Effects of acute carbamazepine administration on haem metabolism in rat liver

Genetic and Functional Evaluation of the Role of FOXO1 in Antituberculosis Drug-Induced Hepatotoxicity

Background

The accumulation of the hepatotoxic substance protoporphyrin IX (PPIX) induced by aminolevulinate synthase 1 (ALAS1) activation is one of the important mechanisms of antituberculosis drug-induced hepatotoxicity (ATDH). Forkhead box protein O1 (FOXO1) may activate ALAS1 transcription. However, little is known about their roles in ATDH; we performed a study to determine the association between polymorphisms in the two genes and ATDH susceptibility. Then, we verified this possible association by cellular functional experiments.

Materials and Methods

Tag single-nucleotide polymorphisms (TagSNPs) in the two genes were genotyped in 746 tuberculosis patients. The frequencies of the alleles, genotypes, genetic models, and haplotype distribution of the variants were compared between the case and control groups. L-02 cells and HepG2 cells were incubated with the indicated concentration of isoniazid (INH) and rifampicin (RIF) for the desired times, and then the expression levels of ALAS1 and FOXO1 mRNAs and proteins were detected. HepG2 cells were transiently transfected with FOXO1 siRNA to observe the effect of changes in the FOXO1 expression on the cell survival rate and ALAS1 expression.

Results

The C allele at rs2755237 and the T allele at rs4435111 in the FOXO1 gene were associated with a decreased risk of ATDH. The expression of ALAS1 in both L-02 cells and HepG2 cells was increased by the coadministration of INH/RIF (600/200 μM) for 24 h. Although FOXO1 expression was reduced slightly by the same treatment, its content in the nucleus was significantly increased. However, the cell survival rate and ALAS1 expression level were not significantly altered by the downregulation of FOXO1 in HepG2 cells.

Conclusions

Variants of the rs4435111 and rs2755237 loci in the FOXO1 gene were associated with susceptibility to ATDH. Coadministration of INH/RIF promoted the transfer of FOXO1 from the cytoplasm to the nucleus, but the functional significance of its nuclear translocation requires further verification.

Hypothesis: Metabolic targeting of 5-aminolevulinate synthase by tryptophan and inhibitors of heme utilisation by tryptophan 2,3-dioxygenase as potential therapies of acute hepatic porphyrias

Metabolic targeting of liver 5-aminolevulinate synthase (5-ALAS) by inhibition of heme utilisation by tryptophan (Trp) 2,3-dioxygenase (TDO) or the use of tryptophan is proposed as a therapy of acute hepatic porphyrias. 5-ALAS, the rate-limiting enzyme of heme biosynthesis, is under negative feedback control by a small regulatory heme pool in the hepatic cytosol. Acute porphyric attacks, precipitated by fasting, certain hormones and some drugs, involve induction of 5-ALAS secondarily to depletion of the above pool, and the resultant elevation of 5-ALA levels initiates the abdominal and neurological symptoms of attacks. By utilising the regulatory heme, cytosolic TDO undermines the feedback control, thus allowing 5-ALAS induction to occur, e.g. upon glucocorticoid induction of TDO during fasting (starvation) and exogenous glucocorticoid administration. Currently, glucose therapy is the preferred strategy for reversing moderate attacks induced by fasting (calorie restriction), with more severe attacks being treated by intravenous heme preparations. Reversal of fasting-induced attacks by glucose is explained by the previously demonstrated reversal of increased heme utilisation by TDO. Inhibitors of this utilisation are therefore potential therapeutic targets in acute attacks and also for maintenance of a symptomless state. Existing TDO inhibitors other than glucose include allopurinol, nicotinamide and recently developed potent inhibitors such as LM10 used in cancer therapy. Based on studies in rats, the hypothesis predicts that the safety or otherwise of drugs in the hepatic porphyrias is determined by their ability to inhibit TDO utilisation of heme under basal conditions or after glucocorticoid induction or heme activation of TDO, in parallel with reciprocal changes in 5-ALAS induction. Tryptophan is also proposed as a potential therapy of acute attacks either alone or as an adjunct to the recently proposed 5-ALAS1 gene silencing. Trp increases heme biosynthesis by enhancing 5-ALA dehydratase activity and, based on a Trp-5-ALA model presented herein, Trp offers several advantages over heme therapy, namely rapid conversion of 5-ALA into heme, a greatly enhanced heme availability, a near complete inhibition of 5-ALAS induction, assumed rapid clearance of 5-ALA and hence accelerated resolution of symptoms of attacks, and finally provision of the neuroprotective metabolite kynurenic acid to neutralise the neurological symptoms. The hypothesis also addresses heme regulation in species lacking the TDO free apoenzyme and its glucocorticoid induction mechanism and proposes detailed assessment of heme biosynthesis in these species. Detailed proposals for testing the hypothesis are presented.

Upregulation of the initiating step of the kynurenine pathway in postmortem anterior cingulate cortex from individuals with schizophrenia and bipolar disorder

Upregulation of the kynurenine pathway has been associated with several etiologies of psychosis, an indication that increased levels of pathway intermediates might be involved in eliciting some psychotic features. In schizophrenia, tryptophan 2,3-dioxygenase (TDO2) was previously identified in postmortem frontal cortex as the enzyme likely responsible for the reported increase in pathway activity in the brain. For this follow-up study of postmortem anterior cingulate gyrus, we have found evidence of increased TDO2 activity in schizophrenia at three different levels of regulation: mRNA, protein, and metabolic product. The results were unaffected by neuroleptic status or smoking history. To make the distinction between mental disorders with psychosis and those without, this study included patients with bipolar disorder and major depression. Compared to the control group, the HPLC, RT-PCR, and immunohistochemistry results show significant elevation of (1) kynurenine in schizophrenia (1.9-fold, P = 0.02), and in bipolar disorder (1.8-fold, P = 0.04), primarily in the bipolar subgroup with psychosis (2.1-fold, P = 0.03); (2) TDO2 mRNA in schizophrenia (1.7-fold; P = 0.049); and (3) the immunohistochemistry values for the density of TDO2-positive white matter glial cells in schizophrenia (P = 0.01) and in major depression (P = 0.03) as well as the density and intensity of glial cells (in both gray and white matter) stained for TDO2 in bipolar disorder (P = 0.02). Unlike the results for schizophrenia and bipolar disorder, the increase in TDO2 protein in the major depression group was not associated with an increase in kynurenine concentration.

Biotin biotransformation to bisnorbiotin is accelerated by several peroxisome proliferators and steroid hormones in rats

Bisnorbiotin and biotin sulfoxide are the major catabolites of biotin for humans, swine, and rats. Increased urinary excretion of bisnorbiotin, biotin sulfoxide, or both have been observed during pregnancy and in patients treated with certain anticonvulsants. We sought more insight into the sites and mechanisms of biotin catabolism by exposing rats in vivo to compounds known to induce classes of enzymes that were candidates to catalyze the biotransformations. Rats were treated with the anticonvulsants phenytoin, phenobarbital, and carbamazepine, the steroid hormones dexamethasone and dehydroepiandrosterone, and the peroxisome proliferators clofibrate and di(2-ethylhexyl)phthalate. [14C]Biotin was injected intraperitoneally at physiologic doses in treated rats and control rats; HPLC and radiometric flow detection were used to specifically identify and quantify [14C]biotin and its metabolites in urine. Treatment effects were assessed by the change in the urinary excretion of [14C]bisnorbiotin and [14C]biotin sulfoxide in response to administration of [14C]biotin. No significant changes resulted from treatment with any of the anticonvulsants. With the steroid hormones and the peroxisome proliferators, [14C]bisnorbiotin excretion increased significantly. These results indicate that biotin is converted into bisnorbiotin in the liver and that this conversion likely occurs in peroxisomes or mitochondria or both via beta-oxidative cleavage, and, in contrast to responses in humans, the enzymes responsible for the formation of biotin sulfoxide in rats are not induced by the anticonvulsants examined here.