Transcriptional regulation of MDR-1 by HOXC6 in multidrug-resistant cells

HDAC and MEK inhibition synergistically suppresses HOXC6 and enhances PD-1 blockade efficacy in BRAFV600E-mutant microsatellite stable colorectal cancer

BACKGROUND

B-Raf proto-oncogene, serine/threonine kinase (BRAF)V600E-mutant microsatellite stable (MSS) colorectal cancer (CRC) constitutes a distinct CRC subgroup, traditionally perceived as minimally responsive to standard therapies. Recent clinical attempts, such as BRAF inhibitors (BRAFi) monotherapy and combining BRAFi with other inhibitors, have yielded unsatisfactory efficacy. This study aims to identify a novel therapeutic strategy for this challenging subgroup.

METHODS

We first performed a large-scale drug screening using patient-derived organoid models and cell lines to pinpoint potential therapies. Subsequently, we investigated the synergistic effects of identified effective inhibitors and probed their cooperative mechanisms. Concurrently, we explored the immune characteristics of BRAFV600E MSS CRC using RNA sequencing and multiplex immunohistochemistry. Finally, we established a CT26 BRAFV637E mouse cell line and validated the efficacy of combining these inhibitors and programmed death 1 (PD-1) blockades in immunocompetent mice.

RESULTS

Drug screening identified histone deacetylase (HDAC) inhibitor and mitogen-activated protein kinase kinase (MEK) inhibitor as significantly effective against BRAFV600E MSS CRC. Further research revealed that these two inhibitors have superior synergistic effects by comprehensively inhibiting the activation of the epidermal growth factor receptor, mitogen-activated protein kinase, and phosphoinositide 3-kinase-protein kinase B pathways and suppressing the key target homeobox C6 (HOXC6). HOXC6, overexpressed in BRAFV600E MSS CRC, regulates the MYC gene and contributes to treatment resistance, tumor growth, and metastasis. Moreover, the combination therapy demonstrated the ability to enhance antitumor immunity by synergistically upregulating the expression of immune activation-related genes, activating the cyclic guanosine monophosphate-adenosine monophosphate synthase/stimulator of interferon genes (cGAS/STING) pathway, and diminishing the tumor cells' DNA mismatch repair capacity. Notably, BRAFV600E MSS CRC was identified to exhibit a distinct immune microenvironment with increased PD-1+ cell infiltration and potential responsiveness to immunotherapy. Echoing the above findings, in vivo, HDAC and MEK inhibitors significantly improved PD-1 blockade efficacy, accompanied by increased CD8+ T-cell infiltration.

CONCLUSIONS

Our findings indicate that combining HDAC inhibitor, MEK inhibitor, and PD-1 blockade is a potential strategy for treating BRAFV600E-mutant MSS CRC, warranting further investigation in clinical settings.

Exploring the prognosis value, immune correlation, and drug responsiveness prediction of homeobox C6 (HOXC6) in lung adenocarcinoma

BACKGROUNDS

Homeobox C6 (HOXC6) is a gene that encodes for a transcription factor involved in various cellular processes, including development and differentiation, and regulates cancer progression. However, the carcinogenesis and effect of HOXC6 in lung adenocarcinoma (LUAD) still need further investigation.

METHODS

The differential HOXC6 expression levels at the mRNA and protein level were explored in multiple public datasets, including The Cancer Genome Atlas (TCGA) and Human Protein Atlas (HPA) dataset. Gene Expression Omnibus (GSE31210), International Cancer Genome Consortium (ICGC) datasets and the LUAD sample from Affiliated Hospital of Guangxi Medical University. We also investigated the relation between HOXC6 expression and clinicopathologic indexes. Furthermore, the correlation of immune infiltration, drug responsiveness and HOXC6 were explored.

RESULTS

The upregulated HOXC6 expressions at mRNA and protein levels were found in LUAD tissues compared to the normal lung tissues. Besides, the relatively shorter overall survival time, worse T and N stages, and lower immune scores were found in the high-expression HOXC6 subgroup. Notably, T cells regulatory (Tregs), Macrophages M0, and Plasma cells had the higher infiltration levels in the high-HOXC6 expression subgroup, while NK cells activated, Monocytes, Dendritic cells resting, and Mast cells resting had the lower infiltration levels. In drug sensitivity analysis, we revealed that LUAD patients with high-HOXC6 expression may be more susceptible to Camptothecin, Cytarabine, Docetaxel, Elesclomol, Rapamycin, Sorafinib, Temsirolimus, and Vorinostat.

CONCLUSIONS

Taken together, there is a great potential for HOXC6 to become a prognosis biomarker and contribute to develop treatment strategies for LUAD patients. Further mechanism exploration and drug development for HOXC6 are needed.

Development of an exosome-related and immune microenvironment prognostic signature in colon adenocarcinoma

Background: The correlation between exosomes and the tumor immune microenvironment has been proved to affect tumorigenesis and progression of colon adenocarcinoma (COAD). However, it remained unclear whether exosomes had an impact on the prognostic indications of COAD patients.

Methods: Expression of exosome-related genes (ERGs) and clinical data were downloaded from The Cancer Genome Atlas (TCGA) and Gene Expression Omnibus (GEO) database. The ERGs associated with prognosis were identified and exosome-related prognostic signature was constructed. Patients in two risk groups were classified according to the risk score calculation formula: Risk score = 1.0132 * CCKBR + 0.2416 * HOXC6 + 0.7618 * POU4F1. The expression of three ERGs was investigated by qRT-PCR. After that, we developed a nomogram predicting the likelihood of survival and verified its predictive efficiency. The differences of tumor immune microenvironment, immune cell infiltration, immune checkpoint and sensitivity to drugs in two risk groups were analyzed.

Results: A prognostic signature was established based on the three ERGs (CCKBR, HOXC6, and POU4F1) and patients with different risk group were distinguished. Survival analysis revealed the negative associated of risk score and prognosis, ROC curve analyses showed the accuracy of this signature. Three ERGs expression was investigated by qRT-PCR in three colorectal cancer cell lines. Moreover, risk score was positively correlated with tumor mutational burden (TMB), immune activities, microsatellite instability level, the expression of immune checkpoint genes. Meanwhile, the expression level of three ERGs and the risk score were markedly related with the sensitive response to chemotherapy.

Conclusion: The novel signature composed of three ERGs with precise predictive capabilities can be used to predict prognosis and provide a promising therapeutic target for improving the efficacy of immunotherapy.

HOXC6-Mediated miR-188-5p Expression Induces Cell Migration through the Inhibition of the Tumor Suppressor FOXN2

Homeobox C6 (HOXC6) is a transcription factor that plays a role in the malignant progression of various cancers. However, the roles of HOXC6 and its regulatory mechanism remain unclear. In this study, we used microRNA (miRNA) regulatory networks to identify key regulatory interactions responsible for HOXC6-mediated cancer progression. In microarray profiling of miRNAs, the levels of miRNAs such as hsa-miR-188-5p, hsa-miR-8063, and hsa-miR-8064 were significantly increased in HOXC6-overexpressing cells. Higher positive expression rates of HOXC6 and miR-188-5p were observed in malignant cancer. We also found that HOXC6 significantly upregulated miR-188-5p expression. The underlying function of HOXC6-mediated miR-188-5p expression was predicted through TargetScan and the MiRNA Database. Overexpression of mir-188-5p inhibited the expression of forkhead box N2 (FOXN2), a tumor suppressor gene. Furthermore, in the luciferase assay, miR-188-5p bound to the 3'-UTR of FOXN2 and was mainly responsible for the dysregulation of FOXN2 expression. Silencing FOXN2 induced cell migration, and the effect of FOXN2 silencing was enhanced when the HOXC6/miR-188-5p axis was induced. These results suggest that HOXC6/miR-188-5p may induce malignant progression in cancer by inhibiting the activation of the FOXN2 signaling pathway.

A forward genetic screen identifies modifiers of rocaglate responsiveness

Rocaglates are a class of eukaryotic translation initiation inhibitors that are being explored as chemotherapeutic agents. They function by targeting eukaryotic initiation factor (eIF) 4A, an RNA helicase critical for recruitment of the 40S ribosome (and associated factors) to mRNA templates. Rocaglates perturb eIF4A activity by imparting a gain-of-function activity to eIF4A and mediating clamping to RNA. To appreciate how rocaglates could best be enabled in the clinic, an understanding of resistance mechanisms is important, as this could inform on strategies to bypass such events as well as identify responsive tumor types. Here, we report on the results of a positive selection, ORFeome screen aimed at identifying cDNAs capable of conferring resistance to rocaglates. Two of the most potent modifiers of rocaglate response identified were the transcription factors FOXP3 and NR1I3, both of which have been implicated in ABCB1 regulation-the gene encoding P-glycoprotein (Pgp). Pgp has previously been implicated in conferring resistance to silvestrol, a naturally occurring rocaglate, and we show here that this extends to additional synthetic rocaglate derivatives. In addition, FOXP3 and NR1I3 impart a multi-drug resistant phenotype that is reversed upon inhibition of Pgp, suggesting a potential therapeutic combination strategy.

The potential value of dequalinium chloride in the treatment of cancer: Focus on malignant glioma

Dequalinium chloride has been known as one kind of antibiotic that displays a broad antimicrobial spectrum and has been clinically proven to be very safe. In recent years, studies have shown that dequalinium chloride can inhibit the growth of malignant tumours, and reports were mainly used for solid tumours. Glioblastoma is the most common malignant neuroepithelial tumour of the central nervous system in adults, and the prognosis of glioblastoma is poor as it has a high resistance to apoptosis. This review summarizes the current understanding of dequalinium chloride-induced cancer cell apoptosis and its potential role in glioblastoma resistance and progression. Particularly, we focus on dequalinium chloride as it exerts a wide range of anti-cancer activity through its ability to target and accumulate in the mitochondria, and it effectively inhibits the growth of glioblastoma cells in vitro and vivo. Dequalinium chloride is an inhibitor of XIAP and can also act as a mitochondrial targeting agent, which gives it an interesting perspective regarding recent advances in the treatment of malignant glioma.

HomeoboxC6 promotes metastasis by orchestrating the DKK1/Wnt/β-catenin axis in right-sided colon cancer

Patients with right-sided colon cancer (RCC) generally have a poorer prognosis than those with left-sided colon cancer (LCC). We previously found that homeobox C6 (HOXC6) was the most significantly upregulated gene in RCC compared to LCC. However, it remains unclear whether HOXC6 plays a role in tumor proliferation and metastasis. Our study aimed to explore the potential oncogenic role and the detailed molecular mechanism of HOXC6 in RCC. In this study, HOXC6 was validated to be overexpressed in RCC and associated with poor prognosis. Furthermore, overexpression of HOXC6 promoted the migration and invasion of colon cancer cells through inducing EMT by activating the Wnt/β-catenin signaling pathway and inhibition of DKK1 secretion. Lastly, we preliminary explored the translational effect of HOXC6 and found that silencing of HOXC6 made HCT116 and HT29 cells more sensitive to irinotecan.

Metformin Derivative HL156A Reverses Multidrug Resistance by Inhibiting HOXC6/ERK1/2 Signaling in Multidrug-Resistant Human Cancer Cells

Multidrug resistance is a significant clinical crisis in cancer treatment and has been linked to the cellular expression of multidrug efflux transporters. The aim of this study was to examine the effects and mechanisms of the metformin derivative HL156A on human multidrug resistance (MDR) cancer cells. Here, HL156A significantly suppressed cell growth and colony formation through G2/M phase cell cycle arrest in MDR cancer cells. HL156A also reduced the wound closure rate and cell migration and induced caspase-3-dependent apoptosis. We found that HL156A inhibited the expression of MDR1 by inhibiting the HOXC6-mediated ERK1/2 signaling pathway and increased the sensitivity to paclitaxel or doxorubicin in MDR cells. Furthermore, HL156A significantly inhibited angiogenesis in a chicken chorioallantoic membrane (CAM) assay. These results suggest the potential of the metformin derivative HL156A as a candidate therapeutic modality for the treatment of human multidrug-resistant cancers.

HOXB7 mediates cisplatin resistance in esophageal squamous cell carcinoma through involvement of DNA damage repair

Background

DNA damage repair is an important mechanism of platinum resistance. HOXB7 is one member of HOX family genes, which are essential developmental regulators and frequently dysregulated in cancer. Recently, its relevance in chemotherapy resistance and DNA damage repair has also been addressed. However, little is known regarding the association between HOXB7 and chemotherapy resistance in esophageal squamous cell carcinoma (ESCC).

Methods

The association between HOXB7 expression detected by immunohistochemisty and tumor regression grade (TRG) and long‐term survival was analyzed in 143 ESCC patients who underwent neoadjuvant chemotherapy. CCK8 assay was used to examine the effect of cisplatin in a panel of four ESCC cell lines. A stable cell strain with HOXB7 knockdown of KYSE150 and KYSE450 was established to explore the effect on cisplatin sensitivity. The interaction of HOXB7 with Ku70, Ku80 and DNA‐PKcs was determined by GST‐pull down, coimmunoprecipitation and immunofluorescent colocalization. Finally, we investigated whether disrupting HOXB7 function by a synthetic peptide HXR9 blocking the formation of HOXB7/PBX could enhance cisplatin sensitivity in vitro and in vivo.

Results

High expression of HOXB7 was associated with cisplatin resistance and worse chemotherapy efficacy. HOXB7 knockdown reinforced cisplatin sensitivity. It was identified that HOXB7 interacts with Ku70, Ku80 and DNA‐PKcs. HOXB7 knockdown was related to the downregulation of Ku70, Ku80 and DNA‐PKcs as well as arrested cell cycle in S phase. HOXB7 inhibition by HXR9 had a synergistic effect to improve cisplatin sensitivity.

Conclusion

HOXB7 may be a biomarker for the prediction of chemoresistance of ESCC and serves as a promising therapeutic target.

Silencing homeobox C6 inhibits colorectal cancer cell proliferation

Homeobox C6 (HOXC6), a member of the homeobox family that encodes highly conserved transcription factors, plays a vital role in various carcinomas. In this study, we used a tissue microarray (TMA) consisting of 462 CRC samples to demonstrate that HOXC6 is more abundantly expressed in colorectal cancer (CRC) tissues than adjacent normal mucosa. Clinicopathological data indicated that higher HOXC6 expression correlated with poor overall survival and was associated with primary tumor location in the right colon, primary tumor (pT) stage 3/4 and primary node (pN) stage 1/2. Multivariate analysis showed that high HOXC6 expression was an independent risk factor for poor CRC patient prognosis. HOXC6 downregulation via lentivirus-mediated expression of HOXC6-targeting shRNA reduced HCT116 cell viability and colony formation in vitro, and reduced growth of subcutaneous xenografts in nude mouse. HOXC6 thus appears to promote CRC cell proliferation and tumorigenesis through autophagy inhibition and mTOR pathway activation.

HOXC6 Overexpression Is Associated With Ki-67 Expression and Poor Survival in NPC Patients

BACKGROUND: Homeobox (HOX) genes are expressed in adult cells and regulate expression of genes involved in cell proliferation as well as cell-cell and cell-extracellular matrix interactions. Dysregulation of HOX gene expression plays important roles in carcinogenesis in a variety of organs. Through data mining on a published transcriptome dataset, this study first identified Homeobox protein Hox-C6 (HOXC6) gene as one of the differentially upregulated genes in nasopharyngeal carcinoma (NPC). We aimed to evaluate HOXC6 expression and its prognostic effect in a large cohort of NPC patients.

METHODS: We retrospectively examined the HOXC6 expression and Ki-67 index by immunohistochemistry in biopsy specimens from 124 patients with non-metastasized NPC. The results were correlated with the clinicopathological variables including disease-specific survival (DSS), metastasis-free survival (MeFS), and local recurrence-free survival (LRFS).

RESULTS: HOXC6 high expression was positively correlated with increased Ki-67 labeling index, and significantly associated with increment of tumor stage (p=0.024), advanced nodal status (p<0.001) and American Joint Committee on Cancer (AJCC) stage (p=0.002). Its expression also correlated with worse prognosis in terms of DSS (p=0.008), MeFS (p=0.0047) univariately. In multivariate analyses, HOXC6 expression still remained prognostically independent to portend worse DSS (p=0.015, hazard ratio=1.988) and MeFS (p=0.036, hazard ratio=1.899), together with stage III-IV (p=0.024, DSS; p=0.043, MeFS).

CONCLUSION: In summary, our results suggest HOXC6 may play a critical role in NPC progression and may serve as a potential prognostic biomarker in NPC patients.

Oncogenic retinoic acid receptor γ knockdown reverses multi-drug resistance of human colorectal cancer via Wnt/β-catenin pathway

Retinoic acid receptor γ (RARγ), a unique member of the nuclear receptor superfamily, plays an important role in the progression of several cancers such as hepatocellular carcinoma, esophageal cancer, and cholangiocarcinoma. However, little is known about the regulatory mechanism of the RARγ expression in colorectal cancer (CRC) progression. In the present study, we found that RARγ was frequently overexpressed in human CRC specimens and CRC cell lines, and it mainly resided in the cytoplasm in CRC specimens. Tissue microarrays showed that RARγ indicated vital clinical significance in CRC. RARγ knockdown neither affected CRC cell proliferation nor blocked the cell cycle of CRC cells. However, RARγ knockdown increased the sensitivity of CRC cells to chemotherapeutics through downregulation of multi-drug resistance 1(MDR1). Further studies suggested that RARγ knockdown resulted in downregulation of MDR1, in parallel with suppression of the Wnt/β-catenin pathway. Moreover, a significantly positive association between RARγ and MDR1 was demonstrated in CRC tissue microarrays. Collectively, these results suggested that overexpression of RARγ contributed to the multidrug chemoresistance of CRC cells, at least in part due to upregulation of MDR1 via activation of the Wnt/β-catenin pathway, indicating that RARγ might serve as a potential therapeutic target for chemoresistant CRC patients.

Investigation of novel LPS-induced differentially expressed long non-coding RNAs in endothelial cells

The molecular mechanisms responsible for sepsis-induced endothelial dysfunction leading to an elevated risk of cardiovascular diseases remain undefined. Endotoxic or septic shock is a potentially lethal complication of systemic infection by Gram-negative bacteria. Lipopolysaccharide (LPS) is a critical glycolipid component of the outer wall of Gram-negative bacteria, and many of the sepsis-associated cellular signals by Gram-negative bacteria are attributed to LPS. Given that LPS has an established role in the pathophysiology of sepsis and long non-coding RNAs (lncRNAs) have been reported to critically regulate vascular homeostasis, a systematic transcriptional survey was conducted to evaluate the impact of LPS stimulation on human endothelial lncRNAs and protein-coding transcripts (mRNAs). LncRNAs and mRNAs from LPS-treated (100 ng/mL; 24 h) human umbilical vein endothelial cells (HUVECs) were profiled with the Arraystar Human lncRNA Expression Microarray V3.0. Of the 30,584 lncRNAs screened, 871 were significantly upregulated and 1068 significantly downregulated (p < 0.05) in response to LPS. In the same HUVEC samples, 733 of the 26,106 mRNAs screened were upregulated and 536 were downregulated. Among the differentially expressed lncRNAs, AL132709.5 was the most upregulated (~70 fold) and CTC-459I6.1 the most downregulated (~28 fold). Bioinformatics analyses indicated that the differentially expressed upregulated mRNAs are primarily enriched in cytokine-cytokine receptor interaction, infectious diseases, TNF signaling pathway, FoxO signaling pathway, and pathways in cancer. This is the first lncRNA and mRNA transcriptome profile of LPS-mediated changes in human endothelial cells. These observations may reveal novel endothelial targets of LPS that may be involved in the vascular pathology of sepsis.

Underexpression of HOXA11 Is Associated with Treatment Resistance and Poor Prognosis in Glioblastoma

Purpose

Homeobox (HOX) genes are essential developmental regulators that should normally be in the silenced state in an adult brain. The aberrant expression of HOX genes has been associated with the prognosis of many cancer types, including glioblastoma (GBM). This study examined the identity and role of HOX genes affecting GBM prognosis and treatment resistance.

Materials and Methods

The full series of HOX genes of five pairs of initial and recurrent human GBM samples were screened by microarray analysis to determine the most plausible candidate responsible for GBM prognosis. Another 20 newly diagnosed GBM samples were used for prognostic validation. In vitro experiments were performed to confirm the role of HOX in treatment resistance. Mediators involved in HOX gene regulation were searched using differentially expressed gene analysis, gene set enrichment tests, and network analysis.

Results

The underexpression of HOXA11 was identified as a consistent signature for a poor prognosis among the HOX genes. The overall survival of the GBM patients indicated a significantly favorable prognosis in patients with high HOXA11 expression (31±15.3 months) compared to the prognoses in thosewith low HOXA11 expression (18±7.3 months, p=0.03). When HOXA11 was suppressed in the GBM cell lines, the anticancer effect of radiotherapy and/or temozolomide declined. In addition, five candidate mediators (TGFBR2, CRIM1, TXNIP, DPYSL2, and CRMP1) that may confer an oncologic effect after HOXA11 suppression were identified.

Conclusion

The treatment resistance induced by the underexpression of HOXA11 can contribute to a poor prognosis in GBM. Further investigation will be needed to confirm the value of HOXA11 as a potential target for overcoming the treatment resistance by developing chemo- or radiosensitizers.

Investigation of TGFβ1-Induced Long Noncoding RNAs in Endothelial Cells

Objective. To evaluate the relationship between TGFβ signaling and endothelial lncRNA expression. Methods. Human umbilical vein endothelial cell (HUVECs) lncRNAs and mRNAs were profiled with the Arraystar Human lncRNA Expression Microarray V3.0 after 24 hours of exposure to TGFβ1 (10 ng/mL). Results. Of the 30,584 lncRNAs screened, 2,051 were significantly upregulated and 2,393 were appreciably downregulated (P < 0.05) in response to TGFβ. In the same HUVEC samples, 2,148 of the 26,106 mRNAs screened were upregulated and 1,290 were downregulated. Of these 2,051 differentially expressed upregulated lncRNAs, MALAT1, which is known to be induced by TGFβ in endothelial cells, was the most (~220-fold) upregulated lncRNA. Bioinformatics analyses indicated that the differentially expressed upregulated mRNAs are primarily enriched in hippo signaling, Wnt signaling, focal adhesion, neuroactive ligand-receptor interaction, and pathways in cancer. The most downregulated are notably involved in olfactory transduction, PI3-Akt signaling, Ras signaling, neuroactive ligand-receptor interaction, and apoptosis. Conclusions. This is the first lncRNA and mRNA transcriptome profile of TGFβ-mediated changes in human endothelial cells. These observations may reveal potential new targets of TGFβ in endothelial cells and novel therapeutic avenues for cardiovascular disease-associated endothelial dysfunction.

The Major Prognostic Features of Nuclear Receptor NR5A2 in Infiltrating Ductal Breast Carcinomas

Background. Gene expression profiles of 181 breast cancer samples were analyzed to identify prognostic features of nuclear receptors NR5A1 and NR5A2 based upon their associated transcriptional networks. Methods. A supervised network analysis approach was used to build the NR5A-mediated transcriptional regulatory network. Other bioinformatic tools and statistical methods were utilized to confirm and extend results from the network analysis methodology. Results. NR5A2 expression is a negative factor in breast cancer prognosis in both ER(-) and ER(-)/ER(+) mixed cohorts. The clinical and cohort significance of NR5A2-mediated transcriptional activities indicates that it may have a significant role in attenuating grade development and cancer related signal transduction pathways. NR5A2 signature that conditions poor prognosis was identified based upon results from 15 distinct probes. Alternatively, the expression of NR5A1 predicts favorable prognosis when concurrent NR5A2 expression is low. A favorable signature of eight transcription factors mediated by NR5A1 was also identified. Conclusions. Correlation of poor prognosis and NR5A2 activity is identified by NR5A2-mediated 15-gene signature. NR5A2 may be a potential drug target for treating a subset of breast cancer tumors across breast cancer subtypes, especially ER(-) breast tumors. The favorable prognostic feature of NR5A1 is predicted by NR5A1-mediated 8-gene signature.

Expression of aryl hydrocarbon receptor nuclear translocator enhances cisplatin resistance by upregulating MDR1 expression in cancer cells

The identification of molecular pathways in cancer cells is important for understanding the cells' underlying biology and for designing effective cancer therapies. We demonstrate that the expression of aryl hydrocarbon receptor nuclear translocator (ARNT) is critical during the development of cisplatin resistance. The reduced expression of ARNT was correlated with cisplatin-induced cell death in drug-sensitive cells. In addition, suppression of ARNT reversed the characteristics of cisplatin-resistant cells, making these cells cisplatin-sensitive, and significantly enhanced caspase-3 activation, DNA fragmentation, and apoptosis. The inhibition of colony formation, regulated by cisplatin, was more significant in ARNT-knockdown cells than in parental cells. In a xenograft analysis of severe combined immunodeficiency mice, cisplatin also efficiently inhibited ARNT-deficient c4 tumors but not ARNT-containing vT2 tumor formation. Furthermore, the downregulation of multidrug resistance 1 (MDR1) expression and retention of drugs in cells caused by suppression of ARNT, resulting in the resensitization of drug-resistant cells to cisplatin, was observed. When overexpressed, ARNT interacted with Sp1 to enhance the expression of MDR1 through Sp1-binding sites on the MDR1 promoter, resulting in a reversal of the effect of cisplatin on cell death. In addition, ARNT-induced MDR1 expression was inhibited in Sp1-knockdown cells. These results reveal previously unrecognized, multifaceted functions of ARNT in establishing the drug-resistant properties of cancer cells by the upregulation of MDR1, highlighting ARNT's potential as a therapeutic target in an important subset of cancers.