Regulation of Na,K-ATPase beta 1 subunit gene transcription by low external potassium in cardiac myocytes. Role of Sp1 AND Sp3

Protein O-GlcNAcylation in the heart

O-GlcNAcylation is a ubiquitous post-translational modification that is extremely labile and plays a significant role in physiology, including the heart. Sustained activation of cardiac O-GlcNAcylation is frequently associated with alterations in cellular metabolism, leading to detrimental effects on cardiovascular function. This is particularly true during conditions such as diabetes, hypertension, cardiac remodelling, heart failure and arrhythmogenesis. Paradoxically, transient elevation of cardiac protein O-GlcNAcylation can also exert beneficial effects in the heart. There is compelling evidence to suggest that a complex interaction between O-GlcNAcylation and phosphorylation also exists in the heart. Beyond direct functional consequences on cardiomyocytes, O-GlcNAcylation also acts indirectly by altering the function of transcription factors that affect downstream signalling. This review focuses on the potential cardioprotective role of protein O-GlcNAcylation during ischaemia-reperfusion injury, the deleterious consequences of chronically elevated O-GlcNAc levels, the interplay between O-GlcNAcylation and phosphorylation in the cardiomyocytes and the effects of O-GlcNAcylation on other major non-myocyte cell types in the heart.

Transcriptional regulators of Na,K-ATPase subunits

The Na,K-ATPase classically serves as an ion pump creating an electrochemical gradient across the plasma membrane that is essential for transepithelial transport, nutrient uptake and membrane potential. In addition, Na,K-ATPase also functions as a receptor, a signal transducer and a cell adhesion molecule. With such diverse roles, it is understandable that the Na,K-ATPase subunits, the catalytic α-subunit, the β-subunit and the FXYD proteins, are controlled extensively during development and to accommodate physiological needs. The spatial and temporal expression of Na,K-ATPase is partially regulated at the transcriptional level. Numerous transcription factors, hormones, growth factors, lipids, and extracellular stimuli modulate the transcription of the Na,K-ATPase subunits. Moreover, epigenetic mechanisms also contribute to the regulation of Na,K-ATPase expression. With the ever growing knowledge about diseases associated with the malfunction of Na,K-ATPase, this review aims at summarizing the best-characterized transcription regulators that modulate Na,K-ATPase subunit levels. As abnormal expression of Na,K-ATPase subunits has been observed in many carcinoma, we will also discuss transcription factors that are associated with epithelial-mesenchymal transition, a crucial step in the progression of many tumors to malignant disease.

Association with {beta}-COP regulates the trafficking of the newly synthesized Na,K-ATPase

Plasma membrane expression of the Na,K-ATPase requires assembly of its α- and β-subunits. Using a novel labeling technique to identify Na,K-ATPase partner proteins, we detected an interaction between the Na,K-ATPase α-subunit and the coat protein, β-COP, a component of the COP-I complex. When expressed in the absence of the Na,K-ATPase β-subunit, the Na,K-ATPase α-subunit interacts with β-COP, is retained in the endoplasmic reticulum, and is targeted for degradation. In the presence of the Na,K-ATPase β-subunit, the α-subunit does not interact with β-COP and traffics to the plasma membrane. Pulse-chase experiments demonstrate that in cells expressing both the Na,K-ATPase α- and β-subunits, newly synthesized α-subunit associates with β-COP immediately after its synthesis but that this interaction does not constitute an obligate intermediate in the assembly of the α- and β-subunits to form the pump holoenzyme. The interaction with β-COP was reduced by mutating a dibasic motif at Lys(54) in the Na,K-ATPase α-subunit. This mutant α-subunit is not retained in the endoplasmic reticulum and reaches the plasma membrane, even in the absence of Na,K-ATPase β-subunit expression. Although the Lys(54) α-subunit reaches the cell surface without need for β-subunit assembly, it is only functional as an ion-transporting ATPase in the presence of the β-subunit.

Regulation of Na,K-ATPase alpha1 subunit gene transcription in response to low K(+): role of CRE/ATF- and GC box-binding proteins

Na,K-ATPase expression is upregulated in mammalian cells as a consequence of persistent inhibition of Na,K-ATPase enzymatic activity by low external K(+). We previously demonstrated that exposure of neonatal rat cardiac myocytes to low K(+) increased Na,K-ATPase alpha1 subunit mRNA content and promoter activity. In this work, we utilized transient transfection studies with rat Na,K-ATPase alpha1 subunit 5'-flanking region deletion plasmids to identify DNA sequences required for low K(+)-mediated stimulation of alpha1 subunit promoter expression in cardiac myocytes. Maximal low K(+)-responsiveness of the alpha1 promoter was found to be dependent on nucleotides from -102 to -62 and a downstream region from +53 to +261. Further analysis of the upstream low K(+)-responsive region using mutant constructs revealed that a CRE/ATF site at -70 to -63 and a GC box motif at -57 to -48 were both required for the effect of low K(+) on alpha1 subunit gene transcription. Electrophoretic mobility shift assays revealed that low K(+) increased binding of transcription factors to the GC box and, to a lesser extent, to the CRE/ATF site. Western blot analysis demonstrated that exposure of cardiac myocytes to low K(+) resulted in increased nuclear content of Sp1, Sp3 and CREB-1. Finally, a selective increase in phosphorylation of Sp1 was found in nuclear extracts from low K(+)-treated cells. We conclude that low K(+)-mediated upregulation of Na,K-ATPase alpha1 subunit gene expression in neonatal rat cardiac myocytes is dependent, in part, on CRE/ATF- and GC box-binding transcription factors.

A high-potassium diet reduces infarct size and improves vascular structure in hypertensive rats

High-potassium diets can improve vascular function, yet the effects of potassium supplementation on ischemic stroke have not been studied. We hypothesized that dietary potassium supplementation would reduce ischemic cerebral infarct size by reversing cerebral artery hypertrophy. Six-week-old male stroke-prone spontaneously hypertensive rats (SHRSP) were fed diets containing 0.79% potassium (LK) or 2.11% potassium (HK) for 6 wk; Wistar-Kyoto (WKY) rats were fed the LK diet. The HK diet did not reduce blood pressure, as measured by telemetry, in the SHRSP. Cerebral ischemia was induced by middle cerebral artery (MCA) occlusion. The resultant infarct was smaller in the HK-SHRSP than in the LK-SHRSP: 55.1 +/- 6.3 vs. 71.4 +/- 2.4% of the hemisphere infarcted (P < 0.05). Infarcts were smaller in WKY rats (33.5 +/- 4.8%) than in LK-SHRSP or HK-SHRSP. The vessel wall of MCAs from LK-SHRSP was hypertrophied compared with WKY rats; this was reversed in HK-SHRSP. RT-PCR analysis of the cerebral vessels showed that expression of platelet-derived growth factor receptors-alpha and -beta, epidermal growth factor receptor, and collagen I and III was increased in the vessels from LK-SHRSP compared with WKY rats and reduced in HK-SHRSP. These results suggest that potassium supplementation provides neuroprotection in a model of ischemic stroke independent of blood pressure and possibly through changes in vascular structure.

Divergent signaling pathways mediate induction of Na,K-ATPase α1 and β1 subunit gene transcription by low potassium

Prolonged inhibition of Na,K-ATPase enzymatic activity by exposure of a variety of mammalian cells to low external K+ yields a subsequent adaptive up-regulation of Na,K-ATPase expression. The aim of this study was to examine the intracellular signal transduction system that is responsible for mediating increased Na,K-ATPase subunit gene expression in primary cultures of neonatal rat cardiac myocytes. In this work, we show long-term inhibition of Na,K-ATPase function with 0.6 mM K+ resulted in hypertrophy of cardiac myocytes and augmentation of Na,K-ATPase alpha1 and beta1 subunit gene expression. Transient transfection experiments in neonatal rat cardiac myocytes demonstrated that low K+ induction of alpha1 and beta1 gene transcription was dependent on intracellular Ca2+ and activation of calcineurin. Based on effects of pharmacological inhibitors, protein kinase A (PKA), extracellular signal-regulated kinase 1/2 (ERK1/2) and histone deacetylase were found to be unique downstream components in the low K+ signal transduction pathway leading to increased alpha1 subunit promoter activity. Similarly, low K+-induced beta1 subunit gene transcription was dependent on activation of protein kinase C (PKC), c-Jun-N-terminal kinase (JNK) and p38 mitogen-activated protein kinase (MAPK). These findings indicate that persistent inhibition of Na,K-ATPase activity with low external K+ activates overlapping and Na,K-ATPase subunit gene-specific signaling pathways in cardiac myocytes.

Evidence of calcium‐dependent pathway in the regulation of human β1,3‐glucuronosyltransferase‐1 (GlcAT‐I) gene expression: a key enzyme in proteoglycan synthesis

The importance of heparan- and chondroitin-sulfate proteoglycans in physiological and pathological processes led to the investigation of the regulation of beta1,3-glucuronosyltransferase I (GlcAT-I), responsible for the completion of glycosaminoglycan-protein linkage tetrasaccharide, a key step prior to polymerization of chondroitin- and heparan-sulfate chains. We have cloned and functionally characterized GlcAT-I 5'-flanking regulatory region. Mutation analysis and electrophoretic mobility shift assays demonstrated the importance of Sp1 motif located at -65/-56 position in promoter activity. Furthermore, we found that elevation of intracellular calcium concentration by the calcium ionophore ionomycin stimulated GlcAT-I gene expression as well as glycosaminoglycan chain synthesis in HeLa cells. Bisanthracycline, an anti-Sp1 compound, inhibited GlcAT-I basal promoter activity and suppressed ionomycin induction, suggesting the importance of Sp1 in calcium induction of GlcAT-I gene expression. Nuclear protein extracts from ionomycin-induced cells exhibited an increased DNA binding of Sp1 factor to the consensus sequence at position -65/-56. Signaling pathway analysis and MEK inhibition studies revealed the important role of p42/p44 MAPK in the stimulation of GlcAT-I promoter activity by ionomycin. The present study identifies, for the first time, GlcAT-I as a target of calcium-dependent signaling pathway and evidences the critical role of Sp1 transcription factor in the activation of GlcAT-I expression.

Sp3 inhibits Sp1-mediated activation of the cardiac troponin T promoter and is downregulated during pathological cardiac hypertrophy in vivo

Combinatorial interactions between cis elements and trans-acting factors are required for regulation of cardiac gene expression during normal cardiac development and pathological cardiac hypertrophy. Sp factors bind GC boxes and are implicated in recruitment and assembly of the basal transcriptional complex. In this study, we show that the cardiac troponin T (cTnT) promoter contains a GC box that is necessary for basal and cAMP-mediated activity of cTnT promoter constructs transfected in embryonic cardiomyocytes. Cardiac nuclear proteins bind the cTnT GC box in a sequence-specific fashion and consist of Sp1, Sp2, and Sp3 protein factors. By chromatin immunoprecipitation, Sp1 binds the cTnT promoter "in vivo." Cotransfected Sp1 trans-activates the cTnT promoter in cardiomyocytes in culture. Sp3 represses Sp1-mediated transcriptional activation of the cTnT gene in embryonic cardiomyocytes. Sp3 repression of Sp1-mediated cTnT promoter activation is dose dependent, inferring a mechanism of competitive binding/inhibition. To evaluate the role of Sp factors in cardiac gene expression in vivo, we have established a clinically relevant animal model of pathological cardiac hypertrophy where the fetal cardiac program is activated. In this animal model, cardiac hypertrophy results from increased left-right shunting, volume loading of the left ventricle, and pressure loading of the right ventricle. Sp1 expression is increased in all four hypertrophied cardiac chambers, whereas Sp3 expression is diminished. This observation is consistent with the in vitro activating function of Sp1 and inhibitory effects of Sp3 on activity of cTnT promoter constructs. Sp factor levels are modulated during the hypertrophic cardiac program in vivo.

Identification of a prostaglandin-responsive element in the Na,K-ATPase beta 1 promoter that is regulated by cAMP and Ca2+. Evidence for an interactive role of cAMP regulatory element-binding protein and Sp1

The Na,K-ATPase is a transmembrane protein responsible for maintaining electrochemical gradients across the plasma membrane in all mammalian cells, a process that is subject to regulation at the transcriptional as well as post-transcriptional level. Included among physiologic regulators in the kidney are prostaglandins. Previously, we demonstrated that prostaglandin E(1) (PGE(1)) increases the activity and expression of the Na,K-ATPase in Madin-Darby canine kidney cells (Taub, M., Borsick, M., Geisel, J., Matlhagela, K., Rajkhowa, T., and Allen, C. (2004) Exp. Cell Res. 299, 1-14; Taub, M. L., Wang, Y., Yang, I. S., Fiorella, P., and Lee, S. M. (1992) J. Cell. Physiol. 151, 337-346). In this work, we present evidence that transcription of the Na,K-ATPase beta(1) subunit is stimulated by PGE(1), an effect that may be mediated through the cAMP and Ca(2+) pathways. Transient transfection studies using 5'-deletion mutants of the human beta(1) subunit promoter indicated that region -100 to -92 containing the sequence AGTCCCTGC (a prostaglandin-responsive element (PGRE)) is required to elicit the stimulatory effects of PGE(1), 8-bromo-cAMP, phorbol 12-myristate 13-acetate, and okadaic acid. Electrophoretic mobility shift assays indicated that both the cAMP regulatory element-binding protein (CREB) and Sp1 bind to the PGRE present within this region of the beta(1) subunit promoter. The involvement of the PGRE and Sp1 sites in regulation by PGE(1) was further confirmed by the increased PGE(1) stimulation that was observed following insertion of the PGRE into a promoter/luciferase construct containing a portion of a heterologous promoter and the fibronectin promoter with four GC boxes. Further evidence suggesting an interaction between Sp1 and CREB was obtained from experiments conducted with pLuc-MCS-beta 72-167, which contains region -167 to -72 in the human beta(1) subunit promoter. The PGE(1) stimulation observed in Madin-Darby canine kidney cells transiently transfected with pLuc-MCS-beta 72-167 was reduced when the two GC boxes immediately upstream from the PGRE were translocated farther upstream. Also consistent with an interaction between CREB and Sp1 are the results of our immunoprecipitation studies indicating that CREB co-immunoprecipitated with Sp1 when an antibody against CREB, Sp1, or the CREB-binding protein was used.

Stimulation of Na,K-ATPase by low potassium requires reactive oxygen species

The signaling pathway that transduces the stimulatory effect of low K+ on the biosynthesis of Na,K-ATPase remains largely unknown. The present study was undertaken to examine whether reactive oxygen species (ROS) mediated the effect of low K+ in Madin-Darby canine kidney (MDCK) cells. Low K+ increased ROS activity in a time- and dose-dependent manner, and this effect was abrogated by catalase and N-acetylcysteine (NAC). To determine the role of ROS in low-K+-induced gene expression, the cells were first stably transfected with expression constructs in which the reporter gene chloramphenicol acetyl transferase (CAT) was under the control of the avian Na,K-ATPase alpha-subunit 1.9 kb and 900-bp 5'-flanking regions that have a negative regulatory element. Low K+ increased the CAT expression in both constructs. Catalase or NAC inhibited the effect of low K+. To determine whether the increased CAT activity was mediated through releasing the repressive effect or a direct stimulation of the promoter, the cells were transfected with a CAT expression construct directed by a 96-bp promoter fragment that has no negative regulatory element. Low K+ also augmented the CAT activity expressed by this construct. More importantly, both catalase and NAC abolished the effect of low K+. Moreover, catalase and NAC also inhibited low-K+-induced increases in the Na,K-ATPase alpha1- and beta1-subunit protein abundance and ouabain binding sites. The antioxidants had no significant effect on the basal levels of CAT activity, protein abundance, or ouabain binding sites. In conclusion, low K+ enhances the Na,K-ATPase gene expression by a direct stimulation of the promoter activity, and ROS mediate this stimulation and also low-K+-induced increases in the Na,K-ATPase protein contents and cell surface molecules.