INTESTINAL APOLIPOPROTEIN AI GENE-TRANSCRIPTION IS REGULATED BY MULTIPLE DISTINCT DNA ELEMENTS AND IS SYNERGISTICALLY ACTIVATED BY THE ORPHAN NUCLEAR RECEPTOR, HEPATOCYTE NUCLEAR FACTOR-4

We have used apolipoprotein genes to investigate the signal transduction mechanisms involved in the control of intestinal specific gene expression. The human apoAI, apoCIII, and apoAIV genes are tandemly organized within a 15-kb DNA segment and are expressed predominantly in the liver and intestine, Transient transfection of various human apoAI gene plasmid constructs into human hepatoma (HepG2) and colon carcinoma (Caco-2) cells showed that apoAI gene transcription is under the control of two separate and distinct cell-specific promoters. The region between nucleotides -192 acid -41 is essential for expression in HepG2 cells, whereas the region from -595 to -192 is essential for expression in Caco-2 cells, A third 0.6, kb DNA fragment in the apoCIII gene promoter region, similar to 5 kb downstream from the human apoAI gene, enhances transcription mediated by either of these two tissue-specific apoAI promoters, In Caco-2 cells, expression of the apoAI gene and activation by the distal enhancer required the presence of a nuclear hormone receptor response element (NHRRE) located in the -214 to -192 apoAI promoter region. Overexpression of the orphan receptor hepatocyte nuclear factor 4 (HNF-4), which binds to the NHRRE, dramatically stimulates apoAl gene expression in Caco-2 cells but not in HepG2 cells, Maximal stimulation of transcription by HNF-4 in Caco-2 cells required the presence of both the intestinal specific promoter, the NHRRE, and distal enhancer elements, Transactivation by HNF-4 thus appears to result from functional synergy between the NHRRE binding HNF-4 and distal DNA elements containing intestinal-specific DNA binding activities, The apoAI gene provides a model system to define the mechanism(s) governing intestinal cell specific gene regulation and the role of nuclear hormone receptors in the establishment and regulation of enterocytic gene transcription.