Evidence that stimulation of 1,25(OH)2D3 production in primary cultures of mouse kidney cells by cyclic AMP requires new protein synthesis

Cadmium decreases SGLT1 messenger RNA in mouse kidney cells

Mouse renal cortical tubule cells in primary culture exposed to cadmium (Cd2+) develop decreased Na(+)-glucose cotransport activity as measured by uptake of the glucose analogue alpha-methyl-glucoside. RNA was isolated from kidney cell cultures, and after reversed transcription, the DNA was amplified with primers to rat SGLT1 (the high affinity isoform of the sodium glucose cotransporter) and mouse beta-actin. Only one product was identified after amplification with the rat SGLT1 primers, which on sequencing was 96% identical to rat SGLT1. Compared to beta-actin, the intensity of the SGLT1 message declined progressively as CdCl2 concentration in the medium increased from 0 to 10 microM. Similar decreases in SGLT1 mRNA were also observed as media zinc (Zn2+) concentrations rose from 0 to 75 microM or as copper (Cu) concentrations increased from 0 to 150 microM. Exposure to 8 microM Cd as Cd-metallothionein (Cd7-MT) also caused a fall in relative SGLT1 mRNA abundance, and at nearly identical internal Cd concentrations of 40-43 pmol/microgram DNA, both Cd7-MT and CdCl2 reduced SGLT1 mRNA to 33% of control. In general, the fall in SGLT1 mRNA was more rapid than the decline in Na(+)-dependent glucose uptake after cells were exposed to Cd2+. These findings suggest that the effects of Cd2+ and other metals on renal glucose transport are related to decreased expression of SGLT1 message.

Parathyroid hormone activation of the 25-hydroxyvitamin D3-1alpha-hydroxylase gene promoter

The DNA flanking the 5' sequence of the mouse 1alpha-hydroxylase gene has been cloned and sequenced. A TATA box has been located at -30 bp and aCCAAT box has been located at -79 bp. The gene's promoter activity has been demonstrated by using a luciferase reporter gene construct transfected into a modified pig kidney cell line, AOK-B50. Parathyroid hormone stimulates this promoter-directed synthesis of luciferase by 17-fold, whereas forskolin stimulates it by 3-fold. The action of parathyroid hormone is concentration-dependent. 1,25-Dihydroxyvitamin D3 does not suppress basal promoter activity and marginally suppresses parathyroid hormone-driven luciferase reporter activity. The promoter has three potential cAMP-responsive element sites, and two perfect and one imperfect AP-1 sites, while no DR-3 was detected. These results indicate that parathyroid hormone stimulates 25-hydroxyvitamin D3-1alpha-hydroxylase by acting on the promoter of the 1alpha-hydroxylase gene.

Loss of parathyroid hormone-stimulated 1,25-dihydroxyvitamin D3 production in aging does not involve protein kinase A or C pathways

Intestinal calcium absorption declines with aging as a result of decreased renal 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] biosynthesis. At least part of the decline in 1,25-(OH)2D3 may be due to acquired resistance to parathyroid hormone (PTH) stimulation of renal 25-hydroxyvitamin D1-hydroxylase (1-OHase) activity. To test whether aging rats can increase 1,25-(OH)2D3 production in response to PTH, male rats of the same litter were fed a normal Ca diet and were sacrificed at 175-225 g (young rats) or 3 months later at 350-425 g (aging rats). At sacrifice, basal serum 1,25-(OH)2D3 levels (88 +/- 16 versus 49 +/- 8 pg/ml, P < 0.05) and in vitro renal proximal tubule 1-OHase activity (178 +/- 15 versus 77 +/- 5 pmol/mg protein/5 minutes, n = 6, P < 0.001) were lower in aging animals. rPTH-(1-34) (10(-11) or 10(-7) M) increased in vitro 1,25-(OH)2D3 secretion by perifused renal proximal tubules from young but not aging rats. For young and aging rats, rPTH-(1-34) (10(-7) M) increased proximal tubule cAMP-dependent protein kinase (PKA) activity, and lower concentrations (10(-11) M) stimulated translocation of protein kinase C (PKC) activity from cytosolic to soluble membrane proximal tubule cell fractions. The results of this study show that PTH activation of 1,25-(OH)2D3 production may involve both signaling pathways, with the PKC pathway responsive to lower concentrations of the hormone. The acquired resistance to PTH stimulation of 1,25-(OH)2D3 production in aging appears not to involve the hormonal activation of PKA or PKC.

Characterization and regulation of the vitamin D hydroxylases

The metabolism of vitamin D is regulated by three major cytochrome P450-containing h hydroxylases-the hepatic 25-hydroxylase, the renal 1α-hydroxylase, and the renal and intestinal 24-hydroxylase. In the liver, the 25-hydroxylation reaction is catalyzed by microsomal and mitochondrial cytochrome P450cc25. The microsomal P450 accepts electrons from the NADPH-cytochrome P450 reductase, and the mitochondrial P450 accepts electrons from NADPH-ferredoxin reductase and ferredoxin. In the kidney, the 1α- and 24-hydroxylation reactions are catalyzed by mitochondrial cytochromes P450cc1α and P450cc24, respectively. The 24-hydroxylase is also found in vitamin D target tissues such as the intestine. The rat hepatic mitochondrial P450cc25 and the rat renal mitochondrial P450cc24 have been purified, and their cDNAs have been cloned and sequenced. 1,25-Dihydroxyvitamin D, the active metabolite of vitamin D, markedly stimulates renal P450cc24 mRNA and 24-hydroxylase activity in the intact animal and in renal cell lines. This stimulation occurs via a receptor-mediated mechanism requiring new protein synthesis. Despite the availability of a clone, no studies have yet been reported of the regulation of hepatic P450cc25 at the mRNA level. The study of one of the most important enzymes in vitamin D metabolism, the renal 1α-hydroxylase which produces the active metabolite, awaits the definitive cloning of the cDNA for the P450cc1α.

Role of protein kinase C in parathyroid hormone stimulation of renal 1,25-dihydroxyvitamin D3 secretion

PTH is a major regulator of renal proximal tubule 1,25(OH)2D3 biosynthesis. However, the intracellular pathways involved in PTH activation of the mitochondrial 25-hydroxyvitamin D3-1 alpha-hydroxylase (1-OHase) remain unknown. PTH can activate both the adenylate cyclase/protein kinase A (PKA) and the plasma membrane phospholipase C/protein kinase C (PKC) pathways. The present study was undertaken to determine whether PKC may mediate PTH activation of renal 25-hydroxyvitamin D3-1 alpha-hydroxylase activity. Rat PTH 1-34 fragment in vitro translocated PKC activity from cytosolic to soluble membrane fraction from freshly prepared rat proximal tubules. Physiologic concentrations (10(-11)-10(-10) M) of rat PTH 1-34 fragment increased PKC translocation three- to fourfold while PKA activity ratio increased at PTH 10(-7) M. PTH stimulation of PKC and PKA was reduced in the presence of staurosporine (10 nM) by 41 and 29%, respectively. Sangivamycin (10 and 50 microM) also reduced PTH-stimulated PKC translocation, but did not alter PKA activity ratio. In vitro perifusion of renal proximal tubules with PTH (10(-11) M) increased 1,25(OH)2D3 steady-state secretion two- to fourfold. Sangivamycin at the same concentration that inhibited PKC translocation by 52% completely inhibited PTH-stimulated 1,25(OH)2D3 secretion. The present studies indicate that the phospholipase C/PKC pathway may mediate PTH stimulation of mammalian renal proximal tubule 1,25(OH)2D3 secretion.

The calcium oxalate crystal growth inhibitor protein produced by mouse kidney cortical cells in culture is osteopontin

Urine contains proteins that inhibit the growth of calcium oxalate (CaOx) crystals and may prevent the formation of kidney stones. We have identified a potent crystal growth inhibitor in the conditioned media from primary cultures of mouse kidney cortical cells. Conditioned media, incubated with the kidney cells for 6-72 h, was assayed for crystal growth inhibition; inhibitory activity increased 15-fold by 24 h. Inhibitory activity was purified from serum-free media containing proteinase inhibitors using anion-exchange and gel-filtration chromatography. A single band of molecular weight 80,000 daltons was seen after SDS-polyacrylamide gel electrophoresis. The sequence of the N-terminal 21 amino acids of this protein matched that of osteopontin (OP), a phosphoprotein initially isolated from bone matrix. Antisera raised to fusion proteins produced by plasmids containing the N-terminal or C-terminal portions of OP cDNA also cross-reacted with the protein purified from cell culture media on western blots. The effect of the purified protein on the growth of CaOx crystals was measured using a constant composition assay. A 50% inhibition of growth occurred at a protein concentration of 0.85 micrograms/ml, and the dissociation constant of the protein with respect to CaOx crystal was 3.7 x 10(-8) M. The concentration of OP in mouse urine, measured using antibodies raised to the purified protein, was approximately 8 micrograms/ml. We conclude that OP is synthesized by kidney cortical tubule cells and functions as a crystal growth inhibitory protein in urine.

Expression of 25-hydroxyvitamin D 24-hydroxylase cytochrome P450 in kidney and intestine. Effect of 1,25-dihydroxyvitamin D and age

To study the mechanism of hormonal regulation of the 25-hydroxyvitamin D 24-hydroxylase, a DNA probe complementary to the published sequence of the recently cloned P450 component [(1991) FEBS Lett. 278, 195] was employed. Young (2 month) and adult (12 month) F344 rats, deficient in 1,25-dihydroxyvitamin D, were given a single dose of 1,25-dihydroxyvitamin D. In young rats, 1,25-dihydroxyvitamin D markedly increased P450 mRNA levels within 3 h in both kidney and intestine, and maximal levels were attained at 16 and 3 h, respectively. In adult animals, maximal induction of mRNA was diminished in the kidney, and the decline was slower in the intestine. Time of maximal induction did not change with age. These studies demonstrate for the first time regulation of the 24-hydroxylase enzyme by 1,25-dihydroxyvitamin D at the level of the mRNA for the cytochrome P450. They also demonstrate that this regulation may change with age.

Renal proximal tubular cells in suspension or in primary culture as in vitro models to study nephrotoxicity

The kidney forms a frequent target for xenobiotic toxicity. The complex biochemical mechanisms underlying nephrotoxicity are best studied in vitro provided that reliable and relevant in vitro models are available. Since most nephrotoxicants affect primarily the cells of the proximal tubules (PTC), much effort has been directed towards the development of in vitro models of PTC. This review focuses on the preparation of PTC and the use of these cells. Discussed are important criteria such as the viability (survival time) of the cells and the parameters to assess toxicity. Recent studies have shown that isolated PTC in suspension are especially suitable for studies on the biochemical mechanisms of 'acute' nephrotoxicity, whereas PTC in primary culture may be used to investigate mechanisms of nephrotoxic damage at very low concentrations, upon prolonged exposure. PTC cultured on porous filter membranes provide new possibilities to study toxicity in relation to cell and transport polarity. Primary cell cultures of human PTC have been set up. Although a further characterization of these systems is needed, recent data indicate their usefulness.

Effect of pH on growth of mouse renal cortical tubule cells in primary culture

We examined the effect of the medium pH on growth of primary cultures of mouse cortical tubule cells grown in defined medium. A significantly higher DNA content was observed within 24 h of lowering medium pH from 7.4 to 6.8 or 7.1 and persisted for the duration of the study. Further studies revealed that either medium acidification or insulin plus prostaglandin E1 nearly doubled uptake of [3H]thymidine in cells deprived of other growth factors for the previous 72-110 h. Moreover, the effects of insulin, prostaglandin E1, and medium acidification on [3H]thymidine uptake of quiescent cells were additive. An alkaline medium pH appeared to have a small but significant effect on cell hypertrophy, since cells exposed to pH 7.4 and 7.7 had a higher protein-to-DNA ratio than cells incubated at a lower pH. Cell pH of monolayers grown on glass slides determined from fluorescence of the carboxyfluorescein analogue 2',7'-biscarboxyethyl-5(6)-carboxyfluorescein (BCECF) was linearly correlated with medium pH, and changes in medium pH resulted in changes in steady-state cell pH of a similar magnitude. Four hours after medium acidification, relative increases in cell Na+ and water content occurred, whereas medium alkalinization led to decreases in cell Na+ and water content. The increases in cell Na+ and cell water content at pH 6.8 could be inhibited by amiloride. We conclude that decreasing the cell pH can be a mitogenic stimulus for renal tubule cells. Medium acidification is accompanied by changes in cell Na+ transport, which may be mediated in part by altered Na+-H+ antiporter activity.

Idiopathic hypercalciuria

Idiopathic hypercalciuria, defined as the urinary excretion of more than 300 mg. calcium per day in men or more than 250 mg. calcium per day in women, or more than 4 mg. calcium per kg. per day, is observed in about 50 per cent of the patients with calcium oxalate/apatite nephrolithiasis and is one of the risk factors for stone formation. These patients do not exhibit hypercalcemia, elevated serum parathyroid hormone concentrations or urinary cyclic adenosine monophosphate excretion nor clinical evidence of sarcoidosis, other granulomas or a malignancy. Hypophosphatemia may be present. Augmented rates of intestinal absorption of dietary calcium account for most of the increments in urinary calcium. Serum 1,25-dihydroxyvitamin D concentrations are in the upper normal range or elevated among many patients and are normal but not suppressed in the others. Activation of 1,25-dihydroxyvitamin D formation may be secondary to hypophosphatemia or other, as yet undefined, factors. Since, 1,25-dihydroxyvitamin D apparently can up-regulate its own receptor, small increments in its synthesis and blood levels could amplify the effect of the hormone to stimulate intestinal calcium absorption. Calcium balances are slightly but significantly negative and urinary hydroxyproline excretion may be increased so that a generalized disorder of calcium homeostasis also involving bone may be present. Additional studies are required to determine the genetic basis for the occurrence of idiopathic hypercalciuria in families, the cause of greater expression of idiopathic hypercalciuria in men and whether environmental factors (high dietary sodium chloride, protein and purified carbohydrate intakes) contribute to the expression of idiopathic hypercalciuria. Although thiazide diuretics, inorganic phosphate, magnesium hydroxide and potassium citrate have provided effective therapy, prospective studies are needed to determine optimum therapy and the optimum duration of treatment.

Normal and abnormal regulation of 1,25-(OH)2D synthesis

1,25-Dihydroxyvitamin D (1,25-(OH)2D) plays a crucial role in the maintenance of blood calcium and phosphorus levels and in normal skeletal mineralization. The concentration of this metabolite in the blood is, by necessity, tightly regulated. The most important stimuli for renal 1,25-(OH)2D synthesis include parathyroid hormone (PTH), its second messenger cyclic adenosine monophosphate (cAMP) and phosphate deprivation. Hypocalcemia and calcitonin, initially thought to act via stimulation of PTH release, have now been shown to directly stimulate 1-hydroxylation. Estrogens also increase 1,25-(OH)2D production, probably by upregulating renal PTH receptors. Inhibitors of the renal 25-(OH)D 1 alpha-hydroxylase include 1,25-(OH)2D itself, hypercalcemia, and phosphate loading. The PTH-vitamin D axis as modulated by the serum ionized calcium level controls adaptation to alterations in dietary calcium and sodium intake and to changes in skeletal turnover based on the level of physical activity. Although normally the renal production of 1,25-(OH)2D is tightly regulated and changes little in response to vitamin D challenge, there are certain conditions in which 1,25-(OH)2D appears to be substrate-dependent. These include hypoparathyroidism, hyperparathyroidism, vitamin D deficiency, sarcoidosis and the anephric state, conditions in which PTH is not well-modulated by alterations in serum ionized calcium or in which extrarenal synthesis of 1,25-(OH)2D occurs. In several disorders, including absorptive hypercalciuria, pseudohypoparathyroidism, hypophosphatemic rickets, and tumoral calcinosis, the regulation of the renal 1 alpha-hydroxylase appears to be altered.