Paclitaxel and SN-38 overcome cisplatin resistance of ovarian cancer cell lines by down-regulating the influx and efflux system of cisplatin.

Cisplatin (DDP) is one of the key drugs used to treat patients with ovarian cancer, although resistance to DDP can occur. Paclitaxel and SN-38 (an active metabolite of irinotecan (CPT-11)) are two drugs that are effective in patients with DDP-resistant ovarian cancer. To study how these drugs may overcome the intrinsic and / or acquired resistance of cancer cells to DDP, we investigated the effect of a combination of DDP with paclitaxel and a combination of DDP with SN-38 on three ovarian cancer cell lines, RTSG (intrinsically resistant cell line), KF (DDP-sensitive cell line), and KFra (acquired resistant cell line obtained from KF). We found that these combinations showed additive to synergistic antitumor activity. A time-dependent platinum (Pt) accumulation was observed in the DDP-sensitive KF cell line, while a decrease occurred in the KFra cell line. Little accumulation was observed in RTSG. Intracellular Pt accumulation was increased in all three cell lines by exposure to paclitaxel or SN-38. Ouabain, a Na(+),K(+)-ATPase inhibitor, decreased Pt accumulation in KF and KFra cell lines and inhibited the paclitaxel- and SN-38-induced increases in Pt accumulation in these cell lines. When we assessed the mRNA levels of the multidrug resistance-associated protein (MRP), which may be an efflux pump for DDP, the combination of paclitaxel or SN-38 with DDP down-regulated these levels, which are up-regulated by DDP alone. These results suggest that paclitaxel and SN-38 overcome DDP resistance of ovarian cell lines by controlling intracellular accumulation of DDP via both the influx and efflux systems.     

Mitochondrial defects in cis-diamminedichloroplatinum(II)-resistant human ovarian carcinoma cells.

We have been studying the membranes of cisplatin (DDP)-resistant 2008 human ovarian carcinoma cells (C13* cells) for alterations that may account for their decreased DDP accumulation. We now report that C13* cells have significant changes in their mitochondrial and plasma membrane potentials and in their mitochondrial morphology. C13* cells accumulated 2.0 +/- 0.1-fold more of the membrane potential marker [3H]tetraphenylphosphonium cation (TPP+) than sensitive cells. In high K+ medium, which depolarizes the plasma membrane but not the mitochondrial membrane, [3H]TPP+ accumulation was still 2.3 +/- 0.1- fold greater in resistant cells, indicating that the mitochondrial membrane potential was higher. In the presence of carbonyl cyanide p-trifluoromethoxyphenyl hydrazone, which depolarizes the mitochondrial membrane but not the plasma membrane, [3H]TPP+ accumulation demonstrated that the plasma membrane potential in C13* cells was elevated as well. These elevations were also present in C8 cells with low-level DDP resistance. After ouabain treatment, exposure to nigericin stimulated [3H]TPP+ accumulation 3-fold in sensitive cells but had no effect in C13* cells, indicating either that: (a) the mitochondrial pH gradient was minimal; or (b) the mitochondrial electric potential was already at a maximal level in C13* cells. Fluorescence microscopy of living cells stained with the mitochondria-specific dye rhodamine 123 revealed that resistant cells had significant changes in their mitochondrial morphology. Electron microscopy also revealed major alterations in the cristae structure. The C13* cells, which were approximately 15-fold resistant to DDP, were 5-fold hypersensitive to the mitochondrial poison rhodamine 123. We conclude that these DDP-resistant 2008 cells have an elevated plasma membrane potential and alterations in their mitochondria as indicated by their membrane potential, morphology, and sensitivity to mitochondrial poisons. These results imply that mitochondria play an important role in the cellular pharmacology of DDP.     

Human tumor cell sensitivity to oleandrin is dependent on relative expression of Na+, K+ -ATPase subunitst

The membrane enzyme Na+, K+ -ATPase is known to help maintain ion homeostasis in mammalian cells. Newly identified functions of this enzyme suggest that inhibition of Na+, K+ -ATPase by cardiac glycosides may be useful to patients with cancer. Twelve human tumor cell lines were chosen to examine determinants of human tumor cell sensitivity to cardiac glycosides. In vitro cell culture models of human glioma HF U251 and U251 cells as well as human parental and modified melanoma BRO cells were also included in these studies. Data derived from both models and twelve tumor cell lines indicated that high expression of Na+, K+ -ATPase alpha 1 isoform in the presence of low alpha 3 expression correlated with increased resistance to inhibition of cell proliferation by cardiac glycosides such as oleandrin, ouabain and bufalin. Interestingly, increased expression of Na+, K+ -ATPase alpha 1 and therefore total Na+, K+ -ATPase activity is associated with increased cellular levels of glutathione. The altered enzyme activity and glutathione content were associated with a delayed and diminished release of cytochrome c and caspase activation. Additionally, an increased colony-forming ability was noted in cells with high levels of Na+, K+ -ATPase alpha 1 expression, suggesting that Na+, K+ -ATPase alpha 1 isoform may be actively involved in tumor growth and cell survival. Its inhibition by cardiac glycosides may provide a strategy for effective cancer therapy.     

Modulation of cis-diamminedichloroplatinum(II) accumulation and sensitivity by forskolin and 3-isobutyl-1-methylxanthine in sensitive and resistant human ovarian carcinoma cells.

We have determined the effect of forskolin, an adenyl cyclase agonist, and 3-isobutyl-1-methylxanthine (IBMX), a phosphodiesterase inhibitor, on the accumulation and cytotoxicity of cisplatin (DDP) in 2008 human ovarian carcinoma cells. In DDP-sensitive 2008 cells, forskolin and IBMX caused 2.1-fold and 2.3-fold increases, respectively, in the short-term accumulation of DDP relative to untreated cells. The inactive analogue, 1,9-dideoxyforskolin, decreased DDP accumulation. Forskolin and IBMX also increased accumulation in A2780 cells. Neither forskolin nor IBMX had any effect on DDP accumulation in DDP-resistant 2008 cells. The effects were detectable as early as 1 min and persisted at 60 min. The concentrations for half-maximal stimulation of DDP accumulation were approximately 0.2 microM for forskolin and 0.2 mM for IBMX. Forskolin caused marked increases in cAMP levels in both sensitive and resistant 2008 cells within 1 min, although there were differences in the subsequent time-courses of the response. Both 2008 cell types had identical cAMP-dependent protein kinase (PKA) activity. These results suggest that there is a target downstream of PKA that is an important participant in DDP accumulation, and that this target is defective or missing in DDP-resistant cells. Following a 1-hr exposure to drugs, forskolin and IBMX at concentrations that were by themselves completely non-toxic increased the slopes of the clonogenic survival vs. DDP concentration curves in 2008 cells 1.9-fold and 3.3-fold, respectively. In DDP-resistant 2008 cells, however, forskolin and IBMX increased the slopes only 1.2 and 2.6-fold, respectively. These effects of forskolin and IBMX on DDP cytotoxicity did not directly correlate with the effects on the 1-hr DDP accumulation which suggested that, in addition to modulating DDP accumulation, these agents increase the cytotoxicity of the intracellular platinum. The results indicate that modulation of cAMP levels can have important effects on DDP accumulation and cytotoxicity in 2008 cells and that these effects are significantly diminished in DDP-resistant cells.     

cis-Diamminedichloroplatinum(II) accumulation in sensitive and resistant human ovarian carcinoma cells

We have characterized the accumulation of cisplatin (DDP) into parent and cisplatin-resistant 2008 human ovarian carcinoma cells. Accumulation of DDP at 1 h was a linear function of concentration from 0.25 to 100 microM DDP in both cell types. DDP-resistant cells that were only 3.3-fold resistant had approximately 50% less accumulated platinum at all concentrations examined. Accumulation of 1.0 microM DDP was linear for approximately 3 h and then slowed but did not reach equilibrium at up to 24 h in either cell type. The decreased DDP accumulation in resistant cells did not appear to be due to increased efflux; similar percentages of platinum were available for exodus in parent and resistant cells. Intracellular metabolites of DDP appeared to be decreased by similar amounts in the resistant cells. Native DDP was not driven uphill into cells against a concentration gradient, suggesting that DDP uptake does not involve primary active transport. The metabolic inhibitors, dinitrophenol and NaF, did not decrease DDP accumulation; iodoacetate had a stimulatory effect. Dinitrophenol, however, in combination with NaF or iodoacetate decreased DDP accumulation. A 30-min exposure to 0.2 mM ouabain also decreased DDP accumulation in both parent and resistant cells. A component of DDP accumulation thus appears to be energy dependent. These studies have identified decreased DDP accumulation as an important mechanism of resistance that is expressed early in the acquisition of DDP resistance in human ovarian carcinoma cells.     

Acquisition of resistance to cisplatin is accompanied by changes in the cellular pharmacology of copper

Impaired uptake of cisplatin (DDP) consistently accompanies the acquisition of resistance to the platinum drugs. The pathways by which DDP enters or exits from cells remain poorly defined. Using three pairs of human ovarian carcinoma cell lines, each consisting of a sensitive parental line and a stably DDP-resistant subline derived by in vitro selection, resistance to DDP was found to be accompanied by cross-resistance to Cu. Accumulation of DDP in the resistant sublines ranged from 38 to 67% of that in the parental line at 1 h, and DNA adduct formation varied from 10 to 38% of that in the sensitive cells. The DDP-resistant cells had 22-56% lower basal levels of copper, and the copper levels were only 27-46% of those observed in the sensitive parental lines after a 24-h exposure to medium supplemented with copper. The initial influx rate for DDP in the three resistant cell lines ranged from 23 to 55% of that in the sensitive cells of each pair; the initial influx rate for copper in the resistant cells varied from 56 to 75% of control. Studies performed using one pair of cell lines demonstrated that for both copper and DDP the initial efflux rate was lower, whereas the terminal efflux rate was higher in the resistant cells. On Western blot analysis all three resistant lines exhibited increased expression of one or the other of the two copper export pumps (ATP7A or ATP7B) with no change in the HAH1 chaperone. We conclude that the acquisition of DDP resistance in ovarian carcinoma is accompanied by alterations in the cellular pharmacology of DDP that are paralleled by similar changes in the uptake and efflux of copper. These results are consistent with the concept that DDP enters and exits from the cell via transporters that normally mediate copper homeostasis.     

Pharmacological modulation of lung cancer cells for potassium ion depletion

BACKGROUND: Depletion of intracellular potassium ions (K+) is necessary for cells to shrink, induce DNA fragmentation and activate caspases, events which are features of apoptosis. MATERIALS AND METHODS: We used 86Rb+ as a K+ analogue to evaluate the possibility of pharmacologically depleting human pulmonary mesothelioma (P31) and small cell lung cancer (U1690) cells of K+, for future use in studies of apoptosis induction. RESULTS: The Na+, K+, 2CI(-)-cotransport inhibitor bumetanide transiently inhibited 86Rb+ influx, but when combined with the Na+, K+, ATPase pump inhibitor ouabain there was a marked and lasting (up to 6 h) 86Rb+ influx inhibition. Cellular K+ efflux was augmented by amphotericin B, digitonin and nigericin. Amphotericin B was an effective 86Rb+ efflux stimulator with low cytotoxicity, whereas digitonin caused cell detachment and nigericin increased LDH release in the U1690 cell line, indicating considerable toxicity of the drugs. CONCLUSION: It is possible to efficiently reduce intracellular K+ by persistent K+ influx inhibition and simultaneous K+ efflux stimulation with clinically available drugs.     

Modulation of thyroid hormone-dependent Na+,K(+)-ATPase induction in cultured human submandibular gland cell lines, HSG cells

The hormonal regulation of Na+,K(+)-ATPase enzyme activities and induction of the alpha subunit protein of the enzyme in the human submandibular gland (HSG) were studied by use of cultured HSG cells. We treated HSG cells with thyroid hormone, androgen, mineralocorticoid, and glucocorticoid, singly or in combination. 3,5,3'-Triiodothyronine (T3), 5 alpha-dihydrotestosterone (DHT), and aldosterone (Ald) induced neither Na+,K(+)-ATPase enzyme activity nor its protein. On the other hand, dexamethasone (Dex) induced both Na+,K(+)-ATPase enzyme activity and the alpha subunit protein level to 128% of the control. The effects of Dex in combination with either T3 or DHT were similar to the effect of Dex alone. Treatment in combination with Dex and Ald increased the enzyme activity and alpha subunit protein level to 160%, synergistically. These increased Na+,K(+)-ATPase enzyme activities were shown to be dependent on their protein levels induced by the hormones. Contrary to the previous evidence that Na+,K(+)-ATPase of ducts in the salivary gland are thyroid hormone inducible, HSG cells had an insignificant response to thyroid hormone in the present study. Also, Na+,K(+)-ATPase enzyme activity and its alpha subunit protein were not induced by any kind of combined treatments with T3. Furthermore, T3 did not cause intracellular calcium mobilization in HSG cells. In view of all data taken together, we suggest that HSG cells lack the thyroid hormone receptor, which is necessary for Na+,K(+)-ATPase induction in human salivary gland.     

Modulation of cisplatin sensitivity and resistance by buthionine sulfoximine and cyclosporin A in human esophageal cancer cells

We established a 2.4-fold cisplatin (CDDP)-resistant human esophageal cancer cell line (TE2R) from the parent TE2 line. CDDP accumulation was reduced in TE2R. The Na+, K+-ATPase inhibitor ouabain inhibited CDDP accumulation in TE2 but not TE2R, suggesting that TE2R may have alterations in the Na+, K+-ATPase and defective CDDP uptake mechanism. Buthionine sulfoximine (BSO) enhanced CDDP sensitivity of both cell lines and cyclosporin A (CsA) modified CDDP resistance in TE2R. These effects were associated with increased CDDP accumulation. Thus, BSO and CsA may be useful for modulation of CDDP sensitivity or resistance in esophageal cancer.     

The antiproliferative riminophenazine agents clofazimine and b669 promote lysophospholipid-mediated inhibition of na+, k+-adenosine triphosphatase-activity in cancer cell-lines in-vitro

The effects of the anti-proliferative, phospholipase A(2) (PLA(2))-activating riminophenazine agents, clofazimine and B669, on the Na+, K+-adenosine triphosphatase activity of the FaDu human pharynx squamous carcinoma cell line have been investigated in vitro. At concentrations of 1.25-10 mu g/ml both agents caused dose-related enhancement of PLA(2), as measured by increased release of lysophosphatidylcholine (LPC), and inhibition of Na+, K+-ATPase in intact cells and isolated membrane preparations. The inhibitory effects of both riminophenazines on the Na+, K+-ATPase activity of FaDu cells were mimicked by reagent LPC and prevented by treatment of the cells with the lysophospholipid-neutralizing agents alpha-tocopherol and lysophospholipase. Riminophenazine-mediated inhibition of Na+, K+-ATPase activity was also observed with the HeLa (human cervix epitheloid carcinoma) and T24 (human transitional cell bladder carcinoma) cell lines. The anti-proliferative activity of clofazimine and B669 is therefore probably achieved by lysophospholipid-mediated inactivation of Na+, K+-ATPase.     

The effect of DNA polymerase inhibitors on the cytotoxicity of cisplatin in human ovarian carcinoma cells

We examined the effect of specific inhibitors of DNA polymerases alpha and delta, and beta, on cisplatin (DDP) cytotoxicity in DDP-sensitive and -resistant human 2008 ovarian carcinoma cells. Under conditions of continuous exposure to drug combinations, neither aphidicolin glycinate (AG) nor dideoxythymidine enhanced the cytotoxicity of DDP in either cell line as determined by clonogenic survival assays. However, when clonogenic survival was determined following short-term drug exposure, AG exhibited strong synergism with DDP in the DDP-resistant, but not the DDP-sensitive cells, as indicated by median effect analysis of the data. DNA polymerase alpha mRNA levels were the same in both cell lines under basal conditions. DDP-sensitive cells, but not DDP-resistant cells, were able to increase their expression of DNA polymerase alpha in response to DDP exposure. Levels of mRNA for DNA polymerase beta and for the human DNA repair gene ERCC-1 were not elevated in resistant cells, either under basal conditions or 18 hr after a 1 hr exposure to IC20 concentrations of DDP. In another human ovarian carcinoma cell line, A2780, AG and DDP were synergistic in both DDP-sensitive and -resistant variants in short-term exposure. We conclude that DNA polymerases alpha and/or delta play a role in the DDP sensitivity of human ovarian carcinoma cells.     

Reduced drug accumulation as a major mechanism of acquired resistance to cisplatin in a human ovarian carcinoma cell line: circumvention studies using novel platinum (II) and (IV) ammine/amine complexes.

Acquired resistance to cisplatin (cis-diamminedichloroplatinum (II)) has been generated in vitro in the 41M human ovarian carcinoma cell line, established from a previously untreated patient. Three cisplatin-resistant variants were selected at approximately 2, 4 and 6-fold resistance (in terms of 50% inhibitory concentrations), in order to study the underlying mechanisms of acquired cisplatin resistance. Compared to the parent line, platinum accumulation following exposure to equimolar concentrations of cisplatin was on average (across the entire concentration range) 2.9, 3.6 and 4.8-fold lower in the 41McisR2, 41McisR4 and 41McisR6 cell lines, respectively. Thus the difference in uptake corresponded closely with their resistance factor in the three resistant variants. Moreover, a significant reduction in platinum accumulation was observed as early as 5 min after exposure to cisplatin in the 41M vs 41McisR6 cell lines. Platinum accumulation was similar in all cell lines following exposure to equitoxic concentrations (2 h IC50) of cisplatin. Enhanced efflux of drug was not observed between the 41M and 41McisR6 cells. In addition, there was no difference in intracellular glutathione (GSH) levels. Our previous studies have shown no indication of metallothionein involvement and the decrease in cisplatin uptake in the 41McisR6 cells was reflected by a similar reduction in DNA interstrand cross-links (ISC) formation. These results suggest that the mechanism of acquired resistance to cisplatin in the 41McisR6 cell line may be predominantly due to reduced drug uptake. The 41McisR6 cells were not found to be cross-resistant to ouabain, a postulated specific inhibitor of sodium-potassium adenosine triphosphatase (Na+, K(+)-ATPase), suggesting that decreased cisplatin accumulation in these cells is probably not regulated by alterations in their Na+, K(+)-ATPase levels, and Na+ potential across the plasma membrane. Cellular accumulation of a novel class of platinum (IV) ammine/cyclohexylamine dicarboxylates, which exhibit enhanced cytotoxicity over cisplatin and completely circumvent resistance to cisplatin in the 41McisR line, was also examined. The data suggests that increased accumulation of these compounds, as a result of their enhanced lipophilicity, could account for the dramatic increase in their potency over cisplatin.     

Reduction of doxorubicin cytotoxicity by ouabain: correlation with topoisomerase-induced DNA strand breakage in human and hamster cells

The cardiac glycoside ouabain, which is a specific inhibitor of the Na+,K+-pump, confers dramatic protection from the cytotoxic effects of doxorubicin (Adriamycin). This effect was documented in cultured A549 cells (human lung adenocarcinoma). CCL210 cells (human fibroblasts), and V79 cells (hamster fibroblasts). Maximum protection from doxorubicin cytotoxicity was achieved using 1 microM ouabain for A549 and CCL210 cells and 300 microM ouabain for V79 cells. These concentrations correlated well with the concentrations of ouabain required to induce Na+,K+-pump blockade, which was assessed using the K+ analogue 86Rb+. This suggests that protection is mediated by pump blockade. Addition of ouabain at the same time as doxorubicin was just as protective as preincubation with ouabain for an hour, demonstrating that the ouabain acts rapidly. Ouabain treatment affected neither influx nor efflux of doxorubicin. Ouabain also had no effect on verapamil-induced inhibition of doxorubicin efflux. However, ouabain partially blocked the verapamil-induced potentiation of the cytotoxic effects of doxorubicin. Therefore, ouabain does not protect by affecting intracellular doxorubicin levels. Fluorescence microscopy showed that the ability of doxorubicin to reach the nucleus was not influenced by ouabain. Alkaline elution studies demonstrated that ouabain greatly decreased doxorubicin-induced DNA strand breakage. Protection from cytotoxicity correlated closely with this decrease in strand breakage. These studies suggest that the stabilization of DNA-topoisomerase II complexes is closely linked to the mechanism of doxorubicin cytotoxicity and that this stabilization is influenced by the intracellular ionic milieu.     

The influence of Na+, K+-pump blockade on doxorubicin-mediated cytotoxicity and DNA strand breakage in human tumor cells

Summary We have previously shown that blockade of the Na⁺, K⁺-pump by the cardiac glycoside ouabain produces doxorubicin resistance and decreases topoisomerase II-mediated DNA strand breakage in hamster cells. To determine if this were a general phenomenon, the effect of pump blockade on doxorubicin resistance was assessed in three human tumor cell lines: A549 lung and HT29 colon adenocarcinomas and U1 melanoma. When cells were exposed to 1 μM ouabain prior to and during incubation with doxorubicin, cytotoxicity was markedly reduced. Ouabain had no effect on either the influx or the efflux of doxorubicin. However, all cell lines showed a ouabain-induced decrease in doxorubicin-induced topoisomerase-mediated DNA strand breakage (SSB). These data suggest that blockade of the Na⁺, K⁺ pump decreases doxorubicin cytotoxicity in human tumor cells by inhibiting topoisomerase-mediated SSB. Furthermore, they indicate that altered ionic gradients are a potential cause of resistance to drugs that use topoisomerase II as a target. This research was supported in part by a grant from the Rackham Foundation. Presented in part at the meeting of the American Association for Cancer Research in May, 1989     

哇巴因抑制结直肠癌多药耐药细胞增殖及侵袭力的研究

  目的  探讨结直肠癌耐药细胞增殖及侵袭力与Na+, K+-ATP酶活性及其β1亚单位和P糖蛋白(P-gp)表达的关系, 及哇巴因增加化疗敏感性的可能机制。  方法  以人结直肠癌亲本细胞(SW480)和耐奥沙利铂细胞(SW480/OxR)为研究对象, 采用MTS法、Transwell小室、生化酶学、实时定量PCR(Real time quantitative, RT-PCR)及流式细胞技术等方法比较SW480细胞与SW480/OxR细胞的增殖及侵袭力、Na+, K+-ATP酶活性及其β₁亚单位和P-gp表达的差异, 观察哇巴因对SW480/OxR细胞上述指标的影响。  结果  与SW480细胞比较, SW480/OxR细胞增殖活力无明显变化(P > 0.05), 而细胞侵袭力却显著增加, Na+, K+-ATP酶活性下降, β₁亚单位表达下调, P-gp表达上调(P均 < 0.01);哇巴因能显著抑制SW480/OxR细胞增殖活力, 减弱其侵袭力, 下调SW480/OxR细胞P-gp蛋白表达, 上调SW480/OxR细胞β₁亚单位蛋白表达, 增加SW480/OxR细胞Na+, K+-ATP酶活性(P均 < 0.01)。  结论  Na+, K+-ATP酶活性下降可能源于β₁亚单位表达下调, 并参与结直肠癌的耐药; 哇巴因能部分逆转结直肠癌耐药细胞MDR, 可能与增加Na+, K+-ATP酶活性及下调P-gp表达有关。      

Immunocytologic staining of cytokeratin in bone marrow aspirates to detect micrometastatic cells fails in patients with metastatic urologic carcinoma

Cisplatin is a key drug in chemotherapy for lung cancer. It has been reported that intracellular accumulation of cisplatin is an important step as a determinant for resistance to cisplatin, which may be modulated by Na+, K+-ATPase activity. And it has been reported that beta-adrenoceptor agonists modulate the Na+, K+-ATPase in some organs. In this study, the effects of a beta-adrenoceptor agonist and an antagonist on membrane Na+, K+-ATPase activity were evaluated using human non-small cell (NSCLC) lung cancer cell lines. In the NSCLC cell lines, sensitivity to cisplatin was improved by treatment with isoproterenol. Na+, K+-ATPase was activated and intracellular accumulation of cisplatin increased with the treatment. But the antagonist, propranolol, did not modulate sensitivity to cisplatin or Na+, K+-ATPase activity. These results suggest that beta-adrenoceptors may be one of the determinant for sensitivity to cisplatin in NSCLC, but endogenous catecholamine dose not play a role in the intracellular accumulation of cisplatin in these cell lines. Exogenous beta-adrenoceptor agonists may improve the antitumor effect of chemotherapy involving cisplatin.     

Increase in intracellular Na+: transmembrane signal for rejoining of DNA strand breaks in proliferating lymphocytes

Two early events in the mitogen-induced entry of murine splenocytes into proliferation are (a) a rapid rise in influx of Na+, causing its total internal concentration to increase by 42 +/- 7% within 2 h of culture with concanavalin A (Con A), and (b) rejoining of some 3000 DNA strand breaks per diploid genome within the same period. Con A did not induce rejoining in low Na+ (less than 9 mM) medium, but the process began directly when Na+ was added, at its usual concentration, to the growth medium. Incubation of cells with ouabain, an inhibitor of the Na+K+-ATPase, or monensin, a Na+ ionophore, caused an increase in the internal Na+ concentration in normal, but not in low, Na+ medium. In the former (but not in the latter) medium, both ouabain and monensin caused rejoining of the DNA strand breaks to occur in resting lymphocytes, i.e., in the absence of mitogen. Stimulation of splenocytes with Con A also resulted in a rapid but transient increase in the level of intracellular free Ca2+. This effect was also observed in the absence of extracellular Na+; however, deprivation of extracellular Ca2+ completely abolished this effect. Moreover, the intracellular free Ca2+ level was significantly higher in cells suspended in Na+-free buffer or medium. Since the Con A-induced rejoining of DNA strand breaks occurred in the absence of extracellular Ca2+ and removal of extracellular Na+ had no inhibitory effect on the Con A-induced increase in the level of intracellular free Ca2+, the Con A-stimulated repair could not have been mediated by the initial increase in Ca2+ influx. The early mitogen-induced increase in the internal Na+ concentration is a necessary and sufficient signal for the rejoining of breaks, an event that must occur before the proliferating lymphocytes can replicate their DNA.     

Synergistic interaction between cisplatin and taxol in human ovarian carcinoma cells in vitro.

Taxol, a unique tubulin active agent, was found to demonstrate a marked schedule-dependent synergistic interaction with cisplatin (DDP) in the killing of human ovarian carcinoma 2008 cells in vitro as determined by median effect analysis. The interaction was highly synergistic when 19 h taxol exposure was followed by 1 h concurrent exposure to taxol and DDP. The combination indices (CIs) on this schedule were 0.11 +/- 0.1, 0.25 +/- 0.15 and 0.39 +/- 0.14 at 20%, 50% and 80% cell kill respectively. However, the interaction was antagonistic when 1 h exposure to DDP was followed by 20 h exposure to taxol, or when cells were exposed to DDP and taxol for 1 h concurrently. When taxol preceded DDP, synergy was also observed with the 11-fold DDP-resistant 2008/C13*5.25 subline, which yielded CI values of 0.21 +/- 0.02, 0.30 +/- 0.11 and 0.31 +/- 0.17 at 20%, 50% and 80% cell kill respectively. At an IC50 concentration, taxol had no effect on [3H]cis-dichloro(ethylenediamine) platinum uptake, on the permeability of the plasma membrane or on glutathione or metallothionein levels in 2008 or 2008/C13*5.25 cells. Mitotic arrest in these cells was observed only at taxol concentrations well above those required for synergy with DDP, suggesting that the mechanism underlying the synergistic interaction was not a taxol-induced alteration in cell cycle kinetics. Of additional interest was the fact that the 2008/C13*5.25 cells were hypersensitive to taxol, and that this was partially explained by an alteration in the biochemical pharmacology of taxol. Although cellular taxol accumulation reached steady state within 2 h in both cell lines, taxol efflux was slower and the taxol was more extensively bound in 2008/C13*5.25 cells than in 2008 cells. In addition, the 2008/C13*5.25 cells had only 55% of the parental levels of beta-tubulin content. However, in another pair of DDP-sensitive and -resistant ovarian cell lines no taxol hypersensitivity and no change in beta-tubulin content was observed, indicating that the DDP-resistant and taxol-hypersensitive phenotypes do not segregate together. We conclude that taxol interacts synergistically with DDP in a manner that is highly schedule dependent, and that the hypersensitivity of 2008/C13*5.25 cells no taxol is unrelated to the mechanism of synergy. These in vitro observations suggest that drug schedule will be an important determinant of the activity and toxicity of the DDP and taxol drug combination in clinical studies.     

Preferential inhibition by quercetin of mitogen-stimulated thymocyte glucose transport.

The ATPase inhibitor quercetin, which inhibits tumor glycolysis, was shown to be a glucose transport inhibitor like the chemically related compound phloretin. Rat thymocyte glucose transport stimulated by the mitogens concanavalin A or ionophore A 23187 was more sensitive than unstimulated transport to quercetin inhibition. The partial inhibition of Na+-, K+- ATPase activity by quercetin observed in tumor cells was confirmed in thymocyte plasma membranes. The specific Na+-, K+- ATPase inhibitor ouabain did not mimic the effect of quercetin on mitogen-stimulated glucose transport but did reduce the effectiveness of concanavalin A as a stimulator of mitochondrial pyruvate oxidation. The results support the idea that glycolytic flux and the activity of plasma membrane ATPase are related but suggest that glucose transport, rather than the Na+-, K+-ATPase, is the rate-limiting reaction in lymphocytes.     

Dependence of etoposide-induced cytotoxicity and topoisomerase II-mediated DNA strand breakage on the intracellular ionic environment

We have found that blockade of the Na+,K+-pump by the cardiac glycoside ouabain protects human A549 and hamster V79 cells from the cytotoxic effects of the topoisomerase II poison etoposide. One thousand-fold higher concentrations of ouabain were required to protect V79 cells compared to A549 cells. Since this difference parallels previously measured differences in pump sensitivity, it suggests that protection is mediated directly through pump blockade. Ouabain affected neither the cellular influx nor efflux of etoposide. However, pump blockade did decrease the formation of etoposide-induced DNA-topoisomerase, II-cleavable complexes, assessed as single and double strand DNA breaks using alkaline and neutral elution. To determine if this decrease were a direct effect of change in ionic environment produced by pump blockade, experiments with isolated nuclei and partially purified topoisomerase II were performed. Etoposide-induced cleavable complex formation and topoisomerase-mediated decatenation were assessed in buffers which mimicked either normal intracellular ionic conditions or those produced by ouabain. Compared to the buffer which resembled the normal intracellular ionic conditions, the buffer that mimicked the conditions produced by pump blockade produced fewer etoposide-mediated cleavable complexes in isolated nuclei and less decatenating activity of partially purified topoisomerase II. These findings demonstrate that inhibition of the Na+,K+-pump causes an alteration in the intracellular ionic environment which decreases the activity of topoisomerase II, thus producing a decrease in etoposide-induced cleavable complex formation and cytotoxicity. Since ionic changes occur inside normal cells during progression through the cell cycle as well as in cells that have undergone transformation, these data suggest that the intracellular ionic environment plays a role in determining the sensitivity of normal and malignant cells to this group of chemotherapeutic agents.