Enhancement of intestinal calcium transport by short-chain fatty acids: roles of Na+/H+ exchanger 3 and transient receptor potential vanilloid subfamily 6.

Small organic molecules in the intestinal lumen, particularly short-chain fatty acids (SCFAs) and glucose, have long been postulated to enhance calcium absorption. Here, we used 45Ca radioactive tracer to determine calcium fluxes across the rat intestine after exposure to glucose and SCFAs. Confirming previous reports, glucose was found to increase the apical-to-basolateral calcium flux in the cecum. Under apical glucose-free conditions, SCFAs (e.g., butyrate) stimulated the cecal calcium fluxes by approximately twofold, while having no effect on proximal colon. Since SCFAs could be absorbed into the circulation, we further determined whether basolateral SCFA exposure rendered some positive actions. It was found that exposure of duodenum and cecum on the basolateral side to acetate or butyrate increased calcium fluxes. Under butyrate-rich conditions, cecal calcium transport was partially diminished by Na+/H+ exchanger 3 (NHE3) inhibitor (tenapanor) and nonselective transient receptor potential vanilloid subfamily 6 (TRPV6) inhibitor (miconazole). To confirm the contribution of TRPV6 to SCFA-stimulated calcium transport, we synthesized another TRPV6 inhibitor that was demonstrated by in silico molecular docking and molecular dynamics to occlude TRPV6 pore and diminish the glucose- and butyrate-induced calcium fluxes. Therefore, besides corroborating the importance of luminal molecules in calcium absorption, our findings provided foundation for development of more effective calcium-rich nutraceuticals in combination with various absorptive enhancers, e.g., glucose and SCFAs.NEW & NOTEWORTHY Organic molecules in the intestinal lumen, e.g., glucose and short-chain fatty acids (SCFAs), the latter of which are normally produced by microfloral fermentation, can stimulate calcium absorption dependent on transient receptor potential vanilloid subfamily 6 (TRPV6) and Na+/H+ exchanger 3 (NHE3). A selective TRPV6 inhibitor synthesized and demonstrated by in silico docking and molecular dynamics to specifically bind to the pore domain of TRPV6 was used to confirm a significant contribution of this channel. Our findings corroborate physiological significance of nutrients and SCFAs in enhancing calcium absorption.

Fibroblast growth factor-21 potentiates the stimulatory effects of 1,25-dihydroxyvitamin D3 on transepithelial calcium transport and TRPV6 Ca2+ channel expression.

Fibroblast growth factor (FGF)-21 is a salient liver-derived endocrine regulator for metabolism of glucose and triglyceride as well as bone remodeling. Previously, certain peptides in the FGF family have been shown to modulate calcium absorption across the intestinal epithelia. Since FGF21 receptor, i.e., FGF receptor-1, is abundantly expressed in the enterocytes, there was a possibility that FGF21 might exert direct actions on the intestine. Herein, a large-scale production of recombinant FGF21 at the multi-gram level was developed in order to minimize variations among various batches. In the oral glucose tolerance test, recombinant FGF21 was found to reduce plasma glucose levels in mice fed high-fat diet. A series of experiments applying radioactive tracer 45Ca in Ussing chamber showed that FGF21 potentiated the stimulatory effect of low-dose 1,25-dihydroxyvitamin D3 [10 nM 1,25(OH)2D3] on the transepithelial calcium transport across intestinal epithelium-like Caco-2 monolayer. FGF21 + 1,25(OH)2D3 also decreased transepithelial resistance, but had no effect on epithelial potential difference or short-circuit current. Furthermore, 1,25(OH)2D3 alone upregulated the Caco-2 mRNA expression of the major apical calcium channels, i.e., transient receptor potential vanilloid subfamily member 6 (TRPV6), which was further elevated by a combination of FGF21 and 1,25(OH)2D3, consistent with the upregulated TRPV6 protein expression in enterocytes of FGF21-treated mice. However, FGF21 was without effects on the mRNA expression of voltage-gated calcium channel 1.3, calbindin-D9k, plasma membrane Ca2+-ATPase 1b, claudin-12 or claudin-15. In conclusion, FGF21 did exert a direct action on the intestinal epithelial cells by potentiating the 1,25(OH)2D3-enhanced calcium transport, presumably through the upregulation of TRPV6 expression.

Modulation of fibroblast growth factor-23 expression and transepithelial calcium absorption in Caco-2 monolayer by calcium-sensing receptor and calcineurin under calcium hyperabsorptive state.

Fibroblast growth factor (FGF)-23 and calcium-sensing receptor (CaSR) have previously been postulated to be parts of a negative feedback regulation of the intestinal calcium absorption to prevent excessive calcium uptake and its toxicity. However, the underlying mechanism of this feedback regulation remained elusive, especially whether it required transcription of FGF-23. Herein, we induced calcium hyperabsorptive state (CHS) by exposing intestinal epithelium-like Caco-2 monolayer to 30 mM CaCl2 and 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] after which FGF-23 mRNA levels and transepithelial calcium flux were determined. We found that CHS upregulated FGF-23 transcription, which was reverted by CaSR inhibitors (Calhex-231 and NPS2143) but without effect on CaSR transcription. Although 10 nM 1,25(OH)2D3 was capable of enhancing transepithelial calcium flux, the higher-than-normal calcium inundation as in CHS led to a decrease in calcium flux, consistent with an increase in FGF-23 protein expression. Administration of inhibitors (≤10 µM CN585 and cyclosporin A) of calcineurin, a mediator of CaSR action to control transcription and production of its target proteins, was found to partially prevent FGF-23 protein production and the negative effect of CHS on calcium transport, while having no effect on FGF-23 mRNA expression. Direct exposure to FGF-23, but not FGF-23 + PD173074 (FGFR1/3 inhibitor), also completely abolished the 1,25(OH)2D3-enhanced calcium transport in Caco-2 monolayer. Nevertheless, CHS and CaSR inhibitors had no effect on the mRNA levels of calcineurin (PPP3CB) or its targets (i.e., NFATc1-4). In conclusion, exposure to CHS induced by high apical calcium and 1,25(OH)2D3 triggered a negative feedback mechanism to prevent further calcium uptake. CaSR and its downstream mediator, calcineurin, possibly contributed to the regulatory process, in part by enhancing FGF-23 production to inhibit calcium transport. Our study, therefore, corroborated the physiological significance of CaSR-autocrine FGF-23 axis as a local feedback loop for prevention of excessive calcium uptake.

Emerging roles of calcium-sensing receptor in the local regulation of intestinal transport of ions and calcium.

Whether the intestinal mucosal cells are capable of sensing calcium concentration in the lumen and pericellular interstitium remains enigmatic for decades. Most calcium-regulating organs, such as parathyroid gland, kidney, and bone, are capable of using calcium-sensing receptor (CaSR) to detect plasma calcium and trigger appropriate feedback responses to maintain calcium homeostasis. Although both CaSR transcripts and proteins are abundantly expressed in the crypt and villous enterocytes of the small intestine as well as the surface epithelial cells of the large intestine, the studies of CaSR functions have been limited to amino acid sensing and regulation of epithelial fluid secretion. Interestingly, several lines of recent evidence have indicated that the enterocytes use CaSR to monitor luminal and extracellular calcium levels, thereby reducing the activity of transient receptor potential channel, subfamily V, member 6, and inducing paracrine and endocrine feedback responses to restrict calcium absorption. Recent investigations in zebra fish and rodents have also suggested the role of fibroblast growth factor (FGF)-23 as an endocrine and/or paracrine factor participating in the negative control of intestinal calcium transport. In this review article, besides the CaSR-modulated ion transport, we elaborate the possible roles of CaSR and FGF-23 as well as their crosstalk as parts of a negative feedback loop for counterbalancing the seemingly unopposed calciotropic effect of 1,25-dihydroxyvitamin D3 on the intestinal calcium absorption.

Parathyroid hormone increases CFTR expression and function in Caco-2 intestinal epithelial cells.

Parathyroid hormone (PTH) enhances cystic fibrosis transmembrane conductance regulator (CFTR)-mediated anion secretion by the human intestinal epithelial cell line Caco-2. With the patch-clamp and Ussing chamber techniques, we investigated how PTH stimulates CFTR activity in Caco-2 cells. Cell-attached recordings revealed that PTH stimulated the opening of CFTR-like channels, while impedance analysis demonstrated that PTH increased apical membrane capacitance, a measure of membrane surface area. Using ion substitution experiments, the PTH-stimulated increase in short-circuit current (Isc), a measure of transepithelial ion transport, was demonstrated to be Cl-- and HCO3--dependent. However, the PTH-stimulated increase in Isc was unaffected by the carbonic anhydrase inhibitor acetazolamide, but partially blocked by the intermediate-conductance Ca2+-activated K+ channel (IKCa) inhibitor clotrimazole. TRAM-34, a related IKCa inhibitor, failed to directly inhibit CFTR Cl- channels in cell-free membrane patches, excluding its action on CFTR. In conclusion, PTH enhances CFTR-mediated anion secretion by Caco-2 monolayers by increasing the expression and function of CFTR in the apical membrane and IKCa activity in the basolateral membrane.

Activation of calcium-sensing receptor by allosteric agonists cinacalcet and AC-265347 abolishes the 1,25(OH)2D3-induced Ca2+ transport: Evidence that explains how the intestine prevents excessive Ca2+ absorption.

Long-term high-calcium intake and intestinal calcium hyperabsorption are hazardous to the body. It is hypothesized that enterocytes possess mechanisms for preventing superfluous calcium absorption, including secretion of negative regulators of calcium absorption and utilization of calcium-sensing receptor (CaSR) to detect luminal calcium. Herein, Caco-2 monolayers were treated with high doses of 1,25(OH)2D3 to induce calcium hyperabsorption or directly exposed to high apical calcium. The expression of counterregulatory factor of calcium absorption, fibroblast growth factor (FGF)-23, was also investigated in the intestine of lactating rats, which physiologically exhibit calcium hyperabsorption. We found that FGF-23 expression was enhanced in all intestinal segments of lactating rats. In Caco-2 monolayers, high apical calcium and 1,25(OH)2D3 induced FGF-23 secretion into culture media. FGF-23 antagonized 1,25(OH)2D3-induced calcium transport and led to a significant, but small, change in paracellular permeability. Furthermore, high-dose 1,25(OH)2D3 upregulated FGF-23 expression, which was prevented by CaSR inhibitors. Activation of apical CaSR by cinacalcet and AC-265347 abolished 1,25(OH)2D3-induced calcium transport in a dose-dependent manner. In conclusion, the intestinal FGF-23 expression was upregulated in conditions with calcium hyperabsorption, presumably to help protect against excessive calcium absorption, while CaSR probably monitored calcium in the lumen and induced FGF-23 production for preventing superfluous calcium uptake.

Prolonged exposure to 1,25(OH)2D3 and high ionized calcium induces FGF-23 production in intestinal epithelium-like Caco-2 monolayer: A local negative feedback for preventing excessive calcium transport.

Overdose of oral calcium supplement and excessive intestinal calcium absorption can contribute pathophysiological conditions, e.g., nephrolithiasis, vascular calcification, dementia, and cardiovascular accident. Since our previous investigation has indicated that fibroblast growth factor (FGF)-23 could abolish the 1,25-dihydroxyvitamin D3 [1,25(OH)2D3]-enhanced calcium absorption, we further hypothesized that FGF-23 produced locally in the enterocytes might be part of a local negative feedback loop to regulate calcium absorption. Herein, 1,25(OH)2D3 was found to enhance the transcellular calcium transport across the epithelium-like Caco-2 monolayer, and this stimulatory effect was diminished by preceding prolonged exposure to high-dose 1,25(OH)2D3 or high concentration of apical ionized calcium. Pretreatment with a neutralizing antibody for FGF-23 prevented this negative feedback regulation of calcium hyperabsorption induced by 1,25(OH)2D3. FGF-23 exposure completely abolished the 1,25(OH)2D3-enhanced calcium transport. Western blot analysis revealed that FGF-23 expression was upregulated in a dose-dependent manner by 1,25(OH)2D3 or apical calcium exposure. Finally, calcium-sensing receptor (CaSR) inhibitors were found to prevent the apical calcium-induced suppression of calcium transport. In conclusion, prolonged exposure to high apical calcium and calcium hyperabsorption were sensed by CaSR, which, in turn, increased FGF-23 expression to suppress calcium transport. This local negative feedback loop can help prevent unnecessary calcium uptake and its detrimental consequences.

CFTR-mediated anion secretion across intestinal epithelium-like Caco-2 monolayer under PTH stimulation is dependent on intermediate conductance K+ channels.

Parathyroid hormone (PTH), a pleiotropic hormone that maintains mineral homeostasis, is also essential for controlling pH balance and ion transport across renal and intestinal epithelia. Optimization of luminal pH is important for absorption of trace elements, e.g., calcium and phosphorus. We have previously demonstrated that PTH rapidly stimulated electrogenic [Formula: see text] secretion in intestinal epithelial-like Caco-2 monolayers, but the underlying cellular mechanism, contributions of other ions, particularly Cl- and K+, and long-lasting responses are not completely understood. Herein, PTH and forskolin were confirmed to induce anion secretion, which peaked within 1-3 min (early phase), followed by an abrupt decay and plateau that lasted for 60 min (late phase). In both early and late phases, apical membrane capacitance was increased with a decrease in basolateral capacitance after PTH or forskolin exposure. PTH also induced a transient increase in apical conductance with a long-lasting decrease in basolateral conductance. Anion secretion in both phases was reduced under [Formula: see text]-free and/or Cl--free conditions or after exposure to carbonic anhydrase inhibitor (acetazolamide), CFTR inhibitor (CFTRinh-172), Na+/H+ exchanger (NHE)-3 inhibitor (tenapanor), or K+ channel inhibitors (BaCl2, clotrimazole, and TRAM-34; basolateral side), the latter of which suggested that PTH action was dependent on basolateral K+ recycling. Furthermore, early- and late-phase responses to PTH were diminished by inhibitors of PI3K (wortmannin and LY-294002) and PKA (PKI 14-22). In conclusion, PTH requires NHE3 and basolateral K+ channels to induce [Formula: see text] and Cl- secretion, thus explaining how PTH regulated luminal pH balance and pH-dependent absorption of trace minerals.

Hepcidin and 1,25(OH)2D3 effectively restore Ca2+ transport in ß-thalassemic mice: reciprocal phenomenon of Fe2+ and Ca2+ absorption.

Previously, ß-thalassemia, an inherited anemic disorder with iron overload caused by loss-of-function mutation of ß-globin gene, has been reported to induce osteopenia and impaired whole body calcium metabolism, but the pathogenesis of aberrant calcium homeostasis remains elusive. Herein, we investigated how ß-thalassemia impaired intestinal calcium absorption and whether it could be restored by administration of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] or hepcidin, the latter of which was the liver-derived antagonist of intestinal iron absorption. The results showed that, in hemizygous ß-globin knockout (BKO) mice, the duodenal calcium transport was lower than that in wild-type littermates, and severity was especially pronounced in female mice. Both active and passive duodenal calcium fluxes in BKO mice were found to be less than those in normal mice. This impaired calcium transport could be restored by 7-day 1,25(OH)2D3 treatment. The 1,25(OH)2D3-induced calcium transport was diminished by inhibitors of calcium transporters, e.g., L-type calcium channel, NCX1, and PMCA1b, as well as vesicular transport inhibitors. Interestingly, the duodenal calcium transport exhibited an inverse correlation with transepithelial iron transport, which was markedly enhanced in thalassemic mice. Thus, 3-day subcutaneous hepcidin injection and acute direct hepcidin exposure in the Ussing chamber were capable of restoring the thalassemia-associated impairment of calcium transport; however, the positive effect of hepcidin on calcium transport was completely blocked by proteasome inhibitors MG132 and bortezomib. In conclusion, both 1,25(OH)2D3 and hepcidin could be used to alleviate the ß-thalassemia-associated impairment of calcium absorption. Therefore, our study has shed light on the development of a treatment strategy to rescue calcium dysregulation in ß-thalassemia.

Intestinal mucosal changes and upregulated calcium transporter and FGF-23 expression during lactation: Contribution of lactogenic hormone prolactin.

As the principal lactogenic hormone, prolactin (PRL) not only induces lactogenesis but also enhances intestinal calcium absorption to supply calcium for milk production. How the intestinal epithelium res-ponses to PRL is poorly understood, but it is hypothesized to increase mucosal absorptive surface area and calcium transporter expression. Herein, lactating rats were found to have greater duodenal, jejunal and ileal villous heights as well as cecal crypt depths than age-matched nulliparous rats. Morphometric analyses in the duodenum and cecum showed that their mucosal adaptations were diminished by bromocriptine, an inhibitor of pituitary PRL release. PRL also upregulated calcium transporter expression (e.g., TRPV6 and PMCA1b) in the duodenum of lactating rats. Since excessive calcium absorption could be detrimental to lactating rats, local negative regulator of calcium absorption, e.g., fibroblast growth factor (FGF)-23, should be increased. Immunohistochemistry confirmed the upregulation of FGF-23 protein expression in the duodenal and cecal mucosae of lactating rats, consistent with the enhanced FGF-23 mRNA expression in Caco-2 cells. Bromocriptine abolished this lactation-induced FGF-23 expression. Additionally, FGF-23 could negate PRL-stimulated calcium transport across Caco-2 monolayer. In conclusion, PRL was responsible for the lactation-induced mucosal adaptations, which were associated with compensatory increase in FGF-23 expression probably to prevent calcium hyperabsorption.

Expression of osteoclastogenic factor transcripts in osteoblast-like UMR-106 cells after exposure to FGF-23 or FGF-23 combined with parathyroid hormone.

As a bone-derived hormone, fibroblast growth factor-23 (FGF-23) negatively regulates phosphate and calcium metabolism, while retaining growth-promoting action for mesenchymal cell differentiation. Elevated FGF-23 levels, together with hyperparathyroidism, are often observed in chronic kidney disease, which is associated with impaired bone mineralization and enhanced bone resorption. Although overexpression of osteoblast-derived osteoclastogenic cytokines might contribute to this metabolic bone disease, whether FGF-23 alone and FGF-23 plus parathyroid hormone (PTH) directly modulated the expression of osteoblast-derived osteoclastogenic genes remained elusive. Herein, we demonstrated the direct effects of FGF-23 on proliferation and mRNA expression of osteoblast-specific differentiation and osteoclastogenic markers in rat osteoblast-like UMR-106 cells in the presence or absence of PTH. FGF-23 was found to suppress UMR-106 cell proliferation, while increasing FGF-23 expression, the latter of which suggested the presence of positive feedback regulation of FGF-23 expression in osteoblasts. FGF-23 also upregulated the mRNA expression of osteoblast differentiation markers (e.g., Runx2, osterix, AJ18, Dlx5, alkaline phosphatase, and osteopontin), osteoclastogenic factors (e.g., MCSF, MCP-1, IL-6, and TNF-α), and bone resorption regulators (RANKL and osteoprotegerin). However, combined PTH and FGF-23 exposure did not alter the levels of FGF-23-induced transcripts, suggesting that both hormones had no additive effect. In conclusion, FGF-23 directly suppressed osteoblast proliferation, while inducing osteoclastogenic gene expression in UMR-106 cells, and the FGF-23-induced transcripts were not altered by long-standing PTH exposure.

Bone microstructural defects and osteopenia in hemizygous ßIVSII-654 knockin thalassemic mice: sex-dependent changes in bone density and osteoclast function.

ß-Thalassemia, a hereditary anemic disorder, is often associated with skeletal complications that can be found in both males and females. The present study aimed to investigate the age- and sex-dependent changes in bone mineral density (BMD) and trabecular microstructure in ß(IVSII-654) knockin thalassemic mice. Dual-energy X-ray absorptiometry and computer-assisted bone histomorphometry were employed to investigate temporal changes in BMD and histomorphometric parameters in male and female mice of a ß(IVSII-654) knockin mouse model of human ß-thalassemia, in which impaired splicing of ß-globin transcript was caused by hemizygous C→T mutation at nucleotide 654 of intron 2. Young, growing ß(IVSII-654) mice (1 mo old) manifested shorter bone length and lower BMD than their wild-type littermates, indicating possible growth retardation and osteopenia, the latter of which persisted until 8 mo of age (adult mice). Interestingly, two-way analysis of variance suggested an interaction between sex and ß(IVSII-654) genotype, i.e., more severe osteopenia in adult female mice. Bone histomorphometry further suggested that low trabecular bone volume in male ß(IVSII-654) mice, particularly during a growing period (1-2 mo), was primarily due to suppression of bone formation, whereas both a low bone formation rate and a marked increase in osteoclast surface were observed in female ß(IVSII-654) mice. In conclusion, osteopenia and trabecular microstructural defects were present in both male and female ß(IVSII-654) knockin thalassemic mice, but the severity, disease progression, and cellular mechanism differed between the sexes.

Running exercise alleviates trabecular bone loss and osteopenia in hemizygous ß-globin knockout thalassemic mice.

A marked decrease in ß-globin production led to ß-thalassemia, a hereditary anemic disease associated with bone marrow expansion, bone erosion, and osteoporosis. Herein, we aimed to investigate changes in bone mineral density (BMD) and trabecular microstructure in hemizygous ß-globin knockout thalassemic (BKO) mice and to determine whether endurance running (60 min/day, 5 days/wk for 12 wk in running wheels) could effectively alleviate bone loss in BKO mice. Both male and female BKO mice (1-2 mo old) showed growth retardation as indicated by smaller body weight and femoral length than their wild-type littermates. A decrease in BMD was more severe in female than in male BKO mice. Bone histomorphometry revealed that BKO mice had decreases in trabecular bone volume, trabecular number, and trabecular thickness, presumably due to suppression of osteoblast-mediated bone formation and activation of osteoclast-mediated bone resorption, the latter of which was consistent with elevated serum levels of osteoclastogenic cytokines IL-1α and -1ß. As determined by peripheral quantitative computed tomography, running increased cortical density and thickness in the femoral and tibial diaphyses of BKO mice compared with those of sedentary BKO mice. Several histomorphometric parameters suggested an enhancement of bone formation (e.g., increased mineral apposition rate) and suppression of bone resorption (e.g., decreased osteoclast surface), which led to increases in trabecular bone volume and trabecular thickness in running BKO mice. In conclusion, BKO mice exhibited pervasive osteopenia and impaired bone microstructure, whereas running exercise appeared to be an effective intervention in alleviating bone microstructural defect in ß-thalassemia.

Pre-suckling calcium supplementation effectively prevents lactation-induced osteopenia in rats.

During lactation, osteoclast-mediated bone resorption and intestinal calcium hyperabsorption help provide extra calcium for lactogenesis. Since the suckling-induced surge of pituitary prolactin (PRL) rapidly stimulates calcium absorption in lactating rats, it is hypothesized that pre-suckling oral calcium supplementation should be an efficient regimen to shift the calcium source from bone to diet, thereby slowing lactation-induced osteopenia. Our results showed that 30-min suckling markedly stimulated maternal duodenal calcium transport, which returned to the baseline at 45 min. Lactating rats given 4 mg/kg per dose calcium via a gavage tube at 90 min pre-suckling 4 doses a day for 14 days prevented a decrease in bone mineral density (BMD) of long bones and vertebrae. On the other hand, a single-dose supplementation, despite the same amount of calcium per day, appeared less effective. Because glucose and galactose further stimulated duodenal calcium transport in lactating rats, pre-suckling calcium supplement containing both sugars successfully normalized plasma ionized calcium and led to better bone gain than that with calcium alone. A histomorphometric study revealed that lactating rats given pre-suckling calcium plus monosaccharide supplement manifested greater trabecular bone volume and thickness and exhibited less eroded surface than in vehicle-treated lactating rats. Beneficial effects of the 14-day calcium supplementation persisted until 6 mo postweaning in dams and also elevated the baseline BMD of the offspring. In conclusion, our proof-of-concept study has corroborated that pre-suckling calcium supplements, especially regimens containing monosaccharides, are efficient in preventing osteopenia in lactating rats and could increase bone density in both breastfeeding mothers and neonates.

Expression of transcripts related to intestinal ion and nutrient absorption in pregnant and lactating rats as determined by custom-designed cDNA microarray.

In pregnancy and lactation, maternal adaptation for the enhancement of intestinal ion and nutrient absorption is of paramount importance for fetal development and lactogenesis. This nutrient hyperabsorption has been reported to result from upregulation of transporter gene expression, in part, under control of lactogenic hormone prolactin (PRL). Since a number of gene families are responsible for ion and nutrient transport in the rat small intestine, we herein developed a custom-designed cDNA microarray (CalGeneArray) to determine the transcriptome responses of duodenal epithelial cells during these reproductive periods, which was subsequently validated by quantitative real-time PCR. We thus designed 277 oligonucleotide probes to detect 113 transcripts related to ion/nutrient transport, bone/calcium metabolism, paracrine regulator, and cell metabolism. Pregnancy was found to upregulate the expressions of several duodenal transporters, e.g., Trpm6, Trpm7, Glut5, and Trpv6. Pregnant rats subjected to 7-day injection of bromocriptine, an inhibitor of PRL release, showed the increased levels of some other transcripts, e.g., insulin-2 and Cyp27b1, compared to untreated pregnant rats. Bromocriptine also increased the mRNA levels of insulin-2, glucose transporter-1 (Sglt1), and Cyp27b1, while decreasing those of Fgfr2c, Atp1b2, and Cldn19 in early lactation. During late lactation, the levels of eight studied transcripts (i.e., NaPi-IIb, Cyp27b1, Cldn18, Casr, Atp1b2, Xpnpep, Pept1, and Trpm7) were altered. In conclusion, the CalGeneArray was powerful to help reveal that pregnancy and lactation modulated the expression of genes related to duodenal nutrient transport and cell metabolism. Our findings supported the physiological significance of PRL in regulating nutrient absorption during pregnancy and lactation.

1,25-Dihydroxyvitamin D3 -induced intestinal calcium transport is impaired in ß-globin knockout thalassemic mice.

Besides being a common haematological disorder caused by a reduction in ß-globin production, ß-thalassemia has been reported to impair body calcium homeostasis, leading to massive bone loss and increased fracture risk. Here, we demonstrated that heterozygous ß-globin knockout thalassemic mice had a lower rate of duodenal calcium absorption compared with the wild-type littermates, whereas the epithelial electrical parameters, including transepithelial resistance, were not affected, suggesting no change in the epithelial integrity and permeability. Daily subcutaneous injection of 1 µg kg(-1) 1,25-dihydroxyvitamin D3 [1,25(OH)2 D3 ] for 3 days enhanced the duodenal calcium absorption in wild-type, but not in thalassemic mice. Although ß-thalassemia increased the mRNA level of divalent metal transporter-1, an iron transporter in the duodenum, it had no effect on the transcripts of ferroportin-1 or the principal calcium transporters. In conclusion, ß-thalassemia impaired the 1,25(OH)2 D3 -dependent intestinal calcium absorption at the post-transcriptional level, which, in turn, contributed to the dysregulation of body calcium metabolism and ß-thalassemia-induced osteopenia.

Extracellular Fe2+ and Fe3+ modulate osteocytic viability, expression of SOST, RANKL and FGF23, and fluid flow-induced YAP1 nuclear translocation.

Iron overload negatively affects bone mass and strength. However, the impact of iron excess on osteocytes-important bone cells for mechanotransduction and remodeling-is poorly understood. Herein, we examined the effects of iron exposure on osteocytes during their maturation process. We discovered that iron overload caused apoptosis of osteocytes in early and late stages of differentiation. Notably, the expression of key proteins for iron entry was downregulated during differentiation, suggesting that mature osteocytes were less susceptible to iron toxicity due to limited iron uptake. Furthermore, iron overload also enriched a subpopulation of mature osteocytes, as indicated by increased expression of Dmp1, a gene encoding protein for bone mineralization. These iron-exposed osteocytes expressed high levels of Sost, Tnfsf11 and Fgf23 transcripts. Consistently, we demonstrated that exogenous FGF23 stimulated the formation and survival of osteoclasts, suggesting its regulatory role in bone resorption. In addition, iron overload downregulated the expression of Cx43, a gene encoding gap junction protein in the dendritic processes, and impaired YAP1 nuclear translocation in response to fluid flow in differentiated osteocytes. It can be concluded that iron overload induces cellular adaptation in differentiating osteocytes, resulting in insensitivity to mechanical stimulation and potential disruption of the balance in bone remodeling.

Tissue-specific expression of senescence biomarkers in spontaneously hypertensive rats: evidence of premature aging in hypertension.

Background: Cellular senescence is an age-related physiological process that contributes to tissue dysfunction and accelerated onset of chronic metabolic diseases including hypertension. Indeed, elevation of blood pressure in hypertension coincides with premature vascular aging and dysfunction. In addition, onsets of metabolic disturbance and osteopenia in patients with hypertension have also been reported. It is possible that hypertension enhances premature aging and causes progressive loss of function in multiple organs. However, the landscape of cellular senescence in critical tissues affected by hypertension remains elusive. Materials and Methods: Heart, liver, bone, hypothalamus, and kidney were collected from spontaneously hypertensive rats (SHR) and age- and sex-matched normotensive Wistar rats (WT) at 6, 12, 24 and 36 weeks of age (n = 10 animals/group). Changes in mRNA levels of senescence biomarkers namely cyclin-dependent kinase (CDK) inhibitors (CDKIs), i.e., Cdkn2a (encoding p16Ink4a) and Cdkn1a (encoding p21cip1) as well as senescence-associated secretory phenotypes (SASPs), i.e., Timp1, Mmp12, Il6 and Cxcl1, were determined. Additionally, bone collagen alignment and hydroxy apatite crystal dimensions were determined by synchrotron radiation small- and wide-angle X-ray scattering (SAXS/WAXS) techniques. Results: Real-time PCR revealed that transcript levels of genes encoding CDKIs and SASPs in the heart and liver were upregulated in SHR from 6 to 36 weeks of age. Expression of Timp1 and Cxcl1 was increased in bone tissues isolated from 36-week-old SHR. In contrast, we found that expression levels of Timp1 and Il6 mRNA were decreased in hypothalamus and kidney of SHR in all age groups. Simultaneous SAXS/WAXS analysis also revealed misalignment of bone collagen fibers in SHR as compared to WT. Conclusion: Premature aging was identified in an organ directly affected by high blood pressure (i.e., heart) and those with known functional defects in SHR (i.e., liver and bone). Cellular senescence was not evident in organs with autoregulation of blood pressure (i.e., brain and kidney). Our study suggested that cellular senescence is induced by persistently elevated blood pressure and in part, leading to organ dysfunction. Therefore, interventions that can both lower blood pressure and prevent cellular senescence should provide therapeutic benefits for treatment of cardiovascular and metabolic consequences.

Oral calcium and vitamin D supplements differentially alter exploratory, anxiety-like behaviors and memory in male rats.

Oral calcium and calcium plus vitamin D supplements are commonly prescribed to several groups of patients, e.g., osteoporosis, fracture, and calcium deficiency. Adequate and steady extracellular calcium levels are essential for neuronal activity, whereas certain forms of calcium supplement (e.g., CaCO3) probably interfere with memory function. However, it was unclear whether a long-term use of ionized calcium (calcium chloride in drinking water ad libitum), vitamin D supplement (oral gavage) or the combination of both affected anxiety and memory, the latter of which was probably dependent on the hippocampal neurogenesis. Here, we aimed to determine the effects of calcium and/or vitamin D supplement on the anxiety- and memory-related behaviors and the expression of doublecortin (DCX), an indirect proxy indicator of hippocampal neurogenesis. Eight-week-old male Wistar rats were divided into 4 groups, i.e., control, calcium chloride-, 400 UI/kg vitamin D3-, and calcium chloride plus vitamin D-treated groups. After 4 weeks of treatment, anxiety-, exploration- and recognition memory-related behaviors were evaluated by elevated pulse-maze (EPM), open field test (OFT), and novel object recognition (NOR), respectively. The hippocampi were investigated for the expression of DCX protein by Western blot analysis. We found that oral calcium supplement increased exploratory behavior as evaluated by OFT and the recognition index in NOR test without any effect on anxiety behavior in EPM. On the other hand, vitamin D supplement was found to reduce anxiety-like behaviors. Significant upregulation of DCX protein expression was observed in the hippocampus of both calcium- and vitamin D-treated rats, suggesting their positive effects on neurogenesis. In conclusion, oral calcium and vitamin D supplements positively affected exploratory, anxiety-like behaviors and/or memory in male rats. Thus, they potentially benefit on mood and memory in osteoporotic patients beyond bone metabolism.

Fibroblast growth factor-23 abolishes 1,25-dihydroxyvitamin D3-enhanced duodenal calcium transport in male mice.

Despite being widely recognized as the important bone-derived phosphaturic hormone, whether fibroblast growth factor (FGF)-23 modulated intestinal calcium absorption remained elusive. Since FGF-23 could reduce the circulating level of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3], FGF-23 probably compromised the 1,25(OH)2D3-induced intestinal calcium absorption. FGF-23 may also exert an inhibitory action directly through FGF receptors (FGFR) in the intestinal cells. Herein, we demonstrated by Ussing chamber technique that male mice administered 1 µg/kg 1,25(OH)2D3 sc daily for 3 days exhibited increased duodenal calcium absorption, which was abolished by concurrent intravenous injection of recombinant mouse FGF-23. This FGF-23 administration had no effect on the background epithelial electrical properties, i.e., short-circuit current, transepithelial potential difference, and resistance. Immunohistochemical evidence of protein expressions of FGFR isoforms 1-4 in mouse duodenal epithelial cells suggested a possible direct effect of FGF-23 on the intestine. This was supported by the findings that FGF-23 directly added to the serosal compartment of the Ussing chamber and completely abolished the 1,25(OH)2D3-induced calcium absorption in the duodenal tissues taken from the 1,25(OH)2D3-treated mice. However, direct FGF-23 exposure did not decrease the duodenal calcium absorption without 1,25(OH)2D3 preinjection. The observed FGF-23 action was mediated by MAPK/ERK, p38 MAPK, and PKC. Quantitative real-time PCR further showed that FGF-23 diminished the 1,25(OH)2D3-induced upregulation of TRPV5, TRPV6, and calbindin-D(9k), but not PMCA(1b) expression in the duodenal epithelial cells. In conclusion, besides being a phosphatonin, FGF-23 was shown to be a novel calcium-regulating hormone that acted directly on the mouse intestine, thereby compromising the 1,25(OH)2D3-induced calcium absorption.