J. Endocrinol. Invest. 15: 313-330, 1992
REVIEW ARTICLE
Thyroid hormones and growth hormone secretion R. Valcavi, M. Zini, and I. Portioli 2a Divisione di Medicina Interna e Sezione Endocrino Metabolica, Arcispedale S. Maria Nuova, Reggio Emilia, Italy
INTRODUCTION
THYROID HORMONE REGULA TlON OF GH GENE EXPRESSION
The fundamental role of thyroid hormones in postnatal growth and development of the mammalian organism is well recognized. In both man and experimental animals, thyroid hormone deficiency is associated with an impairment in the somatic growth. The underlying mechanisms by which thyroid hormones influence growth processes, however, are not well defined. Thyroid hormones stimulate growth with direct effects on induction of specific growth factors, and indirectly through the control of generation and secretion of growth hormone (GH) by the anterior pituitary gland.
In somatotroph tumour cell lines T3 stimulates GH mRNA accumulation (15-19) and increases the rate of GH gene transcription (20-22), Stimulation of the expression of GH gene by thyroid hormone has also been described in cultured rat anterior pituitary cells (23), The effects of thyroid hormone on GH gene expression and GH synthesis vary significantly among mammalian species. In bovine anterior pituitary cells T3 fails to augment basal and GHRH-stimulated GH secretion (24) and cultured human foetal pituitary cells supplemented with T3 show reduced basal GH release and attenuated responses to GHRH (25). Also in human pituitary tumour cells from patients with acromegaly T3 diminishes GH secretion (26), Furthermore, when transfected into rat pituitary cells, human GH gene expression is reduced by T3 administration (27) and the human GH gene promoter is insensitive to thyroid hormone, contrary to rat GH and bovine GH promoters which are induced by T3 (28). These data indicate that, although the most prominent action of thyroid hormone on GH secretion at the somatotroph level is to promote the transcription of GH gene and hence synthesis and accumulation of GH, there is potential heterogeneity in hormonal responses of the same gene in different species.
ACTIONS OF THYROID HORMONE ON CELLULAR GROWTH AND DEVELOPMENT Thyroid hormone is an absolute requirement for normal growth and development of many tissues (1-3). The nervous system is particularly sensitive to this hormonal effect (4-6). Thyroid hormone stimulates cell replication (7) and cell differentiation (8). These growth effects may be related to induction, directly or mediated by GH, of several growth factors (see below), including the production of a growth factor that regulates growth in an autocrine fashion (9). Indeed there is evidence that growth of somatotrophs themselves may be regulated by thyroid hormone. The anterior pituitary of hypothyroid rats reduces somatotrophs by 70% (10). When hypothyroid rats are injected with T3, a marked increase in the DNA synthesis of the somatotrophs occurs after 2-5 days (11 ) and the normal somatotroph population is restored within 5-10 days (12). In GC cells, a cell line derived from a rat somatotrophic tumour, T3 stimulates growth by induction of specific regulatory protein(s) during the early G1 period (13,14).
EFFECTS OF THYROID HORMONE ON PITUITARY GH SYNTHESIS AND SECRETION Thyroid hormone plays a critical role in stimulation of pituitary GH synthesis and secretion, The levels of radioimmunoassayable pituitary and circulating GH are extremely low in hypothyroid animals (29-32), In the rat, pituitary GH content drops to 3% of the normal value within 2 weeks following thyroidectomy (32), becoming undetectable by 24 days (30), Thyroid hormone administration at physiological doses produces a rapid increase in pituitary GH content (30, 32), and a subsequent increase in plas-
Key-words: Thyroid hormones, growth hormone, growth suhst;.:mcAs
Correspondence' Dr. Roberto Valcavi, 2a Divisione di Medlcina Interna e Sezione Endocrino Metabolica, Arcispedale S. Marla Nuova, V.le Umberto I 50, 42100 Reggio Emilia, Italy.
313
R. Va/cavi, M. Zini and /. Portio/i
ma GH concentration several hours later (32). Pituitary GH accumulates in response to i.v. pulse injections of T3 in hypothyroid rats (33). However, increases in pituitary GH content above normal can not be elicited when pharmacological doses of T3 are administered to euthyroid rats (33). Treatment of hypothyroid rat anterior pituitary cell cultures with T3 markedly increases intracellular GH content and increases GH secretion (34). As noted in vivo, the production of GH by pituitary cells does not increase above normal when supraphysiological concentrations of T3 are administered (34). In vivo studies in animals indicate that hypothyroidism also impairs stimulated GH secretion. Thyroid hormone deprivation suppresses GH responses to clonidine (35), GHRH (35-38) and GHRP-6 (39). Small doses of T4 maintain the in vivo but not the in vitro GH responses to GHRH (37). When rats are rendered hyperthyroid by the administration of thyroid hormone GH responses to GHRH are not enhanced (37, 38, 40). Hypothyroid patients show clearly impaired GH responses to GHRH, but, in contrast to the hyperthyroid rat, thyrotoxic patients have a blunted GH response to GHRH (Fig. 1). There is, however, a difference between hypo- and hyperthyroid subjects in their GH responses to GHRH. Hyperthyroid patients have a more sustained and delayed response to GHRH than hypothyroid patients. The abnormalities in hyperthyroidism are due to alterations in the hypothalamic control of GH rather than to pituitary abnormalities (see the following paragraphs).
25 GHRH 100 J.lg i.v.
20 -j
1
::::J 15
"'J'
3. :r:
G
10
5
o
r----30
---"r---,---.---.--..----.--~-__,-~
o
30
60
90
120
time (minutes) o---a'
nOr/no/ subjeels (n = 30)
_ ' hypo/hyro/d pol/en/s (n = 25)
x----x 'hyperthyroId pa/lents (n
= 38)
Fig. 1 - Growth hormone responses to GHRH (1-44) in patients with hypo- and hyperthyroidism. as compared to normal control subjects. Mean ± SEM In both instances the GH responses were subnormal, as assessed by ANOVA However the profile of the curves was consistently different, in that hyperthyroid patients had a delayed serum GH peak.
interact with the GH - IGF-I axis. T4 treatment of hypophysectomized (50) or thyroidectomized (51) rats increases serum Sm-C/IGF-I activity in the absence of GH. However both hormones are required for maximal stimulation (50), and have additive effects on IGF-1 production. Thus administration of GH to hypothyroid rats (52) and humans (53) does not completely rectify growth impairment unless T4 is administered simultaneously. It has been suggested that thyroid hormone regulates the in vivo expression of hepatic IGF-I mRNA, since hepatic IGF-I mRNA levels are suppressed in hypothyroid rats (54). However, this observation does not establish whether thyroid hormone has a direct effect on hepatocytes or acts by stimulating pituitary GH secretion. The latter is suggested by recent data showing that T4 replacement of hypophysectomized GH-deficient rats did not increase hepatic IGF-I mRNA or circulating IGF-I. A supraphysiological dose of T3 did increase hepatic IGF-I mRNA very slightly (55) Only when a single injection of T3 was combined with GH administration, were IGF mRNA levels greater than those of rats injected with GH alone (55).
THYROID HORMONE INFLUENCE ON GROWTH FACTORS Thyroid hormone may regulate several growth factors. Thyroxine administration increases brain Nerve Growth Factor (NGF) content (41,42). It also stimulates Epidermal Growth Factor (EGF) mRNA synthesis in mouse kidney and submandibular gland (43), and causes release of erythropoietic growth factor from leukocytes (44). Somatomedin C/lnsulin-like growth factor-1 (IGF-I) mediates, at least in part, the somatic effects of GH (45). The synthesis of somatomedin C/IGF-1 is regulated by many factors. Growth hormone is the most well known (46), but insulin (47), nutrient status (48), and thyroid hormone are also important. It is likely that thyroid hormone enhances cartilage growth both through enhancing the actions IGF-I and by directly accelerating the differentiation of developing chondrocytes (49). Some of the growth promoting effects of thyroid hormone appear to be the result of their ability to
314
Thyroid hormones and GH secretion
sesses enhancing or inhibitory actions on IGF biological effects. Further data on the regulatory role of thyroid hormone on IGF-BPs are required.
The observation that thyroid hormone stimulates growth minimally, if at all, in the absence of GH (56, 57) confirms the view that thyroid hormones themselves have relatively small effects on IGF-I synthesis and secretion. However direct effects of thyroid hormone on IGF-I production have been demonstrated in certain experimental models, such as foetal hypothalamic culture cells (58) and perfused rat liver (59). Although the primary source of circulating IGF-I appears to be the liver (60), IGF-I production has also been documented in several other tissues and cells (61), including anterior pituitary tissue (62). This suggests that IGF-I may act also in an autocrine or paracrine fashion. Local pituitary IGF synthesis and secretion may be controlled by the thyroid hormone, since it has been shown that thyroid hormone directly regulates IGF-I gene expression in GH3 rat pituitary cells (63). These data indicate that thyroid hormone exerts, at least in part, its growth promoting effects through a permissive role in IGF-I synthesis and secretion.
INFLUENCES ON GH NEUROREGULA TlON BY THYROID HORMONE In addition to their direct effects on GH synthesis in the pituitary, thyroid hormones might influence the neural control of GH secretion. This would be modulated by the release of GHRH and somatostatin from the hypothalamus. Furthermore, a complex network of neuropeptides, neurotransmitters and feedback signals interact with GHRH and somatostatin, thus contributing to the fine-tuning modulation of GH secretion. Thyroid hormones interact at several levels in this network. NEUROPEPTIDES Somatostatin The concept that thyroid hormone has a primary effect on hypothalamic somatostatin is highly controversial. Thyroidectomy or T4 administration does not modify somatostatin levels in the hypophyseal portal blood (73), but hypothalamic tissue from hypothyroid rats is twice as potent as normal tissue in inactivating somatostatin (74). Both normal (25) and increased (22) sensitivity to the suppressive effect of somatostatin on GH secretion has been reported in cultured pituitary cells from hypothyroid rats. A decrease in hypothalamic somatostatin content has been reported in hypothyroid rats by some workers (75) but not by others (21, 76, 77). T3 causes the acute release of somatostatin from the perfused rat hypothalamus (75). However, in foetal rat neuronal cultures, T3 either decreases somatostatin synthesis and release (78), or has a biphasic effect on somatostatin content. At low doses it increases somatostatin content and at high doses it decreases somatostatin content (79). More recently, no differences in hypothalamic somatostatin gene expression have been reported, either in hypo- or in hyperthyroid rats (77).
GH-BINDING PROTEINS, IGF-BINDING PROTEINS AND THYROID HORMONE Specific GH binding proteins (GH-BP) have been recently discovered in plasma (64). The high affinity component of GH-BP is a fragment of the GH receptor (65). The GH-BPs act to prolong the biological half life of GH, dampen the oscillations of plasma GH levels caused by episodic pituitary secretion (66), and may have other as yet unknown effects on GH action. GH-BP correlates positively with the thyroid status. It has been observed recently that GH-BP is decreased in hypo- and increased in hyperthyroid patients (67). GH-BP is a function of circulating GH (68). Thus, the GH deficiency of hypothyroidism may contribute towards GH-BP deficiency in analogy with the low GH-BP levels observed in GH-deficient patients (69). Another possibility is that thyroid hormones influence GH-BP generation and degradation. At present, however, there are no data substantiating this hypothesis. There are a family of IGF binding proteins (IGF-BPs) (70). These bind IGFs in the circulation and are believed to modulate the availability of the IGFs to their target cells. IGF-I carrier protein has been measured in early reports in hypothyroid rats (52) and humans (71) by means of competitive binding assays. In these studies IGF-I carrier protein paralleled IGF-I changes induced by hypothyroidism. However at least 5 IGF-BPs have been isolated in humans more recently (72), each of which pos-
GHRH Hypothalamic GHRH content has been reported to be decreased in hypothyroid rats, and cultured pituitary cells from hypothyroid rats have decreased sensitivity to GHRH (22). Anterior pituitary cells from hypothyroid rats secrete less GH in response to GHRH than anterior pituitary cells from euthyroid rats (22, 23, 26, 80). The GH releasing activity of GHRH can be partially, but not completely, restored
315
R, Va lea vi, M. Zini and I. Portioli
by physiological concentrations of T4 in vitro (80), However, in vitro treatment of rat anterior pituitary cells with T3 strongly enhances the GH response to synthetic GHRH (81), In contrast, GHRH and GHRH-mRNA levels are decreased in hypothalamic fragments from rats injected with pharmacological doses of thyroxine (77), It is likely that these changes reflect a feedback response to the changes in GH pituitary content. This is indicated by the observation that thyroidectomized rats have an increase in hypothalamic GHRH mRNA levels concomitant with a decrease in peptide content (77), The elevated GHRH mRNA levels are decreased by short term thyroxine administration (82), However comparable effects on GHRH mRNA levels are observed by rat GH treatment alone, suggesting that the changes in hypothalamic GHRH gene expression after thyroidectomy are due to the GH deficiency caused by thyroidectomy (82), It is reasonable to speculate that thyroid hormone also modulates receptors for GHRH on the somatotroph cells, This is analogous to the regulation of TRH receptors by T3 (20, 83, 84), Hence hypothyroidism may cause a decrease in the number of somatotroph GHRH receptors, Secondly, the decrease in hypothalamic GHRH content (22) could also result in reduced GHRH delivery to the pituitary, which could contribute to changes in somatotroph GHRH receptors,
TRH 200
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I
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o _30
o
15 30 45 60 Time
90
120
(min)
Fig, 2 - Effect of oral T3 administration (1. 5 il9/k9 b. w. daily for eight days) on mean ± 5EM GH responses to i, v. TRH (20.0. il9) in 10. patients with anorexia nervosa and low serum T3 levels, The shaded area represents the mean GH response ± 250 to i. v. TRH (20.0. Ilg) in 10. matched, normal control subjects. TRH (e-e) and placebo (A-A) before T3 administratIOn. TRH (0-0) and placebo (6-6) following T3 administration. *p < 0.0.5, **p < 0.0.2, ***p < 0..0.1 VS T3 therapy T3 treatment produced a significant reduction in basal serum GH and in the GH responses to TRH
hibit GH release by direct effects on somatotrophs since, as has been reported recently, T3 markedly reduces the GH response to TRH in perfused chicken hemipituitary glands (92),
TRH In hypothyroid rats TRH-induced GH secretion is markedly enhanced both in vivo and in vitro (85, 86), The latter finding probably reflects increased expression of TRH receptors on the somatotroph membranes as a consequence of the lack of thyroid hormone (83, 87), Patients with primary hypothyroidism may release GH in response to i,v, TRH (88), TRH does not usually stimulate GH secretion in normal humans (89); some pathophysiologic conditions, however, are associated with GH-releasing activity of TRH, These include anorexia nervosa, insulin dependent diabetes mellitus and chronic renal failure (89), all of which are also recognized as "low T3" states (90), We have shown that in patients with anorexia nervosa and "low T3" nonthyroidal illness, T3 administration produced a significant reduction in basal serum GH and in GH responses to TRH (91), suggesting that in humans low thyroid hormone concentrations may unmask the activity of TRH as a GH secretagogue (Fig, 2), The suppression of GH secretion by T3 may be mediated at the hypothalamic level by an increase in somatostatin release (75), T3 is also likely to in-
VIP
Pituitary VIP concentrations are markedly increased in hypothyroid rats and this is reversed by administration of physiological doses of T4 (93), It is well established that VIP plays an important role in PRL regulation (94) but there is no firm evidence that VIP is also involved in the neural control of GH secretion, Administration of VIP increases plasma GH levels in patients with acromegaly but not in normal subjects (95), The stimulatory effect may be mediated via GHRH receptors since VIP is closely related to GHRH structurally (89), In a group of hypothyroid patients we did not note GH release after i,v, infusion of VIP, either before treatment or after T4 administration (Fig. 3) It is unlikely, therefore, that VIP has a role in the altered neural control of GH in patients with thyroid diseases, GHRP-6 Recently a synthetic hexapeptide (so called GHRP6) has been developed (96), There may be an en-
316
Thyroid hormones and GH secretion
dogenous peptide with a similar structure. GHRP6 releases GH in a dose-related and specific manner (97) and its action is independent of GHRH. The combined effects of GHRH and GHRP-6 on GH release are additive at maximal concentrations (9698). Somatotroph responsiveness to GHRP-6 is reduced in hypothyroid rats both in vivo and in vitro (25) and the doses of GHRP-6 that are needed to obtain maximal GH responses are much greater than the GHRH doses required for this purpose. As yet, there are no data available on the effects of GHRP-6 on GH secretion in humans with thyroid dysfunction.
80 70 60
n
l
10 normal subjects • 1'
REVIEW ARTICLE
Thyroid hormones and growth hormone secretion R. Valcavi, M. Zini, and I. Portioli 2a Divisione di Medicina Interna e Sezione Endocrino Metabolica, Arcispedale S. Maria Nuova, Reggio Emilia, Italy
INTRODUCTION
THYROID HORMONE REGULA TlON OF GH GENE EXPRESSION
The fundamental role of thyroid hormones in postnatal growth and development of the mammalian organism is well recognized. In both man and experimental animals, thyroid hormone deficiency is associated with an impairment in the somatic growth. The underlying mechanisms by which thyroid hormones influence growth processes, however, are not well defined. Thyroid hormones stimulate growth with direct effects on induction of specific growth factors, and indirectly through the control of generation and secretion of growth hormone (GH) by the anterior pituitary gland.
In somatotroph tumour cell lines T3 stimulates GH mRNA accumulation (15-19) and increases the rate of GH gene transcription (20-22), Stimulation of the expression of GH gene by thyroid hormone has also been described in cultured rat anterior pituitary cells (23), The effects of thyroid hormone on GH gene expression and GH synthesis vary significantly among mammalian species. In bovine anterior pituitary cells T3 fails to augment basal and GHRH-stimulated GH secretion (24) and cultured human foetal pituitary cells supplemented with T3 show reduced basal GH release and attenuated responses to GHRH (25). Also in human pituitary tumour cells from patients with acromegaly T3 diminishes GH secretion (26), Furthermore, when transfected into rat pituitary cells, human GH gene expression is reduced by T3 administration (27) and the human GH gene promoter is insensitive to thyroid hormone, contrary to rat GH and bovine GH promoters which are induced by T3 (28). These data indicate that, although the most prominent action of thyroid hormone on GH secretion at the somatotroph level is to promote the transcription of GH gene and hence synthesis and accumulation of GH, there is potential heterogeneity in hormonal responses of the same gene in different species.
ACTIONS OF THYROID HORMONE ON CELLULAR GROWTH AND DEVELOPMENT Thyroid hormone is an absolute requirement for normal growth and development of many tissues (1-3). The nervous system is particularly sensitive to this hormonal effect (4-6). Thyroid hormone stimulates cell replication (7) and cell differentiation (8). These growth effects may be related to induction, directly or mediated by GH, of several growth factors (see below), including the production of a growth factor that regulates growth in an autocrine fashion (9). Indeed there is evidence that growth of somatotrophs themselves may be regulated by thyroid hormone. The anterior pituitary of hypothyroid rats reduces somatotrophs by 70% (10). When hypothyroid rats are injected with T3, a marked increase in the DNA synthesis of the somatotrophs occurs after 2-5 days (11 ) and the normal somatotroph population is restored within 5-10 days (12). In GC cells, a cell line derived from a rat somatotrophic tumour, T3 stimulates growth by induction of specific regulatory protein(s) during the early G1 period (13,14).
EFFECTS OF THYROID HORMONE ON PITUITARY GH SYNTHESIS AND SECRETION Thyroid hormone plays a critical role in stimulation of pituitary GH synthesis and secretion, The levels of radioimmunoassayable pituitary and circulating GH are extremely low in hypothyroid animals (29-32), In the rat, pituitary GH content drops to 3% of the normal value within 2 weeks following thyroidectomy (32), becoming undetectable by 24 days (30), Thyroid hormone administration at physiological doses produces a rapid increase in pituitary GH content (30, 32), and a subsequent increase in plas-
Key-words: Thyroid hormones, growth hormone, growth suhst;.:mcAs
Correspondence' Dr. Roberto Valcavi, 2a Divisione di Medlcina Interna e Sezione Endocrino Metabolica, Arcispedale S. Marla Nuova, V.le Umberto I 50, 42100 Reggio Emilia, Italy.
313
R. Va/cavi, M. Zini and /. Portio/i
ma GH concentration several hours later (32). Pituitary GH accumulates in response to i.v. pulse injections of T3 in hypothyroid rats (33). However, increases in pituitary GH content above normal can not be elicited when pharmacological doses of T3 are administered to euthyroid rats (33). Treatment of hypothyroid rat anterior pituitary cell cultures with T3 markedly increases intracellular GH content and increases GH secretion (34). As noted in vivo, the production of GH by pituitary cells does not increase above normal when supraphysiological concentrations of T3 are administered (34). In vivo studies in animals indicate that hypothyroidism also impairs stimulated GH secretion. Thyroid hormone deprivation suppresses GH responses to clonidine (35), GHRH (35-38) and GHRP-6 (39). Small doses of T4 maintain the in vivo but not the in vitro GH responses to GHRH (37). When rats are rendered hyperthyroid by the administration of thyroid hormone GH responses to GHRH are not enhanced (37, 38, 40). Hypothyroid patients show clearly impaired GH responses to GHRH, but, in contrast to the hyperthyroid rat, thyrotoxic patients have a blunted GH response to GHRH (Fig. 1). There is, however, a difference between hypo- and hyperthyroid subjects in their GH responses to GHRH. Hyperthyroid patients have a more sustained and delayed response to GHRH than hypothyroid patients. The abnormalities in hyperthyroidism are due to alterations in the hypothalamic control of GH rather than to pituitary abnormalities (see the following paragraphs).
25 GHRH 100 J.lg i.v.
20 -j
1
::::J 15
"'J'
3. :r:
G
10
5
o
r----30
---"r---,---.---.--..----.--~-__,-~
o
30
60
90
120
time (minutes) o---a'
nOr/no/ subjeels (n = 30)
_ ' hypo/hyro/d pol/en/s (n = 25)
x----x 'hyperthyroId pa/lents (n
= 38)
Fig. 1 - Growth hormone responses to GHRH (1-44) in patients with hypo- and hyperthyroidism. as compared to normal control subjects. Mean ± SEM In both instances the GH responses were subnormal, as assessed by ANOVA However the profile of the curves was consistently different, in that hyperthyroid patients had a delayed serum GH peak.
interact with the GH - IGF-I axis. T4 treatment of hypophysectomized (50) or thyroidectomized (51) rats increases serum Sm-C/IGF-I activity in the absence of GH. However both hormones are required for maximal stimulation (50), and have additive effects on IGF-1 production. Thus administration of GH to hypothyroid rats (52) and humans (53) does not completely rectify growth impairment unless T4 is administered simultaneously. It has been suggested that thyroid hormone regulates the in vivo expression of hepatic IGF-I mRNA, since hepatic IGF-I mRNA levels are suppressed in hypothyroid rats (54). However, this observation does not establish whether thyroid hormone has a direct effect on hepatocytes or acts by stimulating pituitary GH secretion. The latter is suggested by recent data showing that T4 replacement of hypophysectomized GH-deficient rats did not increase hepatic IGF-I mRNA or circulating IGF-I. A supraphysiological dose of T3 did increase hepatic IGF-I mRNA very slightly (55) Only when a single injection of T3 was combined with GH administration, were IGF mRNA levels greater than those of rats injected with GH alone (55).
THYROID HORMONE INFLUENCE ON GROWTH FACTORS Thyroid hormone may regulate several growth factors. Thyroxine administration increases brain Nerve Growth Factor (NGF) content (41,42). It also stimulates Epidermal Growth Factor (EGF) mRNA synthesis in mouse kidney and submandibular gland (43), and causes release of erythropoietic growth factor from leukocytes (44). Somatomedin C/lnsulin-like growth factor-1 (IGF-I) mediates, at least in part, the somatic effects of GH (45). The synthesis of somatomedin C/IGF-1 is regulated by many factors. Growth hormone is the most well known (46), but insulin (47), nutrient status (48), and thyroid hormone are also important. It is likely that thyroid hormone enhances cartilage growth both through enhancing the actions IGF-I and by directly accelerating the differentiation of developing chondrocytes (49). Some of the growth promoting effects of thyroid hormone appear to be the result of their ability to
314
Thyroid hormones and GH secretion
sesses enhancing or inhibitory actions on IGF biological effects. Further data on the regulatory role of thyroid hormone on IGF-BPs are required.
The observation that thyroid hormone stimulates growth minimally, if at all, in the absence of GH (56, 57) confirms the view that thyroid hormones themselves have relatively small effects on IGF-I synthesis and secretion. However direct effects of thyroid hormone on IGF-I production have been demonstrated in certain experimental models, such as foetal hypothalamic culture cells (58) and perfused rat liver (59). Although the primary source of circulating IGF-I appears to be the liver (60), IGF-I production has also been documented in several other tissues and cells (61), including anterior pituitary tissue (62). This suggests that IGF-I may act also in an autocrine or paracrine fashion. Local pituitary IGF synthesis and secretion may be controlled by the thyroid hormone, since it has been shown that thyroid hormone directly regulates IGF-I gene expression in GH3 rat pituitary cells (63). These data indicate that thyroid hormone exerts, at least in part, its growth promoting effects through a permissive role in IGF-I synthesis and secretion.
INFLUENCES ON GH NEUROREGULA TlON BY THYROID HORMONE In addition to their direct effects on GH synthesis in the pituitary, thyroid hormones might influence the neural control of GH secretion. This would be modulated by the release of GHRH and somatostatin from the hypothalamus. Furthermore, a complex network of neuropeptides, neurotransmitters and feedback signals interact with GHRH and somatostatin, thus contributing to the fine-tuning modulation of GH secretion. Thyroid hormones interact at several levels in this network. NEUROPEPTIDES Somatostatin The concept that thyroid hormone has a primary effect on hypothalamic somatostatin is highly controversial. Thyroidectomy or T4 administration does not modify somatostatin levels in the hypophyseal portal blood (73), but hypothalamic tissue from hypothyroid rats is twice as potent as normal tissue in inactivating somatostatin (74). Both normal (25) and increased (22) sensitivity to the suppressive effect of somatostatin on GH secretion has been reported in cultured pituitary cells from hypothyroid rats. A decrease in hypothalamic somatostatin content has been reported in hypothyroid rats by some workers (75) but not by others (21, 76, 77). T3 causes the acute release of somatostatin from the perfused rat hypothalamus (75). However, in foetal rat neuronal cultures, T3 either decreases somatostatin synthesis and release (78), or has a biphasic effect on somatostatin content. At low doses it increases somatostatin content and at high doses it decreases somatostatin content (79). More recently, no differences in hypothalamic somatostatin gene expression have been reported, either in hypo- or in hyperthyroid rats (77).
GH-BINDING PROTEINS, IGF-BINDING PROTEINS AND THYROID HORMONE Specific GH binding proteins (GH-BP) have been recently discovered in plasma (64). The high affinity component of GH-BP is a fragment of the GH receptor (65). The GH-BPs act to prolong the biological half life of GH, dampen the oscillations of plasma GH levels caused by episodic pituitary secretion (66), and may have other as yet unknown effects on GH action. GH-BP correlates positively with the thyroid status. It has been observed recently that GH-BP is decreased in hypo- and increased in hyperthyroid patients (67). GH-BP is a function of circulating GH (68). Thus, the GH deficiency of hypothyroidism may contribute towards GH-BP deficiency in analogy with the low GH-BP levels observed in GH-deficient patients (69). Another possibility is that thyroid hormones influence GH-BP generation and degradation. At present, however, there are no data substantiating this hypothesis. There are a family of IGF binding proteins (IGF-BPs) (70). These bind IGFs in the circulation and are believed to modulate the availability of the IGFs to their target cells. IGF-I carrier protein has been measured in early reports in hypothyroid rats (52) and humans (71) by means of competitive binding assays. In these studies IGF-I carrier protein paralleled IGF-I changes induced by hypothyroidism. However at least 5 IGF-BPs have been isolated in humans more recently (72), each of which pos-
GHRH Hypothalamic GHRH content has been reported to be decreased in hypothyroid rats, and cultured pituitary cells from hypothyroid rats have decreased sensitivity to GHRH (22). Anterior pituitary cells from hypothyroid rats secrete less GH in response to GHRH than anterior pituitary cells from euthyroid rats (22, 23, 26, 80). The GH releasing activity of GHRH can be partially, but not completely, restored
315
R, Va lea vi, M. Zini and I. Portioli
by physiological concentrations of T4 in vitro (80), However, in vitro treatment of rat anterior pituitary cells with T3 strongly enhances the GH response to synthetic GHRH (81), In contrast, GHRH and GHRH-mRNA levels are decreased in hypothalamic fragments from rats injected with pharmacological doses of thyroxine (77), It is likely that these changes reflect a feedback response to the changes in GH pituitary content. This is indicated by the observation that thyroidectomized rats have an increase in hypothalamic GHRH mRNA levels concomitant with a decrease in peptide content (77), The elevated GHRH mRNA levels are decreased by short term thyroxine administration (82), However comparable effects on GHRH mRNA levels are observed by rat GH treatment alone, suggesting that the changes in hypothalamic GHRH gene expression after thyroidectomy are due to the GH deficiency caused by thyroidectomy (82), It is reasonable to speculate that thyroid hormone also modulates receptors for GHRH on the somatotroph cells, This is analogous to the regulation of TRH receptors by T3 (20, 83, 84), Hence hypothyroidism may cause a decrease in the number of somatotroph GHRH receptors, Secondly, the decrease in hypothalamic GHRH content (22) could also result in reduced GHRH delivery to the pituitary, which could contribute to changes in somatotroph GHRH receptors,
TRH 200
+ *** . . ***
10
"0>
8
~
6
:I: Co?
4
**_
*
*
~I . _ _~ . ~--l----t~ _____ ~_______ Q T _____ ~
2
.09 i. v,
I
• -
o _30
o
15 30 45 60 Time
90
120
(min)
Fig, 2 - Effect of oral T3 administration (1. 5 il9/k9 b. w. daily for eight days) on mean ± 5EM GH responses to i, v. TRH (20.0. il9) in 10. patients with anorexia nervosa and low serum T3 levels, The shaded area represents the mean GH response ± 250 to i. v. TRH (20.0. Ilg) in 10. matched, normal control subjects. TRH (e-e) and placebo (A-A) before T3 administratIOn. TRH (0-0) and placebo (6-6) following T3 administration. *p < 0.0.5, **p < 0.0.2, ***p < 0..0.1 VS T3 therapy T3 treatment produced a significant reduction in basal serum GH and in the GH responses to TRH
hibit GH release by direct effects on somatotrophs since, as has been reported recently, T3 markedly reduces the GH response to TRH in perfused chicken hemipituitary glands (92),
TRH In hypothyroid rats TRH-induced GH secretion is markedly enhanced both in vivo and in vitro (85, 86), The latter finding probably reflects increased expression of TRH receptors on the somatotroph membranes as a consequence of the lack of thyroid hormone (83, 87), Patients with primary hypothyroidism may release GH in response to i,v, TRH (88), TRH does not usually stimulate GH secretion in normal humans (89); some pathophysiologic conditions, however, are associated with GH-releasing activity of TRH, These include anorexia nervosa, insulin dependent diabetes mellitus and chronic renal failure (89), all of which are also recognized as "low T3" states (90), We have shown that in patients with anorexia nervosa and "low T3" nonthyroidal illness, T3 administration produced a significant reduction in basal serum GH and in GH responses to TRH (91), suggesting that in humans low thyroid hormone concentrations may unmask the activity of TRH as a GH secretagogue (Fig, 2), The suppression of GH secretion by T3 may be mediated at the hypothalamic level by an increase in somatostatin release (75), T3 is also likely to in-
VIP
Pituitary VIP concentrations are markedly increased in hypothyroid rats and this is reversed by administration of physiological doses of T4 (93), It is well established that VIP plays an important role in PRL regulation (94) but there is no firm evidence that VIP is also involved in the neural control of GH secretion, Administration of VIP increases plasma GH levels in patients with acromegaly but not in normal subjects (95), The stimulatory effect may be mediated via GHRH receptors since VIP is closely related to GHRH structurally (89), In a group of hypothyroid patients we did not note GH release after i,v, infusion of VIP, either before treatment or after T4 administration (Fig. 3) It is unlikely, therefore, that VIP has a role in the altered neural control of GH in patients with thyroid diseases, GHRP-6 Recently a synthetic hexapeptide (so called GHRP6) has been developed (96), There may be an en-
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Thyroid hormones and GH secretion
dogenous peptide with a similar structure. GHRP6 releases GH in a dose-related and specific manner (97) and its action is independent of GHRH. The combined effects of GHRH and GHRP-6 on GH release are additive at maximal concentrations (9698). Somatotroph responsiveness to GHRP-6 is reduced in hypothyroid rats both in vivo and in vitro (25) and the doses of GHRP-6 that are needed to obtain maximal GH responses are much greater than the GHRH doses required for this purpose. As yet, there are no data available on the effects of GHRP-6 on GH secretion in humans with thyroid dysfunction.
80 70 60
n
l
10 normal subjects • 1'
