Growth hormone (GH) is produced by the somatotropic cells in the anterior pituitary.
In humans, GH consists of a single peptide of 191 amino acids that is crosslinked by two disulfide bridges. Rat GH is one amino acid shorter and has 66% homology with its human counterpart [93]. The secretion of GH into the circulation is primarily regulated by two peptide hormones with opposite effects: GH-releasing hormone (GHRH), which stimulates the secretion of GH, and somatostatin, which inhibits the secretion of GH. These hormones are produced in the hypothalamus and reach the pituitary via the portal vascular system. The GH secretion is also regulated by a number of other factors, such as insulin-like growth factor-I (IGF-I), thyroid hormone, glucocorticoids, fasting and type 1 and type 2 diabetes mellitus (for review see [94]), indicating the complexity of the regulation of GH secretion.
The GH secretory pattern in females and males
There is a marked sex difference in the pattern of GH secretion in most adult species, including rodents [95] (Figure 5) and man [96]. Male rats secrete GH regularly in large pulses every 3-3.3 hours with low or undetectable levels between the peaks [95, 97]. Female rats, on the other hand, secrete GH in a near continuous fashion with lower pulse amplitudes and higher baseline levels than male rats [95]. The mean plasma level of GH, however, is similar in both sexes [98]. The sexually dimorphic secretory pattern of GH in humans is similar to that in rodents but is less pronounced [96]. The average daily GH concentration, however, is higher in women compared to men [99, 100].
Figure 5. The sex-differentiated GH secretory pattern in rats.
The difference in the secretory pattern of GH does not become apparent until after the onset of puberty. This suggests that the GH secretory pattern is highly controlled by gonadal steroids. Both neonatal and prepubertal gonadectomy of male rats result in elevated baseline GH levels during adult life, which can be completely reversed by sustained testosterone treatment [98]. However, GH pulse height is only decreased after neonatal gonadectomy, with unchanged pulse heights after prepubertal gonadectomy [98]. This suggests that a neonatal testosterone surge is needed to maintain normal GH pulse amplitudes in adult male rats, while a continuous presence of testosterone is necessary for preserving low GH basal levels in adult male rats.
Neonatal gonadectomy of female rats only mildly affects the female GH secretory pattern [98]. However, the importance of estradiol in maintaining the sexually dimorphic GH secretory is shown by the feminisation of the GH secretory pattern in intact males after estradiol treatment, i.e. higher baseline GH levels, lower pulse heights and more frequent GH bursts [101, 102].
The mechanism of sex steroid action is probably by modulation of the GHRH and somatostatin levels, as well as by direct effects on GH secretion in the pituitary. The
GH GH
Time Time
GH GH
Time Time
greater peaks that are observed in male rats compared to female rats may be due to the androgen-induced increase of both GHRH in the hypothalamus and GH secretion from the pituitary [103]. Some studies also show that androgens might enhance the somatostatin levels between peaks, which could account for the lower GH baseline level in male rats. Moreover, GHRH inhibits its own secretion and increases the secretion of somatostatin [104]. These properties in male rats are therefore likely to account for the cyclic GH secretory pattern through feedback mechanisms. In female rats, there is no such cyclic variation in somatostatin levels due to the inhibition by estradiol [105], which explains why female rats secrete GH more continuously than male rats.
GH receptor and GH signalling
The GH receptor is a member of the large cytokine receptor superfamily. This family includes receptors for more than 25 ligands, such as prolactin, multiple interleukins, leptin and erythropoietin [106]. Cytokine receptors are generally composed of an extracellular region, a single transmembrane domain and an intracellular region. A soluble form of the extracellular region of the GH receptor is found in plasma. This glycoprotein is called growth hormone-binding protein (GHBP) and it binds up to 60%
of circulating GH [107].
The initial step in GH signalling is the sequential binding of GH to two GH receptors that results in receptor dimerisation [108]. This moves the receptors in close proximity of each other, which increases the affinity of the GH receptor for the tyrosine kinase JAK2. As the GH receptor itself lacks tyrosine kinase activity, each JAK2 transphosphorylates the other JAK2 molecule and they are both thereby activated [109]. Activated JAK2 proteins subsequently phosphorylate the GH receptor at several tyrosine residues, converting them to docking sites for other signalling molecules [110]. One such molecule is the signal transducer and activator of transcription 5 (STAT5), which upon binding becomes phosphorylated by the action of JAK2 [111, 112]. This phosphorylation event activates STAT5 proteins, triggering their dimerisation, nuclear translocation and activation of gene transcription [112, 113].
There are two closely related isoforms of STAT5, termed STAT5a and STAT5b, of which STAT5b appears to be most important in GH signalling [114]. Other signalling pathways that are activated by GH include the mitogen-activated protein kinase (MAPK) pathway, the insulin receptor substrates (IRS)-1 and IRS-2, and protein kinase C.
The activation phase is usually transient, which means that effective shut off mechanisms by which the GH signalling pathway is inhibited must occur in the cell.
The most important negative regulators of GH signalling are members of the
cytokine-inducible gene family, termed suppressors of cytokine signalling (SOCS) [115]. These proteins are induced in response to GH or other cytokines by increased transcription via JAK-STAT activation. The SOCS proteins subsequently inhibit GH signalling by reducing the kinase activity of JAKs [115]. As the SOCS switch off the signalling pathway that initially led to its production, these proteins are involved in a classical negative feedback loop.
Influence of the GH secretory pattern on GH signalling
The second messenger system of intracellular signalling is activated differently by the intermittent and continuous GH stimulus. This is demonstrated by the activation of STAT5b by the intermittent GH secretory pattern in contrast to the continuous GH secretory pattern [116]. STAT5b has therefore been suggested to play a key role in the regulation of the sexual dimorphic gene expression in the liver that is induced by the male pulsatile GH secretory pattern. The hepatic expression of Cis (cytokine-inducible SH2-containing protein), a member of the SOCS family, is higher in female rats compared to males due to the continuous GH secretory pattern of females [117]. As Cis is responsible for the desensitisation of GH-induced STAT5b signalling, a more pronounced expression of Cis could result in less expression of male-characteristic genes in female rats. There is not much known about signalling that is specifically induced by the female secretory pattern of GH. However, one study shows that incubation of rat hepatocytes with GH stimulates female-specific CYP2C12 expression via upregulation of phospholipase A2 (PLA2). This effect is dependent on the subsequent P450-catalysed formation of an arachidonic acid metabolite [118] that may function as an intracellular second messenger.
Insulin-like and diabetogenic effects of GH
GH exerts both insulin-like and diabetogenic (antiinsulin-like) effects in adipose tissue and skeletal muscle (for review see [119]). The insulin-like effects occur soon after GH exposure and involve increased glucose utilisation and decreased lipolysis. When tissues are exposed to GH for a longer time, responsive cells are turned into unresponsive cells towards insulin-like actions, which is termed the refractory effect of GH. As GH is secreted endogenously throughout the day, causing a constant refractory state, the insulin-like GH effects probably have no physiological role. The late diabetogenic effects of GH that occur after prolonged GH exposure are therefore considered to better reflect the physiological situation. These effects include impaired glucose utilisation, hyperglycemia, stimulation of lipolysis, and induction and maintenance of the refractory state to insulin-like effects. The mechanism by which GH induces refractoriness to the insulin-like effects is thought to occur via upregulation of SOCS-3 and thereby blocking JAK2 activation [120].
GH and lipoprotein metabolism in humans
GH regulates a number of important functions in cholesterol and lipoprotein metabolism (Table 2). This is particularly emphasised during conditions of GH deficiency and GH excess (acromegaly), which are both associated with an abnormal lipid profile. GH-deficient subjects have elevated levels of total cholesterol, LDL cholesterol and triglycerides, and a reduced HDL cholesterol level [2]. This serum lipid profile is atherogenic and GH-deficient patients indeed have an increased risk for CVD [121]. GH replacement therapy in these patients has been shown to be favourable with respect to the changed plasma lipoprotein profile. Most notably, GH administration lowers the LDL cholesterol and increases HDL cholesterol, while plasma triglyceride levels are principally unchanged [122-124]. GH therapy stimulates the secretion of VLDL-apoB from the liver [125], but despite this the plasma concentration of apoB and LDL cholesterol decreases [122, 123]. This is probably due to the increased clearance of these particles after GH treatment [125], which might at least partly be explained by upregulation of LDL receptors [126, 127]. Moreover, GH treatment reduces the activity of CETP in GH-deficient patients, which increases the HDL cholesterol level and further contributes to the decreased LDL cholesterol level [128]. However, GH therapy has also been reported to increase the level of the atherogenic lipoprotein (a) [123, 124], which may contribute to the increased risk for CVD in acromegaly [1]. These patients also have increased triglyceride levels, decreased HDL levels and are often insulin resistant [129, 130], which might further contribute to the increased prevalence of CVD in acromegaly.
GH and lipoprotein metabolism in laboratory animals
GH plays an important role in cholesterol and lipoprotein metabolism also in rodents (Table 2). GH is known to stimulate lipolysis in adipose tissue, which increases the flux of free fatty acids to the liver and other tissues such as skeletal muscle. A continuous GH infusion to Hx rats has also a stimulatory effect on some hepatic lipogenic enzymes in vivo [131-134] and on triglyceride synthesis in hepatic cultures derived from these rats [135, 136]. Although this results in a stimulated secretion of VLDL [136, 137], no increase in serum levels of VLDL has been found in GH-treated Hx rats [138], which is in line with the findings in humans. This indicates that GH increases both production and clearance of VLDL. The increased turnover of VLDL in rats can partly be explained by the fact that GH enhances the editing of apoB mRNA.
This results in an increased proportion of secreted apoB-48-containing VLDL particles [139], known to have a considerably shorter half-life than particles containing apoB-100 [140]. GH also has a stimulatory effect on LPL activity in skeletal muscle, which may further contribute to a more rapid catabolism of secreted VLDL particles [141, 142]. In addition, the secretion of apoE is increased in hepatocytes isolated from Hx rats treated with a continuous infusion of GH [143]. As apoB-48-containing
lipoproteins can be removed via interaction of apoE with LRP and LDL receptors, an increased content of apoE may add to the enhanced turnover of lipoproteins in response to GH. HDL cholesterol levels are increased by GH treatment in both rats [138] and mice [144], which in rats has been shown to be due to the female GH secretory pattern [138]. The LDL cholesterol level is conversely decreased by GH treatment in hypophysectomised rodents [138]. This is probably mainly due to the stimulatory effect that GH exerts on hepatic LDL receptor expression [126, 127], but increased editing of apoB mRNA [139] and increased HL activity [142] may also be involved. In conclusion, even though it is clear that GH increases VLDL secretion from the liver, the mechanisms behind the stimulatory effect of GH on VLDL assembly and secretion is not known.
Table 2. Effects of GH on lipoprotein metabolism in humans and laboratory animals. *Hx rats.
Humans Lab. animals*
HDL cholesterol
↑ ↑
LDL cholesterol
↓ ↓
TG
↔
or↑ ↔
or↑
ApoB
↓ ↓
ApoE
↑* ↑*
Lp(a)
↑*
Hepatic apoB mRNA editing
↑
Hepatic TG synthesis
↑*
VLDL-secretion
↑ ↑*
LDL receptor
↑ ↑
CETP
↓
LPL in adipose tissue
↓ ↔
LPL in skeletal muscle
↔ ↑
*Effect dependent on mode of GH administration