Glucocorticoid inhibition of Na-Pi cotransport in renal epithelial cells is mediated by protein kinase C

Interactions of Phosphate Metabolism With Serious Injury, Including Burns

Approximately 85% of the body's phosphate pool resides within the skeleton. The remaining 15% is stored as high-energy phosphates or in its free form, where it acts as a substrate for adenosine triphosphate (ATP) production. Accordingly, phosphate plays a crucial role in energy metabolism. Trauma and critical illness result in a hypermetabolic state in which energy expenditure increases. The impact of trauma and critical illness on the body's phosphate stores and phosphate-dependent metabolic reactions is poorly understood. We had previously observed that after severe burn trauma, increased energy expenditure is temporally related to a marked reduction in serum concentrations of both parathyroid hormone and fibroblast growth factor 23, both of which have phosphaturic effects. The aim of this article is to describe as far as is known the similarities and differences in phosphate metabolism in different types of injury and to infer what these differences tell us about possible signaling pathways that may link increased phosphate utilization and phosphate retention. © 2017 The Authors. JBMR Plus is published by Wiley Periodicals, Inc. on behalf of the American Society for Bone and Mineral Research.

11β-Hydroxysteroid Dehydrogenase Type 1(11β-HSD1) mediates insulin resistance through JNK activation in adipocytes

Glucocorticoids are used to treat a number of human diseases but often lead to insulin resistance and metabolic syndrome. 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1) is a key enzyme that catalyzes the intracellular conversion of cortisone to physiologically active cortisol. Despite the known role of 11β-HSD1 and active glucocorticoid in causing insulin resistance, the molecular mechanisms by which insulin resistance is induced remain elusive. The aim of this study is to identify these mechanisms in high fat diet (HFD) experimental models. Mice on a HFD were treated with 11β-HSD1 inhibitor as well as a JNK inhibitor. We then treated 3T3-L1-derived adipocytes with prednisone, a synthetic glucocorticoid, and cells with 11β-HSD1 overexpression to study insulin resistance. Our results show that 11β-HSD1 and JNK inhibition mitigated insulin resistance in HFD mice. Prednisone stimulation or overexpression of 11β-HSD1 also caused JNK activation in cultured adipocytes. Inhibition of 11β-HSD1 blocked the activation of JNK in adipose tissue of HFD mice as well as in cultured adipocytes. Furthermore, prednisone significantly impaired the insulin signaling pathway, and these effects were reversed by 11β-HSD1 and JNK inhibition. Our study demonstrates that glucocorticoid-induced insulin resistance was dependent on 11β-HSD1, resulting in the critical activation of JNK signaling in adipocytes.

The transcriptional regulation of regucalcin gene expression

Regucalcin, which is discovered as a calcium-binding protein in 1978, has been shown to play a multifunctional role in many tissues and cell types; regucalcin has been proposed to play a pivotal role in keeping cell homeostasis and function for cell response. Regucalcin and its gene are identified in over 15 species consisting of regucalcin family. Comparison of the nucleotide sequences of regucalcin from vertebrate species is highly conserved in their coding region with throughout evolution. The regucalcin gene is localized on the chromosome X in rat and human. The organization of rat regucalcin gene consists of seven exons and six introns and several consensus regulatory elements exist upstream of the 5'-flanking region. AP-1, NF1-A1, RGPR-p117, β-catenin, and other factors have been found to be a transcription factor in the enhancement of regucalcin gene promoter activity. The transcription activity of regucalcin gene is enhanced through intracellular signaling factors that are mediated through the phosphorylation and dephosphorylation of nuclear protein in vitro. Regucalcin mRNA and its protein are markedly expressed in the liver and kidney cortex of rats. The expression of regucalcin mRNA in the liver and kidney cortex has been shown to stimulate by hormonal factors (including calcium, calcitonin, parathyroid hormone, insulin, estrogen, and dexamethasone) in vivo. Regucalcin mRNA expression is enhanced in the regenerating liver after partial hepatectomy of rats in vivo. The expression of regucalcin mRNA in the liver and kidney with pathophysiological state has been shown to suppress, suggesting an involvement of regucalcin in disease. Liver regucalcin expression is down-regulated in tumor cells, suggesting a suppressive role in the development of carcinogenesis. Liver regucalcin is markedly released into the serum of rats with chemically induced liver injury in vivo. Serum regucalcin has a potential sensitivity as a specific biochemical marker of chronic liver injury with hepatitis. Regucalcin has been proposed to be a key molecule in cellular regulation and metabolic disease.

Regulation of phosphate uptake in primary cultured rabbit renal proximal tubule cells by glucocorticoids: evidence for nongenomic as well as genomic mechanisms

We have investigated the nongenomic as well as the genomic effects of glucocorticoids on phosphate (Pi) uptake in primary rabbit renal proximal tubule cells (PTCs) and have defined the involved signaling pathways. In the present study, cortisol-BSA (cortisol-BSA) (>10(-9) M, 30 min) was found to inhibit Pi uptake in a time- and concentration-dependent manner. However, progesterone-BSA (P(4)-BSA), 17ss-estradiol-BSA (E(2)-BSA), testosterone-BSA (T(4)-BSA), aldosterone, P(4), E(2), and T(4) (10(-9) M, 1 h) had no effect on Pi uptake. In addition, cortisol-BSA (10(-9) M) did not affect either Na(+) uptake or alpha-methylglucopyranoside (alpha-MG) uptake. The cortisol-BSA-induced inhibition of Pi uptake was associated with a decrease in the V(max) for Pi uptake, rather than the K(m). The inhibitory effect of cortisol-BSA was not blocked either by actinomycin D (an inhibitor of transcription), cycloheximide (an inhibitor of translation), or classical glucocorticoid receptor antagonists (RU 486 or P(4)). The cortisol-BSA-induced inhibition of Pi uptake was blocked by two phospholipase C (PLC) inhibitors (neomycin or U73122), and two protein kinase C (PKC) inhibitors (staurosporine or bisindolylmaleimide I) but not by two adenylate cyclase/protein kinase A inhibitors [SQ 22536 (an adenylate cyclase inhibitor) or myristoylated protein kinase A inhibitor amide 14-22]. Furthermore, cortisol-BSA promoted the translocation of PKC from the cytosolic fraction to the membrane fraction, while having no effect on the activity of adenylate cyclase. Our observations may thus be interpreted as indicating that cortisol does indeed inhibit renal Pi uptake via a nongenomic mechanism, which involves the PLC/PKC pathway.

Glucocorticoid modulates Na+/H+ exchange activity in vascular smooth muscle cells by nongenomic and genomic mechanisms

BACKGROUND

In vascular smooth muscle cells (VSMCs), Na+/H+ exchange (NHE) plays an important role in intracellular pH (pHi) regulation. The genomic effect of glucocorticoid (GC) on NHE activity has been suggested in VSMCs. However, the nongenomic and genomic effects of GC on NHE activity and the underlying intracellular signaling mechanisms have not yet been demonstrated in VSMCs. Also, it is not known whether there are specific surface-binding sites of GC to the plasma membrane of VSMCs.

METHODS

The effects of short (3 h)- and long (24 h)-term exposure to corticosterone (CORTI) on NHE activity were studied in cultured rat aortic VSMCs by using pHi measurement with the pH-sensitive fluorescent dye 2'7'-bis(carboxyethyl)-5(6)-carboxyfluorescein. The NHE activity was calculated from the initial rate of Na+-dependent pHi recovery after the acid load.

RESULTS

Short-term exposure of VSMCs to CORTI (10-6 mol/L) increased NHE activity, whereas long-term exposure to CORTI decreased it. The inhibitors of gene transcription (actinomycin D) and of protein synthesis (cycloheximide) did not affect the short-term effect of CORTI on NHE activity, but inhibited the long-term effect of CORTI on NHE activity. The cytosolic GC receptor (GR) antagonist (RU38486) inhibited both the short- and long-term effects of CORTI on NHE activity, but the cytosolic mineralocorticoid receptor antagonist (spironolactone) did not influence either the short- or long-term CORTI effects. Two protein kinase C (PKC) inhibitors (staurosporine A and calphostin C) and PKC down-regulation [24-h pre-exposure to phorbol 12-myristate 13-acetate (PMA)] inhibited both short- and long-term CORTI effects. Exposure to PMA for three hours mimicked the short-term CORTI effect. The short-term CORTI effect was inhibited by the disruptor of microtubule (colchicine), but not by the disruptor of filamentous-actin (cytochalasin B). The long-term exposure to CORTI decreased NHE (NHE-1) mRNA levels to 0.65 times the control level, whereas the short-term exposure to CORTI caused no effect. Scatchard analysis of [3H]CORTI surface binding to VSMCs showed a single class of CORTI binding sites with a Bmax of 876.2 fmol per mg of cell protein and a Kd of 12.2 nmol/L. RU38486 also inhibited [3H]CORTI surface binding to VSMCs.

CONCLUSIONS

In VSMCs, NHE activity is stimulated by short-term exposure to CORTI, but is inhibited by long-term exposure to CORTI. The short-term stimulatory effect of CORTI on NHE activity is independent of gene transcription and protein synthesis, is mediated through the CORTI surface receptor, and occurs through a microtubule-dependent process. The long-term inhibitory effect of CORTI on NHE activity requires gene transcription and protein synthesis and occurs only through the cytosolic GR. The short- and long-term effects of CORTI on NHE activity occur via PKC activation. Therefore, CORTI differentially modulates NHE activity in VSMCs by nongenomic and genomic mechanisms.

Differential regulation of renal sodium-phosphate transporter by glucocorticoids during rat ontogeny

The effects of chronic administration of methylprednisolone (MP) were studied on the ontogeny of the renal type II Na-P(i) transporter (NaPi-2). Immunoblot analysis showed that MP did not alter the expression of NaPi-2 protein levels in suckling and weanling rats; however, there was an approximately 50% decrease in adolescent and adult rats. There was no change in Na-dependent P(i) uptake in brush-border membrane vesicles in suckling rats, but there was an almost twofold decrease in adolescent rats induced by MP treatment. MP administration did not alter mRNA levels in suckling or adolescent rats. Dual injections with the glucocorticoid receptor blocker RU-486 (mifepristone) and MP did not reverse the downregulation of NaPi-2 immunoreactive protein levels in adolescent rats. To control for RU-486 antagonism efficiency, Na/H exchanger isoform 3 (NHE3) protein levels were also assayed after injection with RU-486 and MP. As expected, NHE3 protein levels increased after MP injection; however, the increase was blocked in adolescent rats by RU-486. We conclude that there is an age-dependent responsiveness to glucocorticoids and that the marked decrease in NaPi-2 immunoreactive protein levels and activity in adolescent rats is due to posttranscriptional mechanisms.

Regulatory mechanism of polarized membrane transport by glucocorticoid in renal proximal tubule cells: involvement of [Ca2+]i

We examined the effect of glucocorticoids on brush border membrane transporters and, furthermore, the involvement of Ca2+ in its action in the primary cultured rabbit renal proximal tubule cells (PTCs). Dexamethasone (DEX, 10(-9) M) decreased Pi uptake by 17%; whereas DEX affected neither alpha-methyl-glucopyranoside (alpha-MG) uptake nor Na+ uptake. The DEX-induced inhibition of Pi uptake was due to a decrease of V(max). In contrast, other steroid hormones such as progesterone, testosterone, and 17beta-estradiol (10(-9) M) did not induce inhibition of Pi uptake. In order to examine the involvement of Ca2+ in DEX-induced inhibition of Pi uptake, PTCs were treated with A 23187 (10(-6) M, Ca2+ ionophore). A 23187 also inhibited Pi uptake, mimicking DEX action in Pi uptake. Treatments with W-7 (10(-4) M, calmodulin dependent kinase inhibitor), KN-62 (10(-6) M, Ca2+/calmodulin-dependent protein kinase II inhibitor), and BAPTA/AM (10(-6) M) or TMB-8 (10(-4) M) (intracellular Ca2+ mobilization blockers) blocked the DEX-induced inhibition of Pi uptake. However, nifedifine, methoxyverapamil (10(-6) M, L-type Ca2+ channel blockers), and EGTA (1 mM, extracellular Ca2+ chelator) did not block it. In conclusion, DEX inhibited Pi uptake via, in part, Ca2+/calmodulin pathway mediated by intracellular Ca2+ mobilization in the PTCs.

Protein-RNA interactions determine the stability of the renal NaPi-2 cotransporter mRNA and its translation in hypophosphatemic rats

Hypophosphatemia leads to an increase in type II Na(+)-dependent inorganic phosphate cotransporter (NaPi-2) mRNA and protein levels in the kidney and increases renal phosphate reabsorption. Nuclear transcript run-on experiments showed that the effect of a low phosphate diet was post-transcriptional. In an in vitro degradation assay, renal proteins from hypophosphatemic rats stabilized the NaPi-2 transcript 6-fold compared with control rats and this was dependent upon an intact NaPi-2 3'-untranslated region (UTR). To determine an effect of hypophosphatemia upon NaPi-2 protein synthesis, the incorporation of injected [(35)S]methionine into renal proteins was studied in vivo. Hypophosphatemia led to increased [(35)S]methionine incorporation only into NaPi-2 protein. The effect of hypophosphatemia on translation was studied in an in vitro translation assay, where hypophosphatemic renal proteins led to increased translation of NaPi-2 and other transcripts. NaPi-2 RNA interaction with cytosolic proteins was studied by UV cross-linking and Northwestern gels. Hypophosphatemic proteins led to increased binding of renal cytosolic proteins to the 5'-UTR of NaPi-2 mRNA. Therefore, hypophosphatemia increases NaPi-2 gene expression post-transcriptionally, which correlates with a more stable transcript mediated by the 3'-UTR, and an increase in NaPi-2 translation involving protein binding to the 5'-UTR. These findings show that phosphate regulates gene expression by affecting protein-RNA interactions in vivo.

Type II Na-Pi cotransport is regulated transcriptionally by ambient bicarbonate/carbon dioxide tension in OK cells

The purpose of the present study was to determine whether isohydric changes in HCO3 concentration and PCO2 directly affect apical Na-dependent Pi (Na-Pi) cotransport in OK cells (opossum kidney cell line). Cells were kept at either 44 mM NaHCO3/10% CO2, pH 7.4 (high-HCO3/CO2 condition), or 22 mM NaHCO3/5% CO2, pH 7.4 (low-HCO3/CO2 condition) (for 14-24 h). Incubation in lower HCO3/CO2 concentrations increased Na-Pi cotransport 1.5-fold. The increased Na-Pi cotransport was paralleled by a two- to threefold increased expression of the NaPi-4 transporter protein and a two- to threefold increase in NaPi-4 mRNA abundance. The increase in NaPi-4 mRNA could be completely prevented by incubation in the presence of a transcriptional inhibitor, suggesting that the increase in NaPi-4 mRNA results from an increased NaPi-4 mRNA transcription. In agreement, the NaPi-4 promoter activity was stimulated by 50% at lower HCO3/CO2 concentrations. In conclusion, our data demonstrate that isohydric changes in HCO3 concentration and PCO2 exert a significant, direct cellular effect on Na-Pi cotransport and NaPi-4 protein expression in OK cells by affecting NaPi-4 mRNA transcription.

Characterization of the 5'-flanking region of OK cell type II Na-Pi cotransporter gene

The renal type II Na-Pi cotransport is the rate-limiting step in proximal tubular phosphate (Pi) reabsorption. Among the different "proximal tubular" cell lines, this transporter seem only to be expressed in opossum kidney cells (OK cells). We have isolated the 5'-flanking region of the ok-Npt2 gene (OK cell type II Na-Pi cotransporter) including exons 1-3 and containing a TFIID site (TATA box), a GCCAAT site, an AP1 site, and two microsatellite GGAA repeats. Major transcription initiation sites were determined by primer extension and rapid amplification of 5' cDNA ends (5'-RACE). A 327-bp fragment containing the TFIID and GCAAT element was driving the downstream luciferase reporter gene in homologous transfection assays. Slightly reduced promoter activity was observed with a 198-bp fragment containing the GCAAT element; shorter fragments were without activity. Promoter activity (327-bp fragment) could also be observed in transfections into HeLa cells but not in U937 human macrophage cells, MCT mouse kidney cortex cells, and MDCK cells. Different "physiological" stimuli known to be associated with altered proximal tubular Na-Pi cotransport activity are without effect on transcriptional activity in above homologous transfection experiments.

beta-Adrenoceptors and dexamethasone synergistically stimulate the expression of the angiotensinogen gene in opossum kidney cells

We transiently co-transfected opossom kidney (OK) cells with the plasmid containing the cDNA for beta 1-adrenoceptor (pBC-beta 1 AR) or beta 2-adrenoceptor (pBC-beta 2 AR) and a fusion gene with the 5'-flanking region of the angiotensinogen (ANG) gene linked to a bacterial chloramphenicol acetyl transferase (CAT) coding sequence as a reporter, pOCAT (ANG N-1498/ +18). Co-transfection of plasmid pBC-beta 1 AR or pBC-beta 2 AR alone enhanced the expression of pOCAT (ANG N-1498/+18). The addition of isoproterenol further stimulated the expression of pOCAT (ANG N-1498/ +18) when co-transfected with pBC-beta 1AR, but not with pBC-beta 2AR. Moreover, the addition of a combination of dexamethasone and isoproterenol synergistically stimulated the expression of pOCAT (ANG N-1498/+18) when co-transfected with pBC-beta 1AR, but not when cotransfected with pBC-beta 2AR. The synergistic effect of dexamethasone and isoproterenol was inhibited by the presence of RU 486 (an antagonist of glucocorticoid) or Rp-cAMP (an inhibitor of cAMP-dependent protein kinase A I and II). To localize the putative cAMP-responsive element (CRE) and glucocorticoid responsive element (GRE) in the ANG gene, we constructed the fusion gene by inserting the DNA fragment, ANG N-806 to N-465 upstream of the thymidine kinase (TK) promoter fused to a CAT gene and introduced them with pBC-beta 1AR into OK cells. The addition of dexamethasone or isoproterenol alone stimulated the expression of pTKCAT (ANG N-806/-465). The addition of isoproterenol and dexamethasone synergistically stimulated the transcriptional activity of pTKCAT (N-806/-465). These studies demonstrate that the beta 1-adrenoceptor and dexamethasone act synergistically to stimulate the expression of the ANG gene in OK cells via the putative CRE and GREs in the 5'-flanking region of the rat ANG gene. These data should aid in the understanding of the molecular mechanism(s) of the stimulatory effect of catecholamines/glucocorticoid induced expression of the ANG gene in the kidney.

Insulin-like growth factor I, a unique calcium-dependent stimulator of 1,25-dihydroxyvitamin D3 production. Studies in cultured mouse kidney cells

Previous in vivo and in vitro studies suggest that insulin-like growth factor (IGF-I) could be a regulator of the renal production of 1,25-(OH)2D3. In the present work, the local effect of low nanomolar concentrations of IGF-I on the 25-OH-D3-1 alpha-hydroxylase activity and the mechanism of its action have been investigated. To do so, an in vitro model of mouse proximal tubular cells in primary culture has been developed. These cells bear specific high affinity IGF-I binding sites (apparent Kd = 1.95 +/- 0.46 nM) and express the ability to convert [3H]25-(OH)D3 into [3H]1,25-(OH)2D3 (Km = 139 +/- 15.7 nM). Human recombinant IGF-I (10-100 ng/ml) stimulated both sodium-dependent phosphate uptake and 1,25-(OH)2D3 synthesis by these cells, in a time- and dose-dependent manner. IGF-I did not alter the apparent Michaelis constant but increased the maximum velocity of the 25-OH-D3-1 alpha-hydroxylase activity. This effect required protein synthesis. It was not affected by calphostin or GF109203X, two protein kinase C inhibitors, and was not mimicked by phorbol 12-myristate 13-acetate. In contrast, it was blocked by verapamil, a calcium channel blocker. Calcium depletion of the medium blunted the IGF-I effect but not that of human 1-34 parathyroid hormone 5 x 10(-8) M. IGF-I thus appears to be the first example of a physiological calcium-dependent regulator of the renal metabolism of vitamin D.

Alpha-adrenoceptors and angiotensinogen gene expression in opossum kidney cells

To investigate whether alpha (alpha)-adrenoceptor agonists have a stimulatory effect on the expression of the angiotensinogen (Ang) gene in opossum kidney (OK) cells, we used OK 27 cells with a fusion gene containing the 5'-flanking regulatory sequence of the rat angiotensinogen gene fused with a human growth hormone (hGH) gene as a reporter, pOGH (Ang N-1498/+18), permanently integrated into their genomes. The level of expression of the pOGH (Ang N-1498/+18) was quantitated by the amount of immunoreactive-human growth hormone (IR-hGH) secreted into the medium. The addition of iodoclonidine (alpha 2-adrenoceptor agonist, 10(-13) to 10(-9) M) and phorbol 12-myristate 13-acetate (PMA, 10(-13) to 10(-5) M) stimulated the expression of pOGH (Ang N-1498/+18) in a dose-dependent manner, whereas the addition of phenylephrine (alpha 1-adrenoceptor agonist, 10(-13) to 10(-5) M) had no effect. The stimulatory effect of iodoclonidine was blocked by the presence of yohimbine (alpha 2-adrenoceptor antagonist) and staurosporine (an inhibitor of protein kinase C) but not blocked by the presence of prazosin (alpha 1-adrenoceptor antagonist) or Rp-cAMP (an inhibitor of cAMP-dependent protein kinase A). The addition of iodoclonidine, phenylephrine or PMA had no effect on the expression of pTKGH in OK 13 cells, an OK cell line, into which had been stably integrated a fusion gene, pTKGH containing the promoter/enhancer DNA sequence of the viral thymidine-kinase (TK) gene fused with a human growth hormone gene as a reporter.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of renal Na-HCO3 cotransporter: III. Presence and modulation by glucocorticoids in primary cultures of the proximal tubule

We looked for the presence of the Na-HCO3 cotransporter in primary cultures of the proximal tubule and examined the modulation of this cotransporter by glucocorticoid hormones. Primary cultures of the proximal tubule of the rabbit have Cl-independent, HCO3-dependent 22Na uptake which is DIDS-sensitive. In addition, in cells loaded with BCECF and perfused with Cl-free solution, removal of Na was associated with a decrease in intracellular pH which returned to normal with re-addition of Na. The pH recovery was not inhibited by EIPA but was sensitive to DIDS. These findings are compatible with existence of Na-HCO3 cotransporter in these cells. We examined the role of glucocorticoids on the activity of the Na-HCO3 cotransporter by culturing proximal tubule cells in the presence of hydrocortisone and when confluence was reached, hydrocortisone was deleted from the medium. In the absence of hydrocortisone, the activity of the cotransporter, measured either isotopically or fluorometrically, was significantly decreased, whereas re-addition of hydrocortisone 10(-8) M, restored the activity of the cotransporter to normal levels. The effect of hydrocortisone could not be duplicated by aldosterone, suggesting a glucocorticoid-dependent effect. Dexamethasone, a glucocorticoid without mineralocorticoid activity, stimulated the activity of the cotransporter within physiologic concentrations and this effect was blocked by progesterone. The effect of dexamethasone was time-dependent and was prevented by cycloheximide, a protein synthesis inhibitor. These results demonstrate that primary cultures of the proximal tubule have Na-HCO3 cotransporter activity which is modulated by physiological concentrations of glucocorticoids through a protein synthesis-dependent mechanism.