The SP1 sites of the human apoCIII enhancer are essential for the expression of the apoCIII gene and contribute to the hepatic and intestinal expression of the apoA-I gene in transgenic mice

We have generated transgenic mice carrying wild-type and mutant forms of th e apolipoprotein (apo)A-I/apoCIII gene cluster. Mutations were introduced e ither in one or in three SP1 binding sites of the apoCIII enhancer, In mice carrying the wild-type transgene, major sites of apoA-I mRNA synthesis wer e liver and intestine and minor sites were kidney and, to a lesser extent, other tissues. The major site of chloramphenicol acetyl transferase (CAT) a ctivity (used as a reporter for the apoCIII gene) was liver and minor sites intestine and kidney, A mutation in one SP1 binding site reduced the expre ssion of the apoA-I gene to similar to 23 and 19% in the liver and intestin e, respectively, as compared to the control wild-type. The hepatic expressi on of the CAT gene was not affected whereas the intestinal expression was n early abolished, Mutations in three SP1 binding sites reduced the hepatic a nd intestinal expression of the apoA-I and CAT genes to 14 and 4%, respecti vely, as compared to the wild-type control, and abolished CAT expression in all tissues, The findings suggest that the SP1 sites of the apoCIII enhanc er are required for the expression of the apoCIII gene and also contribute significantly to the hepatic and intestinal expression of the apoA-I gene i n vivo.