The transcriptional control of alpha(1)-proteinase inhibitor expression in mouse liver and macrophages

Abstract

The {dollar}\alpha\sb1{dollar}-protease inhibitor ({dollar}\alpha\sb1{dollar}-PI) proteins of mice are encoded by a group of genes whose members are expressed coordinately in a liver-abundant manner and regulated primarily at the transcriptional level. Homozygous deletion of the human analog, ({dollar}\alpha\sb1{dollar}-antitrypsin) leads to a severe form of early-onset emphysema in affected adults. In order to better understand the developmental and tissue-specific regulation of {dollar}\alpha\sb1{dollar}-PI, such that a possible mouse model of {dollar}\alpha\sb1{dollar}-antitrypsin deficiency could be established, I have undertaken a detailed study of {dollar}\alpha\sb1{dollar}-PI expression in liver and macrophages. Previous work in the human system has shown that the cis-acting transcriptional elements responsible for the liver-specific expression of {dollar}\alpha\sb1{dollar}-antitrypsin are located immediately upstream of the CAP site (within 523 bp). Sequence analysis demonstrated striking homology ({dollar}>{dollar}95%) between the human and mouse sequence up to {dollar}-{dollar}133, after which the sequences diverged markedly. Specific regions of these upstream sequences were identified as potential cis-acting transcriptional elements via a functional assay utilizing transfection of both 5{dollar}\sp\prime{dollar} and small internal deletion minigene constructs. Several different DNA-protein binding assays were used to demonstrate that each functional element identified interacts specifically with proteins found in adult mouse liver nuclei. The expression of {dollar}\alpha\sb1{dollar}-PI in liver was found to be controlled by one negative and at least six positive cis-acting elements, several of which bind proteins contiguously and may, in fact, interact with each other directly. Another major site of {dollar}\alpha\sb1{dollar}-antitrypsin expression in humans is in macrophages. It has been postulated that macrophages may function as the first line of defense in response to local lung tissue damage caused by inflammation. Thus, alveolar macrophages may be able to secrete {dollar}\alpha\sb1{dollar}-antitrypsin as an immediate response to the release of elastase by polymorphonuclear lymphocytes. I have shown via Northern analysis and PCR amplification that although all five {dollar}\alpha\sb1{dollar}-PI genes are expressed in murine macrophages, the overall level of expression is approximately 100,000- to 150,000-fold less than in liver. In addition, the macrophage transcripts utilize the liver-specific promoter exclusively--unlike the human system where {dollar}\alpha\sb1{dollar}-antitrypsin has a macrophage-specific promoter 2500 bp upstream of the liver message CAP site.