Initiators, promoters and the uncertainty principle

Aromatic Amines in Experimental Cancer Research: Tissue-Specific Effects, an Old Problem and New Solutions

Carcinogenic aromatic amines usually produce tumors in specific target tissue, such as 2-acetylaminofluorene (AAF) producing liver tumors in rats, in contrast to some other structurally related arylamines. A hypothesis is presented that explains the mode of action in this rat liver model. Genotoxic and nongenotoxic effects work together and make AAF a complete rat liver carcinogen. The cytotoxic, promoting effects are particularly important. N-Hydroxy-2-aminofluorene and 2-nitrosofluorene, two metabolites of AAF, are able to uncouple the mitochondrial respiratory chain. They entertain a redox cycle that removes electrons from the respiratory chain and impairs ATP production. The dose-dependent opening of the mitochondrial permeability transition pore signals the viability of the cell. If the pore is opened to a certain extent, the cell is eliminated by apoptosis. As a consequence, oval cells proliferate, and as this process is overloaded, the liver transforms into a cirrhosis-like situation and thus provides the conditions under which initiated liver cells develop tumors. Such an interpretation is based on assumptions that have been debated for a long time. Some of these often forgotten developments are reviewed in support of the hypothesis, which allows a more comprehensive view of the complex in vivo situation at a time when in vitro models prevail.

Early hepatic changes induced by nuarimol in rats

It is commonly believed that in short-term tests hepatic cytochrome P-450b inducers stimulate liver enlargement and mitogenesis in the absence of overt hepatotoxic effects. In this investigation male Wistar rats received naurimol (an organochlorine pesticide) in one, three and five oral doses of 31.5, 62.5 and 125 mg kg-body wt. day-, whereupon the effects on liver were determined. The early effects were dose-dependent increases in p-nitroanisole metabolism, hepatocyte proliferation (DNA synthesis and mitotic activity) and liver weight. Five administrations of the lowest does (31.5 mg kg-1 body wt. day-1) did not change liver weight, despite increased p-nitroanisole metabolism and hepatocyte proliferation. In contrast to p-nitroanisole metabolism and hepatomegaly, proliferation was only transient and disappeared even when treatment continued. The increase in binuclear hepatocytes and signs of necrosis suggested that the hepatomitogenic effect of nuarimol reflected a regenerative response, which may simulate the proliferation caused by partial hepatectomy.

The influence of fenarimol on DNA synthesis and mitotic activity in rat liver

Fenarimol administered in one single oral dose of 125 or 62.5 mg kg-1 body wt. day-1, respectively, stimulated rat liver enlargement at a dose-independent rate. Three single doses of fenarimol produced dose-dependent liver growth, whereas five single doses caused no further increase in liver weight. This increase was accompanied by an increase in hepatic DNA synthesis and mitotic activity, with a peak on the first day after the beginning of the experiments. The increase in binuclear hepatocytes and signs of necrosis suggested that the hepatomitogenic effect reflected a compensatory hyperplasia. After both three and five single doses the hepatomitogenic effect was suppressed, as a result of tolerance development.

Binary effects of carcinogens and tumor promoters--a preliminary chemical structural analysis of BCIDB and PCIDB

Identification of potential interactions in exposures, either to simultaneous or sequential combinations of carcinogenic chemicals and mixtures of carcinogen and carcinogenesis-modifying chemical agents, pose formidable difficulties in human health risk assessments. Studies over the past four decades have revealed synergistic interactions among various classes of carcinogenic chemicals. An earlier effort to identify a possible chemical structural basis of interaction of binary combinations of carcinogens led to the development of Binary Carcinogen Interaction Data Base (BCIDB). A recent endeavor to identify a structural basis of interactions of carcinogens and carcinogenesis modifying chemical agents resulted in Promoter-Carcinogen Interaction Data Base (PCIDB). An attempt will be made in this report to briefly discuss chemical class-class interaction and initiator-target organ orientation as recorded in BCIDB and PCIDB.

New insights into the causes of cancer

Recent advances in molecular biologic analysis have led to major new insights concerning the genetic mechanisms underlying the development of cancer. This article examines the current state of our understanding of the genetic basis underlying the possible mechanisms of carcinogenesis and metastasis. The nature of the genetic lesions found in some cancer-causing genes, cancer-inhibiting genes, growth factor genes, and metastasis genes is discussed, as is the impact that these may have on clinical oncology.