A large number of animal models with GH deficiency have become available for study during the past decade and have provided invaluable resources for the investigation of the growth hormone (GH) secretory process and its regulation. The models can be divided into two groups: naturally occurring (genetic) and experimentally generated (transgenic or knockout). Their importance is underscored by the fact that for many, human counterparts have been identified that are associated with clinical disorders of impaired GH secretion. The GH secretory process is a complex mechanism. It is triggered by an increase in cytosolic Ca, resulting in the fusion of the plasma membrane with that of the GH secretory granule and exocytosis of GH into the extracellular space. Two separate pathways are currently recognized as being capable of producing this change: one stimulated by GH-releasing hormone (GHRH) and the other by a yet identified ligand for which synthetic analogs, collectively known as GH secretagogues, exist. GHRH signal transduction is initiated by its binding to a G-protein-coupled, seven transmembrane-spanning receptor. Receptor activation leads to dissociation of the heterotrimeric Ga subunit from its py subunits, stimulation of adenylyl cyclase, generation of cyclic AMP, and phosphorylation and dissociation of the catalytic subunit of protein kinase A (PKA).
Activated PKA initiates the phosphorylation of plasma membrane monovalent ion channels that results in membrane depolarization and entry of extracellular Ca into the cytoplasm. The rise in intracellular Ca culminates in the extrusion of GH-containing secretory granules. GH secretagogues bind to a separate G-protein-coupled receptor only recently identified. This receptor is linked through the heterotrimeric Gj protein to phospholipase C, resulting in phosphoinositol hydrolysis and stimulation of protein kinase C. Activation of this system leads to liberation of intracellular Ca stores and a rise in intracellular free Ca, the point at which the two signaUng pathways converge. Regulation of the two receptors and post-receptor mechanisms has been a subject of great interest. The advantage of genetic and transgenic models, as an adjunct to the use of normal animals, has been the ability to observe consequences of perturbations of individual components of the hypothalamic-pituitary GH axis on GH secretion and the somatotropic signaling mechanisms.
The lit/lit mouse was first described by Beamer and Eicher. This severely growth-retarded animal carries a recessive mutation, with heterozygotes exhibiting a normal phenotype. Pituitary GH content and mRNA levels are markedly decreased (to 5-10% of normal) and the animal is fully responsive to GH. Unstimulated release of GH by primary monolayer cultures of dispersed lit/lit pituitaries was markedly decreased, as compared to somatotropes from normal mice, though when expressed as a percentage of cellular GH content, was about twice normal. Stimulation with GHRH was completely ineffective in increasing GH release or intracellular GH content. Similar findings were observed in vivo. Measurement of intracellular cyclic AMP after GHRH stimulation also failed to demonstrate any increase, as compared to a 20-fold increase observed in normal somatotropes. However, probes of the signal transduction system with sites of action distal to the GHRH receptor, including cholera toxin (which stimulates Gsa), forskolin (which directly stimulates adenylyl cyclase) and cyclic AMP were fully active in lit/lit pituitaries. These results led to the prediction of a mutation in the GHRH receptor. A missense mutation was described several years later, resulting in an Asp->Gly change in the extracellular portion of the receptor that completely abolishes ligand binding. Exposure of primary cultures of lit/lit somatotropes to GHRP-6, the prototype of GH secretagogues, failed to increase GH secretion and injection of GHRP-6 into anesthetized lit/lit mice did not stimulate GH release in vivo. Whereas these findings were initially difficult to explain, more recent data has suggested that maintenance of somatotrope responsiveness to GH secretagogues requires the presence of an intact GHRHGHRH receptor signaHng system for some, yet undefined, function. The dw/dw rat was initially described by Charlton and has some similarities to the lit/lit mouse. It carries an autosomal recessive mutation, exhibits moderate growth retardation, though less severe than in the lit/lit mouse, has decreased pituitary GH content, and is fully responsive to GH. Unstimulated GH release from primary pituitary monolayer cultures is reduced but, when expressed as a percentage of cell content, is twice that from normals.
The dw/dw rat differs from the lit/lit mouse in that GHRH administration in vivo does increase circulating GH levels. The GH secretory response to GHRH in vitro, however, is only 75% of that in normals, even when expressed as a percentage of cell content of the hormone. Cyclic AMP generation in response to GHRH is markedly impaired, increasing only about 50% over basal levels, in contrast to the 50-100 fold seen in normal rat somatotropes. Because of the considerable difference in sensitivity of the GHRH-induced cyclic AMP response to that of GH (the EC50 of GH is about 10 fold less), only a small increase in cyclic AMP appears necessary for a near maximal GH response. Thus, the limited capability of the dw/dw somatotropes to increase cyclic AMP is sufficient to permit a partial GH secretory response. The GHRH signal transduction system of the dw/dw rat was explored using probes with direct effects on Gsa, adenylyl cyclase, protein kinase A and protein kinase C Stimulation by cyclic AMP produced a normal GH response and exposure to forskolin resulted in normal cyclic AMP and GH responses, indicating that adenylyl cyclase activity and the downstream pathways were intact. However, direct stimulation of Ga by cholera toxin and by PGEi resulted in markedly reduced cyclic AMP and GH responses, implying impairment in Gga function. Attempts to define this defect included sequencing of the GHRH receptor and Gga cDNA, assessment of Gsa protein levels, and measurement of adenosinediphosphate ribosylation, a measure of Ga function. However, each of these signaUng components was normal.
Thus, the secretory defect in the dw/dw rat remains undefined. Among the possibilities are a defect in the GsPy subunit that impairs its dissociation from the a subunit, a mutation in a pituitary (somatotrope) specific protein corresponding to the p-adrenergic receptor kinase, or a mutation in adenylyl cyclase that affects activation by Gsa but not by forskolin. In addition to their GH secretory impairment, dw/dw pituitaries contain proportionally fewer somatotropes (5% of total cells) than do normal pituitaries, underscoring the importance of GHRH in somatotrope proUferation. Stimulation of primary cultures of dw/dw pituitary cells by GHRP-6 revealed no change in the EC50 but a 33% reduction in the maximal GH secretory response, when expressed as a percentage of cell hormone content. The comparable reduction in secretory responses to GHRH and GHRP-6 argue that the two responses are linked and that normal function of the GHRH receptor is required for an intact response to GHRP (and presumably other GH secret agogues).
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