GABAA/central benzodiazepine receptor and peripheral benzodiazepine receptor ligands as inducers of phenobarbital-inducible CYP2B and CYP3A

Prevalence and clinical correlates of flunitrazepam-related complex sleep behaviors

AIM

Complex sleep behaviors (CSB) are often associated with the use of hypnotic drugs. This study investigated the prevalence and correlates of CSB among psychiatric patients who were given flunitrazepam.

METHODS

From June 2011 to May 2012, a total of 268 psychiatric outpatients who had received flunitrazepam for at least 3 months were enrolled. Data on occurrence of CSB, demographic characteristics, flunitrazepam dosage and duration of use, psychiatric diagnoses, physical illnesses, and alcohol use were collected. Logistic regression analysis was used to examine the clinical correlates of CSB.

RESULTS

Sixty-six participants (24.6%) reported experiencing CSB. Logistic regression analysis showed that a high dosage (>2 mg/day) of flunitrazepam (odds ratio [OR] = 1.941, 95% confidence interval [CI] = 1.090-3.455, P = 0.024) and alcohol use (OR = 1.948, 95%CI = 1.023-3.709, P = 0.042) were significantly associated with the occurrence of CSB. Sex, age, duration of flunitrazepam use, psychiatric diagnoses, and physical illnesses were not significantly associated with the occurrence of CSB.

CONCLUSION

CSB among flunitrazepam users should be monitored routinely, especially among those receiving a high dosage who also consume alcohol.

Hypothyroidism caused by phenobarbital affects patterns of estrous cyclicity in rats

We found that repeated treatment with phenobarbital (PB), a thyroid modulator, resulted in a persistent estrous stage in the present study. Although the effects of PB in blocking the surge release of luteinizing hormone (LH), inducing anovulation and prolonging the diestrous period has been well established, there is still no research describing the appearance of persistent estrous states in normal cycling rats dosed with PB. To further study this phenomenon, female rats exhibiting regular estrous cycle were administered an oral dose of PB for 14 consecutive days. Consecutively, vaginal smears were observed and rats from all the groups were sacrificed and serum hormone levels for prolactin, progesterone, estradiol, triiodothyronin (T3), thyroxine (T4) and thyroid stimulating hormone (TSH) were measured. Pituitary, thyroid, liver, uteri and ovaries were excised, weighed and further subjected to histological observations. We found that PB induced irregular estrous cycles, especially persistent estrus in rats. Histopathologically, the persistent estrous stages are characterized by persistent vaginal cornification in the vagina, cystic follicles and anovulation in the ovaries. Endocrinologically, serum T3 and T4 levels were significantly lower, and TSH was higher in treated-female rats compared to control females. The serum estradiol level and the estradiol/progesterone ratio tend to increase in treated-females. Furthermore, PB-treated animals with irregular estrous cycle were reduced by T4 replacement. Our data indicate that treatment with PB resulted in hypothyroidism and irregular estrous cycle, particularly a persistent estrous stage in normal cycling female rats.

Fipronil is a powerful uncoupler of oxidative phosphorylation that triggers apoptosis in human neuronal cell line SHSY5Y

Fipronil is a phenylpyrazole insecticide known to elicit neurotoxicity via an interaction with ionotropic receptors, namely GABA and glutamate receptors. Recently, we showed that fipronil and other phenylpyrazole compounds trigger cell death in Caco-2 cells. In this study, we investigated the mode of action and the type of cell death induced by fipronil in SH-SY5Y human neuroblastoma cells. Flow cytometric and western blot analyses demonstrated that fipronil induces cellular events belonging to the apoptosis process, such as mitochondrial potential collapse, cytochrome c release, caspase-3 activation, nuclear condensation and phosphatidylserine externalization. In addition, fipronil induces a rapid ATP depletion with concomitant activation of anaerobic glycolysis. This cellular response is characteristic of mitochondrial injury associated with a defect of the respiration process. Therefore, we also investigated the effect of fipronil on the oxygen consumption in isolated mitochondria. Interestingly, we show for the first time that fipronil is a strong uncoupler of oxidative phosphorylation at relative low concentrations. Thus in this study, we report a new mode of action by which the insecticide fipronil could triggers apoptosis.

Glucocorticoids and phenobarbital induce murine CYP2B genes by independent mechanisms

BACKGROUND

Genes for CYP of the 2B subfamily (CYP2B genes) have long been known to be inducible in murine liver by phenobarbital and phenobarbital-like inducers. More recently, it has become clear that glucocorticoids can also induce these genes by a mechanism independent of that of phenobarbital-like inducers.

OBJECTIVE

To summarize the evidence for the existence of two distinct molecular mechanisms for induction of murine CYP2B genes and to analyze the wider implications of this situation for inducible xenobiotic metabolism.

METHODS

The mechanism of action of phenobarbital-like inducers of murine CYP2B genes is first briefly summarized. The role of glucocorticoids in the induction of various proteins, particularly rat phosphoenolpyruvate carboxykinase, where transcriptional activation is achieved via a glucocorticoid response unit, is also discussed. Finally, recent results are presented on glucocorticoid induction of murine CYP2B genes, including evidence for the presence of a functional glucocorticoid response unit in the rat CYP2B2 gene and for the role of constitutive androstane receptor as an accessory factor in this response.

RESULTS/CONCLUSION

Murine CYP2B genes are seen to respond to two distinct regulatory mechanisms, but much remains to be learned concerning the interactions between these two regulatory loops, as well as the details of glucocorticoid induction.

Effect of prototypical inducers on ligand activated nuclear receptor regulated drug disposition genes in rodent hepatic and intestinal cells

AIM

The aim of this study was to investigate the impact on expression of mRNA and protein by paradigm inducers/activators of nuclear receptors and their target genes in rat hepatic and intestinal cells. Furthermore, assess marked inter laboratory conflicting reports regarding species and tissue differences in expression to gain further insight and rationalise previously observed species differences between rodent and human based systems.

METHODS

Quantitative real time-polymerase chain reaction (QRT-PCR) and immunoblots were used to assess messenger RNA (mRNA) and protein expression for CYP2B2, CYP3A1, CYP3A2, CYP3A9, ABCB1a, ABCB1b, ABCC1, ABCC2, pregnane X receptor (PXR), farnesoid X receptor (FXR) and constituitive androstane receptor (CAR) in rat hepatoma cell line H411E, intestinal cells, Iec-6, and rat primary hepatocytes, in response to exposure for 18 h with prototypical inducers.

RESULTS

Dexamethasone (DEX) and pregnenolone 16alpha carbonitrile (PCN) significantly induced PXR, CYP3A9, ABCB1a and ABCB1b. However, when co-incubated, DEX appeared to restrict PCN-dependent induction. Chenodeoxycholic acid (CDCA) was the only ligand to induce FXR in all three cell types. Despite previously reported species differences between PCN and rifampicin (RIF), both compounds exhibited a similar profile of induction.

CONCLUSION

Data presented herein may explain some of the discrepancies previously reported with respect to species differences from different laboratories and have important implications for study design.

Dexamethasone induction of murine CYP2B genes requires the glucocorticoid receptor

Hepatic cytochrome P450 (P450) enzymes metabolize exogenous and endogenous compounds, and many are inducible by xenobiotics. Their synthesis is tightly regulated, particularly through nuclear receptors. Expression of murine CYP2B genes is strongly activated by treatment with phenobarbital or phenobarbital-like inducers, and a detectable response requires the presence of the constitutive androstane receptor (CAR). However, other compounds can also induce murine CYP2B proteins. For example, dexamethasone is known to induce rat CYP2B1 and CYP2B2 and mouse CYP2B10. Using human HepG2 and rat H4IIEC3 hepatoma cell lines, we found that dexamethasone induction of CYP2B2 and Cyp2b10 luciferase reporters required the glucocorticoid receptor. Given the well known observation that CYP2B genes are not phenobarbital-responsive in cultured cell lines, the dexamethasone responsiveness of CYP2B reporter constructs in cell lines demonstrates in itself that the mechanism of dexamethasone induction is distinct from that of phenobarbital. We also analyzed the relative importance of the phenobarbital response unit (PBRU) and of a known glucocorticoid response element in this response. Both sites contributed to the response, but other sites were required for maximal induction. CAR was also found to act as an accessory factor to stimulate the response to dexamethasone by the glucocorticoid receptor. Furthermore, in H4IIEC3 cells, CAR activated the PBRU in the natural sequence context of the CYP2B2 and Cyp2b10 5' flanks. In summary, there are at least two independent mechanisms of CYP2B induction: one involving phenobarbital and phenobarbital-like inducers and another involving glucocorticoids that induce via the glucocorticoid receptor with CAR acting as an accessory factor.

The peripheral benzodiazepine receptor ligand 1-(2-chlorophenyl-methylpropyl)-3-isoquinoline-carboxamide is a novel antagonist of human constitutive androstane receptor

As a promiscuous xenobiotic sensor, the constitutive androstane receptor (CAR; NR1I3) regulates the expression of multiple drug-metabolizing enzymes and transporters in liver. The constitutively activated nature of CAR in the cell-based transfection assays has hindered its use as a predictor of metabolism-based drug-drug interactions. Here, we have identified 1-(2-chlorophenylmethylpropyl)-3-isoquinoline-carboxamide (PK11195), a typical peripheral benzodiazepine receptor (PBR) ligand, as a selective and potent inhibitor of human (h) CAR. In cell-based transfection assays, PK11195 inhibited the constitutive activity of hCAR more than 80% at the concentration of 10 microM, and the PK11195-inhibited activity was efficiently reactivated by the direct CAR activator, 6-(4-chlorophenyl)imidazo[2,1-b][1,3]thiazole-5-carbaldehyde-O-(3,4-dichlorobenzyl) oxime, but not by the indirect hCAR activator, phenobarbital. Mammalian two-hybrid and GST pull-down assays showed that PK11195 repressed the interactions of hCAR with the coactivators steroid receptor coactivator-1 and glucocorticoid receptor-interacting protein 1 to inhibit hCAR activity. The inhibition by PK11195 specifically occurred to the hCAR: PK1195 strongly activated human pregnane X receptor (PXR), whereas it did not alter the activity of the mouse CAR and mouse PXR. In addition, PBR played no role in the PK11195 inhibition of hCAR because the inhibition fully occurred in the HeLa cells in which the PBR was knocked down by small interfering RNA. In the Car(-/-) mouse liver, PK11195 translocated enhanced yellow fluorescent protein-hCAR into the nucleus. These results are consistent with the conclusion that PK11195 is a novel hCAR-specific antagonist that represses the CAR-coactivator interactions to inhibit the receptor activity inside the nucleus. Thus, PK11195 can be used as a chemical tool for studying the molecular basis of CAR function.

Differences in the expression and inducibility of cytochrome P450 2B isoenzymes in cultured rat brain neuronal and glial cells

Studies initiated to investigate the distribution of cytochrome P450 2B (CYP2B) isoenzymes in rat brain cells revealed significant activity of CYP2B-dependent 7-pentoxyresorufin-O-dealkylase (PROD) in microsomes prepared from both, cultured rat brain neuronal and glial cells. Neuronal cells exhibited 2-fold higher activity of PROD than the glial cells. RT-PCR and immunocytochemical studies demonstrated significant constitutive mRNA and protein expression of CYP2B in cultured neuronal and glial cells. Induction studies with phenobarbital (PB), a known CYP2B inducer, revealed significant concentration dependent increase in the activity of PROD in cultured brain cells with glial cells exhibiting greater magnitude of induction than the neuronal cells. This difference in the increase in enzyme activity was also observed with RT-PCR and immunocytochemical studies indicating differences in the induction of CYP2B1 and 2B2 mRNA as well as protein expression in the cultured brain cells. Furthermore, a greater magnitude of induction was observed in CYP2B2 than CYP2B1 in the brain cells. Our data indicating differences in the expression and sensitivity of the CYP2B isoenzymes in cultured rat brain cells will help in identifying and distinguishing xenobiotic metabolizing capability of these cells and understanding the vulnerability of the specific cell types toward neurotoxins.

Long lasting effects of prenatal exposure to deltamethrin on cerebral and hepatic cytochrome P450s and behavioral activity in rat offspring

Prenatal exposure to different doses (0.25, or 0.5 or 1.0 mg/kg corresponding to 1/320 th or 1/160 th or 1/80 th of LD50) of deltamethrin to the pregnant Wistar rats from gestation day 5 to 21 were found to produce a dose dependent increase in the activity of cytochrome P450 (CYP) dependent 7-ethoxyresorufin-O-deethylase (EROD), 7-pentoxyresorufin-O-dealkylase (PROD) and N-nitrosodimethylamine demethylase (NDMA-D) in brain and liver of offspring postnatally at 3 weeks. The increase in the activity of cytochrome P450 monooxygenases was found to be associated with the increase in the mRNA and protein expression of xenobiotic metabolizing CYP1A, 2B and 2E1 isoenzymes in the brain and liver of offspring. Dose-dependent alterations in the parameters of spontaneous locomotor activity in the offspring postnatally at 3 weeks have indicated that increase in cytochrome P450 activity may lead to the accumulation of deltamethrin and its metabolites to the levels that may be sufficient to alter the behavioral activity of the offspring. Interestingly, the inductive effect on cerebral and hepatic cytochrome P450s was found to persist postnatally up to 6 weeks in the offspring at the relatively higher doses (0.5 and 1.0 mg/kg) of deltamethrin and up to 9 weeks at the highest dose (1.0 mg/kg), though the magnitude of induction was less than that observed at 3 weeks. Alterations in the parameters of spontaneous locomotor activity in the offspring postnatally at 6 and 9 weeks, though significant only in the offspring at 3 and 6 weeks of age, have further indicated that due to the reduced activity of the cytochrome P450s during the ontogeny, the pyrethroid or its metabolites accumulating in the brain may not be cleared from the brain, thereby leading to the persistence in the increase in the expression of cerebral and hepatic cytochrome P450s in the offspring postnatally up to 9 weeks. The data suggests that low dose prenatal exposure to pyrethroids has the potential to produce long lasting effects on the expression of xenobiotic metabolizing cytochrome P450s in brain and liver of the offspring.