Role for c-jun N-terminal kinase in treatment-refractory acute myeloid leukemia (AML): signaling to multidrug-efflux and hyperproliferation

Leukemia-Related Signaling Pathways Among HTLV-1-Derived Adult T Cell Leukemia/Lymphoma and Asymptomatic Carriers in Comparison to Normal Group

Human T cell lymphotropic virus type 1 (HTLV-1) is associated with adult T cell leukemia/lymphoma (ATLL), a fetal malignant infection. Recently, HTLV-1 new asymptomatic carriers (ACs) have frequently been reported among blood donors. Reaching the profound concept of HTLV-1-associated molecular pathogenesis could result in finding novel therapeutic strategies. The current study aimed to determine leukemia-related signaling regulation in ATLL. Thirty participants were evaluated in 3 groups, including 10 ATLL patients, 10 ACs, and 10 normal controls. Blood samples were isolated without any chemotherapy history from ATLL patients. Also, blood samples were recovered from ACs and normal individuals. White blood cells isolation was done on the collected blood samples. After this, RNA was extracted from the prepared samples and used for the cDNA synthesis. TAX and HTLV-1 basic leucine zipper factor as viral genes and cellular genes, including MKP-1, EVI-1, JNK-1, FOXO-1, AKT-1, DEPTOR, MTOR, and JUN, were investigated using real-time PCR. The mean age of ATLL patients was 53.2 ± 7.32 years, and 9 (90%) were male. The EVI-1 and FOXO-1 expression levels were significantly associated with ATLL patients compared with the internal control. However, the significant differences in expression of other genes in the remaining groups were not seen. Discovering viral and cellular signaling pathways that regulate HTLV-1 transformation is essential. A novel therapeutic strategy for ATLL-regulating cellular signaling pathways in vivo could be considered. Therefore, clinical trials using activators and inhibitors of related cellular signaling pathways for cell therapy of ATLL are recommended. It is recommended that more investigation be conducted on FOXO-1 and EVI-1 to target these genes and reveal the molecular pathogenesis of ATLL.

Choosing the Right Cell Line for Acute Myeloid Leukemia (AML) Research

Immortalized cell lines are widely used in vitro tools in oncology and hematology research. While these cell lines represent artificial systems and may accumulate genetic aberrations with each passage, they are still considered valuable models for pilot, preliminary, and screening studies. Despite their limitations, cell lines are cost-effective and provide repeatable and comparable results. Choosing the appropriate cell line for acute myeloid leukemia (AML) research is crucial for obtaining reliable and relevant results. Several factors should be considered when selecting a cell line for AML research, such as specific markers and genetic abnormalities associated with different subtypes of AML. It is also essential to evaluate the karyotype and mutational profile of the cell line, as these can influence the behavior and response to the treatment of the cells. In this review, we evaluate immortalized AML cell lines and discuss the issues surrounding them concerning the revised World Health Organization and the French-American-British classifications.

Dual Targeting of EGFR with PLK1 Exerts Therapeutic Synergism in Taxane-Resistant Lung Adenocarcinoma by Suppressing ABC Transporters

Simple Summary

Our previous studies led us to hypothesize that downregulation of PLK1 expression or its activity can overcome the hurdles of taxane resistance by downregulating ABC transporters. Targeting PLK1 with shRNA or non-functional mutants downregulated ABCB1, ABCC9, and ABCG2 in paclitaxel-resistant lung adenocarcinoma (LUAD^TXR), similar to the downregulation effects from treatment with PLK1 inhibitors. Since EGFR is highly expressed in LUAD^TXR cells, gefitinib was combined with PLK1 inhibitors. Under these conditions, LUAD^TXR cells tend to undergo apoptosis more effectively than parental cells, showing a synergistic effect on downregulation of ABC transporters through c-Myc or AP-1. Clinical data provide evidence for the relationship between survival rates and expressions of PLK1 and EGFR in LUAD patients. Taken together, our data suggest that a combination of gefitinib and PLK1 inhibitors exerts strong synergism in LUAD^TXR, providing a benefit to overcome the limitations associated with taxanes.

Abstract

To overcome the limitations of chemoresistance, combination therapies using druggable targets have been investigated. Our previous studies led us to hypothesize that the downregulation of PLK1 expression or activity can be one strategy to overcome the hurdles of taxane resistance by the downregulation of ABC transporters. To explore this, various versions of PLK1 including a constitutively active version, kinase-dead form, and polo-box domain mutant were expressed in paclitaxel-resistant lung adenocarcinoma (LUAD^TXR). Targeting PLK1 using shRNA or non-functional mutants downregulated ABCB1, ABCC9, and ABCG2 in LUAD^TXR cells, which was similar to the downregulation effects from treatment with PLK1 inhibitors. The high expression of EGFR in LUAD led us to administer gefitinib, showing a markedly reduced EGFR level in LUAD^TXR cells. When gefitinib and PLK1 inhibitors were combined, LUAD^TXR cells tended to undergo apoptosis more effectively than parental cells, showing a synergistic effect on the downregulation of ABC transporters through c-Myc and AP-1. Clinical data provide evidence for the relevance between survival rates and expressions of PLK1 and EGFR in LUAD patients. Based on these results, we suggest that a combination of gefitinib and PLK1 inhibitors exerts strong synergism in LUAD^TXR, which helps to overcome the limitations associated with taxanes.

Meta-analysis of gene signatures and key pathways indicates suppression of JNK pathway as a regulator of chemo-resistance in AML

The pathways and robust deregulated gene signatures involved in AML chemo-resistance are not fully understood. Multiple subgroups of AMLs which are under treatment of various regimens seem to have similar regulatory gene(s) or pathway(s) related to their chemo-resistance phenotype. In this study using gene set enrichment approach, deregulated genes and pathways associated with relapse after chemotherapy were investigated in AML samples. Five AML libraries compiled from GEO and ArrayExpress repositories were used to identify significantly differentially expressed genes between chemo-resistance and chemo-sensitive groups. Functional and pathway enrichment analysis of differentially expressed genes was performed to assess molecular mechanisms related to AML chemotherapeutic resistance. A total of 34 genes selected to be differentially expressed in the chemo-resistance compared to the chemo-sensitive group. Among the genes selected, c-Jun, AKT3, ARAP3, GABBR1, PELI2 and SORT1 are involved in neurotrophin, estrogen, cAMP and Toll-like receptor signaling pathways. All these pathways are located upstream and regulate JNK signaling pathway which functions as a key regulator of cellular apoptosis. Our expression data are in favor of suppression of JNK pathway, which could induce pro-apoptotic gene expression as well as down regulation of survival factors, introducing this pathway as a key regulator of drug-resistance development in AML.

The role of endolysosomal trafficking in anticancer drug resistance

Multidrug resistance (MDR) remains a major obstacle towards curative treatment of cancer. Despite considerable progress in delineating the basis of intrinsic and acquired MDR, the underlying molecular mechanisms remain to be elucidated. Emerging evidences suggest that dysregulation in endolysosomal compartments is involved in mediating MDR through multiple mechanisms, such as alterations in endosomes, lysosomes and autophagosomes, that traffic and biodegrade the molecular cargo through macropinocytosis, autophagy and endocytosis. For example, altered lysosomal pH, in combination with transcription factor EB (TFEB)-mediated lysosomal biogenesis, increases the sequestration of hydrophobic anti-cancer drugs that are weak bases, thereby producing an insufficient and off-target accumulation of anti-cancer drugs in MDR cancer cells. Thus, the use of well-tolerated, alkalinizing compounds that selectively block Vacuolar H⁺-ATPase (V-ATPase) may be an important strategy to overcome MDR in cancer cells and increase chemotherapeutic efficacy. Other mechanisms of endolysosomal-mediated drug resistance include increases in the expression of lysosomal proteases and cathepsins that are involved in mediating carcinogenesis and chemoresistance. Therefore, blocking the trafficking and maturation of lysosomal proteases or direct inhibition of cathepsin activity in the cytosol may represent novel therapeutic modalities to overcome MDR. Furthermore, endolysosomal compartments involved in catabolic pathways, such as macropinocytosis and autophagy, are also shown to be involved in the development of MDR. Here, we review the role of endolysosomal trafficking in MDR development and discuss how targeting endolysosomal pathways could emerge as a new therapeutic strategy to overcome chemoresistance in cancer.

JNK signaling as a target for anticancer therapy

The JNKs are members of mitogen-activated protein kinases (MAPK) which regulate many physiological processes including inflammatory responses, macrophages, cell proliferation, differentiation, survival, and death. It is increasingly clear that the continuous activation of JNKs has a role in cancer development and progression. Therefore, JNKs represent attractive oncogenic targets for cancer therapy using small molecule kinase inhibitors. Studies showed that the two major JNK proteins JNK1 and JNK2 have opposite functions in different types of cancers, which need more specification in the design of JNK inhibitors. Some of ATP- competitive and ATP non-competitive inhibitors have been developed and widely used in vitro, but this type of inhibitors lack selectivity and inhibits phosphorylation of all JNK substrates and may lead to cellular toxicity. In this review, we summarized and discussed the strategies of JNK binding inhibitors and the role of JNK signaling in the pathogenesis of different solid and hematological malignancies.

Pharmacological impact of FLT3 mutations on receptor activity and responsiveness to tyrosine kinase inhibitors

Acute myelogenous leukaemia (AML) is an aggressive blood cancer characterized by the rapid proliferation of immature myeloid blast cells, resulting in a high mortality rate. The 5-year overall survival rate for AML patients is approximately 25%. Circa 35% of all patients carry a mutation in the FLT3 gene which have a poor prognosis. Targeting FLT3 receptor tyrosine kinase has become a treatment strategy in AML patients possessing FLT3 mutations. The most common mutations are internal tandem duplications (ITD) within exon 14 and a single nucleotide polymorphism (SNP) that leads to a point mutation in the D835 of the tyrosine kinase domain (TKD). Variations in the ITD sequence and the occurrence of other point mutations that lead to ligand-independent FLT3 receptor activation create difficulties in developing personalized therapeutic strategies to overcome observed mutation-driven drug resistance. Midostaurin and quizartinib are tyrosine kinase inhibitors (TKIs) with inhibitory efficacy against FLT3-ITD, but exhibit limited clinical impact. In this review, we focus on the structural aspects of the FLT3 receptor and correlate those mutations with receptor activation and the consequences for molecular and clinical responsiveness towards therapies targeting FLT3-ITD positive AML.

Single-cell analysis of AIMP2 splice variants informs on drug sensitivity and prognosis in hematologic cancer

Aminoacyl-tRNA synthetase-interacting multifunctional protein 2 (AIMP2) is a non-enzymatic component required for the multi-tRNA synthetase complex. While exon 2 skipping alternatively spliced variant of AIMP2 (AIMP2-DX2) compromises AIMP2 activity and is associated with carcinogenesis, its clinical potential awaits further validation. Here, we found that AIMP2-DX2/AIMP2 expression ratio is strongly correlated with major cancer signaling pathways and poor prognosis, particularly in acute myeloid leukemia (AML). Analysis of a clinical patient cohort revealed that AIMP2-DX2 positive AML patients show decreased overall survival and progression-free survival. We also developed targeted RNA-sequencing and single-molecule RNA-FISH tools to quantitatively analyze AIMP2-DX2/AIMP2 ratios at the single-cell level. By subclassifying hematologic cancer cells based on their AIMP2-DX2/AIMP2 ratios, we found that downregulating AIMP2-DX2 sensitizes cells to anticancer drugs only for a subgroup of cells while it has adverse effects on others. Collectively, our study establishes AIMP2-DX2 as a potential biomarker and a therapeutic target for hematologic cancer.

Ku, Kim et al develop a method to analyse the ratio of the alternatively spliced variant of AIMP2 to full length AIMP via single-molecule RNA-FISH. They can subclassify hematologic cancer based on AIMP2-DX2/AIMP2 ratio and find that cells with high AIMP2-DX2 ratio can be sensitized to chemotherapy drugs by depleting AIMP2-DX2.

Chronic myeloid leukemia stem cells require cell-autonomous pleiotrophin signaling

Tyrosine kinase inhibitors (TKIs) induce molecular remission in the majority of patients with chronic myelogenous leukemia (CML), but the persistence of CML stem cells hinders cure and necessitates indefinite TKI therapy. We report that CML stem cells upregulate the expression of pleiotrophin (PTN) and require cell-autonomous PTN signaling for CML pathogenesis in BCR/ABL+ mice. Constitutive PTN deletion substantially reduced the numbers of CML stem cells capable of initiating CML in vivo. Hematopoietic cell-specific deletion of PTN suppressed CML development in BCR/ABL+ mice, suggesting that cell-autonomous PTN signaling was necessary for CML disease evolution. Mechanistically, PTN promoted CML stem cell survival and TKI resistance via induction of Jun and the unfolded protein response. Human CML cells were also dependent on cell-autonomous PTN signaling, and anti-PTN antibody suppressed human CML colony formation and CML repopulation in vivo. Our results suggest that targeted inhibition of PTN has therapeutic potential to eradicate CML stem cells.

Solanine reverses multidrug resistance in human myelogenous leukemia K562/ADM cells by downregulating MRP1 expression

Multidrug resistance (MDR) in leukemia cells is a major obstacle to chemotherapeutic treatment. High expression and constitutive activation of multidrug resistance protein 1 (MRP1) has been associated with the development of resistance to anticancer drugs in a number of tumor types. The activity of c-Jun N-terminal kinase 1 (JNK1) is associated with the occurrence of MDR and MRP1 expression. The present study aimed to investigate the ability of solanine to resensitize the Adriamycin^® (ADR)-resistant human myelogenous leukemia cell line K562/ADM to ADR. Results of the Cell Counting Kit-8 assay demonstrated that solanine inhibited K562/ADM cell proliferation. K562/ADM cell sensitivity to ADR was increased following treatment with solanine, indicated by increased intracellular accumulation of ADR. Western blotting demonstrated that treatment with solanine led to reduced MRP1 protein expression, suggesting that solanine-induced ADR accumulation is due to the downregulation of MRP1 expression. Solanine-mediated MRP1 downregulation was observed to be dependent on the JNK signaling pathway. In conclusion, the results of the present study suggest that solanine reverses MDR in K562/ADM cells and may represent a novel therapeutic agent for the treatment of human myelogenous leukemia.

Reactive oxygen species activate differentiation gene transcription of acute myeloid leukemia cells via the JNK/c-JUN signaling pathway

Reactive oxygen species (ROS) and altered cellular redox status are associated with many malignancies. Acute myeloid leukemia (AML) cells are maintained at immature state by differentiation blockade, which involves deregulation of transcription factors in myeloid differentiation. AML cells can be induced to differentiate by phorbol-12-myristate-13-acetate (PMA), which possesses pro-oxidative activity. However, the signaling events mediated by ROS in the activation of transcriptional program during AML differentiation has not been fully elucidated. Here, we investigated AML cell differentiation by treatment with PMA and ROS scavenger N-acetyl-l-cysteine (NAC). We observed elevation of intracellular ROS level in the PMA-treated AML cells, which correlated with differentiated cell morphology and increased CD11b⁺ mature cell population. The effect of PMA can be abolished by NAC co-treatment, supporting the involvement of ROS in the process. Moreover, we demonstrated that short ROS elevation mediated cell cycle arrest, but failed to activate myeloid gene transcription; whereas prolonged ROS elevation activated JNK/c-JUN signaling pathway. Inhibition of JNK suppressed the expression of key myeloid transcriptional regulators c-JUN, SPI-1 and MAFB, and prevented AML cells from undergoing terminal differentiation. These findings provide new insights into the crucial role of JNK/c-Jun signaling pathway in the activation of transcriptional program during ROS-mediated AML differentiation.

Cisplatin induces expression of drug resistance-related genes through c-jun N-terminal kinase pathway in human lung cancer cells

PURPOSE

Change of multidrug resistance-related genes (e.g., lung resistance protein, LRP) and overexpression of anti-apoptotic genes (Bcl-2, Bcl-Xl, XIAP, Survivin) are responsible for cisplatin resistance. In our study, we investigated the mechanism by which cisplatin induces LRP, Bcl-2, Bcl-xL, XIAP, and Survivin expression in human lung adenocarcinoma A549 cells and human H446 small cell lung cancer cells at mRNA and protein levels.

METHODS

In our study, cell proliferation was assessed with CCK-8 assays, and cell apoptosis was assessed with flow cytometric analysis and Annexin-V/PI staining. qPCR was used to complete RNA experiments. Protein expression was assessed with Western blotting.

RESULTS

Cisplatin increased Bcl-2, LRP, and Survivin expression, but decreased Bcl-xL and XIAP expression in a dose-dependent manner. Preincubation with JNK-specific inhibitor, SP600125, significantly inhibited these genes' expression at mRNA and protein levels, enhanced chemosensitivity of lung cancer cells to cisplatin, and promoted cisplatin-induced apoptosis.

CONCLUSION

Our data suggest that the JNK signaling pathway plays an important role in cisplatin resistance. Lung resistance protein (LRP) and anti-apoptotic genes (Bcl-2, Bcl-Xl, XIAP, Survivin) are involved in the process. The results reminded us of a novel therapy target for lung cancer treatment.

Novel tumor-suppressor function of KLF4 in pediatric T-cell acute lymphoblastic leukemia

Acute lymphoblastic leukemia (ALL) is the most common hematological malignancy in pediatric patients. Despite advances in the treatment of this disease, many children with T-cell ALL (T-ALL) die from disease relapse due to low responses to standard chemotherapy and the lack of a targeted therapy that selectively eradicates the chemoresistant leukemia-initiating cells (LICs) responsible for disease recurrence. We reported recently that the reprogramming factor Krüppel-like factor 4 (KLF4) has a tumor-suppressive function in children with T-ALL. KLF4 silencing by promoter deoxyribonucleic acid (DNA) methylation in patients with T-ALL leads to aberrant activation of the mitogen-activated protein kinase kinase MAP2K7 and the downstream c-Jun NH₂-terminal kinase (JNK) pathway that controls the expansion of leukemia cells via c-Jun and activating transcription factor 2. This pathway can be inhibited with small molecules and therefore has the potential to eliminate LICs and eradicate disease in combination with standard therapy for patients with refractory and relapsed disease. The present review summarizes the role of the KLF4-MAP2K7 pathway in T-ALL pathogenesis and the function of JNK and MAP2K7 in carcinogenesis and therapy.

Reserpine requires the D2-type receptor, dop-3, and the exoribonuclease, eri-1, to extend the lifespan in C. elegans

Lifespan extension is an all systems encompassing event. Involvement of reduced insulin/IGF1 signalling is well worked out, first in the model organism Caenorhbaditis elegans followed by other systems including humans. But the role of neuronal component in lifespan extension is not well understood due to the refractory nature of neurons to small RNA interference (sRNAi) in C. elegans. Earlier, we have demonstrated that an antihypertensive drug, reserpine, extends lifespan through modulation of neurotransmitter release, especially, acetylcholine, in C. elegans. Intriguingly, the reserpine mediated lifespan extension (RMLE) does not happen through the known longevity pathways. Here, we report that the D2-type dopamine receptor (DOP-3), which acts through the inhibitory Gprotein coupled (G alpha i) pathway mediated signalling is partly required for RMLE. In the dop-3 loss of function mutant RMLE is shortened. DOP-3 acts through Gαo (goa-1). One of the downstream targets of G protein signalling is the transcription factor, jun-1. MRP-1, an ATP binding cassette transporter, belonging to the multidrug resistance protein family is one of the genes turned on by JUN-1. RMLE is shortened in dop-3-->goa-1-->jun1-->mrp-1 loss of function mutants, elucidating the contribution of dop-3 signalling. The dop-3 receptor system is known to inhibit acetylcholine release. This suggests dopamine receptor, dop-3 could be contributing to the modulation of acetylcholine release by reserpine. ERI-1 is a 3'-5' exoribonuclease, one of the negative regulators of sRNAi, whose loss of function makes neurons amenable to siRNA. In the absence of eri-1, RMLE is shortened. In the dop-3 loss-of-function background, lack of eri-1 completely abolishes RMLE. This suggests that dop-3 and eri-1 act in independent parallel pathways for RMLE and these two pathways are essential and sufficient for the longevity enhancement by reserpine in C. elegans.

Phosphorylation of Glutathione S-Transferase P1 (GSTP1) by Epidermal Growth Factor Receptor (EGFR) Promotes Formation of the GSTP1-c-Jun N-terminal kinase (JNK) Complex and Suppresses JNK Downstream Signaling and Apoptosis in Brain Tumor Cells

Under normal physiologic conditions, the glutathione S-transferase P1 (GSTP1) protein exists intracellularly as a dimer in reversible equilibrium with its monomeric subunits. In the latter form, GSTP1 binds to the mitogen-activated protein kinase, JNK, and inhibits JNK downstream signaling. In tumor cells, which frequently are characterized by constitutively high GSTP1 expression, GSTP1 undergoes phosphorylation by epidermal growth factor receptor (EGFR) at tyrosine residues 3, 7, and 198. Here we report on the effect of this EGFR-dependent GSTP1 tyrosine phosphorylation on the interaction of GSTP1 with JNK, on the regulation of JNK downstream signaling by GSTP1, and on tumor cell survival. Using in vitro and in vivo growing human brain tumors, we show that tyrosine phosphorylation shifts the GSTP1 dimer-monomer equilibrium to the monomeric state and facilitates the formation of the GSTP1-JNK complex, in which JNK is functionally inhibited. Targeted mutagenesis and functional analysis demonstrated that the increased GSTP1 binding to JNK results from phosphorylation of the GSTP1 C-terminal Tyr-198 by EGFR and is associated with a >2.5-fold decrease in JNK downstream signaling and a significant suppression of both spontaneous and drug-induced apoptosis in the tumor cells. The findings define a novel mechanism of regulatory control of JNK signaling that is mediated by the EGFR/GSTP1 cross-talk and provides a survival advantage for tumors with activated EGFR and high GSTP1 expression. The results lay the foundation for a novel strategy of dual EGFR/GSTP1 for treating EGFR+ve, GSTP1 expressing GBMs.

JNK signalling in cancer: in need of new, smarter therapeutic targets

The JNKs are master protein kinases that regulate many physiological processes, including inflammatory responses, morphogenesis, cell proliferation, differentiation, survival and death. It is increasingly apparent that persistent activation of JNKs is involved in cancer development and progression. Therefore, JNKs represent attractive targets for therapeutic intervention with small molecule kinase inhibitors. However, evidence supportive of a tumour suppressor role for the JNK proteins has also been documented. Recent studies showed that the two major JNK proteins, JNK1 and JNK2, have distinct or even opposing functions in different types of cancer. As such, close consideration of which JNK proteins are beneficial targets and, more importantly, what effect small molecule inhibitors of JNKs have on physiological processes, are essential. A number of ATP-competitive and ATP-non-competitive JNK inhibitors have been developed, but have several limitations such as a lack of specificity and cellular toxicity. In this review, we summarize the accumulating evidence supporting a role for the JNK proteins in the pathogenesis of different solid and haematological malignancies, and discuss many challenges and scientific opportunities in the targeting of JNKs in cancer.

SOX4 is a direct target gene of FRA-2 and induces expression of HDAC8 in adult T-cell leukemia/lymphoma

SOX4 is consistently expressed in ATL, is involved in ATL cell growth, and induces genes such as GCRK, NAP1, and HDAC8 in ATL. FRA-2/JUND and SOX4 form an important oncogenic cascade in ATL, leading to upregulation of genes such as HDAC8.

Increased JNK1 signaling pathway is responsible for ABCG2-mediated multidrug resistance in human colon cancer

Multidrug resistance remains a major obstacle to effective chemotherapy of colon cancer. ABCG2, as a half-transporter of the G subfamily of ATP-binding cassette transporter genes (ABC transporters), is known to play a crucial role in multidrug resistance. However, the molecular mechanism of controlling ABCG2 expression in drug resistance of colon cancer is unclear and scarcely reported. In the present study, we systematically investigate the potential role of the c-Jun NH2-terminal kinase (JNK) signal pathway in ABCG2-induced multidrug resistance in colon cancer. In the hydroxycamptothecin (HCPT) resistant cell line SW1116/HCPT from human colon cancer cell line SW1116, ABCG2 is the major factor for multidrug resistance, other than well-studied ABCB1 or ABCC1. Our findings indicate that blocking the JNK pathway by pathway inhibitor SP600125 reduces the expression level and transport function of ABCG2 in drug-resistant cells SW116/HCPT. Notably, the experiments of small interfering RNA directed against JNK1 and JNK2 show that only silence of JNK1 gene has the equal effect as SP600125 on dephosphorylation of transcription factor c-Jun and the expression of ABCG2 protein, while the corresponding phenomena were not observed after silence of JNK2 gene. Meanwhile, SP600125 induces the apoptosis of SW116/HCPT cells by promoting the cleavage of PARP and suppressing the anti-apoptotic protein survivin and bcl-2, and increases the sensitivity of SW1116/HCPT to HCPT. Taken together, our work demonstrated that JNK1/c-jun signaling pathway was involved in ABCG2-mediated multidrug resistance in colon cancer cells. Definitely, inhibition of the JNK1/c-jun pathway is useful for reversing ABCG2-mediated drug resistance in HCPT-resistant colon cancer cells.

Zosuquidar, a novel modulator of P-glycoprotein, does not improve the outcome of older patients with newly diagnosed acute myeloid leukemia: a randomized, placebo-controlled trial of the Eastern Cooperative Oncology Group 3999

Zosuquidar, which modulates P-glycoprotein (P-gp) with minimal delay of anthracycline clearance, may reverse P-gp-mediated resistance in acute myeloid leukemia without increased toxicity. A total of 449 adults older than 60 years with acute myeloid leukemia or high-risk myelodysplastic syndrome enrolled in a randomized placebo-controlled double-blind trial (Eastern Cooperative Oncology Group 3999). Overall survival was compared between patients receiving conventional-dose cytarabine and daunorubicin and either zosuquidar (550 mg; 212 patients) or placebo (221 patients). Median and 2-year overall survival values were 7.2 months and 20% on zosuquidar and 9.4 months and 23% on placebo, respectively (P = .281). Remission rate was 51.9% on zosuquidar and 48.9% on placebo. All cause mortality to day 42 was not different (zosuquidar 22.2% vs placebo 16.3%; P = .158). In vitro modulation of P-gp activity by zosuquidar and expression of P-gp, multidrug resistance-related protein 1, lung resistance protein, and breast cancer resistance protein, were comparable in the 2 arms. Poor-risk cytogenetics were more common in P-gp(+) patients. P-gp expression and cytogenetics were correlated, though independent prognostic factors. We conclude that zosuquidar did not improve outcome in older acute myeloid leukemia, in part, because of the presence P-gp independent mechanisms of resistance. This trial is registered at www.clinicaltrials.gov as #NCT00046930.

The PEA-15 protein regulates autophagy via activation of JNK

PEA-15/PED (phosphoprotein enriched in astrocytes 15 kDa/phosphoprotein enriched in diabetes) is a death effector domain-containing protein which is known to modulate apoptotic cell death. The mechanism by which PEA-15 inhibits caspase activation and increases ERK (extracellular-regulated kinase) activity is well characterized. Here, we demonstrate that PEA-15 is not only pivotal in the activation of the ERK pathway but also modulates JNK (c-Jun N-terminal kinase) signaling. Upon overexpression of PEA-15 in malignant glioma cells, JNK is potently activated. The PEA-15-induced JNK activation depends on the phosphorylation of PEA-15 at both phosphorylation sites (serine 104 and serine 116). The activation of JNK is substantially inhibited by siRNA-mediated down-regulation of endogenous PEA-15. Moreover, we demonstrate that glioma cells overexpressing PEA-15 show increased signs of autophagy in response to classical autophagic stimuli such as ionizing irradiation, serum deprivation, or rapamycin treatment. In contrast, the non-phosphorylatable mutants of PEA-15 are not capable of promoting autophagy. The inhibition of JNK abrogates the PEA-15-mediated increase in autophagy. In conclusion, our data show that PEA-15 promotes autophagy in glioma cells in a JNK-dependent manner. This might render glioma cells more resistant to adverse stimuli such as starvation or ionizing irradiation.

A New alpha5beta1 integrin-dependent survival pathway through GSK3beta activation in leukemic cells

Background

Cell survival mediated by integrin engagement has been implicated in cell adhesion-mediated drug resistance. We have recently demonstrated that the activation of glycogen synthase kinase 3 β (GSK3β) is a new pathway supporting the chemoresistance of leukemic cells adhered to fibronectin.

Methodology and Principal Findings

We show here that in conditions of serum starvation, the fibronectin receptor α₅β₁ integrin, but not α₄β₁, induced activation of GSK3β through Ser-9 dephosphorylation in adherent U937 cells. The GSK3β-dependent survival pathway occurred in adherent leukemic cells from patients but not in the HL-60 and KG1 cell lines. In adhesion, activated GSK3β was found in the cytosol/plasma membrane compartment and was co-immunoprecipitated with α₅ integrin, the phosphatase PP2A and the scaffolding protein RACK1. PP2A and its regulatory subunit B' regulated the Ser-9 phosphorylation of GSK3β. In adherent leukemic cells, α₅β₁ integrin but not α₄β₁ upregulated the resistance to TNFα-induced apoptosis. Both extrinsic and intrinsic apoptotic pathways were under the control of α₅β₁ and GSK3β.

Conclusions and Significance

Our data show that, upon serum starvation, α₅β₁ integrin engagement could regulate specific pro-survival functions through the activation of GSK3β.

Tribbles-1 and -2 are tumour suppressors, down-regulated in human acute myeloid leukaemia

Constitutive MAPK signalling is observed in approximately 50% of acute myeloid leukaemia (AML) cases. JNK activation in particular is associated with treatment failure in AML. Tribbles proteins (trb-1, trb-2 and trb-3) are potent negative regulators of MAPK pathways influencing apoptosis, differentiation and cell-cycle progression. Here we aimed to examine tribbles gene expression in AML and to characterise their role in leukaemic cells. A microarray dataset was interrogated for tribbles expression levels in AML cases and healthy controls. Myeloid cell proliferation and apoptosis were assayed in response to trb-1/trb-2 gene knockdown and overexpression, as well as a physical and functional interaction between trb and C/EBPalpha. Trb-2 expression was reduced in AML compared to healthy controls (correlating with nucleophosmin (NPM1) mutations), while low trb-1 expression was associated with inactive C/EBPalpha. In vitro assays indicated that trb-1/trb-2 are growth restrictive and pro-apoptotic in Me-1 cells, each capable of inhibiting JNK activation. JNK inactivation was itself associated with reduced Bcl-2 Ser70 phosphorylation, a residue which, when phosphorylated, maintains the anti-apoptotic activity of Bcl-2. Consistent with this, tribbles-mediated dephosphorylation of Bcl-2 Ser70 was associated with subsequent apoptosis. Trb-1/trb-2 transcription appeared to be moderately C/EBPalpha-responsive, and physical interaction between C/EBPalpha and trb-1/trb-2 was observed, suggesting a potential for auto-regulation of trb-1 and trb-2 transcription. In conclusion, we propose that trb-1 and trb-2 tumour suppressor activity may be abrogated in a proportion of AML patients. This may lead to enhanced cell survival, and therefore contribute to pathogenesis of the disease. Trb-1/trb-2 may, therefore, represent useful therapeutic targets for the treatment of AML in patients with dys-regulated trb activity.

MAPK signaling pathways in the regulation of hematopoiesis

The MAPKs are a family of serine/threonine kinases that play an essential role in connecting cell-surface receptors to changes in transcriptional programs. MAPKs are part of a three-component kinase module consisting of a MAPK, an upstream MEK, and a MEKK that couples the signals from cell-surface receptors to trigger downstream pathways. Three major groups of MAPKs have been characterized in mammals, including ERKs, JNKs, and p38MAPKs. Over the last decade, extensive work has established that these proteins play critical roles in the regulation of a wide variety of cellular processes including cell growth, migration, proliferation, differentiation, and survival. It has been demonstrated that ERK, JNK, and p38MAPK activity can be regulated in response to a plethora of hematopoietic cytokines and growth factors that play critical roles in hematopoiesis. In this review, we summarize the current understanding of MAPK function in the regulation of hematopoiesis in general and myelopoiesis in particular. In addition, the consequences of aberrant MAPK activation in the pathogenesis of various myeloid malignancies will be discussed.

Isolation of intrinsically active (MEK-independent) variants of the ERK family of mitogen-activated protein (MAP) kinases

MAPKs are key components of cell signaling pathways with a unique activation mechanism: i.e. dual phosphorylation of neighboring threonine and tyrosine residues. The ERK enzymes form a subfamily of MAPKs involved in proliferation, differentiation, development, learning, and memory. The exact role of each Erk molecule in these processes is not clear. An efficient strategy for addressing this question is to activate individually each molecule, for example, by expressing intrinsically active variants of them. However, such molecules were not produced so far. Here, we report on the isolation, via a specifically designed genetic screen, of six variants (each carries a point mutation) of the yeast MAPK Mpk1/Erk that are active, independent of upstream phosphorylation. One of the activating mutations, R68S, occurred in a residue conserved in the mammalian Erk1 (Arg-84) and Erk2 (Arg-65) and in the Drosophila ERK Rolled (Arg-80). Replacing this conserved Arg with Ser rendered these MAPKs intrinsically active to very high levels when tested in vitro as recombinant proteins. Combination of the Arg to Ser mutation with the sevenmaker mutation (producing Erk2(R65S+D319N) and Rolled(R80S+D334N)) resulted in even higher activity (45 and 70%, respectively, in reference to fully active dually phosphorylated Erk2 or Rolled). Erk2(R65S) and Erk2(R65S+D319N) were found to be spontaneously active also when expressed in human HEK293 cells. We further revealed the mechanism of action of the mutants and show that it involves acquisition of autophosphorylation activity. Thus, a first generation of Erk molecules that are spontaneously active in vitro and in vivo has been obtained.

Wiring the cell signaling circuitry by the NF-kappa B and JNK1 crosstalk and its applications in human diseases

Integration of the cell signaling circuitry determines the ultimate response of a cell to extracellular stimuli. The transcription factor nuclear factor-kappa B (NF-kappaB) and mitogen-activated protein kinase JNK1 are major players in the cell signaling circuitry, regulating numerous cellular events and being implicated in the process of many human diseases and certain types of cancer. The interplay between NF-kappaB and JNK1 provides a paradigm that shows how the crosstalk between different signaling pathways decides the function of the cell signaling circuitry. Understanding the wiring of the cell signaling circuitry may hold the key for cell signaling-based therapy of human diseases.

Methylation-controlled J protein promotes c-Jun degradation to prevent ABCB1 transporter expression

Methylation-controlled J protein (MCJ) is a newly identified member of the DnaJ family of cochaperones. Hypermethylation-mediated transcriptional silencing of the MCJ gene has been associated with increased chemotherapeutic resistance in ovarian cancer. However, the biology and function of MCJ remain unknown. Here we show that MCJ is a type II transmembrane cochaperone localized in the Golgi network and present only in vertebrates. MCJ is expressed in drug-sensitive breast cancer cells but not in multidrug-resistant cells. The inhibition of MCJ expression increases resistance to specific drugs by inducing expression of the ABCB1 drug transporter that prevents intracellular drug accumulation. The induction of ABCB1 gene expression is mediated by increased levels of c-Jun due to an impaired degradation of this transcription factor in the absence of MCJ. Thus, MCJ is required in these cells to prevent c-Jun-mediated expression of ABCB1 and maintain drug response.

Mitochondrial c-Jun NH2-terminal kinase prevents the accumulation of reactive oxygen species and reduces necrotic damage in neural tumor cells that lack trophic support

In response to different stress signals, the c-Jun NH(2)-terminal kinase (JNK) can trigger cell death. However, JNK also facilitates the survival and cell cycle progression of tumor cells by mechanisms that are poorly defined. Here, we show that schwannoma RN22 cells can survive and proliferate under serum-free conditions although serum withdrawal rapidly induces mitochondrial fission and swelling. Although the morphologic changes observed in the mitochondria did not trigger cytochrome c release, they were accompanied by an increase in the mitochondrial membrane potential (DeltaPsi(M)) and of immunoreactivity for active JNK in these organelles. Pharmacologic inhibition of JNK provoked a further increase of the DeltaPsi(M), an increase in reactive oxygen species (ROS) production, and a sustained decrease in cell viability due to necrosis. This increase in necrosis was prevented by the presence of ROS scavengers. Immunoreactivity for active JNK was also observed in the mitochondria of neuroblastoma 1E-115 and neuroblastoma 2a neuroblastoma cell lines on serum withdrawal, whereas active JNK was barely detected in serum-deprived fibroblasts. Accordingly, the reduction in neural tumor cell viability induced by JNK inhibition was largely attenuated in serum-deprived fibroblasts. These data indicate that local activation of JNK in the mitochondria can protect against necrotic cell death associated with ROS production, facilitating the growth of neural tumor cells subjected to serum deprivation.

Portrait of multifaceted transporter, the multidrug resistance-associated protein 1 (MRP1/ABCC1)

MRP1 (ABCC1) is a peculiar member of the ABC transporter superfamily for several aspects. This protein has an unusually broad substrate specificity and is capable of transporting not only a wide variety of neutral hydrophobic compounds, like the MDR1/P-glycoprotein, but also facilitating the extrusion of numerous glutathione, glucuronate, and sulfate conjugates. The transport mechanism of MRP1 is also complex; a composite substrate-binding site permits both cooperativity and competition between various substrates. This versatility and the ubiquitous tissue distribution make this transporter suitable for contributing to various physiological functions, including defense against xenobiotics and endogenous toxic metabolites, leukotriene-mediated inflammatory responses, as well as protection from the toxic effect of oxidative stress. In this paper, we give an overview of the considerable amount of knowledge which has accumulated since the discovery of MRP1 in 1992. We place special emphasis on the structural features essential for function, our recent understanding of the transport mechanism, and the numerous assignments of this transporter.

c-Jun NH(2)-terminal kinase 2alpha2 promotes the tumorigenicity of human glioblastoma cells

c-Jun NH(2)-terminal kinases (JNK) are members of the mitogen-activated protein kinase family and have been implicated in the formation of several human tumors, especially gliomas. We have previously shown that a 55 kDa JNK isoform is constitutively active in 86% of human brain tumors and then showed that it is specifically a JNK2 isoform and likely to be either JNK2alpha2 or JNK2beta2. Notably, we found that only JNK2 isoforms possess intrinsic autophosphorylation activity and that JNK2alpha2 has the strongest activity. In the present study, we have further explored the contribution of JNK2 isoforms to brain tumor formation. Analysis of mRNA expression by reverse transcription-PCR revealed that JNK2alpha2 is expressed in 91% (10 of 11) of glioblastoma tumors, whereas JNK2beta2 is found in only 27% (3 of 11) of tumors. Both JNK2alpha2 and JNK2beta2 mRNAs are expressed in normal brain (3 of 3). Using an antibody specific for JNK2alpha isoforms, we verified that JNK2alpha2 protein is expressed in 88.2% (15 of 17) of glioblastomas, but, interestingly, no JNK2alpha2 protein was found in six normal brain samples. To evaluate biological function, we transfected U87MG cells with green fluorescent protein-tagged versions of JNK1alpha1, JNK2alpha2, and JNK2alpha2APF (a dominant-negative mutant), and derived cell lines with stable expression. Each cell line was evaluated for various tumorigenic variables including cellular growth, soft agar colony formation, and tumor formation in athymic nude mice. In each assay, JNK2alpha2 was found to be the most effective in promoting that phenotype. To identify effectors specifically affected by JNK2alpha2, we analyzed gene expression. Gene profiling showed several genes whose expression was specifically up-regulated by JNK2alpha2 but down-regulated by JNK2alpha2APF, among which eukaryotic translation initiation factor 4E (eIF4E) shows the greatest change. Because AKT acts on eIF4E, we also examined AKT activation. Unexpectedly, we found that JNK2alpha2 could specifically activate AKT. Our data provides evidence that JNK2alpha2 is the major active JNK isoform and is involved in the promotion of proliferation and growth of human glioblastoma tumors through specific activation of AKT and overexpression of eIF4E.

Constitutive c-jun N-terminal kinase activity in acute myeloid leukemia derives from Flt3 and affects survival and proliferation

OBJECTIVE

c-jun N-terminal kinase (JNK) has been implicated in proliferation and survival downstream from the tyrosine kinase oncogene, p210 BCR-ABL, in chronic myeloid leukemia. We studied whether a similar relationship between JNK and FMS-like tyrosine kinase 3 (Flt3) describes acute myeloid leukemia (AML).

METHODS

By immunoprecipitation, Flt3 was found to be activated and identified as the potential origin of JNK activity in a heavy majority of JNK+ve AML blasts tested. Often, Flt3 activity is associated with activating mutation of the gene locus. However, statistical linkage tied JNK activity with Flt3 expression levels rather than with mutation. An adaptor network to describe the signal cascade Flt3-to-JNK was uncovered.

RESULTS

Active Flt3 was linked to p85 phosphoinositide-3 (PI-3) kinase, and p85 with cbl and CrkII/CrkL by co-immunoprecipitaton assays from lysates of model cell lines and primary AML blasts. JNK1 co-immunoprecipitated from such lysates with p85-cbl-crkII/L and bound to Crk species SH3 domain in pull-down assay. siRNA-mediated depletion of Flt3 or of cbl, the adaptor at the nexus of this signaling group, inhibited JNK activity on substrate c-jun. Within AML blast cells influenced by Flt3 signaling, selective inhibition of JNK by a small molecule inhibitor, led to proliferative inhibition, apoptosis, and sensitizing cells to the anthracycline, daunorubicin. These effects occurred upon JNK inhibition without off-target inhibition of extracellular signal-regulated kinase or AKT pathways, and p38-kinase activation, an effector in the p53/p14 arf tumor suppressor pathway, was also maintained or augmented.

CONCLUSION

JNK is a bonafide signaling pathway from Flt3 in AML whose function for proliferation and survival is required in a significant AML cohort with active Flt3 signaling, by mutation or overexpression of Flt3.

In vivo pharmacokinetics, activation of MAPK signaling and induction of phase II/III drug metabolizing enzymes/transporters by cancer chemopreventive compound BHA in the mice

Phenolic antioxidant butylated hydroxyanisole (BHA) is a commonly used food preservative with broad biological activities, including protection against chemical-induced carcinogenesis, acute toxicity of chemicals, modulation of macromolecule synthesis and immune response, induction of phase II detoxifying enzymes, as well as its undesirable potential tumor-promoting activities. Understanding the molecular basis underlying these diverse biological actions of BHA is thus of great importance. Here we studied the pharmacokinetics, activation of signaling kinases and induction of phase II/III drug metabolizing enzymes/transporter gene expression by BHA in the mice. The peak plasma concentration of BHA achieved in our current study after oral administration of 200 mg/kg BHA was around 10 microM. This in vivo concentration might offer some insights for the many in vitro cell culture studies on signal transduction and induction of phase II genes using similar concentrations. The oral bioavailability (F) of BHA was about 43% in the mice. In the mouse liver, BHA induced the expression of phase II genes including NQO-1, HO-1, gamma-GCS, GST-pi and UGT 1A6, as well as some of the phase III transporter genes, such as MRP1 and Slcolb2. In addition, BHA activated distinct mitogen-activated protein kinases (MAPKs), c-Jun N-terminal kinase (JNK), extracellular signal-regulated protein kinase (ERK), as well as p38, suggesting that the MAPK pathways may play an important role in early signaling events leading to the regulation of gene expression including phase II drug metabolizing and some phase III drug transporter genes. This is the first study to demonstrate the in vivo pharmacokinetics of BHA, the in vivo activation of MAPK signaling proteins, as well as the in vivo induction of Phase II/III drug metabolizing enzymes/transporters in the mouse livers.

HIV-TAT protein upregulates expression of multidrug resistance protein 1 in the blood-brain barrier

Central nervous system (CNS) complications of human immunodeficiency virus (HIV) infection remain a serious health risk in HIV/acquired immunodeficiency syndrome despite significant advances in highly active antiretroviral therapy (HAART). Specific drugs used for HAART are substrates for the efflux transport systems, such as the multidrug resistance-associated proteins (MRPs), which are present on brain microvascular endothelial cells (BMEC) and astrocytes, that is, the main cell types that form the blood-brain barrier (BBB). Thus, drugs employed in HAART are actively removed from the CNS and do not efficiently inhibit HIV replication in the brain. To study the potential mechanisms of this process, the aim of the present research was to address the hypothesis that HIV Tat protein can contribute to upregulation of MRP expression at the BBB level. Tat is a protein produced and released by HIV-infected cells, which may play an important role in brain vascular pathology in the course of HIV infection. Among the family of MRPs, exposure to Tat specifically induced MRP1 messenger ribonucleic acid and protein expression both in BMEC and astrocytes. These alterations were accompanied by enhanced MRP1-mediated efflux functions. Furthermore, activation of the mitogen-activated protein kinase signaling cascade was identified as the mechanism involved in Tat-mediated overexpression of MRP1. These results indicate that Tat exposure can lead to alterations of the BBB functions and decrease HAART efficacy in the CNS through overexpression of drug efflux transporters.

Doxorubicin induces expression of multidrug resistance-associated protein 1 in human small cell lung cancer cell lines by the c-jun N-terminal kinase pathway

Multidrug resistance (MDR) is a major impediment to successful chemotherapy for lung cancer. Overexpression of multidrug resistance-associated protein 1 (MRP1) appears to be involved in MDR development in lung cancer cells. A number of chemotherapeutic agents including doxorubicin (DOX) were reported to induce MRP1 expression in human lung cancer cells. In our study, we investigated the mechanism by which DOX induces MRP1 expression in human small cell lung cancer (SCLC) cell lines, GLC4 and NCI-H82. These cells expressed MRP1 protein at low levels and were sensitive to DOX. Doxorubicin at 50 nM induced a marked increase in MRP1 expression in 24 hr, and stimulated c-jun N-terminal kinase (JNK) activity. Treatment with a JNK inhibitor, SP600125, significantly inhibited MRP1 induction. Furthermore, transfection with JNK1 and JNK2 antisense oligonucleotides markedly inhibited DOX-induced MRP1 expression. Chromatin immunoprecipitation assays revealed an enhanced recruitment of phosphorylated c-jun to the MRP1 promoter containing the AP-1 site upon DOX stimulation, which was inhibited by pretreatment with SP600125. Surprisingly, GLC4 cells exposed to DOX for 24 hr maintained increased MRP1 expression and resistance to DOX for at least 3 weeks. Pretreatment with SP600125 before DOX stimulation blocked the appearance of the MDR phenotype as well as MRP1 induction in GLC4 cells. These findings suggest that JNK activation may play an essential role for the induction of MRP1 protein in human SCLC cells by chemotherapeutic agents and that combined treatment of a JNK inhibitor with anticancer drugs may prevent the development of MDR by the abrogation of MRP1 induction.

12-O-Tetradecanoylphorbol-13-acetate (TPA)-induced dual-specificity phosphatase expression and AML cell survival

12-O-Tetradecanoylphorbol-13-acetate (TPA) is being developed as a therapeutic agent by virtue of its being a potent modulator of signal transduction in pre-clinical models of AML [Strair RK, Schaar D, Goodell L, Aisner J, Chin KV, Eid J, et al. Administration of a phorbol ester to patients with hematological malignancies: preliminary results from a phase I clinical trial of 12-O-tetradecanoylphorbol-13-acetate. Clin Cancer Res 2002;8:2512-8]. In this report, we identify a subset of primary AML samples that undergoes apoptosis after exposure to TPA and demonstrate that TPA-induced cytotoxicity is associated with modulation of the ERK signaling pathway. Analysis of mitogen-activated protein kinase (MAPK) dual-specificity phosphatases (DUSP), as potential regulators of AML cell signaling, indicates that these genes are coordinately regulated and rapidly induced by TPA in primary AML cells. Therefore, TPA-induced primary AML cytotoxicity is associated with modulation of ERK signaling which may be partially mediated by regulation of phosphatase expression.

Mercury induces multidrug resistance-associated protein gene through p38 mitogen-activated protein kinase

The multidrug resistance-associated protein (MRP1) belongs to a drug efflux membrane pump that confers multidrug resistance to the cells. The MRP1 mediates the cellular efflux of various xenobiotics including heavy metals and mediates cellular resistance to heavy metals. Mercury is a well-known health hazard and an environmental contaminant. Recently, information about the uptake of the heavy metals such as mercury has been suggested. However, little is known regarding molecular mechanisms of exporting mercury. This study was designed to determine if mercury could be extruded by MRP1 in acute myeloid leukemia cells (AML-2). The MRP-1-overexpressing AML-2/DX100 cells showed a higher resistance to mercury than AML-2/WT. Probenecid, which is a specific MRP1 inhibitor, decreased the resistance to mercury. Exposing the AML-2 cells to mercury-induced MRP1 gene expression and production without altering the MRP1 activity. Mercury activated p38 mitogen-activated protein kinase (MAPK) and SB 203580, a specific p38 MAPK inhibitor, blocked the mercury-induced MRP1 production. These results suggest that MRP1 can control mercury and p38 MAPK mediates the mercury-induced MRP1 gene expression.

Targeting the multidrug resistance-1 transporter in AML: molecular regulation and therapeutic strategies

The multidrug resistance-1 (MDR1) gene product, P-glycoprotein (P-gp), and the multidrug resistance-related proteins (MRPs) are members of the adenosine triphosphate (ATP)-binding cassette (ABC) transporter gene superfamily that regulates the trafficking of drugs, peptides, ions, and xenobiotics across cell membrane barriers. Three-dimensional modeling of human MDR1/P-gp indicates that these glycoproteins function as efficient, ATP-dependent gate-keepers, which scan the plasma membrane and its inner leaflet to flip lipophilic substrates to the outer membrane leaflet. Delineation of the adverse prognostic power of MDR1 in adult acute myeloid leukemia (AML) raised hopes that pharmacologic blockade of P-gp would improve the outcome of conventional cytotoxic therapy, perhaps more so than in any other human malignancy. Phase 3 clinical trials investigating first- and second-generation P-gp antagonists have yielded conflicting results, emphasizing the importance of applying preclinical principals to realistically appraise expectations for clinical benefit. Structure-based design strategies and the delineation of transcriptional regulators of survival gene cassettes promise to yield novel, more-effective strategies to overcome drug resistance. Lessons learned from investigations of these and other mechanisms of cellular defense hold promise for a renaissance in the development of targeted therapeutics in acute leukemia.

Downmodulation of ERK activity inhibits the proliferation and induces the apoptosis of primary acute myelogenous leukemia blasts

MAP kinase/ERK kinase (MEK)-extracellular signal-regulated kinase (ERK) kinases are frequently activated in acute myelogenous leukemia (AML), and can have prosurvival function. The purpose of this study was to induce downmodulation of MEK-ERK activation in AML primary blasts in order to detect the effect on cell cycle progression and on the apoptosis of leukemic cells. We investigated 14 cases of AML with high ERK 1/2 activity and four cases with undetectable or very low activity. After 24 h incubation of the AML blasts with high ERK activity using PD98059 (New England BioLabs, Beverly, MA, USA), a selective inhibitor of MEK1 phosphorylation, at concentrations of 20 and 40 microM, we observed a strong decrease in the levels of ERK1/2 activity. A significant decrease of blast cell proliferation compared with untreated controls was found. In contrast, the proliferation of blast cells that expressed low or undetectable levels of ERK activity was not inhibited. Time-course analysis demonstrated that the downmodulation of MEK1/2, ERK1 and ERK2 dual-phosphorylation was evident even after 3 h of treatment with 20 and 40 microM. The cleavage of poly(ADP-ribose) polymerase (PARP), an early sign of apoptosis, appeared after 18 h of PD98059 treatment at concentrations of 20 and 40 microM in eight of the 14 cases. After 24 h of treatment, cleaved PARP appeared in all 14 cases. Time-course analysis of cell cycle progression and apoptosis showed that PD98059 induced a G1-phase accumulation with low or undetectable levels of apoptosis after 24 h incubation; after 48 and 72 h incubation, a significant increase of apoptosis was observed. Thus, the primary effect of ERK downmodulation was a cell cycle arrest followed by the apoptosis of a significant percentage of the leukemic blasts. The preclinical model of leukemia treatment reported in this paper makes further comment with regard to MEK1 inhibition as a useful antileukemic target, and encourages the conducting of in vivo studies and clinical investigations.

Map kinase signaling pathways and hematologic malignancies

Mitogen-activated protein (Map) kinases are widely expressed serine-threonine kinases that mediate important regulatory signals in the cell. Three major groups of Map kinases exist: the p38 Map kinase family, the extracellular signal-regulated kinase (Erk) family, and the c-Jun NH2-terminal kinase (JNK) family. The members of the different Map kinase groups participate in the generation of various cellular responses, including gene transcription, induction of cell death or maintenance of cell survival, malignant transformation, and regulation of cell-cycle progression. Depending on the specific family isoform involved and the cellular context, Map kinase pathways can mediate signals that either promote or suppress the growth of malignant hematopoietic cells. Over the last few years, extensive work by several groups has established that Map kinase pathways play critical roles in the pathogenesis of various hematologic malignancies, providing new molecular targets for future therapeutic approaches. In this review, the involvement of various Map kinase pathways in the pathophysiology of hematologic malignances is summarized and the clinical implications of the recent advances in the field are discussed.

The activation of c-Jun NH2-terminal kinase (JNK) by DNA-damaging agents serves to promote drug resistance via activating transcription factor 2 (ATF2)-dependent enhanced DNA repair

The activating transcription factor 2 (ATF2) is a member of the ATF/cAMP-response element-binding protein family of basic-leucine zipper proteins involved in cellular stress response. The transcription potential of ATF2 is enhanced markedly by NH2-terminal phosphorylation by c-Jun NH2-terminal kinase (JNK) and mediates stress responses including DNA-damaging events. We have observed that four DNA-damaging agents (cisplatin, actinomycin D, MMS, and etoposide), but not the cisplatin isomer, transplatin, which does not readily damage DNA, strongly activate JNK, p38, and extracellular signal-regulated kinase (ERK), and strongly increase phosphorylation and ATF2-dependent transcriptional activity. Selective inhibition studies with PD98059, SB202190, SP600125, and the dominant negative JNK indicate that activation of JNK but not p38 kinase or ERK kinase is required for the phosphorylation and transcriptional activation of ATF2. Stable expression of ATF2 in human breast carcinoma BT474 cells increases transcriptional activity and confers resistance to the four DNA-damaging agents, but not to transplatin. Conversely, stable expression of a dominant negative ATF2 (dnATF2) quantitatively blocks phosphorylation of endogenous ATF2 leading to a marked decrease in transcriptional activity by endogenous ATF2 and a markedly increased sensitivity to the four agents as judged by decreased cell viability. Similarly, application of SB202190 at 50 micro m or SP600125 inhibited JNK activity, blocked transactivation, and sensitized parental cells to the four DNA-damaging drugs. Moreover, the wild type ATF2-expressing clones exhibited rapid DNA repair after treatment with the four DNA-damaging agents but not transplatin. Conversely, expression of dnATF2 quantitatively blocks DNA repair. These results indicate that JNK-dependent phosphorylation of ATF2 plays an important role in the drug resistance phenotype likely by mediating enhanced DNA repair by a p53-independent mechanism. JNK may be a rational target for sensitizing tumor cells to DNA-damaging chemotherapy agents.

Activation of the JNK signaling pathway: breaking the brake on apoptosis

The JNK signaling pathway is involved in regulation of many cellular events, including growth control, transformation and programmed cell death (apoptosis). The role of JNK activation in apoptosis is highly controversial, being suggested to have a pro-apoptotic, anti-apoptotic or no role in this process. It appears that the JNK pathway functions in a cell-type and stimulus-dependent manner and its different components can sometimes play opposing roles in apoptosis. Recent studies reveal that the effect of JNK activation on apoptosis depends on the activity of other signaling pathways like the NF-kappaB pathway. Here we propose a model that can explain how activation of the JNK pathway "breaks the brake" on apoptosis, thereby regulating, but not initiating the apoptotic process.