Growth hormone releasing hormone. Review

Growth hormone-releasing hormone was isolated 1982 from human pancreatic tumours. They were found to consist of three peptides (GHRH1-44, GHRH1-40, GHRH1-37) which in vivo and in vitro were specific stimulators of pituitary growth hormone secretion. These tumor-derived GHRHs were demonstrated to be identical to human hypothalamic GHRHs. Extrahypothalamic GHRH is present in some brain regions and in the gastrointestinal tract. Circulating GHRH is detectable in human plasma, but little is known about its function. Above all binding of GHRH to a specific receptor stimulates growth hormone secretion through formation of cyclic AMP. GHRH secretion is modulated by somatostatin, the somatomedins and growth hormone itself. Following single injection of GHRH1-44 i.v. the equilibration half-time is 1.0 +/- 0.2 min and elimination half-time is 6.8 +/- 1.2 min. Maximal growth hormone response is achieved after injection of 1 microgram/kg GHRH. Using higher GHRH-doses growth hormone can be stimulated via subcutaneous or intranasal application. A single i.v. GHRH-test is not sufficient to prove a pituitary defect since growth hormone can be stimulated following repetitive injections in some cases. About 50% of patients with growth hormone deficiency have a hypothalamic defect of GHRH release. In some of these patients GHRH s.c. can promote linear growth to the same degree as growth hormone treatment.     

Growth hormone therapy

Growth hormone (GH) therapy has revolutionized treatment of children with growth hormone deficiency (GHD). Improved height outcome with final height in the target height range has been achieved in these children. Identification of Creutzfeldt-Jakob disease, a deadly prion mediated disorder, in recipients of pituitary GH accelerated the transition from pituitary derived GH to recombinant GH. Once daily subcutaneous administration of the freeze-dried preparation at evening is the recommended mode of GH therapy. Studies have led to use of higher dose of GH for improving height outcome (0.33 mg/kg/week or 0.14 IU/kg/day) albeit at a significantly high cost. Growth velocity increases from 3–4 cm/year before therapy to 10–12 cm/year during the first two years of therapy and is maintained at 7–8 cm/year after a period of two years. Close follow-up with regular clinical and laboratory monitoring is essential for achieving a desirable height outcome. A theoretical unlimited supply has led to wide spread use of GH in a variety of disorders other than GHD. Initially started in children with Turner syndrome, GH has now been used in chronic renal failure, idiopathic short stature and intrauterine growth restriction besides a wide array of newly emerging indications.     

Growth hormone therapy in hypochondroplasia

Patients with hypochondroplasia present with variable phenotypes. Children with severe short stature and disproportion of the body segments usually have the mutation Asn540Lys. They respond to growth hormone (GH) therapy with an increase in spinal length and, coupled with a surgical leg-lengthening procedure, it is possible for some patients to achieve adult heights within the normal range. Some children who present with proportionate short stature and hypochondroplasia fail to increase their growth rate at puberty, although the growth spurt can be restored by GH therapy. Others, with an identical presentation, seem to grow normally during puberty. At present, there is no way of predicting who will undergo a normal pubertal growth spurt. We therefore monitor all patients during childhood and give GH treatment only to those patients who fail to develop a growth spurt at puberty. Severe cases may occasionally need treatment before puberty if their growth velocity is compromised, but these will probably also be candidates for a surgical leg-lengthening procedure.     

Growth hormone and hypophosphatemic rickets

This review summarizes seven trials of growth hormone (GH) treatment for X-linked hypophosphatemic rickets (XLHR). These trials range in size from 5 to 30 patients; but despite the limited number of patients enrolled, they represent the largest studies to date of growth hormone in this disorder. Conventional treatment in XLHR, oral phosphate and calcitriol, is often unable to normalize serum phosphate concentration fully and many patients fail to reach normal adult height. The studies reviewed report increased growth velocity when exogenous GH is added to conventional therapy, although the independent effect of GH is difficult to evaluate. Younger patients appear to respond better to GH than do older patients. Disproportionate growth of the trunk may be a problem. Some patients with XLHR have received GH for more than 6 years, yet little is known about the impact of GH on adult height. Reported increases in phosphate concentrations following GH in XLHR are of uncertain clinical benefit. While GH appears to be safe in XLHR, long-term benefits remain unclear.     

Growth hormone therapy in hypochondroplasia

Ramaswami U, Hindmarsh PC, Brook CGD. Growth hormone therapy in hypochondroplasia. Acta Pædiatr 1999; Suppl 428: 116–17. Stockholm. ISSN 0803–5326
Patients with hypochondroplasia present with variable phenotypes. Children with severe short stature and disproportion of the body segments usually have the mutation Asn540Lys. They respond to growth hormone (GH) therapy with an increase in spinal length and, coupled with a surgical leg-lengthening procedure, it is possible for some patients to achieve adult heights within the normal range. Some children who present with proportionate short stature and hypochondroplasia fail to increase their growth rate at puberty, although the growth spurt can be restored by GH therapy. Others, with an identical presentation, seem to grow normally during puberty. At present, there is no way of predicting who will undergo a normal pubertal growth spurt. We therefore monitor all patients during childhood and give GH treatment only to those patients who fail to develop a growth spurt at puberty. Severe cases may occasionally need treatment before puberty if their growth velocity is compromised, but these will probably also be candidates for a surgical leg-lengthening procedure. □ Growth, growth hormone therapy, hypochondroplasia, puberty, short stature     

Growth hormone in turner syndrome

We assessed the effect of one year of therapy with recombinant Human Growth Hormone (rhGH) on growth velocity of 16 Indian girls with Turner Syndrome (TS) in a prospective, open trial. Patients received rhGH in a dose of 1 IU (0.3 mg)/kg/week. The mean pretreatment height was 117.1 cms (Z score minus 3.4), height velocity was 3.8 cm per year (Z score minus 2.4), and predicted height was 140 cm. At the end of therapy mean height was 123.9 (Z score minus 3.1), height velocity was 6.7 cm per year (Z score + 1.7), and the predicted height was 142.4 cm. The increment in height velocity with growth hormone therapy was statistically significant (P value = 0.001) and the mean increment in predicted height was 2.4 cm. Our study shows that girls with TS in India benefit from therapy with rhGH.     

Growth hormone binding protein 2001

The present state of knowledge about growth hormone binding proteins (GHBP) is reviewed, with particular emphasis on the high affinity GHBP, which represents the circulating ectodomain of the growth hormone receptor (GHR). GHBP is conserved through vertebrate evolution, is produced in many tissues (especially liver) by either alternative GHR mRNA splicing (rodents) or by proteolytic cleavage from the GHR (humans, rabbits and several other species). The metalloprotease TACE (tumor necrosis factor-alpha converting enzyme) is the likely enzyme responsible for cleavage, but the structural requirements for TACE recognition or catalysis, and hence the precise cleavage point in the GHR, are unknown. GHBP is widely distributed in biological fluids, with marked concentration differences amongst them. GHBP binds about half of the circulating GH under basal conditions but is easily saturated at high GH levels; it subserves complex functions, including a circulating buffer/reservoir function for GH, prolongation of plasma GH half-life, competition with GHRs for GH, and probably unproductive heterodimer formation with the GHR. The net effect of these partly enhancing and partly inhibitory functions on GH action in vivo is complex and difficult to ascertain. Serum GHBP levels roughly parallel GHR expression (particularly in liver) through the life span, with very low levels in fetal life, upregulation to adult levels during childhood, and decline in senescence. Rodent pregnancy is associated with a massive increase in GHBP expression. Although the regulation of GHBP expression/production is not necessarily tightly linked to GHR expression, in general, low GHBP levels occur in conditions associated with GH resistance (e.g., malnutrition, uncontrolled diabetes, catabolic states, renal failure, hypothyroidism). Conversely, obesity, a condition with enhanced GH responsivity, is associated with elevated GHBP levels. This suggests that in many (but not all) instances of abnormal GH action, the GHBP level reflects GHR status. Laron syndrome (genetic GHR deficiency/dysfunction due to mutations in the GHR gene) is associated with low or undetectable GHBP in 75-80% of patients; GHBP measurement can therefore be used diagnostically. Depending on the design of assays for serum GH, endogenous GHBP may interfere to varying degrees, and GH assays should be individually validated and optimized in this regard. The ultimate biological role and physiological significance of the GHBP remain to be established.