Increased Expression Of ABCG2 Research Articles

Multidrug-Resistant Profiles in Non-Small Cell Lung Carcinoma Patient-Derived Cells: Implications for Personalized Approaches with Tyrosine Kinase Inhibitors.

The impact of tyrosine kinase inhibitors (TKIs) on multidrug resistance (MDR) in non-small cell lung carcinoma (NSCLC) is a critical aspect of cancer therapy. While TKIs effectively target specific signaling pathways of cancer cells, they can also act as substrates for ABC transporters, potentially triggering MDR. The aim of our study was to evaluate the response of 17 patient-derived NSCLC cultures to 10 commonly prescribed TKIs and to correlate these responses with patient mutational profiles. Using an ex vivo immunofluorescence assay, we analyzed the expression of the MDR markers ABCB1, ABCC1, and ABCG2, and correlated these data with the genetic profiles of patients for a functional diagnostic approach. NSCLC cultures responded differently to TKIs, with erlotinib showing good efficacy regardless of mutation burden or EGFR status. However, the modulation of MDR mechanisms by erlotinib, such as increased ABCG2 expression, highlights the challenges associated with erlotinib treatment. Other TKIs showed limited efficacy, highlighting the variability of response in NSCLC. Genetic alterations in signaling pathways associated with drug resistance and sensitivity, including TP53 mutations, likely contributed to the variable responses to TKIs. The relationships between ABC transporter expression, gene alterations, and response to TKIs did not show consistent patterns. Our results suggest that in addition to mutational status, performing functional sensitivity screening is critical for identifying appropriate treatment strategies with TKIs. These results underscore the importance of considering drug sensitivity, off-target effects, MDR risks, and patient-specific genetic profiles when optimizing NSCLC treatment and highlight the potential for personalized approaches, especially in early stages.

THU545 Growth Hormone Receptor Antagonist As A Novel Combination Therapy Against Triple Negative Breast Cancer

Abstract Disclosure: P. Mora-Criollo: None. R. Basu: None. J.J. Kopchick: None. Triple Negative breast (TNBC) cancer is classified based on the absence of ER-, PR-, and HER2- receptors and is associated with high metastatic potential. TNBC accounts for approximately 15-20% of new breast cancer diagnoses and is responsible for most breast cancer-related deaths. First-line treatment of anticancer drugs can be effective in 30-70% of patients with TNBC, but during treatment, more than half of the patients develop resistance and progress into metastatic disease. Previous studies have demonstrated the link between growth hormone receptor (GHR) expression and mammary carcinogenesis and established that GHR signaling promotes breast cancer development and progression. Conversely, inhibition of GHR expression has been shown to arrest tumor initiation and reduce metastasis of mammary carcinoma cells. Here we present the feasibility studies for employing GHR as a target for novel combination therapeutic approaches in TNBC. We performed retrospective analysis of GHR transcript expression and survival in TNBC patients, followed by experimental data in cellular models using combinations of Pegvisomant (an FDA-approved GHR antagonist) with chemotherapeutic agents, as a potential therapeutic option in TNBC. Survival probability plots from the TCGA dataset of 405 TNBC samples showed that patients expressing high levels of the GHR showed a significantly poor survival (27 months, p=0.038) compared to patients expressing low GHRs (40.37 months). Additionally, human breast cancer MDA-MB-453 cells, when treated with human (hGH)(2.5 nM) increased activation of JAK2, STAT5, SRC, ERK1/2, and AKT, associated with cancer progression. Importantly, treatment of the cells with Paclitaxel (1 nM) + (hGH)(2.5 nM) increased ABCG2 expression and the epithelial-to-mesenchymal transition (EMT) makers (SLUG, SNAIL, N-cad) associated with drug resistance and tumor progression. On the contrary, when cells were treated with Pegvisomant (500 nM) in combination with Paclitaxel + GH, a significant decrease in protein and RNA expression of ABCG2 as well as EMT markers was observed. Next, serum collected from GH antagonist (GHA) mice and WT as control, were used to treat PY8119 mouse cells in combination with Paclitaxel (1 nM) + bovine (bGH)(2.5 nM). Downregulation of protein and RNA expression levels of ABCG2 as well EMT makers in GHA serum treatment compared to WT was also found. This preliminary data shows that the GHR antagonist, Pegvisomant, in combination with anticancer drugs, can be effective in attacking drug resistance in TNBC. Key words: Pegvisomant, growth hormone receptor Presentation: Thursday, June 15, 2023

Dysregulated Autophagy Leads to Oxidative Stress and Aberrant Expression of ABC Transporters in Women with Early Miscarriage.

Early miscarriage (EMC) is a devastating obstetrical complication. ATP-binding cassette (ABC) transporters mediate cholesterol transfer across the placenta and enhance cell survival by effluxing substrates from target cells in the presence of stressors. Recent evidence reports an intricate interplay between autophagy and ABC transporters. We hypothesized that dysregulated autophagy and oxidative stress (OS) in the placenta leads to abnormal expression of membrane transporters contributing to poor pregnancy survival in EMC. We determined mRNA and protein expression of autophagy genes (Beclin-1/Bcl-2/LC3I/LC3II/p62) and ABC transporters (ABCA1/ABCG1/ABCG2) in placentae from EMC patients (n = 20), term controls (n = 19), first trimester (n = 6), and term controls (n = 5) controls. Oxidative/antioxidant status and biomarkers of oxidative damage were evaluated in maternal serum and placentae from EMC and healthy controls. In EMC, placental expression of LC3II/LC3I as well as of the key autophagy regulatory proteins Beclin-1 and Bcl-2 were reduced, whereas p62 was increased. Both in the serum and placentae of EMC patients, total OS was elevated reflected by increased oxidative damage markers (8-OHdG/malondialdehyde/carbonyl formation) accompanied by diminished levels of total antioxidant status, catalase, and total glutathione. Furthermore, we found reduced ABCG1 and increased ABCG2 expression. These findings suggest that a decreased autophagy status triggers Bcl-2-dependent OS leading to macromolecule damage in EMC placentae. The decreased expression of ABCG1 contributes to reduced cholesterol export to the growing fetus. Increasing ABCG2 expression could represent a protective feedback mechanism under inhibited autophagy conditions. In conclusion, dysregulated autophagy combined with increased oxidative toxicity and aberrant expression of placental ABC transporters affects materno-fetal health in EMC.

Gel-Free 3D Tumoroids with Stem Cell Properties Modeling Drug Resistance to Cisplatin and Imatinib in Metastatic Colorectal Cancer

Researchers have developed several three-dimensional (3D) culture systems, including spheroids, organoids, and tumoroids with increased properties of cancer stem cells (CSCs), also called cancer-initiating cells (CICs). Drug resistance is a crucial issue involving recurrence in cancer patients. Many studies on anti-cancer drugs have been reported using 2D culture systems, whereas 3D cultured tumoroids have many advantages for assessing drug sensitivity and resistance. Here, we aimed to investigate whether Cisplatin (a DNA crosslinker), Imatinib (a multiple tyrosine kinase inhibitor), and 5-Fluorouracil (5-FU: an antimetabolite) alter the tumoroid growth of metastatic colorectal cancer (mCRC). Gene expression signatures of highly metastatic aggregative CRC (LuM1 cells) vs. low-metastatic, non-aggregative CRC (Colon26 and NM11 cells) were analyzed using microarray. To establish a 3D culture-based multiplexing reporter assay system, LuM1 was stably transfected with the Mmp9 promoter-driven ZsGreen fluorescence reporter gene, which was designated as LuM1/m9 cells and cultured in NanoCulture Plate®, a gel-free 3D culture device. LuM1 cells highly expressed mRNA encoding ABCG2 (a drug resistance pump, i.e., CSC/CIC marker), other CSC/CIC markers (DLL1, EpCAM, podoplanin, STAT3/5), pluripotent stem cell markers (Sox4/7, N-myc, GATA3, Nanog), and metastatic markers (MMPs, Integrins, EGFR), compared to the other two cell types. Hoechst efflux stem cell-like side population was increased in LuM1 (7.8%) compared with Colon26 (2.9%), both of which were markedly reduced by verapamil treatment, an ABCG2 inhibitor. Smaller cell aggregates of LuM1 were more sensitive to Cisplatin (at 10 μM), whereas larger tumoroids with increased ABCG2 expression were insensitive. Notably, Cisplatin (2 μM) and Imatinib (10 μM) at low concentrations significantly promoted tumoroid formation (cell aggregation) and increased Mmp9 promoter activity in mCRC LuM1/m9, while not cytotoxic to them. On the other hand, 5-FU significantly inhibited tumoroid growth, although not completely. Thus, drug resistance in cancer with increased stem cell properties was modeled using the gel-free 3D cultured tumoroid system. The tumoroid culture is useful and easily accessible for the assessment of drug sensitivity and resistance.

Breast cancer resistance protein (Bcrp/Abcg2) is selectively modulated by lipopolysaccharide (LPS) in the mouse yolk sac

Bacterial infection alters placental ABC transporters expression. These transporters provide fetal protection against circulating xenobiotics and environmental toxins present in maternal blood. We hypothesized that lipopolysaccharide (LPS-bacterial mimic) alters the yolk sac morphology and expression of key ABC transporters in a gestational-age dependent manner. Yolk sac samples from C57BL/6 mice were obtained at gestational ages (GD) 15.5 and GD18.5, 4 or 24 h after LPS exposure (150ug/kg; n = 8/group). Samples underwent morphometrical, qPCR and immunohistochemistry analysis. The volumetric proportions of the histological components of the yolk sac did not change in response to LPS. LPS increased Abcg2 expression at GD15.5, after 4 h of treatment (p < 0.05). No changes in Abca1, Abcb1a/b, Abcg1, Glut1, Snat1, Il-1β, Ccl2 and Mif were observed. Il-6 and Cxcl1 were undetectable in the yolk sac throughout pregnancy. Abca1, breast cancer resistance protein (Bcrp, encoded by Abcg2) and P-glycoprotein (P-gp/ Abcb1a/b) were localized in the endodermal (uterine-facing) epithelium and to a lesser extent in the mesothelium (amnion-facing), whereas Abca1 was also localized to the endothelium of the yolk sac blood vessels. LPS increased the labeling area and intensity of Bcrp in the yolk sac’s mesothelial cells at GD15.5 (4 h), whereas at GD18.5, the area of Bcrp labeling in the mesothelium (4 and 24 h) was decreased (p < 0.05). Bacterial infection has the potential to change yolk sac barrier function by affecting Bcrp and Abcg2 expression in a gestational-age dependent-manner. These changes may alter fetal exposure to xenobiotics and toxic substances present in the maternal circulation and in the uterine cavity.

Bergenin as a Novel Urate-Lowering Therapeutic Strategy for Hyperuricemia.

Bergenin is a C-glucoside of 4-O-methyl gallic acid isolated from several medicinal plants and has multiple biological activities. The aim of this study was to assess the potential usefulness of bergenin in hyperuricemia. We found that bergenin reduced serum urate levels in hyperuricemia mice by promoting renal and gut uric acid excretion. Bergenin treatment increased Abcg2 expression both in the kidneys and intestine, while the expression of Slc2a9 was suppressed in the kidney and increased in the intestine. Moreover, bergenin induced ABCG2 expression in HK-2 and Caco-2 cells, as well as SLC2A9 in Caco-2 cells, via the activation of PPARγ. Nevertheless, bergenin suppressed SLC2A9 expression in HK-2 cells by inhibiting the nuclear translocation of p53. Furthermore, bergenin decreased the serum levels of IL-6, IL-1β, and TNF-α in hyperuricemia mice, and promoted a polarization shift from the M1 to M2 phenotype in RAW264.7 cells. In conclusion, these findings provide evidence supporting the further development of bergenin as a novel therapeutic strategy for hyperuricemia.

Depressive Disorder promotes Hepatocellular Carcinoma metastasis via upregulation of ABCG2 gene expression and maintenance of self-renewal.

Depressive disorder (DD) is the leading cause of disability worldwide and is the most prevalent mood disorder. Accumulative evidence from epidemiological studies has shown that DD is a risk factor for cancer. However, the role and molecular mechanism of DD in hepatocellular carcinoma (HCC) are still unknown. In this study, 30 mice were randomly divided into two groups: the HCC group and the HCC-DD group. The DD mouse model of HCC was established by induction with reserpine every other day and with monthly doses of diethylnitrosamine (DEN). All of the molecular studies were based on primary cell culture, and the effects of DD on HCC cell proliferation and migration and cancer stem cell (CSC) self-renewal were determined by colony formation, wound healing, and sphere culture assays. We found that the CSC markers ABCG2 and CD133 were upregulated in HCC-DD primary cells compared with HCC primary cells. Moreover, HCC-DD primary cells were more aggressive in terms of metastasis and self-renewal than HCC primary cells. Further study revealed that DD promoted tumor growth and metastasis by activating the AKT signaling pathway followed by an increased ABCG2 expression. Taken together, our novel findings indicate that DD promotes proliferation, self-renewal, and metastasis by upregulating ABCG2 in the AKT pathway.

ABCG2 expression in colorectal adenocarcinomas may predict resistance to irinotecan.

Irinotecan is a key drug for patients with advanced and recurrent colorectal carcinoma. However, the efficacy of irinotecan is not sufficient; partly, as there is no useful marker to predict chemosensitivity to the drug. The aim of the present study was to evaluate whether the expression levels of adenosine triphosphate-binding cassette sub-family G (WHITE) member 2 (Junior blood group) (ABCG2) in primary colorectal tumors predict chemoresistance to irinotecan. Using the resected primary tumor specimens of 189 patients with colorectal cancer, the association between the immunohistochemical expression of ABCG2 protein and the results of the collagen gel droplet embedded culture drug sensitivity test, performed to evaluate the chemosensitivity to SN-38 (an active metabolite of irinotecan), was investigated. Among the 189 patients, 17 received irinotecan-based chemotherapy, and their responses and progression-free survival (PFS) were analyzed. The tumors of patients with increased ABCG2 expression accounted for 60% of the tumors examined, and were significantly more resistant to SN-38, compared with patients with low ABCG2 expression (P<0.001). In a multivariate logistic regression analysis, increased expression of ABCG2 protein was an independent and significant predictor of resistance to SN-38, increasing the risk of resistance by 12-fold. Increased expression of ABCG2 and a low sensitivity to SN-38 was significantly associated with resistance to irinotecan-based chemotherapy (P=0.01 and 0.028, respectively). The median PFS of patients with increased expression of ABCG2 was significantly shorter, compared with patients with low expression levels of ABCG2 (104 vs. 242 days; P=0.047). The increased immunohistochemical expression of ABCG2 in primary tumors may be a useful predictive biomarker of resistance to irinotecan-based chemotherapy for patients with recurrent or metastatic colorectal cancer.

Gemcitabine upregulates ABCG2/BCRP and modulates the intracellular pharmacokinetic profiles of bioluminescence in pancreatic cancer cells.

A lack of methods capable of exploring real-time intracellular drug deposition has since limited the investigation of gemcitabine-induced multidrug resistance in vitro and in vivo. Specifically, resistance induced by D-luciferin, a substrate of the breast cancer resistance protein (ABCG2/BCRP), has recently attracted clinical attention, but further investigation has been limited. Herein, the intracellular pharmacokinetic behavior of D-luciferin was investigated in pancreatic cancer cell lines in real time by using bioluminescence imaging. To achieve this feat, BxPC3 and Panc1 pancreatic cancer cells overexpressing firefly luciferase were treated with gemcitabine in a dose and time gradient manner in vitro. The intracellular pharmacokinetic profiles of each group were then determined through the acquisition of bioluminescent signal intensity of D-luciferin in cells. Simultaneously, key pharmacokinetic parameters including area under the curve, elimination rate constant (K), and mean resident time were calculated according to the noncompartment model. ABCG2 protein levels following gemcitabine treatment were detected through western blot, and gemcitabine showed no significant effect on luciferase activity over dimethyl sulfoxide (DMSO) as a control (P>0.05). However, gemcitabine significantly increased K values while suppressing area under the curve and mean resident time compared with DMSO (P<0.05) and increased ABCG2 expression over DMSO-treated cells. In addition, gemcitabine increased the elimination rate of the ABCG2 substrate, D-luciferin, and decreased D-luciferin accumulation in BxPC3 and Panc1 cells in a dose-response manner. Advances made herein illustrate the versatility of the in-vitro bioluminescent model and its capability to observe the onset of chemoresistance in real time.

Chemotherapeutic drug resistance of side population cells derived from human gallbladder cancer cell line GBC-SD

Objective To investigate the drug resistance of side population cells in human gallbladder cancer cell line GBC-SD and explore its mechanism. Methods Drug sensitivity assays of 5chemotherapeutic agents were performed on side population cells (SP) and non-SP cells of GBC-SD.GBC-SD was cultured and then treated with the chemotherapeutic agent gemcitabine. The frequency of SP by FACS was measured. RT-PCR and Western blotting were used to detect the expression of AB-CG2 in both the SP and the corresponding non-SP subsets. Results After 1 d treatment with 4 chemotherapeutic agents (gemcitabine, cisplatin, 5-fluorouracil and mitoxantrone) in IC50 concentration to GBC-SD cell line, the reproductive ability of SP was higher than that of non-SP (P 0.05). The percentage of SP in GBC-SD treated with chemotherapeutic agent gemcitabine after 3 weeks was sharply elevated by FACS (8.02% ±0.13% vs 0.62% ±0.08%, P<0.05), and the expression of ABCG2mRNA and protein were increased in SP as compared with non-SP. Conclusion SP from human gallbladder cancer cell line GBC-SD, like stem cell, showed a heighten resistance to drugs. Increased expression of ABCG2 was largely responsible for the multi-drug resistance. Key words: Side population cells; Multi-drug resistance; Cancer stem cell

Abstract 3541: Pharmacological chaperones increase membrane expression of ABCG2 harboring the Q141K single nucleotide polymorphism

Abstract ABCG2 is an ATP-binding cassette half-transporter that functions as a xenobiotic transporter. Since ABCG2's discovery in multidrug-resistant cancer cells, the transporter has garnered interest in pharmacogenomics as a component of the blood-brain barrier and the maternal-fetal barrier as well as a modulator of oral drug absorption. A single nucleotide polymorphism (SNP) C421A in ABCG2, resulting in a glutamic acid to leucine mutation at amino acid 141 (Q141K), has been extensively studied and has been found to affect protein expression and ATPase activity, but may also affect protein folding. As misfolded ABCG2 has been shown to be degraded through the endoplasmic reticulum associated protein degradation (ERAD) system (T. Furukawa et al., Pharm Res., 26(2): 469-479, 2008), we postulated that pharmacological chaperones such as mitoxantrone, a known substrate of ABCG2, might increase surface expression of Q141K ABCG2. In this study, we used HEK293 stably transfected with Q141K ABCG2 to examine whether incubating cells expressing the mutated transporter with mitoxantrone or other ABCG2 substrates would serve to “rescue” Q141K ABCG2 and improve trafficking to the cell surface, as we have demonstrated in other mutations that impair trafficking (O. Polgar et al., Biochemistry, 43(29): 9448-9456, 2004). Expression of ABCG2 in cells transfected with wild-type or Q141K ABCG2 was measured by immunoblot analysis and flow cytometry. After treatment with 5 μM of mitoxantrone, cell surface expression of ABCG2 in cells transfected with vector containing Q141K ABCG2 increased, while cells transfected with wild-type ABCG2 showed no change. Increased ABCG2 expression was also observed in membrane preparations from mitoxantrone-treated cells transfected with Q141K ABCG2 compared to wild-type ABCG2 transfected cells. Interestingly, immunoblot analysis showed that when cells transfected with Q141K ABCG2 were incubated with 50 μM of the dietary compounds tert-butyl hydroquinone (TBHQ) and dibenzoylmethane (DBM), increased expression of ABCG2 was observed in membrane preparations, while no change was observed for cells expressing wild-type ABCG2. These results suggest that some dietary compounds may promote proper folding and membrane localization of mutant forms of ABCG2. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 3541.

ABCG2 Is Upregulated in Alzheimer's Brain with Cerebral Amyloid Angiopathy and May Act as a Gatekeeper at the Blood–Brain Barrier for Aβ1–40Peptides

Alzheimer's disease (AD) is characterized by accumulation and deposition of Abeta peptides in the brain. Abeta deposition in cerebrovessels occurs in many AD patients and results in cerebral amyloid angiopathy (AD/CAA). Since Abeta can be transported across blood-brain barrier (BBB), aberrant Abeta trafficking across BBB may contribute to Abeta accumulation in the brain and CAA development. Expression analyses of 273 BBB-related genes performed in this study showed that the drug transporter, ABCG2, was significantly upregulated in the brains of AD/CAA compared with age-matched controls. Increased ABCG2 expression was confirmed by Q-PCR, Western blot, and immunohistochemistry. Abcg2 was also increased in mouse AD models, Tg-SwDI and 3XTg. Abeta alone or in combination with hypoxia/ischemia failed to stimulate ABCG2 expression in BBB endothelial cells; however, conditioned media from Abeta-activated microglia strongly induced ABCG2 expression. ABCG2 protein in AD/CAA brains interacted and coimmunoprecipitated with Abeta. Overexpression of hABCG2 reduced drug uptake in cells; however, interaction of Abeta(1-40) with ABCG2 impaired ABCG2-mediated drug efflux. The role of Abcg2 in Abeta transport at the BBB was investigated in Abcg2-null and wild-type mice after intravenous injection of Cy5.5-labeled Abeta(1-40) or scrambled Abeta(40-1). Optical imaging analyses of live animals and their brains showed that Abcg2-null mice accumulated significantly more Abeta in their brains than wild-type mice. The finding was confirmed by immunohistochemistry. These results suggest that ABCG2 may act as a gatekeeper at the BBB to prevent blood Abeta from entering into brain. ABCG2 upregulation may serve as a biomarker of CAA vascular pathology in AD patients.

Becatecarin (rebeccamycin analog, NSC 655649) is a transport substrate and induces expression of the ATP-binding cassette transporter, ABCG2, in lung carcinoma cells

ABCG2 overexpression has been linked to resistance to topoisomerase inhibitors, leading us to examine the potential interaction between ABCG2 and becatecarin. Interaction with ABCG2 was determined by ATPase assay, competition of [(125)I]iodoarylazidoprazosin (IAAP) photolabeling and flow cytometry. Cellular resistance was measured in 4-day cytotoxicity assays. ABCG2 expression was measured by fluorescent-substrate transport assays and immunoblot. Becatecarin competed [(125)I]-IAAP labeling of ABCG2, stimulated ATPase activity and, at concentrations greater than 10 microM, inhibited ABCG2-mediated transport. Becatecarin-selected A549 Bec150 lung carcinoma cells were 3.1-, 15-, 8-, and 6.8-fold resistant to becatecarin, mitoxantrone, SN-38 and topotecan, respectively. A549 Bec150 cells transported the ABCG2 substrates pheophorbide a, mitoxantrone and BODIPY-prazosin and displayed increased staining with the anti-ABCG2 antibody 5D3 compared to parental cells. Increased ABCG2 expression was confirmed by immunoblot. Our results suggest that becatecarin is transported by ABCG2 and can induce ABCG2 expression in cancer cells.

Increased ABCG2 Expression Could Be Responsible for Resistance to Imatinib Mesylate in Patients with Chronic Myeloid Leukemia Who Do Not Have Mutations in BCR-ABL Kinase Domain

Mutation in the Bcr-Abl kinase domain is one of the most common mechanism of resistance to imatinib mesylate (IM), seen in 30– 90% of patients with chronic myeloid leukemia (CML). A total of 57 patients with CML, 41 in chronic phase, 10 in accelerated phase and 6 in blast crisis, were analyzed for bcr-abl kinase domain (KD) mutations using reverse transcriptase polymerase chain reaction (RT-PCR) amplification of the bcr-abl KD followed by direct sequencing. Twelve different mutations including three novel mutations were identified in the bcr-abl KD in 18 patients. Resistance to IM was considered if the patient had rising white blood cell counts or did not achieve hematological remission at 6 months; or had rising bcr-abl levels (as determined by fluorescence in situ hybridization or real-time quantitative PCR) after the start of IM treatment. In order to evaluate other potential mechanisms of resistance in patients without mutations in the bcr-abl kinase domain, we tested the expression levels of transporter genes known to be involved in IM influx and efflux. mRNA expression levels of efflux transporters, ABCG2 and ABCB1, and influx transporter, hOCT1 were measured by real time quantitative PCR using ABL to normalize the expression levels of the same. Serially diluted cDNA made from HepG2 RNA was used to make the standard curve and amplification efficiencies of the target and the house keeping genes were similar. Each sample was analyzed in duplicate and the experiment was repeated twice. In the patients without mutations in the bcr-abl KD, significantly higher ABCG2 mRNA levels were observed compared to patients with mutations (Table). Transcript levels of ABCB1, hOCT1 or bcr-abl were not significantly different between the two groups. This study suggests that over expression of ABCG2 may be one of the mechanisms of resistance to imatinib in patients without mutations in bcr-abl. Future studies should not only compare the expression of these transporters at diagnosis (before the start of IM treatment) but also at the time of clinical resistance. This will help understand the influence of expression levels of these transporters in achieving haematological or molecular response to increased IM doses.Bcr-abl mutation positive (n=18)Bcr-abl mutation negative (n=39)p valueABCG2: median (range)0.0126 (0.0004–0.45)0.051 (0.0041–0.51)0.034ABCB1: median (range)0.1842 (0.00349–0.51)0.1819 (0.0463–0.68)NShOCT1: median (range)488 (3.4–3394)486 (62–3059)NSBCR-ABL: median (range)53.96 (5.73–179.1)51.3 (2.36–129)NS

Histone Modifications at the ABCG2 Promoter following Treatment with Histone Deacetylase Inhibitor Mirror Those in Multidrug-Resistant Cells

ABCG2 is a ubiquitous ATP-binding cassette transmembrane protein that is important in pharmacology and may play a role in stem cell biology and clinical drug resistance. To study the mechanism(s) regulating ABCG2 expression, we used ChIP to investigate the levels of acetylated histone H3, histone deacetylases (HDAC), histone acetyltransferases, and other transcription regulatory proteins associated with the ABCG2 promoter. Following selection for drug resistance and the subsequent overexpression of ABCG2, an increase in acetylated histone H3 but a decrease in class I HDACs associated with the ABCG2 promoter was observed. Permissive histone modifications, including an increase in histone H3 lysine 4 trimethylation (Me(3)-K4 H3) and histone H3 serine 10 phosphorylation (P-S10 H3), were observed accompanying development of the resistance phenotype. These changes mirrored those in some cell lines treated with a HDAC inhibitor, romidepsin. A repressive histone mark, trimethylated histone H3 lysine 9 (Me(3)-K9 H3), was found in untreated parental cells and cells that did not respond to HDAC inhibition with ABCG2 up-regulation. Interestingly, although all five studied cell lines showed global histone acetylation and MDR1 up-regulation upon HDAC inhibition, only those cells with removal of the repressive mark, and recruitment of RNA polymerase II and a chromatin remodeling factor Brg-1 from the ABCG2 promoter, showed increased ABCG2 expression. In the remaining cell lines, HDAC1 binding in association with the repressive Me3-K9 H3 mark apparently constrains the effect of HDAC inhibition on ABCG2 expression. These studies begin to address the differential effect of HDAC inhibitors widely observed in gene expression studies.