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 transducti on mechanisms involved in the control of intestinal specific gene expr ession. The human apoAI, apoCIII, and apoAIV genes are tandemly organi zed within a 15-kb DNA segment and are expressed predominantly in the liver and intestine, Transient transfection of various human apoAI gen e plasmid constructs into human hepatoma (HepG2) and colon carcinoma ( Caco-2) cells showed that apoAI gene transcription is under the contro l of two separate and distinct cell-specific promoters. The region bet ween nucleotides -192 acid -41 is essential for expression in HepG2 ce lls, 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 prom oter region, similar to 5 kb downstream from the human apoAI gene, enh ances transcription mediated by either of these two tissue-specific ap oAI promoters, In Caco-2 cells, expression of the apoAI gene and activ ation by the distal enhancer required the presence of a nuclear hormon e receptor response element (NHRRE) located in the -214 to -192 apoAI promoter region. Overexpression of the orphan receptor hepatocyte nucl ear factor 4 (HNF-4), which binds to the NHRRE, dramatically stimulate s apoAl gene expression in Caco-2 cells but not in HepG2 cells, Maxima l stimulation of transcription by HNF-4 in Caco-2 cells required the p resence of both the intestinal specific promoter, the NHRRE, and dista l enhancer elements, Transactivation by HNF-4 thus appears to result f rom functional synergy between the NHRRE binding HNF-4 and distal DNA elements containing intestinal-specific DNA binding activities, The ap oAI gene provides a model system to define the mechanism(s) governing intestinal cell specific gene regulation and the role of nuclear hormo ne receptors in the establishment and regulation of enterocytic gene t ranscription.