European Journal of Endocrinology (1997) ISSN

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1 European Journal of Endocrinology (1997) ISSN Effect of the somatostatin analog, octreotide, and of other hormones on the release of the acid-labile subunit of the 150 kda complex by rat hepatocyte in primary culture Antonina M Barreca 1, Adriana Voci 2, Phillip D K Lee 3, Marica Arvigo 1, Valeria Ghigliotti 1, Emilia Fugassa 2, Giulio Giordano 1 and Francesco Minuto 1 1 Department of Endocrinology and Metabolism, 2 Institute of General Physiology, University of Genova, Genova, Italy, and 3 Diagnostic Systems Laboratories, Inc., Webster, Texas 77598, USA (Correspondence should be addressed to A Barreca, DiSEM, University of Genova, Viale Benedetto XV no. 6, I Genova, Italy) Abstract Objective: In normal subjects, the major form of circulating IGF is the GH-dependent 150 kda complex. The liver appears to be the main source of the three components of the 150 kda complex and, in particular, hepatocytes synthesize the insulin-like growth factor (IGF) peptide and the acid-labile subunit (ALS), whereas Kupffer and sinusoidal endothelial cells produce IGF-binding protein-3 (IGFBP-3). We have studied the effects of the somatostatin analog octreotide, IGF-II, des(1 3)IGF-I, transforming growth factor (TGF)-b1 and tri-iodothyronine (T3) on ALS secretion into the medium conditioned by rat hepatocytes in primary culture. Methods: The regulation of ALS release was evaluated in the conditioned medium of adult rat hepatocytes exposed to increasing concentrations of test substances or to vehicle alone (control), after gel filtration in basic conditions, by immunoblot using an antiserum generated against the N-terminal 34 amino acids of human ALS. Results: The results demonstrate that: 1) octreotide in vitro produces a dose-dependent inhibition of both basal and GH-stimulated ALS secretion into the hepatocyte conditioned medium; 2) the release of ALS by adult rat hepatocytes is not affected by the presence during the incubation of des(1 3)IGF-I or IGF-II; 3) an inhibitory effect, although only with very high doses, can be observed after treatment with TGF-b1; and 4) a small but significant increase of ALS released into the medium can be seen when hepatocytes are treated with T3. Conclusions: Evaluation of the effect of substances known to affect the production of IGF peptides, the IGFBPs, or both, on adult rat hepatocytes in primary culture revealed no powerful stimulator, but instead a potent inhibitor of ALS release/synthesis. Our data suggest that the effect of somatostatin on the 150 kda complex is mediated not only by the reduction in GH concentration, but also by a direct inhibition of ALS release or synthesis. European Journal of Endocrinology Introduction The insulin-like growth factor (IGF) binding proteins (IGFBPs) are a family of proteins, present in plasma and other extracellular fluids, that bind IGF peptides (1, 2). To date, the complete primary structures of six distinct IGFBPs have been determined from complementary DNA clones, and the proteins have been named IGFBP-1 to -6 (3). In normal individuals, the major form of circulating IGF is the growth hormone (GH)-dependent complex of 150 kda (150 kda complex) (4 6). The 150 kda complex is a three-subunit structure, consisting of the IGF peptide, a specific IGFBP (IGFBP-3) that is acid stable, and an acid-labile glycoprotein, which does not by itself bind the IGF peptide, but is necessary to reconstitute the whole complex from purified components (6 9). All three components of the 150 kda complex are produced by distinct hepatic cell types and, specifically, hepatocytes synthesize the IGF peptide and the acidlabile subunit (ALS), while Kupffer and sinusoidal endothelial cells produce IGFBP-3 (10 12). Regarding the regulation of the synthesis of the 150 kda complex subunits, it has been established that, besides GH and nutrients (13), several other factors, i.e. epidermal growth factor (EGF) (14), fibroblast growth factor and platelet-derived growth factor can modulate IGF-I production in liver (14) and in other target cells (15), where the IGF peptide can act through autocrine and paracrine mechanisms. In addition, the synthesis of 1997 Society of the European Journal of Endocrinology

2 194 A M Barreca and others EUROPEAN JOURNAL OF ENDOCRINOLOGY (1997) 137 IGFBP-3, the binding subunit of the complex, can be regulated by several factors in liver (12) and in other cell types (16). Clinical data obtained in hypopituitary subjects (17) or in subjects treated with GH, alone or in combination with IGF-I (18, 19), demonstrate that ALS is primarily under the control of GH. Experimental data (20, 21) have confirmed that ALS production by rat liver is stimulated by GH. The possible effects of other hormones on ALS production have not been reported. To ascertain ALS dependence on factors other than GH, we started by evaluating substances known to modulate the other two components of the 150 kda complex. Thus we studied the effects of a somatostatin analog, octreotide, and IGF-II, des(1 3)IGF-I, transforming growth factor (TGF)-b1 and tri-iodothyronine (T3) on the secretion of ALS into the medium conditioned by rat hepatocytes in primary culture. Materials and methods Cell culture Hepatocytes were isolated from male Wistar rats ( g) as described previously (14) and resuspended at a concentration of cells/ml in serum-free Dulbecco s modified Eagle s medium (DMEM) supplemented with essential and non-essential amino acids; 1 ml cell suspension was plated on 60 mm tissue culture dishes previously coated with calf skin collagen (acid soluble, Type III, Sigma Chemical Co., St Louis, MO, USA) and, after 90 min at 37 C, incubation medium was replaced by 1 ml DMEM containing 0.25% BSA. After 24 h of culture, monolayers were exposed to increasing concentrations of test substances: the somatostatin analog, octreotide (Sandostatina, Sandoz, Milano, Italy), GH (human recombinant, Genotropin, Pharmacia, Uppsala, Sweden), IGF-II, des(1 3)IGF-I (GroPep, Adelaide, Australia), TGF-b1 (from human endotoxin tested platelets, Sigma Chemical Co.), T3 (sodium salt cell-culture-tested, Sigma Chemical Co.) or vehicle alone (control); 48 h later the conditioned medium was collected and processed, and cells were counted. To evaluate the time-course of the effect of octreotide on ALS release, hepatocytes were cultured for 3, 6, 12, 24, 36, 48, and 72 h in presence or absence of 10 ng/ml octreotide. Evaluation of ALS content in hepatocyteconditioned media The hepatocyte-conditioned media (1 ml; cells/ ml) obtained from three different cultures for each dose of test substance were gel-filtered on Hitrap columns (Pharmacia) equilibrated with Tris HCl 0.05 mol/l, ph 8.2. Fractions corresponding to the protein peak were lyophilized and reconstituted with distilled H 2 O. Protein content was determined in each sample by the Lowry method. Fifty microlitres (corresponding to 250 ml of original medium) were fractionated under non-reducing conditions on 10% SDS PAGE, and then transferred Figure 1 A. ALS immunoblot of medium conditioned by adult rat hepatocytes. Increasing concentrations ( mg protein) of starting material (SM) and of peak 2 (10 80 mg protein), obtained by ion exchange and gel permeation chromatography, were denatured and fractionated on 10% SDS PAGE, then transferred electrophoretically to nitrocellulose membranes. After the transfer, membranes were incubated with anti-als antiserum and the immunoblot was performed as detailed in Materials and methods. B. IGFBP-3 immunoblot. Hepatocyte-derived ALS, alone or in combination with rigfbp-3 and IGF-I, was incubated overnight at 4 C. Samples were cross-linked by DSS, denatured and fractionated on 10% SDS PAGE, then transferred electrophoretically to nitrocellulose membranes. After the transfer, membranes were incubated with anti-igfbp-3 antiserum and the IGFBP-3 immunoblot was performed as detailed in Materials and methods. Numbers to the left of lanes are markers of molecular mass.

3 EUROPEAN JOURNAL OF ENDOCRINOLOGY (1997) 137 Regulation of ALS release by hepatocytes 195 Figure 3 Effect of octreotide on ALS accumulated in the culture medium as a function of time in culture. Adult rat hepatocytes were cultured for 3, 6, 12, 24, 48 and 72 h in serum-free medium containing 0.25% BSA with or without (control) 10 ng/ml octreotide. Immunoblot analysis of ALS was performed, after gel filtration in basic conditions, on pooled conditioned media from three different cultures for each time, as detailed in Materials and methods. Results of a densitometric scan of the immunoblot show the relative intensity of the bands at the times considered. chemiluminescent (ECL), Amersham), washed as before and immersed for 1 min in the ECL detection solution. Subsequently, membranes were exposed to sensitive film for about 0.25 min to generate immunoblots. Figure 2 Effect of increasing concentrations of octreotide on ALS release by rat hepatocytes in culture. Immunoblot analysis of ALS was performed (upper panel), after gel filtration in basic conditions, on conditioned media from three different cultures for each dose of test substance, as detailed in Materials and methods. The bar graph (lower panel) derived from the densitometric scan of the immunoblot shows the relative intensity of the bands for each dose of octreotide. Numbers to the left of lanes are markers of molecular mass. P < 0:001. electrophoretically to Hybond C extra nitrocellulose membranes (Amersham, Aylesbury, Bucks, UK). After the transfer, non-specific binding sites were blocked by immersing the membranes in Tris-buffered saline Tween (TBS-T: 0.02 mol/l Tris base, mol/l NaCl, 0.5% Tween 20; ph 7.6 with HCl 1 mol/l) containing 5% non-fat dry milk (Bio Rad, Hercules, CA, USA) for 1 h at 22 C on a rotating shaker, washed five times with TBS-T, and then incubated for 16 h at 4 C with a 1: dilution rabbit antiserum to a synthetic N-terminal fragment of human ALS (Diagnostic Systems Laboratories, Inc., Webster, TX, USA) (22) in TBS-T containing 1% BSA. After incubation, the membranes were washed five times with TBS-T and incubated for 1 h at 22 C with 1:3000 dilution of horseradish peroxidase-linked anti-rabbit Ig (enhanced Purification of ALS from hepatocyteconditioned media ALS was partially purified from 20 ml medium conditioned by hepatocytes by ion exchange chromatography, after an overnight dialysis at 4 C against Tris HCl 0.05 mol/l ph 8.2, on a Mono Q column (Pharmacia), using a discontinuous salt gradient. After extensive washing with the equilibrating buffer (Tris HCl 0.05 mol/l, ph 8.2), proteins were eluted by washing the column with Tris HCl 0.05 mol/ l NaCl 0.15 mol/l, and Tris HCl 0.05 mol/l NaCl 0.6 mol/l. All protein peaks were desalted by Hitrap columns equilibrated with Tris HCl 0.05 mol/l, ph 8.2. By immunoblot, ALS activity was revealed in the protein peak 2 eluted from Mono Q. This peak was further purified by gel filtration on a fast performance liquid chromatography Superose 12 column and tested for ability to form the 150 kda complex by immunoblot after affinity cross-linking to recombinant human (rh)igfbp-3 and rigf-i. For cross-linking, 300 ng rhigfbp-3 alone or in combination with 3 mg medium-derived ALS and 300 ng IGF-I were incubated overnight at 4 C. The cross-linking agent, disuccinimidyl suberate (DSS; Pierce Chemical Co., Rockford, IL,

4 196 A M Barreca and others EUROPEAN JOURNAL OF ENDOCRINOLOGY (1997) 137 Figure 4 Effect of increasing concentrations of des(1 3)IGF-I, IGF-II and TGF-b1 on release of ALS by rat hepatocytes in culture. Immunoblot of ALS was performed, after gel filtration in basic conditions, on conditioned media from three different cultures for each dose of test substance, as detailed in Materials and methods. The bar graphs, derived from densitometric analysis, show the relative intensity of the bands for each dose of test substances. USA), was then added to a final concentration of 0.5 mmol/l and the samples were incubated for 20 min at room temperature. The proteins were separated by 10% SDS PAGE, and the IGFBP-3 immunoblot was performed as described for the ALS studies, using 1:5000 dilution anti-igfbp-3 antiserum kindly provided by Celtrix Pharmaceuticals (Santa Clara, CA, USA). Statistical analysis Data are expressed as the mean S.E.M. The response to test substances was analyzed by t-test, with results considered statistically significant at P < Results The ALS immunoblot of increasing concentration ( mg protein) of the starting material and of the active peak (10 80 mg protein), obtained by ion exchange and gel permeation chromatography, showed that the ALS contained in the medium conditioned by rat adult hepatocytes was purified more than 10 times (Fig. 1 A). By IGFBP-3 immunoblot, it was evident that this partially purified ALS (apparent molecular weight kda) was able to form the 150 kda complex with human rigfbp-3 and rigf-i. ALS alone did not show any specific or non-specific cross-reactivity with the anti-igfbp-3 or the anti-immunoglobulin G antiserum used in the blotting (Fig. 1 B). No significant differences in cell number and viability were observed at the end of 72 h incubation in serumfree medium with or without the test substances. When hepatocytes were treated with increasing concentrations of octreotide, a clearly dose-dependent inhibitory effect on ALS release was evident (75 1.6% of the value in control media at 2.5 ng octreotide/ml medium, % at 5 ng/ml, % at 10 ng/ ml, % at 50 ng/ml and 2 1.6% at 100 ng/ml; P < for each dose, Fig. 2). Octreotide was also able to reduce the stimulatory effect ( % that of control media) of 50 ng/ml GH significantly at each dose tested (percent of the value obtained with 50 ng/ml GH: % with octreotide 2.5 ng/ml medium, % at 5 ng/ml, % at 10 ng/ml, % at 50 ng/ml and % at 100 ng/ml; P < 0.02 for each dose). Evaluation of ALS release as a function of time in culture showed that the maximal accumulation was reached after 36 h of culture and persisted for 24 h. In the presence of 10 ng/ml octreotide, this accumulation appeared to be reduced after 12 h of incubation (Fig. 3). The release of ALS by adult rat hepatocytes was not affected by the presence of des(1 3)IGF-I or IGF-II during the incubation (Fig. 4). Quantification by densitometry revealed that neither peptide showed a dosedependent effect (percentages of the value in control media of increasing concentration of des(1 3)IGF-I: , , and % respectively; percentages of the value in control media of increasing concentration of IGF-II: , , and % respectively). An inhibitory effect, although only at very high doses, was observed after treatment with TGF-b1 ( , , , , and % of the value in control media; Fig. 4). A small but significant increase (P ¼ ; and respectively) of ALS released into the medium was seen when hepatocytes were treated with T3 ( , , and % of the value in control media; Fig. 5).

5 EUROPEAN JOURNAL OF ENDOCRINOLOGY (1997) 137 Regulation of ALS release by hepatocytes 197 Figure 5 Upper panel: representative immunoblot analysis showing the effect of increasing concentrations of T3 on release of ALS by rat hepatocytes in culture. Immunoblot of ALS was performed, after gel filtration in basic conditions, on conditioned media from three different cultures for each dose of test substance, as detailed in Materials and methods. Lower panel: bar graph derived from densitometric analysis, showing the relative intensity of the bands for each dose of T3; data obtained in three separate experiments. Numbers to the left of lanes are markers of molecular mass. P < 0:001. Discussion It is proposed that the role of ALS is to stabilize the IGF/ IGFBP-3 complex, thereby modulating the biological availability and action of the IGFs. Moreover, our recent studies have shown that, although ALS itself does not bind to IGF, it appears to regulate the IGF-binding activity of IGFBP-3 (23, 24), thereby potentially modulating the partitioning of the bound and free forms of serum IGF. In view of these considerations, it is important to define the regulation of ALS. It has been established that GH is the primary stimulator of ALS synthesis in humans (17 19), and it has also been demonstrated that rat hepatocytes in culture secrete ALS under GH stimulation, and that EGF and corticosteroids inhibit ALS production and mrna expression (20). In agreement with the finding that human and rat ALS are 70% homologous in the N-terminal region, with conservation of the four cysteine residues (25), the antiserum generated against the N-terminal 34 amino acids of human ALS also identified rat ALS. Indeed, the protein partially purified by using this antiserum from the medium conditioned by rat hepatocytes was able to reconstitute the 150 kda complex in the presence of rigfbp-3 and rigf-i. A clear dose-dependent inhibitory effect was exerted by octreotide a somatostatin analog known to directly inhibit hepatic IGF-I gene expression (26) on ALS release by hepatocytes in culture. This finding is in agreement with the results we observed by comparing the effect of octreotide with that of classic medical treatment (bromocriptine, cabergoline), surgical or radiation therapies in patients with acromegaly (27). Evaluation of the elution profile of immunoreactive IGF-I and IGFBP-3 after gel filtration in neutral conditions revealed that, after treatment, IGF-I and IGFBP-3 were drastically reduced in all treatment groups. However, the octreotide treatment group, unlike the other groups, not only induced a reduction of IGF-I and IGFBP-3 concentration, but also produced a shift of the peak molecular mass from 150 towards 60 kda. Our experimental and clinical data suggest that the effect of somatostatin on the formation of the 150 kda complex is not only mediated by the reduction in GH concentration, but also by a direct inhibition of spontaneous or GH-dependent release or synthesis of ALS. It will be of interest to evaluate the role of extrahypothalamic somatostatin in the regulation of free IGF concentration and, therefore, of its bioavailability through the inhibition of hepatic ALS synthesis. Moreover, data obtained using octreotide showed that this analog induces expression of IGFBP-1, a binding protein able to inhibit IGF metabolic and mitogenic effects by competing for binding with IGF-specific receptors expression in human hepatoma cells (28). In general, these results indicate that somatostatin has a role in restraining the growth-stimulatory actions of IGF-I. Our results with isolated hepatocytes in primary culture show that, unlike IGFBP-3, the release of ALS does not appear to be influenced by IGFs. This is in agreement with the clinical finding of a lack of effect of IGF-I administration on ALS plasma concentration in humans (18, 19). As we have also used des(1 3)IGF-I, a truncated analog of human IGF-I with markedly reduced ability to bind to IGFBPs, the lack of effect of

6 198 A M Barreca and others EUROPEAN JOURNAL OF ENDOCRINOLOGY (1997) 137 IGFs may not be ascribed to the inhibitory effect of the IGFBPs produced by the hepatocytes in culture. However, as adult rat hepatocytes possess only type II/ mannose-6-phosphate receptor (29), we cannot rule out that the lack of in vitro effect of the IGF peptides on the release of ALS is due to the absence of a type I IGF receptor. An inhibitory effect of TGF-b, which has been demonstrated to be stimulatory on IGFBP-3 in cultured fibroblasts (30), was clearly evident, but at doses higher than those at which it usually expresses its biological actions. It has been demonstrated that thyroid hormone, besides increasing GH secretion, also directly stimulates IGF-I synthesis by the liver (31) and Sertoli cells (32). Our finding of a stimulatory effect of T3 on ALS release by hepatocytes in culture offers further support for a direct regulatory role of thyroid hormone on the 150 kda components and can explain, at least in part, the moderately reduced circulating IGF-I concentration in patients with hypothyroidism (33). In conclusion, by evaluating the effect of some substances known to affect the production of IGF peptides, the IGFBPs, or both, on adult rat hepatocytes in primary culture, we did not find any powerful stimulator, but instead a potent inhibitor of the release or synthesis of ALS. Acknowledgements The authors are indebted to Drs A Sommer and C A Maack (Celtrix Pharmaceuticals, Inc., Santa Clara, CA) for providing the recombinant IGFBP-3 and the anti- IGFBP-3 antiserum. This work was supported by research grants from MURST (60% and 40%). References 1 Rechler M. Non-receptor-binding proteins for insulin-like growth factors and other cytokines: modulators of peptide action. 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7 EUROPEAN JOURNAL OF ENDOCRINOLOGY (1997) 137 Regulation of ALS release by hepatocytes 199 Journal of Clinical Endocrinology and Metabolism Dai J & Baxter RC. Molecular cloning of the acid-labile subunit of the rat insulin-like growth factor binding protein complex. Biochemical and Biophysical Research Communications Serri O, Brazeau P, Kachra Z & Posner B. Octreotide inhibits insulin-like growth factor-i hepatic gene expression in the hypophysectomized rat: evidence for a direct and indirect mechanism of action. Endocrinology Barreca A, Cariola G, Ponzani P, Arvigo M, Foppiani L, Giordano G & Minuto F. Effect of octreotide on circulating IGF-I chromatographic profile: evidence for an inhibitory action on the formation of the 150 kda ternary complex. Clinical Endocrinology Ren SG, Ezzat S, Melmed S & Braunstein GD. Somatostatin analog induces insulin-like growth factor binding protein-1 (IGFBP-1) expression in human hepatoma cells. Endocrinology Hartshorn MA, Scott CD & Baxter RC. Immunofluorescent localization of type II insulin-like growth factor receptor in rat liver and hepatoma cells. Journal of Endocrinology Martin JL & Baxter RC. Transforming growth factor-beta stimulates production of insulin-like growth factor-binding protein-3 by human skin fibroblasts. Endocrinology Schalch DS, Heinrich UE, Draznin B, Johnson CJ & Miller LL. Role of the liver in regulating somatomedin activity: hormonal effects on the synthesis and release of insulin-like growth factor and its carrier protein by the isolated perfused rat liver. Endocrinology Palmero S, Prati M, Barreca A, Minuto F, Giordano G & Fugassa E. Thyroid hormone stimulates the production of insulin-like growth factor-i (IGF-I) by immature Sertoli cells. Molecular and Cellular Endocrinology Furlanetto RW, Underwood LE, Van Wyk JJ & D Ercole AJ. The radioimmunoassay for somatomedin C. In Somatomedins and Growth, ch 23, pp Eds G Giordano, JJ Van Wyk & F Minuto. London: Academic Press, Serono Symposia, Received 24 September 1996 Accepted 7 April 1997