GROWTH-HORMONE STIMULATES TRANSCRIPTION OF THE GENE ENCODING THE ACID-LABILE SUBUNIT (ALS) OF THE CIRCULATING INSULIN-LIKE GROWTH FACTOR-BINDING PROTEIN COMPLEX AND ALS PROMOTER ACTIVITY IN RAT-LIVER

The growth-promoting activity of GH, the principal hormonal determinan t of body size, is mediated by insulin-like growth factor I (IGF-I). M ost of the IGF-I in plasma circulates in a 150-kDa complex that contai ns IGF-binding protein-3 (IGFBP-3) and an acid-labile subunit (ALS). T he 150-kDa complex serves as a reservoir of IGF-I and determines its b ioavailability to the tissues. Formation of the 150-kDa complex depend s upon the synthesis of ALS, which is synthesized primarily in liver a nd is regulated by GH. The present study demonstrates that GH stimulat es ALS gene transcription in rat liver and ALS promoter activity in a rat hepatoma cell line. ALS messenger RNA (mRNA) and ALS nuclear trans cripts were decreased to similar extents in the livers of GH-deficient hypophysectomized rats. GH increased hepatic ALS mRNA within 3-4 h to about 65% of the levels seen in sham-operated control rats. To confir m that GH stimulated ALS gene transcription, we transiently transfecte d an ALS promoter-luciferase reporter gene construct into H4-II-E rat hepatoma cells and primary rat hepatocytes. Recombinant human GH (hGH) stimulated promoter activity about 3-fold. In contrast, basal promote r activity was lower, and GH stimulation was absent when the ALS repor ter construct was transfected into ON-responsive 3T3-F442A mouse pread ipocyte fibroblasts. GH stimulation of ALS promoter activity in H4-II- E cells was mediated by functional GN receptors; nonprimate (rat and b ovine) OH gave identical stimulation to hGH, and stimulation by hGH oc curred at physiological concentrations. Reverse transcriptase-PCR anal ysis indicated that ON receptor mRNA was present in H4-II-E cells at a pproximately 40% of the level seen in rat liver. ON also induced the e xpression of the endogenous c-fos gene, indicating that the signaling pathway necessary for the activation of gene expression by ON was inta ct in H4-II-E cells. Thus, H4-II-E cells are a ON-responsive liver cel l line that should provide a useful system in which to study the molec ular mechanism of transcriptional regulation by ON of ALS and other he patic genes.