Subcellular distribution of the long and short isoforms of the growth hormone (GH) receptor in rat adipocytes: both isoforms participate in specific binding of GH.

In rodents the GH receptor gene encodes long and short isoforms of the receptor that arise from alternate splicing of the mRNA transcript. To determine whether the long and short isoforms reside on the plasma membrane, we compared their sensitivity to mild digestion of adipocytes with trypsin. Immunofunctional assays were used to measure each isoform in adipocyte extracts. The GH-binding capacity of the short isoform in cell extracts was more than 5 times that of the long. Digestion of freshly isolated adipocytes with trypsin decreased the specific binding of [125I]human GH by 77%. The GH-binding capacities of the long and short isoforms in cell extracts were diminished by 68% and 18%, respectively. The relative insensitivity of the short isoform to digestion with trypsin suggests that much of it may be inaccessible to trypsin because it is located within the cell. When cells were allowed to recover for 2 h after digestion with trypsin, GH binding was restored. Analysis of cell extracts indicated net synthesis of the long isoform and further loss of the short. Adipocytes lost more than half of the short isoform during a 2-h incubation in vitro regardless of whether the cells were first digested with trypsin. Loss of the short isoform during this period appeared to be limited to the intracellular component, as the amount of the trypsin-sensitive component remained constant. The abundance of RNA transcripts that encode the long and short isoforms of the GH receptor remained unchanged when adipocytes were incubated in vitro for 3 h in the presence or absence of actinomycin-D. Therefore, the acute changes seen in these experiments do not reflect regulation of GH receptor gene transcription. Adipocytes appear to maintain a fixed ratio of the long and short isoforms of the GH receptor on their surface. Each isoform accounts for approximately half of the cell's GH-binding capacity.