Transcriptional Regulation of Chicken Apolipoprotein A-I Gene Expression

Deregulation of a set of critical cellular genes has long been speculated as a cause of the oncogenic transformation induced by v-Jun oncoprotein. In the past several years, extensive efforts have been made to identify such genes. Several target genes have been identified that are specifically associated with the v-Jun induced transformation phenotype in chicken embryo fibroblasts (CEF). We have undertaken the objective to identify and characterize the genes that become deregulated in response to, or as a consequence of, Jun-induced transformation. By exploiting the difference in oncogenic potential between v-Jun and c-Jun in CEF, the chicken apolipoprotein A-I (apoA-I) gene has been identified in our laboratory as one of the target genes whose expression is repressed in response to v-Jun overexpression in CEF. The overall objective of this study is to investigate the underlying molecular mechanisms by which the apoA-I gene expression is regulated, especially by the v-Jun oncoprotein. In this study, we mapped the v-Jun responsive elements within nucleotides −311 to +19 upstream of the apoA-I transcription start site. Biochemical analysis of functional domains of v-Jun indicates that DNA binding specificity of vJun and its ability to heterodimerize with diverse partners are absolutely required for repression of apoA-I transcription. Further, the sequences between amino acid residues 108 to 128 in the amino terminus of v-Jun proteins that contains the acidic region III of its transactivation domain are important for its repressor activity. In addition, an enhancer located between −6.8 kb to −6.0 kb upstream of the transcription start site of the apoA-I gene was identified and characterized. The enhancer is capable of stimulating transcription from the apoA-I promoter in a distance- and orientation-independent manner and is CEF-specific. Sequence information of the enhancer revealed that it is 791-by long and contains many putative binding sites for known transcription factors. Deletion analysis of the enhancer activity indicates that there are many modules, both positive and negative, located within the 791-by enhancer region. Nuclear transcription factors Sp1, C/EBP, HNF-3, and an unknown factor were found that might interact with the cis-acting elements of the enhancer