Growth, growth hormone and the somatomedins: a historical perspective and current concepts

The GH/IGF-1 Axis and Heart Failure

The growth hormone (GH)/insulin-like growth factor 1 (IGF-1) axis regulates cardiac growth, stimulates myocardial contractility and influences the vascular system. The GH/IGF-1 axis controls intrinsic cardiac contractility by enhancing the intracellular calcium availability and regulating expression of contractile proteins; stimulates cardiac growth, by increasing protein synthesis; modifies systemic vascular resistance, by activating the nitric oxide system and regulating non-endothelial-dependent actions. The relationship between the GH/IGF-1 axis and the cardiovascular system has been extensively demonstrated in numerous experimental studies and confirmed by the cardiac derangements secondary to both GH excess and deficiency. Several years ago, a clinical non-blinded study showed, in seven patients with idiopathic dilated cardiomyopathy and chronic heart failure (CHF), a significant improvement in cardiac function and structure after three months of treatment with recombinant GH plus standard therapy for heart failure. More recent studies, including a small double-blind placebo-controlled study on GH effects on exercise tolerance and cardiopulmonary performance, have shown that GH benefits patients with CHF secondary to both ischemic and idiopathic dilated cardiomyopathy. However, conflicting results emerge from other placebo-controlled trials. These discordant findings may be explained by the degree of CHF-associated GH resistance. In conclusion, we believe that more clinical and experimental studies are necessary to exactly understand the mechanisms that determine the variable sensitivity to GH and its positive effects in the failing heart.

Human growth disorders: molecular genetics of the growth hormone-insulin-like growth factor I axis

The critical role of the growth hormone-insulin-like growth factor I axis in controlling somatic growth in humans and other vertebrate species has been known for many years. Through molecular cloning and other related techniques many of the components of this axis have been characterized, with the most recent additions being key transcription factors required for pituitary development and for pituitary-specific gene expression. Several of these genes have been shown to be mutated in familial and sporadic human growth deficiency syndromes, thereby validating the central roles of the encoded proteins in the endocrine pathways regulating somatic growth. The purpose of this review is to highlight these recent advances from the perspective of the molecular genetics of human growth disorders.

Human genital melanocytes as androgen target cells

As some parts of human skin - such as genital and areolar skin - become pigmented after puberty, melanocytes in these regions are thought to be sex hormone target cells. We immunohistochemically localized androgen receptors in the nuclei of cultured human genital melanocytes by monoclonal and polyclonal antibodies. When these cells were incubated with [1,2-3H]-testosterone, the major metabolite in the medium was dihydrotestosterone and 5alpha-reduction predominated over 17beta-oxidation. Androgen receptor and type I 5alpha-reductase mRNAs could be detected in genital melanocytes by reverse transcriptase polymerase chain reaction. The tyrosinase activity was stimulated by the addition of androgen. This stimulation was antagonized by cyproterone acetate, whereas tyrosinase mRNA expression was not affected by androgen. These results indicate that human genital melanocytes are androgen target cells, and that androgen plays a role for pigmentation in the specific regional skin after puberty.

Recombinant growth hormone: a new cardiovascular drug therapy

GH has an important role in normal cardiovascular physiologic functioning, working indirectly through effects on IGF-1. An excess or deficiency of GH causes an increased rate of cardiovascular disease, including cardiomyopathy. A relative GH deficiency in older subjects may also increase cardiovascular morbidity and mortality risk. In replacement doses, GH can enhance myocardial contractility; can decrease peripheral vascular resistance; and can reduce total cholesterol and LDL-cholesterol values and fibrinogen and PAI levels. These effects of GH, coupled with the ability to improve skeletal muscle function and reduce adiposity, make it an attractive treatment for patients with CHF and a potential maintenance drug for elderly people. Clinical trials, including studies with GHRH that may reduce the adverse effects of GH therapy, such as hyperglycemia and hypertension, are now in progress.

Overnight growth hormone secretion in achondroplasia: deconvolution analysis, correlation with sleep state, and changes after treatment of obstructive sleep apnea

The normal profile for overnight GH secretion in achondroplasia has not been previously studied. Factors that have been shown to influence GH secretion include age, obesity, sleep state, and the presence of obstructive sleep apnea (OSA). We assessed GH levels in a group of subjects with achondroplasia, during overnight polysomnography. Nineteen subjects with achondroplasia were studied at 11.3 y of age (median 6.7, range 1.8-30.9). Levels of GH were measured using time-resolved immunofluorometric assay (DELFIA, Pharmacia Biotech Inc.) and analyzed by a deconvolution method. Five subjects were restudied after treatment for OSA. Secretion rates of GH were greater in slow wave (SWS) and rapid eye movement (REM) sleep than in stage one and two (SI-II = light non-REM) sleep (p < 0.01). Total overnight GH secretion decreased with increasing age (r2 = 0.22 p < 0.04). Neither the frequency of arousals, frequency of sleep state transitions nor the severity of OSA correlated with measures of GH secretion. Levels of IGF-I correlated independently with age, body weight (percent ideal), and GH secretion rate (r2 = 0.76, p < 0.001). In a group of five subjects treated for OSA, improved respiratory distress index and reduced sleep state transitions were not associated with significant changes in GH secretion rate by sleep stage; SWS [from 0.62 +/- 0.28 mIU/L/min to 1.02 +/- 0.25 mIU/L/min (NS)] and SI-II sleep [from 0.26 +/- 0.07 mIU/L/min to 0.60 +/- 0.16 mIU/L/min (NS)]. However, in those five subjects, a GH secretion peak during the first 2 h of SWS was initially absent, appearing only after treatment of OSA (1.09 +/- 0.38 mIU/L/min) compared with (2.40 +/- 0.59 mIU/L/min (p = 0.01). A profile of overnight GH secretion is presented for subjects with achondroplasia.

Determination of insulin-like growth factor 1 in dogs using a commercially available immunoradiometric assay1)

The objective of the present study was to characterize a commercially available immunoradiometric assay (IRMA) intended for the determination of human insulin-like growth factor 1 (IGF-1) when used to determine IGF-1 in canine plasma samples. The assay was evaluated by assessment of the precision, accuracy, and detection limit, and by comparing the IGF-1 values observed in healthy dogs and in dogs with acromegaly and hereditary pituitary dwarfism to values reported in the literature. Since extraction of IGF-1 from its binding proteins is essential for reliable test results, the efficiency of the extraction method used in the assay was also examined. The main conclusions can be summarized as follows: The commercially available IRMA method offers a precise determination of the IGF-1 concentration in canine plasma samples with IGF-1 values directly comparable to those obtained by others. The assay accuracy was confirmed for normal dogs by parallelity between IGF-1 concentrations and plasma dilutions, whereas plasma from acromegalic dogs could not fullful parallelity. The detection limit was below the IGF-1 concentrations observed in dogs with hereditary pituitary dwarfism. In plasma samples with low or normal IGF-1 levels, the assay's extraction efficiency was comparable to that found using other extraction methods. In samples with IGF-1 concentrations above usual physiological levels, the extraction efficiency decreased unexplainably. However, this decrease in extraction efficiency seems to be of only minor importance when the measurements are used in the diagnosis of canine acromegaly.

Ethanol suppresses growth hormone-mediated cellular responses in liver slices

Previous studies indicate that both acute and chronic ethanol administration inhibit protein synthesis and decrease the secretion of insulin-like growth factor-1 (IGF-1). Although IGF-1 synthesis and secretion are regulated by growth hormone secretion from the pituitary gland, we assessed whether ethanol inhibits tissue response to growth hormone. Liver slices from male Sprague-Dawley rats were prepared, placed into F-12 media, and incubated at 37 degrees C with [3H]leucine, and either 0.25 or 1 nM rat growth hormone and 0, 37 (physiological levels), or 175 mM (toxic levels) ethanol. Tissues were removed at 0, 15, 30, and 60 min. Protein synthesis increased linearly during this time period, and administration of growth hormone (1 nM) significantly increased protein synthetic rate by 48% (p < 0.01), whereas addition of 37 or 175 mM ethanol attenuated the effects of growth hormone (p < 0.01). Analysis of IGF-1 mRNA indicated a 2-fold increase in response to growth hormone (p < 0.01), whereas ethanol administration decreased the growth hormone-induced rise of IGF-1 mRNA. Ethanol (175 mM) inhibited the release of IGF-1 into the media (p < 0.05). Ethanol did not alter growth hormone receptor binding, and exposure of tissue slices to ethanol did not influence the number of growth hormone receptors or the affinity of growth hormone for its receptor. Our results demonstrate that (1) growth hormone is a potent acute regulator of IGF-1 mRNA and IGF-1 peptide release, (2) ethanol inhibits growth hormone-induced protein synthesis and induction of IGF-1 gene expression, and (3) the inhibitory effects of ethanol on growth hormone occur without changing growth hormone receptor number or binding characteristics. We conclude that ethanol suppresses growth hormone-induced signal transduction, resulting in a decrease in IGF-1 gene expression.

Low circulating levels of insulin-like growth factors and testosterone in chronically institutionalized elderly men

OBJECTIVE

To determine the prevalences of and the associations between hyposomatomedinemia and hypogonadism in healthy young men, healthy old men, and chronically institutionalized old men.

DESIGN

Survey with serial blood tests.

SETTING

Veterans Affairs nursing home and young and old men living in the community.

SUBJECTS

Three groups were studied: healthy young men (20-29 years old, n = 32), healthy old men (59-98 years old, n = 30), and chronically institutionalized old men (59-95 years old, n = 112).

MEASUREMENTS

Plasma insulin-like growth factor-I (IGF-I), total testosterone (TT), free testosterone (FT), and plasma insulin-like growth factor-II (IGF-II) were measured. In subjects with low testosterone level, serum luteinizing hormone (LH) was also determined. In a subset of chronically institutionalized old men with low IGF-I, the serum growth hormone (GH) level was analyzed during the first 4 hours of sleep.

RESULTS

A low IGF-I level (defined as a value below the lower 2.5 percentile of the comparison group) occurred in 85% of the healthy old men when compared with healthy young men (P < 0.001), in 90% of the chronically institutionalized old men when compared with healthy young men (P < 0.001), and in 26% of the chronically institutionalized old men when compared with healthy old men (P < 0.001). In chronically institutionalized old men with low IGF-I compared with healthy young men, nocturnal peaks of serum GH were < 2 ng/mL in most cases. Low TT (defined as a value below the lower 2.5 percentile of the comparison group) occurred in 86% of the healthy old men when compared with healthy young men (P < 0.001), in 88% of the chronically institutionalized old men when compared with healthy young men (P < 0.001), and in 28% of the chronically institutionalized old men when compared with healthy old men (P < 0.001). The results of FT were similar. In 80% of the institutionalized old men with low TT and FT, the serum LH level was low (< 20 mU/mL). In 53% of the institutionalized old men, the IGF-II level was below the lower 2.5 percentile of the healthy old men (P < 0.001). In both healthy and institutionalized old men, IGF-I and IGF-II levels were significantly correlated to each other (r = 0.6), but neither was significantly correlated to TT or FT. In the institutionalized old men, IGF-I was inversely correlated with age and with a diagnosis of dementia; TT and FT were inversely correlated with age and with the degree of dependency in ADL's.

CONCLUSIONS

Compared with healthy young men, most healthy old men have low serum IGF-I, TT, and FT levels. The geriatric hyposomatomedinemia and hypogonadism are more severe in institutionalized old men. In the latter group, both endocrine deficiencies are usually of central origin, but their occurrences are not significantly associated. Healthy old men usually have a low level of IGF-I compared with healthy young men, but a similar level of IGF-II; institutionalized old men are usually low in both values.