Glutamine synthetase shows a striking heterogeneous distribution in normal rat liver as consistently revealed by immunohistochemistry using a specific antiserum against the rat liver enzyme or a cross-reacting antiserum. The effects of zonal liver injury induced by allylformate or CCl4 on this distribution and on the activity of glutamine synthetase as well as of enzymes with different acinar distribution were investigated. Treatment with allylformate or CCl4 at appropriate concentrations led to severe hepatocyte necrosis in the periportal and perivenous zone, respectively, as revealed by histological examination and by the levels of serum marker enzymes. Exposure to allylformate (50 to 100 microliter per kg) for less than 1 day did not change the distribution and activity of glutamine synthetase but reduced the specific activities of the urea cycle enzymes. In contrast, treatment with CCl4 (1,000 microliter per kg) strongly reduced the activity and the acinar region covered by glutamine synthetase but not, for instance, the activities of the urea cycle enzymes. These results in conjunction with the data obtained for other enzymes indicate that a short exposure to these hepatotoxins affects different enzyme activities in close accord with their preferential acinar localization. During prolonged exposure this initial response was often modified due to adaptation. In the case of glutamine synthetase, however, no adaptive appearance of glutamine synthetase in other parts of the acinus could be detected even if the cell population originally expressing this phenotype was destroyed. This extremely inflexible distribution suggests that glutamine synthetase expression is a matter of cell differentiation rather than of modulation by nutritional and hormonal factors (or their acinar gradients) as found for many other hepatic enzymes.

Alanine Transaminase↗

PubMedDate not supplied

Obtaining of monoclonal antibodies by means of DNA immunization.

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Animals↗

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Sequential functional and morphological alterations during hepatocarcinogenesis induced in rats by feeding of a low dose of 2-acetylaminofluorene.

The early cellular events in liver carcinogenesis were studied in Fischer-344 male rats that either were fed 200 ppm 2-acetylaminofluorene (AAF) for up to 10 wk or were fed the carcinogen for 8 wk followed by maintenance for an additional 24 wk. By 1 wk of exposure, AAF caused a reduction in the number of glutamine synthetase (GS)-positive centrilobular hepatocytes, an increase in DNA synthesizing hepatocytes in the central areas of the hepatic lobules, and a shift from multinucleated to mononucleated hepatocytes, although overt hepatocellular necrosis was not evident. By 3 wk, altered hepatocellular foci characterized by deficiencies in iron storage (IS-) and collagen production and by expression of gamma-glutamyl transferase (GGT+) and placental-type glutathione transferase (PGT+) activity appeared. Single PGT+ cells were also found. During continued exposure, foci increased in number, size, and total area with the increases escalating between 8 and 10 wk of exposure. Cessation of AAF exposure at 8 wk resulted in a slight decrease in the number of foci after a further 6 wk of maintenance, but with continued maintenance for another 6 and 12 wk, the number again increased. IS- characterized the majority of foci during carcinogen administration, whereas after cessation of exposure, GGT+ and PGT+ foci predominated. None of the foci were positive for GS. After AAF exposure for 10 wk, a few neoplasms developed and greater numbers occurred after maintenance for a further 24 wk of rats exposed for 8 wk. We conclude the following: (a) the low dose of AAF caused subtle alterations in function and proliferation of normal hepatocytes and converted hepatocytes into focus cells; (b) reduction of the GS+ area is a sensitive indicator of cytotoxicity of AAF; (c) the development of some foci at an early stage depends on a promoting action of AAF, which ceased when the carcinogen was withdrawn, allowing some foci to undergo reversion; (d) a strong linkage exists in expression of IS-, GGT+, and PGT+ in foci; (e) the carcinogenic process accelerates in the absence of any indication of increased cytotoxicity by AAF; and (f) under the conditions of this study, no GS+ foci, adenomas, and carcinomas were found, indicating that no carcinogen-induced expression of GS occurred in these lesions and that GS expression is not linked to other phenotypic abnormalities.

2-Acetylaminofluorene↗