Abstract 3609: Metformin treatment outcomes in relation to obesity and cation transporter expression in mouse models of endometrial cancer

Purpose: Obesity is associated with increased risk and worse outcomes for endometrial cancer (EC). Metformin, a frontline therapy for type 2 diabetes, is thought to have both indirect and direct anti-cancer effects via decreasing circulating insulin/glucose and activating AMPK/inhibiting mTOR signaling, respectively. Due to its positive charge (pKa 12.4) and hydrophilicity (logD -6.13 at pH 7.4), metformin requires cation transporters for cellular uptake where it can then activate AMPK; and thus, variation in transporter expression may be critical for response to metformin for cancer treatment. Thus, we explored the inter-relationship of obesity, cation transporter expression and the anti-tumorigenic efficacy of metformin in a genetically engineered endometrioid EC mouse model.

Methods: LKB1fl/fl p53fl/fl mice were fed a low-fat diet (10% calories from fat) vs. a high-fat diet (60% calories from fat) to mimic diet-induced obesity, starting at 3 weeks of age (n=10 mice/group). AdCre was injected at 6 weeks of age to induce EC. Mice were treated for four weeks with placebo or metformin (200 mg/kg/day, oral gavage) following tumor onset. Cell proliferation/apoptotic markers and downstream targets of AMPK/mTOR signaling were evaluated in the ECs by immunohistochemistry. Cation transporter expression was assessed in the ECs by Western blotting and RNA sequencing.

Results: Obesity led to a doubling of endometrial tumor size in obese vs. lean mice (1.469g vs. 0.75g). Metformin was more potent in inhibiting tumor growth in obese vs. lean mice (78% vs. 60%) compared to the respective controls (p<0.05). Metformin (i) decreased expression of Ki-67 and cyclin D1 (cell proliferation markers) by 65% and 15%, respectively, in tumors from obese mice and by 18% and 5%, respectively, in tumors from lean mice vs. their respective controls (p<0.05); (ii) increased cleaved caspase-3 (marker of apoptosis) in tumors from obese and lean mice (2.3 fold and 2.1 fold) vs. their respective controls (p<0.05); (iii) increased phosphorylated AMPK expression and decreased expression of phosphorylated S6 in ECs from obese and lean mice vs. their respective controls (p<0.05); (iv) increased cation transporter OCT3 and PMAT gene expression in only tumors from obese mice, with a trend towards higher protein expression of OCT1 and 3 and MATE1 and 2 with metformin treatment in ECs from both lean and obese mice.

Conclusion: Our data suggest that metformin has greater anti-tumorigenic efficacy in obese vs. lean mice with EC. In addition, cation transporters were expressed in the endometrial tumors and were affected by metformin treatment. Studies are underway in EC patients to understand the impact of obesity and transporter expression on response rates to metformin in the clinic.

Note: This abstract was not presented at the meeting.

Citation Format: Aruljothi Muralidharan, Hui Guo, Jianjun Han, Lu Zhang, Wenchuan Sun, Yajie Yin, Chunxiao Zhou, Ruth S. Everett, Dhiren R. Thakker, Victoria L. Bae-Jump. Metformin treatment outcomes in relation to obesity and cation transporter expression in mouse models of endometrial cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 3609.

Efficacious dose of metformin for breast cancer therapy is determined by cation transporter expression in tumours

BACKGROUND AND PURPOSE

It has been extensively reported that the leading anti-diabetic drug, metformin, exerts significant anticancer effects. This hydrophilic, cationic drug requires cation transporters for cellular entry where it activates its intracellular target, the AMPK signalling pathway. However, clinical results on metformin therapy (used at antidiabetic doses) for breast cancer are ambiguous. It is likely that the antidiabetic dose is inadequate in patients that have breast tumours with low cation transporter expression, resulting in non-responsiveness to the drug. We postulate that cation transporter expression and metformin dose are key determinants in its antitumour efficacy in breast cancer.

EXPERIMENTAL APPROACH

Antitumour efficacy of metformin was compared between low cation transporter-expressing MCF-7 breast tumours and MCF-7 tumours overexpressing organic cation transporter 3 (OCT3-MCF7). A dose-response relationship of metformin in combination with standard-of-care paclitaxel (for oestrogen receptor-positive MCF-7 breast tumours) or carboplatin (for triple-negative MDA-MB-468 breast tumours) was investigated in xenograft mice.

KEY RESULTS

Metformin had greater efficacy against tumours with higher cation transporter expression, as observed in OCT3-MCF7 versus MCF-7 tumours and MDA-MB-468 versus MCF-7 tumours. In MCF-7 tumours, a threefold higher metformin dose was required to achieve intratumoural exposure that was comparable to exposure in MDA-MB-468 tumours and enhance antitumour efficacy of standard-of-care in MCF-7 tumours versus MDA-MB-468 tumours. Antitumour efficacy correlated with intratumoural AMPK activation and metformin concentration.

CONCLUSIONS AND IMPLICATIONS

An efficacious metformin dose for breast cancer varies among tumour subtypes based on cation transporter expression, which provides a useful guide for dose selection.

Four cation-selective transporters contribute to apical uptake and accumulation of metformin in Caco-2 cell monolayers

Metformin is the frontline therapy for type II diabetes mellitus. The oral bioavailability of metformin is unexpectedly high, between 40 and 60%, given its hydrophilicity and positive charge at all physiologic pH values. Previous studies in Caco-2 cell monolayers, a cellular model of the human intestinal epithelium, showed that during absorptive transport metformin is taken up into the cells via transporters in the apical (AP) membrane; however, predominant transport to the basolateral (BL) side occurs via the paracellular route because intracellular metformin cannot egress across the BL membrane. Furthermore, these studies have suggested that the AP transporters can contribute to intestinal accumulation and absorption of metformin. Transporter-specific inhibitors as well as a novel approach involving a cocktail of transporter inhibitors with overlapping selectivity were used to identify the AP transporters that mediate metformin uptake in Caco-2 cell monolayers; furthermore, the relative contributions of these transporters in metformin AP uptake were also determined. The organic cation transporter 1, plasma membrane monoamine transporter (PMAT), serotonin reuptake transporter, and choline high-affinity transporter contributed to approximately 25%, 20%, 20%, and 15%, respectively, of the AP uptake of metformin. PMAT-knockdown Caco-2 cells were constructed to confirm the contribution of PMAT in metformin AP uptake because a PMAT-selective inhibitor is not available. The identification of four intestinal transporters that contribute to AP uptake and potentially intestinal absorption of metformin is a significant novel finding that can influence our understanding of metformin pharmacology and intestinal drug-drug interactions involving this highly prescribed drug.

Organic cation transporter 1 (OCT1/mOct1) is localized in the apical membrane of Caco-2 cell monolayers and enterocytes

Organic cation transporters (OCTs) are members of the solute carrier 22 family of transporter proteins that are involved in absorption, distribution, and excretion of organic cations. OCT3 is localized in the apical (AP) membrane of enterocytes, but the literature is ambiguous about OCT1 (mOct1) localization, with some evidence suggesting a basolateral (BL) localization in human and mouse enterocytes. This is contrary to our preliminary findings showing AP localization of OCT1 in Caco-2 cell monolayers, an established model of human intestinal epithelium. Therefore, this study aims at determining the localization of OCT1 (mOct1) in Caco-2 cells, and human and mouse enterocytes. Functional studies using OCT1-specific substrate pentamidine showed transporter-mediated AP but not BL uptake in Caco-2 cells and human and mouse intestinal tissues. OCT1 inhibition decreased AP uptake of pentamidine by ∼50% in all three systems with no effect on BL uptake. A short hairpin RNA-mediated OCT1 knockdown in Caco-2 cells decreased AP uptake of pentamidine by ∼50% but did not alter BL uptake. Immunostaining and confocal microscopy in all three systems confirmed AP localization of OCT1 (mOct1). Our studies unequivocally show AP membrane localization of OCT1 (mOct1) in Caco-2 cells and human and mouse intestine. These results are highly significant as they will require reinterpretation of previous drug disposition and drug-drug interaction studies where conclusions were drawn assuming BL localization of OCT1 in enterocytes. Most importantly, these results will require revision of the regulatory guidance for industry in the United States and elsewhere because it has stated that OCT1 is basolaterally localized in enterocytes.

Abstract LB-345: Differences in metformin transporter expression between breast tumor and breast cancer cell lines: selecting a relevant breast cancer cell model for metformin therapy

Purpose: Recent studies show that the anti-diabetic drug metformin induces significant anticancer effects in several cancers including breast cancer via activation of intracellular AMPK. Because of its hydrophilic nature and net positive charge at physiologic pHs, metformin requires cation-selective transporters such as OCT1-3, PMAT and MATE1-2 to enter cells and activate AMPK. Expression of these transporters in cancer cells would determine the intracellular concentrations of metformin that can be achieved for anticancer effects. Therefore, elucidating metformin transport in breast cancer is central to understanding optimum dosing and transporter-associated inter-subject variability in its anticancer efficacy. However, there is limited information on metformin transport in breast cancer. Studies in our laboratory suggest that cation-selective transporters are expressed at negligible levels in MCF-7 cells, a widely used in vitro breast cancer model, which is not consistent with the robust anticancer effect of metformin in xenograft mouse models and clinical studies. Therefore, this study aims at determining a) metformin transporter expression in human breast tumor, b) if metformin transporter expression in breast cancer cell lines is relevant to that in breast tumor tissue and c) there is differential metformin transporter expression in tumor and non-malignant breast tissue. Methods: Total RNA isolated from breast cancer cell lines MCF-7, SK-BR-3, MDA-MB-231, ZR-75-1, BT-474, MDA-MB-435S, MDA-MB-468, BT-20 and BT-549 and from 14 pairs of human breast tumor and adjacent non-malignant breast tissues was subjected to.real-time PCR to determine relative expression levels of OCT1-3, PMAT, MATE1 and 2. Results: Most cell lines expressed low levels of candidate cation-selective transporters. MDA-MB-468 and BT-549 showed high expression of MATE1 and MDA-MB-231 expressed high levels of OCT3 and MATE1. In tumor and non-malignant breast tissue, the predominant transporters were OCT3 and PMAT. Nine of the total 14 tumor samples showed downregulation of OCT3 by 85% and PMAT by 66% compared to non-malignant tissue. Conclusions: The expression profile of cation-selective transporters is different between tumor and normal breast tissue and between breast tumor tissue and cell lines. Therefore, one needs to use caution in choosing a relevant in vitro cell model for investigating anticancer efficacy of metformin whose uptake is transporter-mediated. Expression data suggest that OCT3 and PMAT may play a major role in metformin intracellular uptake in breast cancer. MDA-MB-231 may be a useful model to assess the metformin anticancer effects in breast cancer. Clearly, genetic variability in metformin transporters may lead to differential responses to metformin anticancer therapy.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr LB-345. doi:1538-7445.AM2012-LB-345

Adenovirus infection triggers a rapid, MyD88-regulated transcriptome response critical to acute-phase and adaptive immune responses in vivo

Nearly 50 years ago, the discovery of interferon prompted the notion that host cells innately respond to viral invasion. Since that time, technological advances have allowed this response to be extensively characterized and dissected in vitro. However, these advances have only recently been applied to highly complex, in vivo biological systems. To this end, we exploited high-titer adenovirus (Ad) vectors to globally investigate the innate immune response to nonenveloped viral infection in vivo. Our results indicated a potent cellular transcriptome response shortly after infection, with global assessments revealing significant dysregulation in approximately 15% of the measured transcripts derived from Ad vector-transduced tissue. Bioinformatics-based transcriptome analysis revealed a complex innate response to Ad infection, with induction of proinflammatory responses (and suppression of metabolism and mitochondrial genes) akin to those observed when mice are challenged with lipopolysaccharide. Despite this commonality, there were many unique aspects of the Ad-dependent transcriptome response, including the upregulation of several RNA regulatory mechanisms and apoptosis-related pathways, accompanied by the suppression of lysosomal and endocytic genes. Our results also implicated the Toll-like receptors (TLRs) in these responses, prompting specific investigations into this pathway. By using MyD88KO mice, our results confirmed that Ad-induced dysregulation of five functionally related gene clusters are significantly dependent on this TLR adaptor gene. MyD88 deficiency also resulted in significantly diminished, although not abolished, adaptive and acute-phase immune responses to Ad, confirming the transcriptome data, as well as specifically identifying MyD88 as a significant Ad immunity amplifier and regulator in vivo.

Multiple Innate Inflammatory Responses Induced after Systemic Adenovirus Vector Delivery Depend on a Functional Complement System

Excessive complement activation can result in extreme tissue damage and systemic inflammatory responses, similar to innate immune responses rapidly elicited after systemic adenovirus (Ad) injections. To determine if Ad interactions with the complement system impact upon Ad-induced innate immune responses, we injected Ad into complement-deficient, C3-knockout mice (C3-KO) or wild-type mice (WT) and quantitatively compared multiple anti-Ad innate immune responses in both strains of mice. In Ad-treated WT mice, we noted rapid increases in plasma KC levels (1 h post injection), followed by increases in IL-6, IFN-gamma, RANTES, IL-12(p40), IL-5, G-CSF, and GM-CSF and subsequently thrombocytopenia. Conversely, in Ad-treated C3-KO mice, many of these inflammatory responses were significantly blunted, including the avoidance of Ad-induced thrombocytopenia. Global liver transcriptome responses in Ad-treated WT mice were assessed by RT-PCR-validated gene array analysis and were found to be also significantly affected by the lack of complement activity in Ad-treated C3-KO mice. Finally, our results confirmed the ability of high dose Ads to transduce hepatocytes despite a lack of complement activity. In summary, Ad interactions with the mammalian complement system are significant and likely initiate and/or exacerbate many of the inflammatory responses noted after systemic Ad injections.

Specific modulation of airway epithelial tight junctions by apical application of an occludin peptide

Tight junctions are directly involved in regulating the passage of ions and macromolecules (gate functions) in epithelial and endothelial cells. The modulation of these gate functions to transiently regulate the paracellular permeability of large solutes and ions could increase the delivery of pharmacological agents or gene transfer vectors. To reduce the inflammatory responses caused by tight junction-regulating agents, alternative strategies directly targeting specific tight junction proteins could prove to be less toxic to airway epithelia. The apical delivery of peptides corresponding to the first extracellular loop of occludin to transiently modulate apical paracellular flux has been demonstrated in intestinal epithelia. We hypothesized that apical application of these occludin peptides could similarly modulate tight junction permeability in airway epithelia. Thus, we investigated the effects of apically applied occludin peptide on the paracellular permeability of molecular tracers and viral vectors in well differentiated human airway epithelial cells. The effects of occludin peptide on cellular toxicity, tight junction protein expression and localization, and membrane integrity were also assessed. Our data showed that apically applied occludin peptide significantly reduced transepithelial resistance in airway epithelia and altered tight junction permeability in a concentration-dependent manner. These alterations enhanced the paracellular flux of dextrans as well as gene transfer vectors. The occludin peptide redistributed occludin but did not alter the expression or distribution of ZO-1, claudin-1, or claudin-4. These data suggest that specific targeting of occludin could be a better-suited alternative strategy for tight junction modulation in airway epithelial cells compared with current agents that modulate tight junctions.

Liver toxicities typically induced by first-generation adenoviral vectors can be reduced by use of E1, E2b-deleted adenoviral vectors

Adenoviral vectors from which the E1 region has been deleted ([E1(-)] Ad) are known to induce strong immune responses after systemic delivery. In this study we have evaluated liver toxicities in mice after intravenous injection with high doses of [E1(-)] or modified [E1(-), E2b(-)] Ad vectors (both expressing the bacterial beta-galactosidase [lacZ] marker gene) in C57BL/6, BALB/c, and SCID mice. Our data demonstrate a marked reduction in maximal liver toxicities and pathologies (typically noted at 21 days postinjection) with the use of the [E1(-), E2b(-)] modified vector in all strains of mice tested. Our data also demonstrated that despite the use of the [E1(-), E2b(-)] Ad vector, significant liver toxicities were still observed. To address this issue and the fact that the lacZ gene was perceived as a foreign antigen in the immune-competent C57BL/6 and BALB/c mice, we similarly injected mice tolerant of LacZ (lacZ-TG). In contrast to our studies in C57BL/6 and BALB/c mice, LacZ-TG mice exhibited virtually no evidence of hepatotoxicity after intravenous injection with the [E1(-), E2b(-)] vector, in contrast to use of the [E1(-)] Ad vector. Our results demonstrate that the [E1(-), E2b(-)] Ad vector class can reduce liver toxicities typically ascribed to Ad vector-mediated gene transfer after transfer of a highly immunogenic or foreign gene, whereas transfer of a transgene that is perceived as nonforeign by the host can be delivered with virtually no evidence of toxicity. On the basis of a careful review of the literature, these improvements in vector safety rival those noted with other, more significantly modified Ad vectors described to date.

Strain-specific rate of shutdown of CMV enhancer activity in murine liver confirmed by use of persistent [E1−, E2b−] adenoviral vectors

The systemic delivery of [E1(-)] adenoviral (Ad) vectors encoding a transgene results in efficient viral uptake and abundant transgene expression in the liver. However, [E1(-)]Ad vector persistence is transient due to cytotoxic T lymphocyte (CTL)-mediated loss of the Ad-infected cells. Our laboratory has previously demonstrated that additional modifications to the [E1(-)]Ad vector genome, by deletion of the Ad E2b genes, significantly decreased virus-genome-derived gene expression and simultaneously improved the long-term performance of the resultant [E1(-), E2b(-)]Ad vector. In this study, we confirmed that [E1(-), E2b(-)]Ad vector genomes could persist equally well in C57Bl/6 or Balb/c mouse hepatocytes. Despite vector genome persistence, we observed a strain-dependent variability in the duration of CMV enhancer/promoter-driven transgene expression in the liver. While Balb/c mice rapidly shut down [E1(-), E2b(-)]Ad-derived transgene expression, C57Bl/6 mice allowed for prolonged transgene expression. This occurred even when both strains were crossed into a severe combined immune-deficient background, demonstrating that host adaptive immune responses are not responsible for the phenomenon. Furthermore, differential methylation of the CMV enhancer/promoter was also not demonstrated in either strain of mouse, eliminating this mechanism as causative. Thus, alternative mechanisms for this phenomenon are discussed.

Adenovirus vectors with the 100K gene deleted and their potential for multiple gene therapy applications

The 100K protein has a number of critical roles vital for successful completion of the late phases of the adenovirus (Ad) life cycle. We hypothesized that the introduction of deletions within the 100K gene would allow for the production of a series of new classes of Ad vector, including one that is replication competent but blocked in the ability to carry out many late-phase Ad functions. Such a vector would have potential for several gene therapy applications, based upon its ability to increase the copy number of the transgene encoded by the vector (via genome replication) while decreasing the side effects associated with Ad late gene expression. To efficiently produce 100K-deleted Ad ([100K-]Ad) vectors, an E1- and 100K-complementing cell line (K-16) was successfully isolated. Transfection of an [E1-,100K-]Ad vector genome into the K-16 cells readily yielded high titers of the vector. After infection of noncomplementing cells, we demonstrated that [100K-]Ad vectors have a significantly decreased ability to express several Ad late genes. Additionally, if the E1 gene was present in the infected noncomplementing cells, [100K-]Ad vectors were capable of replicating their genomes to high copy number, but were significantly blocked in their ability to efficiently encapsidate the replicated genomes. Injection of an [E1-,100K-]Ad vector in vivo also correlated with significantly decreased hepatotoxicity, as well as prolonged vector persistence. In summary, the unique properties of [100K-]Ad vectors suggest that they may have utility in a variety of gene therapy applications.

Factors affecting production of luciferase and epitope-tagged IGF-I in porcine muscle after DNA injection

Direct DNA injection into porcine skeletal muscle was investigated as an approach for studying roles of locally produced IGF-I on IGF-binding protein (IGFBP) production. To determine parameters for maximal reporter gene expression, and to investigate the effects of dose, time and weaning on exogenous DNA expression, plasmid DNA encoding firefly luciferase under control of a constitutive promoter and enhancer was injected in skeletal muscle of pigs. Results indicate that injected DNA does not migrate beyond 9 mm from injection sites and that 100 microg DNA injections resulted in optimal luciferase activity. Maximum amounts of recombinant protein were observed 3 days after injection, and were reduced by weaning. Using these data, a second DNA injection study was performed using plasmid DNA containing a cDNA insert for epitope-tagged insulin-like growth factor-I (TIGF-I). Significant quantities of TIGF-I were detected by ELISA and confirmed by western blotting. Both IGFBP-2 and IGFBP-2 mRNA were increased in treated muscle compared with controls. We conclude that increased expression of IGF-I in muscle results in increased IGFBP-2. Furthermore, these data indicate that this in vivo approach of gene transfer results in biologically active recombinant protein production in porcine skeletal muscle, and provides an excellent in vivo model for studying the autocrine and (or) paracrine effects of locally produced growth factors in skeletal muscle.

Epitope-tagged insulin-like growth factor-I expression in muscle

Development of a recombinant insulin like growth factor I (IGF-I) that is distinguishable from its endogenous counterpart would provide a powerful tool for delineating the role of IGF in myogenesis. Therefore, the objective of this study was to create an epitope-tagged IGF-I that retains biological activity and determine whether expression of this construct is possible in muscle tissue following direct DNA injection. Expression vectors were created that encoded porcine IGF-I containing a T7 (11-amino acid) epitope-tag (TIGF). Immunoreactivity of the purified recombinant TIGF was confirmed using monoclonal antibodies. Biological activity was evaluated by examining differentiation of myoblasts cultured with TIGF or transfected with TIGF plasmid DNA. Addition of purified TIGF to myoblast cultures stimulated (P < 0.05) muscle creatine kinase levels similar to insulin (10(-5) M). Likewise, transfection of L6A1 with TIGF DNA hastened (P < 0.01) differentiation compared to control pcDNA-transfected myoblasts. The integrity of the recombinant protein was confirmed using a sandwich-configured enzyme linked immunosorbent assay. Finally, recombinant TIGF DNA was injected in porcine muscle and the ability to detect TIGF protein was evaluated. TIGF expression was detected in muscle fibers of injected porcine muscle. These data show that a T7 amino acid tag placed on the amino terminus of the IGF-I protein remains intact during processing and does not interfere with the biological activity of the molecule. Use of this DNA construct is an excellent tool for investigating the role of IGFs in control muscle development and provides a model to investigate other regulators of animal growth.