Uptake of putrescine by 3T3 and SV3T3 cells and its effect on ornithine decarboxylase activity

Binding of spermidine and putrescine to energized liver mitochondria

The binding of spermidine and putrescine to mitochondrial membranes was studied by applying a thermodynamic model of ligand-receptor interactions developed both for equilibrium and far-from-equilibrium binding processes (V. Di Noto, L. Dalla Via, A. Toninello, and M. Vidali Macromol. Theory Simul. 5, 165-181, 1996). Results demonstrate the presence of two monocoordinated binding sites (S1 and S2) for spermidine and one monocoordinated binding site (S2) for putrescine, all exhibiting high capacity and low affinity. It is proposed that differences in the polyamines' flexibility and hydrophilicity perhaps contributes to the observed variations in their interactions with the two sites. A comparison of the binding parameters of these polyamines with those of spermine reveals differences in the specific function of the S1 and S2 sites, identified in studies of spermine binding (L. Dalla Via, V. Di Noto, D. Siliprandi, and A. Toninello Biochim. Biophys. Acta 1284, 247-252, 1996).

Polyamine transport in mammalian cells. An update

The uptake and release of the natural polyamines putrescine, spermidine and spermine by mammalian cells are integral parts of the systems that regulate the intracellular concentrations of these biogenic amines according to needs. Although a general feature of all tissues, polyamine uptake into intestinal mucosa cells is perhaps the most obvious polyamine transport pathway of physiological and pathophysiological importance. Mutant cell lines lacking the ability to take up polyamines from the environment are capable of releasing polyamines. This indicates that uptake and release are functions of two different transport systems. The isolation of a transporter gene from a mammalian cell line is still lacking. Overaccumulation of polyamines is controlled by release and by a feedback regulation system that involves de novo synthesis of antizyme, a well known protein that also regulates the activity of ornithine decarboxylase. Recent work has demonstrated that Ca(2+)-signalling pathways are also involved. Although there is consensus about the importance of polyamine uptake inhibitors in the treatment of neoplastic disorders, a practically useful uptake inhibitor is still missing. However, the attempts to target tumours, and to increase the selectivity of cytotoxic agents by combining them with the polyamine structure, are promising. New, less toxic and more selective anticancer drugs can be expected from this approach.

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.

Positron emission tomography study of human prostatic adenocarcinoma using carbon-11 putrescine

To evaluate [1-11C]putrescine ([11C]PUT) as a potential tracer for imaging and characterization of human prostatic adenocarcinoma, positron emission tomography (PET) was performed in eight patients and three normal controls. In addition, four of the patients and the three normal controls also had a prostate scan with 2-deoxy-2-[18F]fluoro-D-glucose (18FDG). Three of the patients had undergone resection of the prostate tumor and all of the patients except for one had bone metastasis. Carbon-11 rapidly accumulated in prostate, bone and rectum after injection of [11C]PUT. Maximal uptake was achieved 5 min after injection with minimal washout during the 50 min study period. The uptake of carbon-11 in the prostate of normal controls was significantly higher than that in the patients. However, three of the four patients scanned for metastatic bone lesions showed higher uptake in bone metastasis than in normal bone. Quantitation of 18FDG uptake in the prostate was hindered by the high accumulation of activity in the urinary bladder. [11C]PUT does not appear to be a useful tracer for assessing proliferation of human prostate adenocarcinoma. Its utility in the imaging of other cancers with high polyamine concentration remains to be investigated.

Differential regulation of putrescine uptake in Trypanosoma cruzi and other trypanosomatids

Putrescine uptake in Trypanosoma cruzi epimastigotes is 10 to 50-fold higher than in Leishmania mexicana or Crithidia fasciculata. Polyamine transport in all these trypanosomatids is an energy-dependent process strongly inhibited by the presence of 2,4-dinitrophenol or KCN. Putrescine uptake in T. cruzi and L. mexicana was markedly decreased by the proton ionophore carbonylcyanide m-chlorophenylhydrazone but it was not affected by ouabain, a Na(+)-K+ pump inhibitor. The depletion of intracellular polyamines by treatment of parasite cultures with alpha-difluoromethylornithine elicited a marked induction of putrescine uptake in L. mexicana and C. fasciculata by increasing considerably the Vmax of this process. Conversely, the uptake of putrescine in T. cruzi was essentially unchanged by the same treatment. The differential regulation of putrescine transport in T. cruzi might be related to some distinctive features of polyamine metabolism in this parasite.

Evidence for the existence of distinct transporters for the polyamines putrescine and spermidine in B16 melanoma cells

The uptake of intracellular putrescine and spermidine was examined in B16 melanoma cells. It was found that difluoromethylornithine preferentially induced putrescine transport (28-fold) compared to that for spermidine (3.5-fold). Putrescine uptake was partially Na+ dependent, whereas spermidine uptake was not. Inhibition studies with the two polyamines showed that putrescine was a poor competitive inhibitor of spermidine uptake, exhibiting a Ki of 69-75 microM, whereas the estimated Km for putrescine uptake was only 5.36 microM. By contrast, spermidine inhibition of putrescine transport produced a non-linear Eadie-Scatchard plot suggesting that putrescine was taken up by a spermidine-sensitive and a spermidine-insensitive process. The estimated spermidine Ki for inhibition of the spermidine-sensitive process was 0.125 microM. Using a series of polypyridinium quaternary salts to inhibit transport, no correlation between inhibition of putrescine uptake and inhibition of spermidine uptake was seen. Finally, the photoaffinity label, 1,12-di(N5-azido-2-nitrobenzoyl)spermine selectively inactivated the putrescine transporter(s) without affecting spermidine uptake. From these observations, it was concluded that multiple polyamine transporters are present on B16 melanoma cells and that separate, distinct transporter(s) account for the uptake of putrescine and spermidine in this cell-line following induction with difluoromethylornithine. The present of different transporters for the two polyamines indicates that expression of uptake activity for putrescine and spermidine may be under separate cellular control.

Putrescine uptake and release by a normal rat small intestine crypt cell line, IEC-6

IEC-6 cells were cultured on permeable filter inserts with separate access to the apical and basolateral sides. [3H]Putrescine uptake favored the apical side and its release (in Earle's balanced salt solution containing 0.1% bovine serum albumin) was six times greater in the apical-to-basolateral than in the basolateral-to-apical direction. Release in DMEM did not show this preference. The uptake of [3H]putrescine was stimulated approximately 1.3 times the basal level by 10 mM asparagine (ASN) or 5% dialyzed fetal bovine serum whether the [3H]putrescine was added at a concentration of 1 or 100 nM. The increased uptake was maintained for up to 6 h. When [3H]putrescine was removed after 4 h of uptake, the cells continued to release it into the medium on both sides for up to 4 h. Stimulated cells released only 50% as much as unstimulated cells. Unlabeled putrescine reduced the uptake of [3H]putrescine with an IC50 of 1.81 x 10(-6) M (r = 0.9476) and 1.02 x 10(-6) M (r = 0.9967) for unstimulated and ASN-stimulated cells, respectively. When the intracellular putrescine was reduced by difluoromethylornithine, the uptake of [3H]-putrescine was not changed, but its release was inhibited. Sodium was not required for [3H]putrescine uptake or release. Although the stimulated cells attained intracellular levels of [3H]putrescine which, if expressed as concentration based on cell volume, were up to 500 times the original extracellular concentration, a true concentration gradient could not be proven because 85% of the [3H]putrescine was probably bound to polyanions as shown by butanol extraction.

Characterisation of putrescine uptake by cultured adult mouse hepatocytes

Uptake of polyamines by cultured cells has been shown to be influenced by growth rate and/or differentiation. In this study, we have investigated whether the fully differentiated, non-proliferating adult mouse hepatocyte is capable of accumulating extracellular putrescine. When hepatocytes were cultured from 4 to 48 h, uptake of putrescine was found to increase substantially with time spent in culture. The Vmax for putrescine uptake increased 22-fold during this period with no change in apparent Km. Treatment of the cells with cycloheximide or actinomycin D at concentrations that did not affect cell viability inhibited the induction of putrescine uptake. Endogenous putrescine levels increased from 19.7 nmol/mg DNA after 4 h in culture to over 500 nmol/mg DNA after 48 h in culture. This increase was accompanied by a loss of over 90% of ornithine decarboxylase activity. Spermidine levels did not change over this time period, whereas spermine levels decreased by 35%. Difluoromethylornithine prevented the observed increase in intracellular putrescine but did not affect putrescine uptake. The increase in putrescine transport was not inhibited by culturing the hepatocytes in a high concentration of putrescine, spermidine or spermine. Moreover, the induction process was not stimulated by foetal calf serum but was selectively inhibited by the differentiating agents dimethylsulfoxide and retinoic acid. The results from those studies show that cultured mouse hepatocytes express a putrescine transport system that is poorly regulated by extracellular polyamines. The expression of the transporter requires the synthesis of mRNA and protein, and appears to be related to a time-dependent change in hepatocyte phenotype.

Localization of spermidine uptake in rabbit lung slices

The lungs have a high polyamine transport capability, and the type II pneumocyte has recently been identified as a major site of putrescine uptake and localization (N. A. Saunders, P. J. Rigby, K. F. Ilett, and R. F. Minchin. Lab. Invest. 59: 380-386, 1988). However, recent evidence suggests that multiple polyamine transport systems exist. In the present study, localization of spermidine uptake in rabbit lung was investigated. Although [14C]spermidine was rapidly accumulated by lung slices, it was not significantly metabolized, and no efflux of the accumulated polyamine was apparent. Autoradiographs prepared after [3H]spermidine transport revealed a localization of uptake activity to cells identified by electron microscopy as type II pneumocytes. Spermidine uptake occurred in all type II cells examined and thus appeared to be a characteristic function of this cell type. In contrast, spermidine uptake was virtually absent in the major airways and blood vessels, whereas moderate uptake was associated with pulmonary alveolar macrophages and alveolar tissue. Subsequent purification and culture of type II pneumocytes showed these cells to have significant polyamine uptake activity. In addition, spermidine uptake activity was positively correlated with the proportion of type II cells present at the various stages of their purification. In other studies, cultured pulmonary alveolar macrophages possessed similar uptake activity to that of cultured type II cells. Combined, these data suggest that both type II cells and pulmonary alveolar macrophages may represent major sites of spermidine uptake in vivo. We also suggest that the transport of polyamines by type II cells may reflect a critical role for polyamines in a characteristic function of this cell type.

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.

Paraquat is not accumulated in B16 tumor cells by the polyamine transport system

We have examined the effect of difluoromethylornithine on the ability of B16 melanoma cells to take up putrescine and the 4,4'-dipyridyl herbicide paraquat. Pretreatment with difluoromethylornithine for 24 hr enhanced putrescine uptake by inducing the maximum capacity of the transport system without affecting the Km for the substrate. Paraquat uptake was minor compared with that of putrescine and was not affected by difluoromethylornithine. Neither putrescine, spermidine or spermine at concentrations up to 100 microM inhibited the accumulation of paraquat. However, paraquat competitively inhibited putrescine transport (Ki = 54 +/- 10 microM). Exposure of the B16 melanoma cells for 24 hr to increasing concentrations of paraquat produced a dose-dependent inhibition of DNA synthesis. Difluoromethylornithine pretreatment did not affect paraquat toxicity. These data show that paraquat is not taken up into B16 melanoma cells by the uptake system responsible for transporting putrescine. Moreover, it is likely that the difluoromethylornithine inducible polyamine transport system in B16 melanoma cells is characteristically different to that previously described in normal mammalian lung since the latter is reportedly capable of transporting both putrescine and paraquat.

Spermidine uptake by type II pulmonary epithelial cells in primary culture

The transport pathway for the polyamine spermidine (SPD) was characterized in primary isolates of type II pulmonary epithelial cells from rat lungs. [14C]spermidine was accumulated by type II cells via a temperature-, sodium-, and concentration-dependent saturable pathway, with an apparent Km of 0.48 microM and a maximum velocity (V max) of 0.32 pmol.microgram DNA-1.min-1. SPD uptake was inhibited by gramicidin and by reduced extracellular sodium but was unaffected by alpha-aminoisobutyric acid (AIB), which entered the cells by a similar saturable pathway. Uptake of SPD also was inhibited by the exogenous polyamines putrescine (PUTR) and spermine (SPM), as well as by the methylglyoxal bis(guanylhydrazone) (MGBG) and by paraquat (PQ). The order of potency of these inhibitors was SPM greater than PUTR = MGBG much greater than PQ. The absence of serum reduced the Vmax of the system slightly but had no effect on the apparent Km. In contrast, after 3 days in primary cell culture, the kinetics of SPD transport were altered by decreases in both the Km and Vmax of the uptake process. These observations indicate that type II pulmonary epithelial cells exhibit a pathway of polyamine uptake with general characteristics similar to those observed previously in intact lung tissue and other cell types.

Methylglyoxal-bis(guanylhydrazone)-resistant Chinese hamster ovary cells: genetic evidence that more than a single locus controls uptake

Chinese hamster ovary cells spontaneously resistant to the cytotoxic action of methylglyoxal-bis(guanylhydrazone) have been isolated in a multistep selection scheme. A low-level resistant isolate has been shown to be defective in the ability to accumulate the drug intracellularly. This was reflected in a 10-fold lower Vmax than wild-type cells for drug uptake as well as a slight enhancement of drug efflux. More highly resistant isolates selected from this low-level resistant isolate were totally deficient in the ability to take up the drug. A partial revertant, selected from this low-level resistant isolate, retained some change in the Vmax for uptake but lost the accelerated rate of efflux characteristic of the low-level resistant line. Genetic analysis by somatic cell hybridization indicated that the low-level resistant phenotype was recessive to the wild-type phenotype. In addition, the low-level resistant phenotype could be complemented by a previously isolated highly resistant cell also defective in drug uptake (Mandel and Flintoff (1978) J. Cell. Physiol., 97: 335-344). Taken together, these data suggest that more than one locus controls drug uptake in Chinese hamster ovary cells.

Uptake of exogenous spermidine by rat lungs perfused in situ

Uptake of the polyamine spermidine (SPD) from the pulmonary circulation was characterized by using ventilated rat lungs perfused in situ with Krebs-Henseleit-bicarbonate buffer containing 4.5% bovine serum albumin, 5.6 mM glucose, and 20 amino acids at plasma levels. [14C]SPD was accumulated by the lungs in a time- and concentration-dependent manner. The pathway of SPD uptake exhibited saturation kinetics with an apparent Km in the range of 1 microM and a Vmax of 450-540 pmol/g lung min. SPD uptake was inhibited by the naturally occurring polyamines putrescine and spermine (SPM) and by the inhibitor of polyamine synthesis, methyglyoxal bis(guanylhydrazone) (MGBG). Inhibition of SPD uptake by SPM followed competitive kinetics; although MGBG was also a competitive inhibitor of SPD uptake, MGBG was less effective than SPM. These observations indicate that SPD is taken up from the pulmonary circulation by a carrier-mediated pathway that is inhibited by other natural polyamines and by MGBG and exhibits substrate affinity in the range of plasma SPD concentrations.

Effect of altered inflation on pulmonary uptake of methylglyoxal bis(guanylhydrazone)

The effects of increased pulmonary ventilation on uptake of an exogenous, nonmetabolized polyamine analog, methylglyoxal bis(guanylhydrazone) [MGBG] were investigated in rat lungs perfused in situ with buffer containing 4.5% bovine serum albumin, 5.6 mM glucose and plasma levels of amino acids. The perfusate was equilibrated and the lungs were ventilated with warmed, humidified O2/N2/CO2 (20:75:5). A 28% increase in lung inflation rapidly accelerated MGBG uptake at low (1.5 microM) but not at high (50 microM) substrate concentration, a change which appeared to reflect a decrease in the apparent Km of the uptake pathway. This effect was not associated with acute alterations in pulmonary vascular permeability or resistance, nor with instability of the preparations. Taken with observations made previously, these results suggest that deformation of the tissue by increased inflation may account for increased MGBG uptake by lungs from rats subjected to partial pneumonectomy.

Increased pulmonary uptake of exogenous polyamines after unilateral pneumonectomy

Alterations in pulmonary uptake of the naturally occurring polyamine spermidine and of an exogenous polyamine substrate analogue, methylglyoxal bis(guanylhydrazone) (MGBG), were investigated during the early phase of compensatory lung growth after partial pneumonectomy (PNX) in rats. In lungs perfused in situ 3 days after left PNX, when a small (14%) but significant (P less than 0.01) increase in right lung mass could be detected, uptake of [14C]spermidine and of [14C]MGBG from the pulmonary circulation was increased. MGBG uptake exhibited saturation kinetics (1-50 microM MGBG), both in lungs of control animals apparent Km, 11.3 microM; Vmax, 479 pmol X g-1 X min-1) and on the 1st and 3rd post-PNX days. In both PNX groups, the apparent Km of the uptake pathway was decreased somewhat (8.5 microM), while Vmax increased progressively to 584 and 678 pmol X g-1 X min-1 at days 1 and 3, respectively. The effects of PNX on MGBG uptake were detected as early as 3 h after lung resection and were no longer evident when compensatory lung growth was completed 14 days after surgery. In rats adrenalectomized 5 days before left PNX, an accelerated onset and increased rate of lung restoration were associated with a doubling of the effect of PNX alone on MGBG uptake.

Effects of the polyamine spermine on binding of follicle-stimulating hormone to membrane-bound immature bovine testis receptors

The effect of the polyamine, spermine, on the interaction of human 125I-labeled FSH with membrane-bound receptors derived from bovine calf testes has been examined. Concentrations of spermine less than 0.01 M resulted in a slight but insignificant (P greater than 0.10) enhancement of FSH concentrations of 0.01 M and above caused a progressive reduction of FSH binding. Membrane receptors incubated in the presence of spermine at concentrations inhibitory to human 125I-FSH binding (0.01-0.04 M) resulted in an 8-50% decrease in the apparent FSH receptor concentration and a 10-65% decrease in the affinity constant as determined by computerized analysis of the isothermic ligand-binding data. Within the temperature range 4-20 degrees C, simultaneous addition of spermine (0.025 M) increased the reversibility of human 125I-FSH binding approx. 10% (P less than 0.005). Delayed addition of spermine (0.01-0.04 M) resulted in a dose-related dissociation of human 125I-FSH already bound to its receptor (P less than 0.05). However, preincubation of membrane receptors with spermine (0.002-0.04 M) at 4 degrees C or 34 degrees C followed by washing and addition of human 125I-FSH, resulted in an increase in hormone binding (P less than 0.05) over that of controls. If membrane receptor was incubated at 34 degrees C with spermine in the absence of radioligand, the usual loss of hormone binding was reduced (P less than 0.05), while membrane receptor incubated with spermine at 4 degrees C exhibited hormone binding greater (P less than 0.05) than that observed before treatment. Thus, the mechanism of inhibition of human 125I-FSH binding to membrane receptors appears to be correlated with an increase in reversibility of the membrane receptor-human 125I-FSH complex and is expressed as a decrease in the calculated receptor concentration and affinity constant of that interaction. Second, spermine appears to stabilize the membrane receptor in the absence of ligand, presumably through a membrane effect. These data suggest that spermine, and possibly other polyamines, which are endogenous to eukaryotic cells and undergo increases in concentration following stimulation by trophic hormone may play a role in the modulation of the ligand-membrane receptor interaction, in part, through direct effects on the membrane and/or the receptor.

Polyamine uptake by bovine adrenocortical cells

Bovine adrenocortical cells of fasciculo-reticulata origin in primary culture actively accumulate polyamines from the extracellular medium in an energy-dependent process. At low extracellular concentration (e.g., 1 microM putrescine), the transport system resulted in a several-hundred-fold concentration of polyamine in the cellular compartment within 1-2 h of incubation. Putrescine uptake appeared to be the sum of a sodium-dependent, saturable process, with an apparent Km of about 10 microM and of a non-saturable, sodium-independent component. By contrast, spermine was taken up by the cells mostly in a sodium-independent manner. Cross-competition experiments suggested that both polyamines were at least partly transported by the same system. Using specific corresponding probes, it was shown that the polyamine uptake was independent of the amino acid transport systems of the A, L and N types known in a number of cell systems. Adrenocortical cell polyamine content is known to be modulated by adrenocorticotropin through induction of ornithine decarboxylase activity. The existence of a specific uptake system in these cells opens the possibility of a more rapid pathway for the regulation of cellular polyamine levels. It remains to be examined whether this polyamine transport system is under hormonal control, and whether this can support the suggestion that polyamines may represent a form of intracellular messengers in the mechanism of hormone action.

Carrier-mediated uptake of methylglyoxal bis(guanylhydrazone) by rat lungs perfused in situ

Rat lungs perfused in situ were employed to begin investigations of the pathways by which the tissue takes up circulating polyamines (PA). Uptake kinetics were studied using [14C]methylglyoxal bis(guanylhydrazone) (MGBG), a nonmetabolized substrate analogue thought to enter cells via the PA carrier. Lungs concentrated MGBG from the perfusate at a linear rate for at least 60 min. Uptake was saturable with respect to perfusate MGBG concentration; it exhibited an apparent Km of 12.5 microM and Vmax of 0.6 nmol X g lung-1 X min-1. MGBG (1 microM) uptake was inhibited rapidly and to a similar extent (30-40%) by the naturally occurring PAs spermidine, spermine, or putrescine (50 microM); no additional inhibition of uptake was exerted when all three compounds were present simultaneously (total concentration, 150 microM). No inhibition by 5-hydroxytryptamine was evident. Spermidine produced a half-maximal inhibitory effect at a perfusate concentration of 1.9 microM (vs. 1 microM MGBG). The spermidine-insensitive component of MGBG uptake operated at a Vmax similar to that of the control (total), 1.2 nmol X g-1 X min-1, but the apparent Km was increased 3.5-fold to 44 microM. These observations indicate that MGBG is taken up from the pulmonary circulation by a high-affinity, carrier-mediated, concentrative uptake process that is inhibited, at least in part, by naturally occurring polyamines.

Regulation of polyamine content in cultured fibroblasts

The content of putrescine and of the polyamines (spermidine and spermine) and the activities of their biosynthetic enzymes were measured in 3T3 mouse fibroblasts and SV40-transformed mouse fibroblasts over the entire period from subculturing in fresh medium until confluence. The transformed cells had a substantially higher content of putrescine and spermidine than the 3T3 cells and higher activities of all of the biosynthetic enzymes. However, the ratio of spermine synthase to spermidine synthase was higher in the 3T3 cells, which correlated with their higher spermine-to-spermidine ratio. All of the biosynthetic enzymes increased in activity during cell growth. Ornithine decarboxylase increased 20-fold with a maximum at 24-36 h after culturing whereas S-adenosylmethionine decarboxylase increased 3-fold at the same time. Spermidine synthase increased 10- to 16-fold during the growth period whereas spermine synthase increased 2- to 3-fold. The relative enzyme activities and the changes in total polyamine content suggested that 1) the activity of S-adenosylmethionine decarboxylase limited the production of the polyamines and 2) the relative amounts of spermidine and spermine synthase determined the predominant polyamine that the available decarboxylated S-adenosylmethionine is used to synthesize. When 3T3 cells become quiescent at confluence, there was a substantial fall in the intracellular spermidine level because of a greatly increased excretion of spermidine into the medium. Spermine content also fell because there was an increased conversion of spermine into spermidine, which was then excreted. The specific excretion of spermidine did not occur with the transformed SV-3T3 cells.

Effects of inhibitors of ornithine and S-adenosylmethionine decarboxylases on L6 myoblast proliferation

The role of polyamines in myoblast proliferation was studied by treating cells of Yaffe's L6 line of rat myoblasts with inhibitors of polyamine synthesis. Both an irreversible inhibitor of ornithine decarboxylase--difluoromethyl-ornithine (DFMO)--and a competitive inhibitor of S-adenosyl-methionine decarboxylase--methylglyoxal-bis(guanylhydrazone) (MGBG)--depressed spermidine levels and inhibited myoblast proliferation. Spermine levels were not significantly depressed by either inhibitor and putrescine levels were decreased only by DFMO. Putrescine and spermidine, but not magnesium, prevented inhibition of myoblast proliferation by DFMO and MGBG; determination of 14C-DFMO uptake in the presence and absence of these compounds demonstrated that they did not reduce the rate or extent of inhibitor uptake and thus prevent its inhibition of ornithine decarboxylase. Thus it seems likely that these inhibitors reduce cell proliferation by inhibiting polyamine formation. Addition of spermidine to the cells led to a substantial reduction in the activity of S-adenosyl-methionine-decarboxylase, suggesting that the enzyme is subject to negative regulation by the products of the polyamine biosynthetic pathway. Unexpectedly, addition of spermidine also increased intracellular putrescine levels; this apparently resulted from conversion of spermidine to putrescine. Addition of putrescine or spermidine in the absence of serum did not increase the rate of myoblast proliferation although it did elevate intracellular polyamine levels as expected. We conclude that some threshold level of one or more polyamines (probably spermidine) is necessary but not sufficient for initiation and maintenance of myoblast proliferation in culture.