The doxorubicin-selected lung cancer cell line H69AR is resistant to many chemotherapeutic agents. However, like most tumor samples from individuals with this disease, it does not overexpress P-glycoprotein, a transmembrane transport protein that is dependent on adenosine triphosphate (ATP) and is associated with multidrug resistance. Complementary DNA (cDNA) clones corresponding to messenger RNAs (mRNAs) overexpressed in H69AR cells were isolated. One cDNA hybridized to an mRNA of 7.8 to 8.2 kilobases that was 100- to 200-fold more expressed in H69AR cells relative to drug-sensitive parental H69 cells. Overexpression was associated with amplification of the cognate gene located on chromosome 16 at band p13.1. Reversion to drug sensitivity was associated with loss of gene amplification and a marked decrease in mRNA expression. The mRNA encodes a member of the ATP-binding cassette transmembrane transporter superfamily.

Heme biosynthesis in hepatocytes is controlled by a free heme pool, which regulates δ‐aminolevulinic acid synthase. Porphyrinogenic chemicals deplete the regulatory free heme pool by interacting with cytochrome P‐450 thereby inhibiting heme biosynthesis and/or causing heme breakdown. Recent developments allow us to predict which groups of chemicals are likely to be porphyrinogenic. One group is exemplified by 3,5‐diethoxycarbonyl‐1,4‐dihydro‐2,4,6‐trimethylpyridine. Heterocyclic compounds of this type cause mechanism‐based inactivation of cytochrome P‐450, leading to the formation of N‐alkylporphyrins, with ferrochelatase‐inhibitory activity resulting in lowering the free heme pool. Allylisopropylacetamide exemplifies a second group. Such compounds containing a terminal olefinic or acetylenic group, cause mechanism‐based inactivation of cytochrome P‐450. In the process, the heme moiety of cytochrome P‐450 is destroyed and the free heme pool is lowered. A third group is exemplified by planar polyhalogenated or polycyclic aromatic hydrocarbons. These compounds induce specific cytochrome P‐450 isozymes but are poor substrates. Active oxygen is formed, which interacts with a hepatic substrate to form a uroporphyrinogen decarboxylase inhibitor. Inhibition of this enzyme leads to depletion of the free heme pool.—Marks, G. S.; McCluskey, S. A.; Mackie, J. E.; Riddick, D. S.; James, C. A. Disruption of hepatic heme biosynthesis after interaction of xenobiotics with cytochrome P‐450. FASEB J. 2: 2774‐2783; 1988.

Incubation of 2,4-diethyl-1,2-dihydro-2-methylquinoline (DMDQ) with hepatic microsomes from rats pretreated with phenobarbital, 3-methylcholanthrene, pregnenolone-16 alpha-carbonitrile, or dexamethasone results in minor loss of the cytochrome P-450 chromophore and accumulation of a hepatic pigment. The hepatic pigment consists of the four regioisomers of N-ethylprotoporphyrin IX and minor amounts of the corresponding N-methyl regioisomers. Exposure of chick embryo liver cells to DMDQ results in inhibition of their ferrochelatase activity, induction of their 5-aminolevulinic acid synthase activity, and accumulation of protoporphyrin IX. 1,2-Dihydro-2,2,4-trimethylquinoline (TMDQ) causes negligible loss of cytochrome P-450 in rat liver microsomes but in vivo still produces the four N-methylprotoporphyrin IX regioisomers in low yield. Furthermore, it inhibits ferrochelatase activity, elevates 5-aminolevulinic acid synthase activity, and causes protoporphyrin IX accumulation in cultured chick embryo hepatocytes. One-electron oxidation of the 2,2-dialkyl-1,2-dihydroquinolines to radical cations is postulated to result in N-alkylation of the prosthetic heme group of cytochrome P-450. The N-alkylprotoporphyrins IX thus formed are potent inhibitors of ferrochelatase. Inhibition of ferrochelatase causes the induction of 5-aminolevulinic acid synthase and the accumulation of protoporphyrin IX. Heme alkylation and ferrochelatase inhibition may be generally associated with substrates that are subject to cytochrome P-450 mediated oxidative extrusion of alkyl radicals.

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