Growth hormone receptor synthesis and release in tumorous somatolactotrophs

Abstract

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 GH₃ 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 GH₃ 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 GH₃ cells into incubation media (relative to tissue content) was much greater than GHBP release from rat tissue explants. These results demonstrate GHR expression in GH₃ 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.