Is rhodamine 123 an appropriate fluorescent probe to assess P-glycoprotein mediated multidrug resistance in vinblastine-resistant CHO cells?

The Effects of Different Fluorescent Indicators in Observing the Changes of the Mitochondrial Membrane Potential during Oxidative Stress-Induced Mitochondrial Injury of Cardiac H9c2 Cells

We evaluated the ability of different fluorescent indicators by various analytical instruments, including a laser scanning confocal microscope (LSCM), fluorescence plate reader, and flow cytometer (FCM), to measure the mitochondrial membrane potential (ΔΨm) of cardiac H9c2 cells during oxidative stress-induced mitochondrial injury. The mitochondrial oxygen consumption rate and a transmission electron microscope were used to detect changes in mitochondrial functions and morphology, respectively. Cardiac H9c2 cells were exposed to H₂O₂ (500, 750, 1000, and 1250 μM) to induce mitochondrial oxidative stress injury, and fluorescent indicators including tetramethyl rhodamine ethyl ester (TMRE), 5,5',6,6'-tetrachloro-1,1',3,3'-tetraethylbenzimidazolocarbocyanine iodide (JC-1), and rhodamine 123 (R123) were used to detect changes in ΔΨm using an LSCM, fluorescence plate reader, and FCM. The decrease in ΔΨm caused by H₂O₂ was determined by endpoint and dynamic analyses after staining with JC-1 or TMRE. With the R123 probe, the LSCM could only detect the change in ΔΨm caused by 1000 μM H₂O₂. Moreover, R123 was less effective than JC-1 and TMRE for measurement of ΔΨm by the LSCM. Our data indicated that an LSCM is the most suitable instrument to detect dynamic changes in ΔΨm, whereas all three instruments can detect ΔΨm at the endpoint.

Administration of Vitamin D Metabolites Affects RNA Expression of Xenobiotic Metabolising Enzymes and Function of ABC Transporters in Rats

From studies on different species and in cell culture systems, it has been suggested that vitamin D metabolites might affect the metabolism and elimination of xenobiotics. Although most studies performed on rodents and cell cultures report an upregulation of respective enzymes and transporters, data from the literature are inconsistent. Especially results obtained with sheep differ from these observations. As vitamin D metabolites are widely used as feed additives or therapeutics in livestock animals, we aimed to assess whether these differences indicate species-specific responses or occurred due to the very high dosages used in the rodent studies. Therefore, we applied treatment protocols to rats that had been used previously in sheep or cattle. Forty-eight female rats were divided into three treatment and corresponding placebo groups: (1) a single intraperitoneal injection of 1,25-(OH)2D3 or placebo 12 h before sacrifice; (2) daily supplementation with 25-OHD3 by oral gavage or placebo for 10 days; and (3) a single intramuscular injection of vitamin D3 10 days before sacrifice. In contrast to a previous study using sheep, treatment of rats with 1,25-dihydroxyvitamin D3 did not result in an upregulation of cytochrome P450 3A isoenzymes (CYP3A), but a decrease was found in hepatic and intestinal expressions. In addition, a downregulation of P-glycoprotein (P-gp) and breast cancer resistance protein was found in the brain. Taken together, the stimulating effects of vitamin D metabolites on the expression of genes involved in the metabolism and elimination of xenobiotics reported previously for rodents and sheep could not be reproduced. In contrast, we even observed a negative impact on the expression of CYP3A enzymes and their most important regulator, the pregnane X receptor. Most interestingly, we could demonstrate an effect of treatment with 25-hydroxyvitamin D3 and vitamin D3 on the functional activity of ileal P-glycoprotein (P-gp) using the Ussing chamber technique.

Leishmania amazonensis: Increase in ecto-ATPase activity and parasite burden of vinblastine-resistant protozoa

Leishmania amazonensis is a protozoan parasite that induces mucocutaneous and diffuse cutaneous lesions upon infection. An important component in treatment failure is the emergence of drug-resistant parasites. It is necessary to clarify the mechanism of resistance that occurs in these parasites to develop effective drugs for leishmaniasis treatment. Promastigote forms of L. amazonensis were selected by gradually increasing concentrations of vinblastine and were maintained under continuous drug pressure (resistant cells). Vinblastine-resistant L. amazonensis proliferated similarly to control parasites. However, resistant cells showed changes in the cell shape, irregular flagella and a decrease in rhodamine 123 accumulation, which are factors associated with the development of resistance, suggesting the MDR phenotype. The Mg-dependent-ecto-ATPase, an enzyme located on cell surface of Leishmania parasites, is involved in the acquisition of purine and participates in the adhesion and infectivity process. We compared control and resistant L. amazonensis ecto-enzymatic activities. The control and resistant Leishmania ecto-ATPase activities were 16.0 ± 1.5 nmol Pi × h(-1) × 10(-7) cells and 40.0 ± 4.4 nmol Pi × h(-1) × 10(-7)cells, respectively. Interestingly, the activity of other ecto-enzymes present on the L. amazonensis cell surface, the ecto-5' and 3'-nucleotidases and ecto-phosphatase, did not increase. The level of ecto-ATPase modulation is related to the degree of resistance of the cell. Cells resistant to 10 μM and 60 μM of vinblastine have ecto-ATPase activities of 22.7 ± 0.4 nmol Pi × h(-1) × 10(-7) cells and 33.8 ± 0.8 nmol Pi × h(-1) × 10(-7)cells, respectively. In vivo experiments showed that both lesion size and parasite burden in mice infected with resistant parasites are greater than those of L. amazonensis control cells. Furthermore, our data established a relationship between the increase in ecto-ATPase activity and greater infectivity and severity of the disease caused by vinblastine-resistant L. amazonensis promastigotes. Taken together, these data suggest that ecto-enzymes could be potential therapeutic targets in the struggle against the spread of leishmaniasis, a neglected world-wide public health problem.

Mutational analysis of P-glycoprotein: suppression of caspase activation in the absence of ATP-dependent drug efflux

P-glycoprotein (P-gp) can induce multidrug resistance (MDR) through the ATP-dependent efflux of chemotherapeutic agents. We have previously shown that P-gp can inhibit nondrug apoptotic stimuli by suppressing the activation of caspases. To determine if this additional activity is functionally linked to ATP hydrolysis, we expressed wild-type and ATPase-mutant P-gp and showed that cells expressing mutant P-gp could not efflux chemotherapeutic drugs but remained relatively resistant to apoptosis. CEM lymphoma cells expressing mutant P-gp treated with vincristine showed a decrease in the fraction of cells with apoptotic morphology, cytochrome c release from the mitochondria and suppression of caspase activation, yet still accumulated in mitosis and showed a loss of clonogenic potential. The loss of clonogenicity in vincristine-treated cells expressing mutant P-gp was associated with accumulation of cells in mitosis and the presence of multinucleated cells consistent with mitotic catastrophe. The antiapoptotic effect of mutant P-gp was not affected by antibodies that inhibit the efflux function of the protein. These data are consistent with a dual activity model for P-gp-induced MDR involving both ATPase-dependent drug efflux and ATPase-independent inhibition of apoptosis. The structure-function analyses described herein provide novel insight into the mechanisms of action of P-gp in mediating MDR.

Specificity of doxorubicin versus rhodamine-123 in assessing P-glycoprotein functionality in the LLC-PK1, LLC-PK1:MDR1 and Caco-2 cell lines

The LLC-PK1:MDR1, LLC-PK1 and Caco-2 cell lines were used to investigate whether rhodamine-123 or doxorubicin would be the preferred substrate to study P-glycoprotein (P-gp) functionality in vitro. Both rhodamine-123 and doxorubicin showed highly polarised transport in the Caco-2 cell line and the LLC-PK1:MDR1 cell line, indicating that P-gp is actively transporting these drugs. However, for rhodamine-123 polarised transport was also seen in the monolayers of the wild-type LLC-PK1 cell line, indicating the presence of another active transporter for this compound. Polarised transport of doxorubicin in the Caco-2 and the LLC-PK1:MDR1 cell lines could be inhibited by the P-gp inhibitors SDZ-PSC 833 (PSC 833), cyclosporin A (CsA), verapamil and quinine, but not by the inhibitors for the organic cation carrier systems cimetidine and tetraethylammonium (TEA). Polarised transport of rhodamine-123 in the Caco-2 cell line could only be inhibited by P-gp inhibitors. In the LLC-PK1:MDR1 and LLC-PK1 cell lines transport was also inhibited by inhibitors for the organic cation transport systems. In conclusion, rhodamine-123 is a substrate for both P-gp and the organic cation carrier systems in the kidney cell line. This indicates that rhodamine-123 is not selective enough to study P-gp functionality in cell systems were organic cation carrier systems are also present. Doxorubicin appears to be a more selective P-gp substrate and therefore more useful in studying P-gp functionality in vitro.