Cellular Activities of 20K- and 22K-hGH Do Not Necessarily Correlate with Their Binding Affinities for Rat GH Receptor

Abstract

Even though 20K human growth hormone (20K-hGH) has 3–10% binding affinity for the rat liver and adipose tissue microsomes as compared to 22K-hGH, it was also reported that 20K-hGH has the same potency as 22K-hGH in the hypophysectomized rat weight gain assay. In order to investigate the reason why such controversial data exist, we have studied 20K- and 22K-hGH using the rat GH receptor extracellular domain (rGHR-ECD) and full-length rGHR. When we examined the complex formation of rGHR-ECD with 20K- and 22K-hGH in gel filtration assay, 20K-hGH formed no complex while 22K-hGH formed a 1:1 complex. Next, rGHR cDNA was introduced into Ba/F3 cells and CHO-K1 cells, and stable transfectants (Ba/F3-rGHR and CHO-rGHR) were established. In the proliferation of Ba/F3-rGHR cells, 20K-hGH had 10-fold lower activity than 22K-hGH, which is consistent with their affinities for rGHR. But surprisingly, in the Spi2.1 gene promoter activation in CHO-rGHR cells, 20K- and 22K-hGH had the same activity, which was found not only in stable CHO-rGHR clones but also in CHO-K1 cells transiently expressing rGHR. In conclusion, these results indicate that cellular activities of 20K- and 22K-hGH do not necessarily correlate with their binding affinities for rGHR.

Circulating non-22 kDa growth hormone isoforms after a repeated GHRH stimulus in normal subjects

“…The 20 kDa GH is a naturally occurring isoform lacking residues 32-46 of 22 kDa GH, a region involved in the interface with GH receptor [20,21]. Contradictory results obtained in vivo and in vitro, regarding the potency and the binding affinity of 20 kDa GH to GH receptor have been published [22][23][24]. However, previous studies have shown that although this isoform might have a lower affinity for GH receptor it maintains full agonistic activity [23,24].…”

“…Contradictory results obtained in vivo and in vitro, regarding the potency and the binding affinity of 20 kDa GH to GH receptor have been published [22][23][24]. However, previous studies have shown that although this isoform might have a lower affinity for GH receptor it maintains full agonistic activity [23,24]. On the other hand, 17 kDa and other GH fragments have very low affinity to the GHR, and it has been suggested that they might have distinct biological properties perhaps acting through specific receptors [25].…”

Growth hormone isoforms

“…The 20K-GH form has lower affinity than 22K-GH for the both the human and the rat GHR [47][48][49][50][51]. Similarly, 20K-GH has low affinity for the human GH binding protein (GHBP) [52][53][54][55].…”

Repetitive Stimulation of the Pituitary with Growth-Hormone-Releasing Hormone Alters the Proportion of 22 and 20 Kilodalton Human-Growth Hormone Released

“…It may bind less tightly to the extracellular domain of the GH receptor, but appears to have the same efficacy at the full length receptor [6]. The physiological importance of the different isoforms of GH in humans remains unclear as sensitive and specific immunoassays for 20 K-hGH have only been recently developed [7].…”