Stimulation of Na+/phosphate cotransport in LLC-PK1 cells by 12-O-tetradecanoylphorbol 13-acetate (TPA)

Modulating phosphate consumption, a novel therapeutic approach for the control of cancer cell proliferation and tumorigenesis

Phosphorus, often in the form of inorganic phosphate (Pi), is critical to cellular function on many levels; it is required as an integral component of kinase signaling, in the formation and function of DNA and lipids, and energy metabolism in the form of ATP. Accordingly, crucial aspects of cell mitosis - such as DNA synthesis and ATP energy generation - elevate the cellular requirement for Pi, with rapidly dividing cells consuming increased levels. Mechanisms to sense, respond, acquire, accumulate, and potentially seek Pi have evolved to support highly proliferative cellular states such as injury and malignant transformation. As such, manipulating Pi availability to target rapidly dividing cells presents a novel strategy to reduce or prevent unrestrained cell growth. Currently, limited knowledge exists regarding how modulating Pi consumption by pre-cancerous cells might influence the initiation of aberrant growth during malignant transformation, and if reducing the bioavailability or suppressing Pi consumption by malignant cells could alter tumorigenesis. The concept of targeting Pi-regulated pathways and/or consumption by pre-cancerous or tumor cells represents a novel approach to cancer prevention and control, although current data remains insufficient as to rigorously assess the therapeutic value and physiological relevance of this strategy. With this review, we present a critical evaluation of the paradox of how an element critical to essential cellular functions can, when available in excess, influence and promote a cancer phenotype. Further, we conjecture how Pi manipulation could be utilized as a therapeutic intervention, either systemically or at the cell level, to ultimately suppress or treat cancer initiation and/or progression.

Role of the sodium-dependent phosphate co-transporters and of the phosphate complexes of uranyl in the cytotoxicity of uranium in LLC-PK1 cells

Although uranium is a well-characterized nephrotoxic agent, very little is known at the cellular and molecular level about the mechanisms underlying the uptake and toxicity of this element in proximal tubule cells. The aim of this study was thus to characterize the species of uranium that are responsible for its cytotoxicity and define the mechanism which is involved in the uptake of the cytotoxic fraction of uranium using two cell lines derived from kidney proximal (LLC-PK(1)) and distal (MDCK) tubule as in vitro models. Treatment of LLC-PK(1) cells with colchicine, cytochalasin D, concanavalin A and PMA increased the sodium-dependent phosphate co-transport and the cytotoxicity of uranium. On the contrary, replacement of the extra-cellular sodium with N-methyl-D-glucamine highly reduced the transport of phosphate and the cytotoxic effect of uranium. Uranium cytotoxicity was also dependent upon the extra-cellular concentration of phosphate and decreased in a concentration-dependent manner by 0.1-10 mM phosphonoformic acid, a competitive inhibitor of phosphate uptake. Consistent with these observations, over-expression of the rat proximal tubule sodium-dependent phosphate co-transporter NaPi-IIa in stably transfected MDCK cells significantly increased the cytotoxicity of uranium, and computer modeling of uranium speciation showed that uranium cytotoxicity was directly dependent on the presence of the phosphate complexes of uranyl UO(2)(PO(4))(-) and UO(2)(HPO(4))(aq). Taken together, these data suggest that the cytotoxic fraction of uranium is a phosphate complex of uranyl whose uptake is mediated by a sodium-dependent phosphate co-transporter system.

Interaction between Na+/phosphate-cotransporter and the adrenoceptors in myocardial depression

The primary objective of this study was to test the hypothesis that an increase in the alpha1-adrenoceptor tone would potentiate the myocardial biphasic contractile response to inorganic phosphate [Pi, the substrate of Na/Pi-cotransporter (NP)]. A second aim was to determine whether activation of alpha1-adrenoceptor is necessary for the NP-mediated increase in myocardial contractility (+dP/dt). Earlier study from this laboratory showed that high concentration of Pi (10 mM) produces a biphasic contractile response: initial increase in +dP/dt was followed by decline. In another study, Pi (3.5 mM) potentiated phenylephrine (PHE)-induced increase in +dP/dt. The alpha1-adrenoceptor was not blocked in these studies, and it can still be activated by the electrical stimulation of the sympathetic nerve terminals to the heart. Additionally, alpha1-adrenoceptor-activated increases in the activity of NP have been reported in numerous studies in noncardiac tissues and cell lines; therefore it is not clear whether Pi-induced increase in +dP/dt occurs only in the presence of alpha1-adrenoceptor activation. This study was performed by using isolated perfused rat heart in the condition of controlled extracellular calcium activity (0.72 mM); fixed preload (15 mm Hg); and constant heart rate (280 beats/min) and coronary flow (8 ml/min). The electronically differentiated value of the left ventricular pressure (LVP) signal was used as an index of myocardial contractility. The data show that activation of the alpha1-adrenoceptor is not necessary for the Pi-induced increase in +dP/dt (i.e., NP-mediated increase in +dP/dt has both the alpha1-adrenoceptor-dependent and alpha1-adrenoceptor-independent components. The interaction between alpha1-adrenoceptor agonist (PHE) and Pi (10 mM) did not produce a biphasic myocardial contractile response in the presence of propranolol. Because our earlier data on myocardial biphasic contractile response to 10 mM Pi was obtained when neither the beta- nor the alpha-adrenoceptor was blocked, we carried out more studies to see whether beta-adrenoceptor plays a role in this Pi-induced biphasic response. When both the alpha- and beta-adrenoceptors were activated with norepinephrine (NE), myocardial depression by high Pi concentration was markedly potentiated. This myocardial depression did not occur in the presence of phosphonoformate, a selective inhibitor of NP. It also did not occur when alpha1-adrenoceptor was blocked. Our data suggest that alpha1- and beta-adrenoceptors do not interact with the cardiac NP to potentiate the Pi-induced biphasic contractile response, but they interact in a manner that potentiates Pi-induced myocardial depression.

Dithio-bis-mercaptoethanesulphonate (DIMESNA) does not prevent cellular damage by metabolites of ifosfamide and cyclophosphamide in LLC-PK1 cells

Ifosfamide (IF) is an alkylating cytostatic with urotoxic (haemorrhagic cystitis) and nephrotoxic (Fanconi syndrome) side effects. Cyclophosphamide (CP), a structural isomer of IF, shows urotoxic but no nephrotoxic side effects. The development of haemorrhagic cystitis during therapy with IF or CP can be prevented by the uroprotective drug sodium-2-mercaptoethanesulphonate (MESNA). However, even in the presence of MESNA, Fanconi syndrome may still develop after therapy with IF. Using the renal tubular cell line LLC-PK1, we investigated whether there is a protective effect of either MESNA or of its major metabolite DIMESNA, in combination with metabolites of IF or CP, on thymidine incorporation, uridine incorporation or total protein. DIMESNA, the dimer of MESNA, is the dominant form of the molecule in the circulation; the proximal tubular cell must convert this back to MESNA at the expense of glutathione, before it can exert its uroprotective action. We did not find a protective effect of DIMESNA under any of the experimental conditions tested. LLC-PK1 cells exposed to 3 mmol/l DIMESNA did not convert DIMESNA to MESNA. The toxic effect of the CP metabolite 4-OOH-CP was more pronounced in the presence of DIMESNA than in its absence. MESNA completely prevented the toxic effects of acrolein and of 4-OOH-CP. The toxic effects of 4-OOH-IF and of chloracetaldehyde, two major metabolites of IF, were significantly reduced in the presence of MESNA. However, even at 30-fold molar excess of MESNA over a 4-OOH_IF, thymidine incorporation remained reduced by 40% compared with controls, indicating incomplete protection of tubular cells against metabolites of IF. Similarly, the effect of chloracetaldehyde was not completely reversed by MESNA.(ABSTRACT TRUNCATED AT 250 WORDS)

Toxicity of ifosfamide, cyclophosphamide and their metabolites in renal tubular cells in culture

Ifosfamide (IF) and cyclophosphamide (CP) are highly effective alkylating cytostatic drugs. IF and CP have to be activated through a metabolic step in vivo; numerous metabolites are known. While both IF and its structural isomer CP have severe urotoxic side effects, only IF is also a nephrotoxic drug, causing tubular damage resulting in Fanconi syndrome in some cases. Little information is available regarding the pathogenic mechanism of tubular damage by IF. We used the renal epithelial cell line LLC-PK1, which has many properties of the proximal tubule, in order to investigate the toxicity of IF and CP and of their reactive metabolites 4-hydroxy-IF (4-OH-IF), 4-hydroxy-CP (4-OH-CP), acrolein and chloroacetaldehyde (CAA). Protein content of monolayers, DNA and RNA synthesis were determined by standard techniques (thymidine and uridine incorporation). IF and CP had the lowest toxicities of all compounds tested. Both drugs inhibited thymidine incorporation by about 30% at a concentration of 300 mumol/l after 1 h incubation. 4-OH-IF and 4-OH-CP were significantly more toxic than the parent drugs. Thymidine incorporation, the most sensitive parameter, was reduced by about 70% by 300 mumol/l of either compound. In addition, 4-OH-CP reduced the total protein content of monolayers. 4-OH-IF did not effect protein content and RNA synthesis. Acrolein, the most toxic metabolite tested, reduced all three parameters significantly at concentrations of 50-75 mumol/l after 1 h.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential effects of activation of protein kinase C and cyclic-AMP-dependent protein kinase on sodium-dependent phosphate uptake in NIH 3T3 cells

Activation of protein kinase C (PKC) by phorbol ester (PMA), or by diacylglycerol analogue (OAG) treatment of NIH 3T3 cells resulted in the rapid (within 2-5 min) stimulation (approx. 2-fold) of sodium-dependent phosphate (Pi) transport. Conversely, preincubation of these cells with forskolin and cholera toxin, or incubation with 8-bromo-cAMP, to activate cAMP-dependent protein kinase (PKA), resulted in a decrease in Na+/Pi transport. Activation of either PKC or PKA did not change the Vmax of Pi uptake. However, activation of PKC did result in an increase, while activation of PKA caused a decrease, in the affinity for Pi. These results indicate that there is differential regulation of Na+/Pi uptake in NIH 3T3 cells by activators of PKC (stimulated) and PKA (inhibited) as a consequence of changes in the affinity of the transporter for Pi.

Protein kinase C activation causes inhibition of Na/K-ATPase activity in Madin-Darby canine kidney epithelial (MDCK) cells

To evaluate the influence of protein kinase C (PKC) activation on Na/K-ATPase activity in MDCK cells, we studied the effect of phorbol myristate acetate (PMA) and two diacylglycerol analogues, oleoylacetylglycerol and dioctanoylglycerol, on the enzyme activity. Na/K-ATPase activity was determined by cytochemistry. PMA induced a time- and dose-dependent inhibition of Na/K-ATPase activity and at 100 ng/ml decreased the enzyme activity by 55% of the initial value. These effects were mimicked by oleoylacetylglycerol and dioctanoylglycerol, and were abolished by two inhibitors of PKC, 1-(5-isoquinolinylsulphonyl)-2-methylpiperazine (H7) and sphingosine. A phorbol ester that does not activate PKC, 4 alpha-phorbol 12,13-didecanoate, did not inhibit Na/K-ATPase activity. PMA inhibition persisted in the presence of cycloheximide and actinomycin D but not in the presence of amiloride. Dopamine (10 microM) inhibition of Na/K-ATPase activity was abolished in a dose-dependent manner by sphingosine. Results suggest that in MDCK cells Na/K-ATPase is an effector protein for PKC and that dopamine inhibition of its activity may be mediated by PKC.

Activation of high-affinity uptake of glutamate by phorbol esters in primary glial cell cultures

The effects of 12-O-tetradecanoylphorbol 13-acetate (TPA), a potent activator of protein kinase C, on high-affinity Na(+)-dependent glutamate transport were investigated in primary cultures of neurons and glial cells from rat brain cortex. Incubation of glial cells with TPA led to concentration- and time-dependent increases in the glutamate transport that could be completely suppressed by the addition of the protein kinase C (PKC) inhibitor 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine. The TPA effects could be mimicked by oleoylacetylglycerol and by the diacylglycerol kinase inhibitor R59022. The effects of TPA were potentiated by the Ca2+ ionophore A23187. Under the chosen experimental conditions TPA had no effect on glutamate transport in neurons. We conclude that PKC activates the sodium-dependent high-affinity glutamate transport in glial cells and that it has dissimilar effects on neurons and glial cells.

Extracellular ATP activates coordinated Na+, Pi, and Ca2+ transport in cardiac myocytes

Activation of an ATP receptor has previously been shown to induce cytosolic [Ca2+] transients in rat ventricular myocytes. A slower but larger [Ca2+] increase which can cause cell hypercontraction follows the transient when extracellular Pi is increased. This second phase of the [Ca2+] response is stimulated by ATP or adenosine 5'-(gamma-thio)triphosphate in a medium containing 11.2 mM Pi, but not by high concentrations of 2-methylthio-ATP, which stimulate only the initial [Ca2+] transient. Replacing medium Na+ with N-methyl-D-glucamine suppresses this Pi-dependent [Ca2+] increase following ATP addition, suggesting a causal relationship between Na+ transport and Ca2+ influx. Blocking voltage-sensitive Na+ channels, Na(+)-H+ exchange, or Na(+)-K(+)-Cl- cotransport did not reduce ATP-induced cell hypercontraction in 11.2 mM Pi medium, suggesting that these transporters are not involved. ATP stimulation of Na(+)-Pi cotransport was investigated with isotopic methods. The results were consistent with the hypothesis that extracellular ATP stimulates Na(+)-Pi cotransport, which activates Na(+)-Ca2+ exchange. A novel Pi-dependent ATP receptor-effector system has been demonstrated in cardiac cells, and it may have significant effects on cellular transport, contractility, and bioenergetics.

Inhibition of high-affinity gamma-aminobutyric acid uptake in primary astrocyte cultures by phorbol esters and phospholipase C

The effects of phorbol 12-myristate 13-acetate (PMA), a potent activator of protein kinase C (PKC), on high-affinity Na(+)-dependent gamma-aminobutyric acid (GABA) uptake were investigated in primary cultures of neurons and glial cells from rat brain cortex. Incubation of glial cells with PMA led to concentration- and time-dependent decreases in the GABA transport in glial cells. This effect could be completely suppressed by addition of the PKC inhibitor H7. The PMA effects could be mimicked by oleoylacetylglycerol, the diacylglycerol kinase inhibitor R59022 and exogenous phospholipase C. Treatment with PMA did not affect GABA transport in neuronal cells.

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

Calcium-activated, phospholipid-dependent protein kinase (protein kinase C) has been implicated in the regulation of transport processes in a variety of tissues and cell lines. To establish whether protein kinase C participates in the regulation of renal phosphate transport, we examined the effect of phorbol myristate acetate (PMA), a potent activator of protein kinase C, on phosphate uptake in fresh preparations of mouse renal tubules, and we correlated the changes in transport activity with protein kinase C activation and phosphorylation of endogenous proteins. PMA inhibited Na+-dependent phosphate transport, elicited a rapid translocation of protein kinase C from the cytosolic to the particulate fraction and stimulated the phosphorylation of endogenous substrates in the cytosolic and brush border membrane fractions. Effects of PMA were maximal after a 10 min incubation of the tubules with the activator. 4 alpha-Phorbol, an inert analogue of PMA, did not elicit any of these effects. The present results demonstrate a temporal correlation between inhibition of Na+-dependent phosphate transport, translocation and activation of protein kinase C, and phosphorylation of endogenous proteins in mouse renal tubules. These data suggest that protein kinase C may play a regulatory role in phosphate transport in mammalian kidney.

Activity of calcium-sensitive phospholipid-dependent protein kinase C following nephron loss

The localization and activity of the calcium-sensitive phospholipid-dependent protein kinase C (PKC) were examined following the loss of 50% of functioning nephron mass. Four hours following unilateral nephrectomy in rats, soluble (100,000 g supernatant) proteins in the contralateral kidney were increased by 11% compared to sham operated controls; the increase was 33% 144 hours following surgery. The specific activity of PKC did not change in the cytosol at any of the time periods examined and averaged 63.9 +/- 8.2 pmol.mg-1.min-1 in unilaterally nephrectomized animals four hours following surgery. Four hours following sham surgery total soluble PKC activity averaged 1667.0 +/- 278.4 pmol.kidney-1.min-1, whereas activity averaged 3067.7 +/- 415.4 pmol.kidney-1.min-1 in animals post-nephrectomy (N = 5, P less than 0.04). Similar data was seen 144 hours following surgery. To examine the PKC activity in plasma membranes of proximal tubular cells, brush border membranes were prepared from rat kidney cortex. Twenty-four hours following unilateral nephrectomy, activity averaged 193.8 +/- 14.9 pmol.mg-1.min-1, while activity in membranes isolated from sham operated animals averaged 76.6 +/- 8.0 pmol.mg-1.min-1 (N = 5, P less than 0.001). Similar data was evident 48 hours following surgery. A small increment in activity was seen in the basolateral membrane preparation 24 hours following unilateral nephrectomy but not at 48 hours. These data indicate that cellular PKC activity increases rapidly following reductions in renal mass, and there are selective increments in the brush border membrane of the proximal tubular cell. The localization of PKC to this membrane may have important consequences for adaptations following nephron loss.

Initiation and characterization of primary mouse kidney epithelial cultures

Primary cultures of murine renal epithelial cells were established from a preparation of proximal tubule fragments. Confluent cultures exhibited multiple dome formation, indicating the presence of tight junctions and an intact transcellular transport process. Ultrastructural analysis revealed a monolayer of polarized cells, with a sparse but clearly defined microvillar surface facing the growth medium and a basolateral surface attached to the substratum. Cultures grown on collagen gels did not show domes. The epithelial monolayer exhibited several differentiated functions of the proximal tubule: a) parathyroid hormone (PTH)-stimulated cAMP synthesis; b) production of 24,25-dihydroxyvitamin D3 from 25-hydroxyvitamin D3; c) high alkaline phosphatase activity; and d) Na+-dependent transport of phosphate (Pi) and alpha-methylglucoside (alpha-MG). The sugar uptake was selectively inhibited by phlorizin, a competitive inhibitor of glucose uptake at the luminal membrane. Kinetic analysis revealed independent transport systems for Pi and alpha-MG, with Km values corresponding to the high affinity systems identified in brush border membrane vesicles derived from the proximal tubule. Pi uptake by the epithelial monolayers was regulated by the concentration of Pi in the growth medium. Phorbol esters and PTH did not exert an effect on Pi and alpha-MG transport in mouse primary cultures. The present study demonstrates that primary cultures provide a useful in vitro preparation to investigate renal proximal tubular function.

Intracellular cascades in the parathyroid-hormone-dependent regulation of Na+/phosphate cotransport in OK cells

Parathyroid hormone (PTH) increased intracellular cyclic AMP and reduces Na+/phosphate cotransport activity in OK cells [Malmström & Murer (1986) Am. J. Physiol. 251, C23-C31; Caverzasio, Rizzoli & Bonjour (1986) J. Biol. Chem. 261, 3233-3237]. It was also shown that PTH activates phosphoinositide metabolism in OK cells [Hruska, Moskowitz, Esprit, Civitelli, Westbrook & Huskey (1987) J. Clin. Invest. 79, 230-239]. In the present paper we show that tumour-promoting phorbol esters are effective in reducing Na+/phosphate cotransport. The Ca2+ ionophores A23187 and ionomycin had only a small effect on Na+/phosphate cotransport; added together, A23187 and phorbol esters showed a synergistic action. Phorbol esters and phorbol esters plus ionomycin stimulated prostaglandin synthesis as well as cyclic AMP production; acetylsalicylic acid prevented phorbol-ester-induced prostaglandin synthesis and cyclic AMP production, but had no effect on inhibition of Na+/phosphate cotransport. In suspensions of OK cells, PTH and thrombin produced a rise in intracellular Ca2+. In contrast with PTH, thrombin did not elevate cellular cyclic AMP in suspended OK cells. PTH and thrombin reduced Na+/phosphate cotransport in suspended OK cells. It is suggested that two regulatory cascades are involved in PTH action on Na+/phosphate cotransport: cyclic AMP/kinase A and Ca2+/diacylglycerol/kinase C.

Effects of confluence on phosphate transport capacity in cultured renal cell lines

The LLC-PK1 cell line transports phosphate (Pi), glucose, and amino acids using carriers similar to those in proximal tubular cells. Others have reported that when monolayers reach confluence, hexose transport increases and activity of the A-amino acid transporter falls. The present study evaluates Pi uptake by two continuous cell lines derived from renal proximal tubule, and demonstrates that phosphate uptake falls sharply upon reaching confluence in LLC-PK1 cells but not in cultured opossum kidney (OK) cells. The fall in Pi uptake in LLC-PK1 cells at confluence represents a halving in Vmax for Na-dependent phosphate uptake (2.33 vs. 5.00 nmol/mg protein/5 min) without a change in Km (82 vs. 94 microM). Suppression of phosphate transport in confluent monolayers of LLC-PK1 cells is completely reversed by bringing the cells into suspension. As has been shown for the phorbol ester 12-O-tetradecanoyl-phorbol-13-acetate (TPA), exposure of monolayers to serum stimulates phosphate uptake, but unlike phorbol ester, serum does so without stimulating alanine uptake. OK cells differ from LLC-PK1 in that no change occurs in Pi uptake at confluence, although they resemble LLC-PK1 cells in that sugar uptake rises and alanine uptake falls at confluence. The different temporal patterns for Pi uptake in the two cell lines indicates that developmental change in the uptake of Pi is not linked to that of glucose or alanine.

Activity of calcium/phospholipid dependent protein kinase during rat kidney development

The activity of the calcium sensitive phospholipid dependent protein kinase C (PKC) was studied in cytosol and in the proximal tubular luminal membrane of rats during growth. Cytosolic activity was elevated at 14 and 21 days of age and fell to adult levels by day 60. Luminal brush border membrane activity on the other hand was low on day 14 but reached adult levels by day 21. Changes in brush border membrane PKC activity may have important consequences for the development of electrolyte transport in proximal tubular cells.