Phorbol myristate acetate activates protein kinase C, stimulates the phosphorylation of endogenous proteins and inhibits phosphate transport in mouse renal tubules

The p53 protein is a suppressor of Atox1 copper chaperon in tumor cells under genotoxic effects

The p53 protein is crucial for regulating cell survival and apoptosis in response to DNA damage. However, its influence on therapy effectiveness is controversial: when DNA damage is high p53 directs cells toward apoptosis, while under moderate genotoxic stress it saves the cells from death and promote DNA repair. Furthermore, these processes are influenced by the metabolism of transition metals, particularly copper since they serve as cofactors for critical enzymes. The metallochaperone Atox1 is under intensive study in this context because it serves as transcription factor allegedly mediating described effects of copper. Investigating the interaction between p53 and Atox1 could provide insights into tumor cell survival and potential therapeutic applications in oncology. This study explores the relationship between p53 and Atox1 in HCT116 and A549 cell lines with wild type and knockout TP53. The study found an inverse correlation between Atox1 and p53 at the transcriptional and translational levels in response to genotoxic stress. Atox1 expression decreased with increased p53 activity, while cells with inactive p53 had significantly higher levels of Atox1. Suppression of both genes increased apoptosis, while suppression of the ATOX1 gene prevented apoptosis even under the treatment with chemotherapeutic drugs. The findings suggest that Atox1 may act as one of key elements in promotion of cell cycle under DNA-damaging conditions, while p53 works as an antagonist by inhibiting Atox1. Understanding of this relationship could help identify potential targets in cell signaling pathways to enhance the effectiveness of combined antitumor therapy, especially in tumors with mutant or inactive p53.

7-O-methylpunctatin, a Novel Homoisoflavonoid, Inhibits Phenotypic Switch of Human Arteriolar Smooth Muscle Cells

Remodeling of arterioles is a pivotal event in the manifestation of many inflammation-based cardio-vasculopathologies, such as hypertension. During these remodeling events, vascular smooth muscle cells (VSMCs) switch from a contractile to a synthetic phenotype. The latter is characterized by increased proliferation, migration, and invasion. Compounds with anti-inflammatory actions have been successful in attenuating this phenotypic switch. While the vast majority of studies investigating phenotypic modulation were undertaken in VSMCs isolated from large vessels, little is known about the effect of such compounds on phenotypic switch in VSMCs of microvessels (microVSMCs). We have recently characterized a novel homoisoflavonoid that we called 7-O-methylpunctatin (MP). In this study, we show that MP decreased FBS-induced cell proliferation, migration, invasion, and adhesion. MP also attenuated adhesion of THP-1 monocytes to microVSMCs, abolished FBS-induced expression of MMP-2, MMP-9, and NF-κB, as well as reduced activation of ERK1/2 and FAK. Furthermore, MP-treated VSMCs showed an increase in early (myocardin, SM-22α, SM-α) and mid-term (calponin and caldesmon) differentiation markers and a decrease in osteopontin, a protein highly expressed in synthetic VSMCs. MP also reduced transcription of cyclin D1, CDK4 but increased protein levels of p21 and p27. Taken together, these results corroborate an anti-inflammatory action of MP on human microVSMCs. Therefore, by inhibiting the synthetic phenotype of microVSMCs, MP may be a promising modulator for inflammation-induced arteriolar pathophysiology.

Cellular mechanism underlying LPS-induced inhibition of in vitro L-leucine transport across rabbit jejunum

Lipopolysaccharide(LPS) is a known causative agent of sepsis. In previous studies, we have shown that it reduces L-leucine mediated transport across the rabbit jejunum by about 30%. In this study, the mechanism(s) of LPS inhibition on amino acid transport were analysed in detail. LPS did not inhibit L-leucine transport across brush border membrane vesicles, suggesting the need for an intracellular step. The inhibitory effect of LPS was not altered by the addition of protein kinase A (PKA) inhibitor (IP20, 10—7M) or an analog of cAMP (DB-cAMP, 3 × 10—4M), indicating that the PKA signal transduction pathway was not involved in the LPS effect. However, the inhibitory effect of LPS was suppressed by trifluoroperazine (10—7M), a Ca2+/calmodulin inhibitor and staurosporine (10—7M), an protein kinase C (PKC) inhibitor. Likewise, LPS inhibition disappeared in media without calcium. These results suggest that LPS could inhibit the intestinal uptake of L-leucine across the small intestine in vitro by intracellular processes related to calcium, involving PKC and calmodulin protein.

Activation of protein kinase Cδ leads to increased pancreatic acinar cell dedifferentiation in the absence of MIST1

Pancreatic ductal adenocarcinoma (PDAC) has a 5 year survival rate post-diagnosis of < 5%. Individuals with chronic pancreatitis (CP) are 20-fold more likely to develop PDAC, making it a significant risk factor for PDAC. While the relationship for the increased susceptibility to PDAC is unknown, loss of the acinar cell phenotype is common to both pathologies. Pancreatic acinar cells can dedifferentiate or trans-differentiate into a number of cell types including duct cells, β cells, hepatocytes and adipocytes. Knowledge of the molecular pathways that regulate this plasticity should provide insight into PDAC and CP. MIST1 (encoded by Bhlha15 in mice) is a transcription factor required for complete acinar cell maturation. The goal of this study was to examine the plasticity of acinar cells that do not express MIST1 (Mist1(-/-) ). The fate of acinar cells from C57Bl6 or congenic Mist1(-/-) mice expressing an acinar specific, tamoxifen-inducible Cre recombinase mated to Rosa26 reporter LacZ mice (Mist1(CreERT/-) R26r) was determined following culture in a three-dimensional collagen matrix. Mist1(CreERT/-) R26r acini showed increased acinar dedifferentiation, formation of ductal cysts and transient increases in PDX1 expression compared to wild-type acinar cells. Other progenitor cell markers, including Foxa1, Sox9, Sca1 and Hes1, were elevated only in Mist1(-/-) cultures. Analysis of protein kinase C (PKC) isoforms by western blot and immunofluorescence identified increased PKCε accumulation and nuclear localization of PKCδ that correlated with increased duct formation. Treatment with rottlerin, a PKCδ-specific inhibitor, but not the PKCε-specific antagonist εV1-2, reduced acinar dedifferentiation, progenitor gene expression and ductal cyst formation. Immunocytochemistry on CP or PDAC tissue samples showed reduced MIST1 expression combined with increased nuclear PKCδ accumulation. These results suggest that the loss of MIST1 is a common event during PDAC and CP and events that affect MIST1 function and expression may increase susceptibility to these pathologies.

Functional characterization of the human intestinal NaPi-IIb cotransporter in hamster fibroblasts and Xenopus oocytes

The recently cloned NaPi-IIb cotransporter is an apical membrane protein that is involved in the absorption of phosphate in the intestine. To expedite functional and structural studies, the human intestinal NaPi-IIb cotransporter was stably expressed in hamster fibroblast (PS120) cells. The hNaPi-IIb cDNA stably transfected cells exhibited a 1.8-fold higher sodium-dependent phosphate uptake than vector DNA transfected cells, and had a K(m) for Pi of approximately 106 microM and a K(m) for Na(+) of approximately 34 mM. The hNaPi-IIb cotransporter was also expressed in Xenopus oocytes and it exhibited a K(m) for Pi of approximately 113 microM and a K(m) for Na(+) of approximately 65 mM. The hNaPi-IIb cotransporter expressed in both PS120 cells and oocytes was inhibited by high external pH. Furthermore, the phosphate uptake mediated by the hNaPi-IIb cotransporter was inhibited by 5 mM phosphonoformic acid (PFA), 1 mM arsenate and 100 nM phorbol myristate acetate (PMA). These results demonstrate that the human intestinal NaPi-IIb cotransporter is functional when expressed in hamster fibroblasts, and that this model system may be useful in the future to identify NaPi-IIb cotransporter-specific inhibitors.

The role of the PHEX gene (PEX) in families with X-linked hypophosphataemic rickets

For over a hundred years, the bane of rickets (a disease of bone), has been prominent in those countries that have participated in, and seeded, the industrial revolution. Industrialisation had major effects of the demography of populations, and many people moved to dark, heavily industrialised cities to find work. It soon became apparent that rickets could be cured by supplementing the diet with cod liver oil and exposure to sunlight. This in turn led to the discovery that photoactivation of 7-dehydrocholesterol was required to produce vitamin D, an indispensable regulator of bone mineral metabolism. Although inadequate exposure to light and poor dietary intake are the main causes of rickets and osteomalacia, recent research has confirmed the role of familial, and tumour forms of the disease. This review will describe the recent advances in our knowledge of the molecular defects in X-linked hypophosphataemic rickets (HYP), and oncogenic hypophosphataemic osteomalacia (OHO). Although HYP and OHO have different primary defects, both diseases have similarities that suggest a linked or overlapping pathophysiology. Also, without doubt, the recent cloning of the gene defective in HYP (the PHEX gene), has given researchers a new reagent to explore the molecular regulation of bone and its links to kidney endocrine function. The fact that the PHEX gene codes for a Zn metallopeptidase raises new and intriguing questions, and adds new momentum to the research on diseases of bone mineral metabolism.

Prolactin regulation of mitochondrial aspartate aminotransferase and protein kinase C in human prostate cancer cells

Citrate production is a major physiological function of the prostate that is regulated by testosterone and prolactin. Mitochondrial aspartate aminotransferase (mAAT) is a key enzyme in the metabolic pathway of prostate citrate production. In addition, prolactin stimulates expression of mAAT in the rat lateral prostate. In this report we establish the role of prolactin in the regulation of mAAT in two prostate cancer cell lines, LNCaP and PC-3. LNCaP cells respond to hormonal stimulation with increased secretion of prostate specific products. PC-3 cells, on the other hand, are testosterone independent and apparently do not respond to other growth factors either. Results showed that both LNCaP and PC-3 cells responded to prolactin with increased mAAT activity and an increased steady state level of mAAT mRNA. Prolactin also increased protein kinase C (PKC) activity in both these cell lines. Treatment of LNCaP and PC-3 cells with the phorbol ester 12-O-tetradecanoylphorbol (TPA) caused the same effect on mAAT activity and mRNA level as prolactin. The results suggest that the diacylglycerol-PKC signal transduction system mediates the prolactin effect on mAAT. In addition, these results also show that the prolactin effect on mAAT is independent of androgens since PC-3 cells reportedly lack androgen receptor expression. Thus, these results provide evidence that prolactin is a physiological regulator of prostate function in human as well as rat prostate. In addition, the results also show that though prostate cancer cells are androgen independent, they remain responsive to prolactin. This could have important implications for the treatment and management of prostate cancer.

Renal Na(+)-phosphate cotransporter gene expression in X-linked Hyp and Gy mice

The X-linked Hyp and Gy mutations are murine homologues of X-linked hypophosphatemia (XLH), a dominant disorder of phosphate (Pi) homeostasis characterized by growth retardation, rickets, hypophosphatemia and decreased renal tubular maximum for Pi reabsorption relative to glomerular filtration rate (Tmp/GFR). In Hyp and Gy mice, the decrease in Tmp/GFR is associated with a reduction in renal brush-border membrane (BBM) Na(+)-Pi cotransport that can be ascribed to a decrease in renal-specific, Na(+)-Pi cotransporter (NPT2) mRNA and protein abundance. Although renal NPT2 gene expression is reduced in Hyp and Gy mice, the NPT2 gene does not map to the X chromosome. These findings exclude NPT2 as a candidate gene for murine and human X-linked hypophosphatemias and suggest that genes at the Hyp, Gy and XLH (HYP) loci are involved in regulation of NPT2 gene expression. Both Hyp and Gy mice respond to low Pi diet with an increase in BBM Na(+)-Pi cotransport, NPT2 mRNA and protein. The increase in NPT2 protein in Pi-depleted mice far exceeds the increase in NPT2 mRNA, suggesting that translational or post-translational mechanisms are involved in the adaptive process. NPT2 protein is localized to the apical surface of the proximal tubule, where immunostaining in both normal and Hyp mice is increased in response to low Pi diet. Pi-deprived Hyp and Gy mice fail to show an increase in Tmp/GFR, indicating that adaptation at the BBM is not sufficient for the overall increase in Tmp/GFR in response to low Pi diet.

Candidate 56 and 58 kDa protein(s) responsible for mediating the renal defects in oncogenic hypophosphatemic osteomalacia

The effects of tumor-conditioned media (TCM) derived from cultured cells from an oncogenic hypophosphatemic osteomalacia (OHO) tumor on transformed human kidney cells were investigated. Dose-dependent cell detachment and aggregation occurred in kidney cells cultured in serum-free medium supplemented with TCM, but not in skin fibroblast controls, or in kidney cells cultured in the presence of serum. Kidney cells exposed to TCM in the presence of serum (0.5%) had reduced Na(+)-dependent phosphate cotransport (36%, p < 0.04) and increased 1alpha-hydroxylase activity (48%, p < 0.05). In contrast, TCM had no significant effect on Na(+)-dependent alpha-methyl-glucose transport. To investigate these effects further, serum from an OHO patient, before and after tumor resection, was used to raise polyclonal antiserum to tumor-derived products (preoperative and postoperative antiserum, respectively). Changes in Na(+)-dependent phosphate cotransport and vitamin D metabolism induced by TCM were prevented by the addition of preoperative but not postoperative antisera. Furthermore, Western analysis revealed the presence of two proteins (56-58 kDa) in TCM media screened with preoperative antisera. These proteins were not detected by postoperative antisera and were absent in skin fibroblast control media. Direct inhibition of Na(+)-dependent phosphate cotransport by phosphonoformic acid did not affect 1,25-dihydroxy vitamin D(3) synthesis. These studies provide support for a circulating component affecting phosphate handling and vitamin D metabolism in OHO.

Effect of motilin on the L-leucine transport in rabbit jejunum

Motilin is a gastrointestinal peptide that stimulates the gastrointestinal motility in several species. The aim of the present work has been to determine the effect of motilin on the L-leucine absorption in rabbit jejunum. The results show that motilin inhibits the L-leucine Na(+)-dependent system of transport located in the mucosal border mainly by diminishing the apparent Vmax. Motilin did not directly affect the Na(+)-dependent system of transport, but it seems to act across the protein kinase C (PKC). These results suggest that motilin may act as a regulatory hormone of the intestinal absorption of nutrients.

Effect of phosphate supplementation on the expression of the mutant phenotype in murine X-linked hypophosphatemic rickets

The X-linked Hyp mouse, a murine homologue of X-linked hypophosphatemia in humans, is characterized by rachitic bone disease, hypophosphatemia, impaired renal brush-border membrane Na(+)-phosphate cotransport and abnormal regulation of renal vitamin D metabolism. We demonstrated that short-term phosphate supplementation decreases renal 1,25-dihydroxyvitamin D3 (1,25-(OH)2D) catabolism and increases serum 1,25-(OH)2D levels in Hyp mice (Tenenhouse & Jones 1990). In the present study, we compared several other parameters in normal and Hyp mice fed control (1%) and high (1.6%) phosphate diets for 4 days. Phosphate supplementation significantly raised serum phosphate levels and decreased renal brush-border membrane Na(+)-phosphate but not Na(+)-glucose, cotransport in both genotypes (67% of control diet, p < 0.05). However, under both dietary conditions, the phosphate/glucose transport ratio was significantly reduced in Hyp mice (58% of normal littermates, p < 0.05). Renal PTH-stimulated cAMP accumulation, which was significantly blunted in Hyp mice compared to normal mice under control dietary conditions (p < 0.05), was not altered by phosphate supplementation in either genotype. Serum alkaline phosphatase activity was significantly higher than normal in Hyp mice on the control diet and was further increased in mutants but not in normals fed the high phosphate diet (p < 0.05). Measurements of serum bilirubin and electrophoresis of serum alkaline phosphatase suggested that the elevation in serum alkaline phosphatase activity in phosphate-supplemented Hyp mice represents the bone-derived isozyme.(ABSTRACT TRUNCATED AT 250 WORDS)

[In vivo effects of 24R,25-dihydroxyvitamin D3 on kidney alkaline phosphatase and gamma-glutamyltransferase of hypophysectomized rats]

The effects of 24R, 25-dihydroxyvitamin D3 (24, 25 (OH)2 D3) on alkaline phosphatase (ALP), gamma-glutamyltransferase (GGT) and acid phosphatase (ACP) activities were investigated on renal cortex of hypophysectomized (Hx) rats. ALP activity was increased by +27, +56 and +60% as compared to controls respectively 3, 6 and 12 h after intraperitoneal administration of the secosteroid (10 pmoles/100 g body weight). Stimulations of GGT activity began only after 6 h (+30%) and 12 h (+ 46%). ACP activity was not modified. In vivo, the two enzymatic inductions in kidneys of Hx rats were higher and longer than those obtained in vitro.

The role of protein phosphorylation in renal amino acid transport

Changes in tubular reabsorption of amino acids and other solutes are characteristic of the immature renal tubule and of various hereditary nephropathies. The cellular mechanisms governing these aberrations in renal amino acid transport have not been established. Calcium (Ca2+)-dependent protein kinases are known to phosphorylate membrane-bound carrier proteins, thereby modulating transport of various solutes by the proximal tubule. The role of these enzymes in regulating renal tubular amino acid transport, particularly during kidney development, is unknown. We investigated: (1) the effect of Ca(2+)- and phospholipid-dependent protein kinase [protein kinase C (PKC)] and Ca2+/calmodulin-dependent protein kinase II (CaMKII) on sodium chloride (NaCl)-linked proline transport by renal brush border membrane vesicles (BBMV) from adult rats using the "hypoosmotic shock" technique (lysis of vesicles); (2) the activity, expression and subcellular distribution (cytosol, particulate, BBM) of Ca(2+)-dependent protein kinases in kidneys from 7-day-old and adult rats using MBP 4-14 and autocamtide II phosphorylation assays for PKC and CaMKII, respectively, endogenous protein phosphorylation (using gel electrophoresis and autoradiography) and Western immunoblot analysis to detect PKC and CaMKII. The studies showed: (1) endogenous (membrane-bound) CaMKII and PKC as well as exogenous, highly purified PKC inhibit proline uptake by phosphorylated, lyzed/resealed BBMV when compared with control vesicles; the voltage-clamped, nonelectrogenic component of proline transport was inhibited by PKC- but not CaMKII-mediated phosphorylation; (2) a Ca(2+)-dependent activity of both kinases was evident in all subcellular fractions tested in immature and adult kidneys.(ABSTRACT TRUNCATED AT 250 WORDS)

Renal adaptation to phosphate deprivation: lessons from the X-linked Hyp mouse

The X-linked Hyp mutation, a murine homologue of X-linked hypophosphatemia in humans, is characterized by renal defects in phosphate reabsorption and vitamin D metabolism. In addition, the renal adaptive response to phosphate deprivation in mutant Hyp mice differs from that of normal littermates. While Hyp mice fed a low phosphate diet retain the capacity to exhibit a significant increase in renal brush-border membrane sodium-phosphate cotransport in vitro, the mutants fail to show an adaptive increase in maximal tubular reabsorption of phosphate per volume of glomerular filtrate (TmP/GFR) in vivo. Moreover, unlike their normal counterparts, Hyp mice respond to phosphate restriction with a fall in the serum concentration of 1,25-dihydroxyvitamin D [1,25(OH)2D] that can be ascribed to increased renal 1,25(OH)2D catabolism. The dissociation between the adaptive brush-border membrane phosphate transport response and the TmP/GFR and vitamin D responses observed in Hyp mice is also apparent in X-linked Gy mice and hypophysectomized rats. Based on these findings and the notion that transport across the brush-border membrane reflects proximal tubular function, we suggest that the adaptive TmP/GFR response requires the participation of 1,25(OH)2D or a related metabolite and that a more distal segment of the nephron is the likely target for the 1,25(OH)2D-dependent increase in overall tubular phosphate conservation.

Protein kinase C activity in rat renal proximal tubule cells

The presence of protein kinase C (PKC) in proximal tubule cells of the rat kidney is established by means of immunodetection and by the demonstration of calcium- and phospholipid-dependent, staurosporine-inhibitable histone phosphorylation. The calcium-dependence of renal PKC is described. Maximal activation of the enzyme (178.2 and 258.8 pmol P1 mg-1 min-1 for cytosol and membrane respectively) was achieved with 5 microM of Ca2+. Phorbol 12, 13 dibutyrate (PDBu) translocated PKC from cytosol to membrane in a dose- and time-dependent fashion, while 4 alpha-phorbol 12,13-didecanoate produced no significant effect on translocation. Cytosolic PKC activity was compared in immature and mature tissues (10- and 40-day-old kidneys). Basal activity was found to be significantly higher (P less than 0.05) in immature cells (272.8 vs. 157.5 pmol Pi mg-1 min-1). PDBu at 10(-6) M for 15 min reduced immunoreactivity in the soluble fraction of both groups, which was accompanied by a significant decrease in kinase activity. We speculate that the high PKC activity in the infant kidney plays a role in cell growth.

X-linked hypophosphatemia. A phenotype in search of a cause

XLH is an important disease, it is the subject of several classic articles in the medical sciences (Scriver et al., 1991), and it has been an important stimulus to study renal hypophosphatemias and how they are involved in rickets and osteomalacia (Scriver, 1974; Scriver and Tenenhouse, 1991). Renal transport is the major determinant of phosphate homeostasis in mammals and it is unlikely that this important biochemical parameter would have been left by evolution to a single renal transport system. Together physiologists and geneticists found that the mammalian kidney has several gene products dedicated to phosphate transport. That has implications for biochemists in search of a membrane protein to clone and explain XLH, for example. Let us suppose the transporter affected in XLH is cloned. Will it be the product of the XLH (or Hyp or Gy) locus? One will not know until the transporter gene is mapped. There is no question of the X-chromosome locus product being protein kinase C for example, since it maps to autosomes. But where does one start in the search for the X-chromosome locus? With the elusive putative diffusible factor or with the transporter, or perhaps with an enzyme in vitamin D hormone metabolism? Which goes to say that it is necessary to know the phenotype to arrive at the right locus. Or is it? Sufficient physical mapping of region Xp22.31-p21.3 will eventually lead to positional cloning of the Hyp gene. What will it be?(ABSTRACT TRUNCATED AT 250 WORDS)

Renal Na(+)-phosphate cotransport in X-linked Hyp mice responds appropriately to Na+ gradient, membrane potential, and pH

To investigate the mechanism for the 50% decrease in Vmax of the high-affinity phosphate transport system in the renal brush-border membrane of X-linked Hyp mice, we compared the effects of external Na+ concentration, membrane potential, pH, phosphonoformic acid (PFA), and arsenate on Na(+)-Pi cotransport in brush-border membrane vesicles prepared from normal mice and Hyp littermates. The affinity of the Na(+)-Pi cotransport system for Na+ (apparent Km = 60 +/- 7 and 64 +/- 2 mM for normal and Hyp mice, respectively) and the Na(+)-Pi stoichiometry estimated from Hill plots (2.5 +/- 0.2 and 2.9 +/- 0.6 for normal and Hyp mice, respectively) were similar in brush-border membranes of both strains. Inside-negative membrane potential, generated by anions of different permeabilities, stimulated Na(+)-Pi cotransport and inside-positive membrane potential generated by valinomycin, and a K+ gradient (outside greater than inside) inhibited Na(+)-Pi cotransport to the same extent in brush-border membranes derived from normal mice and Hyp littermates. The pH dependence of Na(+)-Pi cotransport was similar in brush-border membrane vesicles of normal and Hyp mice. The ratio of Na(+)-Pi cotransport measured at pH 7.5 relative to that at pH 6.5 was 2.9 +/- 0.6 in normal mice and 2.9 +/- 0.7 in Hyp mice. PFA was a competitive inhibitor of Na(+)-Pi cotransport in brush-border membranes of both normal and Hyp mice. However, the apparent Ki for PFA was significantly lower in Hyp mice (0.31 +/- 0.01 and 0.19 +/- 0.02 mM in normal and Hyp mice, respectively, P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

X-linked hypophosphataemia: a homologous phenotype in humans and mice with unusual organ-specific gene dosage

XLH (X-linked hypophosphataemia, gene symbol HYP, McKusick 307800, 307810) and its murine counterparts (Hyp and Gy) map to a conserved segment on the X-chromosome (Xp 22.31-p.21.3, human; distal X, mouse). Gene dosage has received relatively little attention in the long history of research on this disease, which began over 50 years ago. Bone and teeth are sites of the principal disease manifestations in XLH (rickets, osteomalacia, interglobular dentin). Newer measures of quantitative XLH phenotypes reveal gene dose effects in bone and teeth with heterozygous values distributed between those in mutant hemizygotes and normal homozygotes. On the other hand, serum phosphate concentrations (which are low in the mutant phenotype and thereby contribute to bone and tooth phenotypes) do not show gene dosage. In Hyp mice serum values in mutant hemizygotes, mutant homozygotes and heterozygotes are similar. Phosphate homeostasis reflects its renal conservation. Renal absorption of phosphate on a high-affinity, Na+ ion-gradient coupled system in renal brush border membrane is impaired and gene dosage is absent at this level; the mutant phenotype is fully dominant. Synthesis and degradation of 1,25(OH)2D are also abnormal in XLH (and Hyp), but gene dosage in these parameters has not yet been measured. An (unidentified) inhibitory trans-acting product of the X-linked locus, affecting phosphate transport and vitamin D metabolism, acting perhaps through cytosolic protein kinase C, could explain the renal phenotype. But why would it have a normal gene dose effect in bone and teeth? Since the locus may have duplicated (to form Hyp and Gy), and shows evidence of variable expression in different organs (inner ear, bone/teeth, kidney), it may have been recruited during evolution to multiple functions.

Carbachol-stimulated phosphorylation of a 170-kDa endogenous protein in avian salt gland cells

The effect of cholinergic stimulation of cellular protein phosphorylation was studied using an intact cell preparation isolated from the avian salt gland. Isolated cells were allowed to incorporate 32Pi into the cellular ATP pool and then challenged with compounds known to induce ion secretion in this tissue. Addition of carbachol resulted in a time- and concentration-dependent (EC50 = 500 nM) increase in 32Pi content of a 170-kDa protein (pp170). The stimulated phosphorylation could be blocked by the inclusion of atropine (100 microM). Subcellular fractionation studies localized pp170 to the plasma membrane fraction of the tissue. The integral nature of this protein was demonstrated by detergent-solubilization experiments with Triton X-100. The possibility that carbachol stimulates phosphorylation of pp170 via activation of protein kinase C (PKC) was investigated. Incubating salt gland cells with 4 beta-phorbol 12-myristate 13-acetate (PMA; 1 microM) or carbachol (100 microM) resulted in a translocation of soluble PKC from the cytosol to a plasma membrane fraction. Addition of either PMA (1 microM) or ionomycin (1 microM) alone did not enhance phosphorylation of pp170. A 4.5-fold increase in the phosphorylation state of pp170 was only observed when PMA and ionomycin were added concurrently. Preincubation of salt gland cells with PKC inhibitors H-7 (50 microM) or staurosporine (10 microM) inhibited the carbachol-stimulated phosphorylation of pp170. These findings suggest that carbachol mediates its secretomimetic effects via activation of PKC and that pp170 may represent a novel integral membrane PKC substrate protein.

Cellular mechanisms in proximal tubular reabsorption of inorganic phosphate

Filtered inorganic phosphate (Pi) is largely reabsorbed in the proximal tubule. Na-Pi cotransport, with a stoichiometry of at least 2:1, mediates uphill transport at the apical membrane; at the basolateral membrane different types of transport systems can be involved in efflux and uptake of Pi from the interstitium. Regulation of transcellular Pi flux involves alteration of the apical Na-Pi cotransport; at least three different cellular control/sensing systems seem to participate in this regulation and are exemplified by parathyroid hormone (PTH)-dependent inhibition, Pi deprivation-dependent increase, and insulin-like growth factor I (IGF-I)-dependent increase in Na-Pi cotransport. For PTH inhibition, recent evidence suggests a role of the phospholipase C/protein kinase C-dependent regulatory cascade in inhibition of Na-Pi cotransport, at least at low PTH concentrations. In addition, an endocytic mechanism seems to be involved in this PTH action. Little is known of the cellular mechanisms in Pi deprivation-dependent and/or IGF-I-dependent increases in Na-Pi cotransport; they are dependent on de novo protein synthesis. Recent experiments involving an expression in Xenopus laevis oocytes led to the identification of an approximately 50 kDa membrane protein that is a good candidate for being involved in brush-border membrane Na-Pi cotransport activity.

Effect of parathyroid hormone on rat renal cAMP-dependent protein kinase and protein kinase C activity measured using synthetic peptide substrates

The actions of parathyroid hormone (PTH) on the renal cortex are thought to be mediated primarily by cAMP-dependent protein kinase (PKA) with some suggestion of a role for protein kinase C (PKC). However, present methods for assaying PKA and PKC in subcellular fractions are insensitive and require large amounts of protein. Recently, a sensitive method for measuring the activity of protein kinases has been reported. This method uses synthetic peptides as substrates and a tandem chromatographic procedure for isolating the phosphorylated peptides. We have adapted this method to study the effect of PTH on PKA and PKC activity using thin slices of rat renal cortex. PTH (250 nM) stimulated cytosolic PKA activity four- to fivefold within 30 s, and PKA activity was sustained for at least 5 min. PTH also rapidly stimulated PKC activity in the membrane fraction and decreased PKC activity in the cytosol. These changes were maximal at 30 s, but unlike changes in PKA, they declined rapidly thereafter. PTH significantly activated PKC only at concentrations of 10 nM or greater. This study demonstrates that PTH does activate PKC in renal tissue, although the duration of activation is much less than for PKA. It also demonstrates that a combination of synthetic peptides with tandem chromatography can be used as a sensitive assay procedure for protein kinase activity in biological samples.

Conserved loci on the X chromosome confer phosphate homeostasis in mice and humans

Several genes expressed in kidney and other tissues determine phosphate homeostasis in extracellular fluid. The major form of inherited hypophosphatemia in humans involves an X-linked locus (HPDR, Xp22.31-p21.3). It has two murine homologues (Hyp and Gy) which map to closely-linked but separate loci (crossover value 0.4%-0.8%). Both murine mutations impair Na(+)-phosphate cotransport in renal brush border membrane; an associated renal disorder of 1,25-dihydroxyvitamin D3 (1,25(OH)2D) metabolism has been characterized in Hyp mice. Whereas experiments with cultured Hyp renal epithelium indicate that the gene is expressed in kidney, studies showing the development of the mutant renal phenotype in normal mice parabiosed to Hyp mice implicate a circulating factor; these findings can be reconciled if the humoral factor is of renal origin. The gene dose effect of HPDR, Hyp and Gy on serum phosphorus values is consistently deviant and heterozygotes resemble affected hemizygotes. The deviant effect is also seen on renal phosphate transport; all mutant females (Hyp/Hyp and Hyp/+) have similar phenotypes. On the other hand, there is a normal gene dose effect of HPDR in mineralized tissue; tooth PRATIO (pulp area/tooth area) values for heterozygotes are distributed between those for affected males and normals. The tooth data imply that the X chromosome locus is expressed in both renal and non-renal cells. The polypeptide product of the X chromosome gene(s) is still unknown.

Protein kinase C in mouse kidney: effect of the Hyp mutation and phosphate deprivation

To test whether protein kinase C plays a role in the regulation of renal brush border membrane phosphate transport and mitochondrial vitamin D metabolism, we examined the activity, distribution and endogenous substrates of protein kinase C in renal subcellular fractions derived from two mouse models exhibiting perturbations in both renal functions. The X-linked Hyp mouse is characterized by reduced phosphate transport and 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) synthesis relative to normal, whereas the phosphate-deprived mouse exhibits elevated phosphate transport and vitamin D hormone synthesis. Protein kinase C activity was higher in renal cytosol of Hyp mice, when compared to normal littermates (358 +/- 11 vs. 244 +/- 31 pmol 32P/mg prot/min, P less than 0.02), whereas genotype differences in brush border membrane and mitochondrial kinase were not apparent. Phosphate deprivation of normal mice elicited a 50% reduction in brush border membrane protein kinase C (from 819 +/- 56 to 460 +/- 48 pmol 32P/mg prot/min, P less than 0.03), an increase in mitochondrial kinase (from 57 +/- 7 to 87 +/- 10 pmol 32P/mg prot/min, P less than 0.03), and no change in cytosolic kinase activity. Phosphate deprivation of Hyp mice led to an increase in mitochondrial protein kinase C (from 72 +/- 7 to 98 +/- 9 pmol 32P/mg prot/min, P less than 0.03) and no change in either brush border membrane or cytosolic kinase activity.(ABSTRACT TRUNCATED AT 250 WORDS)