Jason Zastre

pH Dependence of Organic Anion-Transporting Polypeptide 2B1 in Caco-2 Cells: Potential Role in Antiretroviral Drug Oral Bioavailability and Drug–Drug Interactions

Human intestinal epithelium expresses a number of drug efflux and influx transporters that can restrict and/or facilitate intestinal drug uptake during absorption. Organic anion-transporting polypeptide 2B1 (OATP2B1), a multispecific organic anion uptake transporter localized at the brush-border membrane of intestinal epithelial cells, is known to transport many endogenous substrates (e.g., steroid conjugates) and xenobiotics (e.g., statins). At present, limited information is available on the mechanism of HIV protease inhibitor (PIs) intestinal uptake. In this study, we examined the interaction of PIs with the OATP2B1 transport system in Caco-2 cells, an in vitro model of human intestinal epithelium, and Madin-Darby canine kidney II cells stably transfected with OATP2B1. The expression of OATP2B1 transcript and protein was confirmed by reverse transcription-polymerase chain reaction and immunoblot analysis, respectively. Estrone-3-sulfate (E3S) uptake demonstrated biphasic saturation kinetics in Caco-2 cells, with dissociation constants (KM) of 6 ± 2 μM and 1.5 ± 0.2 mM. Several PIs potently inhibited OATP2B1-mediated transport in Caco-2 cells at clinically relevant IC50 concentrations for ritonavir (0.93 μM), atazanavir (2.2 μM), lopinavir (1.7 μM), tipranavir (0.77 μM), and nelfinavir (2.2 μM). An inwardly directed proton gradient was identified as the driving force of E3S uptake through NH4Cl intracellular acidification studies with a H+:E3S stoichiometry for OATP2B1 of 1:1. In contrast, although atazanavir and ritonavir uptake by Caco-2 cells was stimulated by low extracellular pH, this process was not mediated by OATP2B1 and was not affected by an outwardly directed H+ gradient. Because OATP2B1 exhibits an increasing number of drug substrates, including several statins, alterations of its function by PIs could result in clinically significant drug–drug interactions in the intestine.

Role of Drug Efflux and Uptake Transporters in Atazanavir Intestinal Permeability and Drug-Drug Interactions

ABSTRACTPurposeTo investigate the role of membrane-associated drug transporters in regulating the intestinal absorption of the HIV-1 protease inhibitor, atazanavir, and assess the potential contribution of these transporters in clinical interactions of atazanavir with other protease inhibitors and tenofovir disoproxil fumarate (TDF).MethodsIntestinal permeability of atazanavir was investigated in vitro, using the Caco-2 cell line system grown on Transwell inserts, and in situ, by single-pass perfusion of rat intestinal segments, jejunum and ileum, in the absence or presence of standard transporter inhibitors or antiretroviral drugs.ResultsAtazanavir accumulation by Caco-2 cells was susceptible to inhibition by P-glycoprotein and organic anion transporting polypeptide (OATP) family inhibitors and several antiretroviral drugs (protease inhibitors, TDF). The secretory flux of atazanavir (basolateral-to-apical Papp) was 11.7-fold higher than its absorptive flux. This efflux ratio was reduced to 1.5–1.7 in the presence of P-glycoprotein inhibitors or ritonavir. P-glycoprotein inhibition also resulted in 1.5–2.5-fold increase in atazanavir absorption in situ. Co-administration of TDF, however, reduced atazanavir intestinal permeability by 13–49%, similar to the effect observed clinically.ConclusionsDrug transporters such as P-glycoprotein and OATPs regulate intestinal permeability of atazanavir and may contribute to its poor oral bioavailability and drug-drug interactions with other protease inhibitors and TDF.

Linking vitamin B1 with cancer cell metabolism

The resurgence of interest in cancer metabolism has linked alterations in the regulation and exploitation of metabolic pathways with an anabolic phenotype that increases biomass production for the replication of new daughter cells. To support the increase in the metabolic rate of cancer cells, a coordinated increase in the supply of nutrients, such as glucose and micronutrients functioning as enzyme cofactors is required. The majority of co-enzymes are water-soluble vitamins such as niacin, folic acid, pantothenic acid, pyridoxine, biotin, riboflavin and thiamine (Vitamin B1). Continuous dietary intake of these micronutrients is essential for maintaining normal health. How cancer cells adaptively regulate cellular homeostasis of cofactors and how they can regulate expression and function of metabolic enzymes in cancer is underappreciated. Exploitation of cofactor-dependent metabolic pathways with the advent of anti-folates highlights the potential vulnerabilities and importance of vitamins in cancer biology. Vitamin supplementation products are easily accessible and patients often perceive them as safe and beneficial without full knowledge of their effects. Thus, understanding the significance of enzyme cofactors in cancer cell metabolism will provide for important dietary strategies and new molecular targets to reduce disease progression. Recent studies have demonstrated the significance of thiamine-dependent enzymes in cancer cell metabolism. Therefore, this review discusses the current knowledge in the alterations in thiamine availability, homeostasis, and exploitation of thiamine-dependent pathways by cancer cells.

Hypoxia induced upregulation and function of the thiamine transporter, SLC19A3 in a breast cancer cell line

Adaptive responses within hypoxic tumor microenvironments require the altered expression of Solute Carrier (SLC) transporters to maintain nutrient uptake in support of cellular metabolism and biosynthesis. Using a real time PCR array strategy to further characterize changes in transporter expression within a chronic hypoxia breast cancer cell line model (BT474), we have found a 31 fold increase in the expression of the thiamine transporter, SLC19A3. Thus, further investigations into the expression changes of the thiamine transporters, SLC19A2 and SLC19A3, and the role of hypoxia inducible factor-1 alpha (HIF-1α) regulating their expression were conducted. Chronic culturing of BT474 cells in 1% O2 up to 142 days consistently demonstrated a high level of SLC19A3 expression with a mean of approximately 40 fold with no change in SLC19A2. A corresponding 2 fold increase in thiamine uptake over 15 min was measured in chronic hypoxic BT474 cells compared to normoxia. Acute 1% O2 exposure of BT474 cells up to 72h demonstrated a 7.5 fold increase in SLC19A3 expression. The chemical hypoxia mimetic deferoxamine, resulted in an approximately 70 fold increase in SLC19A3 expression. Stable shRNA knockdown of HIF-1α reduced hypoxia mediated SLC19A3 up-regulation by approximately 3 fold compared to scrambled construct. In conclusion, SLC19A3 transporter expression was observed to be up-regulated under acute, chronic and DFO induced hypoxia. The attenuated increase in SLC19A3 expression after HIF-1α knockdown suggests a role for HIF-1α mediated pathways regulating SLC19A3 gene expression.

HIF1-α-Mediated Gene Expression Induced by Vitamin B1 Deficiency

It is well established that thiamine deficiency results in an excess of metabolic intermediates such as lactate and pyruvate, which is likely due to insufficient levels of cofactor for the function of thiamine-dependent enzymes. When in excess, both pyruvate and lactate can increase the stabilization of the hypoxia-inducible factor 1-alpha (HIF-1α) transcription factor, resulting in the trans-activation of HIF-1α regulated genes independent of low oxygen, termed pseudo-hypoxia. Therefore, the resulting dysfunction in cellular metabolism and accumulation of pyruvate and lactate during thiamine deficiency may facilitate a pseudo-hypoxic state. In order to investigate the possibility of a transcriptional relationship between hypoxia and thiamine deficiency, we measured alterations in metabolic intermediates, HIF-1α stabilization, and gene expression. We found an increase in intracellular pyruvate and extracellular lactate levels after thiamine deficiency exposure to the neuroblastoma cell line SK-N-BE. Similar to cells exposed to hypoxia, there was a corresponding increase in HIF-1α stabilization and activation of target gene expression during thiamine deficiency, including glucose transporter-1 (GLUT1), vascular endothelial growth factor (VEGF), and aldolase A. Both hypoxia and thiamine deficiency exposure resulted in an increase in the expression of the thiamine transporter SLC19A3. These results indicate thiamine deficiency induces HIF-1α-mediated gene expression similar to that observed in hypoxic stress, and may provide evidence for a central transcriptional response associated with the clinical manifestations of thiamine deficiency.

Up-regulation of vitamin B1 homeostasis genes in breast cancer

An increased carbon flux and exploitation of metabolic pathways for the rapid generation of biosynthetic precursors is a common phenotype observed in breast cancer. To support this metabolic phenotype, cancer cells adaptively regulate the expression of glycolytic enzymes and nutrient transporters. However, activity of several enzymes involved in glucose metabolism requires an adequate supply of cofactors. In particular, vitamin B1 (thiamine) is utilized as an essential cofactor for metabolic enzymes that intersect at critical junctions within the glycolytic network. Intracellular availability of thiamine is facilitated by the activity of thiamine transporters and thiamine pyrophosphokinase-1 (TPK-1). Therefore, the objective of this study was to establish if the cellular determinants regulating thiamine homeostasis differ between breast cancer and normal breast epithelia. Employing cDNA arrays of breast cancer and normal breast epithelial tissues, SLC19A2, SLC25A19 and TPK-1 were found to be significantly up-regulated. Similarly, up-regulation was also observed in breast cancer cell lines compared to human mammary epithelial cells. Thiamine transport assays and quantitation of intracellular thiamine and thiamine pyrophosphate established a significantly greater extent of thiamine transport and free thiamine levels in breast cancer cell lines compared to human mammary epithelial cells. Overall, these findings demonstrate an adaptive response by breast cancer cells to increase cellular availability of thiamine.

Ion pair liquid chromatography method for the determination of thiamine (vitamin B1) homeostasis.

A new method for reversed phase HPLC determination of thiamine and its major in vivo phosphorylation products, thiamine monophosphate (TMP) and thiamine pyrophosphate (TPP), was developed using tetrabutylammonium hydroxide as the ion-pairing agent. The separation was performed on a Phenomenex Kinetex EVO C18 column with a gradient of a phosphate-buffered aqueous solution of the ion-pair reagent and methanol. The duty cycle for the assay was 13 min and pyrithiamine was successfully used as the internal standard for the first time in a thiamine HPLC measurement protocol. Detection of the fluorescence derivatives of the analytes as well as the IS allowed for lower detection limits in order to support biological applications in cell culture models. The linearity, sensitivity, specificity, accuracy and precision of the method were evaluated and met the requirements specified by the US Food and Drug Administration. The calibration curves proved to be linear and the method was validated over the range from 1.0-4000 nM for both cells and the media where complete recovery of the analytes was also achieved.

Pharmacological Reversal of Histone Methylation Presensitizes Pancreatic Cancer Cells to Nucleoside Drugs: In Vitro Optimization and Novel Nanoparticle Delivery Studies

We evaluated the potential of an investigational histone methylation reversal agent, 3-deazaneplanocin A (DZNep), in improving the chemosensitivity of pancreatic cancer to nucleoside analogs (i.e., gemcitabine). DZNep brought delayed but selective cytotoxicity to pancreatic cancer cells without affecting normal human pancreatic ductal epithelial (HPDE) cells. Co-exposure of DZNep and gemcitabine induced cytotoxic additivity or synergism in both well- and poorly-differentiated pancreatic cell lines by increased apoptosis. In contrast, DZNep exerted antagonism with gemcitabine against HPDE cells with significant reduction in cytotoxicity compared with the gemcitabine-alone regimen. DZNep marginally depended on purine nucleoside transporters for its cytotoxicity, but the transport dependence was circumvented by acyl derivatization. Drug exposure studies revealed that a short priming with DZNep followed by gemcitabine treatment rather than co-treatment of both agents to produce a maximal chemosensitization response in both gemcitabine-sensitive and gemcitabine-resistant pancreatic cancer cells. DZNep rapidly and reversibly decreased trimethylation of histone H3 lysine 27 but increased trimethylation of lysine 9 in an EZH2- and JMJD1A/2C-dependent manner, respectively. However, DZNep potentiation of nucleoside analog chemosensitization was found to be temporally coupled to trimethylation changes in lysine 27 and not lysine 9. Polymeric nanoparticles engineered to chronologically release DZNep followed by gemcitabine produced pronounced chemosensitization and dose-lowering effects. Together, our results identify that an optimized DZNep exposure can presensitize pancreatic cancer cells to anticancer nucleoside analogs through the reversal of histone methylation, emphasizing the promising clinical utilities of epigenetic reversal agents in future pancreatic cancer combination therapies.

High-dose vitamin B1 reduces proliferation in cancer cell lines analogous to dichloroacetate

PurposeThe dichotomous effect of thiamine supplementation on cancer cell growth is characterized by growth stimulation at low doses and growth suppression at high doses. Unfortunately, how thiamine reduces cancer cell proliferation is currently unknown. Recent focuses on metabolic targets for cancer therapy have exploited the altered regulation of the thiamine-dependent enzyme pyruvate dehydrogenase (PDH). Cancer cells inactivate PDH through phosphorylation by overexpression of pyruvate dehydrogenase kinases (PDKs). Inhibition of PDKs by dichloracetate (DCA) exhibits a growth suppressive effect in many cancers. Recently, it has been shown that the thiamine coenzyme, thiamine pyrophosphate reduces PDK-mediated phosphorylation of PDH. Therefore, the objective of this study was to determine whether high-dose thiamine supplementation reduces cell proliferation through a DCA-like mechanism.MethodsCytotoxicity of thiamine and DCA was assessed in SK-N-BE and Panc-1 cancer cell lines. Comparative effects of high-dose thiamine and DCA on PDH phosphorylation were measured by Western blot. The metabolic impact of PDH reactivation was determined by glucose and lactate assays. Changes in the mitochondrial membrane potential, reactive oxygen species (ROS) production, and caspase-3 activation were assessed to characterize the mechanism of action.ResultsThiamine exhibited a lower IC50 value in both cell lines compared with DCA. Both thiamine and DCA reduced the extent of PDH phosphorylation, reduced glucose consumption, lactate production, and mitochondrial membrane potential. High-dose thiamine and DCA did not increase ROS, but increased caspase-3 activity.ConclusionOur findings suggest that high-dose thiamine reduces cancer cell proliferation by a mechanism similar to that described for dichloroacetate.

P-glycoprotein inhibition by the agricultural pesticide propiconazole and its hydroxylated metabolites: Implications for pesticide–drug interactions

The human efflux transporter P-glycoprotein (P-gp, MDR1) functions as an important cellular defense system against a variety of xenobiotics; however, little information exists on whether environmental chemicals interact with P-gp. Conazoles provide a unique challenge to exposure assessment because of their use as both pesticides and drugs. Propiconazole is an agricultural pesticide undergoing evaluation by the U.S. Environmental Protection Agencys Endocrine Disruptor Screening Program. In this study, the P-gp interaction of propiconazole and its hydroxylated metabolites were evaluated using MDR1-expressing membrane vesicles and NIH-3T3/MDR1 cells. Membrane vesicle assays demonstrated propiconazole (IC50,122.9μM) and its metabolites (IC50s, 350.8μM, 366.4μM, and 456.3μM) inhibited P-gp efflux of a probe substrate, with propiconazole demonstrating the strongest interaction. P-gp mediated transport of propiconazole in MDR1-expressed vesicles was not detected indicating propiconazole interacts with P-gp as an inhibitor rather than a substrate. In NIH-3T3/MDR1 cells, propiconazole (1 and 10μM) led to decreased cellular resistance (chemosensitization) to paclitaxel, a chemotherapeutic drug and known MDR1 substrate. Collectively, these results have pharmacokinetic and risk assessment implications as P-gp interaction may influence pesticide toxicity and the potential for pesticide-drug interactions.