Factors participating in the liver-specific expression of the human apolipoprotein A-II gene and their significance for transcription

We have shown previously that the hepatic and intestinal transcription of the human apolipoprotein (A-II apoA-II) gene in cell cultures is controlled by a complex set of regulatory elements A-N [Chambaz, et al. (1991) J. Biol. Chem. 266, 11676-11685; Cardot, et al. (1991) J. Biol. Chem. 266,24460-24470]. In the present communication, we have assessed the functional importance of each of the regulatory elements. In addition, we have used DNA binding and competition assays and protein fractionation to identify the hepatic nuclear activities which are involved in the regulation of the human apoA-II gene. Such activities may be of general importance for the regulation of liver-specific genes. The DNA binding and competition analysis showed that the regulatory elements M, D, and F bind new activities which have not been identified in apolipoprotein or other liver-specific promoters. These activities have been designated AIIM1 and AIIM2 for element M, AIID1 and AIID2 for element D, and AIIF2 for element F. The activity AIIM2 is present in liver, but absent in CaCo-2 cells. A set of regulatory elements binds activities which resemble liver-enriched or ubiquitous factors previously shown to play important roles in the regulation of their target genes. Thus, element I binds to activities related to NF1, and elements L, C, D, G, AB, and F bind to C/EBP alpha as well as other heat-stable activities. The affinity of the bacterially expressed C/EBP alpha for the various apoA-II regulatory regions follows the order: AIIL approximately equal to AIIC > AIID > AIIF > AIIG > AIIAB. Protein fractionation showed that element J binds at least three hepatic nuclear activities and is also recognized by members of the nuclear receptor family, HNF4, EAR2, EAR3, and ARP1. Another liver-enriched factor, HNF1, was shown previously to bind to element H. Despite the importance of HNF1, HNF4, NF1, and C/EBP alpha in the regulation of numerous other target genes, deletion of the HNF1, NF1, and HNF4 and several C/EBP binding sites did not drastically affect the hepatic transcription of the apoA-II gene. Rather, the hepatic and intestinal transcription is affected severely by deletion of elements A, B, K, L, and N. In addition, the intestinal transcription is affected by deletion of elements C, J, and M. The in vivo physiological importance of these elements will require analysis of their function in transgenic animals. This analysis establishes the organization of several nuclear activities on the human apoA-II promoter. Our findings suggest that the expression of a liver-specific gene such as apoA-II is controlled by a combination of factors and that the relative importance of individual factors is determined by the promoter context. Maximum expression of the apoA-II gene requires the synergism of factors bound to the proximal and distal regulatory elements.