A unifying concept for carcinogenic risk assessments

Carcinogens influence both the initiation of abnormal cells and the subsequent promotion of such cells into neoplasia. Certain other insults seem limited to the stimulation of cellular proliferation and of carcinogenic potentiation. Common examples include surgical, mechanical, chemical, and temperature wounding of tissue followed by healing. In addition, certain hyperplastic growth induced by some chemicals may also enhance tumorigenesis. We propose that the quantification of carcinogenic potentiation may derive from a common-index-quantity estimated according to enhanced cell proliferation resulting from cytotoxicity or toxic hyperplasia induced by a specific exposure. At this time, it is not possible to define, in a restrictive sense, the molecular events which are critical to potentiation but the processes of cell proliferation resulting from cytotoxicity/hyperplasia seem to serve as indices which contain the necessary (and perhaps several secondary) biological responses. The unique advantage is that cell-culture, animal, and human-level studies can be used to evaluate certain parameters of the mathematical model for an untested treatment protocol or chemical insult suspected to be a cofactor in tumorigenesis. The main thrust of this paper is to propose that tumorigenesis should be studied in terms of cellular-population kinetics in response to a biological challenge rather than according to chemical or energetic parameters of that challenge.This approach leads to mathematical equations which can serve as a unifying concept for carcinogenic risk assessments. Sample results, to illustrate the utility of this model, are given for polynuclear aromatic hydrocarbons, trace metals, ionizing radiations, CO, NO, SO₂, O₃, and NO₂. Treatment, here, is for acute exposure conditions, but because the model is mechanistic, other exposure protocols can be addressed by simply adjusting some of the mathematical parameters according to factors estimated from a relative potency comparison of in vitro and in vivo studies best suited to the particular application of interest.     

Granular “endocrine” cells in avian respiratory epithelia

Summary An electron microscopic investigation of the extrapulmonary respiratory tract of embryos and chick of the domestic fowl (Gallus domesticus) has demonstrated for the first time in birds the presence here of a small number of epithelial cells characterised by an aminecontaining type of granule. These granular cells were scattered singly throughout the trachea, syrinx and primary bronchi and seemed more numerous in the caudal part of the airway. In favourable planes of section a small part of the cell was in contact with the luminal surface of the epithelium. The characteristic granular vesicles (approximate diameter 140 nm) appeared to be randomly distributed in the cytoplasm and there was no concentration of vesicles close to the plasma membrane. One of the cells was closely associated with an intraepithelial axon. By fluorescence microscopy, a small number of cells with a similar shape and distribution to the granular cells was observed after administration of 3,4-dihydroxyphenylalanine which may indicate the presence of an amine handlign mechanism in these cells. It is suggested that the granular cells belong to the APUD system of endocrine cells and that they may be modulated by the concentration of gas in the airways.     

The 15 N-terminal amino acids of hexokinase II are not required for in vivo function: analysis of a truncated form of hexokinase II in Saccharomyces cerevisiae

The function of the N-terminal amino acids of Saccharomyces cerevisiae hexokinase II was studied in vivo using strains producing a form of hexokinase II lacking its first 15 amino acids (short form). This short form of hexokinase II was produced from a fusion between the promoter region of the PGK1 gene and the HXK2 coding sequence except the first 15 codons. As expected, the in vitro analysis of the short form protein by gel filtration chromatography indicates that the short protein does not form dimers under conditions where the wild-type protein dimerizes. Kinetic studies show that the enzymatic activities are very similar to wild-type behavior. The physiological experiments performed on the strains containing the fusion allele demonstrate that the short form of the enzyme is similar to the wild-type both in terms of phosphorylation of hexoses and glucose repression. We conclude that the N-terminal amino acids of hexokinase II are not required in vivo either for phosphorylation of hexoses or for glucose repression.