Regulation of human growth hormone receptor gene transcription by human growth hormone binding protein

The hypothesis that growth hormone binding protein (GHBP) has an effect on its own on the regulation of the GH-receptor/GHBP transcription was tested. Three different forms of human GHBP (recombinant non-glycosylated GHBP, recombinant glycosylated GHBP and GHBP purified and extracted from serum) were added in different concentrations determined by LIFA [0 pmol/l; 50 pmol/l (low level), 200 pmol/l (average level) and 500 pmol/l (high level in circulation)] to a human hepatoma cell line (HuH7 cells) cultured in a serum free hormonally-defined medium. Following the incubation with GHBP for 0, 1 and 2 h, GH-receptor expression was quantitatively assessed by using polymerase chain reaction amplification. Treatment with a GHBP concentration of 50 pmol/l resulted in a significant increase of GH-receptor mRNA molecules given as number of molecules x 10(6)/microg total RNA. In contrast, the concentration of 500 pmol/l presented a significant decrease of GH-receptor mRNA molecules, whereas 200 pmol/l GHBP produced a GH-receptor gene expression which was in between the values of the experiments with 50 and 500 pmol/l of GHBP added. Furthermore, the three different forms of human GHBP used provided similar data and, therefore, did not effect in any variation of GH-receptor expression. In addition, nuclear run-on experiments confirmed the changes in GH-receptor expression; and cycloheximide (10 microg/ml) did not alter the transcription indicating that the up and down regulating effects of GHBP on the GH-receptor/GHBP gene transcription was dependent, at least partly, on pre-existing factors and does not require protein synthesis. In conclusion, we present data showing that GHBP on its own has an effect on GH-receptor gene expression.