Characterisation of the rat acid-labile subunit gene

The acid-labile subunit (ALS) of the ternary insulin-like growth factor binding protein (IGFBP) complex has a central role in regulating the bioavailability of circulating IGF. We have shown that gene expression of ALS in vivo and in vitro is regulated by a variety of factors, including growth hormone (GH). Our aim was to isolate and characterise the ALS gene as a step in defining the mechanism of its regulation. Southern analysis of rat genomic DNA suggests that the ALS gene exists as a single copy in the rat genome. In order to isolate this gene we screened 5×10⁵ clones and selected fragments of two genomic clones were sequenced. Comparison of this sequence with the cDNA identified two exons and a single ∼1.1 kb intron. Primer extension experiments suggest two major transcription initiation sites at −539 and −396 nts relative to the translational initiation codon, although there are no consensus TATA-boxes in this region. Analysis of 2.3 kb of 5′ flanking sequence identified two LF-A1 sites which may confer the liver-specific expression of the ALS gene. In addition, there are several elements that may be involved in regulation by growth hormone and cytokines.