help button home button Endocrine Society Endocrinology
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow Purchase Article
Right arrow View Shopping Cart
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 Google Scholar
Google Scholar
Right arrow Articles by Niiori-Onishi, A.
Right arrow Articles by Saito, H.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Niiori-Onishi, A.
Right arrow Articles by Saito, H.
Endocrinology Vol. 140, No. 1 344-349
Copyright © 1999 by The Endocrine Society

ARTICLES

Molecular Mechanisms of the Negative Effect of Insulin-Like Growth Factor-I on Growth Hormone Gene Expression in MtT/S Somatotroph Cells

Akiko Niiori-Onishi, Yasumasa Iwasaki, Noriko Mutsuga, Yutaka Oiso, Kinji Inoue and Hidehiko Saito

First Department of Internal Medicine (A.N.-O., M.N., Y.O., H.S.), Department of Clinical Laboratory Medicine (Y.I.), Nagoya University School of Medicine, Nagoya 466-8550; Department of Cell Regulation, Saitama University (K.I.), Saitama 338-0825, Japan

Address all correspondence and requests for reprints to: Yasumasa Iwasaki, M.D., Department of Clinical Laboratory Medicine, Nagoya University School of Medicine, 65 Tsurumai-cho, Showa-ku, Nagoya 466-8550, Japan. E-mail: iwasakiy{at}mb.infoweb.ne.jp


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Although insulin-like growth factor-I (IGF-I) is shown to have a suppressive effect on GH gene expression at the pituitary level, its molecular mechanism has not yet been clarified. To study the issue, we established a new in vitro system using MtT/S, a recently established rat somatotroph tumor cell line that retains the basic characteristics of somatotroph function. Plasmids containing the GH 5' promoter (~1.75 kb or shorter)-luciferase fusion gene were transfected stably or transiently into the cells, and the effect of IGF-I on the GH promoter activity was estimated by a luciferase assay. The results showed that IGF-I inhibited GH promotor activity (more than 50% suppression) in a time- and dose-related manner. IGF-I also inhibited GH secretion. A study using deletion mutants of the GH promoter revealed that the negative effect was maintained in the shortest construct (-80 to +6), suggesting that IGF-I-related factor is acting at the region very close to the minimal promoter. Interestingly, the negative effect was completely eliminated by a PI3 kinase inhibitor wortmannin (1 µM), whereas a MAP kinase inhibitor PD98059 (20 µM) or S6 kinase inhibitor rapamycin (10 nM) did not influence the effect. Our results suggest that IGF-I suppresses GH gene expression at the transcriptional level and that the PI3 kinase-mediated signaling pathway plays a major role in the negative effect of IGF-I. We believe that our system using MtT/S cells is an excellent experimental model system for studying the cellular and molecular mechanisms of the transcriptional regulation of GH in vitro.


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
GH IS SYNTHESIZED and secreted from the somatotroph in the anterior pituitary mainly under hypothalamic control. Among a variety of hypothalamic factors, GH-releasing factor (GRF) and somatostatin (SRIF) are known to be the two major regulatory substances, the former exerting stimulatory and the latter inhibitory effects on GH secretion (1, 2, 3). GRF is shown to be important for GH gene expression as well (1).

In addition to the hypothalamic factors, insulin-like growth factor-I (IGF-I) is supposed to be involved in GH regulation (4). IGF-I is produced mainly in the liver under the control of GH and is supposed to mediate most of the peripheral effects of GH (5, 6). IGF-I also exerts a negative effect on GH expression at both hypothalamic and pituitary levels, forming a long negative feedback loop (i.e. GRF-GH-IGF-I axis) (7, 8, 9, 10). Yamashita and Melmed (11, 12) have shown previously that IGF-I inhibits both GH synthesis and secretion at the anterior pituitary level. However, the precise molecular mechanism of the negative effect of IGF-I on GH regulation, especially on its gene expression, is not entirely clear, probably because a good somatotroph cell line for in vitro experiments has not been available.

MtT/S is a rat somatotroph tumor cell line established recently by Inoue et al. (13). They showed that this MtT/F84-derived cell line retains most of the characteristics of the pure somatotroph. In fact, MtT/S cells are shown to secrete only GH and not PRL. Furthermore, stimulation by GRF and inhibition by IGF-I of GH secretion are maintained in MtT/S (13) but lost or diminished in the GH3 or GC cell lines that have been used for studying GH gene expression (14, 15).

In this study, we established a new experimental in vitro system for studying GH gene expression using the MtT/S cell line. With both transient and stable gene transfer techniques, we showed that the rat GH gene 5' promoter has substantial transcriptional activity in this cell line. We also found that IGF-I potently suppresses gene expression as well as secretion of GH, indicating that this system is suitable for studying the negative regulation of GH synthesis by IGF-I. Furthermore, we showed that wortmannin-sensitive intracellular signaling pathway(s) mediates the negative effect of IGF-I on GH gene transcription.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Materials
Recombinant human IGF-I was obtained from Fujisawa Pharmaceutical Co., Ltd. (Osaka, Japan). Genistein and wortmannin were from Calbiochem (San Diego, CA). PD98059 and rapamycin were from ICN Biomedicals, Inc. (Costa Mesa, CA). Bacitracin were obtained from Sigma Chemical Co. (St. Louis, MO). [125I]IGF-I were from New England Nuclear (Boston, MA).

Cell culture
MtT/S cells were maintained in a T75 culture flask with DMEM/F12 culture medium (Life Technologies, Grand island, NY) supplemented with 10% horse serum (Life Technologies), 2.5% FBS (Life Technologies) and antibiotics (50 µU/ml penicillin and 50 µg/ml streptomycin) under a 5% CO2/95% air atmosphere at 37 C. Culture medium was changed twice a week, and the cells were subcultured once a week. During each experiment, the cells were plated in poly-D-lysine-coated 3.5-cm diameter culture dishes with approximately 70% confluence, using DMEM/F12 culture medium supplemented with or without 10% horse serum.

Plasmid construction
An approximately 1.75 kb (-1734 to +6; +1 indicates the transcription start site) fragment of the rat GH 5' promoter was isolated from a rat GH gene (16). This longest promoter fragment and a series of shorter fragments (see below) were inserted into the multiple cloning site of the pFlash luciferase plasmid (SynapSys, Burlington, MA).

Stable transfection
MtT/S cells were transfected stably with the plasmid containing approximately 1.75 kb GH 5' promoter-luciferase fusion gene using a lipofection method. The cells of 70% confluence in a T75 flask (poly-D-lysine-coated) were incubated for 2 h in serum-free culture medium containing 10 µg of GH-luciferase plasmid, 5 µg of neomycin-selectable marker (pRSV-Neo), and 45 µl of Tfx-50 reagent (Promega Corp., Madison, WI), followed by addition of culture medium with 10% horse serum. On the next day, DNA-containing medium was removed and the cells were incubated with the medium containing 10% horse serum, 2.5% FBS, and G418 (100 µg/ml; Geneticin, Life Technologies). Three to 4 weeks later when discrete colonies became visible, approximately 20 colonies were isolated and subcultured independently. Luciferase activity of the cells of each clone was estimated, and a representative clone with moderate luciferase activity, designated as MtT/SGL (GH-Luciferase), was used for the subsequent experiments.

Experiments
To study the effect of IGF-I or other reagents on GH gene 5'-promoter activity, MtT/SGL cells were plated in 3.5-cm diameter culture dishes with 10% horse serum, as mentioned above. On the next day, test reagents, in 1000x concentration, or solvent alone, were added directly into the culture medium of each dish, and the cells were incubated for the defined time interval. Genistein and wortmannin were dissolved in sterilized distilled water, PD98059 was done so in DMSO, and IGF-I in 1 mM acetic acid with 0.1% BSA. At the end of incubation, the culture medium was removed, and the cells were harvested for the luciferase assay (see below).

Assays
Luciferase assay was performed as previously described (17, 18). Briefly, the culture medium was completely removed at the end of each experiment, and the cells were harvested with lysis buffer containing 1% (vol/vol) Triton X-100 (Sigma Chemical Co., St. Louis, MO), followed by centrifugation at 18,000 x g for 30 min. For the luciferase assay, 100 µl of each supernatant was added to 400 µl of reaction buffer, and the reactions were started by the injection of 200 µl of luciferin solution containing 0.2 mM D-luciferin (Wako, Osaka, Japan). Light output was measured for 20 sec at room temperature using a luminometer (Berthold Lumat LB9501, Bad Wildbad, Germany). ß-galactosidase (ß-gal) activity was determined using commercially available assay kits (Boehringer Mannheim, Mannheim, Germany). Rat GH was measured by double-antibody RIA (13).

Transient transfection
A series of deletion mutants of the rat GH 5' promoter (-320, -250, -200, -150, -115, -80 to +6)-luciferase fusion genes was constructed, and expressed transiently in MtT/S cells. In each experiment, MtT/S cells were plated in 3.5-cm diameter culture dishes and were incubated with serum-free medium containing 2 µg of one of the GH-luciferase constructs, 1 µg of ß-gal expression vector (pRSV-ß-gal) as an internal control, and 6 µl of TransIT polyamine transfection reagent (Pan Vera Corporation, Madison, WI) for 4 h. Then the medium was replaced with serum-containing medium for up to 48 h, during which time experiments using IGF-I were carried out as in those with MtT/SGL cells. The difference in transfection efficiency among the constructs was corrected by ß-gal activity.

RT-PCR
RNA was isolated from the MtT/SGL cells using TRIzol reagent (Life Technologies), and 1 µg of the total RNA was used for the RT reaction with AMV reverse transcriptase (Toyobo, Osaka, Japan). The expression of IGF-I receptor was then analyzed by PCR with Taq DNA polymerase (Toyobo) using a primer set for the rat IGF-I receptor complementary DNA (cDNA) as previously described (19). The RNA sample without reverse transcriptase was also amplified by PCR as a negative control.

Binding studies
Binding of radiolabeled ligand was performed in cell suspension. MtT/SGL cells (2 x 107) were incubated with [125I ]IGF-I (0.2 ng/ml) and increasing concentrations of unlabeled IGF-I or insulin in a final volume of 400 µl HEPES buffer (pH 8.0; 50 mM HEPES, 1% BSA, 120 mM NaCl, 1.2 mM MgSO4, and 1 mg/ml bacitracin) for 5 h at 4 C. Nonspecific binding was defined as the binding observed in the presence of excess unlabeled IGF-I (1 µM). At the end of the incubation, cells were centrifuged, washed once with PBS, and cell-associated radioactivity was determined by a {gamma}-counter.

Data analysis
Samples in each group of the experiments were in triplicate or quadruplicate. All data were expressed as mean ± SE. When statistical analyses were performed, data were compared by one-way ANOVA with Fisher’s protected least significant difference test, and P values below 0.05 were considered significant.


   Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
RT-PCR analysis of IGF-I receptor in MtT/SGL cells
We studied whether MtT/SGL cells express IGF-I receptor messenger RNA using the RT-PCR technique. As shown in Fig. 1Go, a single band (343 bp) corresponding to cDNA of the rat IGF-I receptor (19) was amplified. It is not likely that this band was derived from contaminated genomic DNA, because no band was amplified from a sample in which reverse transcriptase was not added. This result suggests that MtT/SGL, a new clone of MtT/S, indeed expresses the IGF-I receptor. A similar result was obtained with MtT/S cells used for transient transfection experiments (data not shown).



View larger version (62K):
[in this window]
[in a new window]
  		Figure 1. Expression of the IGF-I receptor analyzed by RT-PCR in MtT/SGL cells. The figure shows photographs of the ethidium bromide-stained products using agarose gel electrophoresis. A, cDNA produced from an RT reaction using total RNA from MtT/SGL cells was amplified using PCR with pairs of oligonucleotide primers specific for rat IGF-I receptor. A DNA fragment with the predicted length (343 bp) was amplified (19 ). B, No band was amplified in the same reaction as A without reverse transcriptase.

 
The time-course effect of IGF-I on GH gene expression
We then studied the effect of IGF-I on the transcriptional regulation of the GH gene in MtT/SGL cells. As shown in Fig. 2Go, IGF-I (10 nM) inhibited GH 5'-promoter activity in a time-related manner. The significant suppression occurred as early as 6 h, and the maximal effect was observed 18 h after the addition of IGF-I, with more than 50% decrease of the transcriptional activity compared with the value at time 0. A significant effect was retained for 30 h, although it gradually became less potent after 24 h.



View larger version (83K):
[in this window]
[in a new window]
  		Figure 2. The time course effect of IGF-I on the GH 5'-promoter activity in MtT/SGL cells. Cells were treated with IGF-I (10 nM) for 0–24 h. *, P < 0.05 vs. basal value.

 
The dose-response effect of IGF-I and insulin on GH gene expression
As shown in Fig. 3Go and 18-h treatment with IGF-I inhibited GH 5'-promoter activity in a dose-dependent manner; IGF-I significantly suppressed it at 1 nM (Fig. 3AGo). We also showed that insulin suppresses GH gene expression (Fig. 3BGo) as previously reported (12), although the effect was obvious only at 10 nM.



View larger version (42K):
[in this window]
[in a new window]
  		Figure 3. The dose response effects of IGF-I or insulin on the rat GH 5'-promotor activity. A, Cells were treated with IGF-I (10 pM to 100 nM) for 18 h. B, Cells were treated with insulin (10 pM to 100 nM) for 18 h. *, P < 0.05 vs. basal value.

 
IGF-I/insulin binding
Binding of [125I]IGF-I was examined in MtT/SGL cells. As shown in Fig. 4Go, increasing amounts of unlabeled IGF-I or insulin displaced [125I]IGF-I binding from the cells. Fifty percent occupancy was observed with unlabeled IGF-I at concentrations of 1–10 nM. Although insulin competed for the binding of [125I]IGF-I as well, higher concentrations (more than 10 nM) were required for 50% occupancy, suggesting that IGF-I preferentially binds to IGF-I receptor over insulin receptor.



View larger version (17K):
[in this window]
[in a new window]
  		Figure 4. Competition for [125I]IGF-I binding by unlabeled ligands in MtT/SGL cells. Cells were incubated with [125I]IGF-I and varying concentrations of unlabeled IGF-I (closed circles) or insulin (open circles) at 4 C for 5 h. Data are expressed as a percentage of maximum specific binding. Each point represents the mean of duplicate determinations.

 
The effect of IGF-I on GH secretion
The effect of IGF-I on GH secretion in MtT/SGL cells was also examined. As shown in Fig. 5Go, incubation of the cells with IGF-I (10 nM) for 24 h potently inhibited GH secretion (more than 50% decrease compared with control).



View larger version (38K):
[in this window]
[in a new window]
  		Figure 5. The effect of IGF-I on GH secretion in MtT/SGL cells. Cells were treated with IGF-I (10 nM) for 24 h. At the end of incubation, culture medium was collected for GH assay. *, P < 0.05 vs. basal value.

 
The influence of kinase inhibitors on the suppressive effect of IGF-I on GH gene expression
To elucidate the post-receptor signal transduction pathway(s) mediating the inhibitory effect of IGF-I, we carried out experiments under treatment with various inhibitors of kinases that are supposed to be involved in the IGF-I signaling. As shown in Fig. 6Go, A and B, the suppressive effect of IGF-I was completely abolished with the treatments with broad-spectrum tyrosine kinase inhibitor genistein (100 µM) or PI3 kinase inhibitor wortmannin (1 µM). On the other hand, MAP kinase inhibitor PD98059 (20 µM) or an S6 kinase inhibitor rapamycin (10 nM) did not influence the suppressive effect of IGF-I.



View larger version (33K):
[in this window]
[in a new window]
  		Figure 6. The effects of various protein kinase inhibitors on the IGF-I-treated GH 5'-promoter activity in MtT/SGL cells. A, Cells were pretreated for 1 h with genistein (100 µM) and PD98059 (20 µM), and then treated with IGF-I (10 nM) as well as the inhibitors for 18 h. B, Cells were pretreated for 1 h with wortmannin (1 µM) or rapamycin (10 nM), and then treated with IGF-I (10 nM) as well as the inhibitors for 18 h. Dark and light bars represent basal and IGF-I treated values, respectively. Gen, genistein; PD, PD98059; Wort, wortmannin; Rapa, rapamycin; *, P < 0.05 vs. basal value.

 
The effects of IGF-I on the activities of various deletion mutants of the GH gene promoter
Finally, we tried to determine the cis-acting element(s) that mediates the suppressive effect of IGF-I using a series of deletion mutants of the GH 5' promoter-luciferase constructs. As shown in Fig. 7Go, the basal promoter activity was increased by eliminating the proximal repressor element (20) (-169 to -152), and was markedly decreased by eliminating one of the Pit-1 consensus sequences (21) (-123 to -116) or Sp-1 consensus sequence (-147 to -129) (22). However, the suppressive effects of IGF-I were maintained in all deletion mutants including the shortest construct (-80 to +6).



View larger version (21K):
[in this window]
[in a new window]
  		Figure 7. Effects of serial deletions of the rat GH 5' promoter (320, 250, 200, 150, 115, 80) on basal or IGF-I-treated gene expression in MtT/SGL cells. MtT/S cells were cotransfected transiently with various rat GH gene promoter-luciferase constructs along with a control vector (pRSV-ß-gal). The cells were then treated with vehicle (dark bars), or IGF-I (10 nM) (light bars) for 18 h. Values were normalized by the ß-gal activity as an internal control. a, thyroid hormone response element (36 ); b, proximal repressor element (20 ); c, Sp-1 (22 ); d, Pit-1 (21 ); e, AP-2 (37 ); f, TATA box. *, P < 0.05 vs. control (vehicle).

 

   Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
In this paper, we established a new in vitro system for studying the transcriptional regulation of the GH gene using MtT/SGL, a clone of the MtT/S rat somatotroph cell line in which the rat GH promoter-luciferase fusion gene was stably incorporated. We found that, in MtT/SGL, the intrinsic IGF-I receptor is expressed and, in fact, IGF-I potently suppresses the GH 5'-promoter activity as well as GH secretion. Furthermore, the data with various kinase inhibitors show that the suppressive effect of IGF-I is mediated by genistein- or wortmannin-sensitive intracellular signalling pathways, suggesting the involvement of protein tyrosine kinase(s) and PI3 kinase.

Previous in vitro studies on GH gene regulation has been carried out using GH3 or GC cell lines (23, 24, 25). Although these cell lines provide good model for GH expression studies, they may not have full characteristics of the somatotroph. In fact, GH3 is a somatomammotroph cell line secreting both GH and PRL (26). GC is a somatotroph cell line that, however, is shown either not to express the GRF receptor or to express intrinsic IGF-I receptor only weakly, and it has been needed sometimes to express the receptors extrinsically for studying the effects of the ligands (15, 24). In contrast, the MtT/S cells used in this study are shown to maintain many features of the pure somatotroph (13), and, as shown in this study, have a marked inhibitory effect of IGF-I on GH secretion. We also found that these cells express intrinsic GRF receptors (Iwasaki, Y., in preparation). Furthermore, the responses of the GH gene promoter to glucocorticoid, thyroid hormone, and some growth factors in MtT/SGL cells are basically similar to those observed in primary culture of rat anterior pituitary cells (data not shown). Thus, the MtT/SGL cell is a useful cell system for studying the cellular and molecular mechanisms of the GH gene transcription.

Our results showed that IGF-I potently (50% or more) inhibited GH gene transcription in MtT/SGL cells, as was previously shown in primary culture of the rat pituitary cells in vitro (11, 12, 27, 28). Furthermore, the effect of IGF-I was likely to be mediated through IGF-I receptor, and not insulin receptor, based on the dose-response and binding studies. Although the regulation of GH involves mainly two hypothalamic factors, i.e. GRF and somatostatin, IGF-I is supposed to play some role as well by forming a long negative feedback loop because the IGF-I receptor is expressed in both the pituitary and the hypothalamus (29, 30, 31). One of the features of the IGF-I effects on GH expression found in this study is the relatively slow onset (6 h or more), raising the possibility that IGF-I may not suppress GH gene transcription directly but rather indirectly through de novo-expressed protein induced by IGF-I.

Although the intracellular signal transduction system of IGF-I or insulin in nonendocrine cells is relatively well characterized (32, 33, 34, 35), that in the somatotroph is not completely clarified. The IGF-I receptor, like the insulin receptor, has tyrosine kinase activity, and the diverse effects of IGF-I are known to be transduced through multiple phosphorylation cascades (4). Previous studies have shown that two major signalling pathways distal to insulin/IGF-I receptors and insulin-receptor substrates (IRS) are PI3 kinase- and MAP kinase-mediated cascades (34, 35). In this study, using relatively selective inhibitors of these kinases, we found that the inhibitory effect of IGF-I on GH gene transcription is mediated through the wortmannin-sensitive pathway, but not through PD98059- (MAP kinase inhibitor) or rapamycin (S6 kinase inhibitor)-sensitive pathways. Because wortmannin is known to be an inhibitor of PI3 kinase, the role of the kinase in the IGF-I effect on the GH gene is strongly suggested, although involvement of some other wortmannin-sensitive kinase(s) cannot completely be ruled out. Direct measurements of various kinase activities will clarify the precise signal transduction system of IGF-I in the somatotroph.

Finally, using deletion mutants of the GH gene promoter, we found that basal transcriptional activity of the gene is changed as expected by the elimination of positive or negative cis-acting elements, further confirming the suitability of this cell line for characterizing the regulation of GH gene expression (20, 21, 22). However, regarding the suppressive effect of IGF-I, we failed to find any definite cis-acting element in the promoter region examined (upstream of -80). This suggests that the binding site of the putative suppressive factor mediating the IGF-I effect is very close to the basic transcriptional machinery. A consensus sequence, an AP-2 like region, is found in this area, but it is reported not to be functional (36). Alternatively, the IGF-I-related factor(s) somehow represses transcription by protein-protein interaction without DNA binding. Further studies are needed to elucidate this molecular mechanism of the negative effect of IGF-I on the GH gene. Nevertheless, the MtT/SGL cells seem to be an excellent tool for studying the issue because this is the only in vitro system in which IGF-I can exert a profound effect on GH expression through the cells’ intrinsic IGF-I receptor.


   Acknowledgments
 
We are indebted to Dr. Reed P. Larsen (Brigham and Women’s Hospital) for providing the rat GH gene promoter, to Drs. Yoshio Ogino (Teikyo University School of Medicine) and Eiichi Araki (Kumamoto University School of Medicine) for valuable technical suggestions, and to Fujisawa Pharmaceutical Co., Ltd. for recombinant human IGF-I.

Received March 6, 1998.


   References
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

  1. Barinaga M, Bilezikjian LM, Vale WW, Rosenfeld MG, Evans RM 1985 Independent effects of growth hormone releasing factor on growth hormone release and gene transcription. Nature 314:279–281[CrossRef][Medline]
  2. Arimura A 1994 Regulation of growth hormone secretion. In: Imura H (ed) The Pituitary Gland, ed 2. Raven Press Ltd, New York, pp 217–259
  3. Harvey S 1995 Growth hormone release: integrative hypothalamic control. In: Harvey S, Scanes CG, Daughaday WH (eds) Growth Hormone. CRC Press, Florida, pp 131–162
  4. Melmed S, Yamasaki H, Fagin J, Namba H, Yamamoto H, Weber M, Morita S, Webster J, Prager D 1996 IGF-I receptor signalling: lessons from the somatotroph. Recent Prog Horm Res 51:189–216
  5. Salmon WD, Daughaday WH 1957 A hormonally controlled serum factor which stimulates sulfate incorporation by cartilage in vitro. J Lab Clin Med 49:825–836[Medline]
  6. Froesch ER, Schmid C, Schwander J, Zapf J 1985 Actions of insulin-like growth factors. Annu Rev Physiol 47:443–467[CrossRef][Medline]
  7. Berelowitz M, Szabo M, Frohman LA, Firestone S, Chu L, Hintz RL 1981 Somatomedin-C mediates growth hormone negative feedback by effects on both the hypothalamus and the pituitary. Science 212:1279–1281[Abstract/Free Full Text]
  8. Tannenbaum GS, Guyda HJ, Posner BI 1983 Insulin-like growth factors: a role in growth hormone negative feedback and body weight regulation via brain. Science 220:77–79[Abstract/Free Full Text]
  9. Abe H, Nolitch ME, Van Wyk JJ, Underwood LE 1983 Human growth hormone and somatomedin C suppress the spontaneous release of growth hormone in unanesthetized rats. Endocrinology 113:1319–1324[Abstract]
  10. Hartman ML, Clayton PE, Johnson ML, Celniker A, Perlman AJ, Alberti KGMM, Thorner MO 1993 A low dose euglycemic infusion of recombinant human insulin-like growth factor I rapidly suppresses fasting-enhanced pulsatile growth hormone secretion in humans. J Clin Invest 91:2453–2462
  11. Yamashita S, Melmed S 1986 Insulin-like growth factor I action on rat anterior pituitary cells: suppression of growth hormone secretion and messenger ribonucleic acid levels. Endocrinology 118:176–182[Abstract]
  12. Yamashita S, Melmed S 1987 Insulin like growth factor I regulation of growth hormone gene transcription in primary rat pituitary cells. J Clin Invest 79:449–452
  13. Inoue K, Hattori M, Sakai T, Inukai S, Fujimoto N, Ito A 1990 Establishment of a series of pituitary clonal cell lines differing in morphology, hormone secretion, and response to estrogen. Endocrinology 126:2313–2320[Abstract]
  14. Melmed S 1984 Insulin suppresses growth hormone secretion by rat pituitary cells. J Clin Invest 73:1425–1433
  15. Zeytin FN, Gick GG, Brazeau P, Ling N, McLaughlin M, Bancroft C 1984 Growth hormone (GH)-releasing factor does not regulate GH release or GH mRNA levels in GH3 cells. Endocrinology 114:2054–2059[Abstract]
  16. Larsen PR, Harney JW, Moor DD 1986 Repression mediates cell-type-specific expression of the rat growth hormone gene. Proc Natl Acad Sci USA 83:8283–8287[Abstract/Free Full Text]
  17. Nordeen SK 1988 Luciferase reporter gene vectors for analysis of promoters and enhancers. Biotechniques 5:454–457
  18. Aoki Y, Iwasaki Y, Katahira M, Oiso Y, Saito H 1997 Regulation of the rat proopiomelanocortin gene expression in AtT-20 cells. I: effects of the common secretagogues. Endocrinology 138:1923–1929[Abstract/Free Full Text]
  19. Reiss K, Meggs LG, Li P, Olivetti G, Capasso J, Anversa P 1994 Upregulation of IGF1, IGF1-receptor, and late growth related genes in ventricular myocytes acutely after infarction in rats. J Cell Physiol 158:160–168[CrossRef][Medline]
  20. Pan WT, Liu Q, Bancroft C 1990 Identification of a growth hormone gene promoter repressor element and its cognate double- and single-stranded DNA-binding proteins. J Biol Chem 265:7022–7028[Abstract/Free Full Text]
  21. Nelson C, Albert VR, Elsholtz HP, Lu LIW, Rosenfeld MG 1988 Activation of cell-specific expression of rat growth hormone and prolactin genes by a common transcription factor. Science 239:1400–1405[Abstract/Free Full Text]
  22. Schaufele F, West BL, Reudelhuber TL 1990 Overlapping Pit-1 and Sp 1 binding sites are both essential to full rat growth hormone gene promotor activity despite mutually exclusive Pit-1 and Sp 1 binding. J Biol Chem 265:17189–17196[Abstract/Free Full Text]
  23. Yamasaki H, Prager D, Gebremedhin S, Moise L, Melmed S 1991 Binding and action of insulin-like growth factor I in pituitary tumor cells. Endocrinology 128:857–862[Abstract]
  24. Yamasaki H, Prager D, Gebremedhin S, Melmed S 1991 Insulin-like growth factor-I (IGF-I) attenuation of growth hormone is enhanced by overexpression of pituitary IGF-I receptors. Mol Endocrinol 5:890–896[CrossRef][Medline]
  25. Webster J, Prager D, Melmed S 1994 Insulin-like growth factor-1 activation of extracellular signal-related kinase-1 and -2 in growth hormone-secreting cells. Mol Endocrinol 8:539–544[Abstract]
  26. Tashjian AH, Bankroft FC, Levine L 1970 Production of both prolactin and growth hormone by clonal strains of rat pituitary tumor cells. J Cell Biol 47:61–70[Abstract/Free Full Text]
  27. Morita S, Yamashita S, Melmed S 1987 Insulin-like growth factor I action on rat anterior pituitary cells: effects of intracellular messengers on growth hormone secretion and messenger ribonucleic acid levels. Endocrinology 121:2000–2006[Abstract]
  28. Namba H, Morita S, Melmed S 1989 Insulin-like growth factor-I action on growth hormone secretion and messenger ribonucleic acid levels: interaction with somatostatin. Endocrinology 24:1794–1799
  29. Rosenfeld RG, Ceda G, Wilson DM, Dollar LA, Hoffman AR 1984 Characterization of high affinity receptors for insulin-like growth factors I and II on rat anterior pituitary cells. Endocrinology 114:1571–1575[Abstract]
  30. Goodyer CG, Stéphano L, Lai WH, Guyda HJ, Posner BI 1984 Characterization of insulin-like growth factor receptors in rat anterior pituitary, hypothalamus, and brain. Endocrinology 114:1187–1195[Abstract]
  31. Rosenfeld RG, Ceda G, Culter CW, Dollar LA, Hoffman AR 1985 Insulin and insulin-like growth factor (somatomedin) receptors on cloned rat pituitary tumor cells. Endocrinology 117:2008–2016[Abstract]
  32. Holman GD, Kasuga M 1997 From receptor to transporter: insulin signalling to glucose transport. Diabetologia 40:991–1003[CrossRef][Medline]
  33. Frolini JR, Ewton DZ, Coolican SA 1996 Growth hormone and the insulin-like growth factor system in myogenesis. Endocr Rev 17:481–517[CrossRef][Medline]
  34. Párrizas M, Saltiel A, LeRoith D 1997 Insulin-like growth factor 1 inhibits apoptosis using the phosphatidylinositol 3'-kinase and mitogen-activated protein kinase pathways. J Biol Chem 272:154–161[Abstract/Free Full Text]
  35. Valverde AM, Lorenzo M, Navarro P, Benito M 1997 Phosphatidylinositol 3-kinase is a requirement for insulin-like growth factor I-induced differentiation, but not for mitogenesis, in fetal brown adipocytes. Mol Endocrinol 11:595–607[Abstract/Free Full Text]
  36. Brent GA, Harney JW, Chen Y, Warne RL, Moore DD, Larsen PR 1989 Mutations of the rat growth hormone promotor which increase and decrease response to thyroid hormone define a consensus thyroid hormone response element. Mol Endocrinol 3:1996–2004[CrossRef][Medline]
  37. Copp RP, Samuels HH 1989 Identification of an adenosine 3', 5'-monophosphate (cAMP)-responsive region in the rat growth hormone gene: evidence for independent and synergistic effects of cAMP and thyroid hormone on gene expression. Mol Endocrinol 3:790–796[CrossRef][Medline]




This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow Purchase Article
Right arrow View Shopping Cart
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 Google Scholar
Google Scholar
Right arrow Articles by Niiori-Onishi, A.
Right arrow Articles by Saito, H.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Niiori-Onishi, A.
Right arrow Articles by Saito, H.

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