Thyroid hormone stimulation of Na/Pi-cotransport in opossum kidney cells

The Causes of Hypo- and Hyperphosphatemia in Humans

Phosphate homeostasis involves several major organs that are the skeleton, the intestine, the kidney, and parathyroid glands. Major regulators of phosphate homeostasis are parathormone, fibroblast growth factor 23, 1,25-dihydroxyvitamin D, which respond to variations of serum phosphate levels and act to increase or decrease intestinal absorption and renal tubular reabsorption, through the modulation of expression of transcellular transporters at the intestinal and/or renal tubular level. Any acquired or genetic dysfunction in these major organs or regulators may induce hypo- or hyperphosphatemia. The causes of hypo- and hyperphosphatemia are numerous. This review develops the main causes of acquired and genetic hypo- and hyperphosphatemia.

Phosphate Transport in Epithelial and Nonepithelial Tissue

Phosphate is an essential nutrient for life and is a critical component of bone formation, a major signaling molecule, and structural component of cell walls. Phosphate is also a component of high-energy compounds (i.e., AMP, ADP, and ATP) and essential for nucleic acid helical structure (i.e., RNA and DNA). Phosphate plays a central role in the process of mineralization, normal serum levels being associated with appropriate bone mineralization, while high and low serum levels are associated with soft tissue calcification. The serum concentration of phosphate and the total body content of phosphate are highly regulated, a process that is accomplished by the coordinated effort of two families of sodium-dependent transporter proteins. The three isoforms of the SLC34 family (SLC34A1-A3) show very restricted tissue expression and regulate intestinal absorption and renal excretion of phosphate. SLC34A2 also regulates the phosphate concentration in multiple lumen fluids including milk, saliva, pancreatic fluid, and surfactant. Both isoforms of the SLC20 family exhibit ubiquitous expression (with some variation as to which one or both are expressed), are regulated by ambient phosphate, and likely serve the phosphate needs of the individual cell. These proteins exhibit similarities to phosphate transporters in nonmammalian organisms. The proteins are nonredundant as mutations in each yield unique clinical presentations. Further research is essential to understand the function, regulation, and coordination of the various phosphate transporters, both the ones described in this review and the phosphate transporters involved in intracellular transport.

Mechanisms of phosphate transport

Over the past 25 years, successive cloning of SLC34A1, SLC34A2 and SLC34A3, which encode the sodium-dependent inorganic phosphate (P_i) cotransport proteins 2a-2c, has facilitated the identification of molecular mechanisms that underlie the regulation of renal and intestinal P_i transport. P_i and various hormones, including parathyroid hormone and phosphatonins, such as fibroblast growth factor 23, regulate the activity of these P_i transporters through transcriptional, translational and post-translational mechanisms involving interactions with PDZ domain-containing proteins, lipid microdomains and acute trafficking of the transporters via endocytosis and exocytosis. In humans and rodents, mutations in any of the three transporters lead to dysregulation of epithelial P_i transport with effects on serum P_i levels and can cause cardiovascular and musculoskeletal damage, illustrating the importance of these transporters in the maintenance of local and systemic P_i homeostasis. Functional and structural studies have provided insights into the mechanism by which these proteins transport P_i, whereas in vivo and ex vivo cell culture studies have identified several small molecules that can modify their transport function. These small molecules represent potential new drugs to help maintain P_i homeostasis in patients with chronic kidney disease - a condition that is associated with hyperphosphataemia and severe cardiovascular and skeletal consequences.

Identification and expression analysis of type II and type III Pi transporters in the opossum kidney cell line

NEW FINDINGS

What is the central question of this study? The opossum kidney (OK) cell line is the main in vitro model of proximal tubular P_i transport, but it is incomplete because only the NaPiIIa P_i transporter has been identified. What is the main finding and its importance? We have cloned and characterized the P_i transporters NaPiIIc, PiT1 and PiT2 from OK cells and have analysed the relevance of the four transporters to P_i transport. All four transporters are involved in the upregulated P_i transport of cells incubated using a low-P_i medium, and only PiT1 is not involved in basal transport.

ABSTRACT

The apical membrane of renal proximal tubular epithelial cells is the main controller of phosphate homeostasis, because it determines the rate of urinary P_i excretion. The opossum kidney (OK) cell line is a good model for studying this function, but only NaPiIIa (NaPi4) has been identified to date as a P_i transporter in this cell line. In this work, we have identified three additional P_i transporters that are present in OK cells: NaPiIIc, PiT1 and PiT2. All three sequences are similar to the corresponding orthologues, but PiT1 is missing the first transmembrane domain. Confluent cells exhibit characteristics of type II P_i transport, which increases with alkalinity and is inhibited by phosphonoformic acid (PFA), and they mainly express NaPiIIa and NaPiIIc, with a low abundance of PiT1 and PiT2. Proliferating cells show a higher expression of PiT1 and PiT2 and a low expression of NaPiIIa and NaPiIIc. Adaptation to a low P_i concentration for 24 h induces the expression of RNA from NaPiIIa and NaPiIIc, which is not prevented by actinomycin D. Small interfering RNA transfections revealed that PiT1 is not necessary for P_i transport, but it is necessary for adaptation to a low P_i , similar to NaPiIIa and PiT2. Our study reveals the complexity of the coordination between the four P_i transporters, the variability of RNA expression according to confluence and the heterogeneous correlation between P_i transport and RNA levels.

The transcriptome of the Didelphis virginiana opossum kidney OK proximal tubule cell line

The OK cell line derived from the kidney of a female opossum Didelphis virginiana has proven to be a useful model in which to investigate the unique regulation of ion transport and membrane trafficking mechanisms in the proximal tubule (PT). Sequence data and comparison of the transcriptome of this cell line to eutherian mammal PTs would further broaden the utility of this culture model. However, the genomic sequence for D. virginiana is not available and although a draft genome sequence for the opossum Monodelphis domestica (sequenced in 2012 by the Broad Institute) exists, transcripts sequenced from both species show significant divergence. The M. domestica sequence is not highly annotated, and the majority of transcripts are predicted rather than experimentally validated. Using deep RNA sequencing of the D. virginiana OK cell line, we characterized its transcriptome via de novo transcriptome assembly and alignment to the M. domestica genome. The quality of the de novo assembled transcriptome was assessed by the extent of homology to sequences in nucleotide and protein databases. Gene expression levels in the OK cell line, from both the de novo transcriptome and genes aligned to the M. domestica genome, were compared with publicly available rat kidney nephron segment expression data. Our studies demonstrate the expression in OK cells of numerous PT-specific ion transporters and other key proteins relevant for rodent and human PT function. Additionally, the sequence and expression data reported here provide an important resource for genetic manipulation and other studies on PT cell function using these cells.

Severe hyponatremia in association with I(131) therapy in a patient with metastatic thyroid cancer

Hyponatremia is a common clinical problem that results from various causes. Hypothyroidism is known to be one of the causes of this disorder. We report a case of metastatic thyroid cancer presenting with severe hyponatremia in association with hypothyroidism induced by pretreatment of I(131) therapy, such as a low-iodine diet and withdrawal of thyroid hormone. Serum arginine vasopressin (AVP) was elevated and urine osmolality was higher than that of serum. Saline infusion and thyroid hormone replacement normalized serum sodium and AVP. Inappropriate secretion of AVP in hypothyroid state was thought to be one of the causes of this hyponatremia.

Thyroid hormones regulate phosphate homoeostasis through transcriptional control of the renal type IIa sodium-dependent phosphate co-transporter (Npt2a) gene

The type IIa renal sodium-dependent phosphate (Na/Pi) co-transporter Npt2a is implicated in the control of serum phosphate levels. It has been demonstrated previously that renal Npt2a protein and its mRNA expression are both up-regulated by the thyroid hormone T3 (3,3',5-tri-iodothyronine) in rats. However, it has never been established whether the induction was mediated by a direct effect of thyroid hormones on the Npt2a promoter. To address the role of Npt2a in T3-dependent regulation of phosphate homoeostasis and to identify the molecular mechanisms by which thyroid hormones modulate Npt2a gene expression, mice were rendered pharmacologically hypo- and hyper-thyroid. Hypothyroid mice showed low levels of serum phosphate and a marked decrease in renal Npt2a protein abundance. Importantly, we also showed that Npt2a-deficient mice had impaired serum phosphate responsiveness to T3 compared with wild-type mice. Promoter analysis with a luciferase assay revealed that the transcriptional activity of a reporter gene containing the Npt2a promoter and intron 1 was dependent upon TRs (thyroid hormone receptors) and specifically increased by T3 in renal cells. Deletion analysis and EMSAs (electrophoretic mobility-shift assays) determined that there were unique TREs (thyroid-hormone-responsive elements) within intron 1 of the Npt2a gene. These results suggest that Npt2a plays a critical role as a T3-target gene, to control phosphate homoeostasis, and that T3 transcriptionally activates the Npt2a gene via TRs in a renal cell-specific manner.

Estrogen downregulates the proximal tubule type IIa sodium phosphate cotransporter causing phosphate wasting and hypophosphatemia

Estrogen treatment causes significant hypophosphatemia in patients. To determine the mechanisms responsible for this effect, we injected ovariectomized rats with either 17beta-estradiol or vehicle for three days. Significant renal phosphate wasting and hypophosphatemia occurred in estrogen-treated rats despite a decrease in their food intake. The mRNA and protein levels of the renal proximal tubule sodium phosphate cotransporter (NaPi-IIa) were significantly decreased in estradiol-treated ad-libitum or pair-fed groups. Estrogen did not affect NaPi-III or NaPi-IIc expression. In ovariectomized and parathyroidectomized rats, 17beta-estradiol caused a significant decrease in NaPi-IIa mRNA and protein expression compared to vehicle. Estrogen receptor alpha isoform blocker significantly blunted the anorexic effect of 17beta-estradiol but did not affect the downregulation of NaPi-IIa. Our studies show that renal phosphate wasting and hypophosphatemia induced by estrogen are secondary to downregulation of NaPi-IIa in the proximal tubule. These effects are independent of food intake or parathyroid hormone levels and likely not mediated through the activation of estrogen receptor alpha subtype.

The adaptation and relationship of FGF-23 to changes in mineral metabolism in Graves' disease

OBJECTIVE

The aim of this study was to observe the changes in bone and mineral metabolism and to confirm the regulation of fibroblast growth factor-23 (FGF-23) in untreated Graves' disease.

PATIENTS AND MEASUREMENTS

The study comprised 39 patients, with or without Graves' disease. The Graves' disease group was made up of 21 newly diagnosed patients, enrolled before starting treatment. Their disease was determined by biochemical and radiological means. The control group was composed of 18 people who were proven to be euthyroid without any diseases affecting bone and mineral metabolism. FGF-23, calcium, phosphate, PTH, 25-hydroxyvitamin D [25(OH)D] and 1,25-dihydroxyvitamin D [1,25(OH)2D] levels and bone turnover markers were compared between these groups.

RESULTS

Serum calcium and phosphate, plasma FGF-23 and free T4 were significantly higher in the Graves' disease group than in the healthy control group (P < 0.05). The bone turnover markers serum osteocalcin and C-terminal cross-linked telopeptide of type 1 collagen (s-CTx) were also significantly elevated in the Graves' disease group, and had a positive correlation with free T4 levels. However, there was no significant decrease in PTH and 1,25(OH)2D in the Graves' disease group. Plasma levels of FGF-23 exhibited a positive correlation with serum phosphate levels and with free T4 levels (P < 0.05).

CONCLUSIONS

These findings suggest that FGF-23 is physiologically related to serum phosphate homeostasis, as indicated indirectly by the changes in bone and mineral metabolism, in untreated Graves' disease.

Cytotoxicity of peroxisome proliferator-activated receptor alpha and gamma agonists in renal proximal tubular cell lines

Fibrates and thiazolidinediones are agonists of peroxisome proliferator-activated receptors (PPAR) alpha and gamma, pharmacologically designed to control dyslipidemia and insulin resistance, respectively. Several works have reported the toxicity of some agonists in a number of tissues. In this work we have analyzed the toxicity of two PPARalpha (WY14643 and clofibrate) and two PPARgamma (pioglitazone and ciglitazone) agonists, using three different renal proximal tubular cell lines: Opossum OK, pig LLC-PK1, and murine MCT. Cell death was determined by the activity of intracellular lactate dehydrogenase. WY14643 and ciglitazone increased cell death with LC50 values of 92-124 microM and 8.6-14.8 microM, respectively, depending on the cell line. Clofibrate and pioglitazone were, however, non-cytotoxic even at concentrations of 10 and 100 higher than the corresponding EC50, which suggests that cell death is independent of PPAR activation. Discrimination between apoptosis or necrosis was analyzed by light microscopy and stress fiber morphology, double staining with acridine orange and ethidium bromide, binding of annexin V, caspase-3 activity, and DNA laddering. With these methods, no signs of apoptosis were observed, which suggests a direct necrosis of the compounds on these renal proximal tubular cell lines.

Interaction of MAP17 with NHERF3/4 induces translocation of the renal Na/Pi IIa transporter to the trans-Golgi

The function of the NaPiIIa renal sodium-phosphate transporter is regulated through a complex network of interacting proteins. Several PDZ domain-containing proteins interact with its COOH terminus while the small membrane protein MAP17 interacts with its NH(2) end. To elucidate the function of MAP17, we identified its interacting proteins using both bacterial and mammalian two-hybrid systems. Several PDZ domain-containing proteins, including the four NHERF proteins, as well as NaPiIIa and NHE3, were found to bind to MAP17. The interactions of MAP17 with the NHERF proteins and with NaPiIIa were further analyzed in opossum kidney (OK) cells. Expression of MAP17 alone had no effect on the NaPiIIa apical membrane distribution, but coexpression of MAP17 and NHERF3 or NHERF4 induced internalization of NaPiIIa, MAP17, and the PDZ protein to the trans-Golgi network (TGN). This effect was not observed when MAP17 was cotransfected with NHERF1/2 proteins. Inhibition of protein kinase C (PKC) prevented expression of the three proteins in the TGN. Activation of PKC in OK cells transfected only with MAP17 induced complete degradation of MAP17 and NaPiIIa. When lysosomal degradation was prevented, both proteins accumulated in the TGN. When the dopamine D1-like receptor was activated with fenoldopam, both NaPiIIa and MAP17 also accumulated in the TGN. Finally, cotransfection of MAP17 and NHERF3 prevented the adaptive upregulation of phosphate transport activity in OK cells in response to low extracellular phosphate. Therefore, the interaction between MAP17, NHERF3/4, and NaPiIIa in the TGN could be an important intermediate or alternate path in the internalization of NaPiIIa.

Post-renal transplantation hypophosphatemia: a review and novel insights

PURPOSE OF REVIEW

This review intends to elucidate the pathophysiologic mechanism of renal phosphorus loss in the post-renal transplantation population. This review will provide new insight in to the pathophysiologic mechanism(s) responsible for the development of this phenomenon and will also explore the pathogenetic role of persistent phosphorus wasting in the development of post-renal transplantation osteodystrophy.

RECENT FINDINGS

Recently, the phosphaturic hormone, fibroblast growth factor-23, has been ascertain to be increased in the sera of patients with chronic kidney and end-stage renal disease. There is new evidence that a non-PTH humoral factor is persistently present in post-renal transplantation patients that is likely responsible for the observed persistent renal phosphorus loss. We offer that fibroblast growth factor-23 (and/or other phosphatonins) is the culprit factor responsible for the phenomenon of persistent hypophosphatemia in post-renal transplantation patients. Moreover, we believe that the phenomenon of persistent renal phosphorus wasting is an important but overlooked cause of osteodystrophy and increased fracture risk in this patient population.

SUMMARY

The pathophysiology of post-renal transplantation phosphorus wasting is complex and to date is still not fully recognized. Further studies of the regulatory mechanism of fibroblast growth factor-23 and its metabolism may offer additional insights into phosphorus homeostasis and its clinical application in the post-renal transplantation population.

Regulation of sodium-proton exchanger isoform 3 (NHE3) by PKA and exchange protein directly activated by cAMP (EPAC)

The Na(+)/H(+) exchanger 3 (NHE3) is expressed in the brush border membrane (BBM) of proximal tubules (PT). Its activity is down-regulated on increases in intracellular cAMP levels. The aim of this study was to investigate the contribution of the protein kinase A (PKA) and the exchange protein directly activated by cAMP (EPAC) dependent pathways in the regulation of NHE3 by adenosine 3',5'-cyclic monophosphate (cAMP). Opossum kidney cells and murine kidney slices were treated with cAMP analogs, which selectively activate either PKA or EPAC. Activation of either pathway resulted in an inhibition of NHE3 activity. The EPAC-induced effect was independent of PKA as indicated by the lack of activation of the kinase and the insensitivity to the PKA inhibitor H89. Both PKA and EPAC inhibited NHE3 activity without inducing changes in the expression of the transporter in BBM. Activation of PKA, but not of EPAC, led to an increase of NHE3 phosphorylation. In contrast, activation of PKA, but not of EPAC, inhibited renal type IIa Na(+)-coupled inorganic phosphate cotransporter (NaPi-IIa), another Na-dependent transporter expressed in proximal BBM. PKA, but not EPAC, induced the retrieval of NaPi-IIa from BBM. Our results suggest that EPAC activation may represent a previously unrecognized mechanism involved in the cAMP regulation of NHE3, whereas regulation of NaPi-IIa is mediated by PKA but not by EPAC.

Fibroblast growth factor-23 in patients with Graves' disease before and after antithyroid therapy: its important role in serum phosphate regulation

OBJECTIVE

Hyperthyroidism is a well-described cause of hyperphosphatemia. We aimed to clarify the physiological role of fibroblast growth factor (FGF)-23 in serum phosphate homeostasis in patients with Graves' disease during the course of treatment for hyperthyroidism.

CONTEXT

The study group comprised 56 patients (45 for a cross-sectional study and 11 for a longitudinal study) with Graves' disease. For the cross-sectional study, patients were assigned, on the basis of their serum phosphate level, to a hypophosphatemia group (n = 14), a normophosphatemia group (n = 16), or a hyperphosphatemia group (n = 15). Serum FGF-23, calcium, phosphate, PTH, and 1,25-dihydroxyvitamin D [1,25(OH)(2)D] levels were compared between the three groups. For the longitudinal study, we assessed changes in these biochemical indices before and after antithyroid treatment.

RESULTS

In the cross-sectional study, the serum FGF-23 level was significantly higher (P < 0.05) in the hyperphosphatemia group than in the other groups (61 +/- 36 ng/liter vs. 31 +/- 22 ng/liter and 30 +/- 9 ng/liter). In the longitudinal study, serum levels of FGF-23 decreased significantly (P < 0.05) from a high of 54 +/- 12 ng/liter before treatment to 29 +/- 14 ng/liter after treatment. In contrast, the serum 1,25(OH)(2)D level increased significantly (P < 0.005) from 55 +/- 22 pmol/liter before treatment to 185 +/- 76 pmol/liter 3 months after treatment. Serum FGF-23 levels were positively correlated with serum phosphate levels (P < 0.0001) and negatively correlated with serum 1,25(OH)(2)D levels (P < 0.0001).

CONCLUSIONS

The significant positive correlation between serum levels of phosphate and FGF-23 indicates that FGF-23 may play an important role in serum phosphate homeostasis by its up-regulation in the hyperphosphatemic condition.

Effects of angiotensin II on NaPi-IIa co-transporter expression and activity in rat renal cortex

The kidney plays a major role in reabsorption of phosphate with the majority occurring in the proximal tubule (PT). The type IIa sodium-phosphate co-transporter (NaPi-IIa) is the main player in PT. The purpose of current study was to determine the effect of angiotensin II (A-II) on membrane expression of NaPi-IIa in the rat renal cortex. A-II (500 ng/kg/min) was chronically infused into the Sprague-Dawley rats by miniosmotic pump for 7 days. The arterial pressure and circulating plasma A-II level along with urine output were markedly increased in A-II rats. There was diuresis but no natriuresis. The phosphate excretion increased sevenfold on day 4 and 5.7-fold on day 7. There was no change in Na-dependent Pi uptake in brush-border membrane (BBM) vesicles between A-II-treated group and control on day 4, however, there was a 43% increase on day 7. Western blot analysis of NaPi-IIa protein abundance showed a parallel pattern: no change after 4 days of treatment and a 48% increase after 7 days of treatment. However, Northern blot analysis of cortical RNA showed no change in NaPi-IIa mRNA abundance on day 7. A-II stimulation of Na/Pi co-transport activity is a result of increases in the expression of BBM NaPi-IIa protein level and that stimulation is most likely mediated by posttranscriptional mechanisms.

Rat kidney MAP17 induces cotransport of Na-mannose and Na-glucose in Xenopus laevis oocytes

Renal reabsorption is the main mechanism that controls mannose homeostasis. This takes place through a specific Na-coupled uphill transport system, the molecular identity of which is unknown. We prepared and screened a size-selected rat kidney cortex cDNA library through the expression of mannose transport in Xenopus laevis oocytes. We have identified a membrane protein that induces high-affinity and specific Na-dependent transport of d-mannose and d-glucose in X. laevis oocytes, most likely through stimulation of the capacity of an endogenous transport system of the oocyte. Sequencing has revealed that the cDNA encodes the counterpart of the human membrane-associated protein MAP17, previously known by its overexpression in renal, colon, lung, and breast carcinomas. We show that MAP17 is a 12.2-kDa nonglycosylated membrane protein that locates to the brush-border plasma membrane and the Golgi apparatus of transfected cells and that it is expressed in the proximal tubules of the kidney cortex and in the spermatids of the seminiferous tubules. It spans twice the cell membrane, with both termini inside the cell, and seems to form homodimers through intracellular Cys55, a residue also involved in transport expression. MAP17 is responsible for mannose transport expression in oocytes by rat kidney cortex mRNA. The induced transport has the functional characteristics of a Na-glucose cotransporter (SGLT), because d-glucose and alpha-methyl-d-glucopyranoside are also accepted substrates that are inhibited by phloridzin. The corresponding transporter from the proximal tubule remains to be identified, but it is different from the known mammalian SGLT-1, -2, and -3.

Renal expression of sodium transporters and aquaporin-2 in hypothyroid rats

Hypothyroidism is associated with significant abnormalities in the renal handling of salt and water. To address the involvement of tubular transport proteins in these abnormalities, rats were rendered pharmacologically hypothyroid and the abundance of major tubular transport proteins was assessed by immunoblot and immunohistochemistry. Hypothyroidism resulted in a marked reduction in kidney size and creatinine clearance along with decreased or unchanged total kidney abundance of the transport proteins. Whereas the proximal tubular type 3 Na/H exchanger (NHE3) and type 2 Na-phosphate cotransporter (NaPi2) stood out by their disproportionately reduced abundance, the bumetanide-sensitive type 2 Na-K-2Cl cotransporter (NKCC2) and aquaporin-2 (AQP2) were unaltered in their total kidney abundance despite a markedly lower kidney mass. The latter proteins in fact showed enhanced immunostaining. Decreased NHE3 and NaPi2 expression was most likely due to a combination of triiodo-l-thyronine (T(3)) deficiency along with a reduced glomerular filtration rate. The increased abundance of NKCC2 and AQP2 may have been caused by an increased action of vasopressin since urinary excretion of this hormone was elevated. On the other hand, the thiazide-sensitive Na-Cl cotransporter; the alpha-, beta-, and gamma-subunits of the amiloride-sensitive epithelial Na channel; and the alpha(1)-subunit of Na-K-ATPase showed a moderate decrease in total kidney abundance that was largely proportional to the smaller kidney mass. Although the observed expression of transporters was associated with a balanced renal sodium handling, altered transporter abundance may become functionally relevant if the hypothyroid kidney is challenged by an additional destabilization of the milieu interieur that has previously been shown to result in an inadequate natriuresis and clinical symptoms.

Glucocorticoid regulation and glycosylation of mouse intestinal type IIb Na-P(i) cotransporter during ontogeny

We sought to characterize expression of an apically expressed intestinal Na-P(i) cotransporter (Na-P(i)-IIb) during mouse ontogeny and to assess the effects of methylprednisolone (MP) treatment. In control mice, Na-P(i) uptake by intestinal brush-border membrane vesicles was highest at 14 days of age, lower at 21 days, and further reduced at 8 wk and 8-9 mo of age. Na-P(i)-IIb mRNA and immunoreactive protein levels in 14-day-old animals were markedly higher than in older groups. MP treatment significantly decreased Na-P(i) uptake and Na-P(i)-IIb mRNA and protein expression in 14-day-old mice. Additionally, the size of the protein was smaller in 14-day-old mice. Deglycosylation of protein from 14-day-old and 8-wk-old animals with peptide N-glycosidase reduced the molecular weight to the predicted size. We conclude that intestinal Na-P(i) uptake and Na-P(i)-IIb expression are highest at 14 days and decrease with age. Furthermore, MP treatment reduced intestinal Na-P(i) uptake approximately threefold in 14-day-old mice and this reduction correlates with reduced Na-P(i)-IIb mRNA and protein expression. We also demonstrate that Na-P(i)-IIb is an N-linked glycoprotein and that glycosylation is age dependent.

Proximal tubular phosphate reabsorption: molecular mechanisms

Renal proximal tubular reabsorption of P(i) is a key element in overall P(i) homeostasis, and it involves a secondary active P(i) transport mechanism. Among the molecularly identified sodium-phosphate (Na/P(i)) cotransport systems a brush-border membrane type IIa Na-P(i) cotransporter is the key player in proximal tubular P(i) reabsorption. Physiological and pathophysiological alterations in renal P(i) reabsorption are related to altered brush-border membrane expression/content of the type IIa Na-P(i) cotransporter. Complex membrane retrieval/insertion mechanisms are involved in modulating transporter content in the brush-border membrane. In a tissue culture model (OK cells) expressing intrinsically the type IIa Na-P(i) cotransporter, the cellular cascades involved in "physiological/pathophysiological" control of P(i) reabsorption have been explored. As this cell model offers a "proximal tubular" environment, it is useful for characterization (in heterologous expression studies) of the cellular/molecular requirements for transport regulation. Finally, the oocyte expression system has permitted a thorough characterization of the transport characteristics and of structure/function relationships. Thus the cloning of the type IIa Na-P(i )cotransporter (in 1993) provided the tools to study renal brush-border membrane Na-P(i) cotransport function/regulation at the cellular/molecular level as well as at the organ level and led to an understanding of cellular mechanisms involved in control of proximal tubular P(i) handling and, thus, of overall P(i) homeostasis.

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.

Recent advances in epithelial sodium-coupled phosphate transport

This review focuses on recent developments in the molecular characterization of renal sodium-phosphate cotransporters and the mechanisms involved in their regulation. Of the three classes of sodium-phosphate cotransporters expressed in the mammalian kidney, the type II transporter, NPT2/Npt2 reflects the characteristics of apical sodium-dependent phosphate transport, and is a target for regulation. Studies in mice in which the Npt2 gene was disrupted by targeted mutagenesis underscore the importance of Npt2 in the maintenance of phosphate homeostasis. Recent advances in our understanding of phosphate transport mechanisms in intestine and bone are also discussed.

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.

Cellular mechanisms of the age-related decrease in renal phosphate reabsorption

The aging process in humans and in the rat is associated with an impairment in renal tubular reabsorption of Pi and renal tubular adaptation to a low Pi diet. The purposes of the present study were to determine whether changes in the abundance of type II Na-Pi contransporter (NaPi-2) protein and/or mRNA play a role in the age-related decrease in Na-Pi cotransport activity, and to further determine the cellular mechanisms of impaired adaptation to a low Pi diet. In studies performed in 3- to 4-month-old young adult rats and 32-to 16-month-old aged rats we found that there was an age-related twofold decrease in proximal tubular apical brush border membrane (BBM) Na-Pi cotransport activity, which was associated with similar decreases in BBM NaPi-2 protein abundance and renal cortical NaPi-2 mRNA level. Immunohisto-chemistry showed lower NaPi-2 protein expression in the BBM of proximal tubules of superficial, midcortical, and juxtamedullary nephrons. We also found that in response to chronic (7 days) and/or acute (4 hr) feeding of a low Pi diet there were similar adaptive increases in BBM Na-Pi cotransport activity and BBM NaPi-2 protein abundance in both young and aged rats. However, BBM Na-Pi cotransport activity and BBM NaPi-2 protein abundance were still significantly lower in aged rats, in spite of a significantly lower serum Pi concentration in aged rats. The results indicate that impaired expression of the type II renal Na-Pi cotransporter protein at the level of the apical BBM plays an important role in the age-related impairment in renal tubular reabsorption of Pi and renal tubular adaptation to a low Pi diet.