Characterization of putrescine and cadaverine export in mammalian cells. A pharmacological approach

c-MYC-Driven Polyamine Metabolism in Ovarian Cancer: From Pathogenesis to Early Detection and Therapy

c-MYC and its paralogues MYCN and MYCL are among the most frequently amplified and/or overexpressed oncoproteins in ovarian cancer. c-MYC plays a key role in promoting ovarian cancer initiation and progression. The polyamine pathway is a bona fide target of c-MYC signaling, and polyamine metabolism is strongly intertwined with ovarian malignancy. Targeting of the polyamine pathway via small molecule inhibitors has garnered considerable attention as a therapeutic strategy for ovarian cancer. Herein, we discuss the involvement of c-MYC signaling and that of its paralogues in promoting ovarian cancer tumorigenesis. We highlight the potential of targeting c-MYC-driven polyamine metabolism for the treatment of ovarian cancers and the utility of polyamine signatures in biofluids for early detection applications.

Evidence for Polyamine, Biogenic Amine, and Amino Acid Adduction Resulting from Metabolic Activation of Diosbulbin B

Dioscorea bulbifera L. (DBL), a traditional Chinese medicine, is a well-known herb with hepatotoxicity, and the biochemical mechanisms of the toxic action remain unknown. Diosbulbin B (DSB), a major component of DBL, can induce severer liver injury which requires cytochrome P450-catalyzed oxidation of the furan ring. It is reported that a cis-enedial reactive intermediate resulting from metabolic activation of DSB can react with thiols and amines to form pyrrole or pyrroline derivatives. In this study, we investigated the interaction of the reactive intermediate with polyamines, biogenic amines, and amino acids involved in the polyamine metabolic pathway, including putrescine, spermidine, spermine, histamine, arginine, ornithine, lysine, glutamine, and asparagine. Seven DSB-derived amine adducts were detected in microsomal incubations supplemented with DSB and individual amines. Six adducts were observed in cultured rat primary hepatocytes after exposure to DSB. DSB was found to induce apoptosis and cell death in time- and concentration-dependent manners. Apparently, the observed apoptosis was associated with the detected amine adduction. The findings facilitate the understanding of the mechanisms of toxic action of DSB.

Tacorin, an extract from Ananas comosus stem, stimulates wound healing by modulating the expression of tumor necrosis factor α, transforming growth factor β and matrix metalloproteinase 2

Wound healing is a complex biological process that involves integration of hemostasis, inflammation, proliferation and tissue remodeling. An extract of pineapple (Ananas comosus) stem demonstrates several therapeutics properties, including acceleration of wound healing. Tacorin is a water crude extract derived from the stem of A. comosus with high protein content. The effect of tacorin on wound healing in vivo was examined using rats with an induced injury. Wound closure was faster with tacorin treatment than in the untreated group. An in vitro study was conducted on mammalian cells (3T3‐L1) to observe the effect of tacorin on cell proliferation. Tacorin was first heated to inactivate its proteolytic activity. It increased the viability of 3T3‐L1 cells in a dose‐dependent manner. Excessive inflammation was suppressed by tacorin as shown by decreased tumor necrosis factor α expression. Treatment with tacorin increased the expression of transforming growth factor β, a major player in tissue remodeling. Moreover, tacorin also reduced the expression of MMP‐2 to accelerate the recovery of the wound. Taken together, tacorin is able to accelerate the wound‐healing process by increasing cell proliferation, suppressing inflammation and accelerating tissue remodeling.

Ornithine: the overlooked molecule in the regulation of polyamine metabolism

We overexpressed a mouse ornithine decarboxylase gene under the control of a constitutive and an estradiol-inducible promoter in Arabidopsis thaliana to increase our understanding of the regulation of polyamine metabolism. Of particular interest was the role of the substrate ornithine not only in the regulation of polyamine biosynthesis, but also in the accumulation of related amino acids in response to short-term induction of this enzyme. We hypothesized that the inducible expression of the transgene would mimic the natural responses of plants to changing conditions, e.g. under stress conditions and during rapid growth. Our results reveal that ornithine, even though present in relatively small quantities (compared with other amino acids of the glutamate-arginine-proline pathway), may not only be the key regulator of polyamine biosynthesis in Arabidopsis, but it may also regulate the entire subset of pathways for glutamate to arginine and to proline. Indirectly, it could also regulate putrescine catabolism, therefore contributing to the γ-aminobutyric acid content of the cells. Furthermore, the induction of mouse ornithine decarboxylase resulted in up- and down-regulation of several amino acids in the transgenic plants. It was learned that the turnover of putrescine in both the wild type and the transgenic plants occurs rapidly, with a half-life of 6-8 h.

Identification and characterization of a diamine exporter in colon epithelial cells

SLC3A2, a member of the solute carrier family, was identified by proteomics methods as a component of a transporter capable of exporting the diamine putrescine in the Chinese hamster ovary (CHO) cells selected for resistance to growth inhibition by high exogenous concentrations of putrescine. Putrescine transport was increased in inverted plasma membrane vesicles prepared from cells resistant to growth inhibition by putrescine compared with transport in inverted vesicles prepared from non-selected cells. Knockdown of SLC3A2 in human cells, using short hairpin RNA, caused an increase in putrescine uptake and a decrease in arginine uptake activity. SLC3A2 knockdown cells accumulated higher polyamine levels and grew faster than control cells. The growth of SLC3A2 knockdown cells was inhibited by high concentrations of putrescine. Knockdown of SLC3A2 reduced export of polyamines from cells. Expression of SLC3A2 was suppressed in human HCT116 colon cancer cells, which have an activated K-RAS, compared with their isogenic clone, Hkh2 cells, which lack an activated K-RAS allele. Spermidine/spermine N(1)-acetyltransferase (SAT1) was co-immunoprecipitated by an anti-SLC3A2 antibody as was SLC3A2 with an anti-SAT1 antibody. SLC3A2 and SAT1 colocalized on the plasma membrane. These data provide the first molecular characterization of a polyamine exporter in animal cells and indicate that the diamine putrescine is exported by an arginine transporter containing SLC3A2, whose expression is negatively regulated by K-RAS. The interaction between SLC3A2 and SAT1 suggests that these proteins may facilitate excretion of acetylated polyamines.

A stable, inducible, dose-responsive ODC overexpression system in human cell lines

ODC is a labile protein subject to rapid turnover, and a conditional expression system providing long-term overexpression may be helpful in further understanding the biochemical properties of this enzyme and elucidating aspects of the polyamine biosynthetic pathway that have otherwise been difficult to study. HEK293 and LNCaP cell lines were engineered to stably and inducibly overexpress ODC using a Tet-on inducible construct. Clones from both cell lines were characterized by evaluating ODC mRNA expression, ODC activity, intracellular and extracellular polyamine levels, SSAT activity and growth kinetics. The ODC-inducible cell lines were time- and dose-responsive providing a mechanism to increase ODC and putrescine accumulation to a desired level in a flexible and controllable manner. The findings demonstrate that LNCaP ODC overexpressing cells maintained over a 100-fold increase in ODC activity and over a 10-fold increase in intracellular putrescine after 6 h. ODC induction at the highest levels was accompanied by a slight decline in intracellular spermidine and spermine levels and this observation was supported by the finding that SSAT activity was induced over 40-fold under these conditions. Growth rate remained unaffected following at least 12 h of ODC overexpression. Similar results were observed in the HEK293 ODC overexpressing cells.

Polyamine flux in Xenopus oocytes through hemi-gap junctional channels

Diverse polyamine transport systems have been described in different cells, but the molecular entities that mediate polyamine influx and efflux remain incompletely defined. We have previously demonstrated that spermidine efflux from oocytes is a simple electrodiffusive process, inhibitable by external Ca2+, consistent with permeation through a membrane cation channel. Hemi-gap junctional channels in Xenopus oocytes are formed from connexin 38 (Cx38), and produce a calcium-sensitive (Ic) current that is inhibited by external Ca2+. Spermidine efflux is also calcium sensitive, and removal of external calcium increases both Ic currents and spermidine efflux in Xenopus oocytes. Injection of Cx38 cRNA or Cx38 antisense oligonucleotides (to increase or decrease, respectively, Cx38 expression) also increases or decreases spermidine efflux in parallel. Spermidine efflux has a large voltage-dependent component, which is abolished with injection of Cx38 antisense oligonucleotides. In addition, spermidine uptake is significantly increased in Cx38 cRNA-injected oocytes in the absence of external calcium. The data indicate that hemi-gap junctional channels provide the Ca2+-inhibited pathway for electrodiffusive efflux of polyamines from oocytes, and it is likely that hemi-gap junctional channels provide Ca2+ and metabolism-sensitive polyamine permeation pathways in other cells.

A streamlined method for the isolation and quantitation of nanomole levels of exported polyamines in cell culture media

A number of years ago, our laboratory published a method for the isolation of small amounts of polyamines from cell culture media using the ion-exchange resin Bio-Rex 70. We have used this technique extensively to study the export of putrescine and cadaverine from cultured mammalian cells. Unfortunately, this method was highly inefficient in isolating the polyamines spermidine and spermine and was incapable of recovering the acetylated polyamine N(1)-acetylspermidine. In response to these shortcomings, we modified our previous protocol to quantitatively isolate the polyamines N(1)-acetylspermidine, putrescine, cadaverine, N(1)-acetylspermine, spermidine, and spermine. The new method, which is much faster to perform and more efficient than the one previously described, employs the use of disposable minicolumns and a single resin washing step using a weak solution of sodium carbonate at pH 9.3. This new protocol also eliminates the column elution step in favor of directly derivatizing the polyamines with dansyl chloride on the ion-exchange resin. High-performance liquid chromatography analysis of the dansylated polyamines isolated by this procedure showed that 75% of N(1)-acetylspermidine and nearly 100% of the other polyamines present in nanomolar levels were recovered from small amounts of cell culture medium. This new protocol is a valuable new tool for the study of the intracellular/extracellular dynamics of polyamine pools in cultured cells. [A detailed laboratory protocol for this procedure (containing all of the information in this paper but in a condensed form) can be requested by e-mailing the authors.]

Characterization of a diamine exporter in Chinese hamster ovary cells and identification of specific polyamine substrates

Export of the diamine putrescine was studied using inside-out plasma membrane vesicles prepared from Chinese hamster cells. Putrescine uptake into vesicles was a saturable and an ATP- and antizyme-independent process. Excess amounts of a series of diamines or monoacetyl spermidine, but not monoacetyl putrescine, spermidine, or spermine, inhibited putrescine transport. Putrescine uptake into vesicles prepared at pH 7.4 was suppressed at pH 5, compared with pH 7.4; was stimulated approximately 2.5-fold at pH 7.4 in vesicles prepared at pH 6.25, compared with vesicles prepared at pH 7.4; and was not inhibited by valinomycin in the presence of potassium ions. Reserpine and verapamil blocked [3H]putrescine uptake into inverted vesicles. Verapamil treatment caused an increase in intracellular contents of putrescine, cadaverine, and N8-acetylspermidine, in unstressed proliferating cells, or of N1-acetylspermidine, in cells subjected to heat shock to induce acetylation of spermidine at N1. These data indicate that putrescine export in Chinese hamster cells is mediated by a non-electrogenic antiporter capable of using protons as the counter ion. Physiological substrates for this exporter include putrescine, cadaverine, and monoacetyl spermidine and have the general structure NH3+-(CH2)n-NH2 + R at acidic or neutral pH.

Tolerance to putrescine toxicity in Chinese hamster ovary cells is associated with altered uptake and export

When Chinese hamster ovary (CHO) cells were cultured with low concentrations of putrescine (< 5 mM) their cell cycle time increased significantly and a fraction of the cells died. A cell line tolerant to the cytotoxic and growth inhibitory effects of millimolar concentrations of putrescine was developed by growing CHO cells over many months in increasing concentrations of the polyamine. A putrescine-tolerant cell line was obtained which was capable of growing in concentrations up to 25 mM putrescine and displayed growth and cell division rates similar to the original untreated/parental CHO cells. The tolerant cells grown in putrescine displayed relatively high intracellular putrescine yet the cell-associated putrescine concentration was estimated to be 10-fold less than the culture medium level. This high concentration of cellular putrescine diminished within 60 min when the cells were changed to non-putrescine-containing media. The putrescine-tolerant phenotype was further characterized in regards to the mechanisms involved in putrescine uptake, efflux, and biosynthesis. The parental and tolerant cell lines had similar or identical levels of cellular spermidine and spermine and no differences in the acetylated polyamine pools or diamine oxidase activity. The activity of ornithine decarboxylase was also similar in the two cell lines in both the presence and the absence of ornithine. The tolerant cells, however, had a decreased uptake rate for putrescine. The tolerant cell line also showed a greatly enhanced ability to export putrescine, especially when treated with ornithine, suggesting that an upregulated polyamine export system may be present in the tolerant cells which could be responsible for the increased cell survival in high putrescine concentrations. The data are discussed in regard to the potential for identifying the transport protein(s) responsible for the maintenance of nontoxic intracellular concentrations of putrescine in a tolerant cell line grown in putrescine.

Putrescine export in Xenopus laevis oocytes occurs against a concentration gradient: evidence for a non-diffusional export process

Putrescine export was found to occur by a non-diffusional highly regulated process using Xenopus oocytes as a model system of polyamine transport. Untreated oocytes were observed to possess high endogenous intracellular putrescine and spermidine levels with no detectable polyamine interconversion or biosynthesis over the assay intervals. The putrescine uptake process demonstrated a rapid saturation within a 5 min interval. Spermidine demonstrated a relatively larger uptake capacity with only a minimal ability to export. A kinetic analysis of the concentration-dependence of the putrescine and spermidine uptake processes indicated that the putrescine uptake process may possess two concurrent uptake components while spermidine uptake may possess a two-component process with an allosteric regulation. Elevated intracellular putrescine levels were observed to decrease against a 10-fold higher extracellular concentration gradient in a rapid and specific manner. No noticeable changes in the intracellular levels of other polyamines were observed over the same time interval. The uptake and export rates of putrescine transport also showed a concurrent, rapid and cyclical regulation. These findings support a non-diffusional putrescine export process which is highly regulated.

A kinetic characterization of putrescine and spermidine uptake and export in human erythrocytes

Using human erythrocytes as a model system for the study of mammalian polyamine transport, detailed kinetic parameters regarding the uptake and export of putrescine and spermidine were determined. The putrescine uptake data indicated a multi-component uptake system comprised of a low-capacity saturable component and a non-saturable component. The saturable putrescine uptake component demonstrated a calculated Km of 21.0 microM and a V(max) of only 6.52 x 10(-13) M/s. The non-saturable linear putrescine uptake rate was defined by a significant pH dependence, a lack of uptake inhibition by related polyamines, and a permeability pi of 3.19 x 10(-8) s-1. These findings suggested that non-saturable putrescine uptake involved a process of simple diffusion. Spermidine uptake exhibited Michaelis-Menten kinetics with a Km and Vmax of 12.5 microM and 1.36 x 10(-12) M/s, respectively. Spermidine uptake did not demonstrate pH dependence and was not significantly inhibited by any of the tested polyamines. The Arrhenius plot of spermidine uptake was determined to be biphasic with calculated activation energies of spermidine uptake of 135.2 kJ/mol for 19-21 degrees C and 59.3 kJ/mol for 21-35 degrees C. These data suggest the possibility of multiple spermidine uptake processes which are not mediated by simple diffusion across the cell membrane. The putrescine export process demonstrated both saturable and non-saturable components. The calculated Km, V(max) and pi for putrescine export were 33.8 microM, 1.19 x 10(-11) M/s and 2.81 x 10(-7) s-1, respectively. The spermidine export process was non-saturable up to intracellular spermidine concentrations of 4 microM. At similar intracellular and extracellular concentrations of putrescine and spermidine, however, export processes displayed rates which were an order of magnitude greater than their respective uptake rates. This finding supports the possible presence of mediated putrescine and spermidine export processes different than simple diffusion.