A GC box in the human sodium iodide symporter gene promoter is essential for full activity

We previously reported that the human sodium iodide symporter (hNIS) 5'-flanking region between -603 and -415 is essential for full expression. In this study, we further localized sequences within this region required for the basal expression of the hNIS promoter and identified a functional GC box. Activity of the hNIS promoter was assessed by transient transfection of luciferase reporter gene constructs with progressive 5' deletions into the human papillary thyroid cancer cell line BHP 2-7. Deletion from -603 to -535 enhanced promoter activity, further deletion to -469 decreased promoter activity, and deletion to -415 nearly abolished promoter activity. The DNA sequence within this critical 55 bp region from -469 to -415 contains a GC box. Introduction of mutations into the GC box of the deletion constructs -603, -535 and -469 decreased promoter activity in both thyroid cells (BHP 2-7 and rat thyroid cell FRTL-5) and nonthyroid cells (human prostate cancer cell LNCaP-2 and human breast cancer cell MCF-7). The magnitude of reduction for the -603 mutation construct was significantly greater than that for the -469 mutation construct in thyroid cells compared to non-thyroid cells. In vitro transcription using nuclear extracts isolated from HeLa cells was reduced from DNA templates with the GC box mutation and nearly abolished from templates with 5' deletion to -415. Identification of proteins interacting with the GC box was performed by gel retardation assays with or without Sp1 specific antibodies. Sp1 and an "Sp1-like" protein bound to the wild-type GC box sequence but not to the GC box mutant. In summary, the GC box is a positive regulatory element in the hNIS basal promoter.

Respiration-dependent utilization of sugars in yeasts: a determinant role for sugar transporters

In many yeast species, including Kluyveromyces lactis, growth on certain sugars (such as galactose, raffinose, and maltose) occurs only under respiratory conditions. If respiration is blocked by inhibitors, mutation, or anaerobiosis, growth does not take place. This apparent dependence on respiration for the utilization of certain sugars has often been suspected to be associated with the mechanism of the sugar uptake step. We hypothesized that in many yeast species, the permease activities for these sugars are not sufficient to ensure the high substrate flow that is necessary for fermentative growth. By introducing additional sugar permease genes, we have obtained K. lactis strains that were capable of growing on galactose and raffinose in the absence of respiration. High dosages of both the permease and maltase genes were indeed necessary for K. lactis cells to grow on maltose in the absence of respiration. These results strongly suggest that the sugar uptake step is the major bottleneck in the fermentative assimilation of certain sugars in K. lactis and probably in many other yeasts.

Role of reactive oxygen species in apoptosis induced by N-ethylmaleimide in HepG2 human hepatoblastoma cells

We have previously reported that N-ethylmaleimide induces apoptosis through activation of K(+), Cl(-)-cotransport in HepG2 human hepatoblastoma cells. In this study, we investigated the role for reactive oxygen species as a mediator of the apoptosis induced by N-ethylmaleimide. N-ethylmaleimide induced a significant elevation of intracellular level of reactive oxygen species. Treatment with antioxidants (N-acetyl cysteine, N,N'-diphenyl-p-phenylenediamine) which markedly suppressed generation of reactive oxygen species, significantly inhibited the N-ethylmaleimide-induced activation of K(+), Cl(-)-cotransport and apoptosis. Inhibitors of NADPH oxidase (diphenylene iodonium, apocynin, D-(+)-neopterine) also significantly blunted the generation of reactive oxygen species, activation of K(+), Cl(-)-cotransport and apoptosis induced by N-ethylmaleimide. These results suggest that reactive oxygen species generated through activation of NADPH oxidase may play a role in the N-ethylmaleimide-induced stimulation of K(+), Cl(-)-cotransport and apoptosis in HepG2 cells.

The KCl cotransporter isoform KCC3 can play an important role in cell growth regulation

The KCl cotransporter (KCC) plays a significant role in the ionic and osmotic homeostasis of many cell types. Four KCC isoforms have been cloned. KCC1 and KCC4 activity is osmolality-sensitive and involved in volume regulation. KCC2, a neuronal-specific isoform, can lower intracellular Cl(-) and is critical for inhibitory GABA responses in the mature central nervous system. KCC3, initially cloned from vascular endothelial cells, is widely but not universally distributed and has an unknown physiological significance. Here we show a tight link between the expression and activity of KCC3 and cell growth by a NIH/3T3 fibroblast expression system. KCC3 activity is sensitive to [(dihydroindenyl)oxy] alkanoic acid (DIOA) and N-ethylmaleimide and is regulated by tyrosine phosphorylation. Osmotic swelling does not activate KCC3, and the process of regulatory volume decrease is refractory to DIOA, indicating that KCC3 is not involved in volume regulation. KCC3 expression enhances cell proliferation, and this growth advantage can be abolished by the inhibition of KCC3 by DIOA. Fluorescence-activated cell sorting measurements and Western blot analysis show DIOA caused a significant reduction of the cell fraction in proliferative phase and a change in phosphorylation of retinoblastoma protein (Rb) and cdc2, suggesting that KCC3 activity is important for cell cycle progression. Insulin-like growth factor-1 up-regulates KCC3 expression and stimulates cell growth. Tumor necrotic factor-alpha down-regulates KCC3 expression and causes growth arrest. These data indicate that KCC3 is an important KCC isoform that may be involved in cell proliferation.

A dominant negative mutant of the KCC1 K-Cl cotransporter: both N- and C-terminal cytoplasmic domains are required for K-Cl cotransport activity

K-Cl cotransport regulates cell volume and chloride equilibrium potential. Inhibition of erythroid K-Cl cotransport has emerged as an important adjunct strategy for the treatment of sickle cell anemia. However, structure-function relationships among the polypeptide products of the four K-Cl cotransporter (KCC) genes are little understood. We have investigated the importance of the N- and C-terminal cytoplasmic domains of mouse KCC1 to its K-Cl cotransport function expressed in Xenopus oocytes. Truncation of as few as eight C-terminal amino acids (aa) abolished function despite continued polypeptide accumulation and surface expression. These C-terminal loss-of-function mutants lacked a dominant negative phenotype. Truncation of the N-terminal 46 aa diminished function. Removal of 89 or 117 aa (Delta(N)117) abolished function despite continued polypeptide accumulation and surface expression and exhibited dominant negative phenotypes that required the presence of the C-terminal cytoplasmic domain. The dominant negative loss-of-function mutant Delta(N)117 was co-immunoprecipitated with wild type KCC1 polypeptide, and its co-expression did not reduce wild type KCC1 at the oocyte surface. Delta(N)117 also exhibited dominant negative inhibition of human KCC1 and KCC3 and, with lower potency, mouse KCC4 and rat KCC2.

Evidence supporting a role for KCl cotransporter in the thick ascending limb of Henle's loop

BACKGROUND

A basolateral Ba(2+)-sensitive KCl cotransporter has previously been proposed as participating in basolateral K+ recycling and transepithelial NaCl reabsorption in the thick ascending limb of Henle's loop (TAL). The aim of the present study was to answer the question as to whether this cotransporter plays a role in transepithelial K+ reabsorption and whether dietary Mg(2+) deficiency, known to regulate the KCl cotransporter in erythrocytes, also regulates KCl transport in the TAL.

METHODS

The effects of a low-Mg(2+) diet were investigated on urinary and plasma K+ concentration in control mice and Mg(2+)-deficient mice. Transepithelial Na+, Cl- and K+ net fluxes (J(Na), J(Cl), J(K)), determined in isolated perfused TALs with electron probe analysis or cation-exchange high-performance liquid chromatography (HPLC) and electrophysiological parameters (V(te), R(te)), were measured in both animal groups. Expression of transcripts for the KCl cotransporter and its possible regulation by low-Mg(2+) were studied by RT-PCR in microdissected mouse cortical TAL (CTAL) and medullary TAL (MTAL) segments.

RESULTS

In isolated perfused CTALs, basolateral Ba(2+) and amiloride induced a large K+ net secretion towards the tubular lumen, paralleled by a 50% decrease in transepithelial NaCl reabsorption. KCC1 transcripts were found in the mouse CTAL and MTAL. A low-Mg(2+) diet led to diminished urinary K+ excretion, lowered plasma K+ concentration and up-regulation of KCC1 transcripts in the TAL. For low-Mg(2+) diet, this upregulation was associated with increased transepithelial K+ reabsorption in the in vitro-perfused CTAL.

CONCLUSIONS

Our study provides evidence that the KCl cotransporter, which is functionally expressed in the TAL, plays an important role in transepithelial K+ reabsorption. Direct inhibition of this transporter by Ba(2+) and its indirect inhibition by amiloride lead to a strong transepithelial K+ secretion and diminished NaCl reabsorption in the TAL. Up-regulation of KCC1 mRNA by dietary Mg(2+) restriction is associated with an increased K+ reabsorption in the in vitro perfused CTAL.

Chronic and acute regulation of Na+/Cl- -dependent neurotransmitter transporters: drugs, substrates, presynaptic receptors, and signaling systems

Na+/Cl- -dependent neurotransmitter transporters, which constitute a gene superfamily, are crucial for limiting neurotransmitter activity. Thus, it is critical to understand their regulation. This review focuses primarily on the norepinephrine transporter, the dopamine transporter, the serotonin transporter, and the gamma-aminobutyric acid transporter GAT1. Chronic administration of drugs that alter neurotransmitter release or inhibit transporter activity can produce persistent compensatory changes in brain transporter number and activity. However, regulation has not been universally observed. Transient alterations in norepinephrine transporter, dopamine transporter, serotonin transporter, and GAT1 function and/or number occur in response to more acute manipulations, including membrane potential changes, substrate exposure, ethanol exposure, and presynaptic receptor activation/inhibition. In many cases, acute regulation has been shown to result from a rapid redistribution of the transporter between the cell surface and intracellular sites. Second messenger systems involved in this rapid regulation include protein kinases and phosphatases, of which protein kinase C has been the best characterized. These signaling systems share the common characteristic of altering maximal transport velocity and/or cell surface expression, consistent with regulation of transporter trafficking. Although less well characterized, arachidonic acid, reactive oxygen species, and nitric oxide also alter transporter function. In addition to post-translational modifications, cytoskeleton interactions and transporter oligomerization regulate transporter activity and trafficking. Furthermore, promoter regions involved in transporter transcriptional regulation have begun to be identified. Together, these findings suggest that Na+/Cl- -dependent neurotransmitter transporters are regulated both long-term and in a more dynamic manner, thereby providing several distinct mechanisms for altering synaptic neurotransmitter concentrations and neurotransmission.

Ontogeny of renal phosphate transport and the process of growth

The kidneys of infants and children reabsorb a high fraction of the filtered phosphate (Pi), as appropriate to the needs of a growing organism. This high Pi reabsorptive rate is associated with a high capacity (Vmax) of the Na+-Pi symport system. At the molecular level this high reabsorptive capacity appears to be due to the presence of a growth-specific Na-Pi cotransporter. Several experimental findings support this assumption. Firstly, the expression of NaPi-2 mRNA is, if anything, lower in the renal cortex of young animals than of adult animals. Secondly, polyA RNA obtained from growing animals depleted of NaPi-2 by specific hybridization with an antisense 16-mer induces Na+-Pi transport in oocytes. No induction of Na+-Pi transport was observed in oocytes injected with hybridized polyA RNA obtained from adult animals. Thirdly, polyA RNA derived from young rats, depleted of NaPi-2 by subtractive hybridization with adult animal renal cortical cDNA, retains its ability to encode for Na+-Pi cotransport in oocytes. Adult animal renal cortical polyA RNA, depleted of NaPi-2 by subtractive hybridization, failed to induce Na+-Pi uptake into oocytes. Fourthly, renal cortical polyA RNA from young animals, depleted of NaPi-2, contains a region that is highly homologous (80%-92%) with the corresponding region of other modulated NaPi (type II) transporters. Fifthly, this region is also present in the polyA RNA obtained from the renal cortex of newborn rats (1st week of life), despite the fact that NaPi-2 is absent at this early age. Lastly, Npt2 (-/-) knockout mice, although hypophosphatemic and phosphaturic, filter and reabsorb Pi at rates exceeding those that can be accounted for by the expression of type I and III transporters. Based on these observations it is reasonable to surmise that the high Vmax of the Na+-Pi cotransport system observed in the young is due to a large extent to the presence of a growth-specific NaPi transporter, homologous but not identical to already cloned type II NaPi transporters.

Molecular aspects in the regulation of renal inorganic phosphate reabsorption: the type IIa sodium/inorganic phosphate co-transporter as the key player

The type IIa sodium/inorganic phosphate co-transporter is the rate-limiting inorganic phosphate transport pathway in renal brush-border membranes, and is thus a key player in overall inorganic phosphate homeostasis. Its regulation is mostly associated with membrane retrieval/reinsertion (traffic) of the transport protein. This membrane traffic is controlled by specific 'motifs' at the level of the transporter protein and probably involves interacting proteins (e.g. for scaffolding, regulation or sorting). The intracellular signaling mechanisms (e.g. the involvement of kinases) and the involvement of the cytoskeleton are not yet understood. Hereditary alterations in renal inorganic phosphate handling can be associated with factors controlling the expression of the brush-border type IIa sodium/inorganic phosphate co-transporter.

The sympathetic neurobiology of essential hypertension: disparate influences of obesity, stress, and noradrenaline transporter dysfunction?

Although the importance of sympathetic nervous activation in the pathogenesis of essential hypertension is well documented, the exact pathophysiology of the sympathetic nervous dysfunction present remains to be delineated. This review details three relatively new findings of disturbed sympathetic neurobiology in hypertension. Adrenaline cotransmission is present in the cardiac sympathetic nerves of patients with essential hypertension, as it is in patients with panic disorder, providing presumptive evidence of exposure to high levels of mental stress in hypertensive patients. In lean patients with hypertension there is also evidence of faulty noradrenaline reuptake into the sympathetic nerves of the heart, an abnormality amplifying the sympathetic neural signal by impairing removal of noradrenaline from the synaptic cleft. If both abnormalities are present in the sympathetic nerves of the kidneys also (which we did not test), there would most probably be a direct contribution to hypertension development. In the kidneys the causal chain between sympathetic overactivity and the development of hypertension is stronger than for the heart. In obesity-related hypertension there is evidence that renal sympathetic tone is high, based on approximately a doubling of the measured rate of spillover of noradrenaline into the renal veins. This increase in sympathetic outflow to the kidneys appears to be a necessary but apparently not a sufficient cause for the development of clinical hypertension, commonly being present also in overweight people with blood pressure in the normotensive range. High renal sympathetic tone in the latter, of course, may well still contribute to elevation of their pressure level, although not on such a scale as to cause clinical hypertension.

PEPT2-mediated uptake of neuropeptides in rat choroid plexus

PURPOSE

The peptide transporter PEPT2 was recently shown to be functionally active in rat choroid plexus, suggesting that it may play a role in neuropeptide homeostasis in the cerebrospinal fluid. This study, therefore, examined the role of PEPT2 in mediating neuropeptide uptake into choroid plexus.

METHODS

Whole-tissue rat choroid plexus uptake studies were performed on GlySar in the absence and presence of neuropeptides and on carnosine.

RESULTS

The neuropeptides NAAG, CysGly, GlyGln, kyotorphin, and carnosine inhibited the uptake of radiolabeled GlySar at 1.0 mM concentrations. In contrast, TRH, [D-Arg2]-kyotorphin, glutathione, and homocarnosine did not inhibit GlySar uptake. Kyotorphin, an analgesic, was a competitive inhibitor of GlySar with a Ki of 8.0 microM. The direct uptake of carnosine was also shown to be mediated by PEPT2 in isolated choroid plexus (Km = 39.3 microM; Vmax = 73.9 pmol/mg/min). Radiolabeled carnosine uptake was inhibited by 1.0 mM concentrations of GlySar or carnosine but not homocarnosine, L-histidine, or beta-alanine.

CONCLUSIONS

These findings indicate that PEPT2 mediates the uptake of a diverse group of neuropeptides in choroid plexus, and suggests a role for PEPT2 in the regulation of neuropeptides, peptide fragments, and peptidomimetics in cerebrospinal fluid.