Tissue Distribution, Turnover, and Glycosylation of the Long and Short Growth Hormone Receptor Isoforms in Rat Tissues1

• Published in: Endocrinology (Philadelphia) Vol. 139; no. 6; pp. 2824 - 2830
• Main Authors: Frick, G. Peter, Tai, Li-Ruey, Baumbach, William R, Goodman, H. Maurice
• Format: Journal Article
• Language: English
• Published: Endocrine Society 01-06-1998
• ISSN: 0013-7227; 1945-7170
• DOI: 10.1210/endo.139.6.6047

Two isoforms of the GH receptor, the full-length receptor
(GHRL) and a short isoform (GHRS) that lacks
the transmembrane and intracellular domains of GHRL, have
been analyzed in rat tissue extracts by Western blotting and
immunoprecipitation. Although quantitative estimates of
GHRS and GHRL based on coprecipitation of[
125I]GH indicated similar amounts of both isoforms in
tissue extracts, the 110 kDa band corresponding to GHRL was
generally not detected on Western blots without enrichment by
immunoprecipitation. Two bands with electrophoretic mobilities
corresponding to 38 and 42 kDa were present in extracts prepared from
liver, muscle, and adipocytes. Western blots of the GH binding protein
in rat serum also revealed two bands, but these had electrophoretic
mobilities corresponding to 44 and 52 kDa. After digestion by
endoglycosidase F, a single band with an electrophoretic mobility
corresponding to 31 kDa was detected in samples from adipocytes, liver
or serum, indicating that GHRS retained in tissues is
glycosylated less extensively than that in rat serum. Digestion with
neuraminidase indicated that the smaller glycoproteins in tissue
extracts lack sialic acid residues that are present in serum samples.
Furthermore, endoglycosidase H degraded GHRS in liver
extracts to a 31 kDa band but did not degrade serum samples, suggesting
that tissues retain a high mannose form of GHRS. The
abundance of GHRS or GHRL in tissues from male,
virgin female, and pregnant rats was estimated from the amount of
125I-GH that was bound to each isoform after
immunoprecipitation. Liver contained more than 10 times as much
GHRS per gram of tissue as fat or muscle. In liver, muscle,
and fat, the amount of GHRS exceeded that of
GHRL, sometimes by as much as 6-fold. GHBP levels in serum
of females exceeded those in males, and rose even higher in pregnant
females. The abundance of GHRS in all tissue extracts
paralleled serum levels. In muscle and fat, the levels of
GHRL did not differ in male, female and pregnant rats,
whereas in liver, the pattern was similar to the GHRS
pattern. In all tissues, pools of GHRS exceeded those of
GHRL by a factor that grew larger as tissue and serum
levels increased.
The half life of GHBP in serum was estimated to be 2.4 h in rats
treated with cycloheximide, whereas that of GHRS was 20 min
in liver and 8.5 h in fat. These results suggest that
GHRS is synthesized in liver 8 times faster than it is
released into serum, whereas synthesis in fat is less than 30% of the
rate at which it is released into serum by all tissues. Therefore,
liver appears to be the major source of GHBP in serum. Although
secretion into the circulatory system accounts for little or perhaps
none of its turnover in some tissues, GHRS pools in tissues
do appear to be regulated, suggesting that GHRS may
function primarily in the cells in which it is synthesized.