Multimodal Imaging Probe Development for Pancreatic β Cells: From Fluorescence to PET

Pancreatic β cells are responsible for insulin secretion and are important for glucose regulation in a healthy body and diabetic disease patient without prelabeling of islets. While the conventional biomarkers for diabetes have been glucose and insulin concentrations in the blood, the direct determination of the pancreatic β cell mass would provide critical information for the disease status and progression. By combining fluorination and diversity-oriented fluorescence library strategy, we have developed a multimodal pancreatic β cell probe PiF for both fluorescence and for PET (positron emission tomography). By simple tail vein injection, PiF stains pancreatic β cells specifically and allows intraoperative fluorescent imaging of pancreatic islets. PiF-injected pancreatic tissue even facilitated an antibody-free islet analysis within 2 h, dramatically accelerating the day-long histological procedure without any fixing and dehydration step. Not only islets in the pancreas but also the low background of PiF in the liver allowed us to monitor the intraportal transplanted islets, which is the first in vivo visualization of transplanted human islets without a prelabeling of the islets. Finally, we could replace the built-in fluorine atom in PiF with radioactive 18F and successfully demonstrate in situ PET imaging for pancreatic islets.

Quantitative Evaluation of Cytochrome P450 3A4 Inhibition and Hepatotoxicity in HepaRG 3-D Spheroids

Predicting drug–drug interactions (DDIs) is an important step during drug development to avoid unexpected side effects. Cytochrome P450 (CYP) 3A4 is the most abundant human hepatic phase I enzyme, which metabolizes >50% of therapeutic drugs. Therefore, it is essential to test the potential of a drug candidate to induce CYP3A4 expression or inhibit its activity. Recently, 3-dimensional (3-D) mammalian cell culture models have been adopted in drug discovery research to assess toxicity, DDIs, and pharmacokinetics. In this study, we applied a human 3-D spheroid culture protocol using HepaRG cells combined with liquid chromatography-tandem mass spectrometry (LC-MS/MS) to assess its ability to predict CYP3A4 inhibition. Levels of midazolam, a specific substrate of CYP3A4, were used to determine the long-term metabolic capacity of CYP3A4. Midazolam was decreased in the 3-D HepaRG culture system by ∼80% over 7 days, whereas its primary metabolite, 1-hydroxymidazolam, increased by ∼40%. Next, we assessed hepatotoxicity by determining the cytotoxicity of known hepatotoxicants in HepaRG spheroids, HepG2 cells, and primary human hepatocytes. Significant differences in cytotoxicity were detected in the system using 3-D HepaRG spheroids. These results suggest that 3-D HepaRG spheroids are a good model for prediction of CYP inhibition and hepatotoxicity in screening of early drug candidates.

Zebrafish as a Screening Model for Testing the Permeability of Blood-Brain Barrier to Small Molecules

The objective of this study was to evaluate the permeability of small molecules into the brain via the blood-brain barrier in zebrafish and to investigate the possibility of using this animal model as a screening tool during the early stages of drug discovery. Fifteen compounds were used to understand the permeation into the brain in zebrafish and mice. The ratio of brain-to-plasma concentration was compared between the two animal models. The partition coefficient (K_p,brain), estimated using the concentration ratio at designated times (0.167, 0.25, 0.5, or 2 h) after oral administrations (per os, p.o), ranged from 0.099 to 5.68 in zebrafish and from 0.080 to 11.8 in mice. A correlation was observed between the K_p,brain values obtained from the zebrafish and mice, suggesting that zebrafish can be used to estimate K_p,brain to predict drug penetration in humans. Furthermore, in vivo transport experiments to understand the permeability glycoprotein (P-gp) transporter-mediated behavior of loperamide (LPM) in zebrafish were performed. The zebrafish, K_{p,brain,30min} of LPM was determined to be 0.099 ± 0.069 after dosing with LPM alone, which increased to 0.180 ± 0.115 after dosing with LPM and tariquidar (TRQ, an inhibitor of P-gp). In mouse, the K_{p,brain,30min} of LPM was determined to be 0.080 ± 0.004 after dosing with LPM alone and 0.237 ± 0.013 after dosing with LPM and TRQ. These findings indicate that the zebrafish could be used as an effective screening tool during the discovery stages of new drugs to estimate their distribution in the brain.

A Novel Small-Molecule Inhibitor Targeting the IL-6 Receptor β Subunit, Glycoprotein 130

IL-6 is a major causative factor of inflammatory disease. Although IL-6 and its signaling pathways are promising targets, orally available small-molecule drugs specific for IL-6 have not been developed. To discover IL-6 antagonists, we screened our in-house chemical library and identified LMT-28, a novel synthetic compound, as a candidate IL-6 blocker. The activity, mechanism of action, and direct molecular target of LMT-28 were investigated. A reporter gene assay showed that LMT-28 suppressed activation of STAT3 induced by IL-6, but not activation induced by leukemia inhibitory factor. In addition, LMT-28 downregulated IL-6-stimulated phosphorylation of STAT3, gp130, and JAK2 protein and substantially inhibited IL-6-dependent TF-1 cell proliferation. LMT-28 antagonized IL-6-induced TNF-α production in vivo. In pathologic models, oral administration of LMT-28 alleviated collagen-induced arthritis and acute pancreatitis in mice. Based on the observation of upstream IL-6 signal inhibition by LMT-28, we hypothesized IL-6, IL-6Rα, or gp130 to be putative molecular targets. We subsequently demonstrated direct interaction of LMT-28 with gp130 and specific reduction of IL-6/IL-6Rα complex binding to gp130 in the presence of LMT-28, which was measured by surface plasmon resonance analysis. Taken together, our data suggest that LMT-28 is a novel synthetic IL-6 inhibitor that functions through direct binding to gp130.

Determination of mesoridazine by liquid chromatography-tandem mass spectrometry and its application to pharmacokinetic study in rats

The object of the present study was to develop and validate an assay method of mesoridazine in rat plasma using liquid chromatography-tandem mass spectrometry (LC-MS/MS). Plasma samples from rats were prepared by simple protein precipitation and injected onto the LC-MS/MS system for quantification. Mesoridazine and chlorpromazine as an internal standard (IS) were separated by a reversed phase C18 column. A mobile phase was composed of 10mM ammonium formate in water and acetonitrile (ACN) (v/v) by a linear gradient system, increasing the percentage of ACN from 2% at 0.4min to 98% at 2.5min with 4min total run time. The ion transitions monitored in positive-ion mode [M+H](+) of multiple-reaction monitoring (MRM) were m/z 387>126 for mesoridazine and m/z 319>86 for IS. The detector response was specific and linear for mesoridazine at concentrations within the range 0.001-4μg/ml and the correlation coefficient (R(2)) was greater than 0.999 and the signal-to-noise ratios for the samples were ≥10. The intra- and inter-day precision and accuracy of the method were determined to be within the acceptance criteria for assay validation guidelines. The matrix effects were approximately 101 and 99.5% from rat plasma for mesoridazine and chlorpromazine, respectively. Mesoridazine was stable under various processing and/or handling conditions. Mesoridazine concentrations were readily measured in rat plasma samples after intravenous and oral administration. This assay method can be practically useful to the pharmacokinetic and/or toxicokinetic studies of mesoridazine.

Preclinical pharmacokinetic characterization of 2-(4-(4-(5-(2-phenyl-5-(trifluoromethyl)oxazole-4-carboxamido)-1H-benzo[d]imidazol-2-yl)phenyl)cyclohexyl) acetic acid, a novel DGAT-1 inhibitor

1. A novel diacylglyceride acyltransferase-1 (DGAT-1) inhibitor, 2-(4-(4-(5-(2-phenyl-5-(trifluoromethyl) oxazole-4-carboxamido)-1H-benzo[d]imidazol-2-yl)phenyl)cyclohexyl) acetic acid (KR-69232), was synthesized for a potential therapeutic use against several metabolic disorders, such as obesity, insulin resistance, and type II diabetes, characterized by excessive triglycerides (TGs) in the blood. 2. The half-lives against phase I metabolism were measured as 75.3 ± 20.9 min and over 120 min in rat and human liver microsomes, respectively. In Caco-2 cell monolayers, extremely low permeability (<0.13 × 10⁻⁶cm/s) was seen in the absorptive direction, predicting limited intestinal absorption of KR-69232. This compound was highly bound to rat and human plasma proteins (>99.8%). 3. With the intravenous administration of KR-69232 in rats (1, 2, and 5 mg/kg), non-linear kinetics were observed at the highest dose, with significantly higher systemic clearance, higher volume of distribution, and lower dose-normalized AUC. Following oral administration, it exhibited low bioavailability (<10%) and was absorbed slowly (T(max), 3.8-5.2 h) over the dose range. We also confirmed that considerable KR-69232 remained in the intestine at T(max), demonstrating its limited absorption into the systemic circulation.

Association study between polymorphisms of CD28, CTLA4 and ICOS and non-segmental vitiligo in a Korean population

CD28 molecule (CD28), cytotoxic T-lymphocyte-associated protein 4 (CTLA4) and inducible T-cell co-stimulator (ICOS) are important regulators of the immune system. Vitiligo, a common autoimmune skin disorder, is characterized by a loss of melanocytes that results in cutaneous white patches. The aim of the present study was to determine whether or not polymorphisms of the CD28, CTLA4 and ICOS genes are associated with non-segmental vitiligo in a Korean population. To determine the relationships between CD28, CTLA4 and ICOS genes and vitiligo, four single nucleotide polymorphisms (SNPs) associated with the CD28 gene [rs1879877 (promoter, -1198), rs3181097 (promoter, -1059), rs2140148 (intron 1) and rs3116494 (intron 2)], two SNPs associated with the CTLA4 gene [rs231777 (intron 1) and rs231779 (intron 1)] and five SNPs associated with the ICOS gene [rs4270326 (intron 3), rs11571314 (intron 3), rs10183087 (3' untranslated region; UTR), rs4404254 (3'UTR) and rs1559931 (3'UTR)] were selected. Two hundred and thirty-one patients with non-segmental vitiligo (NSV) and 405 healthy controls were enrolled. Genotyping was performed using the restriction fragment length polymorphism technique and direct sequencing. SNPStats, Haploview 4.2 and SPSS 18.0 were used to conduct the analyses. Significant differences were noted between CTLA4 (p<0.05) and NSV, but not CD28 and ICOS (p>0.05). However, these associations disappeared after Bonferroni correction. The CD28, CTLA4 and ICOS genes may not be associated with NSV.

Perfluorinated compounds in coastal waters of Hong Kong, South China, and Korea

Perfluorinated compounds (PFCs), such as perfluorooctanesulfonate (PFOS) and related compounds, have recently been identified in the environment. PFOS, the terminal degradation product of many of the PFCs, has been found globally in many wildlife species, as well as open ocean waters, even in remote regions far from sources. In this study, a solid-phase extraction procedure coupled with high-performance liquid chromatography interfaced to high-resolution mass spectrometry was used to isolate, identify, and quantify small concentrations of PFCs in seawater. These techniques were applied to investigate the local sources of PFCs in several industrialized areas of Asia and provide information on how the PFCs are circulated by coastal currents. Ranges of concentrations of PFOS in coastal seawaters of Hong Kong, the Pearl River Delta, including the South China Sea, and Korea were 0.09-3.1, 0.02-12, and 0.04-730 pg/mL, respectively, while those of perfluorooctanoic acid (PFOA) were 0.73-5.5, 0.24-16, and 0.24-320 pg/mL, respectively. Potential sources of PFCs include major industrialized areas along the Pearl River Delta of southern China and major cities of Korea, which are several of the fastest growing industrial and economic regions in the world. Detectable concentrations of PFOS and PFOA in waters of southern China were similar to those in the coastal marine environment of Japan and certain regions in Korea. Concentrations of PFCs in several locations in Korean waters were 10-100-fold greater than those in the other locations on which we report here. The spatial and seasonal variations in PFC concentrations in surface seawaters in the Pearl River Delta and South China Sea indicate the strong influence of the Pearl River discharge on the magnitude and extent of PFC contamination in southern China. All of the concentrations of PFOS were less than those that would be expected to cause adverse effects to aquatic organisms or their predators except for one location in Korea adjacent to an industrialized area. Hazard quotients were from <0.001 to 0.002 for aquatic animals and ranged from <0.001 to 17 for predatory birds.

Childhood meningioma: unusual location, atypical radiological findings, and favorable treatment outcome

OBJECTS

To investigate the characteristics of childhood meningioma, especially, locations, radiological findings, pathological features (including proliferative potential) and outcome, 11 children with meningiomas were retrospectively analyzed.

RESULTS

Unusual location, large size, frequent calcification, and cyst formation were characteristic radiological findings. Gross total resection was achieved in 8 patients, and there was recurrence in 2. Gamma knife radiosurgery was performed on residual and recurrent tumors. MIB-1 indices tended to be high in large tumors. Nine patients had a Karnofsky Performance Scale of more than 70 during the follow-up period of 10 months to 19.5 years. Surgical treatment rendered 4 of 5 epileptic patients seizure free. The childhood meningiomas examined had unusual locations, atypical radiological findings, and various proliferative potentials.

CONCLUSIONS

Complete resection is the treatment of choice. Gamma knife radiosurgery can be a good alternative for residual tumors and small recurrent tumors. The outcome of childhood meningiomas is good after surgery.

Suppression of experimental myasthenia gravis, a B cell-mediated autoimmune disease, by blockade of IL-18

Interleukin-18 (IL-18) is a pleiotropic proinflammatory cytokine that plays an important role in interferon gamma (IFN-gamma) production and IL-12-driven Th1 phenotype polarization. Increased expression of IL-18 has been observed in several autoimmune diseases. In this study we have analyzed the role of IL-18 in an antibody-mediated autoimmune disease and elucidated the mechanisms involved in disease suppression mediated by blockade of IL-18, using experimental autoimmune myasthenia gravis (EAMG) as a model. EAMG is a T cell-regulated, antibody-mediated autoimmune disease in which the nicotinic acetylcholine receptor (AChR) is the major autoantigen. Th1- and Th2-type responses are both implicated in EAMG development. We show that treatment by anti-IL-18 during ongoing EAMG suppresses disease progression. The protective effect can be adoptively transferred to naive recipients and is mediated by increased levels of the immunosuppressive Th3-type cytokine TGF-beta and decreased AChR-specific Th1-type cellular responses. Suppression of EAMG is accompanied by down-regulation of the costimulatory factor CD40L and up-regulation of CTLA-4, a key negative immunomodulator. Our results suggest that IL-18 blockade may potentially be applied for immunointervention in myasthenia gravis.

Blockade of CD40 Ligand Suppresses Chronic Experimental Myasthenia Gravis by Down-Regulation of Th1 Differentiation and Up-Regulation of CTLA-4

Myasthenia gravis (MG) and experimental autoimmune MG (EAMG) are T cell-dependent Ab-mediated autoimmune disorders, in which the nicotinic acetylcholine receptor (AChR) is the major autoantigen. Th1-type cells and costimulatory factors such as CD40 ligand (CD40L) contribute to disease pathogenesis by producing proinflammatory cytokines and by activating autoreactive B cells. In this study we demonstrate the capacity of CD40L blockade to modulate EAMG, and analyze the mechanism underlying this disease suppression. Anti-CD40L Abs given to rats at the chronic stage of EAMG suppress the clinical progression of the autoimmune process and lead to a decrease in the AChR-specific humoral response and delayed-type hypersensitivity. The cytokine profile of treated rats suggests that the underlying mechanism involves down-regulation of AChR-specific Th1-regulated responses with no significant effect on Th2- and Th3-regulated AChR-specific responses. EAMG suppression is also accompanied by a significant up-regulation of CTLA-4, whereas a series of costimulatory factors remain unchanged. Adoptive transfer of splenocytes from anti-CD40L-treated rats does not protect recipient rats against subsequently induced EAMG. Thus it seems that the suppressed progression of chronic EAMG by anti-CD40L treatment does not induce a switch from Th1 to Th2/Th3 regulation of the AChR-specific immune response and does not induce generation of regulatory cells. The ability of anti-CD40L treatment to suppress ongoing chronic EAMG suggests that blockade of CD40L may serve as a potential approach for the immunotherapy of MG and other Ab-mediated autoimmune diseases.

Short peptides with induced beta-turn inhibit the interaction between HIV-1 gp120 and CD4

To identify novel peptides that inhibit the interaction between human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein gp120 and CD4, we constructed a targeted phage-displayed peptide library in which phenylalanine and proline were fixed at the fourth and sixth positions, respectively, because Phe43 and the adjacent beta-turn of CD4 are critical for interaction with gp120. Two synthetic peptides were selected after three rounds of biopanning against gp120, and one of them, G1 peptide (ARQPSFDLQCGF), exhibited specific inhibition of the interaction between gp120 and CD4 with an IC(50) of about 50 microM. Structural analysis using NMR demonstrated that G1 peptide forms a compact cyclic structure similar to the CD4 region interacting with gp120. Two derivatives of G1 peptide, a linear hexameric peptide (G1-6) and a cyclic nonameric peptide (G1-c), were synthesized based on the structure of the G1 peptide. Interestingly, they showed higher inhibitory activities than did G1 peptide with IC(50)'s of 6 and 1 microM, respectively. Thus, this study might provide a new insight into the development of anti-HIV-1 inhibitors.

Short peptides with induced beta-turn inhibit the interaction between HIV-1 gp120 and CD4

To identify novel peptides that inhibit the interaction between human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein gp120 and CD4, we constructed a targeted phage-displayed peptide library in which phenylalanine and proline were fixed at the fourth and sixth positions, respectively, because Phe43 and the adjacent beta-turn of CD4 are critical for interaction with gp120. Two synthetic peptides were selected after three rounds of biopanning against gp120, and one of them, G1 peptide (ARQPSFDLQCGF), exhibited specific inhibition of the interaction between gp120 and CD4 with an IC(50) of about 50 microM. Structural analysis using NMR demonstrated that G1 peptide forms a compact cyclic structure similar to the CD4 region interacting with gp120. Two derivatives of G1 peptide, a linear hexameric peptide (G1-6) and a cyclic nonameric peptide (G1-c), were synthesized based on the structure of the G1 peptide. Interestingly, they showed higher inhibitory activities than did G1 peptide with IC(50)'s of 6 and 1 microM, respectively. Thus, this study might provide a new insight into the development of anti-HIV-1 inhibitors.

Mechanism of nasal tolerance induced by a recombinant fragment of acetylcholine receptor for treatment of experimental myasthenia gravis

Acetylcholine receptor (AChR) is the major autoantigen in myasthenia gravis (MG) and experimental autoimmune MG (EAMG). Here we analyze the mechanisms involved in suppression of ongoing EAMG in rats by nasal administration of a recombinant fragment from the human AChR alpha-subunit. We demonstrate that such a fragment, expressed without a fusion partner, confers nasal tolerance that can be adoptively transferred. Our observations suggest that the underlying mechanism of this nasal tolerance is active suppression involving a shift from a Th1 to a Th2/Th3-regulated AChR-specific response which may be mediated by down regulation of costimulatory factors.

Role of tolerogen conformation in induction of oral tolerance in experimental autoimmune myasthenia gravis

We recently demonstrated that oral or nasal administration of recombinant fragments of the acetylcholine receptor (AChR) prevents the induction of experimental autoimmune myasthenia gravis (EAMG) and suppresses ongoing EAMG in rats. We have now studied the role of spatial conformation of these recombinant fragments in determining their tolerogenicity. Two fragments corresponding to the extracellular domain of the human AChR alpha-subunit and differing in conformation were tested: Halpha1-205 expressed with no fusion partner and Halpha1-210 fused to thioredoxin (Trx), and designated Trx-Halpha1-210. The conformational similarity of the fragments to intact AChR was assessed by their reactivity with alpha-bungarotoxin and with anti-AChR mAbs, specific for conformation-dependent epitopes. Oral administration of the more native fragment, Trx-Halpha1-210, at the acute phase of disease led to exacerbation of EAMG, accompanied by an elevation of AChR-specific humoral and cellular reactivity, increased levels of Th1-type cytokines (IL-2, IL-12), decreased levels of Th2 (IL-10)- or Th3 (TGF-beta)-type cytokines, and higher expression of costimulatory factors (CD28, CTLA4, B7-1, B7-2, CD40L, and CD40). On the other hand, oral administration of the less native fragments Halpha1-205 or denatured Trx-Halpha1-210 suppressed ongoing EAMG and led to opposite changes in the immunological parameters. It thus seems that native conformation of AChR-derived fragments renders them immunogenic and immunopathogenic and therefore not suitable for treatment of myasthenia gravis. Conformation of tolerogens should therefore be given careful attention when considering oral tolerance for treatment of autoimmune diseases.

Prevention of passively transferred experimental autoimmune myasthenia gravis by a phage library-derived cyclic peptide

Many pathogenic antibodies in myasthenia gravis (MG) and its animal model, experimental autoimmune MG (EAMG), are directed against the main immunogenic region (MIR) of the acetylcholine receptor (AcChoR). These antibodies are highly conformation dependent; hence, linear peptides derived from native receptor sequences are poor candidates for their immunoneutralization. We employed a phage-epitope library to identify peptide-mimotopes capable of preventing the pathogenicity of the anti-MIR mAb 198. We identified a 15-mer peptide (PMTLPENYFSERPYH) that binds specifically to mAb 198 and inhibits its binding to AcChoR. A 10-fold increase in the affinity of this peptide was achieved by incorporating flanking amino acid residues from the coat protein as present in the original phage library. This extended peptide (AEPMTLPENYFSERPYHPPPP) was constrained by the addition of cysteine residues on both ends of the peptide, thus generating a cyclic peptide that inhibited the binding of mAb 198 to AcChoR with a potency that is three orders of magnitude higher when compared with the parent library peptide. This cyclic peptide inhibited the in vitro binding of mAb 198 to AcChoR and prevented the antigenic modulation of AcChoR caused by mAb 198 in human muscle cell cultures. The cyclic peptide also reacted with several other anti-MIR mAbs and the sera of EAMG rats. In addition, this peptide blocked the ability of mAb 198 to passively transfer EAMG in rats. Further derivatization of the cyclic peptide may aid in the design of suitable synthetic mimotopes for modulation of MG.

Suppression of ongoing experimental myasthenia by oral treatment with an acetylcholine receptor recombinant fragment

Myasthenia gravis (MG) is an autoimmune disorder in which the nicotinic acetylcholine receptor (AChR) is the major autoantigen. In an attempt to develop an antigen-specific therapy for MG, we administered a nonmyasthenogenic recombinant fragment of AChR orally to rats. This fragment, corresponding to the extracellular domain of the human AChR alpha-subunit (Halpha1-205), protected rats from subsequently induced experimental autoimmune myasthenia gravis (EAMG) and suppressed ongoing EAMG when treatment was initiated during either the acute or chronic phases of disease. Prevention and suppression of EAMG were accompanied by a significant decrease in AChR-specific humoral and cellular responses. The underlying mechanism for the Halpha1-205-induced oral tolerance seems to be active suppression, mediated by a shift from a T-helper 1 (Th1) to a Th2/Th3 response. This shift was assessed by changes in the cytokine profile, a deviation of anti-AChR IgG isotypes from IgG2 to IgG1, and a suppressed AChR-specific delayed-type hypersensitivity response. Our results in experimental myasthenia suggest that oral administration of AChR-specific recombinant fragments may be considered for antigen-specific immunotherapy of myasthenia gravis.

Antigen-specific modulation of experimental myasthenia gravis: nasal tolerization with recombinant fragments of the human acetylcholine receptor alpha-subunit

Myasthenia gravis (MG) and experimental autoimmune myasthenia gravis (EAMG) are antibody-mediated autoimmune diseases in which the nicotinic acetylcholine receptor (AcChoR) is the major autoantigen. The immune response in these diseases is heterogeneous and is directed to a wide variety of T and B cell epitopes of AcChoR. Candidate molecules for specific immunotherapy of MG should, therefore, have a broad specificity. We used recombinant fragments of the human AcChoR, encompassing the extracellular domain of the alpha-subunit, or shorter fragments derived from it, in experiments to modulate EAMG. We have demonstrated that intranasal administration of these recombinant fragments, which represent a major portion of epitopes involved in MG, prevents the induction of EAMG in rats and immunosuppresses an ongoing disease, as assessed by clinical symptoms, weight loss, and muscle AcChoR content. These effects on EAMG were accompanied by a marked reduction in the proliferative T-cell response and IL-2 production in response to AcChoR, in reduced anti-self AcChoR antibody titers and in an isotype switch of AcChoR-specific antibodies, from IgG2 to IgG1. We conclude that nasal tolerance induced by appropriate recombinant fragments of human AcChoR is effective in suppressing EAMG and might possibly be considered as a therapeutic modality for MG.

Modulation of aflatoxin B1 biotransformation by beta-naphthoflavone in isolated rabbit lung cells

The cytotoxic and carcinogenic mycotoxin aflatoxin (AF) B1 (AFB1) is biotransformed by the cytochrome P450 monooxygenases (CYP) to a number of relatively nontoxic metabolites, as well as to the ultimate toxic metabolite, AFB1-8,9-epoxide. In a number of tissues and species, AFB1 hydroxylation to the relatively nontoxic metabolite, AFM1, is induced by beta-naphthoflavone (BNF) treatment. Although the liver is the principle target organ for AFB1 toxicity, the mycotoxin is also toxic and carcinogenic to respiratory tissues. To determine if BNF treatment alters the extent of pulmonary AFB1 bioactivation by enhancing detoxification and thereby decreasing epoxidation, the effects of BNF on pulmonary AFB1 metabolism were examined. Rabbit lung cells, isolated by protease digestion and centrifugal elutriation, were incubated with [3H]AFB1. In nonciliated bronchiolar epithelial (Clara) cell-enriched (45-50%) fractions, [3H]AFM1 production (pmol/mg DNA per 2 h) was increased by prior treatment of rabbits with BNF (80 mg/kg per day, 3 and 2 days before cell isolation) as follows: with 1.0 microM [3H]AFB1; control, 10.6 +/- 2.3; BNF, 30.0 +/- 6.4; with 0.10 microM [3H]AFB1; control, 9.4 +/- 4.7; BNF, 20.6 +/- 5.9. With 1.0 microM [3H]AFB1, prior treatment of animals with BNF abolished formation of [3H]aflatoxicol (AFL) but not [3H]AFQ1. The activation (epoxidation) of [3H]AFB1 was measured indirectly as covalent binding to endogenous DNA. With 1.0 microM [3H]AFB1, treatment of rabbits with BNF did not alter DNA binding (pmol/mg DNA per 2 h) in the Clara cell-enriched fraction: control, 103 +/- 41; BNF, 114 +/- 49. However, with 0.10 microM [3H]AFB1, DNA binding in the same fraction was 47% lower in cells from BNF treated animals: control, 17.4 +/- 4.2; BNF, 9.3 +/- 3.9. Formation of 8,9-dihydro-8,9-dihydroxy-AFB1, and the glutathione conjugate of the aflatoxin epoxide (AFB1-GSH) were not detectable at the AFB1 concentration and time point studied, in cells from either BNF-treated or control rabbits. Incubation of isolated, unseparated lung cells from untreated rabbits with 5.0 to 50 microM BNF decreased [3H]AFB1-DNA binding in the presence of 0.1 microM [3H]AFB1 by 35 to 77%, while lower BNF concentrations did not alter DNA binding. In lung cells isolated from BNF treated rabbits, BNF was not detectable (i.e. < 0.5 microM detection limit). Therefore, the amount of BNF present in isolated rabbit lung cells following in vivo treatment with BNF was below that required to directly inhibit AFB1-DNA adduct formation. The decrease in AFB1-DNA binding from rabbits treated with BNF is apparently due to the selective induction of CYP isozymes and related increases in AFM1 formation, and not to direct inhibition of epoxidation or enhanced conjugation of AFB1-8,9-epoxide with glutathione.