Molecular cloning of the porcine acid-labile subunit (ALS) of the insulin-like growth factor-binding protein complex and detection of ALS gene expression in hepatic and non-hepatic tissues

The acid-labile subunit (ALS) is an approximatey 85 kDa AT-glycoprotein tha t is known primarily as a component of the systemic insulin-like growth fac tor-binding protein (IGFBP) complex. We have amplified, using a PCR, three overlapping porcine ALS genomic DNA fragments that together encode the dist al region of the signal peptide through to the COOH-terminus. The compiled sequence of 1775 nucleotides of the three overlapping DNAs and the deduced amino acid sequence of the mature porcine ALS (pALS) protein exhibited 84/8 1%, 79/77%, 79/78% and 84/79% identities with respect to those of the human , the rat, the mouse and the baboon respectively. Four conserved cysteine r esidues in the NH2-terminal domain and 20 leucine-rich repeats in the centr al domain also were identified at identical positions in the porcine ALS. B y using Northern blot analysis, with a genomic DNA fragment as the probe, i t was determined that a 2.2 kb ALS mRNA was induced in the liver during the late fetal stage, and hepatic ALS mRNA abundance was increased post-natall y. Moreover, hepatic ALS mRNA abundance was increased by daily injection of porcine somatotropin (100 mug/kg body weight) in cross-bred market pigs ea ch weighing approximately 100 kg. The ALS mRNA was not detected by Northern analysis in any non-hepatic tissue examined. However, results of a more se nsitive solution hybridization/RNAse protection assay indicated that low le vels of ALS mRNA were also present in adult muscle, spleen, ovary and uteru s, but not in lung, kidney, oviduct and placenta. Taken together, the prese nt results suggest that although liver is the primary organ that expresses the ALS gene under somatotropin stimulation, some non-hepatic tissues also express the gene at low levels in the pig.