Regulation of human GH receptor gene transcription by 20 and 22 kDa GH in a human hepatoma cell line

The human GH gene is 1.7 kilobase pairs (kb) in length and is composed of f ive exons and four introns. This gene is expressed in the pituitary gland a nd encodes a 22 kDa protein. In addition to this predominant (75%) form, 5- 10% of pituitary GH is present as a 20 kDa protein that has an amino acid ( aa) sequence identical to the 22 kDa form except for a 15 aa internal delet ion of residues 32-46 as a result of an alternative splicing event. Because it has been reported that non-22-kDa GH isoforms might be partly responsib le for short stature and growth retardation in children, the aim of this st udy was to compare the impact of both 22 kDa and 20 kDa GH on GH receptor g ene (GPI receptor/GH binding protein (GHR/GHBP)) expression. Various concen trations of 20 kDa and 22 kDa GH (0, 2, 5, 12.5, 25, 50 and 150 ng/ml) were added to human hepatoma (HuH7) cells cultured in serum-free hormonally def ined medium for 0, 1 and 2 h. Thereafter GHR/GHBP mRNA expression was measu red by quantitative PCR. Addition of either 20 kDa or 22 kDa GH, at low or normal physiological concentrations (0, 2, 5, 12.5, 25 or 50 ng/ml) induced a dose-dependent increase in GHR/GHBP expression. However, a supraphysiolo gical concentration of 20 kDa GH (150 ng/ml) resulted in a significantly lo wer (P<0.05) downregulation of GHR/GHBP gene transcription compared with th e downregulation achieved by this concentration of 22 kDa GH. This differen ce might be explained by a decreased ability to form a 1 : 1 complex with G HR and/or GHBP, which normally occurs at high concentrations of GH. Nuclear run-on experiments and GHBP determinations confirmed the changes in GHR/GH BP mRNA levels. In conclusion, we report that both 20 kDa and 22 kDa GH, in low and normal physiological concentrations, have the same effect on regul ation of GHR/GHBP gene transcription in a human hepatoma cell Line. At a su praphysiological concentration of 150 ng/ml, however, 20 kDa GH has a less self-inhibitory effect than the 22 kDa form.