Synthesis and secretion of albumin in rats during treatment with a carcinogenic dose of N-2-acetylaminofluorene

High-Affinity Low-Capacity and Low-Affinity High-Capacity N-Acetyl-2-Aminofluorene (AAF) Macromolecular Binding Sites Are Revealed During the Growth Cycle of Adult Rat Hepatocytes in Primary Culture

Long-term cultures of primary adult rat hepatocytes were used to study the effects of N-acetyl-2-aminofluorene (AAF) on hepatocyte proliferation during the growth cycle; on the initiation of hepatocyte DNA synthesis in quiescent cultures; and, on hepatocyte DNA replication following the initiation of DNA synthesis. Scatchard analyses were used to identify the pharmacologic properties of radiolabeled AAF metabolite binding to hepatocyte macromolecules. Two classes of growth cycle-dependent AAF metabolite binding sites-a high-affinity low-capacity site (designated Site I) and a low-affinity high-capacity site (designated Site II)-associated with two spatially distinct classes of macromolecular targets, were revealed. Based upon radiolabeled AAF metabolite binding to purified hepatocyte genomic DNA or to DNA, RNA, proteins, and lipids from isolated nuclei, Site IDAY 4 targets (KD[APPARENT] ≈ 2-4×10-6 M and BMAX[APPARENT] ≈ 6 pmol/106 cells/24 h) were consistent with genomic DNA; and with AAF metabolized by a nuclear cytochrome P450. Based upon radiolabeled AAF binding to total cellular lysates, Site IIDAY 4 targets (KD[APPARENT] ≈ 1.5×10-3 M and BMAX[APPARENT] ≈ 350 pmol/106 cells/24 h) were consistent with cytoplasmic proteins; and with AAF metabolized by cytoplasmic cytochrome P450s. DNA synthesis was not inhibited by concentrations of AAF that saturated DNA binding in the neighborhood of the Site I KD. Instead, hepatocyte DNA synthesis inhibition required higher concentrations of AAF approaching the Site II KD. These observations raise the possibility that carcinogenic DNA adducts derived from AAF metabolites form below concentrations of AAF that inhibit replicative and repair DNA synthesis.

Non-random interaction of N-2-acetylaminofluorene and its N-hydroxy metabolite with DNA of rat hepatic parenchymal and non-parenchymal cell nuclei following in vivo administration of carcinogen

Treatment of rats with the hepatocarcinogen, N-2-acetylaminofluorene (AAF) results in development of malignant tumors derived primarily from hepatic parenchymal cells. Following administration of AAF, or its N-hydroxy derivative, in vivo nuclei from parenchymal cell and non-parenchymal cell populations (NI and NII nuclei populations, respectively) were isolated and treated with the endonuclease DNase I. The binding of carcinogen residues to the DNA of nuclease-accessible vs. nuclease-resistant regions of chromatin was evaluated on the basis of the selectivity of DNase I for transcriptionally active DNA. Under the experimental conditions employed DNase I digested approx. 50-60% of the genome of NI nuclei while only 10-20% of the DNA from non-parenchymal cell nuclei (NII) was susceptible to this enzyme. When the DNA of NI and NII nuclei were nick translated following limited digestion with DNase I, a greater degree of transcriptional activity (nuclease accessibility) was found in parenchymal cell nuclei (NI). Following a single injection of rats with [ring-3H]AAF or its N-hydroxy metabolite (N-[ring-3H]-OH-AAF) (1.8 mumol carcinogen/100 g), adducts were preferentially associated with DNA of DNase I resistant regions of target cell nuclei (NI), while preferentially associated with nuclease-accessible regions of non-target cell nuclei (NII). Damage following a single injection persisted for up to 3 days in DNase I-resistant DNA of NI nuclei, carcinogen adducts were rapidly lost from DNase I-accessible DNA of NII nuclei. These studies stress the importance of investigating specificity of carcinogens for particular regions of the genome of cell subpopulations within the target organ.

Alterations in the enzyme activity and polypeptide composition of rat hepatic endoplasmic reticulum during acute exposure to 2-acetylaminofluorene

Studies have been made of the morphology, enzyme activity and protein composition of liver endoplasmic reticulum in rats exposed to acute doses of the carcinogen, 2-acetylaminofluorene (2-AAF). Electron microscopic examination revealed numerous ultrastructural changes in the hepatocyte; most consistent alterations were the disorganisation of endoplasmic reticulum system with apparent increase of smooth endoplasmic reticulum. Administration of 2-AAF to rats immediately depressed microsomal glucose-6-phosphatase activity and eventually induced epoxide hydratase activity 6--7-fold over control activity. The induction was time-dependent and maximal rates of induction were observed at dosages greater than 40 mg/kg body wt. The treatment also induced cytochrome b5 content, NADH and NADPH cytochrome c reductase activities (1.0--1.5-fold). Only very small changes in the total content of cytochrome P-45- were noted. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) of microsomal proteins from 2-AAF pretreated animals showed time-dependent induction of two polypeptides which differed slightly in migration, in the region of Mr = 48000; the fast-migrating induced polypeptide has been identified as epoxide hydratase. Two-dimensional PAGE analysis of microsomal proteins from 2-AAF exposed rats showed a reproducible deletion of a protein with molecular weight in the region of 67000. The basis for the alterations in the protein composition of endoplasmic reticulum in response to 2-AAF treatment is discussed.

Involvement of a lipid-linked intermediate in the transfer of galactose from UDP [(14)C]galactose to exogenous protein in castor bean endosperm homogenates

A crude organelle preparation from germinating castor bean endosperm catalysed the incorporation of galactose from UDP[(14)C]galactose into chloroform/methanol (2:1)-soluble glactolipids. At least two galactolipids were formed. Most of the [(14)C]galactose was present in a galactolipid synthesized by the microsomal membranes, the remainder was present in a second galactolipid synthesized by other cellular membranes, possibly Golgi-derived. The addition of asialo-agalacto-fetuin reduced incorporation of [(14)C]galactose into the microsomal galactolipid with a concomitant increase in microsomal [(14)C]galactoprotein. Asialo-agalacto-fetuin did not affect galactolipid or galactoprotein synthesis by nonmicrosomal fractions. The results suggest that the endoplasmic reticulum is a major site of protein galactosylation in castor bean endosperm cells, and that galactose transfer from UDP-galactose to protein occurs via a lipid-linked intermediate.