Ochratoxin A activates opposing c-MET/PI3K/Akt and MAPK/ERK 1-2 pathways in human proximal tubule HK-2 cells.

Ochratoxin A (OTA) is a mycotoxin produced as a secondary metabolite by filamentous fungi, such as Aspergillus and Penicillium. Because OTA is a common contaminant of food and feeds, humans and animals are frequently exposed to OTA in daily life. It has been classified as a carcinogen in rodents and a possible carcinogen in humans. OTA has been shown to deregulate a variety of different signal transduction pathways in a cell type- and dosage-depending manner resulting in contrasting physiological effects, such as survival or cell death. While the ERK1-2 and JNK/SAPK MAPK pathways are major targets, knowledge about their role in OTA-mediated cell survival and death is fragmented. Similarly, the contribution of the PI3K/Akt pathway to the carcinogenic effect of OTA in proximal tubule cells has not been elucidated in detail. In this study, we demonstrated that OTA induced sustained activation of the PI3K/Akt and MEK/ERK1-2 signaling pathways in a dose- and time-dependent manner in HK-2 cells. Chemical inhibition of ERK1-2 activation or overexpression of dominant-negative and kinase-dead MEK1 leads to increased cell viability and decreased apoptosis in OTA-treated cells. Blockage of PI3K/Akt with Wortmannin aggravated the negative effect of OTA on cell viability and increased the levels of apoptosis. Moreover, we identified the c-MET proto-oncogene as an upstream receptor tyrosine kinase responsible for OTA-induced activation of PI3K/Akt signaling in HK-2 cells. Our data suggest that OTA may potentiate carcinogenesis by sustained activation of c-MET/PI3K/Akt signaling through suppression of apoptosis induced by MEK/ERK1-2 activation in damaged renal proximal tubule epithelial cells.

Seasonal variations in toxic metal levels of two fish species, Mugil cephalus and Mullus barbatus and estimation of risk for children.

Concentrations of five toxic metals were determined in two fish species namely, Mugil cephalus and Mullus barbatus consumed in two neighboring Turkish cities during 2010-2011. Lead concentrations (mean 900 µg kg(-1)) for M. cephalus were found to be higher than the maximum allowances concentration (MAC) of 300 µg kg(-1). Similarly, Cr (mean 410 µg kg(-1)), Ni (mean 331 µg kg(-1)) and Cu (mean 834 µg kg(-1)) concentrations in M. cephalus were significantly higher than in M. barbatus (mean 341 µg Cr kg(-1), 256 µg Ni kg(-1) and 568 µg Cu kg(-1)). Although concentrations of some metals exceed the limits set by the authorities, the estimated non-carcinogenic and carcinogenic health risks by the Target Hazard Quotient and target carcinogenic risk indicate that there is no carcinogenic risk for humans, and the risk of developing cancer over a human lifetime is between 2 and 12 in 1,000,000.

Variations in toxic metal levels of two fish species, Pomatomus saltatrix and Dicentrarchus labrax, and risk estimation for children.

The concentrations of five toxic metals were monthly determined in two fish species, obtained from fish markets in Turkey during 2010-2011. For the determinations, AAS and ICP-AES were used. The obtained lead concentrations for all studied Pomatomus saltatrix (mean 635 microg x kg(-1)) and Dicentrarchus labrax (mean 463 microg x kg(-1)) samples were found to be significantly higher than the maximum allowances concentration (MAC) of 300 microg x kg(-1). Mean chromium (324 microg x kg(-1)) and Cu (940 microg x kg(-1)) concentrations in Pomatomus saltatrix were higher than in Dicentrarchus labrax (268 microg Cr x kg(-1) and 600 microg Cu x kg(-1)) while Ni in Pomatomus saltatrix (216 microg x kg(-1)) was lower in Dicentrarchus labrax (291 microg x kg(-1)). The estimated non-carcinogenic and carcinogenic health risks by the Target Hazard Quotient and target carcinogenic risk indicate that there are no sytemic effects, and the risk of developing cancer over a human lifetime is between 2-9 in 1000000.

Membrane Receptor-Mediated Disruption of Cellular Homeostasis: Changes in Intracellular Signaling Pathways Increase the Toxicity of Ochratoxin A.

Organisms maintain their cellular homeostatic balance by interacting with their environment through the use of their cell surface receptors. Membrane based receptors such as the transforming growth factor ß receptor (TGFR), the prolactin receptor (PRLR), and hepatocyte growth factor receptor (HGFR), along with their associated signaling cascade, play significant roles in retaining cellular homeostasis. While these receptors and related signaling pathways are essential for health of cell and organism, their dysregulation can lead to imbalance in cell function with severe pathological conditions such as cell death or cancer. Ochratoxin A (OTA) can disrupt cellular homeostasis by altering expression levels of these receptors and/or receptor-associated intracellular downstream signaling modulators and/or pattern and levels of their phosphorylation/dephosphorylation. Recent studies have shown that the activity of the TGFR, the PRLR, and HGFR and their associated signaling cascades change upon OTA exposure. A critical evaluation of these findings suggests that while increased activity of the HGFR and TGFR signaling pathways leads to an increase in cell survival and fibrosis, decreased activity of the PRLR signaling pathway leads to tissue damage. This review explores the roles of these receptors in OTA-related pathologies and effects on cellular homeostasis.

Ochratoxin A induces ERK1/2 phosphorylation-dependent apoptosis through NF-κB/ERK axis in human proximal tubule HK-2 cell line.

Ochratoxin A (OTA) is a food contaminant mycotoxin with hazardous effects on human and animal health, primarily affecting the kidneys. OTA's mode of action is not well understood. OTA activates both MAPK/ERK and PI3K/Akt signaling pathways, which play role in apoptosis and cell survival, respectively. OTA is also known to induce toxicity by activating the NF-κB pathway in immune cells. However, its role in determining the cell fate upon OTA exposure in a human kidney cell line (HK-2) has not been fully explored. We made use of pharmacological inhibition of NF-κB to define its role in viability of OTA-treated HK-2 cells. We show that OTA-induced p65 NF-κB subunit translocation into the nucleus in a time-dependent manner using both Western blotting and immunofluorescence (IF). We also document the DNA-binding and reporter gene expression activities of NF-κB by electrophoretic mobility shift (EMSA) and luciferase reporter assays, respectively. Our results indicate that, following 6 h of exposure, OTA fully activates NF-κB pathway and its downstream effectors in HK-2 cells. In addition, Bay11-7085 treatment causes attenuation of the relative levels of OTA-mediated ERK1/2 phosphorylation, suggesting a cross-talk between NF-κB and the MAPK/ERK pathway. Critically, co-treatment of HK-2 cells with OTA and Bay11-7085 leads to the inhibition of OTA-induced apoptosis in a time-dependent manner. Our results support a robust association between NF-κB and the MAPK/ERK pathways in the modulation of apoptotic effects of OTA in HK-2 cells.

Prevalence of Jervell-Lange Nielsen syndrome in children with congenital bilateral sensorineural hearing loss.

OBJECTIVE: Long QT syndrome (LQTS) is an inherited cardiac ion channel disorder (channelopathy) that is characterized by prolonged QT intervals on the electrocardiography (ECG) and possess the risk of sudden cardiac death (SCD). Jervell-Lange Nielsen syndrome (JLNS) is a specific subtype of LQTS that is accompanied by congenital sensorineural hearing loss, inherited autosomal recessively, and higher risk of SCD. In this study, we aimed to investigate JLNS prevalence in deaf children attending special schools for hearing loss, located in our province. METHODS: An ECG screening program was conducted in 6 special schools for children with hearing loss in Istanbul and a total of 440 students between 6 and 18 years old were included. Corrected QT interval (QTc) was calculated using the Bazett formula. Notably, 51 students, detected with any abnormal finding on ECG, were invited to our center for a comprehensive examination. RESULTS: A total of 8 patients were found with a prolonged QT interval. JLNS was diagnosed in 4 (0.9%) patients. In addition, 2 students had already been diagnosed with JLNS at another center earlier. The other 2 students, being siblings, were newly diagnosed with JLNS; and appropriate treatment was initiated. Genetic testing revealed a pathological homozygous mutation in KCNQ1 gene. The younger sibling (Case 1), who possessed a QTc of greater than 500 ms and a history of syncope, which was very suspicious for SCD, was implanted an implantable cardioverter-defibrillator. Propranolol treatment was initiated for both siblings. CONCLUSION: JLNS should be carefully considered and screened, especially in patients with a history of congenital deafness.

Ochratoxin A Sequentially Activates Autophagy and the Ubiquitin-Proteasome System.

Ochratoxin A (OTA) is a carcinogenic mycotoxin, which is produced by Aspergillus and Penicillium genera of fungi and commonly contaminates food and feed. We and others have previously shown that OTA causes sustained activation of PI3K/AKT and MAPK/ERK1-2 signaling pathways in different cell types and animal models. Given the close relationship between cellular signaling activity and protein stability, we were curious whether increased PI3K/AKT and MAPK/ERK1-2 signaling may be the result of OTA-stimulated alterations in proteolytic activity. We show that both of the major proteolytic systems, autophagy, and the ubiquitin-proteasome system (UPS), are activated upon OTA exposure in human kidney proximal tubule HK-2 and mouse embryonic fibroblast (MEF) cells. OTA stimulates transient autophagic activity at early time points of treatment but autophagic activity subsides after 6 h even in the sustained presence of OTA. Interestingly, OTA exposure also results in increased cell death in wild-type MEF cells but not in autophagy-halted Atg5-deficient cells, suggesting that autophagy exerts a pro-death effect on OTA-induced cytotoxicity. In addition, prolonged OTA exposure decreased ubiquitinated protein levels by increasing proteasomal activity. Using purified and cellular proteasomes, we observed enhanced chymotrypsin-, caspase-, and trypsin-like activities of the 26S but not the 20S proteasome in the presence of OTA. However, in the cellular context, increased proteasomal activity depended on prior induction of autophagy. Our results suggest that autophagy and subsequent UPS activation are responsible for sustained activation of PI3K/AKT and MAPK/ERK1-2 pathways through regulating the levels of critical phosphatases VHR/DUSP3, DUSP4, and PHLPP, which are known to be involved in OTA toxicity and carcinogenicity.

A feedback transcriptional mechanism controls the level of the arginine/lysine transporter cat-1 during amino acid starvation.

The adaptive response to amino acid limitation in mammalian cells inhibits global protein synthesis and promotes the expression of proteins that protect cells from stress. The arginine/lysine transporter, cat-1, is induced during amino acid starvation by transcriptional and post-transcriptional mechanisms. It is shown in the present study that the transient induction of cat-1 transcription is regulated by the stress response pathway that involves phosphorylation of the translation initiation factor, eIF2 (eukaryotic initiation factor-2). This phosphorylation induces expression of the bZIP (basic leucine zipper protein) transcription factors C/EBP (CCAAT/enhancer-binding protein)-beta and ATF (activating transcription factor) 4, which in turn induces ATF3. Transfection experiments in control and mutant cells, and chromatin immunoprecipitations showed that ATF4 activates, whereas ATF3 represses cat-1 transcription, via an AARE (amino acid response element), TGATGAAAC, in the first exon of the cat-1 gene, which functions both in the endogenous and in a heterologous promoter. ATF4 and C/EBPbeta activated transcription when expressed in transfected cells and they bound as heterodimers to the AARE in vitro. The induction of transcription by ATF4 was inhibited by ATF3, which also bound to the AARE as a heterodimer with C/EBPbeta. These results suggest that the transient increase in cat-1 transcription is due to transcriptional activation caused by ATF4 followed by transcriptional repression by ATF3 via a feedback mechanism.

[A rare anomaly presented with symptoms of acute abdomen: a third kidney located at right iliac fossa]. / Akut karin tablosu ile kendini gösteren nadir bir anomali: Sag pelvik yerlesimli üçüncü böbrek.

Supernumerary kidney is an extremely rare congenital anomaly of the urinary tract. A 39 year-old woman was admitted to the emergency unit with right lower quadrant pain. Physical examination revealed abdominal tenderness and defense on palpation of the right pelvic region. Blood and urine analysis revealed leukocytosis and urinary tract infection. Ultrasound examination demonstrated a suspicious mass which was thought to be a pseudokidney in the pelvis. Native kidneys were found in the normal anatomic position. Further investigation with computed tomography demonstrated a functioning third kidney which has located at the right iliac fossa in addition to normal excreting right and left kidneys. Acute abdomen like symptoms were secondary to the urinary infection of the third kidney and the urinary infection was successfully treated by antimicrobial and anti-inflammatory medication. We conclude that the infection of pelvic supernumerary kidney may create clinical symptoms of acute abdomen. Although extremely rare, congenital anomalies like supernumerary pelvic kidney should be included in the differential diagnosis.

Metal levels in Trachurus trachurus and Cyprinus carpio in Turkey.

Concentrations of five toxic metals were determined in two fish species from Turkish cities during 2010-2011. The obtained lead concentrations for all of the studied Trachurus trachurus (mean 777 µg kg(-1)) and Cyprinus carpio (mean 439 µg kg(-1)) samples were found to be higher than the maximum level (ML) of 300 µg kg(-1), while Cd concentrations in the same samples were lower than the ML. Mean chromium (501 µg kg(-1)), Ni (272 µg kg(-1)) and Cu (785 µg kg(-1)) concentrations in T. trachurus were significantly higher than in C. carpio (336 µg Cr kg(-1), 229 µg Ni kg(-1) and 394 µg Cu kg(-1)), similar to those of Pb and Cd. Measured Pb concentrations in T. trachurus tissues are significantly higher than the ML, while those of Cd in both T. trachurus and C. carpio species were lower than the ML values.

The hnRNA-binding proteins hnRNP L and PTB are required for efficient translation of the Cat-1 arginine/lysine transporter mRNA during amino acid starvation.

The response to amino acid starvation involves the global decrease of protein synthesis and an increase in the translation of some mRNAs that contain an internal ribosome entry site (IRES). It was previously shown that translation of the mRNA for the arginine/lysine amino acid transporter Cat-1 increases during amino acid starvation via a mechanism that utilizes an IRES in the 5' untranslated region of the Cat-1 mRNA. It is shown here that polypyrimidine tract binding protein (PTB) and an hnRNA binding protein, heterogeneous nuclear ribonucleoprotein L (hnRNP L), promote the efficient translation of Cat-1 mRNA during amino acid starvation. Association of both proteins with Cat-1 mRNA increased during starvation with kinetics that paralleled that of IRES activation, although the levels and subcellular distribution of the proteins were unchanged. The sequence CUUUCU within the Cat-1 IRES was important for PTB binding and for the induction of translation during amino acid starvation. Binding of hnRNP L to the IRES or the Cat-1 mRNA in vivo was independent of PTB binding but was not sufficient to increase IRES activity or Cat-1 mRNA translation during amino acid starvation. In contrast, binding of PTB to the Cat-1 mRNA in vivo required hnRNP L. A wider role of hnRNP L in mRNA translation was suggested by the decrease of global protein synthesis in cells with reduced hnRNP L levels. It is proposed that PTB and hnRNP L are positive regulators of Cat-1 mRNA translation via the IRES under stress conditions that cause a global decrease of protein synthesis.

Ribosome stalling regulates IRES-mediated translation in eukaryotes, a parallel to prokaryotic attenuation.

It was previously shown that the mRNA for the cat-1 Arg/Lys transporter is translated from an internal ribosome entry site (IRES) that is regulated by cellular stress. Amino acid starvation stimulated cat-1 translation via a mechanism that requires translation of an ORF in the mRNA leader and remodeling of the leader to form an active IRES (the "zipper model" of translational control). It is shown here that slowing of the leader peptide elongation rate, either by cycloheximide or the introduction of rare codons, stimulated translation of the downstream ORF. These results suggest that ribosome stalling in the upstream ORF causes mRNA remodeling and formation of an active IRES. This control is reminiscent of translation attenuation in prokaryotic operons, where inhibition of translation elongation can regulate both mRNA translation and gene transcription by altering mRNA structure.

Serum intercellular adhesion molecule-1 levels in acute rheumatic fever.

In order to determine the role of soluble intercellular adhesion molecule-1 (sICAM-1) in the pathogenesis and course of acute rheumatic fever (ARF), serum levels of ICAM-1 were measured in 30 patients at onset of ARF, in remission and during inactive periods of the disease (group 1), in 20 patients who had had ARF at least a year beforehand and had no evidence of exacerbation of the disease (group 2) and in 20 healthy children. Serum levels of sICAM-1 were increased in group 1, peaking in the active phase of the disease and declining during remission to the inactive phase of the disease when they were still significantly higher than in the controls, despite their ESR and fibrinogen levels having fallen to normal limits. The levels in group 2 were similar to those in the healthy controls. We consider that ICAM-1 plays a role in the pathogenesis of ARF and that improvement of ARF might best be determined by serum sICAM-1 levels, even when clinical and other laboratory test results have returned to normal, but additional studies are needed to clarify this hypothesis.

Regulation of cationic amino acid transport: the story of the CAT-1 transporter.

The discovery of the function of the receptor for the ecotropic retrovirus as a membrane transporter for the essential amino acids lysine and arginine was a landmark finding in the field of molecular nutrition. This finding indicated that cationic amino acid transporters (CATs) act pathologically as viral receptors. The importance of this transporter was further supported by knockout mice that were not viable after birth. CAT-1 was the first amino acid transporter to be cloned; several other CATs were later characterized biochemically and molecularly. These transporters mediate the bidirectional transport of cationic amino acids, thus supporting important metabolic functions, such as synthesis of proteins, nitric oxide (NO) synthesis, polyamine biosynthesis, and interorgan amino acid flow. This review briefly describes the advances in the regulation of cationic amino acid transport, focusing on the molecular mechanisms that regulate the CAT-1 transporter. Of particular interest to this review is the regulation of CAT-1 by nutritional stresses, such as amino acid availability. The studies that are reviewed conclude that the CAT-1 gene is essential for cell survival during stress because it allows cells to resume growth as soon as amino acids become available.

Regulation of internal ribosomal entry site-mediated translation by phosphorylation of the translation initiation factor eIF2alpha.

Initiation of translation from most cellular mRNAs occurs via scanning; the 40 S ribosomal subunit binds to the m(7)G-cap and then moves along the mRNA until an initiation codon is encountered. Some cellular mRNAs contain internal ribosome entry sequences (IRESs) within their 5'-untranslated regions, which allow initiation independently of the 5'-cap. This study investigated the ability of cellular stress to regulate the activity of IRESs in cellular mRNAs. Three stresses were studied that cause the phosphorylation of the translation initiation factor, eIF2alpha, by activating specific kinases: (i) amino acid starvation, which activates GCN2; (ii) endoplasmic reticulum (ER) stress, which activates PKR-like ER kinase, PERK kinase; and (iii) double-stranded RNA, which activates double-stranded RNA-dependent protein kinase (PKR) by mimicking viral infection. Amino acid starvation and ER stress caused transient phosphorylation of eIF2alpha during the first hour of treatment, whereas double-stranded RNA caused a sustained phosphorylation of eIF2alpha after 2 h. The effects of these treatments on IRES-mediated initiation were investigated using bicistronic mRNA expression vectors. No effect was seen for the IRESs from the mRNAs for the chaperone BiP and the protein kinase Pim-1. In contrast, translation mediated by the IRESs from the cationic amino acid transporter, cat-1, and of the cricket paralysis virus intergenic region, were stimulated 3- to 10-fold by all three treatments. eIF2alpha phosphorylation was required for the response because inactivation of phosphorylation prevented the stimulation. It is concluded that cellular stress can stimulate translation from some cellular IRESs via a mechanism that requires the phosphorylation of eIF2alpha. Moreover, there are distinct regulatory patterns for different cellular mRNAs that contain IRESs within their 5'-untranslated regions.

Nutritional control of mRNA stability is mediated by a conserved AU-rich element that binds the cytoplasmic shuttling protein HuR.

The cationic amino acid transporter, Cat-1, is a high affinity transporter of the essential amino acids, arginine and lysine. Expression of the cat-1 gene increases during nutritional stress as part of the adaptive response to starvation. Amino acid limitation induces coordinate increases in stability and translation of the cat-1 mRNA, at a time when global protein synthesis decreases. It is shown here that increased cat-1 mRNA stability requires an 11 nucleotide AU-rich element within the distal 217 bases of the 3'-untranslated region. When this 217-nucleotide nutrient sensor AU-rich element (NS-ARE) is present in a chimeric mRNA it confers mRNA stabilization during amino acid starvation. HuR is a member of the ELAV family of RNA-binding proteins that has been implicated in regulating the stability of ARE-containing mRNAs. We show here that the cytoplasmic concentration of HuR increases during amino acid starvation, at a time when total cellular HuR levels decrease. In addition, RNA gel shift experiments in vitro demonstrated that HuR binds to the NS-ARE and binding was dependent on the 11 residue AU-rich element. Moreover, HuR binding to the NS-ARE in extracts from amino acid-starved cells increased in parallel with the accumulation of cytoplasmic HuR. It is proposed that an adaptive response of cells to nutritional stress results in increased mRNA stability mediated by HuR binding to the NS-ARE.

Regulation of internal ribosome entry site-mediated translation by eukaryotic initiation factor-2alpha phosphorylation and translation of a small upstream open reading frame.

Adaptation to amino acid deficiency is critical for cell survival. In yeast, this adaptation involves phosphorylation of the translation eukaryotic initiation factor (eIF) 2alpha by the kinase GCN2. This leads to the increased translation of the transcription factor GCN4, which in turn increases transcription of amino acid biosynthetic genes, at a time when expression of most genes decreases. Here it is shown that translation of the arginine/lysine transporter cat-1 mRNA increases during amino acid starvation of mammalian cells. This increase requires both GCN2 phosphorylation of eIF2alpha and the translation of a 48-amino acid upstream open reading frame (uORF) present within the 5'-leader of the transporter mRNA. When this 5'-leader was placed in a bicistronic mRNA expression vector, it functioned as an internal ribosomal entry sequence and its regulated activity was dependent on uORF translation. Amino acid starvation also induced translation of monocistronic mRNAs containing the cat-1 5'-leader, in a manner dependent on eIF2alpha phosphorylation and translation of the 48-amino acid uORF. This is the first example of mammalian regulation of internal ribosomal entry sequence-mediated translation by eIF2alpha phosphorylation during amino acid starvation, suggesting that the mechanism of induced Cat-1 protein synthesis is part of the adaptive response of cells to amino acid limitation.

Transcriptional control of the arginine/lysine transporter, cat-1, by physiological stress.

Cells respond to physiological stress by phosphorylating the alpha subunit of the translation initiation factor eIF2. This adaptive response inhibits protein synthesis and up-regulates genes essential for cell survival. Cat-1, the transporter for the essential amino acids, arginine and lysine, is one of the up-regulated genes. We previously showed that stress increases cat-1 expression by coordinated stabilization of the mRNA and increased mRNA translation. This induction is triggered by amino acid depletion and the unfolded protein response (UPR), which is caused by unfolded proteins in the endoplasmic reticulum. We show here that cat-1 gene transcription is also increased by cellular stress. Our studies demonstrate that the cat-1 gene promoter/regulatory region is TATA-less and is located in a region that includes 94 bases of the first exon. Transcription from this promoter is stimulated 8-fold by cellular stress. An amino acid response element within the first exon is shown to be required for the response to amino acid depletion but not to the UPR. The stimulation of transcription by amino acid depletion requires activation of GCN2 kinase, which phosphorylates eIF2alpha. This phosphorylation also induces translation of the cat-1 mRNA, demonstrating that stress-induced transcriptional and translational control of cat-1 are downstream targets of a signaling pathway initiating with eIF2alpha phosphorylation. Our studies show that the increase in cat-1 gene expression by cellular stress involves at least three types of coordinate regulation: regulation of transcription, regulation of mRNA stability, and regulation of mRNA translation.

The zipper model of translational control: a small upstream ORF is the switch that controls structural remodeling of an mRNA leader.

Transport of the essential amino acids arginine and lysine is critical for the survival of mammalian cells. The adaptive response to nutritional stress involves increased translation of the arginine/lysine transporter (cat-1) mRNA via an internal ribosome entry site (IRES) within the mRNA leader. Induction of cat-1 IRES activity requires both translation of a small upstream open reading frame (uORF) within the IRES and phosphorylation of the translation initiation factor eIF2alpha. We show here that translation of the upstream ORF unfolds an inhibitory structure in the mRNA leader, eliciting a conformational change that yields an active IRES. The IRES, whose activity is induced by amino acid starvation, is created by RNA-RNA interactions between the 5' end of the leader and downstream sequences. This study suggests that the structure of the IRES is dynamic and regulation of this RNA structure is a mechanism of translational control.