Characterization of a polyamine transport system in murine embryonic palate mesenchymal cells

Recent advances in the molecular biology of metazoan polyamine transport

Very limited molecular knowledge exists about the identity and protein components of the ubiquitous polyamine transporters found in animal cells. However, a number of reports have been published over the last 5 years on potential candidates for metazoan polyamine permeases. We review the available evidence on these putative polyamine permeases, as well as establish a useful "identikit picture" of the general polyamine transport system, based on its properties as found in a wide spectrum of mammalian cells. Any molecular candidate encoding a putative "general" polyamine permease should fit that provided portrait. The current models proposed for the mechanism of polyamine internalization in mammalian cells are also briefly reviewed.

Identification and Characterization of a Polyamine Permease from the Protozoan Parasite Leishmania major

The proteins that mediate polyamine translocation into eukaryotic cells have not been identified at the molecular level. To define the polyamine transport pathways in eukaryotic cells we have cloned a gene, LmPOT1, that encodes a polyamine transporter from the protozoan pathogen, Leishmania major. Sequence analysis of LmPOT1 predicted an unusual 803-residue polytopic protein with 9-12 transmembrane domains. Expression of LmPOT1 cRNA in Xenopus laevis oocytes revealed LmPOT1 to be a high affinity transporter for both putrescine and spermidine, whereas expression of LmPOT1 in Trypanosoma brucei stimulated putrescine uptake that was sensitive to inhibition by pentamidine and proton ionophores. Immunoblot analysis established that LmPOT1 was expressed predominantly in the insect vector form of L. major, and immunofluorescence demonstrated that LmPOT1 was localized predominantly to the parasite plasma membrane. To our knowledge this is the first molecular identification and characterization of a cell surface polyamine transporter in eukaryotic cells.

Comparative uptake of polyamines by prostate and non-prostate cancer cell lines

The Km and Vmax of [14C]-radiolabeled polyamines were determined for PC-3 and AT3B-1 cell lines. With PC-3 Km values are in the following order: ornithine> spermidine> spermine> putrescine, while with AT3B-1 it was spermidine> ornithine> spermine> putrescine. To determine which of these polyamines exhibit higher accumulation, the relative uptake of all the four amines was studied with prostate (PC-3, AT3B-1, LNCaP) and non-prostate (MCF-7, KLN-205, OVCAR) cell lines at 10 and 20 microM after 1 hour. Spermine and spermidine accumulated at higher levels in prostate (AT3B-1 and LNCaP) over non-prostate cell lines (p < 0.01). Putrescine accumulated more in PC-3 and LNCaP than the non-prostate cancer cells.

The presence of an Na+/spermine antiporter in the rat renal brush-border membrane

This study was aimed at determining the driving force for spermine transport in rat renal proximal tubular brush-border membrane. The uptake of spermine and trientine, a spermine-like drug used for treating Wilson's disease, into rat renal brush-border membrane vesicles was significantly stimulated by an outwardly directed Na+ gradient. The Na+-dependent uptake was temperature dependent and saturable. A kinetic analysis of the initial uptake of spermine with an Na+ gradient gave a Km value of 1.44 microM and a Vmax value of 6.31 pmol (mg protein)(-1)/30s. The Na+ dependent uptake of [3H]spermine was inhibited by spermine, trientine and tetraethylene-pentamine. Substrates of the H+/organic cation transporter (cimetidine and tetraethyl-ammonium), physiological polyamines (putrescine and spermidine) with 2 or 3 amino groups and aminoglycosides (amikacin and tobramicin) with 4 or 5 cationic amines did not affect the uptake of spermine in the presence of an outwardly directed Na+ gradient. These results suggest that the renal tubular secretion of spermine is mediated by an Na+/spermine antiport system which is specific for a straight-chain polyamine compound with more than 4 amino groups.

Extracellular calcium stimulates Na(+)-dependent putrescine uptake in B16 melanoma cells

The regulation of putrescine transport in difluoromethylornithine-treated B16 melanoma cells by extracellular Ca2+ has been investigated. It was found that physiological concentrations of Ca2+ were essential for optimum uptake of putrescine and spermidine, Mg2+, albeit at higher concentrations, also could potentiate polyamine transport. The maximum rate of putrescine uptake increased from 1698 +/- 67 pmol/min/min/mg DNA in the absence of Ca2+ to 3100 +/0 98 pmol/min/mg DNA in the presence of 0.5 mM Ca2+. There was no change in Km. While Ca2+ enhanced transport of both putrescine and spermidine it did not affect the uptake of deoxyglucose, thymidine or leucine. Putrescine did not alter Ca2+ fluxes suggesting that the two cations do not share a common transport system. The effects of Ca2+ on putrescine uptake appeared to be mediated extracellularly firstly because Ca2+ did not potentiate putrescine uptake in the presence of A23187 and secondly, because the effects of Ca2+ were completely inhibited by the lanthanide Tb3+, which binds to calcium-dependent proteins and does not readily cross biological membranes, Ca2+ did not affect putrescine transport in the absence of extracellular Na+. Moreover, the rate of putrescine uptake in the absence of Ca2+ was similar to that in the absence of extracellular Na+. The results from this study indicate that polyamine transport is stimulated by extracellular Ca2+ and suggest that Ca2+ is required for activity of the Na(+)-dependent transporter only. This transporter appears to possess a regulatory binding site for divalent cations.

Putrescine active uptake system in the Trypanosomatid Crithidia fasciculata

Using the insect Trypanosomatid Crithidia fasciculata as a model parasite of mammalian pathogenic flagellates, i.e. Leishmania and Trypanosoma spp., we have studied the kinetic and regulatory characteristics of the polyamine uptake system. Putrescine transport was age-dependent with maximum expression values at the proliferative logarithmic phase. Putrescine transport in Crithidia fasciculata was energy-dependent and against a putrescine concentration gradient. The integrity of the membrane sulfhydryl groups was absolutely required for optimum transport rates. The specificity of this mechanism was studied in the presence of a series of different chain length aliphatic diamines, showing the high specificity for putrescine and the poor effect of this series at the highest concentration analyzed as well as the higher polyamines spermidine and spermine. Finally, the well-known inhibitor of polyamine biosynthesis, DFMO, led to an upward regulation of putrescine uptake correlating with the depletion of intracellular polyamine pool. In addition, the presence of high concentrations of putrescine in the culture medium produced a downward regulation of this system.

Photoaffinity labeling of a cell surface polyamine binding protein

Intracellular polyamine pools are partially maintained by an active transport apparatus that is specific for and regulated by polyamines. Although mammalian transport activity has been characterized by kinetic studies, the actual protein itself has yet to be identified, purified, or cloned. As one approach to this problem, we attempted photoaffinity labeling of plasma membrane proteins using two specifically designed and synthesized polyamine conjugates as photoprobes. The first is a spermidine conjugate bearing the photoreactive moiety 4-azidosalicylic acid at the N4 position via an alkyl linkage, and the second is a norspermine conjugate with 4-azidosalicylic acid at the N4 position via an acyl linkage. Labeling of murine L1210 lymphocytic leukemia cells was carried out at 4 degrees C to promote selective alkylation of cell surface proteins. Separation of plasma membrane proteins from cells cross-linked with the N4-spermidine conjugate by SDS-polyacrylamide gel electrophoresis revealed two heavily labeled proteins at approximately 118 and approximately 50 kDa (designated p118 and p50, respectively). Band p118 was more well defined and much more intensely labeled. Analogous proteins were also observed in human U937 lymphoma cells. Specificity of labeling was strongly suggested by competition with polyamines and analogs during labeling and further indicated by the nearly identical labeling of the same protein by the N1-norspermine photoprobe but not by the unconjugated photoreagent. Neuraminidase pretreatment of L1210 cells increased mobility of the p118, suggesting that it was glycosylated and, thus, of plasma membrane origin. In transport-deficient L1210 cells, p118 and p50 were found to have a slightly higher molecular mass and were accompanied by a less distinct protein band (approximately 100 kDa). These findings indicate the presence of a polyamine binding protein at the surface of murine and human leukemia cells, which could be directly or indirectly related to the polyamine transport apparatus.

Inorganic cation dependence of putrescine and spermidine transport in human breast cancer cells

The mechanism of polyamine uptake in mammalian cells is still poorly understood. The role of inorganic cations in polyamine transport was investigated in ZR-75-1 human breast cancer cells. Although strongly temperature dependent, neither putrescine nor spermidine uptake was mediated by a Na+ cotransport mechanism. In fact, Na+ and cholinium competitively inhibited putrescine uptake relative to that measured in a sucrose-based medium. On the other hand, ouabain, H+, Na+, and Ca2+ ionophores, as well as dissipation of the K+ diffusion potential, strongly inhibited polyamine uptake in keeping with a major role of membrane potential in that process. Polyamine transport was inversely dependent on ambient osmolality at near physiological values. Putrescine transport was inhibited by 70% by decreasing extracellular pH from 7.2 to 6.2, whereas spermidine uptake had a more acidic optimum. Deletion of extracellular Ca2+ inhibited putrescine uptake more strongly than chelation of intracellular Ca2+. In fact, bound divalent cations were absolutely required for polyamine transport, as shown after brief chelation of the cell monolayers with EDTA. Either Mn2+, Ca2+, or Mg2+ sustained putrescine uptake activity with high potency (Km = 50-300 microM). Mn2+ was a much stronger activator of spermidine than putrescine uptake, suggesting a specific role for this metal in polyamine transport. Other transition metals (Co2+, Ni2+, Cu2+, and Zn2+) were mixed activators/antagonists of carrier activity, while Sr2+ and Ba2+ were very weak agonists, while not interfering with Ca2+/Mg(2+)-dependent transport. Thus, polyamine uptake in human breast tumor cells is negatively affected by ionic strength and osmolality, and is driven, at least in part, by the membrane potential, but not by the Na+ electrochemical gradient. Moreover, the polyamine carrier, or a tightly coupled accessory component, appears to have a high-affinity binding site for divalent cations, which is essential for the uptake mechanism.

Role of ATP and sodium in polyamine transport in bovine pulmonary artery smooth cells

Increased polyamine transport may be a key mechanism driving elevations in lung cell polyamine content necessary for the development of chronic hypoxic pulmonary hypertension. Bovine pulmonary artery smooth muscle cells (PASMCs) in culture exhibit two carriers for polyamines, a non-selective one shared by the three polyamines, putrescine (PUT), spermidine (SPD), and spermine (SPM), and another that is selective for SPD and SPM. Hypoxia appears to up-regulate both carriers. In this study, we examined the role of ATP and the Na+ gradient in regulating polyamine transport in control PASMCs and in PASMCs with polyamine transport augmented by culture under hypoxic conditions (Po2: 15-30 torr). Inhibition of ATP synthesis with dinitrophenol+iodoacetate profoundly reduced polyamine uptake in both control and hypoxic PASMCs. Putrescine uptake was somewhat more sensitive to iso-osmotic replacement of extracellular Na+ with choline chloride or sucrose than were SPD or SPM in both hypoxic and standard cells, but under no conditions did Na+ replacement substantially alter polyamine uptake. Treatment of PASMCs with ouabain, a Na(+)-K+ ATPase inhibitor, or with gramicidin, a Na+ ionophore, minimally attenuated polyamine transport, whereas the Na+/K+ ionophore monensin increased polyamine uptake in standard, but not in hypoxic, cells. In general, the reduction in the extracellular Na+ content or ionophore-induced increases in Na+ permeability had a greater suppressive effect on polyamine transport in hypoxic cells than in standard cells, suggestive of the induction of Na(+)-dependent polyamine carriers by hypoxia. These observations indicate that the activities of the two putative polyamine transport pathways in standard PASMCs, as well as their up-regulation by hypoxia, require ATP synthesis. In addition, it appears that polyamine transport in PASMCs is composed of two components: one a prominent sodium-independent transporter and the other a relatively minor component that is sodium dependent. The latter may be activated by hypoxic exposure in combination with the induction of new polyamine carriers.

Endothelial polyamine uptake: selective stimulation by L-arginine deprivation or polyamine depletion

Uptake of putrescine and spermidine by cultured porcine aortic endothelial cells was time dependent and linear for 60 min. Transport, against a 5- to 10-fold concentration gradient, demonstrated both saturable and non-saturable components. Apparent concentration giving one-half maximal transport (Kt) values for putrescine and spermidine were 9 and 0.6 microM, respectively. Transport was reduced at 0 degrees C, suggesting that the process is energy requiring; inhibition by N-ethylmaleimide or p-chloromercuribenzoate suggested a requirement for sulfydryl groups. Transport of putrescine, but not spermidine, was partially activated by Na+. Spermidine and spermine did not inhibit putrescine uptake, and putrescine and spermine did not inhibit spermidine uptake, suggesting the presence of a separate transporter for each polyamine. Pretreatment with DL-2-difluoromethy-lornithine increased the uptake of putrescine but not spermidine. The endothelial cell putrescine transporter is thus sensitive to polyamine depletion, suggesting that transport from the extracellular space may be an important source of polyamines. L-Ornithine or L-arginine were not inhibitory, indicating that polyamine and cationic amino acid transport is mediated by independent systems. The sensitivity of putrescine transport to L-arginine but not to L-ornithine deprivation suggests that intracellular levels of arginine rather than ornithine regulate polyamine metabolism and transport in these cells. Thus factors that affect arginine utilization may also influence polyamine metabolism.

Polyamines in nerve terminals and secretory granules isolated from neurohypophyses

In isolated nerve terminals from ox neurohypophyses the following concentrations of polyamines [pmol (microgram protein)-1 (mean +/- SEM)] were found: spermine: 2.07 +/- 0.14 (n = 3), spermidine: 0.22 +/- 0.01 (n = 4), putrescine: 0.20 +/- 0.01 (n = 4). In secretory granules isolated from the same tissue, the concentrations were: spermine: 0.57 +/- 0.02 (n = 3), spermidine: 0.07 +/- 0.04 (n = 3), putrescine: 0.13 +/- 0.04 (n = 3). After incubation of isolated nerve terminals with the polyamines, they were taken up as a function of time and concentration, approaching saturation at high concentrations. The kinetic parameters of their synthesizing enzyme, ornithine decarboxylase, in ox neurohypophyseal nerve terminals (apparent Km 0.75 mM and Vmax 22.5 pmol mg protein-1 h-1) were comparable to those previously found in cerebral cortex of rats. When isolated, hemilobes from rat neurohypophyses were incubated in a medium which contained spermidine (5 mM), and were stimulated by 56 mM K+, release of vasopressin was smaller than in control experiments. However, after removal of spermidine and after restimulation, 50 min after initial stimulation, the release was significantly elevated. It is suggested that polyamines may take part in modulation of vasopressin release.

Uptake of polyamines by human endothelial cells. Characterization and lack of effect of agonists of endothelial function

Uptake of polyamines by confluent monolayers of human umbilical-vein endothelial cells (HUVECs) was found to be time-, temperature- and concentration-dependent, energy-requiring, and saturable. Kinetic constants were putrescine Kt 3 +/- 1 microM, Vmax. 15 +/- 7 pmol/h per microgram of protein; spermidine, 0.7 +/- 0.2, 12 +/- 3; spermine, 1 +/- 0.7, 11 +/- 4. Putrescine uptake was inhibited by spermine or spermidine, whereas uptake of spermine or spermidine was not inhibited by 20 microM-putrescine. These data suggest the existence of two carriers, one shared by spermine and spermidine, and one capable of transporting all three polyamines. Pretreatment of HUVECs with thrombin (less than or equal to 10 units/ml; 1 h), bradykinin (less than or equal to 10 microM; 1 h), interleukin-1 (less than or equal to 100 units/ml; 2 h) or phorbol 12-myristate 13-acetate (less than or equal to 1.0 microM; 1 h), all known agonists of endothelial function, had no significant effect on polyamine uptake. These responses may be of importance in angiogenesis and wound healing, and could have pharmacological significance, for there is a growing interest in the use of polyamines or polyamine analogues as therapeutic agents.

Ca2+/calmodulin regulation of putrescine uptake in cultured gastrointestinal epithelial cells

Regulation of putrescine uptake in a small intestinal crypt cell line, IEC-6 cells, was examined. Uptake of [14C]putrescine was measured throughout a normal growth curve and was found to be inversely related to growth. Kinetic analysis at low and high cell density revealed the inhibition of uptake in confluent cells was due to a five-fold reduction in Vmax of uptake, 199.5 vs. 43.1 pmol.10(5) cells-1.h-1, respectively. Three gastrointestinal hormones, gastrin, secretin, and cholecystokinin, produced partial inhibition of [14C]putrescine uptake. Conversely, treatment of quiescent cells with 5% fetal bovine serum to stimulate growth did not affect uptake. Influence of putrescine uptake on free ionized intracellular Ca2+ ([Ca2+]i) was measured by microspectrofluorometry using the Ca(2+)-sensitive fluoroprobe fura-2. Basal [Ca2+]i was calculated to be 112 nM and increased rapidly to 313 nM upon addition of 10 microM putrescine. Preventing the rise in [Ca2+]i using an intracellular Ca2+ buffer, 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester, decreased [14C]putrescine uptake to 29.5 +/- 5.3% of control values. 45Ca2+ flux experiments and measurement of transport in 0 Ca2+ and 0.5 mM EDTA suggested an intracellular source of calcium was mobilized during putrescine uptake. Finally, use of the putative calmodulin antagonist N-(6-aminohexyl)-5-chloro-l-naphthalenesulfonamide caused a dose-dependent inhibition of [14C]putrescine uptake with 50% inhibitory concentration of approximately 7 microM. These data suggest that putrescine uptake in IEC-6 cells may be regulated by a Ca2+/calmodulin-dependent mechanism.

Quantification and localization of ornithine decarboxylase in the embryonic palate

Ornithine decarboxylase (ODC; EC4.1.1.17), the key enzyme in polyamine biosynthesis, and intracellular polyamines increase rapidly and markedly in tissues and cells that are actively proliferating as well as differentiating and decrease as these processes cease. ODC activity has also been implicated as playing a role in the proliferation and differentiation of cells derived from the developing palate. Ornithine decarboxylase activity was thus quantified and ODC localized in the developing murine palate in vivo. Levels of ODC activity showed little variation during the ontogeny of the palate, averaging 126 pmol CO2/mg protein/hr. When difluoromethylornithine (DFMO), an irreversible inhibitor of ODC activity, was administered to pregnant mice throughout the period of palate development (days 11-14), palatal tissue ODC activity was reduced by 85%. No craniofacial malformations were observed, however. The lack of a teratogenic effect by DFMO treatment could be due to sufficient remaining ODC activity in craniofacial tissue and/or maintenance of intracellular polyamine levels by the activity of a polyamine transport system. The activity of this system was demonstrated by the ability of palatal tissue in vivo to take up radiolabeled putrescine. The presence of a polyamine transport system was previously suggested by the demonstration of such a system in palate mesenchymal cells in vitro. Dramatic temporal and spatial shifts in tissue patterns of immunolocalization for ODC in developing palatal tissue were also seen. Immunostaining for ODC was evenly distributed in oral, nasal, and medial edge palate epithelial cells on day 12 of gestation. The basal aspects of epithelial cells were, however, more intensely stained. Mesenchymal cells exhibited a peri-nuclear immunostaining pattern. On days 12 and 13 of gestation, the staining patterns for ODC in palate epithelial and mesenchymal cells were comparable. On day 14 of gestation, all regions of the palate epithelium, particularly the medial edge epithelia, were immunostained for ODC, whereas the intensity of staining in the mesenchymal cells was significantly reduced. This study represents essential initial observations toward understanding the role that ODC may play in normal craniofacial development.

Biologic basis for a risk assessment model for cleft palate

A biologic model for palatogenesis is presented, intended as a basis for risk assessment. It comprises a sequence of developmental stages: growth and migration of neural crest cells, downward growth of palatal buds, elevation of palatal shelves, and differentiation of the epithelium followed by shelf fusion. Several events representing these stages and amenable to mathematical translation may be measurable in the form of biomarkers such as DNA and protein synthesis, phospholipid metabolism, and signal transducing systems. Interrupting components of the model will result in cleft palate. Teratogens with known mechanisms of action are compared with the model. The quantitative risk of cleft palate is conceived as a sequence of mathematical probabilities that any stage of the model runs an abnormal course. Stage-specific probabilities are determined by a chemical's potency and dose, and by duration of exposure and gestational age. Species or strain sensitivity may be expressed as quantitative differences in model parameters. Although the model is designed for cleft palate, the risk model may also estimate a multiple response risk to the same exposures.

Regulation of the Na(+)-dependent and the Na(+)-independent polyamine transporters in renal epithelial cells (LLC-PK1)

We have studied the regulation of the Na(+)-dependent and Na(+)-independent polyamine transport pathways in the renal LLC-PK1 cell line. Most of the experiments were performed in the presence of 5 mM DL-2-difluoromethylornithine (DFMO) in order to inhibit the cellular synthesis of polyamines. The activity of both transporters as measured by putrescine uptake was increased by growth-promoting stimuli and decreased by exogenous polyamines. The time course of the increase in uptake activity induced by fetal calf serum could be fitted by a single exponential, and the process was three times faster for the Na(+)-dependent than for the Na(+)-independent transporter. Maximum activity was reached after more than 24 h. This increase could be inhibited by actinomycin D and by cycloheximide. Other growth-promoting stimuli, such as subconfluent cell density, as well as growth factors also induced an increase in the transport activity. Particularly, there was a marked stimulation of the Na(+)-dependent pathway by epidermal growth factor in combination with insulin. On the other hand, the transport activity decayed very rapidly upon addition of exogenous polyamines (t1/2 less than 60 min). The diamine putrescine was much less effective in this respect than the polyamines spermidine and spermine. The non-metabolizable substrate methylglyoxal bis(guanylhydrazone) did not induce a decay of the transport activity, but it protected the Na(+)-dependent pathway against the polyamine-induced decay. Inhibition of the protein synthesis by cycloheximide did not induce a rapid decrease of the transport activity; neither did it affect the polyamine-induced decay. These observations suggest that this polyamine-induced decay is not owing to an inhibitory effect on the rate of synthesis of the transporters, but rather to a degradation or an inactivation of the transporters. The polyamine-induced decay slowed down at lower cell density. This effect was particularly pronounced for the Na(+)-dependent transporter. Since the uptake of polyamines was increased at low cell density, the decreased rate of decay in this condition pleads against a simple mechanism of transinhibition by the substrate. In conclusion, both transport pathways were similarly affected by the regulatory parameters, but the Na(+)-dependent transporter was more rapidly and more effectively regulated. The numerous interacting regulatory steps furthermore suggest a physiological role for these transporters, such as an involvement in urinary polyamine disposal.

Characterization of putrescine- and spermidine-transport systems of a rat pancreatic acinar tumoral cell line (AR4-2J)

Polyamines are polycationic molecules essential for cell growth and differentiation. Recent work has focused on cell polyamine-transport systems as a way to regulate intracellular polyamine levels. In this study, we demonstrate the presence of two different active transporters for putrescine and spermidine in a rat tumoral cell line (AR4-2J). The first has a Km of 3.1 microM and a Vmax of 3.7 pmol/15 min per micrograms of DNA for putrescine and the second a Km of 0.42 microM and a Vmax of 4.7 pmol/15 min per micrograms of DNA for spermidine. Competition studies performed between the polyamines confirm the difference between these two carriers; one has an equal affinity for the three main polyamines, and the other has a lower affinity for putrescine. Amino acids do not share this transport system, which is Na(+)-independent. Choline chloride inhibits selectively and in a dose-responsive manner the uptake of putrescine without affecting that of spermidine. These data demonstrate that AR4-2J cells possess two polyamine transporters; one is specific for aminopropyl groups (spermidine and spermine), and the other is choline-sensitive, but cannot discriminate between aminobutyl (putrescine) and aminopropyl groups.

Uptake and metabolism of putrescine in confluent LLC-PK1 cells

Putrescine is taken up by confluent pig kidney (LLC-PK1) cells at roughly equal rates over both Na(+)-dependent and Na(+)-independent pathways. The former is sensitive to 1 mM amiloride, but the latter is not. Uptake rates are similar at both the apical and basolateral surfaces. The principal fate of the putrescine is oxidative deamination, yielding a product that appears to be either gamma-aminobutyraldehyde or delta 1-pyrroline. Most of the remainder is converted to products tentatively identified as spermidine, spermine, or another unidentified product; these products as well as putrescine itself are lost from the cell at either surface. Changing the extracellular pH in the range of 6.8-8.0 has no affect on putrescine uptake. Cells acidified to intracellular pH 6.8 show a reduced capacity to incorporate radioactivity, an effect that may be due to inhibition of diamine oxidase. Depletion of ATP stores by treating cells with 2-deoxy-D-glucose and NaN3 does not reduce putrescine uptake, suggesting that the mechanism is not a primary active transporter. The Na(+)-dependent component of uptake is inhibited by 5-50 microM Hg2+ in a dose-dependent manner. p-Chloromercuribenzene sulfonic acid (p-CMBS) at high concentrations (500-1,000 microM) does not affect Na(+)-independent uptake but in the presence of Na+ depresses total uptake more than Na+ depletion alone, suggesting that Na+ enhances the binding of p-CMBS to both transporters. Spermidine and spermine compete with putrescine for uptake, but a variety of other organic bases and amino acids do not, indicating that polyamines are transported by mechanisms distinct from the transporters for those other compounds.

Regulation of polyamine transport in Chinese hamster ovary cells

Control Chinese hamster ovary (CHO) cells and mutant CHO cells lacking ornithine decarboxylase activity (CHODC-) were used to study the regulation of polyamine uptake. It was found that the transport system responsible for this uptake was regulated by intracellular polyamine levels and that this regulation was responsible for the maintenance of physiological intracellular levels under extreme conditions such as polyamine deprivation or exposure to exogenous polyamines. Polyamine transport activity was enhanced by decreases in polyamine content produced either by inhibition of ornithine decarboxylase with alpha-difluoromethylornithine in CHO cells or via polyamine starvation of CHODC- cells. The provision of exogenous polyamines resulted in rapid and large increases in intracellular polyamine content followed by decreased polyamine transport activity. Soon after this decrease in uptake activity, intracellular polyamine levels then fell to near control values. Cells grown in the presence of exogenous polyamines maintained intracellular polyamine levels at values similar to those of control cells. Protein synthesis was necessary for the increase in transport in response to polyamine depletion, but appeared to play no role in decreasing polyamine transport. Bis(ethyl) polyamine analogues mimicked polyamines in the regulation of polyamine transport but this process was relatively insensitive to regulation by methylglyoxal bis(guanylhydrazone), a spermidine analogue known to enter cells via this transport system and to accumulate to very high levels.

Transport systems for polyamines in the established renal cell line LLC-PK. Polarized expression of an Na(+)-dependent transporter

We present evidence for the existence of an Na(+)-dependent transporter and an Na(+)-independent transporter for polyamines in LLC-PK1 cells. Both transporters could be discriminated by their sensitivity to inhibitors, particularly rho-chloromercuriphenyl sulphate and various polycationic molecules. By using cell monolayers grown on a permeable filter support, we have found that the Na(+)-dependent polyamine uptake occurred preferentially from the basolateral side. The Na(+)-independent uptake, on the other hand, occurred to the same extent from either the apical or the basolateral side.

Expression of a human gene for polyamine transport in Chinese-hamster ovary cells

A molecular-genetic approach towards isolating mammalian polyamine-transport genes and their encoded proteins was devised involving the production of Chinese-hamster ovary (CHO) cells expressing a human polyamine-transport protein. CHO cells and a polyamine-transport-deficient CHO mutant cell line (CHOMG) were equally sensitive to the antiproliferative effects of alpha-difluoromethylornithine (DFMO), which blocked endogenous polyamine synthesis. Exposure to exogenous polyamines increased intracellular polyamine levels and reversed this DFMO-induced cytostasis in the CHO cells, but not in the CHOMG cells. CHOMG cells were therefore transfected with human DNA (isolated from HT-29 colon carcinoma cells) and cells expressing the human polyamine-transport system were identified by the ability of these cells to grow in a medium containing DFMO and polyamines. A number of different positive clones were identified and shown to have the capacity for polyamine uptake and an increased sensitivity to the toxic effects of the polyamine analogue methylglyoxal bis(guanylhydrazone). Differences in these properties between the clones are consistent with a multiplicity of polyamine-transport systems. Some clones also showed a change in growth characteristics, which may indicate a relationship between genes involved in the polyamine-transport system and in cell proliferation.

Properties and physiological function of the polyamine transport system

Polyamine transport was examined in Chinese hamster ovary (CHO) cells because of the unique potential these cells hold for utilizing genetic approaches to study the mechanisms of polyamine transport, its regulation, and its function. Parental (control) CHO cells were shown to contain a polyamine transport system with characteristics consistent with polyamine-uptake properties described in other cell types. Polyamines appear to cross the plasma membrane via an energy-requiring transport system specific for putrescine, spermidine, spermine, and their analogues. A mutant line, CHOMG, selected for resistance to the toxicity of methylglyoxal bis(guanylhydrazone), was shown to lack a functional polyamine transport system. CHOMG cells provided the negative controls necessary to examine the role of polyamine transport in maintenance of intracellular polyamine levels and in the regulation of the polyamine metabolic enzymes. It was found that the repression of ornithine decarboxylase activity by polyamines and the induction of spermidine/spermine-N1 acetyltransferase by polyamine analogues including bis(ethyl)spermine derivatives required the presence of a functional polyamine transport system. The CHO-CHOMG model was also shown to provide a means for establishing the importance of the polyamine transport system in the toxicity of polyamine analogues. The inability of alpha-difluoromethylornithine-treated CHOMG cells to utilize extracellular polyamines to replenish depleted intracellular polyamine levels suggested a means by which polyamine transport-positive cells may be identified. Such a selection procedure will permit the use of CHOMG cells in the isolation of genes encoding proteins involved in polyamine transport.

Epidermal growth factor: modulator of murine embryonic palate mesenchymal cell proliferation, polyamine biosynthesis, and polyamine transport

Polyamines (putrescine, spermidine, and spermine) are normal cellular constituents able to modulate cellular proliferation and differentiation in a number of tissues and cell types. This investigation explores the response of murine embryonic palate mesenchymal (MEPM) cells to epidermal growth factor (EGF) in terms of biosynthesis of putrescine and its transport across the plasma membrane and tests the hypothesis that polyamine transport can serve as an alternative mechanism (other than biosynthesis) for elevating intracellular polyamines during stimulation of MEPM cellular proliferation. MEPM cells treated with EGF were stimulated to proliferate and showed a dose- and time-dependent stimulation of ornithine decarboxylase (ODC) which was maximal at 4-6 hours. EGF also stimulated the initial rate of putrescine transport in a dose- and time-dependent manner. This stimulation was found to be maximal 3 hours after treatment and specific for the putrescine transport system. The kinetic parameters of putrescine transport shifted from 2.52 microM (Km) and 23.6 nmol/mg protein/15 minutes (Vmax) in nonstimulated cells to 4.48 microM (Km) and 39.8 nmol/mg protein/15 minutes (Vmax) in EGF-treated cells. This kinetic shift did not require de novo protein or RNA synthesis, as cycloheximide (10 micrograms/ml) and actinomycin D (50 micrograms/ml) had little effect on the ability of EGF to stimulate the initial rate of putrescine uptake. The rate of transport, however, was found to be inversely related to cell density. The addition of exogenous putrescine concomitantly with EGF blocked the induction of ODC, while in the presence of difluoromethylornithine (DFMO) (irreversible inhibitor of ODC) the initial rate of putrescine transport remained elevated throughout the time course studied. This stimulation of putrescine uptake caused by polyamine deprivation was reversed by exogenous putrescine and Ca++ while alpha-aminoisobutyric acid (AIB) further stimulated the rate of uptake. EGF's ability to stimulate cellular DNA synthesis was inhibited by DFMO. If DFMO-treated cells were stimulated with EGF in the presence of exogenous putrescine, this stimulatory effect was preserved. These studies indicate that the rate of polyamine transportation is highly responsive to a signal which initiates biosynthesis of polyamines. Further, this transportation system provides a compensatory mechanism allowing the cell to increase intracellular levels of polyamines when environmental conditions inhibit biosynthesis or when polyamines are abundant.

Polyamine transport systems in the LLC-PK1 renal epithelial established cell line

LLC-PK1 cells were brought to a quiescent state by treatment with DL-2-difluoromethylornithine (DFMO), a specific inhibitor of L-ornithine decarboxylase (ODC). The inhibition of ODC, which is the key enzyme for polyamine synthesis, strongly reduced the cellular content of putrescine and spermidine. The cells resumed DNA-synthesis followed by mitosis when exogenous putrescine was added. DFMO treatment strongly stimulated the putrescine uptake capability. A kinetic analysis of the initial uptake rates revealed a saturable Na+-dependent and a saturable Na+-independent pathway on top of non-saturable diffusion. The stimulation by DFMO was exclusively due to an effect on the Vmax values of the saturable pathways. The Na+-dependent transporter had a higher affinity for putrescine (apparent Km = 4.7 +/- 0.7 microM) than the Na+-independent transporter (apparent Km = 29.8 +/- 3.5 microM). As a consequence, although the latter transporter had a higher Vmax, the Na+-dependent transport was more important at a physiological putrescine concentration. Putrescine uptake by both transporters was inhibited with similar relative affinities by spermidine, spermine as well as by the antileukemic agent, methylglyoxal bis(guanylhydrazone), but not by amino acids. The activity of the Na+-dependent transporter was very much dependent on SH-group reagents, whereas the Na+-independent transporter was not affected. Both transporters were inhibited by metabolic inhibitors and by ionophores but the Na+-dependent transporter was affected to a greater extent. For both transporters there was a down-regulation in response to exogenous putrescine. This suggests that the polyamine transporters in LLC-PK1 are adaptively regulated and may contribute to the regulation of the cellular polyamine level and cellular proliferation.