Induction of calbindin D-28K in Madin-Darby bovine kidney cells by 1,25(OH)2D3

Hydrogen sulfide inhibits calcium and phosphorus loss after fracture by negatively regulating glucocorticoid/glucocorticoid receptor α

AIMS

Post-fracture calcium and phosphorus excretion is greater than influx, which might be caused by stress. Glucocorticoid is known to enhance calcium and phosphorous excretion, and hydrogen sulfide (H₂S) has been shown to exert inhibitory effects on glucocorticoid. Therefore, this study explored whether H₂S could inhibit calcium and phosphorus loss after fracture by regulating glucocorticoid and/or its receptor.

MAIN METHODS

The following properties were analyzed in rats with femur fractures: serum and urinary calcium and phosphorus (by colorimetry); bone turnover markers alkaline phosphatase, serum type 1 collagen amino terminal peptide, type 1 procollagen carboxy terminal peptide, and anti-tartaric acid phosphatase (by ELISA); factors related to calcium-phosphorus metabolism including glucocorticoid, parathyroid hormone, calcitonin, fibroblast growth factor 23, and 1,25(OH)₂D₃ (by ELISA); and sulfhydration of glucocorticoid receptor α in the kidney (by immunoprecipitation linked biotin-switch assay), after supplementing with mifepristone, the H₂S donor GYY4137 or H₂S generating enzyme inhibitors aminooxyacetic acid and propargylglycine.

KEY FINDINGS

Serum H₂S decreased and glucocorticoid secretion increased in rats post-fracture. The glucocorticoid receptor inhibitor mifepristone partly blunted calcium and phosphorus loss. Furthermore, supplementation with GYY4137 reduced glucocorticoid secretion; inhibited glucocorticoid receptor α activity by sulfhydration; downregulated vitamin D 1α-hydroxylase expression; and upregulated 24-hydroxylase, calbindin-D28k, and sodium phosphate cotransporter 2a expression in the kidney; thereby inhibiting calcium and phosphorus loss induced by fracture. Moreover, inhibiting endogenous H₂S generation showed opposite effects.

SIGNIFICANCE

Our findings suggest that H₂S antagonized calcium and phosphorus loss after fracture by reducing glucocorticoid secretion and inhibiting glucocorticoid receptor α activity by sulfhydration.

Effect of endoplasmic reticulum stress on metabolic and stress signaling and kidney-specific functions in Madin-Darby bovine kidney cells

Recent studies demonstrated induction of endoplasmic reticulum (ER) stress in tissues of cows after parturition, but knowledge about the effect of ER stress on important cellular processes, such as critical signaling and metabolic pathways, in cattle is scarce. Thus, the present study aimed to investigate the effect of ER stress induction on nuclear factor-κB (NF-κB), nuclear factor E2-related factor 2 (Nrf2), and sterol regulatory element-binding protein (SREBF1) pathway in Madin-Darby bovine kidney (MDBK) cells, a widely used in vitro model in ruminant research. To consider the kidney origin of MDBK cells, the effect on renal distal tubular cell-specific functions, such as transport processes and regulation of 1,25(OH)₂D₃ levels, was also studied. Treatment of MDBK cells with 2 different ER stress inducers, thapsigargin (TG) and tunicamycin (TM), strongly induced ER stress as evident from induction of ER stress target genes, increased phosphorylation of PKR-like ER kinase, and enhanced splicing of X-box binding protein 1. The TM decreased the protein concentration of NF-κB p50 and the mRNA levels of the NF-κB target genes. Likewise, TG decreased the mRNA concentration of tumor necrosis factor and tended to decrease NF-κB p50 protein and mRNA levels of NF-κB target genes. The mRNA levels of most of the Nrf2 target genes investigated were reduced by TG and TM in MDBK cells. Both ER stress inducers reduced the mRNA levels of SREBF1 and its target genes in MDBK cells. Interestingly, TG decreased, but TM increased the mRNA level of the Ca²⁺ binding protein calbindin 1, whereas the mRNA level of the plasma membrane Ca²⁺-transporting ATPase 1 remained unchained. The mRNA level of the cytochrome P450 component 24A1 involved in 1α-hydroxylation of 25(OH)D₃ was strongly elevated, whereas the mRNA level of the cytochrome P450 component 27A1 catalyzing the breakdown of 1,25(OH)₂D₃ was markedly reduced by both ER stress inducers. The concentration of 1,25(OH)₂D₃ in the supernatant of MDBK cells was increased by approximately 15% by both TG and TM. The present study indicates that under conditions of ER stress, critical signaling pathways, such as NF-κB, Nrf2, and SREBF1, are inhibited, whereas the formation of 1,25(OH)₂D₃ is stimulated in bovine MDBK cells. Future studies are necessary to clarify the physiological relevance of these findings.

Critical Role of Calbindin-D28k in Calcium Homeostasis Revealed by Mice Lacking Both Vitamin D Receptor and Calbindin-D28k

Calbindin (CaBP)-D28k and CaBP-D9k are cytosolic vitamin D-dependent calcium-binding proteins long thought to play an important role in transepithelial calcium transport. However, recent genetic studies suggest that CaBP-D28k is not essential for calcium metabolism. Genetic ablation of this gene in mice leads to no calcemic abnormalities. Genetic inactivation of the vitamin D receptor (VDR) gene leads to hypocalcemia, secondary hyperparathyroidism, rickets, and osteomalacia, accompanied by 90% reduction in renal CaBP-D9k expression but little change in CaBP-D28k. To address whether the role of CaBP-D28k in calcium homeostasis is compensated by CaBP-D9k, we generated VDR/CaBP-D28k double knockout (KO) mice, which expressed no CaBP-D28k and only 10% of CaBP-D9k in the kidney. On a regular diet, the double KO mice were more growth-retarded and 42% smaller in body weight than VDRKO mice and died prematurely at 2.5-3 months of age. Compared with VDRKO mice, the double KO mice had higher urinary calcium excretion and developed more severe secondary hyperparathyroidism and rachitic skeletal phenotype, which were manifested by larger parathyroid glands, higher serum parathyroid hormone levels, much lower bone mineral density, and more distorted growth plate with more osteoid formation in the trabecular region. On high calcium, high lactose diet, blood-ionized calcium levels were normalized in both VDRKO and the double KO mice; however, in contrast to VDRKO mice, the skeletal abnormalities were not completely corrected in the double KO mice. These results directly demonstrate that CaBP-D28k plays a critical role in maintaining calcium homeostasis and skeletal mineralization and suggest that its calcemic role can be mostly compensated by CaBP-D9k.

Estrogen and androgen regulation of plasma membrane calcium pump activity in immortalized distal tubule kidney cells

The ATP dependent plasma membrane calcium pump (PMCA) is a regulator of renal calcium reabsorption. The effect of estrogen and dihydrotestosterone to increase the activity of the PMCA in membrane vesicle preparations from a distal tubule cell line was investigated. 17beta Estradiol (10(-10)M) increased PMCA activity (1.5 +/- 0.2-fold increase compared to control) with 24 h, but not 1 or 5 h, of exposure, an effect that was blocked by the addition of the estrogen antagonist ICI 164384. alpha Estradiol did not increase PMCA activity. Dihydrotestosterone (10(-11)M ) resulted in a dose dependent increase in PMCA activity (1.5+/-0.1-fold increase compared to control) with 24h, but not 1 or 5h, of exposure, an effect that was blocked by the androgen receptor agonist flutamide. Testosterone (10(-5)M) also increased PMCA activity (1.9+/-0.3-fold increase compared to control). Neither estrogen nor dihydrotestosterone increased PMCA protein expression in MDBK cells, indicating that these hormones increase PMCA activity by regulating PMCA activity rather than PMCA expression. These results demonstrate receptor dependent stimulatory effects of both estrogen and dihydrotestosterone to increase PMCA activity. and have significance for our understanding of estrogen and androgen deficient states on calcium transport.

Effects of vitamin D receptor inactivation on the expression of calbindins and calcium metabolism

Hypocalcemia, rickets, and osteomalacia are major phenotypic abnormalities in vitamin D receptor (VDR)-null mice. In an attempt to understand the abnormal regulation of calcium metabolism in these animals, we examined the expression of calbindins (CaBP) as well as calcium handling in the intestine and kidney of VDR null mice. In adult VDR-null mice, intestinal and renal CaBP-D9k expression was reduced by 50 and 90%, respectively, at both the mRNA and protein levels compared with wild-type littermates, whereas renal CaBP-D28k expression was not significantly changed. Intestinal calcium absorption was measured by the rate of (45)Ca disappearance from the intestine after an oral dose of the isotope. (45)Ca absorption was similar in VDR-null and wild-type mice, but the amount of (45)Ca accumulated in the serum and bone was 3-4 times higher in wild-type mice than in VDR-null mice. Despite the hypocalcemia, the urinary excretion of calcium in VDR-null mice was not different from that in wild-type mice. Moreover, 1 wk of a high-calcium diet treatment that normalized the serum ionized calcium level of VDR-null mice increased the urinary calcium level of these mutant mice to twofold higher than that of wild-type mice on the same diet, suggesting impaired renal calcium conservation in VDR-null mice. These data demonstrate that renal CaBP-D9k, but not CaBP-D28k, is highly regulated by the VDR-mediated action of 1,25-dihydroxyvitamin D(3). Furthermore, the results also suggest that impaired calcium conservation in the kidney may be the most important factor contributing to the development of hypocalcemia in VDR-null mice, and CaBP-D9k may be an important mediator of calcium reabsorption in the kidney.

Metabolism alters the selectivity of angiotensin-(1-7) receptor ligands for angiotensin receptors

The present study examined whether metabolism of the putative angiotensin-(1-7) receptor agonist and antagonist [angiotensin-(1-7) and D-alanine(7) angiotensin-(1-7), respectively] altered their ability to interact with angiotensin AT(1), AT(2), and AT(4) receptor subtypes. Both angiotensin-(1-7) and D-alanine(7) angiotensin-(1-7) competed with low affinity for (125)I-sarcosine(1), isoleucine(8) angiotensin II binding to AT(1) and AT(2) receptors in rat liver and adrenal medulla membranes, respectively, and competed with low affinity for (125)I-angiotensin IV binding to AT(4) receptors in bovine kidney epithelial cell membranes. In vitro renal metabolism of the angiotensin-(1-7) receptor ligands (incubating peptides with rat cortical tissue homogenates) had minimal influence on low-affinity binding to AT(1) and AT(2) receptors, yet caused a significant and dramatic shift toward high-affinity binding for AT(4) receptors. Low-affinity angiotensin II binding to the AT(4) receptor was also shifted toward high-affinity binding following renal metabolism of the peptide. Conversely, angiotensins with high affinity for the AT(4) receptor (e.g., angiotensin IV) were shifted toward low-affinity binding states following peptide metabolism. Incubation of (125)I-angiotensin-(1-7) with rat cortical tissue generated the high-affinity AT(4) receptor ligand (125)I-angiotensin-(3-7), whereas the renal metabolism of (125)I-angiotensin II generated both (125)I-angiotensin-(3-7) and (125)I-angiotensin IV. These results reveal that renal metabolism of angiotensin-(1-7) receptor ligands and angiotensin II yields products that have high affinity for the AT(4) receptor and could potentially contribute to the biologic actions of the parent peptide in the kidney.

Calcitriol upregulates expression and activity of the 1b isoform of the plasma membrane calcium pump in immortalized distal kidney tubular cells

The plasma membrane calcium pump (PMCA) is ubiquitously expressed in calcium transporting epithelia. PMCA is encoded by four distinct genes (PMCA1-4) which can be further posttranscriptionally modified. PMCA1b is the only isoform of PMCA1 expressed in kidney and intestine. Calcitriol upregulates PMCA protein expression and activity and PMCA1 mRNA expression in the intestine. Calcitriol has a similar effect on kidney distal tubule PMCA activity in vivo but the cellular basis for this effect has not been studied. PMCA expression in Madin-Darby bovine kidney (MDBK) cells, a distal kidney tubule cell line, was compared with a proximal tubule (LLC-PK1) and embryonic (HEK 293) kidney cell line. Only MDBK cells express PMCA1b mRNA and PMCA protein. In MDBK cells, calcitriol increased steady state expression of PMCA1b mRNA and protein and upregulated the functional activity of PMCA on calcium transport to a similar degree. Furthermore, calcitriol enhanced PMCA1b mRNA stability. These data are consistent with in vivo localization studies demonstrating the distal kidney tubule localization of PMCA protein. Furthermore, they indicate that calcitriol is an important regulator of PMCA activity in the kidney distal tubule by a pathway that includes translation and posttranscriptional modification of PMCA1b.

Selective decrease of mRNAs encoding plasma membrane calcium pump isoforms 2 and 3 in rat kidney

BACKGROUND

Although the existence of multiple isoforms of plasma membrane calcium ATPase (PMCA) is now well documented, their biological functions are not yet known. In this study, we set out to investigate the potential role of PMCA isoforms, previously identified in renal cortical tissue, in tubular reabsorption of calcium (Ca2+).

METHODS

With use of reverse transcription-polymerase chain reaction analysis, we determined levels of mRNAs encoding isoforms of PMCA1 through PMCA4 in renal cortex, liver, and brain of rats with hypercalciuria induced by feeding with a low-phosphate diet (LPD) as compared with Ca2+-retaining rats that were fed a high-phosphate diet (HPD).

RESULTS

We observed that in hypercalciuric LPD-fed rats, the mRNAs encoding isoforms PMCA2b and PMCA3(a + c) are significantly lower (Delta approximately-50%) than in HPD-fed hypocalciuric rats, whereas no changes in mRNAs encoding isoforms PMCA1b and PMCA4 were observed, and mRNA encoding calbindin 28 kDa was increased. On the other hand, the content of mRNAs encoding PMCA2b and PMCA3(a + c) in liver and brain, respectively, was not changed.

CONCLUSION

These findings are evidence that expression of PMCA isoforms in the kidney can be selectively modulated in response to pathophysiologic stimuli. The association of a decrease in mRNA encoding PMCA2b and PMCA3(a + c) with hypercalciuria suggests that the two PMCA isoforms may be operant in tubular reabsorption of Ca2+ and its regulation.

The renal function of 25-hydroxyvitamin D3-1alpha-hydroxylase

The active, hormonal form of vitamin D, 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) has numerous pleiotropic actions including the regulation of calcium homeostasis, control of bone cell differentiation and modification of immune responses. Synthesis of 1,25(OH)2D3 from the major circulating metabolite, 25-hydroxyvitamin D3 (25(OH)D3), is catalysed by the mitochondrial cytochrome P450 enzyme 25-hydroxyvitamin D-1alpha-hydroxylase (1alpha-HYD). Although 1alpha-HYD activity has been demonstrated at several ectopic sites, circulating levels of 1,25(OH)2D3 appear to reflect the expression of this enzyme in the kidney. The tight regulation of 1alpha-HYD in both renal and ectopic tissues has made studies of the expression and regulation of this enzyme remarkably difficult. However, the recent cloning of mouse, rat and human cDNAs for 1alpha-HYD has stimulated renewed interest in the molecular endocrinology of 1,25(OH)2D3 production. Analysis of the 1alpha-HYD sequence has revealed homology with the liver enzyme vitamin D-25-hydroxylase, and the ubiquitously expressed vitamin D-24-hydroxylase. Furthermore, mutations causing the inherited disorder vitamin D-dependent rickets type 1, also known as pseudo-vitamin D deficiency rickets have been described for the 1alpha-HYD gene and these have been mapped to chromosome 12q14 by linkage analysis. The availability of sequence information for the 1alpha-HYD gene has also facilitated the development of new molecular tools which will help to clarify key functions of the enzyme. Specific issues such as tissue distribution and regulatory pathways are discussed in this review, with particular emphasis on the role of 1alpha-HYD in renal calcium/phosphate homeostasis.

Transcriptional and posttranscriptional regulation of insulin-like growth factor binding protein-3 by cyclic adenosine 3',5'-monophosphate: messenger RNA stabilization is accompanied by decreased binding of a 42-kDa protein to a uridine-rich domain in the 3'-untranslated region

The Madin Darby bovine kidney (MDBK) cell line was used to investigate the mechanisms underlying the cAMP regulation of insulin-like growth factor binding protein-3 (IGFBP-3) gene expression. Treatment of confluent monolayers either with forskolin or cAMP produced a 60- to 75-fold induction of IGFBP-3 mRNA and protein levels. This effect did not require new protein synthesis as inhibition of translation by cycloheximide actually caused a 2-fold increase in the cAMP induction. The rates of IGFBP-3 gene transcription, assessed by nuclear run-on assays, increased approximately 15-fold in cells exposed to cAMP. In addition, the half-life of the IGFBP-3 mRNA transcript was increased approximately 3-fold in the presence of cAMP. Gel mobility shift and competition experiments revealed the specific binding of an approximately 42-kDa cytoplasmic protein factor to the 3'-untranslated region (3'-UTR) of the IGFBP-3 mRNA. A 21-nucleotide uridine-rich segment that contained no AUUUA motif was sufficient for the specific binding. The binding activity of this protein was reduced after cAMP treatment but was increased by phosphatase treatment. In conclusion, the cAMP induction of IGFBP-3 mRNA in MDBK cells occurred at both the transcriptional and posttranscriptional levels. The IGFBP-3 mRNA stabilization in MDBK cells probably involved the phosphorylation of a member of the family of U-rich region mRNA-binding proteins and is the first reported member whose RNA-binding activity is reduced by cAMP.

The effect of 1,25-vitamin D3 on calbindin-D and calcium-metabolic variables in the rat

Intraperitoneal injection of 1,25-(OH)2D3 4 micrograms/kg was given to 84 calcium- and vitamin D-repleted Wistar rats and samples of plasma, duodenal mucosa and renal tissue were taken after 0, 3, 6, 12, 24, 48 and 96 hr (n = 12 at each time interval). Plasma-ionized Ca increased after 6 hr, reached a maximum after 24 hr and returned to the initial values after 96 hr. The concentrations of renal calbindin-D28k and intestinal calbindin-D9k did not increase until 48 hr after injection and remained elevated until 96 hr after. Therefore, significantly elevated concentrations of the cytosolic calbindin-D were found at a time with normal values of plasma Ca. The present data suggest that calbindin-D does not alone increase the transcellular Ca transport and, therefore, supports the view that calbindin-Ds may serve as Ca buffer proteins.

Retinoid X receptors in the kidney: their protein expression and functional significance

Retinoid X receptors (RXRs) heterodimerize with 1,25-dihydroxyvitamin D3 (VD) receptor (VDR), and play important roles in VD-regulated transactivation. VD acts on many tissues including kidney for the regulation of calcium homeostasis. In the kidney, the expression of VDR in the tubular cells has been well studied. In contrast, little is known about the localization and the functional significance of RXRs there. In order to elucidate these questions, we first performed immunohistochemical analyses of rat kidney using isoform-specific antimouse RXR antibodies we have previously reported. Interestingly, all RXR isoforms, predominantly RXR alpha, mainly localized to the proximal and the distal tubules, but not to the glomeruli. The serial section staining using anti-VDR antibody showed the colocalization of RXR alpha and VDR in those tubular cells. In order to elucidate the functional significance of endogenous receptors in the tubular cells, we next performed transient transfection studies using the tubular-cell derived Madin-Darby bovine kidney cells, which express both endogenous VDR and RXR. We transfected a reporter plasmid containing direct repeat 3 (DR3) sequence, to which only RXR/VDR heterodimer can bind, and found that VD and 9-cis retinoic acid, as well as VD and RXR selective agonist LG100153, had an additive effect for the DR3 transactivation. Taken together, we speculate that endogenous RXRs co-localize with VDR, and coregulate VD-dependent genes in the tubular cells of the kidney as RXR/VDR heterodimer.