Abstract
Expression in mice of transgenes directed by regulatory regions of the rat aldolase B gene requires the presence of a B element located in the first intron, while constructs devoid of this intronic enhancer are silent. Histo- and immunochemical staining of transgenic tissue sections showed that the longer transgene was expressed in the proximal tubular cells of the kidney, enterocytes located in small intestine villi and liver parenchymal cells. In the liver, a maximal expression was observed in perivenous hepatocytes, while the transgene was weakly active in periportal hepatocytes, which reproduced the pattern of functional zonation already reported for other glycolytic and gluconeogenic genes in the liver. We also established that the transgene retained the necessary elements for a correct chronological expression during development but was lacking elements necessary for activation by high carbohydrate diet. Instead, transgene expression was paradoxically stimulated in fasted animals, suggesting that the endogenous gene, which must be active under both glycolytic and gluconeogenic conditions, could possess distinct elements activating it in fasted as well as in carbohydrate-fed animals; the former element might be conserved in the transgene and the latter one might be lost.
Highlights
Aldolase B is the isoform of fructose 1,6-bisphosphate aldolase, which is specific to hepatocytes, proximal tubular cells of the kidney, and enterocytes, where it plays an essential role in fructose metabolism
We show that this element is absolutely required for the expression in transgenic mice of constructs in which the chloramphenicol acetyltransferase (CAT) gene is put under the control of aldolase B regulatory regions
The main result of these studies in transgenic mice is that the intronic element B is indispensable to the expression of the transgenes; 1600AB/CAT, 232ABC/CAT, 1600AB/CAT, and 232A100B/CAT transgenes are expressed in the liver and kidney of most transgenic lines while no 1600AC/ CAT, 232A600/CAT, and 232A100/CAT transgenes were active in any line
Summary
Aldolase B is the isoform of fructose 1,6-bisphosphate aldolase, which is specific to hepatocytes, proximal tubular cells of the kidney, and enterocytes, where it plays an essential role in fructose metabolism. The HNF1 and HNF3 binding sites overlap each other, and their occupancy is mutually exclusive, HNF1 being a transcriptional activator whose effect is counteracted by HNF3 (Gregori et al, 1993, 1994) This competition between HNF1 and HNF3 could explain why the activity of this promoter tested by transient expression is very weak in hepatocytes and hepatoma cells (Gregori et al, 1991, 1994). We show that this element is absolutely required for the expression in transgenic mice of constructs in which the chloramphenicol acetyltransferase (CAT) gene is put under the control of aldolase B regulatory regions (aldolase B/CAT constructs). The expression of the transgene was localized to proximal tubules of the kidney, enterocytes in the upper region of intestine villi, and in the liver to pericentrolobular hepatocytes
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