Growth hormone-releasing factor stimulates proliferation of somatotrophs in vitro.

Abstract

The mitogenic effect of the hypothalamic peptides growth hormone-releasing factor (GRF) and somatostatin on cultured growth hormone (GH)-producing cells (somatotrophs) was studied. Using autoradiographic detection of [3H]thymidine uptake and immunocytochemical identification of GH-producing cells, we show that 5 nM GRF causes a 20-fold increase in the percentage of somatotrophs labeled with [3H]thymidine. The total number of somatotrophs in GRFtreated cultures was increased by 60%. Somatostatin had no measurable effect on the labeling index by itself, but it partly inhibited the GRF-induced increase in both the labeling index and the total number of cells. Forskolin caused an increase in both the percentage of somatotrophs with a [3H]thymidinelabeled nucleus and the somatotroph number similar to that caused by GRF. GH secretion as well as cellular GH content in the GRF-or forskolin-treated cells increased with culture time over the entire period, whereas secretion and content of GH gradually decreased in control or somatostatin-treated cultures during the entire culture period. These data suggest that GRF and somatostatin regulate the mitotic activity of GH-producing cells and that the effect of GRF is possibly mediated by cyclic AMP.The biosynthesis and secretion of growth hormone (GH) in the anterior pituitary gland is under complex hormonal regulation. The hypothalamic peptide growth hormone-releasing factor (GRF) stimulates both release (1-3) and synthesis (4, 5) of GH, as well as transcription of the GH gene (6, 7), whereas somatostatin depresses GH secretion (8). Patients with GRF-producing tumors (9-12) and transgenic mice expressing large amounts of GRF (13) have chronically increased plasma GRF with resulting high levels of plasma GH, pituitary enlargement, increased somatic growth, and acromegaly. This suggests but does not establish that GRF stimulates the growth ofpituitary somatotrophs, the cells that secrete GH. Not only might the effects ofGRF in these in vivo studies be indirect or require the interaction of other mitogenic substances, but also other products of the GRFproducing cells might be involved. We therefore chose to explore the effect of long-term GRF administration on the growth and function of cultured rat somatotrophs. The results show that GRF stimulates the mitotic activity in somatotrophs as measured by cell counts in pituitary cultures treated with [3H]thymidine and immunohistochemically stained for GH. The specificity of this response was indicated by the observation that somatostatin attenuates the growth-promoting effect of GRF, whereas prepro-GRF-(75-104) had no measurable influence. MATERIALS AND METHODSCell Culture. Rat anterior pituitary cells from male Sprague-Dawley rats (Holtzmann, Madison, WI) were prepared as described (14) and seeded in two-well slides, coated with an extracellular matrix from bovine pituitary endothelial cells. Cells were cultured in 1.5 ml of minimal essential medium containing D-valine instead of L-valine (GIBCO) and supplemented with 5 nM ...

Changes in Endogenous Growth Hormone Secretion and Onset of Puberty in Transgenic Rats Expressing Human Growth Hormone Gene.

“…It is also possible that low rGH levels in our transgenic rats were due to inhibition of somatotoroph proliferation. GRF stimulates proliferation of somatotorophs, and this mitogenic effect is partially inhibited by somatostatin [29]. It will be worth examining the expression of SRIF and GRF in our transgenic rats to elucidate the precise mechanisms of endogenous rGH suppression.…”

Effect of Growth Hormone (GH)-Releasing Peptide (GHRP) on the Release of GH from Cultured Anterior Pituitary Cells in Cattle.

“…These two hypothalamic peptides are released from nerve endings in the median eminence and reach the anterior pituitary via the hypophysial-portal circulation [1][2][3]. Moreover, it has been reported that GHRH and SRIF regulate mitotic activities of GH-producing cells and that the effect of GHRH is possibly mediated by cyclic AMP [4]. A hexapeptide, His-D-Trp-AlaTrp-D-Phe-Lys-NH2…”

Discovery of growth hormone-releasing hormones and receptors in nonmammalian vertebrates

“…The primary function of GHRH is to stimulate GH synthesis and secretion from anterior pituitary somatotrophs via specific interaction with its receptor, GHRH-R (5). In addition, GHRH activates cell proliferation, cell differentiation, and growth of somatotrophs (6)(7)(8). There are many other reported activities of GHRH such as modulation of appetite and feeding behavior, regulation of sleeping (9,10), control of jejunal motility (11), and increase of leptin levels in modest obesity (12).…”