Gene Activations of CYP2B1 and CYP3A1 by Dihydropyridine Calcium Channel Antagonists in the Rat Liver: the Structure-Activity Relationship

Docosahexaenoic acid down-regulates phenobarbital-induced cytochrome P450 2B1 gene expression in rat primary hepatocytes via the sphingomyelinase/ceramide pathway

Docosahexaenoic acid (DHA) regulates the expression of cytochrome P450 2B1 (CYP 2B1) in rat primary hepatocytes in response to xenobiotics. Ceramide, a lipid signaling molecule, is involved in various physiological processes and can be generated by the hydrolysis of sphingomyelin via sphingomyelinase (SMase). DHA activates SMase and increases ceramide formation in vitro. Ceramides differentially enhance adenylyl cyclase activity in vitro depending on the chain length of their fatty acids. In addition, the cAMP-dependent PKA pathway down-regulates CYP 2B1 expression induced by phenobarbital (PB). In the present study, we determined the effect of DHA on SMase transactivation and the downstream pathway in CYP 2B1 expression induced by PB. SMase was activated by DHA 2 h after treatment, and D609 (an SMase inhibitor) attenuated the inhibition of PB-induced CYP 2B1 expression by DHA. Ceramide formation reached a maximum 3 h after DHA administration. C2-ceramide dose-dependently inhibited PB-induced CYP 2B1 expression and increased intracellular cAMP concentrations. SQ22536 (an adenylyl cyclase inhibitor) and H89 (a PKA-specific inhibitor) partially reversed the inhibition of PB-induced CYP 2B1 expression by C2-ceramide. These results suggest that stimulation of SMase, generation of ceramide and activation of the cAMP-dependent PKA pathway are involved in the inhibition exerted by DHA.

Induction of hepatic Cyp2b and Cyp3a subfamily enzymes by nicardipine and nifedipine in mice

1. Nicardipine (Nic) or nifedipine (Nif) was given to male and female C57BL/6J mice by a single gavage at doses of 100, 200 and 400 micromolkg(-1), and changes in the levels of mRNA and apoprotein of hepatic cytochrome P450 (P450) enzymes, including Cyp2b9, Cyp2b10, Cyp3a11 and Cyp3a41, were examined by reverse transcriptase-polymerase chain reaction (RT-PCR) and Western blotting, respectively. Furthermore, hepatic microsomal activities for pentoxyresorufin O-dealkylation (PROD) and nifedipine oxidation, which are mainly mediated by Cyp2b and Cyp3a subfamily enzymes, respectively, were measured. 2. Results from RT-PCR analysis revealed that Nic, but not Nif, showed a capacity for activating the Cyp3a11 gene in either sex of mice and that both chemicals could induce a male-selective activation of Cyp2b10 gene, although they had no capacity for activating the Cyp2b9 and Cyp3a41 genes in either sex. 3. Increased levels of the mRNAs of Cyp2b10 and Cyp3a11 were closely correlated with those of apoprotein and activity of the corresponding P450 subfamily enzymes. 4. The study demonstrated for the first time that Nic, but not Nif, showed the ability to induce Cyp3a11 in both sexes of mice, although both Nif and Nic led to a male-selective induction of Cyp2b10, and that Nic and Nif had no ability to induce Cyp2b9 and Cyp3a41 in either sex.

INDUCTION OF HEPATIC CYTOCHROME P450 ISOFORMS BY NICARDIPINE AT THERAPEUTIC DOSES IN SPONTANEOUSLY HYPERTENSIVE RATS

Nicardipine hydrochloride (Nic), a calcium channel antagonist, is used for the treatment of hypertension. In the present study, we estimated its effects on the levels and activities of hepatic cytochrome P450 isoforms in spontaneously hypertensive rats given p.o. with Nic at a dose of 0.5, 2.5, 5, or 12.5 mg/kg at 24-hr intervals for 14 days. Therapeutic effects on the development of hypertension were observed at doses of 5 and 12.5 mg/kg/day. Significant increases in the levels of mRNAs and enzyme activities of hepatic P450 isoforms, CYP1A1 and/or CYP1A2, by 14-day repetitive treatment with Nic were observed at lower therapeutic doses, whereas the increase in protein levels for CYP1A2 was observed at a higher therapeutic dose of 12.5 mg/kg/day. Likewise, the activities of hepatic CYP2B and CYP3A subfamily enzymes were increased by the 14-day-treatment of Nic only at a therapeutic dose (12.5 mg/kg/day), whereas their mRNA and protein levels were increased at lower therapeutic doses. To date, the dihydropyridine family, including Nic, has been believed to have inhibitory effects on the activity of various cytochrome P450 enzymes, especially human CYP3A4. However, the present findings demonstrate for the first time that Nic-repetitive treatments at a therapeutic dose result in significant increases in the expressions and activities of hepatic CYP1A, CYP2B, and CYP3A subfamily enzymes. Therefore, the effects of dihydropyridine family on cytochrome P450 enzymes have to be further validated to provide information on its safe and beneficial therapeutic application.