help button home button Endocrine Society Endocrinology
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
Endocrinology, doi:10.1210/en.2007-0946
This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Right arrow Citation Map
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow Request Copyright Permission
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Luque, R. M.
Right arrow Articles by Kineman, R. D.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Luque, R. M.
Right arrow Articles by Kineman, R. D.
Endocrinology Vol. 148, No. 12 5998-6006
Copyright © 2007 by The Endocrine Society

Gender-Dependent Role of Endogenous Somatostatin in Regulating Growth Hormone-Axis Function in Mice

Raul M. Luque and Rhonda D. Kineman

Department of Medicine, Section of Endocrinology, Diabetes and Metabolism, University of Illinois at Chicago and the Research and Development Division, Jesse Brown Veterans Affairs Medical Center, Chicago, Illinois 60612

Address all correspondence and requests for reprints to: Rhonda D. Kineman, Ph.D., Jesse Brown Veterans Affairs Medical Center, Research and Development Division, M.P 151, West Side, Suite 6215, 820 South Damen Avenue, Chicago, Illinois 60612. E-mail: Kineman{at}uic.edu.


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results and Discussion
 References
 
It has been previously reported that male and female somatostatin (SST) knockout mice (Sst–/–) release more GH, compared with Sst+/+ mice, due to enhanced GH-secretory vesicle release. Endogenous SST may also regulate GH secretion by directly inhibiting GHRH-stimulated GH gene expression and/or by modulating hypothalamic GHRH input. To begin to explore these possibilities and to learn more about the gender-dependent role of SST in modulating GH-axis function, hypothalamic, pituitary, and liver components of the GH-axis were compared in male and female Sst+/+ and Sst–/– mice. Pituitary mRNA levels for GH and receptors for GHRH and ghrelin were increased in female Sst–/– mice, compared with Sst+/+ controls, and these changes were reflected by an increase in circulating GH and IGF-I. Elevated levels of IGF-I in female Sst–/– mice were associated with elevated hepatic mRNA levels for IGF-I, as well as for GH and prolactin receptors. Consistent with the role of GH/IGF-I in negative feedback regulation of hypothalamic function, GHRH mRNA levels were reduced in female Sst–/– mice, whereas cortistatin (CST) mRNA levels were unaltered. In contrast to the widespread impact of SST loss on GH-axis function in females, only circulating GH, hypothalamic CST, and hepatic prolactin receptor expression were up-regulated in Sst–/– male mice, compared with Sst+/+ controls. These results confirm and extend the sexually dimorphic role of SST on GH-axis regulation, and suggest that CST, a neuropeptide that acts through SST receptors to inhibit GH secretion, may serve a compensatory role in maintaining GH-axis function in Sst–/– male mice.


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results and Discussion
 References
 
SOMATOSTATIN (SST) WORKS at the level of the pituitary to inhibit GHRH-stimulated GH synthesis and release, as well as basal GH secretion, depending on the species studied and the dose tested (1, 2). SST may also mediate GH output by acting as a classical neurotransmitter to modulate GHRH neuronal activity (3, 4, 5, 6, 7, 8, 9). The reciprocal effects of SST and GHRH on pituitary GH output are thought to be responsible for the pulsatile nature of GH release, which is gender dependent (10, 11). GH release in males tends to be highly organized, with high amplitude pulses and low baseline values. In contrast, GH release in females is more disorganized, with increased pulse frequency and elevated baseline values. It is believed that females have less SST output from the hypothalamus, allowing for an increase in baseline GH release. This theory is based in part on reports showing that female rats and mice have lower SST mRNA levels in the periventricular nucleus and lower immunodetectable SST in the median eminence, compared with males (12, 13, 14, 15). The importance of SST in maintaining gender-dependent GH pulse patterns and the importance of the pulse pattern on subsequent GH actions are illustrated by the observation that baseline GH values are increased in male SST knockout mice (Sst–/–), and this change is associated with a shift in GH-dependent hepatic gene expression to resemble that of female mice (16). In that same study, the authors report that GH release was also elevated in female Sst–/–, and this change further enhanced the female-dependent pattern of hepatic gene expression. Therefore, endogenous SST is important in both males and females in regulating GH secretion patterns, critical for GH function.

Although it is clear that SST is an important regulator of gender-dependent GH output, questions remain regarding the mechanism by which SST mediates these effects. These questions arise from reports showing that hypothalamic explants from female rats release more SST under basal and K+ stimulated conditions, compared with male explants (14), despite their reported decrease in hypothalamic SST expression [mRNA and peptide (12, 13, 14, 15)]. In addition, pituitary cell cultures from female rats were reported to be more sensitive to the inhibitory actions of SST on GHRH-stimulated GH release, compared with cultures prepared from male rats (17). Given the apparent discrepancies in SST output and function in male and female mice, the following study was conducted to understand better the mechanisms by which endogenous SST mediates gender-dependent GH-axis function. To this end we compared the expression patterns of key hypothalamic, pituitary, and liver components of the GH-axis between male and female wild-type mice (C57Bl/6J), and examined the impact of SST deficiency on these endpoints by comparing the GH-axis of Sst–/– mice with that of their SST intact (Sst+/+) littermates. Given that the loss of SST or its receptors also impacts metabolic hormones (18, 19, 20, 21, 22), we also explored the possibility that gender-dependent differences in metabolic endpoints may indirectly modulate GH-axis function in Sst–/– mice.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results and Discussion
 References
 
Animals
All experimental procedures were approved by the Animal Care and Use Committees of the University of Illinois at Chicago and the Jesse Brown Veterans Affairs Medical Center. Animals were housed under a 12-h light, 12-h dark cycle (lights on 0700 h) and provided food and water ad libitum. Male and female C57Bl/6J mice were purchased from Jackson Laboratory (Bar Harbor, ME) at 8 wk of age. Mice homozygous for the SST-null mutation (Sst–/–) (18), in a C57Bl/6 background, were generously provided by Dr. Ute Hochgeschwender (Oklahoma Medical Research Foundation, Oklahoma City, OK), and these mice were bred to C57Bl/6J mice to establish a breeding colony. Sst+/– mice were crossbred to obtain Sst+/+ and Sst–/– littermates for this study. Genotypes were determined by PCR of tail-snip DNA, using primers and genotyping protocol reported in The Jackson Laboratory web site (http://jaxmice.jax.org/pub-cgi/protocols/protocols.sh?objtype=protocol&protocol_id=210). All mice were handled daily, at least 1 wk before being killed, to acclimatize them to personnel and handling procedures. At 10 wk of age, mice were weighed and killed by decapitation (0800–1100 h, under fed conditions), without anesthesia, and blood, pituitary, hypothalamus, and liver were collected for further analysis. In addition, sc fat (abdominal and inguinal fat pads) and ip fat (urogenital fat pad) were dissected, and weights recorded.

Assessment of circulating hormones
Trunk blood was mixed with 15 µl MiniProtease inhibitor (Roche, Nutley, NJ) and placed on ice until centrifugation at 13,000 rpm, for 10 min. Plasma was stored at –80 C, until evaluation for GH (Diagnostic Systems Laboratories, Inc., mouse/rat ELISA; Webster, TX), IGF-I (Immunodiagnostic Systems Ltd. Octeia rat/mouse ELISA; Fountain Hills, AZ), or IGF-I 100T RIA Kit (Nichols Institute Diagnostics, San Clemente, CA), corticosterone (IDS Octeia rat/mouse ELISA) and insulin (LINCO rat/mouse ELISA; St. Charles, MO). An aliquot of whole blood was also used to assess glucose levels using a SureStep Glucometer (Johnson & Johnson, Milpitas, CA).

Tissue processing for mRNA analysis by quantitative real-time (qrt) RT-PCR
Hypothalami, pituitaries, and livers were collected and frozen in liquid nitrogen and stored at –80 C, until analysis of mRNA levels by qrt RT-PCR. Tissues were processed for recovery of total RNA using the Absolutely RNA RT-PCR Miniprep Kit (Stratagene, La Jolla, CA), with deoxyribonuclease treatment as previously described (23, 24). The amount of RNA recovered was determined using the Ribogreen RNA quantification kit (Molecular Probes, Eugene, OR). Total RNA (1 µg) was reversed transcribed (RT), in a 20-µl vol using random hexamer primers, with enzyme and buffers supplied in the cDNA First Strand Synthesis kit (MRI Fermentas, Hanover, MD). cDNA was treated with RNAseH (MRI Fermentas), and duplicate aliquots (1 µl) were amplified by qrt RT-PCR, in which samples were run against synthetic standards to estimate mRNA copy number. Details regarding the development, validation, and application of a qrt RT-PCR to measure expression levels of mouse GH, GHRH receptor (GHRH-R), ghrelin receptor (GHS-R), GHRH, SST, and cyclophilin A (used as a housekeeping gene) have been previously reported (23, 24). Specific primer sequences, GenBank accession numbers, and product sizes for mouse cortistatin (CST), IGF-I, GH receptor (GH-R), and prolactin receptor (PRL-R) also used in this study were as follows: CST (forward 5'-AAGAGACCCTCGTCCACCAA-3' and reverse 5'-ACCAGGCAAGGAAAGTCAGAAG-3', NM_007745, 213 bp); IGF-I (forward 5'-TCGTCTTCACACCTCTTCTACCT-3' and reverse 5'-ACTCATCCACAATGCCTGTCT-3', NM_010512, 202 bp); GH-R (forward 5'-GATTTTACCCCCAGTCCCAGTTC-3' and reverse 5'-GACCCTTCAGTCTTCTCATCCACA-3', BC075720, 198 bp); and PRL-R (forward 5'-TGGGAGATCCACTTCACAGG-3' and reverse 5'-GGCCACAATGATCCACACA-3', L14811, 189 bp). The thermocycling profile consisted of one cycle of 95 C for 10 min, 40 cycles of 95 C for 30 sec, 61 C for 1 min, and 72 C for 30 sec, followed by a graded temperature dependent dissociation to verify that only one product was amplified. To determine the starting copy number of cDNA, RT samples were PCR amplified, and the signal was compared with that of a standard curve run on the same plate. Standard curves were constructed for all transcripts examined. In addition, total RNA samples that were not reverse transcribed, and a no DNA control were run on each plate to control for genomic DNA contamination and to monitor potential exogenous contamination, respectively. The absolute cDNA copy number for each transcript was adjusted by cyclophilin cDNA copy number, to control for the amount of RNA used in the RT reaction and the efficiency of the RT reaction, in which cyclophilin copy number did not vary between groups within tissue types (data not shown).

Northern blots for GH mRNA
To confirm the impact of gender and genotype on pituitary GH mRNA levels, total pituitary RNA (0.5 µg) was size separated on an agarose/3[N-morpholino]propanesulfonic acid/formaldehyde gel, and transferred to Immobilon-NY membranes (Millipore, Billerica, MA) and hybridized with a probe generated by ATP/CTP-P32 Hexalabel Plus DNA labeling (MRI Fermentas) of a mouse GH cDNA generated by RT-PCR (forward 5'-GGCTGCTGACACCTACAAAGA-3' and reverse 5'-CTTGAGGATCTGCCCAACAC-3', GenBank accession no. NM_008117, 346 bp) using total RNA from a mouse pituitary as a template. The blot was stripped and hybridized with a radiolabeled mouse cyclophilin cDNA probe (forward 5'-GCGTCTCCTTCGAGCTGTTT-3' and reverse 5'-TCTTGCTGGTCTTGCCATTC-3', GenBank accession no. NM_008907, 408 bp).

Pituitary cell cultures
Individual pituitaries of male and female Sst+/+ and Sst–/– mice were dispersed into single cells as previously described (23, 25). Cells were either resuspended in {alpha}-MEM (Invitrogen, Grand Island, NY) and plated on poly-L-coated microscope slides (50,000 cells/ 25 µl/ slide), incubated for 45 min, and then fixed in paraformaldehyde (15 min) for use in immunocytochemical analysis of GH, PRL, and ACTH as previously described (25), or plated at 50,000 cells per well in {alpha}-MEM containing 10% horse serum (Sigma, St. Louis, MO), 0.15% BSA (Sigma), and penicillin-streptomycin (Invitrogen) for culture, as previously reported (23). After a 24-h incubation, cultures were rinsed in fresh warm (37 C) serum-free media and then media were replaced again with serum free media (four wells per pituitary; n = 2 pituitaries per sex per genotype). Cultures were incubated for an additional 4 h, and media were collected to assess basal GH release (Diagnostic Systems Laboratories mouse/rat GH ELISA).

Data presentation and statistical analysis
The mRNA data shown in this report were generated in three separate experiments: 1) male vs. female wild-type mice purchased from Jackson Laboratories; 2) Sst+/+ vs. Sst–/– male mice generated by an in-house breeding colony; and 3) Sst+/+ vs. Sst–/– female mice generated by an in-house breeding colony. Therefore, comparisons were only made between samples processed in parallel, including time of tissue collection, RT, and PCR amplification. Given that this is the first report that compares GH-axis endpoints in male and female mice, means of raw values for mRNA copy numbers of interest (adjusted by cyclophilin mRNA copy numbers) are presented for the wild-type mice to allow for comparison of the expression level of each transcript within tissue type. However, because separate studies were performed comparing these same endpoints in male Sst+/+ and Sst–/– mice and female Sst+/+ and Sst–/– mice bred in house, it is not valid to compare the raw values across gender in these groups. Therefore, we chose to present the data as Sst–/– values compared with Sst+/+ values within gender, in which Sst+/+ values were set at 100. Data were assessed for heterogeneity of variance, and if found, values were log transformed. Differences between groups were assessed by unpaired Student’s t test or two-way ANOVA, followed by Newman-Keuls test for multiple comparisons in the case of GH release from pituitary cell culture, and metabolic hormones and endpoints. All values are expressed as mean ± SEM. All statistical analyses were performed using the GB-STAT software package (Dynamic Microsystems, Inc., Silver Spring, MD).


   Results and Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results and Discussion
References
 
In vivo and in vitro GH output in the presence and absence of endogenous SST
As previously reported (16, 18), mean circulating GH levels calculated from single random samples were significantly increased in both male and female Sst–/– mice, compared with Sst+/+ controls (Fig. 1AGo). Interestingly, the increase in circulating GH levels in Sst–/– females was reflected in a 50% increase in circulating IGF-I levels, compared with Sst+/+ controls, whereas IGF-I levels did not significantly differ between male Sst–/– and Sst+/+ mice (Fig. 1BGo). These gender-dependent effects of SST loss on circulating IGF-I levels are a novel finding, in that Low et al. (16) reported serum IGF-I levels of Sst+/+ and Sst–/– mice did not differ "when the data are collapsed across sex."


Figure 1
View larger version (20K):
[in this window]
[in a new window]

  		FIG. 1. Serum GH (A) and IGF-I (B) levels and basal GH release from primary pituitary cell cultures (C) from male and female Sst+/+ and Sst–/– mice. Circulating hormone levels (A and B) are presented as mean ± SEM of samples taken from four to 24 mice per group. IGF-I values shown (B) were determined by a commercial ELISA kit and confirmed using an RIA kit from another manufacturer (data not shown). In vitro GH release (C) was evaluated in two independent experiments, and data are presented as the mean ± SEM of media GH concentration after a 4-h incubation (three to four wells per experiment). Asterisks (*, P < 0.05; **, P < 0.01) indicated values significantly differs from Sst+/+ controls within gender; a, indicates significant difference between genders (P < 0.05).

 
In vitro, pituitary cell cultures prepared from male Sst+/+ mice did release more GH, compared with cultures prepared from Sst+/+ female mice (Fig. 1CGo), consistent with the larger proportion of somatotropes in male pituitaries, as observed by us and others (26, 27). Interestingly, the enhanced GH release observed in both male and female Sst–/– mice in vivo (Fig. 1AGo) was maintained in vitro (Fig. 1CGo), in which GH release from female Sst–/– cultures achieved levels comparable to those observed in male Sst+/+ cultures. The enhanced GH output in Sst–/– mice could not be accounted for by changes in pituitary size, as assessed by gross morphology and by pituitary cell numbers recovered after enzymatic dispersion (Table 1Go). In addition, differences in GH output between genotypes were not associated with changes in the composition of pituitary cell types, as assessed by the proportion of GH, PRL, and ACTH immunopositive cells (Table 1Go). Together, these data suggest that endogenous SST is not critical for the normal development and expansion of the pituitary somatotrope population but is important in regulating the amount of GH released by individual somatotropes. The fact that enhanced GH release observed in Sst–/– mice is maintained in vitro suggests that the absence of hypothalamic SST input in vivo has permanently changed the internal programming of the somatotrope, and/or it opens the possibility that locally produced SST, as previously reported by others (16, 28, 29, 30, 31), may play a role in controlling GH-secretory vesicle release in vitro. And indeed, we also observed low but detectable levels of SST mRNA in male and female pituitaries from C57Bl/6J mice (males 496 ± 132 and females 186 ± 83 copies per 0.05 µg RNA) and Sst+/+ mice bred in house (males 234 ± 26 and females 204 ± 58 copies per 0.05 µg RNA), whereas SST was not detectable in pituitaries from Sst–/– mice.


View this table:
[in this window]
[in a new window]

  		TABLE 1. Total number of cells recovered after enzymatic dispersion of pituitaries from male and female Sst+/+ and Sst–/– mice, and proportion of immunoreactive GH, PRL, and ACTH cells (n = 4–6 per group)

 
Pituitary phenotype
To explore further the impact of endogenous SST on somatotrope function in the context of a gender-dependent environment, we compared the expression levels of GH, GHRH-R, and GHS-R by qrt RT-PCR in pituitary extracts of wild-type male and female mice. As shown in Fig. 2Go (left panel), GH mRNA levels were greater in males, as compared with females, consistent with the greater proportion of somatotropes (Table 1Go) and the enhanced in vitro GH release (Fig. 1CGo) observed in males, compared with females. In addition, female pituitaries expressed lower levels of GHS-R mRNA than male pituitaries, whereas GHRH-R expression did not differ between genders. In the absence of endogenous SST (Sst–/–), the expression of pituitary GH, GHRH-R, and GHS-R in male mice did not differ from that observed in Sst+/+ controls (Fig. 2Go, middle panel). This observation supports the conclusion of Low et al. (16), that enhanced GH release in Sst–/– mice is due to enhanced GH-secretory, vesicle release. However, in contrast to males, pituitary GH, GHRH-R, and GHS-R mRNA levels in female Sst–/– pituitaries were elevated, compared with their Sst+/+ controls (Fig. 2Go, right panel). The gender-dependent effect of SST knockout on GH mRNA levels was confirmed by Northern blot analysis [males Sst+/+ 1.75 ± 0.17 vs. Sst–/– 1.66 ± 0.09 (not significant) and females Sst+/+ 4.51 ± 0.53 vs. Sst–/– 7.87 ± 0.6 (P < 0.05); values are arbitrary units calculated by adjusting the GH signal by the cyclophilin signal]. These observations extend the work of Low et al. (16), demonstrating that enhanced GH output observed in female Sst–/– mice may not only be due to an increase in GH-secretory vesicle release but can also be attributed to an increase in GH synthesis. In addition, the female-specific change in GH expression may in part be driven by enhanced somatotrope sensitivity to GHRH and ghrelin, via augmentation of their receptor expression.


Figure 2
View larger version (27K):
[in this window]
[in a new window]

  		FIG. 2. Pituitary GH, GHRH-R, and GHS-R mRNA levels in male and female wild-type mice (left panel, n = 7 per sex) and Sst+/+ and Sst–/– littermates (middle and right panels, respectively, n = 4–10 per genotype per sex), as assessed by qrt RT-PCR. Values shown in the left panel are mean ± SEM of absolute mRNA copy numbers (adjusted by cyclophilin mRNA copy number), whereas values shown in the middle and right panels are expressed as percentage of Sst+/+ within gender. *, P < 0.05; **, P < 0.01.

 
Because females are thought to have less hypothalamic SST input (12, 13, 14, 15), leading to higher baseline GH release (10, 11), the question arises as to why somatotrope expression of GH, GHRH-R, and GHS-R in female mice is more sensitive than that of male mice to SST loss. This may in part be explained by gender-dependent pituitary sensitivity to SST, based on a report showing that the EC50 of SST in inhibiting GHRH-stimulated GH release from primary pituitary cell cultures of female rats was 45 times lower than that observed in male pituitary preparations (17), i.e. female somatotropes were more sensitive to SST. It has also been inferred by the examination of GH patterns after the administration of GHRH agonists in humans (32) or GHRH antiserum in rats (33, 34), that elevated baseline GH output in females is due to sustained (tonic) input of GHRH, compared with high pulse, low nadir GHRH input in males. Therefore, we might speculate that when tonic release of GHRH is unopposed by SST in the Sst–/– female mouse, it would allow for an increase in GH, GHRH-R, and GHS-R expression, in which all transcripts have been shown to be directly stimulated by GHRH (24-h exposure) in primary pituitary cell cultures (35, 36, 37, 38). Augmentation of GH, GHRH-R, and GHS-R expression in the female Sst–/– pituitary might also be facilitated by circulating estrogen, in which all transcripts have been directly stimulated by estrogen treatment (39, 40, 41). However, it could also be argued that enhanced IGF-I negative feedback in female Sst–/– mice (Fig. 1BGo) should act to antagonize the stimulatory effects of unopposed GHRH on somatotrope expression (42, 43). This possibility is minimized by the observation that basal GH output in female humans is less sensitive to IGF-I infusion compared with GH output in males (34). Therefore, if like humans the female mouse pituitary is less sensitive to IGF-I negative feedback, the elevation in circulating IGF-I levels observed in the female Sst–/– mouse may not be sufficient to block the stimulatory actions of unopposed GHRH on somatotrope function. Finally, we cannot ignore the possibility that elimination of central SST may have altered the neuronal circuitry in the brain that is critical in maintaining gender-dependent somatotrope function.

Hypothalamic phenotype
To determine if the gender-dependent changes observed in somatotrope function of Sst–/– mice could be attributed to changes in hypothalamic input, expression levels of SST and GHRH were compared between wild-type (C57Bl/6J) male and female mice (Fig. 3Go, left panel), and in male and female littermates with (Sst+/+) and without (Sst–/–) endogenous SST (Fig. 3Go, middle and right panels, respectively). In addition, the impact of gender and genotype on CST expression was also examined, in that CST has been expressed in the hypothalamus of humans and mice (31, 44), and can bind and activate SST receptors, thereby mimicking the actions of SST on GH secretion (45, 46, 47). Hypothalamic expression of SST, GHRH, and CST mRNA did not differ between wild-type male and female mice (Fig. 3Go, left panel), which is consistent with reports examining SST and GHRH expression in whole hypothalamic extracts from mice (9), but differs from others using semiquantitative RT-PCR (16), and reports using in situ hybridization showing that the SST neurons within the periventricular nucleus of male mice (48) and rats (14, 49) express 20–50% higher levels of SST compared with SST neurons of females.


Figure 3
View larger version (29K):
[in this window]
[in a new window]

  		FIG. 3. Hypothalamic SST, GHRH, and CST mRNA levels in male and female wild-type mice (left panel, n = 7 per sex), and Sst+/+ and Sst–/– littermates (middle and right panels, respectively, n = 5–7 per genotype per sex), as assessed by qrt RT-PCR. Values shown in the left panel are mean ± SEM of absolute mRNA copy numbers (adjusted by cyclophilin mRNA copy number), whereas values shown in the middle and right panels are expressed as percentage of Sst+/+ within gender. *, P < 0.05; **, P < 0.01. nd, Not detectable.

 
Nonetheless, SST was undetectable in hypothalamic extracts of male and female Sst–/– mice (Fig. 3Go, middle and right panels, respectively). The absence of endogenous SST did not significantly alter GHRH mRNA levels in males but did significantly suppress GHRH mRNA levels in females (Fig. 3Go, middle and right panels, respectively). This sexually dimorphic response may be due to the fact that female, but not male, Sst–/– mice display elevations in circulating IGF-I (refer to Fig. 1Go), which could act in concert with elevated GH to feedback negatively at the level of the hypothalamus to suppress GHRH expression (50, 51). Although the female pituitary may be less sensitive to the inhibitory actions of IGF-I (as discussed previously), it has also been suggested that hypothalamic negative feedback of IGF-I on GHRH input may be greater in females (34), a theory consistent with the present findings. It is also possible that the variable sensitivity of hypothalamic GHRH expression in male and female Sst–/– mice may be related directly to GHRH neuronal sensitivity to SST input, in that 80% of GHRH neurons in female mice are reported to express the sst2A receptor, compared with 30% of GHRH neurons in male mice (9).

Although SST clearly inhibits GH release at the level of the pituitary, the exact role that intrahypothalamic SST signaling plays in regulating GH release via modulation of GHRH remains controversial. Evidence supporting an inhibitory role of sst2A signaling in GH-negative feedback is based on a report showing that the GHS-R agonist, MK-0677, stimulated c-fos expression in the arcuate nucleus, and this response could be inhibited by GH pretreatment in intact, but not in sst2A null mice (7). However, in that report neither GHRH neuronal activity nor endogenous GH output was examined. In contrast to the putative inhibitory actions of intrahypothalamic SST on GH-axis function, multiple reports support a stimulatory role. Specifically, SST treatment of rat hypothalamic explants with SST enhances GHRH release (6), intracerebroventricular delivery of SST stimulates GH release (3, 4, 5), antiserum against GHRH blocks GH release induced by central SST delivery (5), and intracerebroventricular infusion of sst1, but not sst2, antisense oligonucleotides suppresses GH release (8). These observations, together with our current findings that GHRH mRNA levels are lower in Sst–/– female mice compared with their male counterparts, promote the speculation that hypothalamic SST may in fact be required to maintain optimal GHRH neuronal activity in a gender-dependent fashion.

Although it is clear that much remains to be determined regarding the central role of SST in regulating gender-dependent GH-axis function, our current observation that hypothalamic CST expression is enhanced in Sst–/– male mice, but not Sst–/– female mice (Fig. 3Go), provides a plausible explanation for the relative resistance of the male GH-axis to SST elimination. Because CST has been shown to bind and activate SST receptors and inhibit basal and GHRH-stimulated GH release (45, 46, 47), the increase in CST expression in the Sst–/– male mice may act to compensate for SST loss, thereby attenuating GHRH-stimulated GH, GHRH-R, and GHS-R expression. However, it should be noted that this theoretical compensation by CST was not sufficient to block the increase in circulating GH levels in male Sst–/– mice, compared with Sst+/+ controls (Fig. 1AGo). It should also be mentioned that Low and colleagues (16, 31) stated that they did not observe any effect of gender or genotype (Sst–/– vs. Sst+/+) on hypothalamic CST expression, as measured by semiquantitative RT-PCR. The discrepancy between our current findings and previous reports (16, 31) could be related to differences in mouse strains and techniques used to assess CST mRNA levels and/or the fact that pooled hypothalamic samples were used in the previous report, whereas in the current report, CST mRNA levels were assessed in individual hypothalamic extracts. Nonetheless, our current observations present the possibility that CST, as well as SST, could play a role in gender-dependent regulation of GH-axis function.

Liver phenotype
As previously reported (16, 52, 53), liver PRL-R mRNA levels are greater in females compared with males (Fig. 4Go, left panel), whereas there was no clear gender-dependent effect on IGF-I and GH-R expression. In the absence of endogenous SST, there was a significant increase in the levels of IGF-I, GH-R, and PRL-R mRNA in female Sst–/– mice compared with female Sst+/+ controls. In contrast, only PRL-R mRNA levels were significantly increased in male Sst–/– mice compared with controls. The elevated levels of liver PRL-R mRNA in male Sst–/– mice confirm the work of Low et al. (16), who reported that the lack of endogenous SST "feminized" GH output in males (higher nadir and mean values), leading to the feminization of hepatic PRL-R expression.


Figure 4
View larger version (26K):
[in this window]
[in a new window]

  		FIG. 4. Liver IGF-I, GH-R, and PRL-R mRNA levels in male and female wild-type mice (left panel, n = 5–6 per sex), and Sst+/+ and Sst–/– littermates (middle and right panels, respectively, n = 4–5 per genotype per sex), as assessed by qrt RT-PCR. Values shown in the left panel are mean ± SEM of absolute mRNA copy numbers (adjusted by cyclophilin mRNA copy number), whereas values shown in the middle and right panels are expressed as percentage of Sst+/+ within gender. *, P < 0.05; **, P < 0.01.

 
GH has also been shown to enhance hepatic GH-R expression in a variety of species, including mice (54), in which hepatic GH-R expression is positively correlated with IGF-I mRNA levels (55). These findings are consistent with our current observation that elevated GH and IGF-I levels in Sst–/– females are associated with elevated levels of liver GH-R and IGF-I mRNA (Fig. 4Go). As with GH-regulated PRL-R expression, GH pattern is also critical in regulating hepatic GH-R synthesis, as illustrated by reports showing that continuous, but not intermittent, GH delivery enhances GH-R expression in rats and mice (52, 56). Although technical difficulties in serial blood sampling preclude accurate assessment of GH pulse patterns in mice, the fact that male Sst–/– livers did not respond to enhanced circulating GH levels by augmentation of GH-R and IGF-I expression suggests that the pattern of GH delivery in male and female Sst–/– mice may still differ, and/or a specific gonadal milieu is required to dictate the relative sensitivity of the liver to enhanced GH input. This latter possibility is based on the multiple reports showing that estrogens directly stimulate hepatic GH-R expression (for review, see Ref. 57). Therefore, we speculate that augmentation of GH release, as well as alterations in GH release pattern, in the context of estrogens act to directly enhance hepatic GH-R expression in Sst–/– female mice, which would lead to increased sensitivity to GH input sufficient enough to stimulate IGF-I synthesis.

Finally, we cannot exclude the possibility that the loss of endogenous SST may have a direct influence on hepatic function. This possibility is based on our observations that the mouse liver does express low, but detectable, levels of all SST receptor subtypes (data not shown), consistent with reports of SST receptor expression in rat liver (58, 59, 60). These receptors appear to be functionally relevant in that Murray et al. (60) reported that treatment of primary rat hepatocyte cultures with SST and octreotide blocked GH-induced IGF-I production, and this inhibition was associated with a decrease in GH binding and intracellular signaling. These observations, coupled with our current results showing that GH-R and IGF-I mRNA levels are elevated in the female Sst–/– mouse, suggest that endogenous SST may also serve to directly modulate hepatic sensitivity to GH input in a gender-dependent fashion, by suppressing GH-R synthesis and signaling.

Metabolic phenotype
Endogenous SST not only regulates GH-axis function but also regulates metabolism, based on previous studies showing that corticosterone and total ghrelin levels are elevated in Sst–/– mice, compared with Sst+/+ controls (18, 19), and SST directly inhibits insulin release via sst2A and sst5 (20, 21, 22). Therefore, it is possible that some of the gender-dependent effects of SST loss on GH-axis function may be indirectly mediated by sexually dimorphic changes in metabolic regulation. To begin to test this possibility, we assessed total body weight, sc (abdominal and inguinal) and ip (urogenital) fat depot weight and circulating glucose, insulin, corticosterone, and total ghrelin levels in ad libitum-fed male and female Sst+/+ and Sst–/– mice, and the values are presented in Table 2Go. The loss of SST resulted in a significant increase in sc fat weight and circulating corticosterone and total ghrelin levels, whereas glucose levels were less than Sst+/+ controls, in both males and females. The reduced glucose levels observed in Sst–/– mice would be consistent with enhanced insulin release in the absence of SST, which may account for the increase in fat weight. However, there was no significant effect of genotype on circulating insulin levels under fed conditions. However, it should be noted that more detailed analyses, such as a glucose tolerance test, may be required to truly assess the impact of SST loss on insulin secretion and how such changes would impact body composition. Nonetheless, for the focus of the current study, the fact that male and female Sst–/– mice showed the same directional change in the metabolic endpoints tested indicates that it is unlikely that gender-dependent differences in metabolic endpoints play a major role in the gender-dependent GH-axis phenotype of the Sst–/– mouse.


View this table:
[in this window]
[in a new window]

  		TABLE 2. Body weight, fat weight, glucose, and metabolic hormones in ad libitum-fed male and female Sst+/+ and Sst–/– mice

 
Summary
Our current observations clearly demonstrate that endogenous SST is more critical in female mice, than male mice, in regulating pituitary GH, GHRH-R, and GHS-R expression. These pituitary changes are indeed physiologically relevant, in that they are translated into elevations in circulating GH levels, resulting in alterations in GH-regulated hepatic function, leading to an increase in GH-R/IGF-I expression and circulating IGF-I levels. The enhanced sensitivity of the female GH-axis to SST loss may be related to the gonadal steroid environment that serves to modulate hypothalamic and pituitary sensitivity to GH/IGF-I negative feedback and hepatic sensitivity to GH pulse pattern. It is also possible that the relative insensitivity of the male GH-axis to the loss of endogenous SST may be due in part to a compensatory increase in hypothalamic CST.


   Acknowledgments
 
We thank Dr. Ute Hochgeschwender (Oklahoma Medical Research Foundation, Oklahoma City, OK) who generously provided us with the original Sst–/– mice.


   Footnotes
 
This work was supported by the National Institutes of Health National Institute of Diabetes and Digestive and Kidney Diseases 30677 (to R.D.K.) and the "Secretaria de Universidades, Investigación y Tecnología de la Junta de Andalucia" (to R.M.L.).

Disclosure Statement: The authors have nothing to disclose.

First Published Online August 30, 2007

Abbreviations: CST, Cortistatin; GH-R, GH receptor; GHRH-R, GHRH receptor; GHS-R, ghrelin receptor; PRL-R, prolactin receptor; qrt, quantitative real-time; RT, reverse transcription; SST, somatostatin.

Received July 12, 2007.

Accepted for publication August 21, 2007.


   References
Top
 Abstract
 Introduction
 Materials and Methods
 Results and Discussion
 References
 

  1. Muller EE, Locatelli V, Cocchi D 1999 Neuroendocrine control of growth hormone secretion. Physiol Rev 79:511–607[Abstract/Free Full Text]
  2. Castano JP, Delgado-Niebla E, Duran-Prado M, Luque RM, Sanchez-Hormigo A, Gracia-Navarro F, Garcia-Navarro S, Kineman RD, Malagon MM 2005 New insights in the mechanism by which SRIF influences GH secretion. J Endocrinol Invest 28:10–13[Medline]
  3. Abe H, Kato Y, Iwasaki Y, Chihara K, Imura H 1978 Central effect of somatostatin on the secretion of growth hormone in the anesthetized rat. Proc Soc Exp Biol Med 159:346–349[CrossRef][Medline]
  4. Lumpkin MD, Negro-Vilar A, McCann SM 1981 Paradoxical elevation of growth hormone by intraventricular somatostatin: possible ultrashort-loop feedback. Science 211:1072–1074[Abstract/Free Full Text]
  5. Murakami Y, Kato Y, Kabayama Y, Inoue T, Koshiyama H, Imura H 1987 Involvement of hypothalamic growth hormone (GH)-releasing factor in GH secretion induced by intracerebroventricular injection of somatostatin in rats. Endocrinology 120:311–316[Abstract/Free Full Text]
  6. Pecori Giraldi F, Frohman LA 1995 Discordant effects of endogenous and exogenous somatostatin on growth hormone-releasing hormone secretion from perifused mouse hypothalami. Neuroendocrinology 61:566–572[Medline]
  7. Zheng H, Bailey A, Jiang MH, Honda K, Chen HY, Trumbauer ME, Van Der Ploeg HT, Schaeffer JM, Leng G, Smith RG 1997 Somatostatin receptor subtype 2 knockout mice are refractory to growth hormone-negative feedback on arcuate nucleus. Mol Endocrinol 11:1709–1717[Abstract/Free Full Text]
  8. Lanneau C, Bluet-Pajot MT, Zizzari P, Csaba Z, Dournaud P, Helboe L, Hoyer D, Pellegrini E, Tannenbaum GS, Epelbaum J, Gardette R 2000 Involvement of the Sst1 somatostatin receptor subtype in the intrahypothalamic neuronal network regulating growth hormone secretion: an in vitro and in vivo antisense study. Endocrinology 141:967–979[Abstract/Free Full Text]
  9. Bouyer K, Loudes C, Robinson ICAF, Epelbaum J, Faivre-Bauman A 2006 Sexually dimorphic distribution of sst2A somatostatin receptors on growth hormone-releasing hormone neurons in mice. Endocrinology 147:2670–2674[Abstract/Free Full Text]
  10. Jansson J-O, Eden S, Isaksson O 1985 Sexual dimorphism in the control of growth hormone secretion. Endocr Rev 6:128–150[Abstract/Free Full Text]
  11. Hartman ML, Veldhuis JD, Thorner MO 1993 Normal control of growth hormone secretion. Horm Res 40:37–47[Medline]
  12. Hasegawa O, Sugihara H, Minami S, Wakabayashi I 1992 Masculinization of growth hormone (GH) secretory pattern by dihydrotestosterone is associated with augmentation of hypothalamic somatostatin and GH-releasing hormone mRNA levels in ovariectomized adult rats. Peptides 13:475–481[CrossRef][Medline]
  13. Nurhidayat, Tsukamoto Y, Sigit K, Sasaki F 1999 Sex differentiation of growth hormone-releasing hormone and somatostatin neurons in the mouse hypothalamus: an immunohistochemical and morphological study. Brain Res 821:309–321[CrossRef][Medline]
  14. Murray HE, Simonian SX, Herbison AE, Gillies GE 1999 Correlation of hypothalamic somatostatin mRNA expression and peptide content with secretion: sexual dimorphism and differential regulation by gonadal factors. J Neuroendocrinol 11:27–33[CrossRef][Medline]
  15. Chowen JA, Frago LM, Argente J 2004 The regulation of GH secretion by sex steroids. Eur J Endocrinol 151(Suppl 3):U95–U100
  16. Low MJ, Otero-Corchon V, Parlow AF, Ramirez JL, Kumar U, Patel YC, Rubinstein M 2001 Somatostatin is required for masculinization of growth hormone-regulated hepatic gene expression but not of somatic growth. J Clin Invest 107:1571–1580[Medline]
  17. Kerrigan JR, Martha PM, Krieg RJ, Rogol AD, Evans WS 1989 Somatostatin inhibition of growth hormone secretion by somatotropes from male, female, and androgen receptor-deficient rats: evidence for differing sensitivities. Endocrinology 125:3078–3083[Abstract/Free Full Text]
  18. Zeyda T, Diehl N, Paylor R, Brennan MB, Hochgeschwender U 2001 Impairment in motor learning of somatostatin null mutant mice. Brain Res 906:107–117[CrossRef][Medline]
  19. Luque RM, Gahete MD, Hochgeschwender U, Kineman RD 2006 Evidence that endogenous SST inhibits ACTH and ghrelin expression by independent pathways. Am J Physiol Endocrinol Metab 291:E395–E403
  20. Strowski MZ, Parmar RM, Blake AD, Schaeffer JM 2000 Somatostatin inhibits insulin and glucagon secretion via two receptors subtypes: an in vitro study of pancreatic islets from somatostatin receptor 2 knockout mice. Endocrinology 141:111–117[Abstract/Free Full Text]
  21. Tirone TA, Norman MA, Moldovan S, DeMayo FJ, Wang XP, Brunicardi FC 2003 Pancreatic somatostatin inhibits insulin secretion via SSTR-5 in the isolated perfused mouse pancreas model. Pancreas 26:e67–e73
  22. Strowski MZ, Kohler M, Chen HY, Trumbauer ME, Li Z, Szalkowski D, Gopal-Truter S, Fisher JK, Schaeffer JM, Blake AD, Zhang BB, Wilkinson HA 2003 Somatostatin receptor subtype 5 regulates insulin secretion and glucose homeostasis. Mol Endocrinol 17:93–106[Abstract/Free Full Text]
  23. Luque RM, Kineman RD 2006 Impact of obesity on the growth hormone axis: evidence for a direct inhibitory effect of hyperinsulinemia on pituitary function. Endocrinology 147:2754–2763[Abstract/Free Full Text]
  24. Luque RM, Park S, Kineman RD 2007 Severity of the catabolic condition differentially modulates hypothalamic expression of growth hormone-releasing hormone in the fasted mouse: potential role of neuropeptide Y and corticotropin-releasing hormone. Endocrinology 148:300–309[Abstract/Free Full Text]
  25. Luque RM, Amargo G, Ishii S, Lobe C, Franks R, Kiyokawa H, Kineman RD 2007 Reporter expression, induced by a growth hormone promoter-driven Cre recombinase (rGHp-Cre) transgene, questions the developmental relationship between somatotropes and lactotropes in the adult mouse pituitary gland. Endocrinology 148:1946–1953[Abstract/Free Full Text]
  26. Kineman RD, Aleppo G, Frohman LA 1996 The tyrosine hydroxylase-human growth hormone (GH) transgenic mouse as a model of hypothalamic GH deficiency: growth retardation is the result of a selective reduction in somatotrope numbers despite normal somatotrope function. Endocrinology 137:4630–4636[Medline]
  27. Kuwahara S, Sari DK, Tsukamoto Y, Tanaka S, Sasaki F 2004 Age-related changes in growth hormone (GH)-releasing hormone and somatostatin neurons in the hypothalamus and in GH cells in the anterior pituitary of female mice. Brain Res 1025:113–122[CrossRef][Medline]
  28. Peillon F, Le Dafniet M, Garnier P, Brandi AM, Moyse E, Birman P, Blumberg-Tick J, Grouselle D, Joubert-Bression D 1989 Receptors and neurohormones in human pituitary adenomas. Horm Res 31:13–18[Medline]
  29. Peillon F, Le Dafniet M, Garnier P, Feinstein MC, Donnadieu M, Barret A, Gautron JP, Brandi AM, Benlot C, Lagoguey A 1989 [Neurohormones coming from the normal and tumoral human anterior pituitary. Secretion and regulation in vitro]. Pathol Biol (Paris) 37:840–845 (French)[Medline]
  30. Dalm VAS, van Hagen M, de Krijger RR, Kros JM, Van Koetsveld PM, Van der Lely AJ, Lamberts SW, Hofland LJ 2004 Distribution pattern of somatostatin and cortistatin mRNA in human central and peripheral tissues. Clin Endocrinol (Oxf) 60:625–629[CrossRef][Medline]
  31. Ramirez JL, Mouchantaf R, Kumar U, Otero-Corchon V, Rubinstein M, Low MJ, Patel YC 2002 Brain somatostatin receptors are up-regulated in somatostatin-deficient mice. Mol Endocrinol 16:1951–1963[Abstract/Free Full Text]
  32. Jessup SK, Dimaraki EV, Symons KV, Barkan AL 2003 Sexual dimorphism of growth hormone (GH) regulation in humans: endogenous GH-releasing hormone maintains basal GH in women but not in men. J Clin Endocrinol Metab 88:4776–4780[Abstract/Free Full Text]
  33. Painson J-C, Tannenbaum GS 1991 Sexual dimorphism of somatostatin and growth hormone releasing factor signaling in the control of pulsatile growth hormone secretion in the rat. Endocrinology 128:2858–2866[Abstract/Free Full Text]
  34. Jaffe CA, Ocampo-Lim B, Guo W, Krueger K, Sugahara I, DeMott-Friberg R, Bermann M, Barkan AL 1998 Regulatory mechanisms of growth hormone secretion are sexually dimorphic. J Clin Invest 102:153–164[Medline]
  35. Barinaga M, Yamonoto G, Rivier C, Vale W, Evans R, Rosenfeld MG 1983 Transcriptional regulation of growth hormone gene expression by growth hormone-releasing factor. Nature 306:84–85[CrossRef][Medline]
  36. Lasko CM, Korytko AI, Wehrenberg WB, Cuttler L 2001 Differential GH-releasing hormone regulation of GHRH receptor mRNA expression in the rat pituitary. Am J Physiol Endocrinol Metab 280:E626–E631
  37. Yan M, Hernandez M, Xu R, Chen C 2004 Effect of GHRH and GHRP-2 treatment in vitro on GH secretion and levels of GH, pituitary transcription factor-1, GHRH-receptor, GH-secretagogue-receptor and somatostatin receptor mRNAs in ovine pituitary cells. Eur J Endocrinol 150:235–242[Abstract]
  38. Kineman RD, Luque RM 2007 Evidence that ghrelin is as potent as growth hormone (GH)-releasing hormone (GHRH) in releasing GH from primary pituitary cell cultures of a nonhuman primate (Papio anubis), acting through intracellular signaling pathways distinct from GHRH. Endocrinology 148:4440–4449[Abstract/Free Full Text]
  39. Lloyd RV, Cano M, Landefeld TD 1988 The effects of estrogens on tumor growth and on prolactin and growth hormone mRNA expression in rat pituitary tissues. Am J Pathol 133:397–406[Abstract]
  40. Petersenn S, Rasch AC, Penshorn M, Beil FU 2001 Genomic structure and transcriptional regulation of human growth hormone secretagogue receptor. Endocrinology 142:2649–2659[Abstract/Free Full Text]
  41. Childs GV, Irutayanathan M, Akhter N, Unabia G, Whitehead-Johnson B 2005 Bipotential effects of estrogen on growth hormone synthesis and storage in vitro. Endocrinology 146:1780–1708[Abstract/Free Full Text]
  42. Wallenius K, Sjogren K, Peng X-D, Park S, Wallenius V, Liu J-L, Umaerus M, Wennbo H, Isaksson O, Frohman LA, Kineman RD, Ohlsson C, Jansson J-O 2001 Liver-derived IGF-I regulates GH secretion at the pituitary level in mice. Endocrinology 142:4762–4770[Abstract/Free Full Text]
  43. Luque RM, Gahete MD, Valentine RJ, Kineman RD 2006 Examination of the direct effects of metabolic factors on somatotrope function in a non-human primate model, Papio anubis. J Mol Endocrinol 37:25–38[Abstract/Free Full Text]
  44. Allia E, Tarabra E, Volante M, Cerrato M, Ghigo E, Muccioli G, Papotti M 2005 Expression of cortistatin and MrgX2, a cortistatin receptor, in human neuroendocrine tissues and related tumours. J Pathol 207:336–345[CrossRef][Medline]
  45. Luque RM, Peinado JR, Gracia-Navarro F, Broglio F, Ghigo E, Kineman RD, Malagon MM, Castano JP 2006 Cortistatin mimics somatostatin by inducing a dual, dose-dependent stimulatory and inhibitory effect on growth hormone secretion in somatotropes. J Mol Endocrinol 36:547–556[Abstract/Free Full Text]
  46. Rubinfeld H, Hadani M, Barkai G, Taylor JE, Culler MD, Shimon I 2006 Cortistatin inhibits growth hormone release from human fetal and adenoma pituitary cells and prolactin secretion from cultured prolactinomas. J Clin Endocrinol Metab 91:2257–2263[Abstract/Free Full Text]
  47. Grottoli S, Gasco V, Broglio F, Baldelli R, Ragazzoni F, Gallenca F, Mainolfi A, Prodam F, Muccioli G, Ghigo E 2006 Cortistatin-17 and somatostatin-14 display the same effects on growth hormone, prolactin, and insulin secretion in patients with acromegaly or prolactinoma. J Clin Endocrinol Metab 91:1595–1599[Abstract/Free Full Text]
  48. Bennett E, McGuinness L, Gevers EF, Thomas GB, Robinson ICAF, Davey HW, Luckman SM 2005 Hypothalamic STAT proteins: regulation of somatostatin neurons by growth hormone via STAT5b. J Neuroendocrinol 17:186–194[CrossRef][Medline]
  49. Chowen-Breed JA, Steiner RA, Clifton DK 1989 Sexual dimorphism and testosterone-dependent regulation of somatostatin gene expression in the periventricular nucleus of the rat brain. Endocrinology 125:357–362[Abstract/Free Full Text]
  50. Sato M, Frohman LA 1993 Differential effects of central and peripheral administration of growth hormone (GH) and insulin-like growth factor on hypothalamic GH-releasing hormone and somatostatin gene expression in GH-deficient dwarf rats. Endocrinology 133:793–799[Abstract/Free Full Text]
  51. Ghigo MC, Torsello A, Grilli R, Luoni M, Locatelli V, Muller EE 1997 Effects of GH and IGF-I administration on GHRH and somatostatin mRNA levels: I. A study on ad libitum fed and starved adult male rats. J Endocrinol Invest 20:144–150[Medline]
  52. Norstedt G, Palmiter R 1984 Secretory rhythm of growth hormone regulates sexual differentiation of mouse liver. Cell 36:805–812[CrossRef][Medline]
  53. Robertson JA, Haldosen L-A, Wood TJJ, Steed MK, Gustafsson J-A 1990 Growth hormone pretranslationally regulates the sexually dimorphic expression of the prolactin receptor gene in rat liver. Mol Endocrinol 4:1235–1239[Abstract/Free Full Text]
  54. Talamantes F, Ortiz R 2002 Structure and regulation of expression of the mouse GH receptor. J Endocrinol 175:55–59[Abstract]
  55. Iida K, Del Rincon JP, Kim D-S, Itoh E, Nass R, Coschigano KT, Kopchick JJ, Thorner MO 2004 Tissue-specific regulation of growth hormone (GH) receptor and insulin-like growth factor-I gene expression in the pituitary and liver of GH-deficient (lit/lit) mice and transgenic mice that overexpress bovine GH (bGH) or a bGH antagonist. Endocrinology 145:1564–1570[CrossRef][Medline]
  56. Maiter D, Walker JL, Adam E, Moatsstaats B, Mulumba N, Ketelslegers JM, Underwood LE 1992 Differential regulation by growth hormone (GH) of insulin-like growth factor I and GH receptor/binding protein gene expression in rat liver. Endocrinology 130:3257–3264[Abstract/Free Full Text]
  57. Meinhardt UJ, Ho KK 2007 Regulation of growth hormone action by gonadal steroids. Endocrinol Metab Clin North Am 36:57–73[CrossRef][Medline]
  58. Bruno JF, Xu Y, Song J, Berelowitz M 1993 Tissue distribution of somatostatin receptor subtype messenger ribonucleic acid in the rat. Endocrinology 133:2561–2567[Abstract/Free Full Text]
  59. Song S-H, Leng X-S, Li T, Qin Z-Z, Peng J-R, Zhao L, Wei Y-H, Yu X 2004 Expression of subtypes of somatostatin receptors in hepatic stellate cells. World J Gastroenterol 10:1663–1665[Medline]
  60. Murray RD, Kim K, Ren SG, Chelly M, Umehara Y, Melmed S 2004 Central and peripheral actions of somatostatin on the growth hormone-IGF-I axis. J Clin Invest 114:349–356[CrossRef][Medline]



This article has been cited by other articles:


Home page
 EndocrinologyHome page
R. M. Luque, B. S. Soares, X.-d. Peng, S. Krishnan, J. Cordoba-Chacon, L. A. Frohman, and R. D. Kineman
Use of the Metallothionein Promoter-Human Growth Hormone-Releasing Hormone (GHRH) Mouse to Identify Regulatory Pathways that Suppress Pituitary Somatotrope Hyperplasia and Adenoma Formation due to GHRH-Receptor Hyperactivation
Endocrinology, July 1, 2009; 150(7): 3177 - 3185.
[Abstract] [Full Text] [PDF]

Home page
 DiabetesHome page
G. A. Rutter
Regulating Glucagon Secretion: Somatostatin in the Spotlight
Diabetes, February 1, 2009; 58(2): 299 - 301.
[Full Text] [PDF]

Home page
 J. Clin. Endocrinol. Metab.Home page
J. D. Veldhuis, D. M. Keenan, J. N. Bailey, A. Adeniji, J. M. Miles, R. Paulo, M. Cosma, and C. Soares-Welch
Estradiol Supplementation in Postmenopausal Women Attenuates Suppression of Pulsatile Growth Hormone Secretion by Recombinant Human Insulin-like Growth Factor Type I
J. Clin. Endocrinol. Metab., November 1, 2008; 93(11): 4471 - 4478.
[Abstract] [Full Text] [PDF]

This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Right arrow Citation Map
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow Request Copyright Permission
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Luque, R. M.
Right arrow Articles by Kineman, R. D.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Luque, R. M.
Right arrow Articles by Kineman, R. D.

HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
Endocrinology Endocrine Reviews J. Clin. End. & Metab.
Molecular Endocrinology Recent Prog. Horm. Res. All Endocrine Journals