Stress Granule Core Protein-Derived Peptides Inhibit Assembly of Stress Granules and Improve Sorafenib Sensitivity in Cancer Cells.

Upon a variety of environmental stresses, eukaryotic cells usually recruit translational stalled mRNAs and RNA-binding proteins to form cytoplasmic condensates known as stress granules (SGs), which minimize stress-induced damage and promote stress adaptation and cell survival. SGs are hijacked by cancer cells to promote cell survival and are consequently involved in the development of anticancer drug resistance. However, the design and application of chemical compounds targeting SGs to improve anticancer drug efficacy have rarely been studied. Here, we developed two types of SG inhibitory peptides (SIPs) derived from SG core proteins Caprin1 and USP10 and fused with cell-penetrating peptides to generate TAT-SIP-C1/2 and SIP-U1-Antp, respectively. We obtained 11 SG-inducing anticancer compounds from cell-based screens and explored the potential application of SIPs in overcoming resistance to the SG-inducing anticancer drug sorafenib. We found that SIPs increased the sensitivity of HeLa cells to sorafenib via the disruption of SGs. Therefore, anticancer drugs which are competent to induce SGs could be combined with SIPs to sensitize cancer cells, which might provide a novel therapeutic strategy to alleviate anticancer drug resistance.

Current and potential roles of RNA modification-mediated autophagy dysregulation in cancer.

Autophagy, a cellular lysosomal degradation and survival pathway, supports nutrient recycling and adaptation to metabolic stress and participates in various stages of tumor development, including tumorigenesis, metastasis, and malignant state maintenance. Among the various factors contributing to the dysregulation of autophagy in cancer, RNA modification can regulate autophagy by directly affecting the expression of core autophagy proteins. We propose that autophagy disorder mediated by RNA modification is an important mechanism for cancer development. Therefore, this review mainly discusses the role of RNA modification-mediated autophagy regulation in tumorigenesis. We summarize the molecular basis of autophagy and the core proteins and complexes at different stages of autophagy, especially those involved in cancer development. Moreover, we describe the crosstalk of RNA modification and autophagy and review the recent advances and potential role of the RNA modification/autophagy axis in the development of multiple cancers. Furthermore, the dual role of the RNA modification/autophagy axis in cancer drug resistance is discussed. A comprehensive understanding and extensive exploration of the molecular crosstalk of RNA modifications with autophagy will provide important insights into tumor pathophysiology and provide more options for cancer therapeutic strategies.

Roles of RNA Methylations in Cancer Progression, Autophagy, and Anticancer Drug Resistance.

RNA methylations play critical roles in RNA processes, including RNA splicing, nuclear export, nonsense-mediated RNA decay, and translation. Regulators of RNA methylations have been shown to be differentially expressed between tumor tissues/cancer cells and adjacent tissues/normal cells. N6-methyladenosine (m6A) is the most prevalent internal modification of RNAs in eukaryotes. m6A regulators include m6A writers, m6A demethylases, and m6A binding proteins. Since m6A regulators play important roles in regulating the expression of oncogenes and tumor suppressor genes, targeting m6A regulators can be a strategy for developing anticancer drugs. Anticancer drugs targeting m6A regulators are in clinical trials. m6A regulator-targeting drugs could enhance the anticancer effects of current chemotherapy drugs. This review summarizes the roles of m6A regulators in cancer initiation and progression, autophagy, and anticancer drug resistance. The review also discusses the relationship between autophagy and anticancer drug resistance, the effect of high levels of m6A on autophagy and the potential values of m6A regulators as diagnostic markers and anticancer therapeutic targets.

The Role of mTORC1 Pathway and Autophagy in Resistance to Platinum-Based Chemotherapeutics.

Cisplatin (cis-diamminedichloroplatinum I) is a platinum-based drug, the mainstay of anticancer treatment for numerous solid tumors. Since its approval by the FDA in 1978, the drug has continued to be used for the treatment of half of epithelial cancers. However, resistance to cisplatin represents a major obstacle during anticancer therapy. Here, we review recent findings on how the mTORC1 pathway and autophagy can influence cisplatin sensitivity and resistance and how these data can be applicable for the development of new therapeutic strategies.

Synergistic effect of anticancer drug resistance and Wnt3a on primary ciliogenesis in A549 cell-derived anticancer drug-resistant subcell lines.

Primary cilia, antenna-like cellular sensor structures, are generated from the mother centriole in the G0/G1 cell-cycle phase under control by cellular signaling pathways involving Wnt, hedgehog, and platelet-derived growth factor. Although primary ciliary dynamics have been reported to be closely related to ciliopathy and tumorigenesis, the molecular basis for the role of primary cilia in human disease is lacking. To clarify how Wnt3a affects primary ciliogenesis in anticancer drug-resistant cells, we derived specific drug-resistant subcell lines from A549 human lung cancer cells using anticancer drugs doxorubicin, dasatinib, and paclitaxel (A549/Dox, A549/Das, and A549/Pac, respectively). The primary cilia-containing cell population and primary cilia length increased in the A549/Dox and A549/Pac subcell lines under increased MDR1 expression, when compared to those in the parental A549 cells. In the A549/Das subcell line, primary cilia length increased but the cell population was not affected. In addition, Wnt3a increased primary cilia-containing cell population and primary cilia length in A549/Dox, A549/Das, and A549/Pac cells, without change of cell growth. Abnormal shapes of primary cilia were frequently observed by anticancer drug resistance and Wnt3a stimulation. Taken together, our results indicate that anticancer drug resistance and Wnt3a affect primary ciliogenesis synergistically, suggesting a potential new strategy for overcoming anticancer drug resistance.

[Mechanisms of Survival of Cytomegalovirus-Infected Tumor Cells].

Human cytomegalovirus (HCMV) DNA and proteins are often detected in malignant tumors, warranting studies of the role that HCMV plays in carcinogenesis and tumor progression. HCMV proteins were shown to regulate the key processes involved in tumorigenesis. While HCMV as an oncogenic factor just came into focus, its ability to promote tumor progression is generally recognized. The review discusses the viral factors and cell molecular pathways that affect the resistance of cancer cells to therapy. CMV inhibits apoptosis of tumor cells, that not only promotes tumor progression, but also reduces the sensitivity of cells to antitumor therapy. Autophagy was found to facilitate either cell survival or cell death in different tumor cells. In leukemia cells, HCMV induces a "protective" autophagy that suppresses apoptosis. Viral factors that mediate drug resistance and their interactions with key cell death pathways are necessary to further investigate in order to develop agents that can restore the tumor sensitivity to anticancer drugs.

Chromosome alignment-maintaining phosphoprotein CHAMP1 plays a role in cell survival through regulating Mcl-1 expression.

Antimitotic drugs such as vinca alkaloids and taxanes cause mitotic cell death after prolonged mitotic arrest. However, a fraction of cells escape from mitotic arrest by undergoing mitotic slippage, which is related to resistance to antimitotic drugs. Tipping the balance to mitotic cell death thus can be a way to overcome the drug resistance. Here we found that depletion of a mitotic regulator, CHAMP1 (chromosome alignment-maintaining phosphoprotein, CAMP), accelerates the timing of mitotic cell death after mitotic arrest. Live cell imaging revealed that CHAMP1-depleted cells died earlier than mock-treated cells in the presence of antimitotic drugs that resulted in the reduction of cells undergoing mitotic slippage. Depletion CHAMP1 reduces the expression of antiapoptotic Bcl-2 family proteins, especially Mcl-1. We found that CHAMP1 maintains Mcl-1 expression both at protein and mRNA levels independently of the cell cycle. At the protein level, CHAMP1 maintains Mcl-1 stability by suppressing proteasome-dependent degradation. Depletion of CHAMP1 reduces cell viability, and exhibits synergistic effects with antimitotic drugs. Our data suggest that CHAMP1 plays a role in the maintenance of Mcl-1 expression, implying that CHAMP1 can be a target to overcome the resistance to antimitotic drugs.

MCM10 compensates for Myc-induced DNA replication stress in breast cancer stem-like cells.

Cancer stem-like cells (CSCs) induce drug resistance and recurrence of tumors when they experience DNA replication stress. However, the mechanisms underlying DNA replication stress in CSCs and its compensation remain unclear. Here, we demonstrate that upregulated c-Myc expression induces stronger DNA replication stress in patient-derived breast CSCs than in differentiated cancer cells. Our results suggest critical roles for mini-chromosome maintenance protein 10 (MCM10), a firing (activating) factor of DNA replication origins, to compensate for DNA replication stress in CSCs. MCM10 expression is upregulated in CSCs and is maintained by c-Myc. c-Myc-dependent collisions between RNA transcription and DNA replication machinery may occur in nuclei, thereby causing DNA replication stress. MCM10 may activate dormant replication origins close to these collisions to ensure the progression of replication. Moreover, patient-derived breast CSCs were found to be dependent on MCM10 for their maintenance, even after enrichment for CSCs that were resistant to paclitaxel, the standard chemotherapeutic agent. Further, MCM10 depletion decreased the growth of cancer cells, but not of normal cells. Therefore, MCM10 may robustly compensate for DNA replication stress and facilitate genome duplication in cancer cells in the S-phase, which is more pronounced in CSCs. Overall, we provide a preclinical rationale to target the c-Myc-MCM10 axis for preventing drug resistance and recurrence of tumors.

New insights into the pharmacological, immunological, and CAR-T-cell approaches in the treatment of hepatocellular carcinoma.

The tyrosine kinase inhibitor (TKI) sorafenib continues to be the anchor drug in the treatment of advanced stage hepatocellular carcinoma (HCC). Other TKIs as well as immune checkpoint inhibitors (ICIs) have also been approved, however the response rates remain poor and heterogeneous among HCC patients, largely due to antitumor drug resistance. Studies aimed at identifying novel biomarkers and developing new strategies to improve the response to current treatment and to overcome drug resistance, are urgently needed. Germline or somatic mutations, neoantigens, and an immunotolerogenic state against constant inflammatory stimuli in the liver, are crucial for the anti-tumor response. A pharmacogenetic approach has been attempted considering germline polymorphisms in genes encoding for proteins involved in drug-targeted pathways. Single gene and comprehensive multi-gene somatic profiling approaches have been adopted in HCC to identify tumor sensitivity scores and immunogenic profiles that can be exploited for new biomarkers and innovative therapeutic approaches. However, the high genomic heterogeneity of tumors and lack of molecularly targeted agents, hamper the discovery of specific molecular markers of resistance to therapy. Adoptive cell therapy with chimeric antigen receptor redirected T (CAR-T) cells targeting specific tumor-associated antigens (TAAs) was proposed recently. The specificity of the chosen TAA, an efficient homing of CAR-T cells to the tumor site, and the ability of CAR-T cells to survive in the tumor microenvironment are central factors in the success of CAR-T therapy. The current review describes the principal systemic treatments for HCC and the molecular evidence regarding potential predictive host and somatic genetic markers, as well as the emerging strategy of liquid biopsy for disease monitoring. Novel immunotherapeutic approaches for HCC treatment, including the use of ICIs and CAR-T, as well as strategies to overcome drug resistance, are discussed.

Hybrid Drugs-A Strategy for Overcoming Anticancer Drug Resistance?

Despite enormous progress in the treatment of many malignancies, the development of cancer resistance is still an important reason for cancer chemotherapy failure. Increasing knowledge of cancers' molecular complexity and mechanisms of their resistance to anticancer drugs, as well as extensive clinical experience, indicate that an effective fight against cancer requires a multidimensional approach. Multi-target chemotherapy may be achieved using drugs combination, co-delivery of medicines, or designing hybrid drugs. Hybrid drugs simultaneously targeting many points of signaling networks and various structures within a cancer cell have been extensively explored in recent years. The single hybrid agent can modulate multiple targets involved in cancer cell proliferation, possesses a simpler pharmacokinetic profile to reduce the possibility of drug interactions occurrence, and facilitates the process of drug development. Moreover, a single medication is expected to enhance patient compliance due to a less complicated treatment regimen, as well as a diminished number of adverse reactions and toxicity in comparison to a combination of drugs. As a consequence, many efforts have been made to design hybrid molecules of different chemical structures and functions as a means to circumvent drug resistance. The enormous number of studies in this field encouraged us to review the available literature and present selected research results highlighting the possible role of hybrid drugs in overcoming cancer drug resistance.

Preclinical therapeutic targets in diffuse midline glioma.

Diffuse midline gliomas (DMG) are rapidly fatal tumors of the midbrain in children, characterized by a diffuse growing pattern and high levels of intrinsic resistance to therapy. The location of these tumors, residing behind the blood-brain barrier (BBB), and the limited knowledge about the biology of these tumors, has hindered the development of effective treatment strategies. However, the introduction of diagnostic biopsies and the implementation of autopsy protocols in several large centers world-wide has allowed for a detailed characterization of these rare tumors. This has resulted in the identification of novel therapeutic targets, as well as major advances in understanding the biology of DMG in relation to therapy resistance. We here provide an overview of the cellular pathways and tumor-specific aberrations that have been targeted in preclinical DMG research, and discuss the advantages and limitations of these therapeutic strategies in relation to therapy resistance and BBB-penetration. Therewith, we aim to provide researchers with a framework for successful preclinical therapy development.

5-Oxo-hexahydroquinoline Derivatives and Their Tetrahydroquinoline Counterparts as Multidrug Resistance Reversal Agents.

Cancer is a leading cause of death worldwide. Multidrug resistance (MDR) is a main reason of chemotherapy failure in many patients and is often related to overexpression of ATP-binding cassette (ABC) transporters, including P-glycoprotein (P-gp/ABCB1). Agents that are capable of modulation of the activity of these transporters might be effective in overcoming MDR. In this study, a new set of 1,4,5,6,7,8-hexahydro 5-oxo quinoline-3-carboxamide derivatives bearing 4-methylthiazole moiety and their tetrahydroquinoline counterparts were synthesized. MDR reversal activity of these 16 newly synthesized derivatives was tested in P-gp overexpressing MES-SA-DX5 human uterine sarcoma cells by flow cytometric determination of Rhodamine123 efflux. The effect of the most potent compounds in induction of apoptosis and alterations of cell cycle was examined in these cells by a flow cytometric method. Inherent cytotoxicity of the synthesized compounds was evaluated against MCF-7, A-549 and K562 cancer cell lines, as well as MES-SA-DX5 and their parental non-resistant MES-SA and also HEK-293 non-cancerous cells by MTT assay. Compounds A1 and A2 with 5-oxo-hexahydroquinoline structure bearing 2,4-dichlorophenyl and 4-bromophenyl moieties, respectively, and their tetrahydroquinoline counterparts B1 and B2 significantly blocked P-gp efflux, induced apoptosis and showed the highest cytotoxicities against MES-SA-DX5 cells. However, only A2 and B2 compounds were relatively selective against cancer and MDR cells as compared to non-resistant and non-cancerous cells. These findings demonstrate that 5-oxo-hexahydroquinoline and 5-oxo-tetrahydroquinoline derivatives represent promising agents with therapeutic potential in drug resistant cancers.

High expression of CD44v9 and xCT in chemoresistant hepatocellular carcinoma: Potential targets by sulfasalazine.

CD44v9 is expressed in cancer stem cells (CSC) and stabilizes the glutamate-cystine transporter xCT on the cytoplasmic membrane, thereby decreasing intracellular levels of reactive oxygen species (ROS). This mechanism confers ROS resistance to CSC and CD44v9-expressing cancer cells. The aims of the present study were to assess: (i) expression status of CD44v9 and xCT in hepatocellular carcinoma (HCC) tissues, including those derived from patients treated with hepatic arterial infusion chemoembolization (HAIC) therapy with cisplatin (CDDP); and (ii) whether combination of CDDP with sulfasalazine (SASP), an inhibitor of xCT, was more effective on tumor cells than CDDP alone by inducing ROS-mediated apoptosis. Twenty non-pretreated HCC tissues and 7 HCC tissues administered HAIC therapy with CDDP before surgical resection were subjected to immunohistochemistry analysis of CD44v9 and xCT expression. Human HCC cell lines HAK-1A and HAK-1B were used in this study; the latter was also used for xenograft experiments in nude mice to assess in vivo efficacy of combination treatment. CD44v9 positivity was significantly higher in HAIC-treated tissues (5/7) than in non-pretreated tissues (2/30), suggesting the involvement of CD44v9 in the resistance to HAIC. xCT was significantly expressed in poorly differentiated HCC tissues. Combination treatment effectively killed the CD44v9-harboring HAK-1B cells through ROS-mediated apoptosis and significantly decreased xenografted tumor growth. In conclusion, the xCT inhibitor SASP augmented ROS-mediated apoptosis in CDDP-treated HCC cells, in which the CD44v9-xCT system functioned. As CD44v9 is typically expressed in HAIC-resistant HCC cells, combination treatment with SASP with CDDP may overcome such drug resistance.

Microenvironmental Niches and Sanctuaries: A Route to Acquired Resistance.

A tumor vasculature that is functionally abnormal results in irregular gradients of metabolites and drugs within the tumor tissue. Recently, significant efforts have been committed to experimentally examine how cellular response to anti-cancer treatments varies based on the environment in which the cells are grown. In vitro studies point to specific conditions in which tumor cells can remain dormant and survive the treatment. In vivo results suggest that cells can escape the effects of drug therapy in tissue regions that are poorly penetrated by the drugs. Better understanding how the tumor microenvironments influence the emergence of drug resistance in both primary and metastatic tumors may improve drug development and the design of more effective therapeutic protocols. This chapter presents a hybrid agent-based model of the growth of tumor micrometastases and explores how microenvironmental factors can contribute to the development of acquired resistance in response to a DNA damaging drug. The specific microenvironments of interest in this work are tumor hypoxic niches and tumor normoxic sanctuaries with poor drug penetration. We aim to quantify how spatial constraints of limited drug transport and quiescent cell survival contribute to the development of drug resistant tumors.

Evaluation of Novel Benzo-annelated 1,4-dihydropyridines as MDR Modulators in Cancer Cells.

BACKGROUND: Multidrug resistance (MDR) is the main problem in anticancer therapy today. Causative transmembrane efflux pumps in cancer cells have been reconsidered as promising anticancer target structures to restore anticancer drug sensitivity by various strategies, including MDR modulators. MDR modulators interfere with the efflux pumps and improve the cellular efficiency of chemotherapeutics. So far, only a few candidates have gone through clinical trials with disappointing results because of low specificity and toxic properties. AIM: This study aimed to find Novel MDR modulators to effectively combat multidrug resistance in cancer cells. OBJECTIVE: We synthesized various novel benzo-annelated 1,4-dihydropyridines to evaluate them as MDR modulators towards ABCB1 in cancer cells. METHODS: Synthesized compounds were purified by column chromatography. The MDR modulation of ABCB1 was determined in cellular efflux assays using the flow cytometry technique and cellular fluorescent measurements by the use of each fluorescent substrate. RESULTS: Compounds were yielded in a two-step reaction with structurally varied components. Further, substituent- dependent effects on the determined MDR inhibiting properties towards ABCB1 were discussed. Cellular studies prove that there is no toxicity or restoration of cancer cell sensitivity towards the used anticancer drug. CONCLUSION: Novel MDR modulators could be identified with favorable methoxy and ester group functions. Their use in both ABCB1 non-expressing and overexpressing cells proves a selective toxicity-increasing effect of the applied anticancer agent in the ABCB1 overexpressing cells, whereas the toxicity effect of the anticancer drug was almost unchanged in the non-expressing cells. These results qualify our novel compounds as perspective anticancer drugs compared to MDR modulators with nonselective toxicity properties.

Cellular Dormancy in Cancer: Mechanisms and Potential Targeting Strategies.

Cancer is a leading cause of disease-related mortality worldwide. Drug resistance is one of the primary reasons for the failure of anticancer therapy. There are a number of underlying mechanisms for anticancer drug resistance including genetic/epigenetic modifications, microenvironmental factors, and tumor heterogeneity. In the present scenario, researchers have focused on these novel mechanisms and strategies to tackle them. Recently, researchers have recognized the ability of cancer to become dormant because of anticancer drug resistance, tumor relapse, and progression. Currently, cancer dormancy is classified into "tumor mass dormancy" and "cellular dormancy." Tumor mass dormancy represents the equilibrium between cell proliferation and cell death under the control of blood supply and immune responses. Cellular dormancy denotes the state in which cells undergo quiescence and is characterized by autophagy, stress-tolerance signaling, microenvironmental cues, and epigenetic modifications. Cancer dormancy has been regarded as the stem of primary or distal recurrent tumor formation and poor clinical outcomes in cancer patients. Despite the insufficiency of reliable models of cellular dormancy, the mechanisms underlying the regulation of cellular dormancy have been clarified in numerous studies. A better understanding of the biology of cancer dormancy is critical for the development of effective anticancer therapeutic strategies. In this review, we summarize the characteristics and regulatory mechanisms of cellular dormancy, introduce several potential strategies for targeting cellular dormancy, and discuss future perspectives.

Targeting TRIP13 for overcoming anticancer drug resistance (Review).

Cancer is one of the greatest dangers to human wellbeing and survival. A key barrier to effective cancer therapy is development of resistance to anti­cancer medications. In cancer cells, the AAA+ ATPase family member thyroid hormone receptor interactor 13 (TRIP13) is key in promoting treatment resistance. Nonetheless, knowledge of the molecular processes underlying TRIP13­based resistance to anticancer therapies is lacking. The present study evaluated the function of TRIP13 expression in anticancer drug resistance and potential methods to overcome this resistance. Additionally, the underlying mechanisms by which TRIP13 promotes resistance to anticancer drugs were explored, including induction of mitotic checkpoint complex surveillance system malfunction, promotion of DNA repair, the enhancement of autophagy and the prevention of immunological clearance. The effects of combination treatment, which include a TRIP13 inhibitor in addition to other inhibitors, were discussed. The present study evaluated the literature on TRIP13 as a possible target and its association with anticancer drug resistance, which may facilitate improvements in current anticancer therapeutic options.

The mystery of titan hunter: Rationalized striking of the MAPK pathway via Newly synthesized 6-Indolylpyridone-3-Carbonitrile derivatives.

MAPK pathway sparkles with RTK activation, passes through subsequent downstream RAS-RAF-MEK-ERK signaling cascades, with consequent direct and indirect CDK4/6 signaling activation, and ends with cell survival, division, and proliferation. However, the emergence of anomalies such as mutations or overexpression in one or more points of the pathway could lead to cancer development and drug resistance. Therefore, designing small inhibitors to strike multitudinous MAPK pathway steps could be a promising synergistic strategy to confine cancer. In this study, twelve 6-indolylpyridone-3-carbonitrile candidates were synthesized and assessed in vitro for antineoplastic activity using four cancer cell lines. The initial antiproliferative screening revealed that compounds 3g, 3h, and 3i were the most potent candidates (GI% Avg = 70.10, 73.94, 74.33%, respectively) compared to staurosporine (GI% Avg = 70.99%). The subsequent safety and selectivity assessment showed that 3h exhibited sub-micromolar inhibition against lung cancer cells (HOP-92 GI50 = 0.75 µM) and 13.7 times selectivity toward cancerous cells over normal cells. As a result, 3h was nominated for deep mechanistic studies which evidenced that compound 3h impressively blocks multiple keystones of the MAPK pathway with nanomolar potency (EGFRWT IC50 = 281 nM, c-MET IC50 = 205 nM, B-RAFWT IC50 = 112 nM, and CDK4/6 IC50 = 95 and 184 nM, respectively). Surprisingly, 3h showed a remarkable potency against mutated EGFR and B-RAF, being 4 and 1.3 more selective to the mutated enzymes over the wild-type forms (EGFRT790M IC50 = 69 nM and B-RAFV600E IC50 = 83 nM). Ultimately, combined molecular docking and molecular dynamics (MD) calculations were executed to inspect the mode of binding and the complex stability of 3h towards the keystones of the MAPK pathway.

Mechanisms of Survival of Cytomegalovirus-Infected Tumor Cells.

Human cytomegalovirus (HCMV) DNA and proteins are often detected in malignant tumors, warranting studies of the role that HCMV plays in carcinogenesis and tumor progression. HCMV proteins were shown to regulate the key processes involved in tumorigenesis. While HCMV as an oncogenic factor just came into focus, its ability to promote tumor progression is generally recognized. The review discusses the viral factors and cell molecular pathways that affect the resistance of cancer cells to therapy. CMV inhibits apoptosis of tumor cells, that not only promotes tumor progression, but also reduces the sensitivity of cells to antitumor therapy. Autophagy was found to facilitate either cell survival or cell death in different tumor cells. In leukemia cells, HCMV induces a "protective" autophagy that suppresses apoptosis. Viral factors that mediate drug resistance and their interactions with key cell death pathways are necessary to further investigate in order to develop agents that can restore the tumor sensitivity to anticancer drugs.

Suppression of the doxorubicin response by hypoxia-inducible factor-1α is strictly dependent on oxygen concentrations under hypoxic conditions.

Hypoxia-inducible factor-1α (HIF-1α) and p53 are involved in anticancer drug resistance under hypoxic conditions. Here, we found that the cytotoxicity of anticancer drugs (doxorubicin, gemcitabine, and cisplatin) was lower at 1% O2 than at 5% O2. We examined the effects of these drugs on HIF-1α and p53 expression under different hypoxic oxygen concentrations. At 5% O2, the drugs decreased HIF-1α expression and increased p53 levels. At 1% O2, the drugs increased HIF-1α expression but did not alter p53 levels. When the HIF-1α protein was stabilized by DMOG under normoxic conditions, doxorubicin did not increase the level of p53 expression. These results show that the maintenance of HIF-1α expression blocked doxorubicin-dependent increases in p53 expression. We hypothesized the mechanism of HIF-1α protein translation might be different between at 5% and at 1% O2, because many reports indicate that the same mechanism of HIF-1α protein stabilization occurs under hypoxic conditions, such as 5% and 1% O2. The level of phosphorylated-4E-BP1, which causes translation of HIF-1α, was higher at 1% O2 than at 5% O2. Our results suggest that the sensitivity of tumor cells to anticancer drugs is dependent oxygen concentrations under hypoxic conditions, and involves 4E-BP1-dependent stabilization of the HIF-1α protein.