A regulatory element in the ApoCIII promoter that directs hepatic specific transcription binds to proteins in expressing and nonexpressing cell types.

To better understand the mechanisms that determine cell type-specific gene expression, we have examined the transcriptional activity of a 13-nucleotide long sequence element, designated C3P, located in the promoter of the apoCIII gene. We demonstrate that this element is required for high levels of apoCIII gene expression in hepatic cells and is sufficient to determine hepatic specific expression when introduced into a heterologous promoter. A protein was identified in hepatic cell nuclear extracts, designated AF-1, that binds to this sequence and is presumably responsible for its transcriptional activity in hepatic cells. Even though the C3P element is not active in HeLa cells, a protein with C3P binding specificity was identified in HeLa cell nuclear extracts. While the HeLa protein is similar to the hepatic AF-1 in its binding specificity and relative abundance, it has approximately twice the molecular weight of the hepatic protein, indicating that they are different proteins or different forms of the same protein. A variety of murine tissue types, including those that do not express the apoCIII gene, were found to contain C3P binding proteins. We conclude that the cell type-specific activity of the C3P element is not due to the absence of C3P binding proteins in nonexpressing cells but is the result of qualitative differences in C3P binding proteins in different cell types.