No abstract

Cell proliferation is pivotal to all stages of the carcinogenesis process and is one of the primary characteristics of the promotion stage of cancer development. Both a two-stage model of initiation and promotion for analysis of early preneoplasia and a three-stage initiation-promotion-progression model of hepatocarcinogenesis were used to address the effect of the liver tumor-promoting agent phenobarbital (PB) on hepatic cellular proliferation. Male rats were subjected to a 70% partial hepatectomy and 10 mg diethylnitrosamine (DEN)/kg or the solvent alone and were administered PB for 4-8 months. Analysis of bromodeoxyuridine (BrdU) incorporation (1 h pulse) in liver within (focal) and not within (non-focal) altered hepatic foci (AHF) demonstrated a labeling index in AHF of 2% in DEN-initiated rats; the non-focal labeling index of placental glutathione S-transferase expressing hepatocytes was 0.3-0.6%. The focal labeling index was constant over the 8 month period of promotion. Inasmuch as one characteristic of promotion is the reversibility of the induced effects on clonal expansion of initiated cells, groups of rats initially promoted with PB were maintained in the absence of continued promotion for 4 or 8 months prior to being killed. Assessment of the focal labeling index after cessation of PB treatment indicated a drop in the index from 2.3% to 0.7%. When a progressor agent, ethylnitrosourea, was given at the time PB was discontinued for 4 or 8 months, a significant change in focal labeling index was not observed relative to the index in AHF when the animals were killed immediately after 8 months of PB promotion. Thus, cell proliferation plays an integral role in both the promotion and progression stages of multistage rat hepatocarcinogenesis and is influenced by administration of promoting and progressor agents.

Camptothecin (1), a potent antitumor alkaloid, is known to inhibit topoisomerase I, an enzyme that relaxes supercoiled DNA. Modifications have been made to the B, D, and E rings of this natural product. Specifically, compounds 2-10 either have an ester moiety in place of the E ring lactone, a methyl ester attached to position 14, a saturated (or nonexistent) deaza B ring, or contain a combination of these permutations. We have conducted in vitro assays against the topoisomerase I relaxation reaction which verify the necessity for a lactone in the E ring. Furthermore, steric requirements at position 14 are shown to be crucial for activity, and planarity of the A and B rings of camptothecin is also implicated in the ability of the drug to inhibit topoisomerase I. Speculation on the nature of the drug binding pocket is presented.

Inhibition of eukaryotic topoisomerase I by daunomycin and structurally modified daunomycin analogues is demonstrated. These studies suggest that binding of the drug to DNA, at low ratios of drug to base pairs of DNA, is sufficient for inhibition that is nonspecific, i.e., inhibition that does not involve the trapping of covalent DNA-topoisomerase I cleavable complexes. Inhibition of numerous DNA-processing enzymes by DNA-binding agents is also implicated.