Growth hormone receptor synthesis and release in tumorous somatolactotrophs

The growth hormone (GH) receptor (GHR) gene is expressed in pituitary somatotrophs and lactotrophs, in which GHR/GH-binding protein (GHBP) immunoreactivity is primarily located in secretory granules. The possibility that the GHR gene may be similarly expressed in GH(3) cells was therefore investigated, since these tumorous pituitary cells are used as models of GH- and prolactin-secreting cells but lack secretory granules. GHR/GHBP gene transcripts were detected in GH(3) cells and were homologous to hepatic GHR/GHBP transcripts. Immunoreactive GHRs were also detected by Western Blotting and GHBP/GHR immunoreactivity was localized by immunogold electron microscopy within each intracellular compartment. Although the immunoreactive GHR/GHBP content of these cells was less than that in rat tissues, GHBP release from GH(3) cells into incubation media (relative to tissue content) was much greater than GHBP release from rat tissue explants. These results demonstrate GHR expression in GH(3) cells, comparable with that in normal pituitary cells. These tumorous somatolactotrophs could thus provide a model for studies on GHR/GHBP synthesis and release and for studying GH effects on pituitary function. The release of GHBP from pituitary cells also indicates, for the first time, an extra-hepatic source of the plasma GHBP.