An effective method for the separation of insulin-like growth factors 1 and 2 during the purification process

The separation of human insulin-like growth factors hIGF-1 and hIGF-2 was greatly improved by an additional purification step using the cation exchanger Mono-S (FPLC) compared to previous studies. Cross-reactions between hIGF-1 and hIGF-2 were strongly reduced. The more highly purified hIGF-1 had a cross-reaction of less than 1% in the RIA for hIGF-2, and was equivalent to recombinant hIGF-1. The pure hIGF-2 had a cross-reaction of less than 1% in the RIA for hIGF-1. In the human placental hIGF-2 radioreceptor assay, the hIGF-1 polypeptide completed less than 1% with hIGF-2 when the type 1 IGF receptor was blocked with insulin.

Insulin-like growth factor (IGF) binding protein complementary deoxyribonucleic acid from human HEP G2 hepatoma cells: predicted protein sequence suggests an IGF binding domain different from those of the IGF-I and IGF-II receptors

The primary structure of an insulin-like growth factor (IGF) binding protein produced by human HEP G2 hepatoma cells has been deduced from the cDNA sequence. The 234 amino acid protein has a predicted molecular mass of 25,274 and contains a single, distinctive cysteine-rich region. The N-terminal sequence of this protein is quite similar to the limited sequence data available for a rat IGF binding protein produced by BRL-3A cells and suggests a common ancestral origin. In contrast, the HEP G2 IGF binding protein sequence bears no similarity to the N-terminal 15 amino acids of a 53 kilodalton binding protein purified from human plasma. Comparison of full-length protein sequences for the IGF-I and IGF-II receptors with that of the HEP G2 IGF binding protein also fails to demonstrate any significant similarities among these three proteins, and suggests that each contains a unique binding domain for the IGF peptides.

Purification, partial sequences and properties of chicken insulin-like growth factors

Insulin-like growth factor-I (IGF-I) and IGF-II have been purified to homogeneity from chicken serum as a step towards the characterization of the roles for these peptides in the growth process. Chicken IGF-I had about half the efficacy of bovine/human IGF-I in a bioassay and in radioimmunoassays with bovine IGF-I as radioligand. Chicken IGF-II competed for the binding of bovine IGF-II to cell receptors while chicken IGF-I reacted minimally in this IGF-II radioreceptor assay. Further evidence of homology was obtained by N-terminal sequence analysis of the first 31 and 35 amino acids of chicken IGF-I and IGF-II respectively. Chicken IGF-I had the same N-terminal as human IGF-I, with the exception of the substitution of serine for asparagine at residue 26. Chicken IGF-II had a unique N-terminal tetrapeptide Tyr-Gly-Thr-Ala, but from residues 5-30 the sequence was identical to that reported for residues 6-31 of human IGF-II. Substitutions also occurred corresponding to residues 32, 33, 35 and 36 of human IGF-II. A variant form of chicken IGF-II that had the same N-terminal pentapeptide as human IGF-II was also detected.

Insulin-like growth factors 1 and 2 in bovine colostrum. Sequences and biological activities compared with those of a potent truncated form

1. Insulin-like growth factors 1 and 2 (IGF-1 and IGF-2) together with a truncated form of IGF-1 were purified to homogeneity from bovine colostrum. 2. Two forms of IGF-1 were totally resolved from IGF-2 in the purification by h.p.l.c. involving cation-exchange and reverse-phase columns. 3. The complete amino acid sequences for all three forms of IGF were determined. The sequence of bovine IGF-1 was found to be identical with that of human IGF-1, and that of the variant lacked the N-terminal tripeptide Gly-Pro-Glu (-3N:IGF-1). Bovine IGF-2 was found to differ in three residues of the C-domain compared with human IGF-2, with serine, isoleucine and asparagine substituted for alanine, valine and serine respectively at positions 32, 35 and 36. 4. Protein synthesis in L6 rat myoblasts was stimulated and protein degradation inhibited in a co-ordinate response with all three IGFs. The relative potency in both processes was -3N:IGF-1 greater than IGF-1 greater than IGF-2. A similar order of potency was obtained for the stimulation of DNA synthesis by -3N:IGF-1 and IGF-1. The approximately 10-fold effect on biological activity of removing the N-terminal tripeptide is unexpected in view of current information on IGF-1 structure and function.

Isolation and characterization of insulin-like growth factor-II from human bone

Human bone was sequentially extracted with 4 M guanidine hydrochloride to remove nonmineralized tissue components, 0.5 M EDTA to dissolve the mineral phase, 4 M guanidine hydrochloride to remove matrix associated proteins and finally a combination of 4 M guanidine hydrochloride and 0.5 M EDTA to remove residual proteins. The extracts were examined for the presence of factors that were able to stimulate the incorporation of [3H] thymidine into DNA and [14C] leucine into protein in a cloned rat bone cell culture system. The majority of the bioactivity was found in the first guanidine hydrochloride extract (59 +/- 12%) while the second guandine hydrochloride extract contained 27 +/- 8%. In addition to several known growth factors already reported to be present in bone (transforming growth factor-beta and insulin-like growth factor-I) insulin-like growth factor-II was identified by its chromatographic, electrophoretic and immunological properties as well as by N-terminal sequence data. The insulin-like growth factor-II levels (802 +/- 112 micrograms/kg wet weight bone) were 10 fold higher than that found for insulin-like growth factor-I (84 +/- 23 micrograms/kg wet weight).

Growth-stimulating effects of somatomedin-/insulin-like peptides in Snell dwarf mice

The effect of the somatomedin-/insulin-like growth factors IGF-I, IGF-II and N2, as well as of semi-purified SM fractions separated by isoelectric focusing derived from human Cohn IV on different growth parameters, have been studied in the Snell dwarf mouse. HPLC-pure IGF-II, N2 and IGF-I stimulate to a similar extent the sulphate incorporation into costal cartilage, the osteochondral junction and epiphyseal cartilage. After 4 weeks of treatment, increase in body length and weight as well as the weights of several organs is obtained with SM fractions, focusing at acid and neutral pH, and containing mainly IGF-II- and less than 5% IGF-I-like peptides. Fractions containing mainly IGF-I-like peptides and focusing at basic pH at the dosage used seem to be less stimulatory on most of these parameters. The rump/tail ratio and weight/length ratio is comparable to that obtained after treatment with human growth hormone (hGH). hGH induced a significant stimulation of the weight of the liver, kidneys, heart, thymus and spleen. The acid and neutral SM fractions induced growth of the liver, kidneys and spleen. The basic fractions only produced a significant weight gain in kidneys and spleen. The skinfold thickness is stimulated by the SM preparations and only slightly by hGH.

Isolation and characterization of insulin-like growth factor I (somatomedin-C) from cultures of fetal rat calvariae

Cultured bones have been shown to secrete local regulators of bone remodeling, such as beta 2-microglobulin, transforming growth factor-beta, and insulin-like growth factor (IGF), but the IGF secreted has not been characterized. In the present study, IGF from medium conditioned by 21-day-old fetal rat calvariae was isolated and characterized. IGF was purified using dialysis, gel filtration, and reverse phase HPLC. Amino acid composition was compatible with that of IGF I (somatomedin-C), and amino-terminal sequence analysis revealed homology with IGF-I. The concentration of IGF-I in the calvarial culture medium was 1 nM and was suppressed by cycloheximide. Calvaria-derived rat IGF I at 20 nM stimulated DNA and collagen synthesis by 42% and 26%, respectively, in monolayer cultures of osteoblast-rich rat parietal bone cells. This study indicates that locally produced IGF-I regulates bone formation in cultures of 21-day-old fetal rat calvariae.

The polymorphic pattern of somatomedins during human development

Human somatomedins (Sm) are heterogeneous on separation by chromatofocussing. Besides the 'classic' insulin-like growth factor I and II (IGF-I/Sm-C and IGF-II), a number of minor peaks emerge which can be classified as IGF-I/Sm-C-like or as IGF-II-like. The aim of the current study was to investigate whether or not polymorphism of somatomedins is present in individuals and whether or not the polymorphic pattern changes during development. Serum extracts from normal healthy children and adults were fractionated by chromatofocussing and the various somatomedin-like peptides were quantitated by specific radioimmunoassays for IGF-I/Sm-C or IGF-II. The results demonstrate 1) that heterogeneity of somatomedins is a common phenomenon existing in all individuals studied, and 2) that the polymorphic patterns of the IGF-I/Sm-C-family and of the IGF-II-family remain rather stable during development, although minor changes are evident.

Purification and characterization of a unique high molecular weight form of insulin-like growth factor II

A form of insulin-like growth factor II (IGF-II) with a mol wt of 15,000 has been purified to homogeneity from human Cohn fraction IV1-4. This protein has an amino-terminal sequence through the first 28 residues that is identical to 7.5K IGF-II. The amino acid composition of 15K IGF-II, however, indicates that its carboxyl-terminal region may be different from that predicted from the analysis of IGF-II cDNA clones. The affinities of 15K IGF-II for receptors on rat placental membranes and for an IGF-binding protein that was isolated from the medium of cultured buffalo rat liver cells were similar to those of the 7.5K form of the growth factor. A best-fit analysis of data from the binding of the two mol wt forms of IGF-II to receptors on rat placental membranes by the LIGAND program was consistent with a model in which 7.5K and 15K IGF-II bound to one site with Kd values of 0.27 +/- 0.03 and 0.38 +/- 0.04, respectively. There was an indication that 15K IGF-II also bound to a second low affinity site on the membrane. In mitogenesis assays performed on human fibroblasts isolated from the skin of two fetuses of an early gestational age, 15K IGF-II stimulated the incorporation of [3H]thymidine into DNA at a half-maximal concentration, i.e. ED50, of 5.7 and 5.0 nM. In these experiments, the ED50 values for 7.5K IGF-II were 8.7 and 15 nM.

A method for the purification of bovine somatomedin C

The large scale purification of bovine somatomedin C has been achieved using a protocol that includes cross-flow ultrafiltration of fresh bovine plasma, treatment of the concentrated plasma with formic acid and ethanol, removal of the insoluble material by high speed centrifugation, cross-flow ultrafiltration of the formic acid and ethanol-soluble proteins, ion exchange chromatography, gel filtration and preparative isoelectric focusing. Thirty L of bovine plasma containing 2.16 kg protein were processed to yield approximately 170 micrograms of highly purified bovine somatomedin C. This represents an 840,000-fold purification of this peptide. The purified peptide has a molecular weight of 10,200 daltons, an isoelectric point of 8.5 and a specific activity of 11,750 Units/mg protein.

Characterization of insulin-like growth factor binding to human granulosa cells obtained during in vitro fertilization

Insulin and IGF-I affect in vitro ovarian stromal and follicular cell function in several species. We previously characterized insulin receptors on human granulosa cells obtained from in vitro fertilization procedures but were unable to demonstrate specific binding of IGF-I. Following modification of the assay conditions, we now report specific, high affinity IGF-1 binding sites on human granulosa cells. Substitution of equimolar concentrations of sucrose for sodium chloride in the buffer solution increased binding of IGF but not insulin in equilibrium assays. Maximal specific IGF-I binding was 2.69 +/- 0.30%/10(5) cells (SEM, n = 9) with half-maximal inhibition of binding at 2 ng/ml IGF-I. Unlabeled insulin recognized the type I IGF receptor with low affinity. An IGF-I receptor monoclonal antibody (alpha IR-3) inhibited 125I-IGF-I but not 125I-insulin binding. Affinity crosslinking followed by SDS/PAGE under reducing conditions revealed IGF-I binding at a molecular weight compatible with the alpha subunit of the type I IGF receptor and with a pattern of inhibition by various ligands that paralleled the equilibrium binding assays. IGF-I receptors are present on freshly isolated human ovarian granulosa cells obtained following pharmacologic stimulation with gonadotrophin according to the protocols of in vitro fertilization. The biologic function of these receptors currently is being investigated.

Partial characterization of a somatomedin-like peptide from the medium of cultured rat Sertoli cells

A peptide that is recognized by antibodies to human somatomedin-C/insulin-like growth factor I (Sm-C/IGF-I) has been partially purified from cultured Sertoli cells prepared from sexually immature rats. The mol wt of this peptide is about 25,000, as determined by gel filtration chromatography and immunoblot analysis of samples resolved by polyacrylamide gel electrophoresis. Isoelectric focusing indicated that the isoelectric point of this peptide was near neutrality. However, a smaller peptide of mol wt 8,000 that cross-reacted with antibodies to Sm-C/IGF-I, was released after gel filtration in acetic acid. Similarly, reverse phase HPLC on a C18 column under acidic conditions released a Sm-C/IGF-I immunoreactive peptide of 8,000 mol wt. This smaller species apparently resulted from the dissociation of this peptide from a binding protein. Unlike the larger neutral form, the isoelectric point of the smaller peptide was 9.8. This pI is similar to the GH-dependent Sm-C/IGF-I peptide isolated from rat serum. The small peptide, unlike the larger form, reacted in a parallel manner to human Sm-C/IGF-I in the Sm-C/IGF-I RIA and radioreceptor assays. In addition, the 8,000 mol wt peptide behaved as a progression factor in the BALB/c-3T3 assay and competed with [125I]Sm-C/IGF-I for binding to the type I Sm-C/IGF-I receptor from cultured rat Sertoli cells. In summary, results of this study demonstrate that rat Sertoli cells in culture secrete a peptide that is the rat equivalent of human Sm-C/IGF-I.

Purification and biologic properties of fibroblast somatomedin

Cultured human fibroblasts produce a peptide growth factor that cross-reacts with antisera to human somatomedin-C (Sm-C). To determine the identity of this species and compare its molecular properties to pure Sm-C, 2 liters of conditioned medium derived from human fibroblast monolayers were concentrated (X10) by ultrafiltration. The concentrated conditioned medium was purified further by CM-Sephadex ion-exchange chromatography. Following elution in 1.0 M NaCl, pH 8.0, the active material was purified by gel filtration on Sephadex G-150. The active fractions which eluted at Kd 0.45 (Mr estimated at 32,000) were further purified by isoelectric focusing. Two peaks of activity electrofocused at pI 5.4 and 7.2, respectively. The pI 5.4 peak contained only binding protein activity. The active fractions from the neutral pool were further purified by reverse-phase high pressure liquid chromatography on a C-18 Bondapak with a linear gradient of acetonitrile (10-60%). The active single peak which eluted at 55% acetonitrile gave a single band when analyzed by polyacrylamide gel electrophoresis. This material stimulated [3H]thymidine incorporation into human fibroblast DNA with approximately 3.2 times the potency of pure Sm-C but was equipotent in stimulating BALB/c 3T3 fibroblasts. It was degraded by fibroblast cultures at a slower rate compared to Sm-C, although it had a similar affinity for Sm-C-binding protein. We conclude that human fibroblasts produce two peptides that react with anti-Sm-C antibody but are chemically distinct from Sm-C. The greater response to fibroblast somatomedin may be due to its affinity for somatomedin-binding protein and slower degradation. These findings may have implications for understanding the regulation of human fibroblast replication.

Partial purification and characterization of a neutral insulin-like growth factor. Biochemical and biological properties

A peptide with an isoelectric point of 6.5-7.0 was purified from Cohn fraction IV on the basis of its capacity to cross react with labelled insulin to human placental cell membrane receptors. It possesses insulin-like activity in the adipocyte bioassay (30 mU insulin equivalent/mg of protein) which is in the same order as its activity in the insulin radioreceptorassay (25.5 mU/mg). Somatomedin bioactivity is 40 U/mg in the porcine cartilage assay. In contrast, although in quiescent human fibroblast this peptide preparation has 6% of the mitogenic potency of somatomedin-C/insulin-like growth factor I on a weight basis, cross-reactivity in radioimmunoassay for somatomedin-C/insulin-like growth factor I, insulin-like growth factor II and insulin are very low. It is concluded that this peptide, although exhibiting the major biological characteristics of an insulin-like growth factor is different from the hitherto described somatomedins.

Somatomedin C/insulin-like growth factor I: simplified purification procedure and biological activities of the purified growth factor

Human somatomedin C has been purified from Cohn fraction IV paste by a simplified procedure using chromatofocusing, hydroxylapatite chromatography and reverse-phase high performance chromatography. The purified material has a specific activity by somatomedin C radioimmunoassay of 9160 units/mg (1 unit is defined as the amount of somatomedin present in 1 ml normal adult male human serum), representing a 650,000-fold purification, and possesses sulphation, mitogenic and insulin-like activities (specific activities of 3388 units/mg, 832 units insulin equivalents/mg and 1122 units/mg respectively). Somatomedin C is shown to be a potent stimulator of DNA synthesis (50% maximum stimulation at 150 fmol/ml) in isolated chondrocytes derived from costal cartilage, a major physiological target tissue.

Characterization of somatomedins from human fetal brain: identification of a variant form of insulin-like growth factor I

A fetal form of somatomedin (insulin-like growth factor) that crossreacts in the fetal brain radioreceptor assay has been proposed to exist in humans. Using this assay to monitor activity during purification, we have isolated a variant form of insulin-like growth factor I (IGF-I) from human fetal brain tissue. The variant IGF-I showed potent crossreaction in the fetal brain radioreceptor assay and stimulated DNA synthesis in fetal brain cells in vitro. Structural analysis revealed the variant IGF-I to have a truncated NH2-terminal region compared to IGF-I isolated from serum. An additional peptide, which displayed less potent crossreaction in the fetal brain radioreceptor assay, was also isolated from the human fetal brain. Partial amino acid sequence analysis revealed identity to insulin-like growth factor II.

Isolation and characterization of variant IGF-1 as well as IGF-2 from adult human brain

The forms of somatomedin present in the adult human brain have been characterized in this study. Two peptides were purified by acidification, size exclusion chromatography, affinity chromatography, FPLC and HPLC. Structural analysis identified these peptides as the variant form of IGF-1 with a truncated N-terminal region earlier isolated from human fetal brain and IGF-2. The presence of the truncated IGF-1 variant and IGF-2 in the human CNS suggests their role as neuropeptides.

Affinity chromatography with amniotic fluid somatomedin binding protein in the purification of insulin-like growth factor I

A simplified procedure has been developed for the isolation of insulin-like growth factor I from human plasma by use of affinity chromatography with the somatomedin binding protein. After acidification of human plasma and separation of insulin-like growth factor I and endogenous binding protein by cation exchange chromatography on SP-Sephadex the material was passed through a column packed with pure human amniotic fluid binding protein covalently coupled to Sepharose. The bound insulin-like growth factors I and II were eluted by 1M acetic acid and separated on a Mono S cation exchange column by use of a salt gradient. The 30 micrograms insulin-like growth factor I and 18 micrograms insulin-like growth factor II recovered from 1 liter plasma gave an overall recovery of 30% for insulin-like growth factor I but only 2.5% for insulin-like growth factor II.

Modulation of in vitro growth of murine myeloid leukemia by an autologous substance immunochemically cross-reactive with insulin and antiinsulin serum

Murine myeloid leukemia secretes a substance immunochemically cross-reactive with insulin (SICRI) both in vivo and in serum-free media. High SICRI concentrations in peripheral blood of tumorous animals do not affect circulating glucose levels. In culture, DNA synthesis rate per leukemic cell is proportional to cell density and is reduced by antiinsulin serum. Culture medium conditioned by leukemia cells as well as SICRI affinity purified from this medium stimulate DNA synthesis in cultured leukemia cells. It appears that autocrine stimulation of murine myeloid leukemia can be mediated in part by an insulin-related growth factor.

Isolation and partial characterization of six somatomedin-like peptides from human plasma Cohn fraction IV

Six somatomedin-like peptides were purified from human plasma Cohn fraction IV by a six-step procedure which included ethanol precipitation, reversed-phase extraction, gel filtration, chromatofocusing and reversed-phase high pressure liquid chromatography (HPLC). Purification was monitored with a competitive protein binding assay using a crude preparations of somatomedin carrier protein. The peptides isolated were homogeneous by reversed-phase HPLC and sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE). Their apparent isoelectric points determined by chromatofocusing were 9.2 (Sm I), (Sm II), 8.2 (Sm III), 6.7 (Sm IV), 6.3 (Sm V), and 6.15 (Sm VI). SDS-PAGE under reducing conditions revealed that they are composed of a single peptide chain with apparent molecular weights of 6800 for Sm I, II and IV and 6400 for Sm III, V, and VI. They were equally potent in the porcine costal cartilage in vitro bioassay. The basic peptides (Sm I-III) were significantly more active in radioimmunoassays for somatomedin C (SmC) and insulin-like growth factor I C-peptide (IGF-I (30 - 41], while only the slightly acidic peptides were active in a radioimmunoassay for insulin-like growth factor II C-peptide (IGF-II (33-40]. When receptor binding was tested with human placental cell membranes and Sm III as tracer, the basic peptides were significantly more potent than Sm IV-VI. With rat liver cell membranes and Sm V as tracer the slightly acidic peptides were more potent. These findings suggest 1) that human plasma may contain other somatomedin-like peptides besides the major components IGF-I/SmC and IGF-II, and 2) that the basic peptides are structurally related to IGF-I/SmC and the slightly acidic peptides are related to IGF-II.

Identification of a peptide fragment from the carboxyl-terminal extension region (E-domain) of rat proinsulin-like growth factor-II

A fragment of the carboxyl-terminal extension region (E-peptide) of rat proinsulin-like growth factor-II has been purified from medium conditioned by cultured BRL-3A rat liver cells. The fragment, identified by microsequence analysis, was discovered in a biologically active fraction of insulin-like growth factor II (IGF-II). The fragment begins at position 117 in pro-IGF-II, two amino acids downstream from an Arg-Arg potential prohormone processing site. A synthetic analogue of the E-peptide at high concentrations stimulates [3H]thymidine incorporation in NIL8 hamster cells, raising the possibility that the E-peptide might bind with low affinity to a mitogen receptor. Peptides from the E-regions of pro-IGF-I and pro-IGF-II should be useful for development of radioimmunoassays for measurement of the somatic production of IGF-I and IGF-II, analogous to the radioimmunoassay for the insulin C-peptide.

The nature and regulation of the receptors for insulin-like growth factors

Two subtypes of IGF receptors have been identified. Type I IGF receptors have a Mr greater than 300,000 and are composed of disulfide-linked 130,000-dalton (alpha) and approximately 90,000-dalton (beta) subunits. The alpha subunit binds hormone; the beta subunit appears to have intrinsic tyrosine kinase activity and to be autophosphorylated. Type I receptors preferentially bind IGF-I but also bind IGF-II and, more weakly, insulin. Type II IGF receptors consist of a 250,000-dalton protein that contains internal disulfide bonds but is not linked to other membrane components. Type II receptors bind IGF-II with higher affinity than IGF-I. They do not interact with even very high concentrations of insulin. Type I IGF receptors and insulin receptors are homologous structures. They have similar subunit structure. Both receptors bind IGFs and insulin. They have similar (but not identical) antigenic determinants. Both receptors are downregulated by IGFs and insulin. Both receptors are affected in certain patients with genetically determined insulin resistance. Type II IGF receptors do not appear to be homologous to type I receptors. They differ in structure, peptide binding specificity, and antigenic determinants. Type II receptors do not appear to be downregulated. Although type II receptors appear to be phosphorylated in intact cells, they do not possess intrinsic tyrosine protein-kinase activity. Insulin acutely upregulates type II IGF receptors in intact rat adipose cells by effecting a redistribution of receptors cycling between a large intracellular pool and the plasma membrane. Insulin and the IGFs elicit the same biological responses, either by cross-reacting with one of the receptors for the heterologous ligand or by concurrent activation of convergent effector pathways by binding to the homologous receptor. Which mechanism is utilized appears to depend more on the tissue than on the biological response. Insulin desensitizes rat hepatoma cells to the actions of insulin and IGFs, mediated by both insulin and IGF receptors, by mechanisms distal to hormone binding and possibly common to IGF and insulin effector pathways.

Neonatal rat islet cell cultures synthesize insulin-like growth factor I

Monolayer cultures of islet B-cells were established from neonatal rat pancreas. Serum-free media conditioned by these cultures for 72 h were concentrated and fractionated on Sephadex G-50 at acid pH into a high-molecular-weight pool containing binding protein for insulin-like growth factors (IGFs) and a low-molecular-weight pool containing IGFs. IGF activity in the IGF pool was demonstrated by a specific radioreceptor assay using rat liver plasma membranes and 125I-labeled rat IGF-II. The IGF in islet cell media was characterized further by radioimmunoassays specific for human IGF-I and for rat IGF-II. Islet cell IGF was identified as predominantly IGF-I or a closely related species and not IGF-II. Levels of approximately 15-50 ng IGF-I (based on human IGF-I standard)/10(6) islet cells accumulated in media after 72 h, and presumably represented synthesis by the islet cells. Concentrations of IGF-I attained in culture media, approximately 0.1 ng/ml, were sufficient to stimulate [3H]thymidine incorporation into B-cells. Growth hormone did not consistently increase IGF-I synthesis, suggesting that the previously described effects of growth hormone on islet cell replication do not result from stimulation of IGF-I synthesis by islet cells. Thus, although the IGF-I synthesized by islet cells may be a physiologically relevant growth factor for these cells, the mitogenic effects of growth hormone in islet cells appear to be independent and not mediated by IGF-I.