THE APOLIPOPROTEIN A-I C-III/A-IV GENE-CLUSTER - APOC-III AND APOA-IVEXPRESSION IS REGULATED BY 2 COMMON ENHANCERS/

Genetic, epidemiological and clinical evidence have clearly demonstrat ed the importance of the human apolipoprotein (ape) A-I/C-III/A-IV gen e cluster in lipid metabolism and heart attack. The transcriptional re gulation of these genes determines the level of the encoded proteins a nd therefore influences the concentration of triglycerides and cholest erol. Here, we analyze the existence of transcription control elements in the 6.6 kb apoC-III/A-IV intergenic region and their influence on the expression of both genes. Two main positive common control element s were found to modulate apoC-III and apoA-IV expression in HepG2 and in Caco-2 cells: the previously described apoC-III enhancer, located 0 .8 kb upstream from the cap site of the gene, and a newly detected act ivating region located in the center of the intergenic sequence. The a ctivity of both elements is highly increased by the hepatic and intest inal transcription factor HNF-4. Analysis of a 641 bp fragment contain ing the central element showed that it has the properties of a tissue- specific enhancer. Liver nuclear proteins interact with seven DNA bind ing sites present in this enhancer and HNF-4 specifically interacts wi th one of these sites. A third positive element, situated immediately upstream from the apoA-IV minimal promoter, is also activated by HNF-4 ; however, this element is not involved in apoC-III expression. In add ition, two negative regions were identified, one located near the apoA -IV gene and the other one between the apoC-III enhancer and the newly identified central enhancer. In conclusion, negative and positive con trol elements are located in the apoC-III/A-IV intergenic region, incl uding two enhancers important for the expression of the two genes. The se results add new evidence that common regulatory elements for the ex pression of the apoA-I, apoC-III and apoA-IV genes are interspersed th roughout the cluster. (C) 1997 Elsevier Science B.V.