Cytosolic persistence of mouse brain CYP1A1 in chronic heme deficiency

Abstract

Previous work has demonstrated that the function of extrahepatic cytochrome P450 CYP1A1 is dependent on the availability of heme. CYP1A1 is involved in the activation of polyaromatic hydrocarbons. In the present study we used a transgenic mouse model with chronic impairment of heme synthesis -female porphobilinogen deaminase-deficient (PBGD -/-) mice -to investigate the effects of limited heme in untreated and b-naphthoflavone (b-NF)-treated animals on the function of CYP1A1 in brain. The heme content of PBGD -/-mice was diminished in the liver and brain compared to wild types. In the liver, partial heme deficiency led to less potent induction of CYP1A1 mRNA after b-NF treatment. In the brain, CYP1A1 protein was detected not only at the endoplasmic reticulum (ER), but also in the cytosol of PBGD -/-mice. Furthermore, 7-deethylation of ethoxyresorufin, an indicator of CYP1A1 metabolic activity, could be restored by heme in cytosol of PBGD -/-mouse brain. Independent of the genotype, we found only one cyp1a1 gene product, indicating that the cytosolic appearance of CYP1A1 most likely did not originate from mutant alleles. We conclude that heme deficiency in the brain leads to incomplete heme saturation of CYP1A1, which causes its improper incorporation into the ER membrane and persistence in the cytosol. It is suggested that diseases caused by relative heme deficiency, such as hepatic porphyrias, may lead to impaired hemoprotein function in brain.

Brain Drug-Metabolizing Cytochrome P450 Enzymes are Active In Vivo, Demonstrated by Mechanism-Based Enzyme Inhibition

“…These brain enzymes are unlikely to contribute to overall drug metabolism, however they may contribute to interindividual variation in drug response through local in situ metabolism (Britto and Wedlund, 1992). Although brain CYPs have been shown to be enzymatically active in vitro, it is not clear that there are sufficient necessary cofactors and coenzymes in close enough proximity for them to be active in vivo, nor whether there is sufficient endogenous heme in the brain for induced brain CYPs to be correctly targeted and inserted into appropriate membranes, and to be functional (Meyer et al, 2002(Meyer et al, , 2005. Here we have shown for the first time that both constitutive and induced CYPs are enzymatically functional in vivo.…”

“…As such, it is not clear that there are sufficient necessary cofactors in close enough proximity for brain CYPs to be active in vivo. In the brain, endogenous heme levels have been shown to be rate limiting toward normal CYP function and appropriate membrane insertion, suggesting that basal and induced brain CYPs are not always functional (Meyer et al, 2002(Meyer et al, , 2005.…”

Neurite Degeneration Induced by Heme Deficiency Mediated via Inhibition of NMDA Receptor-Dependent Extracellular Signal-Regulated Kinase 1/2 Activation

“…The importance of heme in mammalian neurobiology, however, extends beyond its functions as a direct constituent of mitochondrial cytochromes to being a prosthetic group in a large number of proteins in oxygen sensing and metabolism, detoxification, control of oxidative damage, growth and differentiation, and production of CO and nitric oxide (Padmanaban et al, 1989;Jover et al, 2000;Meyer et al, 2005). A small proportion of heme forms a regulatory pool that can act in a signaling mode by binding to heme-regulatory motifs in a number of proteins, including BK ion channels (Tang et al, 2003), transcription factors such as NPAS2 (neuronal PAS domain protein 2) and Bach1 (BTB and CNC homology 1) (Ogawa et al, 2001;Dioum et al, 2002), and circadian clock and receptor-associated proteins (Kaasik and Lee, 2004;Ghosh et al, 2005).…”

Early Failure ofN-Methyl-d-aspartate Receptors and Deficient Spine Formation Induced by Reduction of Regulatory Heme in Neurons

“…F, expression of CYP4X1 gene in control, heme-deficient neurons continuously treated with SA (2-14 DIV) and neurons treated with SA and heme for 12 days (2-14 DIV) measured by real-time PCR; ␤-actin was used as an endogenous reference gene. at ASPET Journals on May 11, 2018 molpharm.aspetjournals.org persistence of the protein (Meyer et al, 2005), and involvement of heme in transcriptional regulation (Dwarki et al, 1987). Our data did not contradict these suggestions but indicated that more time is required to develop heme deficiency in these functional compartments.…”

“…Deficiency of heme is detrimental to neurons and is a contributory factor in cell aging (Chernova et al, 2006), Alzheimer's disease (Atamna and Frey, 2004), and drug-induced neurotoxicity (Meyer et al, 2005). It also diminishes neuronspecific gene expression, alters cellular signaling, and induces apoptosis (Zhu et al, 2002).…”