Alternatively spliced forms in the cytoplasmic domain of the human growth hormone (GH) receptor regulate its ability to generate a soluble GH-binding protein.

Abstract

Alternatively spliced forms in the cytoplasmic domain of the human growth hormone (GH) receptor regulate its ability to generate a soluble GH-binding protein ( ABSTRACTThe mechanism underlying the generation of soluble growth hormone binding protein (GHBP) probably differs among species. In rats and mice, it involves an alternatively spliced mRNA, whereas in rabbits, it involves limited proteolysis of the membrane-bound growth hormone receptor (GHR). In humans, this latter mechanism is favored, as no transcript coding for a soluble GHR has been detected so far. To test this hypothesis, we analyzed COS-7 cells transiently expressing the full-length human (h) GHR and observed specific GH-binding activity in the cell supernatants. Concomitantly, an alternatively spliced form in the cytoplasmic domain of GHR, hGHR-tr, was isolated from several human tissues. hGHR-tr is identical in sequence to hGHR, except for a 26-bp deletion leading to a stop codon at position 280, thereby truncating 97.5% of the intracellular domain of the receptor protein. When compared with hGHR, hGHR-tr showed a significantly increased capacity to generate a soluble GHBP. Interestingly, this alternative transcript is also expressed in liver from rabbits, mice, and rats, suggesting that, in these four species, proteolysis of the corresponding truncated transmembrane GHR is a common mechanism leading to GHBP generation. These findings support the hypothesis that GHBP may at least partly result from alternative splicing of the region encoding the intracellular domain and that the absence of a cytoplasmic domain may be involved in increased release of GHBP.

Metalloprotease-mediated GH Receptor Proteolysis and GHBP Shedding

“…That is, despite infectious overexpression: 1) rbGHR del 297-406 and rbGHR 1-274-Myc-His underwent basal and PMA-induced cleavage and GHBP shedding with kinetics similar to what we previously showed; 2) PMA-induced proteolysis and GHBP shedding was inhibited by IC3; and 3) GH antagonized PMA's ability to cause receptor cleavage. In addition, rbGHR 1-274-Myc-His is analogous to a naturally occurring alternatively spliced GHR form that is known to effectively yield proteolytically shed GHBP in other systems (31,32).…”

“…It is known that a naturally occurring GHR splice variant lacking nearly the entire cytoplasmic domain generates substantial amounts of proteolytically shed GHBP (31,32). For remnant purification, we designed and adenovirally expressed a rbGHR mutant, rbGHR 1-274-Myc-His , that encodes the first 274 residues of the receptor followed by C-terminal Myc and His tags.…”

Impact of gender and androgen status on IGF-I levels in normal and GH-deficient adults

“…Recent studies have, however, suggested that the circulating binding protein is also cleaved from a truncated receptor lacking 97.5% of the intracellular part, which is a result of an alternatively spliced mRNA (27,28). High levels of GHBP have been found in conditions with low GH levels, including simple obesity (7,29) and vice versa in, for example, acromegaly and insulin-dependent diabetes mellitus (7,(30)(31)(32)(33).…”

Species-specific Alternative Splice Mimicry at the Growth Hormone Receptor Locus Revealed by the Lineage of Retroelements during Primate Evolution

“…24, and data not shown); this latter finding therefore further supports our molecular model inferring a key role of these sequences of retroviral origin in the genesis of GHRd3 transcripts. Such a species-specific GHR structural difference, which underlies the existence of the sole GHRfl allele in several species, relates to the evolutionary differences of the mechanisms by which the GHR gene gives rise to a soluble growth hormone-binding protein (29,30), thereby making this gene a particularly interesting model to investigate its encoded isoforms through evolution.…”