AMP-activated protein kinase (AMPK) in energy homeostasis, we hypothesized that targeting glycolysis (MCT-blockade) in AMPK-null (Ampk^−/−) cells should kill tumour cells from ‘ATP crisis’. We show that Ampk^−/−-Ras-transformed mouse embryonic fibroblasts (MEFs) maintained ATP levels and viability when glycolysis was inhibited. In MCT-inhibited MEFs treated with OXPHOS inhibitors the ATP level and viability collapsed in both Ampk^+/+ and Ampk^−/− cells. We therefore propose that the intracellular acidification resulting from lactic acid sequestration mimicks AMPK by blocking mTORC1, a major component of an ATP consuming pathway, thereby preventing ‘ATP crisis’. Finally we showed that genetic disruption of Mct4 and/or Ampk dramatically reduced tumourigenicity in a xenograft mouse model suggesting a crucialrolefor these two actors in establishment of tumours in a nutrient-deprived environment. These findings demonstrated that blockade of lactate transport is an efficient anti-cancer strategy that highlights the potential in targeting Mct4 in a context of impaired AMPK activity.

Knockout of Vdac1 activates hypoxia-inducible factor through reactive oxygen species generation and induces tumor growth by promoting metabolic reprogramming and inflammation

Background

Mitochondria are more than just the powerhouse of cells; they dictate if a cell dies or survives. Mitochondria are dynamic organelles that constantly undergo fusion and fission in response to environmental conditions. We showed previously that mitochondria of cells in a low oxygen environment (hypoxia) hyperfuse to form enlarged or highly interconnected networks with enhanced metabolic efficacy and resistance to apoptosis. Modifications to the appearance and metabolic capacity of mitochondria have been reported in cancer. However, the precise mechanisms regulating mitochondrial dynamics and metabolism in cancer are unknown. Since hypoxia plays a role in the generation of these abnormal mitochondria, we questioned if it modulates mitochondrial function. The mitochondrial outer-membrane voltage-dependent anion channel 1 (VDAC1) is at center stage in regulating metabolism and apoptosis. We demonstrated previously that VDAC1 was post-translationally C-terminal cleaved not only in various hypoxic cancer cells but also in tumor tissues of patients with lung adenocarcinomas. Cells with enlarged mitochondria and cleaved VDAC1 were also more resistant to chemotherapy-stimulated cell death than normoxic cancer cells.

Results

Transcriptome analysis of mouse embryonic fibroblasts (MEF) knocked out for Vdac1 highlighted alterations in not only cancer and inflammatory pathways but also in the activation of the hypoxia-inducible factor-1 (HIF-1) signaling pathway in normoxia. HIF-1α was stable in normoxia due to accumulation of reactive oxygen species (ROS), which decreased respiration and glycolysis and maintained basal apoptosis. However, in hypoxia, activation of extracellular signal-regulated kinase (ERK) in combination with maintenance of respiration and increased glycolysis counterbalanced the deleterious effects of enhanced ROS, thereby allowing Vdac1^−/− MEF to proliferate better than wild-type MEF in hypoxia. Allografts of RAS-transformed Vdac1^−/− MEF exhibited stabilization of both HIF-1α and HIF-2α, blood vessel destabilization, and a strong inflammatory response. Moreover, expression of Cdkn2a, a HIF-1-target and tumor suppressor gene, was markedly decreased. Consequently, RAS-transformed Vdac1^−/− MEF tumors grew faster than wild-type MEF tumors.

Conclusions

Metabolic reprogramming in cancer cells may be regulated by VDAC1 through vascular destabilization and inflammation. These findings provide new perspectives into the understanding of VDAC1 in the function of mitochondria not only in cancer but also in inflammatory diseases.

Electronic supplementary material

The online version of this article (doi:10.1186/s40170-015-0133-5) contains supplementary material, which is available to authorized users.

The Na(+)/HCO3(-) Co-Transporter SLC4A4 Plays a Role in Growth and Migration of Colon and Breast Cancer Cells

The hypoxic and acidic tumor environment necessitates intracellular pH (pHi) regulation for tumor progression. Carbonic anhydrase IX (CA IX; hypoxia-induced) is known to facilitate CO2 export and generate HCO3(-) in the extracellular tumor space. It has been proposed that HCO3(-) is re-captured by the cell to maintain an alkaline pHi . A diverse range of HCO3(-) transporters, coupled with a lack of a clear over-expression in cancers have limited molecular identification of this cellular process. Here, we report that hypoxia induces the Na(+)/HCO3(-) co-transporter (NBCe1) SLC4A4 mRNA expression exclusively in the LS174 colon adenocarcinoma cell line in a HIF1α dependent manner. HCO3(-) dependent pHi recovery observations revealed the predominant use of an NBC mechanism suggesting that reversal of a Cl(-)/HCO3(-) exchanger is not utilized for tumor cell pHi regulation. Knockdown of SLC4A4 via shRNA reduced cell proliferation and increased mortality during external acidosis and spheroid growth. pHi recovery from acidosis was partially reduced with knockdown of SLC4A4. In MDA-MB-231 breast cancer cells expressing high levels of SLC4A4 compared to LS174 cells, SLC4A4 knockdown had a strong impact on cell proliferation, migration, and invasion. SLC4A4 knockdown also altered expression of other proteins including CA IX. Furthermore the Na(+)/HCO3(-) dependent pHi recovery from acidosis was reduced with SLC4A4 knockdown in MDA-MB-231 cells. Combined our results indicate that SLC4A4 contributes to the HCO3(-) transport and tumor cell phenotype. This study complements the on-going molecular characterization of the HCO3(-) re-uptake mechanism in other tumor cells for future strategies targeting these potentially important drug targets.

Hypoxia-inducible factor-1 and associated upstream and downstream proteins in the pathophysiology and management of glioblastoma

Glioblastoma multiforme (GBM) is a highly aggressive brain tumor with an exceptionally poor patient outcome despite aggressive therapy including surgery, radiation, and chemotherapy. This aggressive phenotype may be associated with intratumoral hypoxia, which probably plays a key role in GBM tumor growth, development, and angiogenesis. A key regulator of cellular response to hypoxia is the protein hypoxia-inducible factor–1 (HIF-1). An examination of upstream hypoxic and nonhypoxic regulation of HIF-1 as well as a review of the downstream HIF-1– regulated proteins may provide further insight into the role of this transcription factor in GBM pathophysiology. Recent insights into upstream regulators that intimately interact with HIF-1 could provide potential therapeutic targets for treatment of this tumor. The same is potentially true for HIF-1–mediated pathways of glycolysis-, angiogenesis-, and invasion-promoting proteins. Thus, an understanding of the relationship between HIF-1, its upstream protein regulators, and its downstream transcribed genes in GBM pathogenesis could provide future treatment options for the care of patients with these tumors.

AMPK-HDAC5 pathway facilitates nuclear accumulation of HIF-1α and functional activation of HIF-1 by deacetylating Hsp70 in the cytosol

Hypoxia-inducible factor 1 (HIF-1) transcriptionally promotes production of adenosine triphosphate (ATP) whereas AMPK senses and regulates cellular energy homeostasis. A histone deacetylase (HDAC) activity has been proven to be critical for HIF-1 activation but the underlying mechanism and its role in energy homesostasis remain unclear. Here, we demonstrate that HIF-1 activation depends on a cytosolic, enzymatically active HDAC5. HDAC5 knockdown impairs hypoxia-induced HIF-1α accumulation and HIF-1 transactivation, whereas HDAC5 overexpression enhances HIF-1α stabilization and nuclear translocation. Mechanistically, we show that Hsp70 is a cytosolic substrate of HDAC5; and hyperacetylation renders Hsp70 higher affinity for HIF-1α binding, which correlates with accelerated degradation and attenuated nuclear accumulation of HIF-1α. Physiologically, AMPK-triggered cytosolic shuttling of HDAC5 is critical; inhibition of either AMPK or HDAC5 impairs HIF-1α nuclear accumulation under hypoxia or low glucose conditions. Finally, we show specifically suppressing HDAC5 is sufficient to inhibit tumor cell proliferation under hypoxic conditions. Our data delineate a novel link between AMPK, the energy sensor, and HIF-1, the major driver of ATP production, indicating that specifically inhibiting HDAC5 may selectively suppress the survival and proliferation of hypoxic tumor cells.

The biological functions of Naa10 - From amino-terminal acetylation to human disease

N-terminal acetylation (NTA) is one of the most abundant protein modifications known, and the N-terminal acetyltransferase (NAT) machinery is conserved throughout all Eukarya. Over the past 50 years, the function of NTA has begun to be slowly elucidated, and this includes the modulation of protein-protein interaction, protein-stability, protein function, and protein targeting to specific cellular compartments. Many of these functions have been studied in the context of Naa10/NatA; however, we are only starting to really understand the full complexity of this picture. Roughly, about 40% of all human proteins are substrates of Naa10 and the impact of this modification has only been studied for a few of them. Besides acting as a NAT in the NatA complex, recently other functions have been linked to Naa10, including post-translational NTA, lysine acetylation, and NAT/KAT-independent functions. Also, recent publications have linked mutations in Naa10 to various diseases, emphasizing the importance of Naa10 research in humans. The recent design and synthesis of the first bisubstrate inhibitors that potently and selectively inhibit the NatA/Naa10 complex, monomeric Naa10, and hNaa50 further increases the toolset to analyze Naa10 function.

Combined expression of metastasis related markers Naa10p, SNCG and PRL-3 and its prognostic value in breast cancer patients

Combinations of multiple biomarkers representing distinct aspects of metastasis may have better prognostic value for breast cancer patients, especially those in late stages. In this study, we evaluated the protein levels of N-α-acetyltransferase 10 protein (Naa10p), synuclein-γ (SNCG), and phosphatase of regenerating liver-3 (PRL-3) in 365 patients with breast cancer by immunohistochemistry. Distinct prognostic subgroups of breast cancer were identified by combination of the three biomarkers. The Naa10p+SNCG-PRL-3- subgroup showed best prognosis with a median distant metastasis-free survival (DMFS) of 140 months, while the Naa10p-SNCG+PRL-3+ subgroup had the worst prognosis with a median DMFS of 60.5 months. Multivariate analysis indicated Naa10p, SNCG, PRL-3, and the TNM classification were all independent prognostic factors for both DMFS and overall survival (OS). The three biomarker combination of Naa10p, SNCG and PRL-3 performed better in patients with lymph node metastasis, especially those with more advanced tumors than other subgroups. In conclusion, the combined expression profile of Naa10p, SNCG and PRL-3, alone or in combination with the TNM classification system, may provide a precise estimate of prognosis of breast cancer patients.

Autoacetylation regulates differentially the roles of ARD1 variants in tumorigenesis

ARD1 is an acetyltransferase with several variants derived from alternative splicing. Among ARD1 variants, mouse ARD1(225) (mARD1(225)), mouse ARD1(235) (mARD1(235)), and human ARD1(235) (hARD1(235)) have been the most extensively characterized and are known to have different biological functions. In the present study, we demonstrated that mARD1(225), mARD1(235), and hARD1(235) have conserved autoacetylation activities, and that they selectively regulate distinct roles of ARD1 variants in tumorigenesis. Using purified recombinants for ARD1 variants, we found that mARD1(225), mARD1(235), and hARD1(235) undergo similar autoacetylation with the target site conserved at the Lys136 residue. Moreover, functional investigations revealed that the role of mARD1(225) autoacetylation is completely distinguishable from that of mARD1(235) and hARD1(235). Under hypoxic conditions, mARD1(225) autoacetylation inhibited tumor angiogenesis by decreasing the stability of hypoxia-inducible factor-1α (HIF-1α). Autoacetylation stimulated the catalytic activity of mARD1(225) to acetylate Lys532 of the oxygen-dependent degradation (ODD) domain of HIF-1α, leading to the proteosomal degradation of HIF-1α. In contrast, autoacetylation of mARD1(235) and hARD1(235) contributed to cellular growth under normoxic conditions by increasing the expression of cyclin D1. Taken together, these data suggest that autoacetylation of ARD1 variants differentially regulates angiogenesis and cell proliferation in an isoform-specific manner.

Nuclear translocation of hARD1 contributes to proper cell cycle progression

Arrest defective 1 (ARD1) is an acetyltransferase that is highly conserved across organisms, from yeasts to humans. The high homology and widespread expression of ARD1 across multiple species and tissues signify that it serves a fundamental role in cells. Human ARD1 (hARD1) has been suggested to be involved in diverse biological processes, and its role in cell proliferation and cancer development has been recently drawing attention. However, the subcellular localization of ARD1 and its relevance to cellular function remain largely unknown. Here, we have demonstrated that hARD1 is imported to the nuclei of proliferating cells, especially during S phase. Nuclear localization signal (NLS)-deleted hARD1 (hARD1ΔN), which can no longer access the nucleus, resulted in cell morphology changes and cellular growth impairment. Notably, hARD1ΔN-expressing cells showed alterations in the cell cycle and the expression levels of cell cycle regulators compared to hARD1 wild-type cells. Furthermore, these effects were rescued when the nuclear import of hARD1 was restored by exogenous NLS. Our results show that hARD1 nuclear translocation mediated by NLS is required for cell cycle progression, thereby contributing to proper cell proliferation.

HEXIM1 down-regulates hypoxia-inducible factor-1α protein stability

We have previously reported on the inhibition of HIF-1α (hypoxia-inducible factor α)-regulated pathways by HEXIM1 [HMBA (hexamethylene-bis-acetamide)-inducible protein 1]. Disruption of HEXIM1 activity in a knock-in mouse model expressing a mutant HEXIM1 protein resulted in increased susceptibility to the development of mammary tumours, partly by up-regulation of VEGF (vascular endothelial growth factor) expression, HIF-1α expression and aberrant vascularization. We now report on the mechanistic basis for HEXIM1 regulation of HIF-1α. We observed direct interaction between HIF-1α and HEXIM1, and HEXIM1 up-regulated hydroxylation of HIF-1α, resulting in the induction of the interaction of HIF-1α with pVHL (von Hippel-Lindau protein) and ubiquitination of HIF-1α. The up-regulation of hydroxylation involves HEXIM1-mediated induction of PHD3 (prolyl hydroxylase 3) expression and interaction of PHD3 with HIF-1α. Acetylation of HIF-1α has been proposed to result in increased interaction of HIF-1α with pVHL and induced pVHL-mediated ubiquitination, which leads to the proteasomal degradation of HIF-1α. HEXIM1 also attenuated the interaction of HIF-1α with HDAC1 (histone deacetylase 1), resulting in acetylation of HIF-1α. The consequence of HEXIM1 down-regulation of HIF-1α protein expression is attenuated expression of HIF-1α target genes in addition to VEGF and inhibition of HIF-1α-regulated cell invasion.

Hypoxia promotes tumor cell survival in acidic conditions by preserving ATP levels

The efficacy of targeting pH disruption to induce cell death in the acidic and hypoxic tumor microenvironment continues to be assessed. Here we analyzed the impact of varying levels of hypoxia in acidic conditions on fibroblast and tumor cell survival. Across all cell lines tested, hypoxia (1% O(2)) provided protection against acidosis induced cell death compared to normoxia. Meanwhile severe hypoxia (0.1% O(2)) removed this protection and in some cases exacerbated acidosis-induced cell death. Differential survival between cell types during external acidosis correlated with their respective intracellular pH regulating capabilities. Cellular ATP measurements were conducted to determine their contribution to cell survival under these combined stresses. In general, hypoxia (1% O(2)) maintained elevated ATP levels in acidic conditions while severe hypoxia did not. To further explore this interaction we combined acidosis with ATP depletion using 2-deoxyglucose and observed an enhanced rate of cell mortality. Striking results were also observed with hypoxia providing protection against cell death in spite of a severe metabolic stress induced by a combination of acidosis and oligomycin. Finally, we demonstrated that both HIF1α and HIF2α expression were drastically reduced in hypoxic and acidic conditions indicating a sensitivity of this protein to cellular pH conditions. This knockdown of HIF expression by acidosis has implications for the development of therapies targeting the disruption of cellular pH regulation. Our results reinforce the proof of concept that acidosis and metabolic disruption affecting ATP levels could be exploited as a tumor cell killing strategy.

Inverse correlation between Naa10p and MMP-9 expression and the combined prognostic value in breast cancer patients

To analyze the expression profiles of N-a-acetyltransferase 10 protein (Naa10p/ARD1) and matrix metalloproteinase 9 (MMP-9) in human breast cancer and evaluate their possible prognostic values in breast cancer patients. Quantitative RT-PCR was used to evaluate mRNA expression of Naa10p and MMP-9 in 55 cases of fresh breast cancer tissues, and immunohistochemistry was performed for detecting Naa10p and MMP-9 proteins on breast cancer with tissue microarray containing 80 specimens. Furthermore, Naa10p and MMP-9 were measured in 253 breast cancer tissues from patients with up to 15-year follow-up. Survival curves were generated using the Kaplan-Meier method. Multivariate analysis was performed by using the Cox proportional hazard regression model to assess the prognostic values of Naa10p and MMP-9. Both Naa10p and MMP-9 expression in breast cancer tissues were significantly higher than those in the matched non-cancerous tissues (p = 0.000 for both). There was an inverse correlation between Naa10p and MMP-9 expression at mRNA and protein levels (p = 0.000 for both). Patients with MMP-9- positive expression had a poorer overall survival (OS) and disease-free survival (DFS) than those with MMP-9-negative expression (p = 0.001 for both). However, patients with Naa10p-positive expression had better OS and DFS (p = 0.000 for both). In addition, Naa10p-positive/MMP-9- negative patients had the best OS and DFS (p = 0.000 for both). In multivariate survival analysis, TNM stage, Naa10p expression, MMP-9 expression, and combined expression status of Naa10p/MMP-9 were independent prognostic factors related to OS (p = 0.000, 0.007, 0.012, and 0.000, respectively) and DFS (p = 0.000, 0.002, 0.014, and 0.000, respectively).The expression level of Naa10p was inversely correlated with that of MMP-9 in human breast cancer samples. Combined analysis of Naa10p and MMP-9 had a significantly increased value for determining the prognosis of breast cancer patients.

MiR-210 promotes a hypoxic phenotype and increases radioresistance in human lung cancer cell lines

The resistance of hypoxic cells to radiotherapy and chemotherapy is a major problem in the treatment of cancer. Recently, an additional mode of hypoxia-inducible factor (HIF)-dependent transcriptional regulation, involving modulation of a specific set of micro RNAs (miRNAs), including miR-210, has emerged. We have recently shown that HIF-1 induction of miR-210 also stabilizes HIF-1 through a positive regulatory loop. Therefore, we hypothesized that by stabilizing HIF-1 in normoxia, miR-210 may protect cancer cells from radiation. We developed a non-small cell lung carcinoma (NSCLC)-derived cell line (A549) stably expressing miR-210 (pmiR-210) or a control miRNA (pmiR-Ctl). The miR-210-expressing cells showed a significant stabilization of HIF-1 associated with mitochondrial defects and a glycolytic phenotype. Cells were subjected to radiation levels ranging from 0 to 10 Gy in normoxia and hypoxia. Cells expressing miR-210 in normoxia had the same level of radioresistance as control cells in hypoxia. Under hypoxia, pmiR-210 cells showed a low mortality rate owing to a decrease in apoptosis, with an ability to grow even at 10 Gy. This miR-210 phenotype was reproduced in another NSCLC cell line (H1975) and in HeLa cells. We have established that radioresistance was independent of p53 and cell cycle status. In addition, we have shown that genomic double-strand breaks (DSBs) foci disappear faster in pmiR-210 than in pmiR-Ctl cells, suggesting that miR-210 expression promotes a more efficient DSB repair. Finally, HIF-1 invalidation in pmiR-210 cells removed the radioresistant phenotype, showing that this mechanism is dependent on HIF-1. In conclusion, miR-210 appears to be a component of the radioresistance of hypoxic cancer cells. Given the high stability of most miRNAs, this advantage could be used by tumor cells in conditions where reoxygenation has occurred and suggests that strategies targeting miR-210 could enhance tumor radiosensitization.

Clinical implications of arrest-defective protein 1 expression in hepatocellular carcinoma: a novel predictor of microvascular invasion

OBJECTIVE

The associations between arrest-defective protein 1 (ARD1) gene expression and the clinicopathological characteristics and clinical outcomes of 94 patients undergoing hepatectomy for hepatocellular carcinoma (HCC) were investigated.

METHODS

ARD1 mRNA levels in HCC and corresponding non-cancerous tissues were quantified by real-time PCR. The gene expression of the tumor relative to that in the non-tumor tissues was calculated using the 2(-)(ΔΔ)(CT) method. The subjects were classified into high expression (2(-)(ΔΔ)(CT) > 1, n = 38) and low expression (2(-)(ΔΔ)(CT) ≤ 1, n = 56) groups.

RESULTS

The HCCs did not differ from matched liver tissues in terms of ARD1 mRNA levels. The high expression group had more often microvascular invasion than the low expression group (32 vs. 14%; p = 0.045). The two groups did not differ significantly in terms of other patient or tumor variables. The median follow-up period was 92.1 months. The 5-year recurrence-free and overall survival rates were 34 and 76% for the high expression group, respectively, which were similar to the rates of the low expression group (46 vs. 73%, p = 0.98 and p = 0.52, respectively).

CONCLUSIONS

Intratumoral ARD1 mRNA overexpression was involved in the microvascular invasion process in patients with HCC, although it did not associate strongly with postresectional outcomes.

Combined phenotype of 4 markers improves prognostic value of patients with colon cancer

INTRODUCTION

Combination of multiple biomarkers representing distinct aspects of tumor biology will have a better prognostic value. This study was to identify prognostic subgroups of colon adenocarcinoma by combined analysis of synuclein-gamma (SNCG), a human homologue of piwi (Hiwi), phosphatase of regenerating liver-3 (PRL-3), arrest-defective protein 1, homolog A (ARD1) and clinicopathologic features in 225 colon adenocarcinoma specimens.

METHODS

Immunohistochemistry for 4 tumor markers was performed in whole tissue sections from 225 colon adenocarcinoma patients with complete clinicopathologic data and up to 10-year follow-up. The immunohistochemical expression patterns were examined individually and in multimarker combinations. Univariate and multivariate analyses were performed to identify independent predictive markers of poor outcome.

RESULTS

With the tumor marker positive rate [32.0% (62/225) for SNCG; 76.9% (173/225) for combined SNCG/Hiwi/PRL-3/ARD1] and the detecting accuracy [61.9% (252/407) for SNCG; 82.6% (336/407) for combined SNCG/Hiwi/PRL-3/ARD1] increasing, incremental value of combined SNCG/Hiwi/PRL-3/ARD1 (P < 0.001; hazard ratios (HR), 3.2) to poor outcome was found. Stratified by lymph node, Hiwi alone (P = 0.004; HR, 3.2) led to poor outcome in patients without lymph node metastasis (LN-), and SNCG (P < 0.001; HR, 2.5) had independently poor prognostic value for patients with lymph node metastasis (LN+).

CONCLUSIONS

Multimarker phenotypes improved tumor positive rate, detecting accuracy and prognostic value. In addition, a subgroup of more aggressive tumors can be identified by evaluating Hiwi level in LN- cancer, and SNCG level in LN+ cancer.

Inhibition of SIRT1 impairs the accumulation and transcriptional activity of HIF-1α protein under hypoxic conditions

Sirtuins and hypoxia-inducible transcription factors (HIF) have well-established roles in regulating cellular responses to metabolic and oxidative stress. Recent reports have linked these two protein families by demonstrating that sirtuins can regulate the activity of HIF-1 and HIF-2. Here we investigated the role of SIRT1, a NAD+-dependent deacetylase, in the regulation of HIF-1 activity in hypoxic conditions. Our results show that in hepatocellular carcinoma (HCC) cell lines, hypoxia did not alter SIRT1 mRNA or protein expression, whereas it predictably led to the accumulation of HIF-1α and the up-regulation of its target genes. In hypoxic models in vitro and in in vivo models of systemic hypoxia and xenograft tumor growth, knockdown of SIRT1 protein with shRNA or inhibition of its activity with small molecule inhibitors impaired the accumulation of HIF-1α protein and the transcriptional increase of its target genes. In addition, endogenous SIRT1 and HIF-1α proteins co-immunoprecipitated and loss of SIRT1 activity led to a hyperacetylation of HIF-1α. Taken together, our data suggest that HIF-1α and SIRT1 proteins interact in HCC cells and that HIF-1α is a target of SIRT1 deacetylase activity. Moreover, SIRT1 is necessary for HIF-1α protein accumulation and activation of HIF-1 target genes under hypoxic conditions.

Gcn5p-dependent acetylation induces degradation of the meiotic transcriptional repressor Ume6p

Acetyltransferases induce transcription by enhancing the activity of transcriptional activators and opening chromatin domains. A third avenue is described by which gene activation is accomplished by acetylation through the targeted destruction of the Ume6p repressor.

Ume6p represses early meiotic gene transcription in Saccharomyces cerevisiae by recruiting the Rpd3p histone deacetylase and chromatin-remodeling proteins. Ume6p repression is relieved in a two-step destruction process mediated by the anaphase-promoting complex/cyclosome (APC/C) ubiquitin ligase. The first step induces partial Ume6p degradation when vegetative cells shift from glucose- to acetate-based medium. Complete proteolysis happens only upon meiotic entry. Here we demonstrate that the first step in Ume6p destruction is controlled by its acetylation and deacetylation by the Gcn5p acetyltransferase and Rpd3p, respectively. Ume6p acetylation occurs in medium lacking dextrose and results in a partial destruction of the repressor. Preventing acetylation delays Ume6p meiotic destruction and retards both the transient transcription program and execution of the meiotic nuclear divisions. Conversely, mimicking acetylation induces partial destruction of Ume6p in dextrose medium and accelerates meiotic degradation by the APC/C. These studies reveal a new mechanism by which acetyltransferase activity induces gene expression through targeted destruction of a transcriptional repressor. These findings also demonstrate an important role for nonhistone acetylation in the transition between mitotic and meiotic cell division.

Protein N-terminal acetyltransferases in cancer

The human N-terminal acetyltransferases (NATs) catalyze the transfer of acetyl moieties to the N-termini of 80-90% of all human proteins. Six NAT types are present in humans, NatA-NatF, each is composed of specific subunits and each acetylates a set of substrates defined by the N-terminal amino-acid sequence. NATs have been suggested to act as oncoproteins as well as tumor suppressors in human cancers, and NAT expression may be both elevated and decreased in cancer versus non-cancer tissues. Manipulation of NATs in cancer cells induced cell-cycle arrest, apoptosis or autophagy, implying that these enzymes target a variety of pathways. Of particular interest is hNaa10p (human ARD1), the catalytic subunit of the NatA complex, which was coupled to a number of signaling molecules including hypoxia inducible factor-1α, β-catenin/cyclin D1, TSC2/mammalian target of rapamycin, myosin light chain kinase , DNA methyltransferase1/E-cadherin and p21-activated kinase-interacting exchange factors (PIX)/Cdc42/Rac1. The variety of mechanistic links where hNaa10p acts as a NAT, a lysine acetyltransferase or displaying a non-catalytic role, provide insights to how hNaa10p may act as both a tumor suppressor and oncoprotein.

Glycogen Synthesis is Induced in Hypoxia by the Hypoxia-Inducible Factor and Promotes Cancer Cell Survival

The hypoxia-inducible factor 1 (HIF-1), in addition to genetic and epigenetic changes, is largely responsible for alterations in cell metabolism in hypoxic tumor cells. This transcription factor not only favors cell proliferation through the metabolic shift from oxidative phosphorylation to glycolysis and lactic acid production but also stimulates nutrient supply by mediating adaptive survival mechanisms. In this study we showed that glycogen synthesis is enhanced in non-cancer and cancer cells when exposed to hypoxia, resulting in a large increase in glycogen stores. Furthermore, we demonstrated that the mRNA and protein levels of the first enzyme of glycogenesis, phosphoglucomutase1 (PGM1), were increased in hypoxia. We showed that induction of glycogen storage as well as PGM1 expression were dependent on HIF-1 and HIF-2. We established that hypoxia-induced glycogen stores are rapidly mobilized in cells that are starved of glucose. Glycogenolysis allows these “hypoxia-preconditioned” cells to confront and survive glucose deprivation. In contrast normoxic control cells exhibit a high rate of cell death following glucose removal. These findings point to the important role of hypoxia and HIF in inducing mechanisms of rapid adaptation and survival in response to a decrease in oxygen tension. We propose that a decrease in pO₂ acts as an “alarm” that prepares the cells to face subsequent nutrient depletion and to survive.

Inactivation of androgen-induced regulator ARD1 inhibits androgen receptor acetylation and prostate tumorigenesis

Androgen signaling through androgen receptor (AR) is critical for prostate tumorigenesis. Given that AR-mediated gene regulation is enhanced by AR coregulators, inactivation of those coregulators is emerging as a promising therapy for prostate cancer (PCa). Here, we show that the N-acetyltransferase arrest-defect 1 protein (ARD1) functions as a unique AR regulator in PCa cells. ARD1 is up-regulated in human PCa cell lines and primary tumor biopsies. The expression of ARD1 was augmented by treatment with synthetic androgen (R1881) unless AR is deficient or is inhibited by AR-specific siRNA or androgen inhibitor bicalutamide (Casodex). Depletion of ARD1 by shRNA suppressed PCa cell proliferation, anchorage-independent growth, and xenograft tumor formation in SCID mice, suggesting that AR-dependent ARD1 expression is biologically germane. Notably, ARD1 was critical for transcriptionally regulating a number of AR target genes that are involved in prostate tumorigenesis. Furthermore, ARD1 interacted physically with and acetylated the AR protein in vivo and in vitro. Because AR-ARD1 interaction facilitated the AR binding to its targeted promoters for gene transcription, we propose that ARD1 functions as a unique AR regulator and forms a positive feedback loop for AR-dependent prostate tumorigenesis. Disruption of AR-ARD1 interactions may be a potent intervention for androgen-dependent PCa therapy.

HIF1α and HIF2α: sibling rivalry in hypoxic tumour growth and progression

Hypoxia-inducible factors (HIFs) are broadly expressed in human cancers, and HIF1α and HIF2α were previously suspected to promote tumour progression through largely overlapping functions. However, this relatively simple model has now been challenged in light of recent data from various approaches that reveal unique and sometimes opposing activities of these HIFα isoforms in both normal physiology and disease. These effects are mediated in part through the regulation of unique target genes, as well as through direct and indirect interactions with important oncoproteins and tumour suppressors, including MYC and p53. As HIF inhibitors are currently undergoing clinical evaluation as cancer therapeutics, a more thorough understanding of the unique roles performed by HIF1α and HIF2α in human neoplasia is warranted.

Histone deacetylase inhibitors: the epigenetic therapeutics that repress hypoxia-inducible factors

Histone deacetylase inhibitors (HDACIs) have been actively explored as a new generation of chemotherapeutics for cancers, generally known as epigenetic therapeutics. Recent findings indicate that several types of HDACIs repress angiogenesis, a process essential for tumor metabolism and progression. Accumulating evidence supports that this repression is mediated by disrupting the function of hypoxia-inducible factors (HIF-1, HIF-2, and collectively, HIF), which are the master regulators of angiogenesis and cellular adaptation to hypoxia. Since HIF also regulate glucose metabolism, cell survival, microenvironment remodeling, and other alterations commonly required for tumor progression, they are considered as novel targets for cancer chemotherapy. Though the precise biochemical mechanism underlying the HDACI-triggered repression of HIF function remains unclear, potential cellular factors that may link the inhibition of deacetylase activity to the repression of HIF function have been proposed. Here we review published data that inhibitors of type I/II HDACs repress HIF function by either reducing functional HIF-1α levels, or repressing HIF-α transactivation activity. In addition, underlying mechanisms and potential proteins involved in the repression will be discussed. A thorough understanding of HDACI-induced repression of HIF function may facilitate the development of future therapies to either repress or promote angiogenesis for cancer or chronic ischemic disorders, respectively.

hNaa10p contributes to tumorigenesis by facilitating DNMT1-mediated tumor suppressor gene silencing

Hypermethylation-mediated tumor suppressor gene silencing plays a crucial role in tumorigenesis. Understanding its underlying mechanism is essential for cancer treatment. Previous studies on human N-alpha-acetyltransferase 10, NatA catalytic subunit (hNaa10p; also known as human arrest-defective 1 [hARD1]), have generated conflicting results with regard to its role in tumorigenesis. Here we provide multiple lines of evidence indicating that it is oncogenic. We have shown that hNaa10p overexpression correlated with poor survival of human lung cancer patients. In vitro, enforced expression of hNaa10p was sufficient to cause cellular transformation, and siRNA-mediated depletion of hNaa10p impaired cancer cell proliferation in colony assays and xenograft studies. The oncogenic potential of hNaa10p depended on its interaction with DNA methyltransferase 1 (DNMT1). Mechanistically, hNaa10p positively regulated DNMT1 enzymatic activity by facilitating its binding to DNA in vitro and its recruitment to promoters of tumor suppressor genes, such as E-cadherin, in vivo. Consistent with this, interaction between hNaa10p and DNMT1 was required for E-cadherin silencing through promoter CpG methylation, and E-cadherin repression contributed to the oncogenic effects of hNaa10p. Together, our data not only establish hNaa10p as an oncoprotein, but also reveal that it contributes to oncogenesis through modulation of DNMT1 function.

Peptide mimic isolated by autoantibody reveals human arrest defective 1 overexpression is associated with poor prognosis for colon cancer patients

Tumor-associated antigens, which induce the generation of autoantibodies, are useful as cancer biomarkers in early detection and prognostic prediction of cancer. To isolate a novel cancer marker, we used serum antibodies from colon cancer patients to screen a phage display peptide library. A positive peptide 249C (VPLYSNTLRYGF) that could specifically react with serum from colon cancer patients was isolated, and the corresponding antigen-human arrest defective 1 (ARD1A), which shares an identical LYSNTL motif with 249C, was identified. Both immunological assays and three-dimensional structure analysis showed that the LYSNTL region is an epitope of ARD1A. Using ELISA and immunohistochemistry, we found anti-ARD1A antibody levels in serum from patients with colon cancer were significantly higher than those in healthy volunteers (P < 0.001), and ARD1A expression was detected in 84.1% (227/270) of colon cancer tissues compared with 22.7% (55/242) of matched noncancerous tissues (P < 0.001) and 4.8% (2/42) of benign lesions (P < 0.001). Furthermore, multivariate analysis with Cox proportional hazards regression models revealed that ARD1A-positive patients had significantly shortened overall survival (OS) (HR, 1.91, P = 0.039) and borderline significantly shortened disease-free survival (DFS) (HR, 1.70; P = 0.068). Kaplan-Meier survival curves also showed that ARD1A expression was associated significantly with shortened DFS (P = 0.037) and OS (P = 0.019). These results indicate that ARD1A is a novel tumor-associated antigen and a potential prognostic factor for colon cancer.

Hypoxia-inducible factors: post-translational crosstalk of signaling pathways

Hypoxia-inducible factor-1 (HIF-1) has a central role in the mammalian program by which cells respond to hypoxia in both physiological and pathological situations. HIF-1 transcriptional activity, protein stabilization, protein-protein interaction, and cellular localization are mainly modulated by Post-translational modifications such as hydroxylation, acetylation, phosphorylation, S-nitrosylation, and SUMOylation. Here, we summarize current knowledge about Post-translational HIF-1 regulation and give additional information about useful methods to determine some of these various modifications.

Correlation of expression of human arrest-defective-1 (hARD1) protein with breast cancer

Human arrest-defective-1 (hARD1) was reported to be important in regulating cell cycle and promoting lung cancer cell proliferation. Here we have investigated the correlation between hARD1 and breast cancer. Analysis with 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) and flow cytometry (FCM) demonstrated that overexpression of hARD1 was associated with increased proliferation of MCF-7 cell, a human breast cancer cell line. Western blotting and immunohistochemical staining assay showed that hARD1 presented higher in breast cancer tissue than the adjacent tissue; accumulation of hARD1 protein was higher in 86% (37/43) of breast cancer, far more than noncancer samples. Our results suggest that hARD1 might play an important role in breast cancer carcinogenesis.

Hypoxia, hypoxia-inducible factors (HIF), HIF hydroxylases and oxygen sensing

This article outlines the need for a homeostatic response to alterations in cellular oxygenation. It describes work on erythropoietin control that led to the discovery of the hypoxia-inducible transcription factor (HIF-1) and the parallel recognition that this system was responsive to a widespread oxygen-sensing mechanism. Subsequently, multiple HIF isoforms have been shown to have overlapping but non-redundant functions, controlling expression of genes involved in diverse processes such as angiogenesis, vascular tone, metal transport, glycolysis, mitochondrial function, cell growth and survival. The major role of prolyl and asparaginyl hydroxylation in regulating HIFs is described, as well as the identification of PHD1-3 and FIH as the oxygen-sensing enzymes responsible for these hydroxylations. Current understanding of other processes that modulate overall HIF activity, including influences from other signalling mechanisms such as kinases and nitric oxide levels, and the existence of a variety of feedback loops are outlined. The effects of some mutations in this pathway are documented as is knowledge of other substrates for these enzymes. The importance of PHD1-3 and FIH, and the large family of 2-oxoglutarate and iron(II)-dependent dioxygenases of which they are a part, in biology and medicine are discussed.

Phosphorylation of ARD1 by IKKbeta contributes to its destabilization and degradation

IkappaB kinase beta (IKKbeta), a major kinase downstream of various proinflammatory signals, mediates multiple cellular functions through phosphorylation and regulation of its substrates. On the basis of protein sequence analysis, we identified arrest-defective protein 1 (ARD1), a protein involved in apoptosis and cell proliferation processes in many human cancer cells, as a new IKKbeta substrate. We provided evidence showing that ARD1 is indeed a bona fide substrate of IKKbeta. IKKbeta physically associated with ARD1 and phosphorylated it at Ser209. Phosphorylation by IKKbeta destabilized ARD1 and induced its proteasome-mediated degradation. Impaired growth suppression was observed in ARD1 phosphorylation-mimic mutant (S209E)-transfected cells as compared with ARD1 non-phosphorylatable mutant (S209A)-transfected cells. Our findings of molecular interactions between ARD1 and IKKbeta may enable further understanding of the upstream regulation mechanisms of ARD1 and of the diverse functions of IKKbeta.

Composition and biological significance of the human Nalpha-terminal acetyltransferases

Protein N^α-terminal acetylation is one of the most common protein modifications in eukaryotic cells, occurring on approximately 80% of soluble human proteins. An increasing number of studies links N^α-terminal acetylation to cell differentiation, cell cycle, cell survival, and cancer. Thus, N^α-terminal acetylation is an essential modification for normal cell function in humans. Still, little is known about the functional role of N^α-terminal acetylation. Recently, the three major human N-acetyltransferase complexes, hNatA, hNatB and hNatC, were identified and characterized. We here summarize the identified N-terminal acetyltransferase complexes in humans, and we review the biological studies on N^α-terminal acetylation in humans and other higher eukaryotes.

N-acetyltransferase ARD1-NAT1 regulates neuronal dendritic development

ARD1 and NAT1 constitute an N-acetyltransferase complex where ARD1 holds the enzymatic activity of the complex. The ARD1-NAT1 complex mediates N-terminal acetylation of nascent polypeptides that emerge from ribosomes after translation. ARD1 may also acetylate the internal lysine residues of proteins. Although ARD1 and NAT1 have been found in the brain, the physiological role and substrates of the ARD1-NAT1 complex in neurons remain unclear. Here we investigated role of N-acetyltransferase activity in the process of neuronal development. Expression of ARD1 and NAT1 increased during dendritic development, and both proteins colocalized with microtubules in dendrites. The ARD1-NAT1 complex displayed acetyltransferase activity against a purified microtubule fraction in vitro. Inhibition of the complex limited the dendritic extension of cultured neurons. These findings suggest that the ARD1-NAT1 complex has acetyltransferase activity against microtubules in dendrites. Regulation by acetyltransferase activity is a novel mechanism that is required for dendritic arborization during neuronal development.