COUP-TFI and COUP-TFII regulate expression of the NHE through a nuclear hormone responsive element with enhancer activity

Isoforms of the orphan nuclear receptor COUP‑TFII differentially modulate pancreatic cancer progression

The expression of the nuclear receptor transcription factor (TF) COUP‑TFII is broadly associated with cell differentiation and cancer development, including of pancreatic ductal adenocarcinoma (PDAC), a devastating disease with one of the poorest prognoses among cancers worldwide. Recent studies have started to investigate the pathological and physiological roles of a novel COUP‑TFII isoform (COUP‑TFII_V2) that lacks the DNA‑binding domain. As the role of the canonical COUP‑TFII in PDAC was previously demonstrated, the present study evaluated whether COUP‑TFII_V2 may have a functional role in PDAC. It was demonstrated that COUP‑TFII_V2 naturally occurs in PDAC cells and in primary samples, where its expression is consistent with shorter overall survival and peripheral invasion. Of note, COUP‑TFII_V2, exhibiting nuclear and cytosolic expression, is linked to epithelial to mesenchymal transition (EMT) and cancer progression, as confirmed by nude mouse experiments. The present results demonstrated that COUP‑TFII_V2 distinctively regulates the EMT of PDAC and, similarly to its sibling, it is associated with tumor aggressiveness. The two isoforms have both overlapping and exclusive functions that cooperate with cancer growth and dissemination. By studying how PDAC cells switch from one isoform to the other, novel insight into cancer biology was gained, indicating that this receptor may serve as a novel possible target for PDAC management.

SLC9 Gene Family: Function, Expression, and Regulation

The Slc9 family of Na⁺ /H⁺ exchangers (NHEs) plays a critical role in electroneutral exchange of Na⁺ and H⁺ in the mammalian intestine as well as other absorptive and secretory epithelia of digestive organs. These transport proteins contribute to the transepithelial Na⁺ and water absorption, intracellular pH and cellular volume regulation as well as the electrolyte, acid-base, and fluid volume homeostasis at the systemic level. They also influence the function of other membrane transport mechanisms, affect cellular proliferation and apoptosis as well as cell migration, adherence to the extracellular matrix, and tissue repair. Additionally, they modulate the extracellular milieu to facilitate other nutrient absorption and to regulate the intestinal microbial microenvironment. Na⁺ /H⁺ exchange is inhibited in selected gastrointestinal diseases, either by intrinsic factors (e.g., bile acids, inflammatory mediators) or infectious agents and associated bacterial toxins. Disrupted NHE activity may contribute not only to local and systemic electrolyte imbalance but also to the disease severity via multiple mechanisms. In this review, we describe the cation proton antiporter superfamily of Na⁺ /H⁺ exchangers with a particular emphasis on the eight SLC9A isoforms found in the digestive tract, followed by a more integrative description in their roles in each of the digestive organs. We discuss regulatory mechanisms that determine the function of Na⁺ /H⁺ exchangers as pertinent to the digestive tract, their regulation in pathological states of the digestive organs, and reciprocally, the contribution of dysregulated Na⁺ /H⁺ exchange to the disease pathogenesis and progression. © 2018 American Physiological Society. Compr Physiol 8:555-583, 2018.

The orphan nuclear receptor COUP-TFII coordinates hypoxia-independent proangiogenic responses in hepatic stellate cells

BACKGROUND & AIMS

Hepatic stellate cell (HSC) transdifferentiation into collagen-producing myofibroblasts is a key event in hepatic fibrogenesis, but the transcriptional network that controls the acquisition of the activated phenotype is still poorly understood. In this study, we explored whether the nuclear receptor chicken ovalbumin upstream promoter-transcription factor II (COUP-TFII) is involved in HSC activation and in the multifunctional role of these cells during the response to liver injury.

METHODS

COUP-TFII expression was evaluated in normal and cirrhotic livers by immunohistochemistry and Western blot. The role of COUP-TFII in HSC was assessed by gain and loss of function transfection experiments and by generation of mice with COUP-TFII deletion in HSC. Molecular changes were determined by gene expression microarray and RT-qPCR.

RESULTS

We showed that COUP-TFII is highly expressed in human fibrotic liver and in mouse models of hepatic injury. COUP-TFII expression rapidly increased upon HSC activation and it was associated with the regulation of genes involved in cell motility, proliferation and angiogenesis. Inactivation of COUP-TFII impairs proliferation and invasiveness in activated HSC and COUP-TFII deletion in mice abrogate HSC activation and angiogenesis. Finally, co-culture experiments with HSC and liver sinusoidal endothelial cells (SEC) showed that COUP-TFII expression in HSC influenced SEC migration and tubulogenesis via a hypoxia-independent and nuclear factor kappaB-dependent mechanism.

CONCLUSION

This study elucidates a novel transcriptional pathway in HSC that is involved in the acquisition of the proangiogenic phenotype and regulates the paracrine signals between HSC and SEC during hepatic wound healing.

LAY SUMMARY

In this study, we identified an important regulator of HSC pathobiology. We showed that the orphan receptor COUP-TFII is an important player in hepatic neoangiogenesis. COUP-TFII expression in HSC controls the crosstalk between HSC and endothelial cells coordinating vascular remodelling during liver injury.

TRANSCRIPT PROFILING

ArrayExpress accession E-MTAB-1795.

Atrial-like cardiomyocytes from human pluripotent stem cells are a robust preclinical model for assessing atrial-selective pharmacology

Drugs targeting atrial-specific ion channels, K_v1.5 or K_ir3.1/3.4, are being developed as new therapeutic strategies for atrial fibrillation. However, current preclinical studies carried out in non-cardiac cell lines or animal models may not accurately represent the physiology of a human cardiomyocyte (CM). In the current study, we tested whether human embryonic stem cell (hESC)-derived atrial CMs could predict atrial selectivity of pharmacological compounds. By modulating retinoic acid signaling during hESC differentiation, we generated atrial-like (hESC-atrial) and ventricular-like (hESC-ventricular) CMs. We found the expression of atrial-specific ion channel genes, KCNA5 (encoding Kv1.5) and KCNJ3 (encoding K_ir 3.1), in hESC-atrial CMs and further demonstrated that these ion channel genes are regulated by COUP-TF transcription factors. Moreover, in response to multiple ion channel blocker, vernakalant, and K_v1.5 blocker, XEN-D0101, hESC-atrial but not hESC-ventricular CMs showed action potential (AP) prolongation due to a reduction in early repolarization. In hESC-atrial CMs, XEN-R0703, a novel K_ir3.1/3.4 blocker restored the AP shortening caused by CCh. Neither CCh nor XEN-R0703 had an effect on hESC-ventricular CMs. In summary, we demonstrate that hESC-atrial CMs are a robust model for pre-clinical testing to assess atrial selectivity of novel antiarrhythmic drugs.

Na⁺/H⁺ exchangers and intracellular pH in perinatal brain injury

Encephalopathy consequent on perinatal hypoxia–ischemia occurs in 1–3 per 1,000 term births in the UK and frequently leads to serious and tragic consequences that devastate lives and families, with huge financial burdens for society. Although the recent introduction of cooling represents a significant advance, only 40 % survive with normal neurodevelopmental function. There is thus a significant unmet need for novel, safe, and effective therapies to optimize brain protection following brain injury around birth. The Na⁺/H⁺ exchanger (NHE) is a membrane protein present in many mammalian cell types. It is involved in regulating intracellular pH and cell volume. NHE1 is the most abundant isoform in the central nervous system and plays a role in cerebral damage after hypoxia–ischemia. Excessive NHE activation during hypoxia–ischemia leads to intracellular Na⁺ overload, which subsequently promotes Ca²⁺ entry via reversal of the Na⁺/Ca²⁺ exchanger. Increased cytosolic Ca²⁺ then triggers the neurotoxic cascade. Activation of NHE also leads to rapid normalization of pH_i and an alkaline shift in pH_i. This rapid recovery of brain intracellular pH has been termed pH paradox as, rather than causing cells to recover, this rapid return to normal and overshoot to alkaline values is deleterious to cell survival. Brain pH_i changes are closely involved in the control of cell death after injury: an alkalosis enhances excitability while a mild acidosis has the opposite effect. We have observed a brain alkalosis in 78 babies with neonatal encephalopathy serially studied using phosphorus-31 magnetic resonance spectroscopy during the first year after birth (151 studies throughout the year including 56 studies of 50 infants during the first 2 weeks after birth). An alkaline brain pH_i was associated with severely impaired outcome; the degree of brain alkalosis was related to the severity of brain injury on MRI and brain lactate concentration; and a persistence of an alkaline brain pH_i was associated with cerebral atrophy on MRI. Experimental animal models of hypoxia–ischemia show that NHE inhibitors are neuroprotective. Here, we review the published data on brain pH_i in neonatal encephalopathy and the experimental studies of NHE inhibition and neuroprotection following hypoxia–ischemia.

Enhancer/promoter activities of the long/middle wavelength-sensitive opsins of vertebrates mediated by thyroid hormone receptor β2 and COUP-TFII

Cone photopigments (opsins) are crucial elements of, and the first detection module in, color vision. Individual opsins have different wavelength sensitivity patterns, and the temporal and spatial expression patterns of opsins are unique and stringently regulated. Long and middle wavelength-sensitive (L/M) opsins are of the same phylogenetic type. Although the roles of thyroid hormone/TRß2 and COUP-TFs in the transcriptional regulation of L/M opsins have been explored, the detailed mechanisms, including the target sequence in the enhancer of L/M opsins, have not been revealed. We aimed to reveal molecular mechanisms of L/M opsins in vertebrates. Using several human red opsin enhancer/promoter-luciferase reporter constructs, we found that TRß2 increased luciferase activities through the 5'-UTR and intron 3-4 region, whereas the presence of T3 affected only the intron 3-4 region-dependent luciferase activity. Furthermore, COUP-TFII suppressed intron 3-4 region-dependent luciferase activities. However, luciferase expression driven by the mouse M opsin intron 3-4 region was only slightly increased by TRß2, and rather enhanced by COUP-TFII. To determine whether these differential responses reflect differences between primates and rodents, we examined the enhancer/promoter region of the red opsin of the common marmoset. Interestingly, while TRß2 increased 5'-UTR- or intron 3-4 region-driven luciferase expression, as observed for the human red opsin, expression of the latter luciferase was not suppressed by COUP-TFII. In fact, immunostaining of common marmoset retinal sections revealed expression of COUP-TFII and red opsin in the cone cells.

Multiple roles of COUP-TFII in cancer initiation and progression

Chicken ovalbumin upstream promoter transcription factor II (COUP-TFII) is an orphan nuclear receptor that acts as a transcriptional activator or repressor in a cell type-dependent manner. Best characterized for its role in the regulation of angiogenesis during mouse development, COUP-TFII also plays important roles in glucose metabolism and cancer. Expression of COUP-TFII is altered in various endocrine conditions. Cell type-specific functions and the regulation of COUP-TFII expression result in its varying physiological and pathological actions in diverse systems. Evidence will be reviewed for oncogenic and tumor-suppressive functions of COUP-TFII, with roles in angiogenesis, metastasis, steroidogenesis, and endocrine sensitivity of breast cancer described. The applicability of current data to our understanding of the role of COUP-TFII in cancer will be discussed.

Involvement of COUP-TFs in Cancer Progression

The orphan receptors COUP-TFI and COUP-TFII are members of the nuclear receptor superfamily that play distinct and critical roles in vertebrate organogenesis, as demonstrated by loss-of-function COUP-TFI and/or COUP-TFII mutant mice. Although COUP-TFs are expressed in a wide range of tissues in adults, little is known about their functions at later stages of development or in organism homeostasis. COUP-TFs are expressed in cancer cell lines of various origins and increasing studies suggest they play roles in cell fate determination and, potentially, in cancer progression. Nevertheless, the exact roles of COUP-TFs in these processes remain unclear and even controversial. In this review, we report both in vitro and in vivo data describing known and suspected actions of COUP-TFs that suggest that these factors are involved in modification of the phenotype of cancer cells, notably of epithelial origin.

Molecular biology of the myocardial Na+/H+ exchanger

The mammalian Na(+)/H(+) exchanger is a pH regulatory membrane protein that uses the sodium gradient to translocate one intracellular proton in exchange for one extracellular sodium. There are nine isoforms of the protein with varying tissue and cellular distribution, some isoforms are predominantly intracellular. In the myocardium, the Na(+)/H(+) exchanger type 1 isoform (NHE1) is the only plasma membrane isoform present in significant quantities. It plays an important role during ischemia/reperfusion damage to the myocardium and has recently been implicated in myocardial hypertrophy. The NHE1 gene is made from 12 exons and a differentially spliced version mediates Na(+)/Li(+) exchange. The NHE1 promoter is regulated by several transcription factors. In the myocardium, transcription factors both proximal and distal to the start site affect expression, including AP-2 and a thyroid responsive element. Recently, reactive oxygen species have also been shown to be important regulators of the NHE1 promoter. Structural and functional analysis of the NHE1 protein has shown that transmembrane segments IV, VII and IX are important in ion transport and susceptibility to pharmacological inhibition. NHE1 protein and mRNA levels are elevated by cardiac ischemia/reperfusion, hypertrophy and acidosis. Understanding the mechanism by which NHE1 mediates transport and its regulation of expression will give novel insights into its contributions in cardiovascular disease.

Structural and functional analysis of the Na+/H+ exchanger

The mammalian NHE (Na+/H+ exchanger) is a ubiquitously expressed integral membrane protein that regulates intracellular pH by removing a proton in exchange for an extracellular sodium ion. Of the nine known isoforms of the mammalian NHEs, the first isoform discovered (NHE1) is the most thoroughly characterized. NHE1 is involved in numerous physiological processes in mammals, including regulation of intracellular pH, cell-volume control, cytoskeletal organization, heart disease and cancer. NHE comprises two domains: an N-terminal membrane domain that functions to transport ions, and a C-terminal cytoplasmic regulatory domain that regulates the activity and mediates cytoskeletal interactions. Although the exact mechanism of transport by NHE1 remains elusive, recent studies have identified amino acid residues that are important for NHE function. In addition, progress has been made regarding the elucidation of the structure of NHEs. Specifically, the structure of a single TM (transmembrane) segment from NHE1 has been solved, and the high-resolution structure of the bacterial Na+/H+ antiporter NhaA has recently been elucidated. In this review we discuss what is known about both functional and structural aspects of NHE1. We relate the known structural data for NHE1 to the NhaA structure, where TM IV of NHE1 shows surprising structural similarity with TM IV of NhaA, despite little primary sequence similarity. Further experiments that will be required to fully understand the mechanism of transport and regulation of the NHE1 protein are discussed.

Zinc finger transcription factor Egr-1 is involved in stimulation of NHE2 gene expression by phorbol 12-myristate 13-acetate

The apical membrane Na(+)/H(+) exchanger isoforms NHE2 and NHE3 are involved in transepithelial Na(+) absorption in the intestine. However, they exhibit differences in their pattern of tissue expression and regulation of their activity by various molecular signals. To study the mechanisms involved in the transcriptional regulation of these genes, we characterized cis-acting elements within the human NHE2 promoter that regulate NHE2 promoter expression in C2BBe1 cells. A small DNA region (-85/+249) was involved in the regulation of basal transcriptional activity of the NHE2 promoter as determined by transient transfection assays. RT-PCR analysis showed that NHE2 mRNA was upregulated in response to phorbol 12-myristate 13-acetate (PMA). Results from actinomycin D-treated cells indicated that the regulation of the NHE2 gene by PMA occurs in part at the transcriptional level. Furthermore, PMA treatment led to a 100% increase in promoter activity through elements located on the -415/+249 DNA fragment. A PMA-induced nuclear factor that bound to the NHE2 promoter was identified as the transcription factor Egr-1. We identified two PMA response elements in the -415/+1 promoter region that bind to Sp1 and Sp3 in untreated nuclear extracts and to Egr-1 in PMA-treated nuclear extracts. In cotransfection experiments, Egr-1 was able to transactivate the NHE2 promoter. Our data indicate that Egr-1 may play a key role in regulated expression of the human NHE2 gene.

The pig CYP2E1 promoter is activated by COUP-TF1 and HNF-1 and is inhibited by androstenone

Functional analysis of the pig cytochrome P4502E1 (CYP2E1) promoter identified two major activating elements. One corresponded to the hepatic nuclear factor 1 (HNF-1) consensus binding sequence at nucleotides -128/-98 and the other was located in the region -292/-266. The binding of proteins in pig liver nuclear extracts to a synthetic double-stranded oligonucleotide corresponding to this more distal activating sequence was studied by electrophoretic mobility shift assay. The minimum protein binding sequence was identified as TGTTCTGACCTCTGGG. Gel super-shift assays identified the protein binding to this site as chick ovalbumin upstream promoter transcription factor 1 (COUP-TF1). Androstenone inhibited promoter activity in transfection experiments only with constructs which included the COUP-TF1 binding site. Androstenone inhibited COUP-TF1 binding to synthetic oligonucleotides but did not affect HNF-1 binding. The results offer an explanation for the inhibition of CYP2E1 protein expression by androstenone in isolated pig hepatocytes and may be relevant to the low expression of hepatic CYP2E1 in those pigs which accumulate high levels of androstenone in vivo.

Chicken ovalbumin upstream promoter-transcription factor I represses the transcriptional activity of the human muscle glycogen phosphorylase promoter in C2C12 cells

The responsiveness of the 1.13 kb proximal human muscle glycogen phosphorylase (MGP) gene promoter to the chicken ovalbumin upstream promoter-transcription factor (COUP-TF) repressor, known to be ablated during muscle cell differentiation, was examined. Constitutive expression of COUP-TFI repressed the activity of the promoter in C2C12 muscle cells and sequential deletion analysis mapped the sensitive region between nucleotides -362 and -185, which included a putative consensus COUP-TF binding half-site at -198/-193. Mutation of this site abolished transcriptional response to COUP-TFI of the -362 construct. A -209/-180 probe bound in vitro to COUP-TFI and to protein extracts from proliferating but not fusing myoblasts. Thus, COUP-TF may be involved in repression of the human MGP gene promoter at the myoblast stage.

Regulation of Expression of the Na+/H+ Exchanger by Thyroid Hormone

The Na+/H+ exchanger is a pH regulatory protein with a ubiquitous distribution in eukaryotic cells. Several isoforms of the Na+/H+ exchanger are known. The first isoform to be characterized and cloned, NHE1, is present on the plasma membrane of cells and functions to remove one intracellular proton in exchange for one extracellular sodium ion. It is involved in pH regulation, cell growth, differentiation, and cell migration. NHE1 is also involved in the cycle of damage that occurs in the heart with ischemic heart disease. Recent studies have shown that the Na+/H+ exchanger is regulated in response to thyroid hormone. Reduction in circulating thyroid hormone levels reduces the amount of both protein and mRNA of NHE1. Conversely, an elevation of thyroid hormone levels has the opposite effects. Transcriptional regulation of NHE1 expression has been demonstrated. The NHE1 promoter contains a TR alpha(1) binding site located between -841 to -800 bp. This element responds positively to TR alpha(1). This regulation of the NHE1 promoter by thyroid hormone is proposed to be responsible for postnatal changes in expression of the Na+/H+ exchanger.

Expression of Sec61alpha in F9 and P19 teratocarcinoma cells after retinoic acid treatment

Nascent procollagen peptides and other secretory proteins are transported across the endoplasmic reticulum (RE) membrane through a protein-conducting channel called the translocon. Sec61alpha, a multispanning membrane translocon protein, has been implicated as essential for translocation of polypeptides chains into the cisterns of the ER. However, it is not known whether Sec61alpha is ubiquitously expressed in collagen producing teratocarcinoma cells. Furthermore, the production, expression, and utilization of Sec61alpha may depend on the cell differentiation stage. Stem cells from many cultured teratocarcinoma cell lines such as F9 and P19 cells are capable of differentiation in response to low retinoic acid concentrations. This differentiation of the tumorigenic stem cells results in tumorigenicity loss. For this study, mouse F9 and P19 teratocarcinoma cells were grown in culture medium treated with or without retinoic acid. Expression of Sec61alpha was determined by reverse trancriptase polimerase chain reaction (RT-PCR). In untreated conditions, F9 cells expressed undetected Sec61alpha amounts. It was also demonstrated that Sec61alpha expression is stimulated in F9 cells after retinoic acid treatment for 72 hours. No changes were found in Sec61alpha expression in P19 cells after retinoic acid treatment. These data indicate that the expression of Sec61alpha is enhanced with retinoic acid induced differentiation of F9 teratocarcinoma cells.

Structure and function of the NHE1 isoform of the Na+/H+ exchanger

The Na+/H+ exchanger is a ubiquitous, integral membrane protein involved in pH regulation. It removes intracellular acid, exchanging a proton for an extracellular sodium ion. There are seven known isoforms of this protein that are the products of distinct genes. The first isoform discovered (NHE1) is ubiquitously distributed throughout the plasma membrane of virtually all tissues. It plays many different physiological roles in mammals, including important functions in regulation of intracellular pH, in heart disease, and in cytoskeletal organization. The first 500 amino acids of the protein are believed to consist of 12 transmembrane helices, a membrane-associated segment, and two reentrant loops. A C-terminal regulatory domain of approximately 315 amino acids regulates the protein and mediates cytoskeletal interactions. Studies are underway to determine the amino acid residues important in NHE1 function. At present, it is clear that transmembrane segment IV is important in NHE1 function and that transmembrane segments VII and IX are also involved in transport. Further experiments are required to elucidate the mechanism of transport and regulation of this multifunctional protein.

[Ion transporters and cardiovascular diseases: pH control or modulation of intracellular calcium concentration]

The regulation of the intracellular pH is under tight control by several ion transport systems including the sodium-proton exchanger, the sodium-bicarbonate cotransporter and the chlore-bicarbonate anion exchanger. While the activation of the anion exchange induces a cellular acidification, both the sodium-proton exchanger and the sodium-bicarbonate cotransporter are responsible for a protection against acidosis by extruding protons or importing bicarbonate. These transporters are transmembrane proteins whose activity is regulated by several mechanisms including phosphorylation, calcium binding and which are involved in several pathophysiologic processes such as ischemia, hypertrophy and arrhythmias. Recent studies suggest that the activation of these transporters during various diseases induces an increase in intracellular calcium concentration. Therefore, inhibiting these transporters could represent novel therapeutic strategies for the treatment of cardiovascular diseases.

Transcriptional regulation of Na+/H+ exchanger expression in the intact mouse

We examined regulation of the Na+/H+ exchanger (NHE1 isoform) in the developing mouse. We generated transgenic mice with the Na+/H+ exchanger promoter directing expression of the beta-Galactosidase reporter. We found that expression of the Na+/H+ exchanger was maximum in the heart and liver of 12-day-old embryonic mice. Similar results were found in mice using the green fluorescent protein reporter driven by the Na+/H+ exchanger promoter. Detailed examination of the myocardium revealed that the GFP reporter protein was expressed in the cytoplasm of cardiomyocyte cells. We examined NHE1 protein expression in transgenic mice lacking the transcription factors AP-2alpha or the transcription factor COUP-TF1. Eighteen-day-old AP-2alpha heterozygote mice show no large changes in NHE1 expression in heart, lung, liver, kidney and brain. In contrast, 18-day-old embryos from AP-2alpha null mice showed a large increase in Na+/H+ exchanger protein expression in the brain. NHE1 protein levels in COUP-TF1 knockout embryos did not differ from wild type embryos. The results suggest that AP-2alpha and COUP-TF1 are not critical to NHE1 expression in the late stage embryo and that other related transcription factors may function in regulation of the Na+/H+ exchanger.

Thyroid hormone receptor alpha 1 regulates expression of the Na+/H+ exchanger (NHE1)

In this paper we examine the role of thyroid hormone in regulating expression of the Na+/H+ exchanger. Thyroid hormone has been reported to regulate the activity of the Na+/H+ exchanger messenger RNA in some cell types. Treatment of cardiac myocytes with 3,5',3'-triiodothyronine results in an increased expression of Na+/H+ exchanger protein. Also, compared with euthyroid animals, hypothyroid rats express decreased amounts of the Na+/H+ exchanger protein. To examine the mechanisms involved in regulating expression of the Na+/H+ exchanger, we have characterized the regulation of a distal element of the NHE1 promoter by the thyroid hormone receptor. We have previously shown that a -1085/-800 nucleotide (nt) region of the promoter is a modular element with a -841/-800 nt activating element. Using electrophoretic mobility shift assay, we show that this element interacts with thyroid hormone receptor TRalpha(1), a nuclear hormone receptor. The addition of exogenous TRalpha increased transcriptional activity of the -841/-800 nt element of the Na+/H+ exchanger promoter. We show that TRalpha binds to a region on the -841/-800 nt element that is near, but not identical, to the previously identified chicken ovalbumin upstream promoter transcription factor-binding site. Our results are the first demonstration that thyroid hormone and the thyroid hormone receptor TRalpha(1) regulate expression of the Na+/H+ exchanger.