Abstract
GHR-P63 ('growth hormone-regulated protein of 63,000 daltons') is an acidic glycoprotein secreted by rat hepatocytes whose synthesis is abolished upon hypophysectomy. The sequence of its mRNA including the entire coding and 3' untranslated regions was determined from a nearly full-length lambda gt11-cDNA clone. The sequence contained two ATGs in frame giving rise to two overlapping coding regions which could encode precursor polypeptides of 416 and 406 amino acid residues (MrS = 46549 and 45371). These potential translation initiation codons appeared to be functional both in vitro and in intact cells since two precursors of the correct size were immunoprecipitated as products of mRNA translation. The unglycosylated precursors were converted into intermediate intracellular forms of about 56,000 daltons containing N-linked oligosaccharide side chains and thereafter into the secretory form of approximately equal to 63,000 daltons. Strong sequence homologies, both at the nucleotide and the amino acid levels were found between GHR-P63 and several serum protease inhibitors, more particularly mouse contrapsin and human alpha 1-antichymotrypsin. In agreement with sequence data, GHR-P63 purified from rat blood by affinity chromatography was found to carry an anti-trypsin activity. GHR-P63 mRNA, virtually undetectable in hepatocytes from hypophysectomized rats, could be hormonally re-induced to subnormal levels both in vivo by treating animals with hormones and in vitro by incubating the defective cells with hormones. Growth hormone was absolutely required but had a weak effect when used alone. Glucocorticoids which had no effect per se, strongly potentiated the action of growth hormone. Nuclear run-off experiments suggest that hormones regulated GHR-P63 mRNA levels by acting mostly, if not exclusively, on gene transcription.