Effects of 3-(2-phenylethyl)-4-methylsydnone and related sydnones on heme biosynthesis

Inactivation of chick embryo hepatic cytochrome P450 1A, 2H and 3A following in ovo administration of 3,5-diethoxycarbonyl-1,4-dihydro-2,6-dimethyl-4-ethylpyridine and 3-[2-(2,4,6-trimethylphenyl)thioethyl]-4-methylsydnone

Rat hepatic cytochrome P450 (P450) isozymes 1A1, 2C6, 2C11, 3A1 and 3A2 are targets for mechanism-based inactivation by the porphyrinogenic compound 3,5-diethoxycarbonyl-1,4-dihydro-2,6-dimethyl-4-ethylpyridine (4-ethyl DDC). It is of interest to determine whether similar P450 isozymes are targets of porphyrinogenic drugs in the chick embryo liver. The chick embryo expresses P450 2H1/2 isozymes, which are similar to the rat P450 2B1/2 isozymes, a polycyclic aromatic hydrocarbon-inducible P450 1A isozyme, and a pregnenolone 16 alpha-carbonitrile-inducible P450 3A isozyme. We have found previously that chick embryo hepatic P450 1A and 3A isozymes are targeted for in vitro mechanism-based inactivation by 4-ethyl DDC and by the sydnone 3-[2-(2,4,6-trimethylphenyl)thioethyl]-4-methylsydnone (TTMS). Marked differences have been observed between the in vitro and in vivo effects of porphyrinogenic drugs on P450 isozymes. Thus, the first objective of this study was to determine whether chick embryo hepatic P450 1A and 3A isozymes are subject to in ovo inactivation by these porphyrinogenic compounds. Our second objective was to determine whether the chick embryo hepatic P450 2H isozyme(s) was subject to in ovo and in vitro inactivation by 4-ethyl DDC and TTMS. Using hepatic microsomes prepared from beta-naphthoflavone-, dexamethasone-, phenobarbital-, and glutethimide-induced 19-day-old chick embryos, we found that total P450 content was decreased significantly in microsomes prepared from all treatment groups following in ovo administration of 4-ethyl DDC and TTMS. Moreover, in ovo administration of both 4-ethyl DDC and TTMS caused a significant decrease of 7-ethoxyresorufin O-deethylase, erythromycin N-demethylase, and benzphetamine N-demethylase activities, which are selective catalytic markers for chick embryo hepatic P450 1A, 3A and 2H isozymes, respectively. In addition, in vitro administration of 4-ethyl DDC and TTMS caused mechanism-based inactivation of benzphetamine N-demethylase activity in microsomes from phenobarbital- and glutethimide-treated chick embryos, showing that the chick embryo hepatic P450 2H isozyme is a target for mechanism-based inactivation. Therefore, it was concluded that the chick embryo hepatic P450 1A, 2H and 3A isozymes serve as targets for both in ovo and in vitro mechanism-based inactivation by 4-ethyl DDC and TTMS.

Evidence for mechanism-based inactivation of rat and chick embryo hepatic cytochrome P4501A and P4503A by dihydropyridines, sydnones, and dihydroquinolines

Rat hepatic P4501A1 and 3A1/2 have been shown previously to be targets for mechanism-based inactivation by the 4-alkyl analogues of 3,5-diethoxycarbonyl-1,4-dihydro-2,4,6-trimethylpyridine (DDC), namely, 4-ethyl DDC and 4-isopropyl DDC. In this study we have shown that rat hepatic P4501A and P4503A are targets for mechanism-based inactivation by the sydnones, 3-[2-(2,4,6-trimethylphenyl)thioethyl]-4-methylsydnone (TTMS) and 3-(2-phenylethyl)-4-methylsydnone (PEMS). The dihydroquinoline, 2,4-diethyl-2-methyl-1,2-dihydroquinoline (DMDQ), caused mechanism-based inactivation of rat hepatic P4501A but not of P4503A. The P4501A isozyme(s) of chick embryo liver was found to share the ability of rat liver P4501A to serve as a target for mechanism-based inactivation by the dihydropyridines, 4-ethyl DDC and 4-isopropyl DDC, the sydnones, TTMS and PEMS, and the dihydroquinoline, DMDQ. A P4503A-like isozyme of chick embryo liver shared the ability of the rat liver P4503A isozyme(s) to serve as a target for mechanism-based inactivation by the dihydropyridines, 4-ethyl DDC and 4-isopropyl DDC, and the sydnone, TTMS, but not of the sydnone PEMS. The dihydropyridine, DDC, was found to serve as a mechanism-based inactivator of the chick embryo P4501A isozyme(s), but not of the P4503A isozyme(s), in contrast to its previously reported inactivity with both the rat hepatic P4501A1 and 3A1/2 isozymes.

Elevation of delta-aminolevulinic acid synthase and cytochrome PB1 P450 messenger RNA levels by dihydropyridines, dihydroquinolines, sydnones, and N-ethylprotoporphyrin IX

A series of compounds that increase the activity of delta-aminolevulinic acid synthase (ALAS) in chick embryo hepatocyte cultures were studied for their effects on steady-state levels of mRNA for ALAS and phenobarbital-inducible cytochrome PB1 P450. N-Ethylprotoporphyrin IX (N-EtPP), which is believed to lower heme levels by inhibition of ferrochelatase (FC), had little effect on steady-state ALAS mRNA levels. 3,5-Diethoxycarbonyl-1,4-dihydro-2,6-dimethyl-4- isobutylpyridine (4-isobutyl DDC), which is believed to lower heme levels by repetitive destruction of the heme moiety of cytochrome P450, increased steady-state levels of ALAS mRNA levels approximately 2-fold. 3,5-Diethoxycarbonyl-1,4-dihydro-2,6-dimethyl-4-ethylpyridine (4-ethyl DCC) which inhibits FC activity and destroys the heme moiety of cytochrome P450, increased ALAS mRNA levels approximately 4-fold. A combination of N-EtPP and 4-isobutyl DDC produced a synergistic increase in ALAS mRNA levels to approximately 6-fold over control levels. The synergistic increase in ALAS activity observed previously with this combination can be explained, at least in part, by a synergistic increase in ALAS mRNA levels. Other porphyrinogenic agents, which function as mechanism-based inactivators of cytochrome P450 and elevate ALAS activity, were found to elevate ALAS mRNA. These compounds included 3-[2-(2,4,6-trimethylphenyl)thioethyl]-4-methylsydnone (TTMS), 2,4-diethyl-2-methyl-1,2-dihydroquinoline (DMDQ), and 2,2,4-trimethyl-1,2,dihydroquinoline (TMDQ). The elevation of ALAS mRNA by these porphyrinogenic agents is probably due to their lowering of cellular heme levels by a combination of ferrochelatase inhibition and repetitive destruction of the heme moiety of cytochrome P450. The lowering of heme levels should result in an enhancement of ALAS mRNA half-life as it has been demonstrated by others that heme shortens the half-life of ALAS mRNA. It was of interest that some of these drug treatments also caused an elevation in steady-state levels of cytochrome PB1 P450 mRNA; the exception was TTMS, which along with its analogue 3-(2-phenylethyl)-4-methylsydnone (PEMS), did not alter cytochrome PB1 P450 mRNA levels. Increases in steady-state levels of cytochrome PB1 P450 mRNA subsequent to increases in steady-state levels of ALAS mRNA were observed with 4-ethyl DDC, 4-isobutyl DDC, DMDQ, and TMDQ. The data obtained with N-EtPP and a combination of N-EtPP and 4-isobutyl DDC on cytochrome PB1 P450 mRNA levels do not support the contention that heme functions as a positive regulator of cytochrome P450 gene expression.