Na,K-ATPase mRNA beta 1 expression in rat myocardium--effect of thyroid status

Emerging Roles of RNA 3'-end Cleavage and Polyadenylation in Pathogenesis, Diagnosis and Therapy of Human Disorders

A crucial feature of gene expression involves RNA processing to produce 3′ ends through a process termed 3′ end cleavage and polyadenylation (CPA). This ensures the nascent RNA molecule can exit the nucleus and be translated to ultimately give rise to a protein which can execute a function. Further, alternative polyadenylation (APA) can produce distinct transcript isoforms, profoundly expanding the complexity of the transcriptome. CPA is carried out by multi-component protein complexes interacting with multiple RNA motifs and is tightly coupled to transcription, other steps of RNA processing, and even epigenetic modifications. CPA and APA contribute to the maintenance of a multitude of diverse physiological processes. It is therefore not surprising that disruptions of CPA and APA can lead to devastating disorders. Here, we review potential CPA and APA mechanisms involving both loss and gain of function that can have tremendous impacts on health and disease. Ultimately we highlight the emerging diagnostic and therapeutic potential CPA and APA offer.

A polymorphic 3'UTR element in ATP1B1 regulates alternative polyadenylation and is associated with blood pressure

Although variants in many genes have previously been shown to be associated with blood pressure (BP) levels, the molecular mechanism underlying these associations are mostly unknown. We identified a multi-allelic T-rich sequence (TRS) in the 3’UTR of ATP1B1 that varies in length and sequence composition (T_22-27 and T₁₂GT ₃GT₆). The 3’UTR of ATP1B1 contains 2 functional polyadenylation signals and the TRS is downstream of the proximal polyadenylation site (A2). Therefore, we hypothesized that alleles of this TRS might influence ATP1B1 expression by regulating alternative polyadenylation. In vitro, the T₁₂GT ₃GT₆ allele increases polyadenylation at the A2 polyadenylation site as compared to the T₂₃ allele. Consistent with our hypothesis, the relative abundance of the A2-polyadenylated ATP1B1 mRNA was higher in human kidneys with at least one copy of the T₁₂GT ₃GT₆ allele than in those lacking this allele. The T₁₂GT ₃GT₆ allele is also associated with higher systolic BP (beta = 3.3 mmHg, p = 0.014) and diastolic BP (beta = 2.4 mmHg, p = 0.003) in a European-American population. Therefore, we have identified a novel multi-allelic TRS in the 3’UTR of ATP1B1 that is associated with higher BP and may mediate its effect by regulating the polyadenylation of the ATP1B1 mRNA.

Effect of thyroid hormone on Mg(2+) homeostasis and extrusion in cardiac cells

The present study investigated the effect of alteration in thyroid hormone level on Mg(2+) homeostasis in cardiac ventricular myocytes. Hyperthyroid conditions increased cardiac myocytes total Mg(2+) content by ~14% as compared to cells from eu-thyroid animals. The excess Mg(2+) was localized predominantly within cytoplasm and mitochondria, and was mobilized into the extracellular compartment by addition of isoproterenol (ISO) or cAMP but not phenylephrine (PHE). Hypothyroid conditions, instead, decreased cardiac myocytes total Mg(2+) content by ~10% as compared to cells from eu-thyroid animals. Also in this case, cytoplasm and mitochondria were the two cellular pools predominantly affected. Under hypothyroid conditions, administration of ISO or cAMP resulted in a decreased Mg(2+) extrusion as compared to that observed in cardiac cells from eu-thyroid animals. Similar changes in cellular Mg(2+) content and transport were observed in cardiac ventricular myocytes isolated from hyper- and hypo-thyroid animals, as well as in cultures of H9C2 cells rendered hyper- or hypo-thyroid under in vitro conditions. Supplementation of thyroid hormone to hypothyroid animals restored Mg(2+) level and transport to levels comparable to those observed in eu-thyroid animals. Taken together, these results indicate that changes in thyroid hormone level have a major effect on Mg(2+) homeostasis and compartmentation in cardiac cells. The enlarged Mg(2+) mobilization via beta- but not alpha(1)-adrenergic receptor stimulation further suggests that beta- and alpha(1)-adrenergic receptors target selectively different Mg(2+) compartments within the cardiac myocyte. These results provide a new rationale to interpret changes in cardiac function under hyper- or hypo-thyroid conditions.

Effect of thyroid hormone on the distribution and activity of Na, K-ATPase in ventricular myocardium

Employing detergent-free sucrose-density gradient fractionation method we isolated cholesterol-rich lighter membrane fractions containing approximately 10% of protein, approximately 30% of cholesterol in membranes of ventricular myocardium. Cholesterol-rich lighter membrane fractions contain >70% of Na, K-ATPase and caveolins 1 and 3 and <10% of beta-actin. Treatment of hypothyroid rats with T(3) increased the relative abundance of both alpha1 and beta1 Na, K-ATPase subunits in total membranes by 4- to 5-fold (with no change in caveolin-3), and resulted in 1.9-fold increase in enzyme activity. T(3)-induced Na, K-ATPase subunits were preferentially distributed to the lighter fractions (#s 4, 5 and 6); and increased abundance of alpha1 and beta1 were 34-70% and 43-68%, respectively. We conclude that the activity of Na, K-ATPase is not uniform in cardiac membranes, and while a significant amount of Na, K-ATPase is present in cardiac cholesterol-rich membrane fractions, the intrinsic activity is significantly less than the enzyme present in relatively cholesterol-poor membranes.

Control of Na+-K+-ATPase beta 1-subunit expression: role of 3'-untranslated region

Using in vitro translation and cell transfection assays, we previously demonstrated that the Na+ -K+ -ATPase beta1 mRNA species containing its longest 3'-untranslated region (UTR) exhibited the lowest translational efficiency. Here, employing deletions and in vivo expression assays, using direct injection of plasmids into rat ventricular myocardium, we identified a 143-nt segment located in the distal 3'-UTR of beta1 mRNA that was associated with decreased luciferase expression; interestingly, this segment contains three AUUUA motifs. Using RNA-protein binding assays and UV cross-linking of cRNA with cytosolic proteins of rat heart, we identified an approximately 38-kDa protein that specifically bound to the cRNA encoding the 143-nt segment of beta1 mRNA 3'-UTR. Mutation of three nucleotides located in the middle region of the 143-nt segment, which was predicted to greatly disrupt a putative stem-loop structure of the cRNA in this region, was associated with reduced binding of the mutated cRNA to the protein migrating at approximately 38 kDa. The cRNA encoding a segment of cyclooxygenase-2 mRNA 3'-UTR containing six AUUUA sequences did not bind the protein migrating at approximately 38 kDa and did not compete with the binding of the wild-type 143-nt beta(1) cRNA to the protein. The above results suggest that the 143-nt segment in the distal segment of the 3'-UTR of beta1 mRNA may play an important role in the control of beta1-subunit expression.

Thyroid hormone regulation of cardiac bioenergetics: role of intracellular creatine

The effect of thyroid hormone (T(3)) on the content of myocardial creatine (Cr), Cr phosphate (CrP), and high-energy adenine nucleotides and on cardiac function was examined. In the hearts of control and T(3)-treated rats perfused in vitro, while "low" and "high" contractile work was performed, T(3) treatment resulted in a approximately 50% reduction in CrP, Cr, total Cr content (Cr + CrP), and in the CrP-to-Cr ratio. In addition, there was a slight fall in myocardial content of ATP and a large rise in calculated free ADP (fADP), resulting in a significant decrease in the ATP-to-fADP ratio in the hearts of hyperthyroid compared with euthyroid rats. Moreover, there was a substantial decrease in the level of ATP in hearts of T(3)-treated rats under high work conditions. Importantly, the ratio of cardiac work to oxygen consumption was not altered by thyroid status. Treatment with T(3) also resulted in an almost threefold reduction in the content of Na(+)/Cr transporter mRNA in the ventricular myocardium and skeletal muscle but not in the brain. We conclude with the following: 1) changes in the expression of the Na(+)/Cr transporter mRNA correlate with Cr + CrP in the myocardium; 2) hearts of hyperthyroid rats contain lower levels of ATP and higher levels of fADP under both low and high work conditions but no reduction in efficiency of work output; and 3) the reduction in Cr and ATP in hearts of hyperthyroid rats may be the basis for the reduced maximal work capacity of the hyperthyroid heart.

Triiodothyronine supplementation in patients undergoing cardiopulmonary bypass

Patients undergoing cardiopulmonary bypass may develop clinically significant physiologic alterations in the perioperative period, including alteration of thyroid hormone concentrations. Alterations in the concentration of thyroid hormones are of concern due to the effects of these hormones on cardiac function. Hypothyroidism is associated with a decrease in cardiac performance; therefore, supplementation with the active thyroid hormone triiodothyronine (T3) in patients undergoing cardiopulmonary bypass has been investigated to improve outcomes. In addition, T3 has been studied as an agent to reduce the frequency of atrial fibrillation after cardiopulmonary bypass.

Different Na, K-ATPase mRNA(beta1) species exhibit unique translational efficiencies

We have previously identified five Na, K-ATPase beta1-mRNA species that are expressed in the rat heart, kidney, and brain. These mRNAs which are unequal in their abundance have an identical coding region but differ in the length of their 5'- and 3'-untranslated regions (UTRs). In this study we examined the possibility that the beta1-mRNA species exhibit differential translational efficiencies. We constructed expression plasmids encoding each of the five mRNAs and transcribed and translated them in vitro. Using rabbit reticulocyte system we determined the translation of the different mRNAs under conditions optimized for each beta1-cRNA and under an equivalent (standard) condition. The longest beta1-cRNA species (initiating at the first transcription start site and ending at the last [fifth] poly(A) site) exhibited the lowest relative translational efficiency averaging 0.2 +/- 0.05 units/mol of cRNA compared to the shortest beta1-cRNA species initiating at the first transcription start site and ending at the first poly(A) signal (with an assigned relative value of 1.0). These results suggested that the different translation rates of beta1-mRNAs may be due to their 3'-UTRs. To further define the role of beta1-3'-UTR, chimeric luciferase constructs containing different segments of the beta1-3'-UTR were transiently transfected into Clone 9 cells. Compared to the chimeric construct containing the shortest beta1-3'-UTR segment (ending at the first poly(A) site), the construct containing the full-length beta1-3'-UTR exhibited a luciferase expression of 0.23 +/- 0.04. To control for potential changes in the abundance of the expressed chimeric mRNAs which may lead to differences in luciferase expression, luciferase activity was normalized against chimeric luciferase-mRNA content measured in mixtures of cells stably transfected with the above constructs. The ratio of luciferase activity/chimeric luciferase-mRNA content in cells expressing the construct containing the entire beta1-3'-UTR region was 0.17 that in cells expressing chimeric luciferase mRNA containing beta1-3'-UTR up to the first poly(A) signal (P < 0.05). We conclude that the translational efficiency of the different beta1-mRNA species is negatively regulated by the 3'-UTR of the mRNA and that a regulating region appears to be localized between the second and fifth poly(A) signals of beta1-mRNA.

Thyroid hormone regulation of myocardial Na/K-ATPase gene expression

Employing published methods for isolation of cardiac myocyte nuclei from adult rat ventricular myocardium with the use of mechanical disruption without digestive enzymes, we obtained transcriptionally active cardiac myocyte nuclei with sufficient yield and purity. The relative content of Na/K-ATPase subunit mRNAs (alpha 1, alpha 2, and beta 1) in ventricular myocardium of euthyroid rats closely matched the relative rates of transcription of the respective subunit genes determined by nuclear run-on assay. Treatment of hypothyroid rats with T(3)to elicit hyperthyroidism was associated with 2.9-, 7.5-, and seven-fold increases in the contents of alpha 1-, alpha 2, beta 1-mRNAs, respectively. In contrast, rates of transcription of the subunit genes were not changed significantly by T(3), while transcription of the 18 S ribosomal gene was stimulated identical with three-fold by the treatment. A quantitative reverse transcription-polymerase chain reaction assay for measurement of primary RNA transcripts of the beta 1 gene was developed employing a rat genomic DNA fragment that contains the first exon and part of the first intron of the beta 1 gene. The relative abundance of beta 1 primary transcripts did not change in RNA isolated from hypothyroid, euthyroid, and hyperthyroid rats. It is concluded that: (1) The relative contents of Na/K-ATPase subunit mRNAs in euthyroid adult myocardium is primarily controlled at the transcriptional level, and (2) T(3)-induced increases in the contents of Na/K-ATPase subunit mRNAs in the heart is not associated with increased rates of transcription of the subunit genes, and the effect is mediated at the post-transcriptional level.

Apolipoprotein AII enrichment of HDL enhances their affinity for class B type I scavenger receptor but inhibits specific cholesteryl ester uptake

Apolipoproteins of high density lipoprotein (HDL) and especially apolipoprotein (apo)AI and apoAII have been demonstrated as binding directly to the class B type I scavenger receptor (SR-BI), the HDL receptor that mediates selective cholesteryl ester uptake. However, the functional relevance of the binding capacity of each apolipoprotein is still unknown. The human adrenal cell line, NCI-H295R, spontaneously expresses a high level of SR-BI, the major apoAI binding protein in these cells. As previously described for murine SR-BI, free apoAI, palmitoyl-oleoyl-phosphatidylcholine (POPC)-AI, and HDL are good ligands for human SR-BI. In vitro displacement of apoAI by apoAII in HDLs or in Lp AI purified from HDL by immunoaffinity enhances their ability to compete with POPC-AI to bind to SR-BI and also enhances their direct binding capacity. The next step was to determine whether the higher affinity of apoAII for SR-BI correlated with the specific uptake of cholesteryl esters from these HDLs. Free apoAII and, to a lesser extent, free apoAI that were added to the cell medium during uptake experiments inhibited the specific uptake of [(3)H]cholesteryl esters from HDL, indicating that binding sites on cells were the same as cholesteryl ester uptake sites. In direct experiments, the uptake of [(3)H]cholesteryl esters from apoAII-enriched HDL was highly reduced compared with the uptake from native HDL. These results demonstrate that in the human adrenal cell line expressing SR-BI as the major HDL binding protein, efficient apoAII binding has an inhibitory effect on the delivery of cholesteryl esters to cells.

Acid-labile ATP and/or ADP/P(i) binding to the tetraprotomeric form of Na/K-ATPase accompanying catalytic phosphorylation-dephosphorylation cycle

The Na/K-ATPase has been shown to bind 1 and 0.5 mol of (32)P/mol of alpha-chain in the presence [gamma-(32)P]ATP and [alpha-(32)P]ATP, respectively, accompanied by a maximum accumulation of 0.5 mol of ADP-sensitive phosphoenzyme (NaE1P) and potassium-sensitive phosphoenzyme (E2P). The former accumulation was followed by the slow constant liberation of P(i), but the latter was accompanied with a rapid approximately 0.25 mol of acid-labile P(i) burst. The rubidium (potassium congener)-occluded enzyme (approximately 1.7 mol of rubidium/mol of alpha-chain) completely lost rubidium on the addition of sodium + magnesium. Further addition of approximately 100 microM [gamma-(32)P]ATP and [alpha-(32)P]ATP, both induced 0.5 mol of (32)P-ATP binding to the enzyme and caused accumulation of approximately 1 mol of rubidium/mol of alpha-chain, accompanied by a rapid approximately 0.5 mol of P(i) burst with no detectable phosphoenzyme under steady state conditions. Electron microscopy of rotary-shadowed soluble and membrane-bound Na/K-ATPases and an antibody-Na/K-ATPase complex, indicated the presence of tetraprotomeric structures (alphabeta)(4). These and other data suggest that Na/K-ATP hydrolysis occurs via four parallel paths, the sequential appearance of (NaE1P:E.ATP)(2), (E2P:E.ATP:E2P:E. ADP/P(i)), and (KE2:E.ADP/P(i))(2), each of which has been previously referred to as NaE1P, E2P, and KE2, respectively.