P-glycoprotein in acute myeloid leukaemia: therapeutic implications of its association with both a multidrug-resistant and an apoptosis-resistant phenotype

Understanding mechanisms of resistance to FLT3 inhibitors in adult FLT3-mutated acute myeloid leukemia to guide treatment strategy

The presence of FLT3 mutations, including the most common FLT3-ITD (internal tandem duplications) and FLT3-TKD (tyrosine kinase domain), is associated with an unfavorable prognosis in patients affected by acute myeloid leukemia (AML). In this setting, in recent years, new FLT3 inhibitors have demonstrated efficacy in improving survival and treatment response. Nevertheless, the development of primary and secondary mechanisms of resistance poses a significant obstacle to their efficacy. Understanding these mechanisms is crucial for developing novel therapeutic approaches to overcome resistance and improve the outcomes of patients. In this context, the use of novel FLT3 inhibitors and the combination of different targeted therapies have been studied. This review provides an update on the molecular alterations involved in the resistance to FLT3 inhibitors, and describes how the molecular monitoring may be used to guide treatment strategy in FLT3-mutated AML.

The influence of BCL2, BAX, and ABCB1 gene expression on prognosis of adult de novo acute myeloid leukemia with normal karyotype patients

BACKGROUND

Deregulation of the apoptotic process underlies the pathogenesis of many cancers, including leukemia, but is also very important for the success of chemotherapy treatment. Therefore, the gene expression profile of main apoptotic factors, such as anti-apoptotic BCL2 (B-cell lymphoma protein 2) and pro-apoptotic BAX (BCL2-associated X), as well as genes involved in the multi-drug resistance (ABCB1), could have significant impact on the prognosis and could be used as targets for specific therapy.

PATIENTS AND METHODS

We analyzed the expression of BCL2, BAX, and ABCB1 in bone-marrow samples collected at diagnosis from 51 adult patients with acute myeloid leukemia with normal karyotype (AML-NK) using real-time polymerase chain reaction method, and examined their prognostic potential.

RESULTS

Increased expression of BCL2 (BCL2 ⁺) was associated with the presence of chemoresistance (p = 0.024), while patients with low BAX expression were more prone to relapse (p = 0.047). Analysis of the combined effect of BCL2 and BAX expression showed that 87% of patients with BAX/BCL2 ^low status were resistant to therapy (p = 0.044). High expression of ABCB1 was associated with BCL2 ⁺ status (p < 0.001), and with absence FLT3-ITD mutations (p = 0.019).

CONCLUSIONS

The present analysis of BCL2, BAX, and ABCB1 gene expression profiles is the first study focusing solely on AML-NK patients. Preliminary results showed that patients with high BCL2 expression are likely to experience resistance to chemotherapy, and may benefit from specific anti-BCL2 treatment. Further investigations conducted on a larger number of patients could elucidate actual prognostic significance of these genes in AML-NK patients.

Gambogenic Acid Inhibits Basal Autophagy of Drug-Resistant Hepatoma Cells and Improves Its Sensitivity to Adriamycin

Gambogenic acid (GNA) is extracted from plant Gamboge, has a wide range of anti-tumor effects. In this paper, we study the inhibitory effect of GNA on the BEL-7402/ADM of hepatoma resistant cell lines and further study the mechanism of action. Cell viability test represented that GNA could improve the sensitivity of hepatoma drug-resistant cell line BEL-7402/ADM to Adriamycin (ADM), and further study by 4',6-diamidino-2-phenylindole (DAPI) staining and flow cytometry found that GNA could improve the effect of ADM on promoting apoptosis in BEL-7402/ADM cells, and the activation of apoptosis-related protein was significantly increased, and the ratio of Bax to Bcl-2 was significantly increased. Monodansylcadaverine staining and transmission electron microscopy showed that the basal autophagy level of BEL-7402/ADM cells was higher than that of BEL-7402 cells. Further detection of protein expression found that the intracellular LC3-II to LC3-I ratio and Beclin 1 protein expression increased in the combination of GNA and ADM, but the protein level of p62 increased significantly. GNA inhibit protective autophagy in BEL-7402/ADM cells and promote the role of ADM in inducing apoptosis, thereby increasing ADM sensitivity to BEL-7402/ADM cells, and the effect of GNA inhibition of autophagy may be achieved by inhibiting the degradation of autophagosomes.

Emerging Nano-Based Strategies Against Drug Resistance in Tumor Chemotherapy

Drug resistance is the most significant causes of cancer chemotherapy failure. Various mechanisms of drug resistance include tumor heterogeneity, tumor microenvironment, changes at cellular levels, genetic factors, and other mechanisms. In recent years, more attention has been paid to tumor resistance mechanisms and countermeasures. Nanomedicine is an emerging treatment platform, focusing on alternative drug delivery and improved therapeutic effectiveness while reducing side effects on normal tissues. Here, we reviewed the principal forms of drug resistance and the new possibilities that nanomaterials offer for overcoming these therapeutic barriers. Novel nanomaterials based on tumor types are an excellent modality to equalize drug resistance that enables gain more rational and flexible drug selectivity for individual patient treatment. With the emergence of advanced designs and alternative drug delivery strategies with different nanomaterials, overcome of multidrug resistance shows promising and opens new horizons for cancer therapy. This review discussed different mechanisms of drug resistance and recent advances in nanotechnology-based therapeutic strategies to improve the sensitivity and effectiveness of chemotherapeutic drugs, aiming to show the advantages of nanomaterials in overcoming of drug resistance for tumor chemotherapy, which could accelerate the development of personalized medicine.

A comparative study of anti-leukemic effects of kaempferol and epigallocatechin-3-gallate (EGCG) on human leukemia HL-60 cells

OBJECTIVE

Acute promyelocytic leukemia (APL) is among the most threatening hematological malignant cancers. Defects in cell growth and apoptotic pathways lead to the pathogenesis of the disease as well as its resistance to therapy; therefore, it is a good model for examining pro-apoptotic agents. The present study compared the molecular mechanism induced by kaempferol and epigallocatechin gallate (EGCG) as well as all-trans retinoic acid (ATRA), in HL-60 leukemia cells during five days.

MATERIALS AND METHODS

Cell viability was determined by resazurin assay following treatment with ATRA (10 µM), EGCG, and kaempferol (12.5-100 µM), and apoptosis was detected by the ANX V/PI kit. Moreover, the levels of genes involved in apoptosis (PI3K, AKT, BCL2, BAX, P21, PTEN, CASP3, CASP8, and CASP9) and multi-drug resistance (MDR, ABCB1 and ABCC1) were assessed by using real-time PCR test.

RESULTS

Based on the findings, kaempferol decreased cell viability and increased apoptosis in HL60 cells more than EGCG. Apoptosis was induced via extrinsic and intrinsic pathways in HL60 cells by kaempferol and EGCG. In addition, kaempferol and EGCG increased apoptosis and inhibited MDR in a concentration- and time-dependent manner.

CONCLUSION

Kaempferol at high concentrations can be taken into consideration for treating patients with APL as compared with EGCG.

2-Thiopyrimidine/chalcone hybrids: design, synthesis, ADMET prediction, and anticancer evaluation as STAT3/STAT5a inhibitors

A novel 2-thiopyrimidine/chalcone hybrid was designed, synthesised, and evaluated for their cytotoxic activities against three different cell lines, K-562, MCF-7, and HT-29. The most active cytotoxic derivatives were 9d, 9f, 9n, and 9p (IC50=0.77-1.74 µM, against K-562 cell line), 9a and 9r (IC50=1.37-3.56 µM against MCF-7 cell line), and 9a, 9l, and 9n (IC50=2.10 and 2.37 µM against HT-29 cell line). Compounds 9a, 9d, 9f, 9n, and 9r were further evaluated for their cytotoxicity against normal fibroblast cell line WI38. Moreover, STAT3 and STAT5a inhibitory activities were determined for the most active derivatives 9a, 9d, 9f, 9n, and 9r. Dual inhibitory activity was observed in compound 9n (IC50=113.31 and 50.75 µM, against STAT3 and STAT5a, respectively). Prediction of physicochemical properties, drug likeness score, pharmacokinetic and toxic properties was detected.

Study of the mechanisms of crocetin-induced differentiation and apoptosis in human acute promyelocytic leukemia cells

Crocetin, the major carotenoid in saffron, exhibits potent anticancer effects. However, the antileukemic effects of crocetin are still unclear, especially in primary acute promyelocytic leukemia (APL) cells. In the current study, the potential antipromyelocytic leukemia activity of crocetin and the underlying molecular mechanisms were investigated. Crocetin (100 µM), like standard anti-APL drugs, all-trans retinoic acid (ATRA, 10 µM) and As₂ O ₃ (arsenic trioxide, 50 µM), significantly inhibited proliferation and induced apoptosis in primary APL cells, as well as NB4 and HL60 cells. The effect was associated with the decreased expressions of prosurvival genes Akt and BCL2, the multidrug resistance (MDR) proteins, ABCB1 and ABCC1 and the inhibition of tyrosyl-DNA phosphodiesterase 1 (TDP1), while the expressions of proapoptotic genes CASP3, CASP9, and BAX/BCL2 ratio were significantly increased. In contrast, crocetin at relatively low concentration (10 µM), like ATRA (1 µM) and As ₂ O ₃ (0.5 µM), induced differentiation of leukemic cells toward granulocytic pattern, and increased the number of differentiated cells expressing CD11b and CD14, while the number of the immature cells expressing CD34 or CD33 was decreased. Furthermore, crocetin suppressed the expression of clinical marker promyelocytic leukemia/retinoic acid receptor-α ( PML/RARα) in NB4 and primary APL cells, and reduced the expression of histone deacetylase 1 ( HDAC1) in all leukemic cells. The results suggested that crocetin can be considered as a candidate for future preclinical and clinical trials of complementary APL treatment.

1,3,4-oxadiazole/chalcone hybrids: Design, synthesis, and inhibition of leukemia cell growth and EGFR, Src, IL-6 and STAT3 activities

A new series of 1,3,4-oxadiazole/chalcone hybrids was designed, synthesized, identified with different spectroscopic techniques and biologically evaluated as inhibitors of EGFR, Src, and IL-6. The synthesized compounds showed promising anticancer activity, particularly against leukemia, with 8v being the most potent. The synthesized compounds exhibited strong to moderate cytotoxic activities against K-562, KG-1a, and Jurkat leukemia cell lines in MTT assays. Compound 8v showed the strongest cytotoxic activity with IC₅₀ of 1.95 µM, 2.36 µM and 3.45 µM against K-562, Jurkat and KG-1a leukemia cell lines, respectively. Moreover; the synthesized compounds inhibited EGFR, Src, and IL-6. Compound 8v was most effective at inhibiting EGFR (IC₅₀ = 0.24 μM), Src (IC₅₀ = 0.96 μM), and IL-6 (% of control = 20%). Additionally, most of the compounds decreased STAT3 activation.

Blockade of JAK2/STAT3 intensifies the anti-tumor activity of arsenic trioxide in acute myeloid leukemia cells: Novel synergistic mechanism via the mediation of reactive oxygen species

Reactive oxygen species (ROS) are essential mediators of crucial cellular processes including apoptosis, proliferation, survival and cell cycle. Their regulatory role in cancer progression has seen in different human malignancies such as acute myeloid leukemia (AML). AML patients suffer from high resistance of the tumors against routine therapeutics including ATO. ATO enhance reactive oxygen species levels and induce apoptosis and suppresses proliferation in AML cells. However, some pathways such as JAK2/STAT3 ease anti-tumor activity of ATO by reducing reactive oxygen species amount and protecting the cell from apoptosis. In the present study, we use ruxolitinib (potent JAK2 inhibitor) to increase the sensitivity of AML cells to ATO treatment. We test, the effect of this combination on metabolic activity, proliferation, colony formation, cell cycle distribution, apoptosis, oxidative stress and DNA damage. Our results showed that combination of ATO with ruxolitinib synergistically reduced metabolic activity, proliferation and survival of AML cell lines. This combination induced G1/S cell cycle arrest because of reactive oxygen species elevation and GSH reduction. Besides, enhancement of reactive oxygen species increased apoptosis rate in combination samples. We uncovered that the synergistic anti-tumor effect of ATO and ruxolitinib in AML cells mediates via reactive oxygen species elevation and DNA damage. Overall, our results show that the combinatorial therapy of AML cells is more effective than solo-targeted therapy.

Expression of some ATP-binding cassette transporters in acute myeloid leukemia

Hematopoietic cells express ATP binding cassette (ABC) transporters in relation to different degrees of differentiation. One of the known multidrug resistance mechanisms in acute myeloid leukemia (AML) is the overexpression of efflux pumps belonging to the superfamily of ABC transporters such as ABCB1, ABCG2 and ABCC1. Although several studies were carried out to correlate ABC transporters expression with drug resistance, little is known about their role as markers of diagnosis and progression of the disease. For this purpose we investigated the expression, by real-time PCR, of some ABC genes in bone marrow samples of AML patients at diagnosis and after induction therapy. At diagnosis, ABCG2 was always down-regulated, while an up regulated trend for ABCC1 was observed. After therapy the examined genes showed a different expression trend and approached the values of healthy subjects suggesting that this event could be considered as a marker of AML regression. The expression levels of some ABC transporters such as ABCC6, seems to be related to gender, age and to the presence of FLT3/ITD gene mutation.

P-glycoprotein (ABCB1) and Oxidative Stress: Focus on Alzheimer's Disease

ATP-binding cassette (ABC) transporters, in particular P-glycoprotein (encoded by ABCB1), are important and selective elements of the blood-brain barrier (BBB), and they actively contribute to brain homeostasis. Changes in ABCB1 expression and/or function at the BBB may not only alter the expression and function of other molecules at the BBB but also affect brain environment. Over the last decade, a number of reports have shown that ABCB1 actively mediates the transport of beta amyloid (Aβ) peptide. This finding has opened up an entirely new line of research in the field of Alzheimer's disease (AD). Indeed, despite intense research efforts, AD remains an unsolved pathology and effective therapies are still unavailable. Here, we review the crucial role of ABCB1 in the Aβ transport and how oxidative stress may interfere with this process. A detailed understanding of ABCB1 regulation can provide the basis for improved neuroprotection in AD and also enhanced therapeutic drug delivery to the brain.

Kaempferol increases apoptosis in human acute promyelocytic leukemia cells and inhibits multidrug resistance genes

Acute promyelocytic leukemia (APL) is one of the most life-threatening hematological malignancies. Defects in the cell growth and apoptotic pathways are responsible for both disease pathogenesis and treatment resistance. Therefore, pro-apoptotic agents are potential candidates for APL treatment. Kaempferol is a flavonoid with antioxidant and anti-tumor properties. This study was designed to investigate the cytotoxic, pro-apoptotic, and differentiation-inducing effects of kaempferol on HL-60 and NB4 leukemia cells. Resazurin assay was used to determine cell viability following treatment with kaempferol (12.5-100 μM) and all-trans retinoic acid (ATRA; 10 μM; used as a positive control). Apoptosis and differentiation were also detected using propidium iodide and NBT staining techniques, respectively. Furthermore, the expression levels of genes involved in apoptosis (PI3 K, AKT, BCL2, BAX, p53, p21, PTEN, CASP3, CASP8, and CASP9), differentiation (PML-RAR and HDAC1), and multi-drug resistance (ABCB1 and ABCC1) were determined using quantitative real-time PCR. The protein expressions of Bax/Bcl2 and casp3 were confirmed using Western blot. The results showed that kaempferol decreased cell viability and increased subG1 population in the tested leukemic cells. This effect was associated with decreased expression of Akt, BCL2, ABCB1, and ABCC1 genes, while the expression of CASP3 and BAX/BCL-2 ratio were significantly increased at both gene and protein levels. Kaempferol promoted apoptosis and inhibited multidrug resistance in a concentration-dependent manner, without any differential effect on leukemic cells. In conclusion, this study suggested that kaempferol may be utilized as an appropriate alternative for ATRA in APL patients.

Chemotherapy-Resistant Human Acute Myeloid Leukemia Cells Are Not Enriched for Leukemic Stem Cells but Require Oxidative Metabolism

Chemotherapy-resistant human acute myeloid leukemia (AML) cells are thought to be enriched in quiescent immature leukemic stem cells (LSC). To validate this hypothesis in vivo, we developed a clinically relevant chemotherapeutic approach treating patient-derived xenografts (PDX) with cytarabine (AraC). AraC residual AML cells are enriched in neither immature, quiescent cells nor LSCs. Strikingly, AraC-resistant preexisting and persisting cells displayed high levels of reactive oxygen species, showed increased mitochondrial mass, and retained active polarized mitochondria, consistent with a high oxidative phosphorylation (OXPHOS) status. AraC residual cells exhibited increased fatty-acid oxidation, upregulated CD36 expression, and a high OXPHOS gene signature predictive for treatment response in PDX and patients with AML. High OXPHOS but not low OXPHOS human AML cell lines were chemoresistant in vivo. Targeting mitochondrial protein synthesis, electron transfer, or fatty-acid oxidation induced an energetic shift toward low OXPHOS and markedly enhanced antileukemic effects of AraC. Together, this study demonstrates that essential mitochondrial functions contribute to AraC resistance in AML and are a robust hallmark of AraC sensitivity and a promising therapeutic avenue to treat AML residual disease.Significance: AraC-resistant AML cells exhibit metabolic features and gene signatures consistent with a high OXPHOS status. In these cells, targeting mitochondrial metabolism through the CD36-FAO-OXPHOS axis induces an energetic shift toward low OXPHOS and strongly enhanced antileukemic effects of AraC, offering a promising avenue to design new therapeutic strategies and fight AraC resistance in AML. Cancer Discov; 7(7); 716-35. ©2017 AACR.See related commentary by Schimmer, p. 670This article is highlighted in the In This Issue feature, p. 653.

Turning the gun on cancer: Utilizing lysosomal P-glycoprotein as a new strategy to overcome multi-drug resistance

Oxidative stress plays a role in the development of drug resistance in cancer cells. Cancer cells must constantly and rapidly adapt to changes in the tumor microenvironment, due to alterations in the availability of nutrients, such as glucose, oxygen and key transition metals (e.g., iron and copper). This nutrient flux is typically a consequence of rapid growth, poor vascularization and necrosis. It has been demonstrated that stress factors, such as hypoxia and glucose deprivation up-regulate master transcription factors, namely hypoxia inducible factor-1α (HIF-1α), which transcriptionally regulate the multi-drug resistance (MDR), transmembrane drug efflux transporter, P-glycoprotein (Pgp). Interestingly, in addition to the established role of plasma membrane Pgp in MDR, a new paradigm of intracellular resistance has emerged that is premised on the ability of lysosomal Pgp to transport cytotoxic agents into this organelle. This mechanism is enabled by the topological inversion of Pgp via endocytosis resulting in the transporter actively pumping agents into the lysosome. In this way, classical Pgp substrates, such as doxorubicin (DOX), can be actively transported into this organelle. Within the lysosome, DOX becomes protonated upon acidification of the lysosomal lumen, causing its accumulation. This mechanism efficiently traps DOX, preventing its cytotoxic interaction with nuclear DNA. This review discusses these effects and highlights a novel mechanism by which redox-active and protonatable Pgp substrates can utilize lysosomal Pgp to gain access to this compartment, resulting in catastrophic lysosomal membrane permeabilization and cell death. Hence, a key MDR mechanism that utilizes Pgp (the "gun") to sequester protonatable drug substrates safely within lysosomes can be "turned on" MDR cancer cells to destroy them from within.

Research Progress on the Role of ABC Transporters in the Drug Resistance Mechanism of Intractable Epilepsy

The pathogenesis of intractable epilepsy is not fully clear. In recent years, both animal and clinical trials have shown that the expression of ATP-binding cassette (ABC) transporters is increased in patients with intractable epilepsy; additionally, epileptic seizures can lead to an increase in the number of sites that express ABC transporters. These findings suggest that ABC transporters play an important role in the drug resistance mechanism of epilepsy. ABC transporters can perform the funcions of a drug efflux pump, which can reduce the effective drug concentration at epilepsy lesions by reducing the permeability of the blood brain barrier to antiepileptic drugs, thus causing resistance to antiepileptic drugs. Given the important role of ABC transporters in refractory epilepsy drug resistance, antiepileptic drugs that are not substrates of ABC transporters were used to obtain ABC transporter inhibitors with strong specificity, high safety, and few side effects, making them suitable for long-term use; therefore, these drugs can be used for future clinical treatment of intractable epilepsy.

Identification of prognostic subgroups among acute myeloid leukemia patients with intermediate risk cytogenetics using a flow-cytometry-based assessment of ABC-transporter function

BACKGROUND

A median fluorescence intensity ratio (MFIR) which measures the efflux of mitoxantrone (an ATP Binding Cassette (ABC) transporter substrate) with and without ABC transporter inhibition correlates with expression of MDR1 and BCRP in acute myeloid leukemia (AML) blasts.

METHODS

This study evaluates the impacts of the MFIR on AML outcomes and its interaction with detection of the FLT3 ITD.

RESULTS

Among 200 newly diagnosed AML patients, an MFIR of ≥ 1.9 (MFIR+) was detected in 60 (30%) leukemic blast samples. In multivariate analysis, MFIR was an independent prognostic factor for response to induction chemotherapy (OR=7.2, P<0.00001), DFS (HR=2.3, P=0.004) and OS (HR=2.2, P=0.0005) with the main effect being in the 141 patients with intermediate risk cytogenetics. Among intermediate risk cytogenetics patients: MFIR+ outcomes were similar to unfavorable cytogenetic risk (CR, 53% vs. 52%, P=1.0; OS, 11 vs. 9 months, P=0.79). MFIR status can further stratify the prognostic risk for patients with or without FLT3 ITD mutation.

CONCLUSIONS

MFIR has value in predicting outcomes including DFS and OS as well as induction failure. This is particularly true for patients with intermediate risk cytogenetics and when combined with assessment for the FLT3-ITD mutation.

Progress in defining multidrug resistance in leukemia

Multidrug resistance (MDR) is a naturally occurring defense phenomenon by which cells battle against chemically foreign substances (xenobiotics), including some cytotoxic drugs. Membrane transporter hyperactivity is a major contributor to MDR and is the primary target of both diagnostic and therapeutic interventions. Multi-xenobiotic resistance can be exploited as several fluorescent indicator probes are extruded by the same drug transporters, making it possible to quantitatively measure MDR activity in cell lines and clinical samples by flow cytometry. The literature on MDR is reported in a number of different formats, making it difficult to compare data from various groups. This article will briefly review the pathomechanism, then focus upon the diagnostic approach, the interpretation of results from clinical samples and correlations with other variables. The authors believe that a standardized MDR assay, as well as a suitable monitoring test, may become a prognostic marker in several types of leukemia.

Investigation of the mechanism involved in the As2O3-regulated decrease in MDR1 expression in leukemia cells

Arsenic trioxide (As2O3) inhibits the expression of P-glycoprotein (P-gp) in leukemia cells; however, the mechanism behind this inhibition is unclear. The present study aimed to explore the effect of As2O3 on the expression and regulation of P-gp in leukemia cells, and elucidate the mechanism of the reversal of drug resistance. In the present study, electrophoretic mobility shift assay results indicated that p65 binds to the NF-κB binding site of MDR1, specifically in K562/D cells. Expression of p65 and phosphorylated IκB was reduced, while the expression of IκB was increased in K562/D cells treated with As2O3. The activity of luciferase increased up to 9-fold with 40 ng/ml TNF-α, and it was suppressed by ~25% following treatment with 1 µM As2O3. These findings suggest that As2O3 reverses the P-gp-induced drug resistance of leukemia cells through the NF-κB pathway. As2O3 may inhibit the activity of phosphorylase to inhibit IκB phosphorylation, thereby inhibiting NF-κB activity and MDR1 gene expression, leading to reversal of drug resistance.

Distinct poor prognostic subgroups of acute myeloid leukaemia, FLT3-ITD and P-glycoprotein-positive, have contrasting levels of FOXO1

Regulation of ABCB1 (P-glycoprotein/Pgp) in AML was investigated. In a historical cohort with Pgp and transcriptional regulator expression profiling data available (n=141), FOXO1 correlated with Pgp protein expression. This was confirmed in an independent cohort (n=204). Down-regulation (siRNA) or hyperactivation (nicotinamide) of FOXO1 led to corresponding changes in Pgp. Low FOXO1 expression correlated with FLT3-ITDs (p<0.001) and siRNA inhibition of FLT3-ITD up-regulated FOXO1. As FOXO1 is a key growth regulator, it may underpin biological differences between Pgp-positive clones (low WBC and primary resistant disease) and clones with a FLT3-ITD (associated with a high WBC and early relapse).

FLT3-ITD and MLL-PTD influence the expression of MDR-1, MRP-1, and BCRP mRNA but not LRP mRNA assessed with RQ-PCR method in adult acute myeloid leukemia

Fms-like tyrosine kinase 3-internal tandem duplication (FLT3-ITD) and mixed-lineage leukemia gene-partial tandem duplication (MLL-PTD) are aberrations associated with leukemia which indicate unsatisfactory prognosis. Downstream regulatory targets of FLT3-ITD and MLL-PTD are not well defined. We have analyzed the expression of MDR-1, multidrug resistant protein-1 (MRP-1), breast cancer resistance protein (BCRP), and lung resistance protein (LRP) messenger RNA (mRNA) in relation to the mutational status of FLT3-ITD and MLL-PTD in 185 acute myeloid leukemia (AML) adult patients. The real-time quantitative polymerase chain reaction method was performed to assess the expression of the MDR-1, MRP-1, BCRP, and LRP mRNA, and the results were presented as coefficients calculated using an intermediate method according to Pfaffl's rule. Significantly higher expressions of MDR-1 mRNA were found in patients who did not harbor FLT3-ITD (0.20 vs. 0.05; p = 0.0001) and MRP-1 mRNA in patients with this mutation (0.96 vs. 0.70; p = 0.002) and of BCRP mRNA in patients with MLL-PTD (0.61 vs. 0.38; p = 0.03). In univariate analysis, the high expression of MDR-1 mRNA (≥0.1317) negatively influenced the outcome of induction therapy (p = 0.05), whereas the high expression of BCRP mRNA (≥1.1487) was associated with a high relapse rate (RR) (p = 0.013). We found that the high expression of MDR-1 (≥0.1317), MRP-1 (≥0.8409), and BCRP mRNA (≥1.1487) significantly influenced disease-free survival (DFS; p = 0.059, 0.032, and 0.009, respectively) and overall survival (0.048, 0.014, and 0.059, respectively). Moreover, a high expression of BCRP mRNA (≥1.1487) proved to be an independent prognostic factor for RR (p = 0.01) and DFS (p = 0.002) in multivariate analysis. The significant correlation between the expression of MDR-1, MRP-1, and BCRP mRNA and FLT3-ITD or MLL-PTD in AML patients requires further investigation.

P-glycoprotein-dependent resistance of cancer cells toward the extrinsic TRAIL apoptosis signaling pathway

The TNF-related apoptosis-inducing ligand (TRAIL or Apo2L) preferentially cause apoptosis of malignant cells in vitro and in vivo without severe toxicity. Therefore, TRAIL or agonist antibodies to the TRAIL DR4 and DR5 receptors are used in cancer therapy. However, many malignant cells are intrinsically resistant or acquire resistance to TRAIL. It has been previously proposed that the multidrug transporter P-glycoprotein (Pgp) might play a role in resistance of cells to intrinsic apoptotic pathways by interfering with components of ceramide metabolism or by modulating the electrochemical gradient across the plasma membrane. In this study we investigated whether Pgp also confers resistance toward extrinsic death ligands of the TNF family. To this end we focused our study on HeLa cells carrying a tetracycline-repressible plasmid system which shuts down Pgp expression in the presence of tetracycline. Our findings demonstrate that expression of Pgp is a significant factor conferring resistance to TRAIL administration, but not to other death ligands such as TNF-α and Fas ligand. Moreover, blocking Pgp transport activity sensitizes the malignant cells toward TRAIL. Therefore, Pgp transport function is required to confer resistance to TRAIL. Although the resistance to TRAIL-induced apoptosis is Pgp specific, TRAIL itself is not a direct substrate of Pgp. Pgp expression has no effect on the level of the TRAIL receptors DR4 and DR5. These findings might have clinical implications since the combination of TRAIL therapy with administration of Pgp modulators might sensitize TRAIL resistant tumors.

Murine P-glycoprotein deficiency alters intestinal injury repair and blunts lipopolysaccharide-induced radioprotection

P-glycoprotein (P-gp) has been reported to increase stem cell proliferation and regulate apoptosis. Absence of P-gp results in decreased repair of intestinal epithelial cells after chemical injury. To further explore the mechanisms involved in the effects of P-gp on intestinal injury and repair, we used the well-characterized radiation injury model. In this model, injury repair is mediated by production of prostaglandins (PGE(2)) and lipopolysaccharide (LPS) has been shown to confer radioprotection. B6.mdr1a(-/-) mice and wild-type controls were subjected to 12 Gy total body X-ray irradiation and surviving crypts in the proximal jejunum and distal colon were evaluated 3.5 days after irradiation. B6.mdr1a(-/-) mice exhibited normal baseline stem cell proliferation and COX dependent crypt regeneration after irradiation. However, radiation induced apoptosis was increased and LPS-induced radioprotection was blunted in the C57BL6.mdr1a(-/-) distal colon, compared to B6 wild-type controls. The LPS treatment induced gene expression of the radioprotective cytokine IL-1α, in B6 wild-type controls but not in B6.mdr1a(-/-) animals. Lipopolysaccharid-induced radioprotection was absent in IL-1R1(-/-) animals, indicating a role for IL-1α in radioprotection, and demonstrating that P-gp deficiency interferes with IL-1α gene expression in response to systemic exposure to LPS.

Cerebral expression of drug transporters in epilepsy

Over-expression of drug efflux transporters at the level of the blood-brain barrier (BBB) has been proposed as a mechanism responsible for multidrug resistance. Drug transporters in epileptogenic tissue are not only expressed in endothelial cells at the BBB, but also in other brain parenchymal cells, such as astrocytes, microglia and neurons, suggesting a complex cell type-specific regulation under pathological conditions associated with epilepsy. This review focuses on the cerebral expression patterns of several classes of well-known membrane drug transporters such as P-glycoprotein (Pgp), and multidrug resistance-associated proteins (MRPs) in the epileptogenic brain. Both experimental and clinical evidence of epilepsy-associated cerebral drug transporter regulation and the possible mechanisms underlying drug transporter regulation are discussed. Knowledge of the cerebral expression patterns of drug transporters in normal and epileptogenic brain will provide relevant information to guide strategies attempting to overcome drug resistance by targeting specific transporters.

Drug resistance: still a daunting challenge to the successful treatment of AML

Resistance to chemotherapy remains a challenging issue for patients and their physicians. P-glycoprotein (Pgp, MDR1, ABCB1), as well as a family of structurally and functionally related proteins, are plasma membrane transporters able to efflux a variety of substrates from the cell cytoplasm, including chemotherapeutic agents. The discovery of ABCB1 made available a potential target for pharmacologic down-regulation of efflux-mediated chemotherapy resistance. In patients with acute myeloid leukemia (AML), a neoplasm characterized by proliferation of poorly differentiated myeloid progenitor cells, leukemic cells often express ABCB1 at high levels, which may lead to the development of resistance to chemotherapy. Thus, AML seemed to be a likely cancer for which the addition of drug efflux inhibitors to the chemotherapeutic regimen would improve outcomes in patients. Despite this rational hypothesis, the majority of clinical trials evaluating this strategy have failed to reach a positive endpoint, most recently the Eastern Cooperative Oncology Group E3999 trial. Here we review data suggesting the importance of ABCB1 in AML, address the failure of clinical trials to support a therapeutic strategy aimed at modulating ABCB1-mediated resistance, and consider the type of research that should be conducted in this field going forward.

Synthesis of a dual functional anti-MDR tumor agent PH II-7 with elucidations of anti-tumor effects and mechanisms

Multidrug resistance mediated by P-glycoprotein in cancer cells has been a major issue that cripples the efficacy of chemotherapy agents. Aimed for improved efficacy against resistant cancer cells, we designed and synthesized 25 oxindole derivatives based on indirubin by structure-activity relationship analysis. The most potent one was named PH II-7, which was effective against 18 cancer cell lines and 5 resistant cell lines in MTT assay. It also significantly inhibited the resistant xenograft tumor growth in mouse model. In cell cycle assay and apoptosis assay conducted with flow cytometry, PH II-7 induced S phase cell cycle arrest and apoptosis even in resistant cells. Consistently revealed by real-time PCR, it modulates the expression of genes related to the cell cycle and apoptosis in these cells, which may contributes to its efficacy against them. By side-chain modification and FITC-labeling of PH II-7, we were able to show with confocal microscopy that not only it was not pumped by P-glycoprotein, it also attenuated the efflux of Adriamycin by P-glycoprotein in MDR tumor cells. Real-time PCR and western blot analysis showed that PH II-7 down-regulated MDR1 gene via protein kinase C alpha (PKCA) pathway, with c-FOS and c-JUN as possible mediators. Taken together, PH II-7 is a dual-functional compound that features both the cytotoxicity against cancer cells and the inhibitory effect on P-gp mediated drug efflux.

P-glycoprotein depresses cisplatin sensitivity in L1210 cells by inhibiting cisplatin-induced caspase-3 activation

Multidrug resistance (MDR) is a phenomenon in which cells become resistant to cytostatic drugs and other substances with diverse chemical structures and cytotoxicity mechanisms. The most often observed molecular mechanism for MDR includes high levels of P-glycoprotein (P-gp)--an ABCB1 member of the ABC drug transporter family. Overexpression of P-gp in neoplastic tissue is an obstacle to chemotherapeutic treatment. Herein, we were focused on differences in apoptosis induced by cisplatin (no substrate for P-gp) between P-gp-positive and P-gp-negative L1210 cells. P-gp-positive cells were obtained by either L1210 cell adaptation to vincristine (R) or L1210 cell transfection with the human gene for P-gp (T) and compared with parental L1210 cells (S). R and T cells were more resistant to CisPt than S cells. R and T cell resistance to CisPt-induced apoptosis could not be reversed by verapamil (a well-known P-gp inhibitor), which excludes P-gp transport activity as a cause of CisPt resistance. CisPt induced a more pronounced entry into apoptosis in S than R and T cells, which was measured using the annexin-V/propidium iodide apoptosis kit. CisPt induced more pronounced caspase-3 activation in S than R and T cells. CisPt did not induce changes in the P-gp protein level for R and T cells. While similar levels of Bax and Bcl-2 proteins were observed in P-gp-negative and P-gp-positive cells, CisPt induced a more significant decrease in Bcl-2 levels for S cells than P-gp-positive cells. Expression of p53 and its molecular chaperone Hsp90 were more pronounced in R and T than S cells. Moreover, CisPt enhanced the upregulation of p53 and Hsp90 in R and T cells to a higher degree than S cells. Apoptosis was shown to be the prevalent mode of cell death in S, R and T cells by the typical DNA fragmentation and cell ultrastructure changes. All of the above findings indicate that P-gp, independent of its drug efflux activity, induced changes in cell regulatory pathways that confer a partial loss of cisplatin sensitivity.

Critical role for P-glycoprotein expression in hematopoietic cells in the FVB.Mdr1a(-/-) model of colitis

OBJECTIVE

P-glycoprotein (P-gp), the functional product of the multidrug resistance gene (MDR), is a transmembrane protein that extrudes substrates from the intracellular environment. P-gp is expressed on the apical surface of epithelial cells and on cells from the hematopoietic lineage. Human MDR polymorphisms have been associated with the increased risk of inflammatory bowel disease, and FVB/N animals deficient in mdr1a expression develop spontaneous colitis. Previous studies using adult bone marrow chimeras indicated that colitis development in this animal model was contingent on P-gp deficiency in radiation-resistant epithelial cells; however, the use of adult animals may mask the role of hematopoietic immune cells in colitis initiation, due to preexisting epithelial abnormalities.

SUBJECTS AND METHODS

To assess the importance of P-gp expression in intestinal epithelial and hematopoietic-derived cells on colitis induction in FVB.mdr1a(-/-) animals, we developed a neonatal model of bone marrow reconstitution. FVB/N and FVB.mdr1a(-/-) adult and neonatal animals were lethally irradiated and reconstituted with bone marrow from FVB/N or FVB.mdr1a(-/-) donors. Animals were observed for 20 weeks.

RESULTS

Adult FVB/N animals deficient in P-gp expression in hematopoietically derived immune cells developed colitis similar to adult animals deficient in P-gp expression in radiation-resistant epithelial/stromal cells. Neonatal animals deficient in P-gp expression in hematopoietically derived immune cells developed a more histologically significant colitis than those deficient in P-gp expression in epithelial tissue.

CONCLUSIONS

The use of a neonatal model of bone marrow reconstitution has revealed a critical role for P-gp expression in hematopoietically derived immune cells in colitis development in the FVB.mdr1a(-/-) model.

Corpora amylacea deposition in the hippocampus of patients with mesial temporal lobe epilepsy: A new role for an old gene?

BACKGROUND

Mesial temporal lobe epilepsy (MTLE) is the most common medically refractory epilepsy syndrome in adults, and hippocampal sclerosis (HS) is the most frequently encountered lesion in patients with MTLE. Premature accumulation of corpora amylacea (CoA), which plays an important role in the sequestration of toxic cellular metabolites, is found in the hippocampus of 50-60% of the patients who undergo surgery for medically refractory MTLE-HS. However, the etiopathogenesis and clinical importance of this phenomenon are still uncertain. The ABCB1 gene product P-glycoprotein (P-gp) plays a prominent role as an antiapoptotic factor in addition to its efflux transporter function. ABCB1 polymorphism has been found to be associated with downregulation of P-gp expression. We hypothesized that a similar polymorphism will be found in patients with CoA deposition, as the polymorphism predisposes the hippocampal neuronal and glial cells to seizure-induced excitotoxic damage and CoA formation ensues as a buffer response.

MATERIALS AND METHODS

We compared five single nucleotide polymorphisms in the ABCB1 gene Ex06+139C/T (rs1202168), Ex 12 C1236T (rs1128503), Ex 17-76T/A (rs1922242), Ex 21 G2677T/A (rs2032582), Ex26 C3435T (rs1045642) among 46 MTLE-HS patients of south Indian ancestry with and without CoA accumulation.

RESULTS

We found that subjects carrying the Ex-76T/A polymorphism (TA genotype) had a five-times higher risk of developing CoA accumulation than subjects without this genotype (Odds ratio 5.0, 95% confidence intervals 1.34-18.55; P = 0.016).

CONCLUSION

We speculate that rs1922242 polymorphism results in the downregulation of P-gp function, which predisposes the hippocampal cells to seizure-induced apoptosis, and CoA gets accumulated as a buffer response.

Analysis of the interaction of induction regimens with p-glycoprotein expression in patients with acute myeloid leukaemia: results from the MRC AML15 trial

Retrospective analyses in non-randomised cohorts suggest that regimens containing fludarabine/Ara C and/or idarubicin/ara C may be more effective than daunorubicin/AraC (DA)-containing regimens in cases of acute myeloid leukaemia (AML) overexpressing p-glycoprotein (Pgp). We prospectively measured Pgp protein and function by flow cytometry in CD45-gated blasts from 434 AML15 trial patients randomised to remission induction therapy with two courses of FLAG-Ida or DA±etoposide (DA/ADE). In all, 34% were positive for Pgp protein and 38% for function. Pgp protein-positive cases had a higher incidence of resistant disease (14% vs 5%), adjusted odds ratio 2.67 (1.14–6.24). There was a trend towards a higher cumulative incidence of relapse at 5 years for Pgp-positive cases (46% vs 55%), adjusted hazard ratio 1.42 (0.98–2.07) (P=0.06). For patients treated with FLAG-Ida, the complete remission (CR) rate was 86% for both Pgp-positive and Pgp-negative patients. In patients treated with DA/ADE, 78% of Pgp-positive and 90% of Pgp-negative cases achieved CR (P=0.06). In analyses of overall survival, there was no interaction between treatment received and Pgp expression. Data for Pgp function followed similar trends. Our data suggest that FLAG-Ida may improve the remission rate for Pgp-positive AML, but the malignant clone is reduced rather than eradicated such that the relapse rate remains high in Pgp-positive patients.

Targeting the brain--surmounting or bypassing the blood-brain barrier

The constituents of the blood-brain barrier, including its efflux transporter system, can efficiently limit brain penetration of potential CNS therapeutics. Effective extrusion from the brain by transporters is a frequent reason for the pharmaceutical industry to exclude novel compounds from further development for CNS therapeutics. Moreover, high transporter expression levels that are present in individual patients or may be generally associated with the pathophysiology seem to be a major cause of therapeutic failure in a variety of CNS diseases including brain tumors, epilepsy, brain HIV infection, and psychiatric disorders. Increasing knowledge of the structure and function of the blood-brain barrier creates a basis for the development of strategies which aim to enhance brain uptake of beneficial pharmaceutical compounds. The different strategies discussed in this review aim to modulate blood-brain barrier function or to bypass constituents of the blood-brain barrier.

Pharmacokinetic and pharmacodynamic implications of P-glycoprotein modulation

Modulation of P-glycoprotein (Pgp)-mediated transport has significant pharmacokinetic implications for Pgp substrates. Pharmacokinetic alterations may be at the systemic (blood concentrations), regional (organ or tissue concentrations), or local (intracellular concentrations) level. Regardless of the particular location of Pgp modulation, changes in substrate pharmacokinetics will have the potential to alter the magnitude and duration of pharmacologic effect (pharmacodynamics). It is important to understand each of the aspects of Pgp modulation for a given Pgp substrate in order to predict the degree to which Pgp modulation may affect that substrate, to minimize untoward effects associated with that modulation, or to exploit that modulation for specific therapeutic advantage.

Targeting regulation of ABC efflux transporters in brain diseases: a novel therapeutic approach

Blood-brain barrier efflux transporters limit the brain penetration and efficacy of various central nervous system drugs. In several CNS diseases, therapy- or pathophysiology-associated transcriptional activation of efflux transporters further strengthens the barrier function. Targeting the regulatory pathways that drive efflux transporter expression in different diseases represents an intriguing approach for prevention of these events thereby promoting delivery to the brain and enhancing or restoring drug efficacy. In particular, the approach holds the promise to preserve basal transporter expression and activity, which is of specific relevance in view of the protective function of efflux transport. The elucidation of the signaling cascades involved in transporter regulation is a major presupposition for the development of preventive strategies. Orphan nuclear receptors as well as the Wnt/beta-catenin signaling pathway have been implicated in drug-induced changes in transporter expression. Targeting these xenobiotic sensors is therefore discussed as a means to optimize brain delivery and therapeutic outcome. Relevant progress has also been made with the identification of key signaling events that drive P-glycoprotein expression in response to pathophysiological mechanisms. In the epileptic brain, complex signaling events involving cyclooxygenase-2 activity trigger P-glycoprotein expression in response to glutamate release and activation of endothelial NMDA receptors. Moreover, reactive oxygen species and inflammatory cytokines have been identified as regulatory factors which might affect P-glycoprotein in several CNS diseases. Recent data substantiated several interesting targets in the respective signaling cascades thereby rendering a basis for the ongoing development of innovative approaches to optimize central nervous system drug brain penetration and efficacy.

Analysis of factors that affect in vitro chemosensitivity of leukaemic stem and progenitor cells to gemtuzumab ozogamicin (Mylotarg) in acute myeloid leukaemia

Relapse in acute myeloid leukaemia (AML) is considered to result from the persistence of drug-resistant leukaemic stem and progenitor cells (LSPC) within a bone marrow 'niche' microenvironment. Identifying novel agents that have the potential to target these LSPC in their niche microenvironment will aid in the characterization of candidate agents for post-remission chemotherapy. Using an in vitro model, we found that 48-h culture with gemtuzumab ozogamicin (Mylotarg) resulted in a 34% reduction in CD34(+)CD38(-)CD123(+) LSPC number, whereas normal CD34(+)CD38(-) haemapoietic stem cells were insensitive to this agent. As there was considerable heterogeneity in LSPC response to Mylotarg treatment, various factors potentially underpinning the differential response were assessed. LSPC that overexpressed CD33 (P=0.01), which were P-glycoprotein-negative (P=0.008) and with internal tandem duplication (ITD) of the FLT3 gene (FLT3/ITD) status (P=0.006) responded better to Mylotarg treatment. LSPC from patient samples that have these combined characteristics as well as low LSPC burden showed significantly more chemosensitivity to Mylotarg compared with all other cases (P=0.002). In multivariate analysis, LSPC burden and FLT3 status were found to be predictors of LSPC chemosensitivity to Mylotarg treatment (P<0.0001). In conclusion, we have shown heterogeneity in the LSPC compartment of AML patients underpinning differential in vitro sensitivity to Mylotarg.

Pim-1 expression and monoclonal antibody targeting in human leukemia cell lines

OBJECTIVE

Based on our previous findings that Pim-1 was expressed on the cell surface and could be targeted with a highly specific anti-Pim-1 monoclonal antibody (P9), this study aims to evaluate the possibility that Pim-1 could be targeted for the treatment of human leukemia.

MATERIALS AND METHODS

Pim-1 expression was investigated in a series of human leukemia cell lines with immunohistochemistry and flow cytometry. The inhibitory effect of P9 on cell proliferation was evaluated with (3)H-thymidine incorporation assay. Cell apoptosis was assayed with Annexin-V/propidium iodide dual staining. The in vivo effect of P9 was evaluated with xenograft tumor models in severe combined immunodeficient mice.

RESULTS

Pim-1 expression varied depending on the cell lines and correlated with the inhibitory effects mediated by P9. An association between Pim-1 expression and drug resistance was observed. Although the drug-resistant CEM/A7R cells were highly resistant to cytotoxic P-glycoprotein substrates, their growth was inhibited by P9 as demonstrated by in vitro proliferation assay and in vivo inhibition of xenograft tumors. P9 had little effect on P-glycoprotein expression and intracellular Rhodamine 123 accumulation, but it inhibited the phosphorylation of Bad and induced apoptosis.

CONCLUSIONS

Pim-1 is variably expressed in leukemia cell lines and associated with drug resistance. Targeting Pim-1 with monoclonal antibody could be explored for the treatment of leukemia and may represent a novel strategy to overcome drug resistance.

Differential susceptibility of P-glycoprotein deficient mice to colitis induction by environmental insults

BACKGROUND

P-glycoprotein (P-gp), the product of the multidrug resistance gene (MDR), is an ATP-dependent transmembrane pump, which is expressed in multiple cell lineages including epithelial and hematopoetic cells. The human MDR gene is located on chromosome 7 (7q21.1), a susceptibility loci for inflammatory bowel disease (IBD). A significant number of IBD patients carry mutations in this gene and P-gp-deficient FVB/N mice develop a severe spontaneous colitis, characterized by impaired intestinal barrier function and immune reactivity to intestinal bacterial antigens.

METHODS

In this work we explored the role of mouse strain, as well as environmental insults, on the development of colonic inflammation in the absence of P-gp. Among the induction methods utilized, dextran sodium sulfate (DSS) disrupts the intestinal epithelium, while piroxicam is a nonsteroidal antiinflammatory (NSAID) drug that inhibits prostaglandin production and initiates colitis in IL10-deficient animals. Helicobacter bilis is a known mediator of bacterial-induced colitis.

RESULTS

We demonstrate that crossing this mutation onto the C57BL/6 strain confers protection from spontaneous colitis. C57BL/6.mdr1a-deficient animals demonstrated increased histological inflammation, colonic shortening, fecal blood, and reduced body weight after 7 days of treatment with 2.25% DSS. C57BL/6.mdr1a-deficient mice treated with piroxicam or infected with H. bilis showed no weight loss, or alterations in colonic histology.

CONCLUSIONS

These data indicate that the effects of P-gp deficiency are significantly modulated by background strain influences, but that the epithelium continues to have increased susceptibility to chemical injury in the C57BL/6 model.

Sequential influences of leukemia-specific and genetic factors on p-glycoprotein expression in blasts from 817 patients entered into the National Cancer Research Network acute myeloid leukemia 14 and 15 trials

PURPOSE

P-glycoprotein (Pgp) is a major prognostic factor for chemotherapy failure in acute myeloid leukemia (AML). This study compared the influence of genetic and leukemia-specific factors on Pgp.

EXPERIMENTAL DESIGN

Eight hundred and seventeen samples were studied prospectively for Pgp protein expression and function and G1199A, G2677T, and C3435T polymorphisms in the encoding gene ABCB1.

RESULTS

Age, low WBC count, high bcl-2, secondary AML and myelodysplastic syndrome, and adverse cytogenetics all correlated strongly with high Pgp (MRK16) protein expression. However, ABCB1 3435TT homozygosity was negatively correlated with Pgp. Pgp protein is only expressed in 41% of samples such that the negative effect of the polymorphism was not seen at baseline Pgp levels but was marked in the upper 41% of samples (MRK16 Deltamean fluorescence intensity of 75th centile sample = 9 units for TT variant samples and 26 units for CC/CT; P = 0.003). However, no association was found between genetic factors and Pgp function using rhodamine 123 accumulation.

CONCLUSIONS

The genetic polymorphism 3435TT (which results in unstable mRNA) has a significant effect on Pgp expression, but this is only seen in approximately 40% of cases in which mRNA and protein are detectable. Moreover, leukemia-specific factors, such as low WBC count and poor risk cytogenetics, have a much greater effect than genetic polymorphisms on Pgp expression in AML blasts.

Statins induce lethal effects in acute myeloblastic leukemia [corrected] cells within 72 hours

HMG-CoA Reductase inhibitors (statins) induce apoptosis in acute myeloid leukemia (AML) cells in vitro; however, the concentrations associated with cell death in AML cells are higher than those clinically tolerated during prolonged therapy. We therefore wished to determine whether short exposures to lovastatin might induce cell death in AML cells at clinically attainable concentrations. The time and concentration dependence of HL60 and U937 cells was determined and showed that cell death was delayed. IC₅₀ values and IC₉₀ values determined on day 6 suggested that the sensitivity of AML cells to statins may occur at lower concentrations than previously reported. After 72 h, mevalonate did not rescue AML cells from cytotoxic concentrations of statins, suggesting that, although cell death was delayed, lovastatin induced lethal effects within 72 h. In conjunction with previously reported Phase I studies, the data presented here suggest that the high-dose, short course statins may be useful for the treatment of patients with AML.

ATP-binding-cassette transporters in hematopoietic stem cells and their utility as therapeutical targets in acute and chronic myeloid leukemia

ATP-binding-cassette (ABC) transporters are evolutionary extremely well-conserved transmembrane proteins that are highly expressed in hematopoietic stem cells (HSCs). The physiological function in human stem cells is believed to be protection against genetic damage caused by both environmental and naturally occurring xenobiotics. Additionally, ABC transporters have been implicated in the maintenance of quiescence and cell fate decisions of stem cells. These physiological roles suggest a potential role in the pathogenesis and biology of stem cell-derived hematological malignancies such as acute and chronic myeloid leukemia. This paper reviews the (patho)physiological role of ABC transporters in human normal and malignant HSCs and discusses its implications for their utility as therapeutical targets to eradicate leukemic stem cells in these diseases.

Expression of mdr1 is required for efficient long term regeneration of dystrophic muscle

The mouse mdr1a and mdr1b genes are expressed in skeletal muscle, though their precise role in muscle is unknown. Dystrophic muscle is characterized by repeated cycles of degeneration and regeneration. To explore the role of the mdr1 genes during muscle regeneration, we have created a triple knockout mouse lacking the mdr1a, mdr1b, and the dystrophin genes. The resulting ReX mice developed normally and were fertile. However, as adults, ReX had a higher proportion of degenerating muscle fibers and greater long-term loss of muscle mass than mdx. ReX muscles were also characterized by a reduced proportion of muscle side population (mSP) cells, of myogenic cells, and a reduced capacity for muscle regeneration. We found too that mSP cells derived from dystrophic muscle are more myogenic than those from normal muscle. Thus, in dystrophic muscle, the mdr1 gene plays an important role in the preservation of the mSP and of the myogenic regenerative potential. Moreover, our results suggest a hitherto unappreciated role of mdr1 in precursor cells of regenerating tissue; they therefore provide an important clue to the physiological significance of mdr1 expression in stem cells.

Tariquidar (XR9576): a P-glycoprotein drug efflux pump inhibitor

P-glycoprotein actively transports structurally unrelated compounds out of cells, conferring the multidrug resistance phenotype in cancer. Tariquidar is a potent, specific, noncompetitive inhibitor of P-glycoprotein. Tariquidar inhibits the ATPase activity of P-glycoprotein, suggesting that the modulating effect is derived from the inhibition of substrate binding, inhibition of ATP hydrolysis or both. In clinical trials, tariquidar is tolerable and does not have significant pharmacokinetic interaction with chemotherapy. In patients, inhibition of P-glycoprotein has been demonstrated for 48 h after a single dose of tariquidar. Studies to assess a possible increase in toxicity of chemotherapy and the impact of P-glycoprotein inhibition on tumor response and patient outcome are ongoing. Tariquidar can be considered an ideal agent for testing the role of P-glycoprotein inhibition in cancer.

Novel antiseizure drug mechanisms

The amount of new knowledge being generated regarding brain mechanisms in general, and epileptic mechanisms in particular, is enormous. Anticonvulsant drugs are ineffective in approximately a third of people with epilepsy. To our knowledge, strategies for preventing epilepsy after an initial insult are nonexistent. In this review, we briefly examine some recent novel concepts for preventing seizures, which might lead to enhanced anticonvulsant drug therapy. We start with some known seizure mechanisms that have yet to yield widely used anticonvulsant drugs, including potassium channels, chloride cotransporters, extracellular space constriction, gap junctions and magnesium. Pharmacoresistance is then discussed, focusing on the upregulation of drug-resistance proteins (a concept with significant therapeutic appeal) and the drug-target hypothesis. Two further areas that hold great promise for future therapeutics are sex hormones and inflammatory processes. The genetics of epilepsy are currently being elaborated, providing potential novel anticonvulsant targets. Prevention being better than a cure, we discuss epileptogenesis and its treatment. Given the astounding progress of neuroscience research, one hopes for many new therapeutics for our intractable epileptic patients.