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.