Doxorubicin selection for MDR1/P-glycoprotein reduces swelling-activated K+ and Cl- currents in MES-SA cells

Role of ion transport in control of apoptotic cell death

Cell shrinkage is a hallmark and contributes to signaling of apoptosis. Apoptotic cell shrinkage requires ion transport across the cell membrane involving K(+) channels, Cl(-) or anion channels, Na(+)/H(+) exchange, Na(+),K(+),Cl(-) cotransport, and Na(+)/K(+)ATPase. Activation of K(+) channels fosters K(+) exit with decrease of cytosolic K(+) concentration, activation of anion channels triggers exit of Cl(-), organic osmolytes, and HCO3(-). Cellular loss of K(+) and organic osmolytes as well as cytosolic acidification favor apoptosis. Ca(2+) entry through Ca(2+)-permeable cation channels may result in apoptosis by affecting mitochondrial integrity, stimulating proteinases, inducing cell shrinkage due to activation of Ca(2+)-sensitive K(+) channels, and triggering cell-membrane scrambling. Signaling involved in the modification of cell-volume regulatory ion transport during apoptosis include mitogen-activated kinases p38, JNK, ERK1/2, MEKK1, MKK4, the small G proteins Cdc42, and/or Rac and the transcription factor p53. Osmosensing involves integrin receptors, focal adhesion kinases, and tyrosine kinase receptors. Hyperosmotic shock leads to vesicular acidification followed by activation of acid sphingomyelinase, ceramide formation, release of reactive oxygen species, activation of the tyrosine kinase Yes with subsequent stimulation of CD95 trafficking to the cell membrane. Apoptosis is counteracted by mechanisms involved in regulatory volume increase (RVI), by organic osmolytes, by focal adhesion kinase, and by heat-shock proteins. Clearly, our knowledge on the interplay between cell-volume regulatory mechanisms and suicidal cell death is still far from complete and substantial additional experimental effort is needed to elucidate the role of cell-volume regulatory mechanisms in suicidal cell death.

Deregulation of apoptotic volume decrease and ionic movements in multidrug-resistant tumor cells: role of chloride channels

Changes in cell volume and ion gradients across the plasma membrane play a pivotal role in the initiation of apoptosis. Here we explore the kinetics of apoptotic volume decrease (AVD) and ion content dynamics in wild-type (WT) and multidrug-resistant (MDR) Ehrlich ascites tumor cells (EATC). In WT EATC, induction of apoptosis with cisplatin (5 μM) leads to three distinctive AVD stages: an early AVD1 (4–12 h), associated with a 30% cell water loss; a transition stage AVDT (∼12 to 32 h), where cell volume is partly recovered; and a secondary AVD2 (past 32 h), where cell volume was further reduced. AVD1 and AVD2 were coupled to net loss of Cl−, K+, Na+, and amino acids (ninhydrin-positive substances), whereas during AVDT, Na+ and Cl− were accumulated. MDR EATC was resistant to cisplatin, showing increased viability and less caspase 3 activation. Compared with WT EATC, MDR EATC underwent a less pronounced AVD1, an augmented AVDT, and a delay in induction of AVD2. Changes in AVD were associated with inhibition of Cl− loss during AVD1, augmented NaCl uptake during AVDT, and a delay of Cl− loss during AVD2. Application of the anion channel inhibitor NS3728 inhibited AVD and completely abolished the differences in AVD, ionic movements, and caspase 3 activation between WT and MDR EATC. Finally, the maximal capacity of volume-regulated anion channel was found to be strongly repressed in MDR EATC. Together, these data suggest that impairment of AVD, primarily via modulation of NaCl movements, contribute to protection against apoptosis in MDR EATC.

Impaired activity of volume-sensitive Cl− channel is involved in cisplatin resistance of cancer cells

The platinum-based drug cisplatin is a widely used anticancer drug which acts by causing the induction of apoptosis. However, resistance to the drug is a major problem. In this study we show that the KCP-4 human epidermoid cancer cell line, which serves as a model of acquired resistance to cisplatin, has virtually no volume-sensitive, outwardly rectifying (VSOR) chloride channel activity. The VSOR chloride channel's molecular identity has not yet been determined, and semi-quantitative RT-PCR experiments in this study suggested that the channel corresponds to none of three candidate genes. However, because it is known that the channel current plays an essential role in apoptosis, we hypothesized that lack of the current contributes to cisplatin resistance in these cells and that its restoration would reduce resistance. To test this hypothesis, we attempted to restore VSOR chloride current in KCP-4 cells. It was found that treatment with trichostatin A (TSA), a histone deacetylase inhibitor, caused VSOR chloride channel function to be partially restored. Treatment of the cells with both TSA and cisplatin resulted in an increase in caspase-3 activity at 24 h and a decrease in cell viability at 48 h. These effects were blocked by simultaneous treatment of the cells with a VSOR chloride channel blocker. These results indicate that restoration of the channel's functional expression by TSA treatment leads to a decrease in the cisplatin resistance of KCP-4 cells. We thus conclude that impaired activity of the VSOR chloride channel is involved in the cisplatin resistance of KCP-4 cancer cells.

Lysosomal accumulation of drugs in drug-sensitive MES-SA but not multidrug-resistant MES-SA/Dx5 uterine sarcoma cells

Sequestration of drugs in intracellular vesicles has been associated with multidrug-resistance (MDR), but it is not clear why vesicular drug accumulation, which depends upon intracellular pH gradients, should be associated with MDR. Using a human uterine sarcoma cell line (MES-SA) and a doxorubicin (DOX)-resistant variant cell line (Dx-5), which expresses p-glycoprotein (PGP), we have addressed the relationship between multidrug resistance, vesicular acidification, and vesicular drug accumulation. Consistent with a pH-dependent mechanism of vesicular drug accumulation, studies of living cells vitally labeled with multiple probes indicate that DOX and daunorubicin (DNR) predominately accumulate in lysosomes, whose lumenal pH was measured at < 4.5, but are not detected in endosomes, whose pH was measured at 5.9. However, vesicular DOX accumulation is more pronounced in the drug-sensitive MES-SA cells and minimal in Dx5 cells even when cellular levels of DOX are increased by verapamil treatment. While lysosomal accumulation of DOX correlated well with pharmacologically induced differences in lysosome pH in MES-SA cells, lysosomal accumulation was minimal in Dx5 cells regardless of lysosomal pH. We found no differences in the pH of either endosomes or lysosomes between MES-SA and Dx5 cells, suggesting that, in contrast to other MDR cell systems, the drug-resistant Dx5 cells are refractory to pH-dependent vesicular drug accumulation. These studies demonstrate that altered endomembrane pH regulation is not a necessary consequence of cell transformation, and that vesicular sequestration of drugs is not a necessary characteristic of MDR.

Phosphorylation of P-glycoprotein by PKA and PKC modulates swelling-activated Cl- currents

Several proteins belonging to the ATP-binding cassette superfamily can affect ion channel function. These include the cystic fibrosis transmembrane conductance regulator, the sulfonylurea receptor, and the multidrug resistance protein P-glycoprotein (MDR1). We measured whole cell swelling-activated Cl- currents (ICl,swell) in parental cells and cells expressing wild-type MDR1 or a phosphorylation-defective mutant (Ser-661, Ser-667, and Ser-671 replaced by Ala). Stimulation of protein kinase C (PKC) with a phorbol ester reduced the rate of increase in ICl,swell only in cells that express MDR1. PKC stimulation had no effect on steady-state ICl,swell. Stimulation of protein kinase A (PKA) with 8-bromoadenosine 3',5'-cyclic monophosphate reduced steady-state ICl, swell only in MDR1-expressing cells. PKA stimulation had no effect on the rate of ICl,swell activation. The effects of stimulation of PKA and PKC on ICl,swell were additive (i.e., decrease in the rate of activation and reduction in steady-state ICl,swell). The effects of PKA and PKC stimulation were absent in cells expressing the phosphorylation-defective mutant. In summary, it is likely that phosphorylation of MDR1 by PKA and by PKC alters swelling-activated Cl- channels by independent mechanisms and that Ser-661, Ser-667, and Ser-671 are involved in the responses of ICl,swell to stimulation of PKA and PKC. These results support the notion that MDR1 phosphorylation affects ICl,swell.

Antisense to MDR1 mRNA reduces P-glycoprotein expression, swelling-activated C1- current and volume regulation in bovine ciliary epithelial cells

Native ciliary epithelial cells from the ciliary epithelium of the eye exhibit anti-P-glycoprotein (P-gp) immunofluorescence. We have used an antisense 'knock-down' approach to investigate the relationship between P-gp and the volume-activated chloride current (IC1,swell) and its role in volume regulation. An antisense oligonucleotide to the human multidrug resistance (MDR1) gene, taken up by the cells in a dose-dependent manner, reduced P-gp immunofluorescence, inhibited IC1,swell and significantly increased the latency of activation of IC1,swell. Increasing the hypotonic stress did not result in an increased activation of ICl,swell. MDR1 antisense 'knock-down' also reduced the ability of the cells to volume regulate following a hypotonic challenge. These cells are known to express at least two volume-activated chloride channels, and the data suggest that P-gp is involved in the activation pathway of a subset of channels that contribute to whole-cell IC1,swell and participate in volume regulation.

FROM VACUOLAR GS-X PUMPS TO MULTISPECIFIC ABC TRANSPORTERS

While the concept of H+-coupling has dominated studies of energy-dependent organic solute transport in plants for over two decades, recent studies have demonstrated the existence of a group of organic solute transporters, belonging to the ATP-binding cassette (ABC) superfamily, that are directly energized by MgATP rather than by a transmembrane H+-electrochemical potential difference. Originally identified in microbial and animal cells, the ABC superfamily is one of the largest and most widespread protein families known. Competent in the transport of a broad range of substances including sugars, peptides, alkaloids, inorganic anions, and lipids, all ABC transporters are constituted of one or two copies each of an integral membrane sector and cytosolically oriented ATP-binding domain. To date, two major subclasses, the multidrug resistance-associated proteins (MRPs) and multidrug resistance proteins (MDRs) (so named because of the phenotypes conferred by their animal prototypes), have been identified molecularly in plants. However, only the MRPs have been defined functionally. This review therefore focuses on the functional capabilities, energetics, organization, and regulation of the plant MRPs. Otherwise known as GS-X pumps, or glutathione-conjugate or multispecific organic anion Mg2+-ATPases, the MRPs are considered to participate in the transport of exogenous and endogenous amphipathic anions and glutathionated compounds from the cytosol into the vacuole. Encoded by a multigene family and possessing a unique domain organization, the types of processes that likely converge and depend on plant MRPs include herbicide detoxification, cell pigmentation, the alleviation of oxidative damage, and the storage of antimicrobial compounds. Additional functional capabilities might include channel regulation or activity, and/or the transport of heavy metal chelates. The identification of the MRPs, in particular, and the demonstration of a central role for ABC transporters, in general, in plant function not only provide fresh insights into the molecular basis of energy-dependent solute transport but also offer the prospect for manipulating and investigating many fundamental processes that have hitherto evaded analysis at the transport level.

Functional significance of cell volume regulatory mechanisms

To survive, cells have to avoid excessive alterations of cell volume that jeopardize structural integrity and constancy of intracellular milieu. The function of cellular proteins seems specifically sensitive to dilution and concentration, determining the extent of macromolecular crowding. Even at constant extracellular osmolarity, volume constancy of any mammalian cell is permanently challenged by transport of osmotically active substances across the cell membrane and formation or disappearance of cellular osmolarity by metabolism. Thus cell volume constancy requires the continued operation of cell volume regulatory mechanisms, including ion transport across the cell membrane as well as accumulation or disposal of organic osmolytes and metabolites. The various cell volume regulatory mechanisms are triggered by a multitude of intracellular signaling events including alterations of cell membrane potential and of intracellular ion composition, various second messenger cascades, phosphorylation of diverse target proteins, and altered gene expression. Hormones and mediators have been shown to exploit the volume regulatory machinery to exert their effects. Thus cell volume may be considered a second message in the transmission of hormonal signals. Accordingly, alterations of cell volume and volume regulatory mechanisms participate in a wide variety of cellular functions including epithelial transport, metabolism, excitation, hormone release, migration, cell proliferation, and cell death.

P-glycoprotein is not a swelling-activated Cl- channel; possible role as a Cl- channel regulator

1. The whole-cell configuration of the patch-clamp technique was used to determine if P-glycoprotein (Pgp) is a swelling-activated Cl- channel. 2. Hamster pgp1 cDNA was transfected into a mouse fibroblast cell line resulting in expression of functional Pgp in the plasma membrane. This cell line was obtained without exposure to chemotherapeutic agents. 3. Swelling-activated whole-cell Cl- current (ICl,swell) was elicited by lowering the bath osmolality. ICl,swell was characterized in detail in the pgp1-transfected mouse cell line and compared with that of its parental cell line. Expression of Pgp did not modify the magnitude or properties of ICl,swell, except that addition of the anti-Pgp antibody C219 to the pipette solution inhibited this current by 75% only in the Pgp-expressing cells. 4. ICl,swell in the mouse Pgp-expressing cell line was compared with that in a Pgp-expressing hamster fibroblast cell line. The characteristics of ICl,swell (voltage dependence, blocker sensitivity, anion selectivity sequence, requirement for hydrolysable ATP) in Pgp-expressing cells were different between the two cell lines. These results suggest that the channel(s) responsible for ICl,swell are different between the two cell lines. In addition, C219 inhibited ICl,swell in both Pgp-expressing cell lines, even though they seem to express different swelling-activated Cl- channels. 5. We conclude that firstly, Pgp is not a swelling-activated Cl- channel; secondly, it possibly functions as a Cl- channel regulator; and thirdly, ICl,swell is underlined by different Cl- channels in different cells.

Cell-volume regulation: P-glycoprotein--a cautionary tale

P-glycoprotein turns out not to be 'VSOAC', a known channel activated by cell swelling; it does seem to influence cell-volume recovery after swelling, but the physiological importance of this effect is presently unclear.