GHRH Receptor Gene Research Articles

Inhibitory effects of antagonistic analogs of GHRH on GH3 pituitary cells overexpressing the human GHRH receptor.

GH3 rat pituitary tumor cells produce GH and prolactin (PRL), but lack the GHRH receptor (GHRH-R). We expressed human GHRH-R (hGHRH-R) in GH3 cells using recombinant adenoviral vectors and studied the effects of GHRH antagonists. The mRNA expression of the GHRH-R gene in the cells was demonstrated by RT-PCR. An exposure of the GH3 cells infected with hGHRH-R to 10(-10), 10(-9) and 10(-8) m hGHRH for 1 or 2 h in culture caused a dose-dependent elevation of the intracellular cAMP concentration and the cAMP efflux. Exposure to hGHRH also elicited dose-dependent increases in GH and PRL secretion from these cells. Neither the uninfected nor the antisense hGHRH-R-infected control cells exhibited cAMP, GH and PRL responses to GHRH stimulation. GHRH antagonists JV-1-38 and jv-1-36 applied at 3x10(-8) m for 3 h, together with 10(-9) m GHRH, significantly inhibited the GHRH-stimulated cAMP efflux from the hGHRH-R-infected cells by 36 and 80% respectively. The more potent antagonist JV-1-36 also decreased the intracellular cAMP levels in these cells by 55%. Exposure to JV-1-36 for 1 h nullified the stimulatory effect of GHRH on GH secretion and significantly inhibited it by 64 and 77% after 2 and 3 h respectively. In a superfusion system, GHRH at 10(-10), 10(-9) and 10(-8) m concentrations induced prompt and dose-related high cAMP responses and smaller increases in the spontaneous GH secretion of the hGHRH-R-infected cells. Antagonists JV-1-36 and JV-1-38 applied at 3x10(-8) m for 15 min, together with 10(-9) m GHRH, inhibited the GHRH-stimulated cAMP response by 59 and 35% respectively. This work demonstrates that GHRH antagonists can effectively inhibit the actions of GHRH on the hGHRH-R. Our results support the view that this class of compounds would be active clinically.

Pituitary morphology in the GH deficiency for mutation in the growth hormone-releasing hormone receptor gene

Pituitary morphology in the GH deficiency for mutation in the growth hormone-releasing hormone receptor gene

Effects of growth hormone treatment on the pituitary expression of GHRH receptor mRNA in uremic rats

A decreased ability of pituitary cells to secrete growth hormone (GH) in response to growth hormone releasing hormone (GHRH) stimulation has been shown in young uremic rats. The aim of the current study was to examine the effect of uremia and GH treatment on pituitary GHRH receptor expression. Pituitary GHRH receptor mRNA levels were analyzed by RNase protection assay in young female rats made uremic by subtotal nephrectomy, either untreated (UREM) or treated with 10 IU/kg/day of GH (UREM-GH), and normal renal function animals fed ad libitum (SAL) or pair-fed with the UREM group (SPF). Rats were sacrificed 14 days after the second stage nephrectomy. Renal failure was confirmed by concentrations (X +/- SEM) of serum urea nitrogen (mmol/L) and creatinine (micromol/L) in UREM (20 +/- 1 and 89.4 +/- 4.5) and UREM-GH (16 +/- 1 and 91.4 +/- 6.9) that were much higher (P < 0.001) than those of sham animals (SAL, 3 +/- 0 and 26.5 +/- 2.2; SPF, 4 +/- 0 and 26.5 +/- 2.1). UREM rats became growth retarded as shown by a daily longitudinal tibia growth rate below (P < 0.05) that observed in SAL animals (156 +/- 3 vs. 220 +/- 5 microm/day). GH treatment resulted in significant growth rate acceleration (213 +/- 6 microm/day). GHRH receptor mRNA levels were no different among the SAL (0.43 +/- 0.03), SPF (0.43 +/- 0.08) and UREM (0.44 +/- 0.04) groups, whereas UREM-GH rats had significantly higher values (0.72 +/- 0.07). The status of pituitary GHRH receptor is not modified by nutritional deficit or by severe uremia causing growth retardation. By contrast, the growth promoting effect of GH administration is associated with stimulated GHRH receptor gene expression.

Lipid profiles in untreated severe congenital isolated growth hormone deficiency through the lifespan.

Growth hormone deficiency (GHD) is associated with adverse changes in lipid profile. However, changes in lipids through life in a homogeneous group of GHD subjects have not been defined. We examined lipid levels in a group of untreated severely GHD patients with a mutation in the GHRH receptor gene from a rural community in North-east Brazil. Lipid profiles in 15 GHD subjects [eight children and adolescents (one male), age (median [range]) 13.2 (5.4-19.9) years; seven adults (one male), age 47 (33-66) years] were compared with those in 29 indigenous controls from the same extended kindred [17 children and adolescents (six male), age 10.2 (5.3-18.4) years; 12 adults (eight male), age 54.5 (33-80) years]. All GHD subjects had a peak GH response of < 0.5 ng/ml in response to an insulin tolerance test and extremely reduced IGF-1 levels (median 5.5 ng/ml). Data were compared between cohorts and with an age- and sex-matched white American reference population. Abnormalities were confined to plasma total cholesterol (TC) and low-density lipoprotein cholesterol (LDL-C) levels. More GHD children had levels of plasma TC and LDL-C above the 95th percentile for our reference population (3/8 and 4/7, respectively) compared to controls (0/17 and 1/15, respectively) (P < 0.05). In the adults, median TC and LDL-C levels were higher in the GHD than controls (P < 0.05) (6.3 vs. 4.1 mmol/l; 4.4 vs. 2.7 mmol/l, respectively). Median Z-scores, calculated using values from the reference population, were not different between GHD children and adults for both TC (+0.8 vs.+0.4) and LDL-C (+1.4 vs.+0.7). The lipid profile in children as well as in adults with very severe GHD is adversely modified. There would appear to be no significant worsening of the lipid abnormality with duration of GHD or achievement of adulthood.

Familial Isolated Growth Hormone Deficiency Is Associated with Increased Systolic Blood Pressure, Central Obesity, and Dyslipidemia

To assess the metabolic and cardiovascular consequences of GH deficiency (GHD) on cardiovascular risk factors, we studied a homogeneous population with GHD due to a homozygous defect in the GHRH receptor gene. Anthropometric, metabolic, and cardiovascular measurements (at rest, during treadmill exercise, and during orthostatic stress) and echocardiographic data were obtained from 16 GH-naive, GH-deficient (GHD) adults and 31 age-, sex-, and body mass index-matched control (CO) subjects. The percentage of fat mass, waist to hip ratio, and total and low density lipoprotein cholesterol were higher in the GHD group. However, high density lipoprotein cholesterol, triglyceride, and fasting glucose levels were similar between groups, and fasting insulin and homeostasis model assessment of insulin resistance (HOMAIR) were lower in the GHD group. Systolic blood pressure (SBP) was higher in the GHD group, but no difference in diastolic blood pressure or heart rate (HR) existed. Blood pressure and HR responses to exercise did not differ between groups. During passive orthostatic stress the decrease in SBP was higher in the GHD than in the CO group, whereas an increase in diastolic blood pressure was not observed in the GHD group. Moreover, the increase in HR was blunted in the GHD compared with the CO group. Left ventricular mass and mass index were lower in the GHD group. In conclusion, this genetically homogeneous isolated GHD population presents a syndrome characterized by central obesity, dyslipidemia, and elevated SBP but reduced cardiac dimensions compared with controls.

Familial Isolated Growth Hormone Deficiency Is Associated with Increased Systolic Blood Pressure, Central Obesity, and Dyslipidemia

To assess the metabolic and cardiovascular consequences of GH deficiency (GHD) on cardiovascular risk factors, we studied a homogeneous population with GHD due to a homozygous defect in the GHRH receptor gene. Anthropometric, metabolic, and cardiovascular measurements (at rest, during treadmill exercise, and during orthostatic stress) and echocardiographic data were obtained from 16 GH-naive, GH-deficient (GHD) adults and 31 age-, sex-, and body mass index-matched control (CO) subjects. The percentage of fat mass, waist to hip ratio, and total and low density lipoprotein cholesterol were higher in the GHD group. However, high density lipoprotein cholesterol, triglyceride, and fasting glucose levels were similar between groups, and fasting insulin and homeostasis model assessment of insulin resistance (HOMA(IR)) were lower in the GHD group. Systolic blood pressure (SBP) was higher in the GHD group, but no difference in diastolic blood pressure or heart rate (HR) existed. Blood pressure and HR responses to exercise did not differ between groups. During passive orthostatic stress the decrease in SBP was higher in the GHD than in the CO group, whereas an increase in diastolic blood pressure was not observed in the GHD group. Moreover, the increase in HR was blunted in the GHD compared with the CO group. Left ventricular mass and mass index were lower in the GHD group. In conclusion, this genetically homogeneous isolated GHD population presents a syndrome characterized by central obesity, dyslipidemia, and elevated SBP but reduced cardiac dimensions compared with controls.

Is GHRH receptor essential to GHRP-2-induced GH secretion in primary cultured rat pituitary cells?

It is still controversial in rat whether the stimulation of GH secretion by GH-releasing peptides (GHRP) requires both GHRP receptor (GHRP-R) and GH-releasing hormone receptor (GHRH-R). To clarify this issue, we have postulated that inhibition of GHS-R or GHRH-R gene transcription should block GHRP-2-induced GH secretion. Rat pituitary cells were incubated for 3 days in the presence or absence of antisense 18-mer phosphorothiate oligonucleotides (ONs) complementary to the codon region of GHS-R or GHRH-R mRNAs. A significant decrease in GHRH-R and GHS-R mRNA levels was found in corresponding antisense-treated cells compared with the control cells treated with sense ON. Treatment with antisense GHS-R ON reduced (but not abolished) GHRP-2-induced GH secretion although GHRH-induced GH secretion was not altered. GHRH-stimulated GH secretion was totally abolished by the treatment with antisense GHRH-R ON, whereas GHRP-2 induced GH secretion was not affected. Treatment of cells with both GHS-R and GHRH-2 ONs however completely inhibited GHRH and GHRP-2-stimulated GH secretion. These results suggest that GHRH-R is vital for GHRH-induced GH secretion but only partially involved in GHRP-2-stimulated GH secretion under the condition of down-regulation of GHS-R gene transcription.

Characterization and expression of the bovine growth hormone-releasing hormone (GHRH) receptor

The hypothalamic hormone, growth hormone-releasing hormone (GHRH) and its pituitary receptor are principal regulators of pituitary growth hormone (GH) synthesis and release. In the present study, we cloned and sequenced a complete bovine pituitary GHRH receptor cDNA in order to study its expression in cattle. The lengths of the exons in the bovine GHRH receptor gene were determined by comparison of the cloned cDNA with genomic sequences obtained from a bovine genomic library clone. As in other species, the bovine cDNA sequence encodes a 423-amino acid protein containing seven hydrophobic domains characteristic of a G protein-coupled receptor. The predicted bovine amino acid sequence shares 93, 90, 89, 87, and 85% identity with the ovine, porcine, human, rat and mouse sequences, respectively. Expression of the receptor in bovine ileum, ovary, anterior pituitary, testis, hypothalamus, pancreas and liver was examined by RT-PCR. Of those tissues examined, GHRH receptor expression was detected in the anterior pituitary gland and hypothalamus. To gain a better understanding of GHRH receptor gene regulation in ruminants, we examined the effect of bovine somatotropin (bST) treatment on pituitary GHRH receptor expression in dairy heifers using relative and real-time RT-PCR. In the present study, bST treatment of dairy heifers resulted in no significant decline in pituitary GHRH receptor expression.

A composite hormone response element regulates transcription of the rat GHRH receptor gene.

To further elucidate the molecular mechanisms underlying the transcriptional regulation of the GHRH receptor (GHRH-R) gene, hormonal regulation of the promoter activity of this gene was examined. An approximately 3-kb genomic fragment spanning the promoter region of the gene was sequenced and the transcription start site was determined by RT-PCR and RNase protection assay. A major start site was localized at -105 (relative to the translation initiation codon, ATG), and a pit-1 binding sequence characteristic of pituitary specific genes was found at -155 to -146. Deletion and mutation studies demonstrated this site to be functional. In the presence of dexamethasone, the GHRH-R promoter (from -2935 to -11) directed luciferase expression in MtT-S cells, a somatotropic cell line, but not in the PC12 cells that normally do not express GHRH-R. While T(3), all trans-RA, and 9cis-RA alone weakly enhanced the reporter gene expression, each of these substances was found to act as a synergistic enhancer in the presence of dexamethasone. Additional deletion and mutation analyses demonstrated a functional RA response element at -1090 to -1074. Two functional glucocorticoid response elements and a T(3) response element were found in an 80-bp 5'-flanking sequence of the pit-1 site. Interestingly, it is suggested that the 6-bp half-site AGGACA (from -209 to -204) functions as a 3'-half-site of T(3) response element as well as a 5'-half-site of one of the glucocorticoid response elements.

Decreased Expression of the GHRH Receptor Gene Due to a Mutation in a Pit-1 Binding Site

Decreased Expression of the GHRH Receptor Gene Due to a Mutation in a Pit-1 Binding Site

Regulation of GHRH receptor gene expression in the neonatal and adult rat pituitary.

The growth hormone releasing hormone (GHRH) receptor gene is essential for normal growth, and its expression is developmentally regulated. The factors that control GHRH receptor expression in the neonatal pituitary are not well understood. This study focuses on the regulation of GHRH receptor gene expression by thyroid hormone, glucocorticoids, insulin-like growth factor-I (IGF-I) and IGF-II in rat pituitary cell cultures. In newborn pituitaries, both T3 and hydrocortisone (24 h) caused a dose-dependent increase in GHRH receptor mRNA abundance, reaching levels 4.8-fold (P<0.001) and 6.5-fold (P<0.001) over corresponding controls. T3 and hydrocortisone also stimulated GHRH receptor expression in adult (70 day) pituitary cell cultures, consistent with our earlier findings. IGF-I treatment suppressed the inductive effects of T3 (P<0.02) and hydrocortisone (P<0.03) on GHRH receptor expression in adult pituitaries but not in neonatal pituitaries. Unlike IGF-I, IGF-II treatment had no effect on T3-induced or hydrocortisone-induced GHRH receptor expression in either neonates or adults. Taken together, these results indicate that (1) thyroid hormone and hydrocortisone act directly at the neonatal pituitary as potent stimulators of GHRH receptor gene expression, (2) IGF-I, but not IGF-II, acts at the pituitary to suppress GHRH receptor mRNA expression and (3) the effects of IGF-I on GHRH receptor gene expression are developmentally determined.

Growth hormone-releasing peptide-2 stimulates GH secretion in GH-deficient patients with mutated GH-releasing hormone receptor.

GH-releasing peptides (GHRPs) are synthetic peptides that bind to specific receptors and thereby stimulate the secretion of pituitary GH. In vivo it is uncertain whether these peptides act directly on somatotroph cells or indirectly via release of GHRH from the hypothalamus. In this study we compared the pituitary hormone response to GHRP-2 in 11 individuals with isolated GH deficiency (GHD) due to a homozygous mutation of the GHRH receptor (GHRH-R) gene and in 8 normal unrelated controls. Basal serum GH levels were lower in the GHD group compared with controls [0.11 +/- 0.11 (range, <0.04 to 0.38) vs. 0.59 +/- 0.76 microg/L (range, 0.04-2.12 microg/L); P = 0.052]. After GHRP-2 administration there was a 4.5-fold increase in serum GH relative to baseline values in the GHD group (0.49 +/- 0.41 vs. 0.11 +/- 0.11 microg/L; P = 0.002), which was significantly less than the 79-fold increase in the control group (46.8 +/- 17.6 vs. 0.59 +/- 0.76 microg/L; P = 0.008). Basal and post-GHRP-2 serum levels of ACTH, cortisol, and PRL were similar in both groups. Basal levels of serum TSH were significantly higher in the GHD group than in the control group (3.23 +/- 2.21 vs. 1.37 +/- 0.34 microIU/mL; P = 0.003). TSH levels in both groups did not change after GHRP-2 administration. These results suggest that an intact GHRH signaling system is not an absolute requirement for GHRP-2 action on GH secretion and that GHRP-2 has a GHRH-independent effect on pituitary somatotroph cells.

Insulin-like growth factor 1 and growthhormone-releasing hormone (GHRH) inhibit GHRH receptor gene expression in cultured rat pituitary cells

Insulin-like growth factor 1 and growthhormone-releasing hormone (GHRH) inhibit GHRH receptor gene expression in cultured rat pituitary cells

Differential GH-releasing hormone regulation of GHRH receptor mRNA expression in the rat pituitary.

To understand the capacity of growth hormone-releasing hormone (GHRH) to regulate expression of the GHRH receptor, we studied the effects of GHRH on GHRH receptor mRNA expression in immature and adult rats by use of pituitary cell culture and immunoneutralization approaches. Pituitary cell cultures from neonatal (2-day-old) and adult (70-day-old) rats were treated with GHRH for 4, 24, or 72 h. The effect of GHRH on GHRH receptor mRNA expression depended on the duration of GHRH exposure in both age groups; short-term (4 h) GHRH treatment significantly reduced GHRH receptor mRNA expression (P < 0.05), whereas intermediate treatment (24 h) restored GHRH receptor mRNA to basal levels, and long-term treatment (72 h) stimulated GHRH receptor mRNA expression (P < 0.02). The long-term stimulatory effect of GHRH on GHRH receptor mRNA expression required the presence of serum in the culture medium, and, in the absence of serum, the stimulatory effect was completely abolished. Moreover, the capacity of the pituitary to increase GHRH receptor mRNA expression in response to 72-h GHRH treatment was age dependent, with neonatal pituitaries exhibiting a much greater stimulatory effect than adult pituitaries (P < 0.025). Immunoneutralization of endogenous GHRH significantly reduced GHRH receptor mRNA expression in neonatal (P < 0.004), juvenile (P < 0.003), and mature (P < 0.004) pituitaries compared with age-matched controls. Taken together, these results indicate that GHRH is a potent regulator of GHRH receptor gene expression in immature and mature pituitaries; however, the nature and direction of GHRH regulation of its receptor depend significantly on several variables, including the duration of GHRH exposure, the presence of permissive components in serum, and the developmental stage of the pituitary.

Study of the Multiple Endocrine Neoplasia Type 1, Growth Hormone-Releasing Hormone Receptor, Gs , and Gi2 Genes in Isolated Familial Acromegaly

Study of the Multiple Endocrine Neoplasia Type 1, Growth Hormone-Releasing Hormone Receptor, Gs , and Gi2 Genes in Isolated Familial Acromegaly

Three new mutations in the gene for the growth hormone (gh)-releasing hormone receptor in familial isolated gh deficiency type ib.

Isolated GH deficiency (IGHD) is familial in 5-30% of cases. The majority of patients have the type IB form, characterized by autosomal recessive transmission, low but measurable serum concentrations of GH, and responsiveness to exogenous GH therapy. Unique mutations in the gene encoding the GHRH receptor (GHRHR) have previously been described in 2 kindreds with IGHD IB. However, the prevalence of GHRHR mutations in patients with IGHD IB is unknown. We analyzed 30 families with IGHD IB in which more than 1 member was affected. Linkage analysis was performed in 28 of the families, and in 3 families sibling pair analysis indicated linkage to the GHRHR gene locus. These 3 families as well as 2 families in which linkage analysis was not performed were screened for mutations in the 13 coding exons, the intron-exon boundaries, and 327 bases of the promoter of the GHRHR gene. We identified novel GHRHR missense mutations in 2 of the 3 kindreds with informative linkage and in 1 family in which linkage had not been performed. In 1 family affected members were homozygous for a mutation in codon 144 that replaces leucine with histidine (L144H). Affected subjects in a second family were compound heterozygotes, carrying both the L144H mutation and a second mutation in codon 242 that replaces phenylalanine with cysteine. Affected subjects in a third family were homozygous for a mutation that replaces alanine at codon 222 with glutamic acid. All 3 mutations segregated with the IGHD phenotype. All 3 mutant receptors were expressed in CHO cells, and each failed to show a cAMP response after treatment of the cells with GHRH. These results demonstrate that missense mutations in the GHRHR gene are a cause of IGHD IB, and that defects in the GHRHR gene may be a more common cause of GH deficiency than previously suspected.

Familial Isolated Growth Hormone Deficiency Is Associated with Hypertension, Central Obesity and Dyslipidemia But not the Classical Syndrome X

69 We have described a splice mutation in the GHRH receptor gene (G to A at position +1 , intron 1) in Itabaianinha county, Sergipe, Brazil. These genetically homogeneous individuals provide a unique opportunity to investigate the effects of isolated growth hormone deficiency [GHD] on cardiovascular risks. Therefore, we studied 16 dwarfs with GHD and no past of growth hormone therapy (8M:8F,49±14 yrs) and 31 age-matched normal controls [C] (12M:19F,44±12 yrs). We have examined insulin-like growth factor 1(IGF-1), arterial blood pressure, body mass index (BMI), waist-to-hip ratio (WHR), body composition by near infra-red interactance,leptin,serum lipids, uric acid, fasting glucose and insulin. IGF-1 in GHD was markedly lower (2.7±1.8 vs 154.9±82 ng/mL; p&lt;0.001). Systolic blood pressure was higher in GHD (147±37 vs 129±24 mmHg; p=0.046) but diastolic blood pressure was similar (84±16 vs 80±16 mmHg; p=NS). BMI (22.9±4.1 vs 24.4±3.8 Kg/m 2 ;p=NS) and leptin (11±8.9 vs 10.2±11.3 ng/mL;p=NS) were similar (GHD vs C), but WHR (0.97±0.08 vs 0.90±0.08; p=0.01) and % of fat mass (34±7 vs 21±10 %;p&lt;0.001)were higher in GHD. Total cholesterol (236±53 vs 195±38 mg/dL;P=0.003) and LDL-C (165 ±47 vs 133±32 mg/dL;p=0.009) were higher in GHD but no differences were observed in HDL-C (40±9 vs 38±7 mg/dL;p=NS), apo B (1.16±0.36 vs 1.33±0.96 g/L; p=NS) and triglycerides (158±117 vs 115±62 mg/dL;p=NS). Uric acid was higher in GHD (4.0±1.2 vs 3.3±0.8; p=0.02). Despite central obesity and hypertension, fasting glucose (86±11 vs 87±14 mg/dL;p=NS),fasting insulin (2.9±2.4 vs 4.3±3.4 μU/mL;p=NS) and insulin/glucose ratio (0.03±0.03 vs 0.05±0.03μU/mL;p=NS) were smaller but not significantly in GHD. We conclude that GHD due to this GHRH receptor mutation is associated with systolic hypertension, central obesity and dyslipidemia with some features different from the classical syndrome X. These data suggest that in GHD another mechanism different from insulin resistance participate in the pathogenesis of hypertension and metabolic alterations.

A Polymorphism in the Growth Hormone (GH)-Releasing Hormone (GHRH) Receptor Gene Is Associated with Elevated Response to GHRH by Human Pituitary Somatotrophinomas in Vitro

A previous study has suggested that a G to A base change at position 169 of the GHRH-receptor gene in human somatotrophinomas is a mutation and confers hypersensitivity to GHRH. The alternative base converts codon 57 from GCG to AGC, resulting in replacement of alanine (Ala) with threonine (Thr). In the present study, two of five human GH-secreting somatotrophinomas were found to possess the codon 57 AGC sequence. The GCG allele was also detected, indicating heterozygosity. However, the patients' normal blood-derived DNA also yielded the same sequence pattern, indicating that the Ala ⇒ Thr amino acid change is a normal polymorphism, and not a somatic mutation. Nevertheless, in vitro, the tumors possessing the Ala ⇒ Thr amino acid change responded very strongly to GHRH in terms of cAMP formation, being increased 40- and 200-fold, in comparison to the 2-fold increases by tumors without the alternative GHRH-receptor sequence. Likewise, the in vitro response of GH secretion to GHRH was elevated. One of the two tumors with the alternative Thr residue, and the highest responder to GHRH, possessed a gsp mutation, despite the fact that these defects are thought to reduce responsiveness to GHRH. These results fail to confirm that the GCG ⇒ AGC at codon 57 of the GHRH-receptor gene is a mutation, but do support the concept that the alternative form with Thr confers increased sensitivity to GHRH.

Familial growth hormone deficiency with mutated GHRH receptor gene: clinical and hormonal findings in homozygous and heterozygous individuals from Itabaianinha.

To characterize clinically and hormonally the syndrome of autosomal recessive familial growth hormone deficiency (FGHD) recently identified in Itabaianinha, Sergipe, Brazil, caused by a novel mutation (mt) that inactivates the growth hormone-releasing hormone receptor (GHRH-R) gene. Clinical and hormonal evaluations were performed in 21 FGHD individuals (mt/mt group) aged 8 to 63 years, 13 heterozygotes for the GHRH-R mutation (wt/mt group) and 5 homozygotes for the wild type (wt) allele (wt/wt group), identified by genotyping of peripheral blood leukocyte DNA. Clinical and hormonal characterization included physical examination and measurement of GH, IGF-I, IGF binding protein-3 (IGFBP-3), cortisol, prolactin, LH, FSH, and free thyroxine (FT4). Clinical features were consistent with isolated growth hormone deficiency. Height was significantly reduced in the mt/mt group compared with the wt/mt group (mean height standard deviation score (SDS) +/- s.d.: -7.35+/-1.37 vs -1.84+/-1.44 respectively, P<0. 0001), and the wt/wt group (-1.85 +/- 0.81, P=0.0007). The height of the 13 wt/mt subjects did not differ from the 5wt/wt individuals. Serum GH, IGF-I, IGF-I SDS, IGFBP-3 and IGFBP-3 SDS were all significantly lower in the mt/mt group than in the wt/mt and wt/wt groups. Two affected children treated with GH for 1 year showed a normal growth response. Serum IGF-I and IGF-I SDS were lower in wt/mt compared with wt/wt group, but did not reach statistical significance. IGF-I and IGF-I SDS correlated inversely with age in wt/mt group. FGHD due to an autosomal recessive GHRH-R gene mutation leads to marked dwarfism, phenotypically and hormonally indistinguishable from other forms of isolated GH deficiency. Heterozygotes for the GHRH-R mutation appear to have a partial defect in the GH/IGF axis, with no apparent height impairment.

Structure and function of the growth-hormone-releasing hormone receptor

Growth-hormone-releasing hormone (GHRH) stimulates growth hormone (GH) secretion and GH synthesis and is also thought to cause somatotroph proliferation. Specific high-affinity binding sites for GHRH have been demonstrated on pituitary membranes using iodinated GHRH analogs. The complementary DNA encoding for the human GHRH receptor (GHRH-R) was recently cloned. The open reading frame was shown to extend 1269 bp and thus to encode a protein of 423 amino acids with a predicted molecular weight of 47 kDa. Expression is restricted to specific tissues. Analysis of the genomic structure revealed that the human GHRH-R gene spans 15 kb and consists of 13 exons. The 5'-flanking region of the human GHRH-R gene was recently characterized. Transcriptional regulation of the GHRH-R is discussed in this review. Mechanisms of signal transduction for control of GH transcription and secretion are presented. Furthermore, the role of the GHRH-R in proliferation and differentiation of the somatotrophic pituitary cell as well as in disease is examined.