LLC-PK1 Pig Kidney Cells Research Articles

Sustained Pituitary T3 Production Explains the T4-mediated TSH Feedback Mechanism.

The regulation of thyroid activity and thyroid hormone (TH) secretion is based on feedback mechanisms that involve the anterior pituitary TSH and medial basal hypothalamus TSH-releasing hormone. Plasma T3 levels can be "sensed" directly by the anterior pituitary and medial basal hypothalamus; plasma T4 levels require local conversion of T4 to T3, which is mediated by the type 2 deiodinase (D2). To study D2-mediated T4 to T3 conversion and T3 production in the anterior pituitary gland, we used mouse pituitary explants incubated with 125I-T4 for 48 hours to measure T3 production at different concentrations of free T4. The results were compared with cultures of D1- or D2-expressing cells, as well as freshly isolated mouse tissue. These studies revealed a unique regulation of the D2 pathway in the anterior pituitary gland, distinct from that observed in nonpituitary tissues. In the anterior pituitary, increasing T4 levels reduced D2 activity slightly but caused a direct increase in T3 production. However, the same changes in T4 levels decreased T3 production in human HSkM cells and murine C2C12 cells (both skeletal muscle) and mouse bone marrow tissue, which reached zero at 50 pM free T4. In contrast, the increase in T4 levels caused the pig kidney LLC-PK1 cells and kidney fragments to proportionally increase T3 production. These findings have important implications for both physiology and clinical practice because they clarify the mechanism by which fluctuations in plasma T4 levels are transduced in the anterior pituitary gland to mediate the TSH feedback mechanism.

Zinc trafficking: 1,10-phenanthroline, glutathione, and other metal binding ligands form adducts with proteomic Zn2+

Hypotheses were tested that the proteome of pig kidney LLC-PK1 cells (i) contains Zn-proteins that react with a diversity of native and pharmacologically active metal-binding ligands to form ternary complexes and (ii) includes proteins that bind Zn2+ nonspecifically and together form ternary adducts with a variety of metal-binding agents. The method to observe ternary complex formation with Zn-proteins and proteome•Zn involved preformation of fluorescent TSQ [6-Methoxy-(8-p-toluenesulfonamido)quinoline]-Zn-proteins and/or proteome•Zn-TSQ adducts followed by competitive reaction with selected ligands. The loss of TSQ-dependent fluorescence signaled the replacement of TSQ by the competing ligand in the starting adducts. In vitro, 1,10-phenanthroline competed effectively with TSQ for binding to Zn-proteins in the proteome. The successful competition of 1,10-phenanthroline with TSQ-Zn-proteins was also observed in cells. Similarly, 1,10-phenanthroline was shown to bind to a sizable fraction of Zn2+ associated adventitiously with proteome (proteome•Zn). Other synthetic ligands that bind to Zn-proteins and proteome•Zn include 2,2-bipyridyl, 8-hydroxyquinoline, 2,2'-dicarboxypyridine, and pyrithione. Such results suggest that ligand binding to such sites may play a role in the observed biological effects of these and other metal-binding molecules. Although cysteine does not significantly compete with TSQ, glutathione displaces TSQ from Zn-proteins and proteome•Zn at concentrations well below those found in cells, implying that ternary complex formation involving glutathione may be physiologically significant.

Zinc trafficking 1. Probing the roles of proteome, metallothionein, and glutathione.

The cellular trafficking pathways that conduct zinc to its sites of binding in functional proteins remain largely unspecified. In this study, the hypothesis was investigated that nonspecific proteomic binding sites serve as intermediates in zinc trafficking. Proteome from pig kidney LLC-PK1 cells contains a large concentration of such sites, displaying an average conditional stability constant of 1010-11, that are dependent on sulfhydryl ligands to achieve high-affinity binding of zinc. As a result, the proteome competes effectively with induced metallothionein for Zn2+ upon exposure of cells to extracellular Zn2+ or during in vitro direct competition. The reaction of added Zn2+ bound to proteome with apo-carbonic anhydrase was examined as a potential model for intracellular zinc trafficking. The extent of this reaction was inversely dependent upon proteome concentration and under cellular conditions thought to be negligible. The rate of reaction was strictly first order in both Zn2+ and apo-carbonic anhydrase, and also considered to be insignificant in cells. Adding the low molecular weight fraction of cell supernatant to the proteome markedly enhanced the speed of this reaction, a phenomenon dependent on the presence of glutathione (GSH). In agreement, inclusion of GSH accelerated the reaction in a concentration-dependent manner. The implications of abundant high-affinity binding sites for Zn2+ within the proteome are considered in relation to their interaction with GSH in the efficient delivery of Zn2+ to functional binding sites and in the operation of fluorescent zinc sensors as a tool to observe zinc trafficking.

Detection of Zn2+ release in nitric oxide treated cells and proteome: dependence on fluorescent sensor and proteomic sulfhydryl groups.

Nitric oxide (NO) is both an important regulatory molecule in biological systems and a toxic xenobiotic. Its oxidation products react with sulfhydryl groups and either nitrosylate or oxidize them. The aerobic reaction of NO supplied by diethylamine NONOate (DEA-NO) with pig kidney LLC-PK1 cells and Zn-proteins within the isolated proteome was examined with three fluorescent zinc sensors, zinquin (ZQ), TSQ, and FluoZin-3 (FZ-3). Observations of Zn2+ labilization from Zn-proteins depended on the specific sensor used. Upon cellular exposure to DEA-NO, ZQ sequestered about 13% of the proteomic Zn2+ as Zn(ZQ)2 and additional Zn2+ as proteome·Zn-ZQ ternary complexes. TSQ, a sensor structurally related to ZQ with lower affinity for Zn2+, did not form Zn(TSQ)2. Instead, Zn2+ mobilized by DEA-NO was exclusively bound as proteome·Zn-TSQ adducts. Analogous reactions of proteome with ZQ or TSQ in vitro displayed qualitatively similar products. Titration of native proteome with Zn2+ in the presence of ZQ resulted in the sole formation of proteome·Zn-ZQ species. This result suggested that sulfhydryl groups are involved in non-specific proteomic binding of mobile Zn2+ and that the appearance of Zn(ZQ)2 after exposure of cells and proteome to DEA-NO resulted from a reduction in proteomic sulfhydryl ligands, favoring the formation of Zn(ZQ)2 instead of proteome·Zn-ZQ. With the third sensor, FluoZin-3, neither Zn-FZ-3 nor proteome·Zn-FZ-3 was detected during the reaction of proteome with DEA-NO. Instead, it reacted independently with DEA-NO with a modest enhancement of fluorescence.

Newport Green, a fluorescent sensor of weakly bound cellular Zn(2+): competition with proteome for Zn(2).

Newport Green (NPG) is a recognized sensor of cellular Zn(2+) that displays fluorescence enhancement upon binding to Zn(2+). Because of its modest affinity for Zn(2+), the extent of its capacity to bind cellular Zn(2+) is unclear. The present study investigated the range of reactivity of NPG(ESTER) with cells, isolated (Zn)-proteome, and model Zn-proteins. The sensor accumulated in pig kidney LLC-PK1 cells and was slowly (>40 min) hydrolyzed to the fluorescent, acid form, NPG(ACID). The powerful, cell permeant Zn(2+) chelator, N,N,N',N'-tetrakis(2-pyridylmethyl)-ethane-1,2-diamine (TPEN) failed to quench the growing fluorescence emission, indicating that Zn-NPG(ACID) had not formed and NPG-Zn-protein adduct species probably were not present. Furthermore, NPG(ACID) did not bind to Zn-carbonic anhydrase or Zn-alcohol dehydrogenase, two proteins that form adducts with some other sensors. Strikingly, most of the NPG(ACID) that had been converted from NPG(ESTER) was detected in the extracellular medium not the cells. As a result, after cells were incubated with NPG(ESTER) and then Zn-pyrithione to raise the internal concentration of mobile Zn(2+), Zn-NPG(ACID) was only observed in the external medium. Residual cellular NPG(ACID) was unable to bind extra intracellular Zn(2+) delivered by pyrithione. Proteome isolated from the sonicated cell supernatant was also unreactive with NPG(ACID). Titration of proteome or glutathione with Zn(2+) in the presence of NPG(ACID) revealed that NPG(ACID) only weakly competes for mobile Zn(2+) in the presence of these cellular components. In addition, when proteomic Zn(2+) was released by a nitric oxide donor or N-ethyl-maleimide, little Zn(2+) was detected by NPG(ACID). However, exposure to nitric oxide independently enhanced the fluorescence properties of NPG(ACID). Thus, the biochemical properties of NPG related to cellular Zn(2+) chelation deepen the question of how it functions as a Zn(2+) sensor.

Toxic metal proteomics: Reaction of the mammalian zinc proteome with Cd2 +

The hypothesis was tested that Cd2+ undergoes measureable reaction with the Zn-proteome through metal ion exchange chemistry. The Zn-proteome of pig kidney LLC-PK1 cells is relatively inert to reaction with competing ligands, including Zinquin acid, EDTA, and apo-metallothionein. Upon reaction of Cd2+ with the Zn-proteome, Cd2+ associates with the proteome and near stoichiometric amounts of Zn2+ become reactive with these chelating agents. The results strongly support the hypothesis that Cd2+ displaces Zn2+ from native proteomic binding sites resulting in the formation of a Cd-proteome. Mobilized Zn2+ becomes adventitiously bound to proteome and available for reaction with added metal binding ligands. Cd-proteome and Zn-metallothionein readily exchange metal ions, raising the possibility that this reaction restores functionality to Cd-proteins. In a parallel experiment, cells were exposed to Cd2+ and pyrithione briefly to generate substantial proteome-bound Cd2+. Upon transition to a Cd2+ free medium, the cells generated new metallothionein protein over time that bound most of the proteomic Cd2+ as well as additional Zn2+.

Insulin-Mediated Oxidative Stress and DNA Damage in LLC-PK1 Pig Kidney Cell Line, Female Rat Primary Kidney Cells, and Male ZDF Rat Kidneys In Vivo

Hyperinsulinemia, a condition with excessively high insulin blood levels, is related to an increased cancer incidence. Diabetes mellitus is the most common of several diseases accompanied by hyperinsulinemia. Because an elevated kidney cancer risk was reported for diabetic patients, we investigated the induction of genomic damage by insulin in LLC-PK1 pig kidney cells, rat primary kidney cells, and ZDF rat kidneys. Insulin at a concentration of 5nM caused a significant increase in DNA damage in vitro. This was associated with the formation of reactive oxygen species (ROS). In the presence of antioxidants, blockers of the insulin, and IGF-I receptors, and a phosphatidylinositol 3-kinase inhibitor, the insulin-mediated DNA damage was reduced. Phosphorylation of protein kinase B (PKB or AKT) was increased and p53 accumulated. Inhibition of the mitochondrial and nicotinamide adenine dinucleotide phosphatase oxidase-related ROS production reduced the insulin-mediated damage. In primary rat cells, insulin also induced genomic damage. In kidneys from healthy, lean ZDF rats, which were infused with insulin to yield normal or high blood insulin levels, while keeping blood glucose levels constant, the amounts of ROS and the tumor protein (p53) were elevated in the high-insulin group compared with the control level group. ROS and p53 were also elevated in diabetic obese ZDF rats. Overall, insulin-induced oxidative stress resulted in genomic damage. If the same mechanisms are active in patients, hyperinsulinemia might cause genomic damage through the induction of ROS contributing to the increased cancer risk, against which the use of antioxidants and/or ROS production inhibitors might exert protective effects.

Antiepileptic Drug Transport—Of Mice and Men

Differences in the Transport of the Antiepileptic Drugs Phenytoin, Levetiracetam and Carbamazepine by Human and Mouse P-Glycoprotein. Baltes S, Gastens AM, Fedrowitz M, Potschka H, Kaever V, Loscher W. Neuropharmacology 2007;52(2):333–346. In view of the important role of P-glycoprotein (P-gp) and other drug efflux transporters for drug distribution and resistance, the identification of compounds as substrates of P-gp-mediated transport is one of the key issues in drug discovery and development, particularly for compounds acting on the central nervous system. In vitro transport assays with P-gp-transfected kidney cell lines are widely used to evaluate the potential of compounds to act as P-gp substrates or inhibitors. Furthermore, such cell lines are also frequently utilized as a substitute for more labor-intensive in vitro or in vivo models of the blood–brain barrier (BBB). Overexpression of P-gp or members of the multidrug resistance protein (MRP) family at the BBB has been implicated in the mechanisms underlying resistance to antiepileptic drugs (AEDs) in patients with epilepsy. Therefore, it is important to know which AEDs are substrates for P-gp or MRPs. In the present study, we used monolayers of polarized MDCKII dog kidney or LLC-PK1 pig kidney cells transfected with cDNA containing either human MDR1, MRP2 or mouse mdr1a and mdr1b sequences to measure the directional transport of AEDs. Cyclosporin A (CsA) and vinblastine were used as reference standards for P-gp and MRP2, respectively. The AEDs phenytoin and levetiracetam were directionally transported by mouse but not human P-gp, whereas CsA was transported by both types of P-gp. Carbamazepine was not transported by any type of P-gp and did not inhibit the transport of CsA. In contrast to vinblastine, none of the AEDs was transported by MRP2 in transfected kidney cells. The data indicate that substrate recognition or transport efficacy by P-gp differs between human and mouse for certain AEDs. Such species differences, which are certainly not restricted to human and mouse, may explain, at least in part, the controversial data that have been previously reported for AED transport by P-gp in preparations from different species. However, because transport efficacy of efflux transporters such as P-gp or MRP2 may not only differ between species but also between tissues, the present data do not exclude that the AEDs examined are weak substrates of P-gp or MRP2 at the human BBB. Valproic Acid Is Not a Substrate for P-Glycoprotein or Multidrug Resistance Proteins 1 and 2 in a Number of In Vitro and In Vivo Transport Assays. Baltes S, Fedrowitz M, Tortos CL, Potschka H, Loscher W. J Pharmacol Exp Ther 2007;320(1):331–343. The antiepileptic drug valproic acid (VPA) is widely used in the treatment of epilepsy, bipolar disorders, and migraine. However, rather high doses are required for the clinical effects of VPA, which is due to its relatively inefficient delivery to the brain. The poor brain distribution of VPA is thought to reflect an asymmetric transport system at the blood–brain barrier (BBB). Based on recent data from in vitro experiments, multidrug resistance proteins (MRPs) have been proposed to be involved in the efflux transport of VPA at the BBB. In the present study, we used different experimental in vitro and in vivo strategies to evaluate whether VPA is a substrate for MRPs or the efflux transporter P-glycoprotein (P-gp). In contrast to known P-gp or MRP substrates, such as cyclosporin A or vinblastine, no directional transport of VPA was observed in cell monolayer efflux assays using the kidney cell lines Madin Darby canine kidney II and LLC-PK1, which had been transfected with either human or mouse cDNAs for the genes encoding P-gp, MRP1, or MRP2. Likewise, no indication for efflux transport of VPA was obtained in a rat microdialysis model, using inhibitors of either P-gp or MRPs. Furthermore, a significant role of MRP2 in brain efflux of VPA was excluded by using MRP2-deficient rats. Our data do not support the hypothesis that MRP1 or MRP2 is involved in the efflux of VPA from the brain. Thus, the molecular identity of the putative transporter(s) mediating the active efflux of VPA from the brain remains to be elucidated.

Regulation of Inositol 1,4,5-Trisphosphate Receptor-mediated Calcium Release by the Na/K-ATPase in Cultured Renal Epithelial Cells

It is known that the Na/K-ATPase alpha1 subunit interacts directly with inositol 1,4,5-triphosphate (IP(3)) receptors. In this study we tested whether this interaction is required for extracellular stimuli to efficiently regulate endoplasmic reticulum (ER) Ca(2+) release. Using cultured pig kidney LLC-PK1 cells as a model, we demonstrated that graded knockdown of the cellular Na/K-ATPase alpha1 subunit resulted in a parallel attenuation of ATP-induced ER Ca(2+) release. When the knockdown cells were rescued by knocking in a rat alpha1, the expression of rat alpha1 restored not only the cellular Na/K-ATPase but also ATP-induced ER Ca(2+) release. Mechanistically, this defect in ATP-induced ER Ca(2+) release was neither due to the changes in the amount or the function of cellular IP(3) and P2Y receptors nor the ER Ca(2+) content. However, the alpha1 knockdown did redistribute cellular IP(3) receptors. The pool of IP(3) receptors that resided close to the plasma membrane was abolished. Because changes in the plasma membrane proximity could reduce the efficiency of signal transmission from P2Y receptors to the ER, we further determined the dose-dependent effects of ATP on protein kinase Cepsilon activation and ER Ca(2+) release. The data showed that the alpha1 knockdown de-sensitized the ATP-induced ER Ca(2+) release but not PKCepsilon activation. Moreover, expression of the N terminus of Na/K-ATPase alpha1 subunit not only disrupted the formation of the Na/K-ATPase-IP(3) receptor complex but also abolished the ATP-induced Ca(2+) release. Finally, we observed that the alpha1 knockdown was also effective in attenuating ER Ca(2+) release provoked by angiotensin II and epidermal growth factor.

Differences in the transport of the antiepileptic drugs phenytoin, levetiracetam and carbamazepine by human and mouse P-glycoprotein

In view of the important role of P-glycoprotein (Pgp) and other drug efflux transporters for drug distribution and resistance, the identification of compounds as substrates of Pgp-mediated transport is one of the key issues in drug discovery and development, particularly for compounds acting on the central nervous system. In vitro transport assays with Pgp-transfected kidney cell lines are widely used to evaluate the potential of compounds to act as Pgp substrates or inhibitors. Furthermore, such cell lines are also frequently utilized as a substitute for more labor-intensive in vitro or in vivo models of the blood–brain barrier (BBB). Overexpression of Pgp or members of the multidrug resistance protein (MRP) family at the BBB has been implicated in the mechanisms underlying resistance to antiepileptic drugs (AEDs) in patients with epilepsy. Therefore, it is important to know which AEDs are substrates for Pgp or MRPs. In the present study, we used monolayers of polarized MDCKII dog kidney or LLC-PK1 pig kidney cells transfected with cDNA containing either human MDR1, MRP2 or mouse mdr1a and mdr1b sequences to measure the directional transport of AEDs. Cyclosporin A (CsA) and vinblastine were used as reference standards for Pgp and MRP2, respectively. The AEDs phenytoin and levetiracetam were directionally transported by mouse but not human Pgp, whereas CsA was transported by both types of Pgp. Carbamazepine was not transported by any type of Pgp and did not inhibit the transport of CsA. In contrast to vinblastine, none of the AEDs was transported by MRP2 in transfected kidney cells. The data indicate that substrate recognition or transport efficacy by Pgp differs between human and mouse for certain AEDs. Such species differences, which are certainly not restricted to human and mouse, may explain, at least in part, the controversial data which have been previously reported for AED transport by Pgp in preparations from different species. However, because transport efficacy of efflux transporters such as Pgp or MRP2 may not only differ between species but also between tissues, the present data do not exclude that the AEDs examined are weak substrates of Pgp or MRP2 at the human BBB.

The livestock photosensitizer, phytoporphyrin (phylloerythrin), is a substrate of the ATP-binding cassette transporter ABCG2

Hepatogenous photosensitization occurs in livestock following damage to the liver or biliary apparatus that results in impaired excretion of phytoporphyrin (phylloerythrin), a photosensitizer. Based on earlier observations that porphyrin-based photosensitizers are substrates of the ATP-binding cassette transporter ABCG2, we examined the ability of the hepatic transporters ABCB1 (P-glycoprotein) and ABCG2 to transport phytoporphyrin. Transport of phytoporphyrin was blocked by the ABCG2-specific inhibitor fumitremorgin C (FTC) in human embryonic kidney cells transfected with full length human ABCG2, while no transport by cells transfected with human ABCB1 was noted. FTC-inhibited transport of phytoporphyrin was also demonstrated in ABCG2-expressing LLC-PK1 pig kidney cells, consistent with the idea that the pig orthologue, like human ABCG2, transports the photosensitizer. ABCG2 expression was confirmed by immunohistochemistry in the hepatocytes of cow, pig and sheep livers. We conclude that phytoporphyrin is a substrate for ABCG2 and that the transporter is likely responsible for its biliary excretion.

Trabectedin (ET-743, Yondelis™) is a substrate for P-glycoprotein, but only high expression of P-glycoprotein confers the multidrug resistance phenotype

Trabectedin (ET-743, Yondelis) is a novel anticancer drug currently undergoing phase II and III investigations. There are various and conflicting reports whether trabectedin is a substrate for P-glycoprotein (P-gp), an important factor in drug disposition and multi-drug resistance (MDR). We have now unambiguously shown that trabectedin is a P-gp substrate by investigating vectorial transport over monolayers of LLC-PK1 pig kidney and Madine-Darby Canine kidney (MDCK) cells and the mdr1a and/or MDR1 transfected subclones. We further characterized the cytotoxic effects and cellular accumulation of trabectedin in these cell lines as well as in a panel of other cell lines with high or moderate expression levels of P-gp. Trabectedin displayed the typical MDR phenotype only in highly P-gp expressing cell lines, but not in cell lines with expression levels more closely conforming to clinical samples, suggesting that P-gp will not confer resistance to trabectedin in cancer patients.

ALTERED DE NOVO SPHINGOLIPID BIOSYNTHESIS IS INVOLVED IN THE SERUM DEPRIVATION-INDUCED CELL DEATH IN LLC-PK1 CELLS

Fumonisin B1, a specific inhibitor of ceramide synthase, and ISPI (Myriocin), a serine palmitoyltransferase inhibitor, modulate the de novo sphingolipid biosynthesis pathway. This study was conducted to determine whether serum deprivation-induced cell death is regulated by de novo sphingolipid biosynthesis in pig kidney LLC-PK1 cells. Serum withdrawal from the culture medium produced cell death in LLC-PK1 cells. Fumonisin B1 at concentrations ranging from 5 µM to 30 µM delayed until 48 h this cell death resulting from the absence of fetal bovine serum (FBS) in cell culture. Pretreatment of cultured cells with fumonisin B1 in the presence of serum for 24 h increased by approximately 70% this cytoprotective activity of fumonisin B1 against serum deprivation-induced cell death. Serum deprivation increased sphingolipid biosynthesis threefold compared to 5% serum-enriched culture. Fumonisin B1 at 5–30 µM lowered the content of total complex sphingolipids to levels of 50% and 77% of the content in serum-enriched culture, although the concentration of intracellular free sphinganine was elevated. ISP1 alone at greater than 1 nM concentration reduced total complex sphingolipid content to values in LLC-PK1 cells grown in the presence of 5% FBS. The results suggest that the de novo complex sphingolipid biosynthesis modulated by either fumonisin B1 or ISP1 may regulate serum deprivation-induced cell death in LLC-PK1 cells.

Prostaglandin D2 synthase induces apoptosis in pig kidney LLC-PK1 cells

Prostaglandin D(2) synthase (PGD(2)S), a unique member of the lipocalin family, is found at elevated levels in the serum of patients with renal impairment and has recently been implicated as a new biochemical marker of renal insufficiency. The aim of this study was to investigate the apoptotic effects of PGD2S on a pig kidney epithelial cell line (LLC-PK1) and to investigate the effects of prostaglandins and growth factors on this process. Apoptosis was detected by terminal deoxynucleotidyl transferase (TdT)-mediated dUTP nick end-labeling (TUNEL), annexin V staining, and electron microscopy. A four- to fivefold increase in apoptosis was observed in PGD(2)S-treated cells as compared with controls and the apoptosis appeared to act via caspase-3. A cyclooxygenase-2 inhibitor, anti-PGD(2)S antibody, and selenium all significantly inhibited the apoptosis induced by PGD(2)S; however, none had any effect on the apoptosis induced by the known apoptotic inducer camptothecin. Furthermore, prostaglandins E(1) and E(2), known to induce mitogen-activated protein (MAP) kinase phosphorylation and exhibit cytoprotective effects, both inhibited PGD(2)S-induced apoptosis, while prostaglandin H(2) had no significant effect. Growth factors such as insulin, insulin-like growth factor-1, and platelet-derived growth factor also decreased PGD(2)S-induced apoptosis. In addition, PGD(2)S isolated from human serum seemed slightly more effective at inducing apoptosis than recombinantly expressed protein. We report on the induction of apoptosis by PGD(2)S in LLC-PK1 pig kidney epithelial cells, and speculate that the accumulation of PGD(2)S in the serum of kidney failure patients may further exacerbate renal problems and is most likely regulated by other prostaglandins and growth factors.

Effects of 24 R,25-Dihydroxyvitamin D 3 on Alkaline Phosphatase Activity in Pig Renal Epithelial LLC-PK1 Cells in Culture

1. The effects of 24 R,25-dihydroxyvitamin D 3 [24,25(OH) 2D 3] on alkaline phosphatase activity (ALP) were evaluated in pig kidney LLC-PK1 cells in culture. 2. The vitamin D 3 metabolite increased ALP activity in these cells, whereas no effect of the hormone was observed on γ-glutamyltranspeptidase and acid phosphatase activities. 3. ALP activity was stimulated after 3- to 12-hr incubation in the presence of 10 −9 mol/l 24,25(OH) 2D 3 with a maximum after 6 hr. 4. The hormonal induction of ALP activity was prevented by pretreatment of cells by actinomycin D. 5. It is proposed that 24,25(OH) 2D 3 could increase ALP activity by de novo protein synthesis.

Availability of PSC833, a substrate and inhibitor of P-glycoproteins, in various concentrations of serum.

P-glycoproteins are membrane-associated transporters that can render cells resistant to a variety of chemotherapeutic drugs. Reversal agents are (preferably nontoxic) drugs that can inhibit these P-glycoproteins and thereby overcome multidrug resistance. PSC833, a cyclosporin A analog, is a reversal agent that has shown potential in in vitro experiments and in clinical trials. We tested PSC833 to determine whether it is a transported substrate of human and murine P-glycoproteins associated with multidrug resistance (encoded by the human MDR1 gene and its murine homolog, mdr1a) and whether it can completely inhibit these P-glycoproteins under simulated in vivo conditions. Monolayers of polarized LLC-PK1 pig kidney cells transfected with complementary DNA containing either MDR1 or mdr1a sequences were used to measure the directional transport of P-glycoprotein substrates under various serum conditions. In contrast to two previous studies, we found that PSC833 is transported by both the MDR1 and the mdr1a P-glycoproteins, albeit at a low rate. PSC833 has a very high affinity for the MDR1 P-glycoprotein, and its Michaelis constant (Km) for transport is 50 nM, fourfold lower than for cyclosporin A. Inhibition of drug transport by PSC833 is approximately eightfold less effective in 100% fetal bovine serum than in tissue culture medium containing 10% serum. The concentration of PSC833 necessary to fully inhibit transport of digoxin and paclitaxel (Taxol) under complete (i.e., 100%) serum conditions is higher than the plasma concentrations achieved in clinical trials. Although PSC833 binds efficiently to the MDR1 P-glycoprotein and is released only sluggishly, the high concentrations of PSC833 necessary to inhibit this P-glycoprotein under complete serum conditions in our in vitro system suggest that it may be difficult for PSC833 alone to produce total inhibition of P-glycoprotein activity in patients.

Activation of caspases in pig kidney cells infected with wild-type and CrmA/SPI-2 mutants of cowpox and rabbitpox viruses.

The cowpox virus (CPV) CrmA and the equivalent rabbitpox virus (RPV) SPI-2 proteins have anti-inflammatory and antiapoptosis activity by virtue of their ability to inhibit caspases, including the interleukin-1beta-converting enzyme (ICE; caspase-1). Infection of LLC-PK1 pig kidney cells with a CPV CrmA mutant, but not with wild-type (wt) CPV, results in the induction of many of the morphological features of apoptosis (C. A. Ray and D. J. Pickup, Virology 217:384-391, 1996). In our study, LLC-PK1 cells infected with CPV delta crmA, but not those infected with wt CPV, showed induction of poly(ADP-ribose) polymerase (PARP)- and lamin A-cleaving activities and processing of the CPP32 (caspase-3) precursor to a mature 18-kDa form. Surprisingly, infection of LLC-PK1 cells with either wt RPV (despite the presence of the SPI-2 protein) or RPV delta SPI-2 resulted in cleavage activity against PARP and lamin A and the appearance of the mature subunit of CPP32/caspase-3. The biotinylated specific peptide inhibitor Ac-Tyr-Val-Lys(biotinyl)-Asp-2,6-dimethylbenzoyloxymethylketone [AcYV(bio)KD-aomk] labeled active caspase subunits of 18, 19, and 21 kDa in extracts from LLC-PK1 cells infected with CPV delta crmA, wt RPV, or RPV delta SPI-2 but not wt CPV. Mixed infection of LLC-PK1 cells with wt RPV and wt CPV gave no PARP-cleaving activity, and all PARP cleavage mediated by SPI-2 and CrmA mutants of RPV and CPV, respectively, could be eliminated by coinfection with wt CPV. These results suggest that the RPV SPI-2 and CPV CrmA proteins are not functionally equivalent and that CrmA, but not SPI-2 protein, can completely prevent apoptosis in LLC-PK1 cells under these conditions.

The ATP-depleting reagent iodoacetamide induces the degradation of protein kinase C alpha (PKCα) in LLC-PK1 pig kidney cells

The alkylating reagent iodoacetamide, a potent inhibitor of sulfhydryl proteases, was found to stimulate the selective degradation of protein kinase C alpha (PKCα) isoform (80 KDa). Treatment of LLC-PK 1 cells with iodoacetamide (0.5–15 mM) for 30–90 minutes at room temperature followed by western blotting on total cell homogenate, revealed the appearance of an 50 KDa band that was still recognized with the antibody. However, iodoacetamide (15 mM) resulted in the total disappearance of the 80 KDa protein. Serine protease inhibitors, metalloprotease inhibitors and leupeptin failed to prevent the degradation of PKCα. The degradation persisted at 4 °C and in the absence of Ca 2+. Iodoacetamide had no direct effect on purified PKCα. PKC activities in iodoacetamide-treated cells were also inhibited. In conclusion, the degradation of PKCα is a novel phenomenon. The degradation process could not be prevented by known protease inhibitors or in the absence of Ca 2+ or by incubation at 4 °C and appears to involve interactions with unknown cellular intermediates.

Elevated Sphingoid Bases and Complex Sphingolipid Depletion as Contributing Factors in Fumonisin-Induced Cytotoxicity

Fumonisin B1is an inhibitor of ceramide synthase, a key enzyme inde novosphingolipid biosynthesis and reacylation of free sphingosine. The purpose of this study was to determine the contribution of increased intracellular free sphinganine and decreased complex sphingolipids on cell growth and cell death induced by fumonisin B1in pig kidney LLC-PK1cells. Fumonisin B1caused an increase in intracellular free sphinganine which preceded depletion of complex sphingolipids, inhibition of cell growth, and cell death. The effects on cell growth and cell death were well correlated with the increase in free sphingoid bases and depletion of complex sphingolipids. Exogenously added sphinganine mimicked the effects of fumonisin, but β-chloroalanine, an inhibitor of serine palmitoyltransferase which is the first enzyme inde novosphingolipid biosynthesis, also inhibited cell growth and increased cell death. When added simultaneously, β-chloroalanine reduced the fumonisin-induced sphinganine increase by approximately 90%; however, it exacerbated the decrease in more complex sphingolipids. The effects of fumonisin on cell growth and cell death were only partially prevented by β-chloroalanine (∼50 to 60%). The results suggest that both the elevation of free sphingoid bases and the decrease in complex sphingolipids contribute to the decreased cell growth and cytolethality of fumonisin B1in pig kidney LLC-PK1cells.

Comparative Effects of Cd2+and Cd–Metallothionein on Cultured Kidney Tubule Cells

The effects of Cd2+and Cd–metallothionein on two cultured cells with proximal tubule characteristics, mouse kidney cortical cells and pig kidney LLC-PK1cells, have been compared. Cd2+inhibits Na+–glucose cotransport in LLC-PK1cells and in the process decreases the number of binding sites for [3H]phlorizin, a competitive inhibitor of glucose for the Na+–glucose cotransporter. During 24 hr incubation and over a range of concentrations in the two cell types, only Cd2+inhibited Na+–glucose cotransport even when approximately equal concentrations of intracellular Cd resulted from these treatments. Indeed, at low concentrations of Cd–metallothionein in mouse cells, transporter activity was elevated. Extension of incubations to 72 hr in mouse cells led to increased Cd uptake and reduction in cell density with both sources of Cd but only a progressive decline in Na+–glucose cotransport activity with Cd2+. Zn–metallothionein was without effect under comparable conditions. Both forms of Cd were accumulated by these cells, with the large majority of the metal ion localizing in metallothionein as a Cd, Zn–protein in LLC-PK1cells. Under equal exposure conditions, the net uptake of Cd from Cd2+was an order of magnitude greater than Cd from Cd–metallothionein in the two cell types. It is evident that the mechanisms of toxicity of Cd2+and Cd–metallothionein as well as their modes of uptake differ in these two cell types.