20 kDa human growth hormone (20K hGH) stimulates insulin-like growth factor-I (IGF-I) gene expression at lower concentrations than 22K hGH in hGH receptor-expressing Ba/F3 cells

Growth hormone (GH) secreted from the pituitary is essential for postnatal growth in animals. GH exerts its actions by a direct effect on target organ s and by stimulating insulin-like growth factor I (IGF-I) production. In th e human pituitary, there is a naturally occurring variant protein which has a molecular mass of 20 kDa (20K hGH) besides the major 22 kDa hGH (22K hGH ), but the physiological actions of 20K hGH are still poorly understood. In this study we have examined its effects on the IGF-I mRNA expression in th e pro B-cell line Ba/F3 cells stably expressing hGH receptor (Ba/F3-hGHR). Ba/F3-hGHR cells were incubated for 2 h with a series of various concentrat ions (10 pM similar to 10 nM) of 20K or 22K hGH. The IGF-I mRNA expression in the Ba/F3-hGHR cells was detected by the RT-PCR method. IGF-I gene expre ssion was increased by 20K and 22K hGH stimulation, but not by PRL or IL-3 in the Ba/F3-hGHR. And this effect was not observed in parental Ba/F3 cells . Lower concentrations of 20K hGH more strongly induced IGF-I gene expressi on than 22K-hGH. These results suggest that 20K and 22K hGH stimulate the I GF-I gene expression in the Ba/F3-hGHR through hGH receptors, and that the stronger effect of 20K hGH than that of 22K hGH in enhancing the IGF-I gene expression may be correlated with a 20K hGH specific receptor dimerization mechanism.