Insulin stimulation of aminoisobutyric acid transport in human skin fibroblasts is mediated through both insulin and type I insulin-like growth factor receptors

A monoclonal antibody to the human type I insulin-like growth factor (IGF-I) receptor (alpha IR-3) was used to distinguish actions of insulin and IGF-I that are mediated through insulin as opposed to IGF-I receptors on human skin fibroblasts. Both insulin and IGF-I stimulate uptake of the nonmetabolized alanine analog alpha-aminoisobutyric acid (AIB) in these cells. alpha IR-3 inhibited AIB uptake stimulated by both of these hormones in a dose-dependent manner. However, the pattern of hormone action in the presence of alpha IR-3 differed for the two hormones. In the case of IGF-I, alpha IR-3 potently inhibited AIB uptake at low hormone concentrations, but this inhibition was overcome by high hormone concentrations, consistent with impairment of IGF-I action through the IGF-I receptor. In the case of insulin, the action of low concentrations (i.e. 10 ng/ml) was not inhibited, but that of higher insulin concentrations was, suggesting a dual receptor mechanism of cell stimulation by insulin. alpha IR-3 will be an important tool in further studies of the biology of the IGF-I receptor in normal and abnormal human cells.     

Monoclonal antibody to the type I insulin-like growth factor (IGF-I) receptor blocks IGF-I receptor-mediated DNA synthesis: clarification of the mitogenic mechanisms of IGF-I and insulin in human skin fibroblasts.

Insulin and insulin-like growth factor type I (IGF-I) stimulate an overlapping spectrum of biological responses in human skin fibroblasts. Although insulin and IGF-I are known to stimulate the incorporation of [3H]thymidine into DNA in these cells, the identity of the receptor(s) that mediates this effect has not been fully clarified. The mouse anti-human IGF-I receptor antibody alpha IR-3 binds with specificity to IGF-I but not to insulin receptors in human placental membranes; it also specifically inhibits the binding of 125I-labeled IGF-I but not 125I-labeled insulin to suspensions of human skin fibroblasts in a dose-dependent manner. alpha IR-3 competitively inhibits IGF-I-mediated stimulation of [3H]thymidine incorporation into DNA. This inhibition is dependent on the concentration of alpha IR-3 and in the presence of a fixed antibody concentration can be partially overcome by high concentrations of IGF-I. In contrast, at concentrations of less than 1 microgram/ml, the effect of insulin to stimulate [3H]thymidine incorporation is not inhibited by alpha IR-3. However, the incremental effects of higher concentrations (greater than 1 microgram/ml) of insulin on [3H]thymidine incorporation are inhibited by alpha IR-3. alpha IR-3 is a highly specific antagonist of IGF-I receptor-mediated mitogenesis in human skin fibroblasts. By using this antibody, it is shown directly that insulin can act through the IGF-I receptor to stimulate DNA synthesis but can also activate this effect through the insulin receptor itself.     

Autocrine regulation of human tumor cell proliferation by insulin-like growth factor II: an in-vitro model.

We have shown that a pleomorphic cell line of abnormal human karyotype derived from a stomach carcinoma (LIM-1839) proliferates in serum-free medium, expresses insulin-like growth factor II (IGF-II) mRNA, and secretes IGF-II (up to 56 ng/ml in serum-free conditioned medium, as measured in a rat liver RRA. No detectable levels of IGF-I can be measured in serum-free conditioned medium by RIA. These cells also secrete IGF-binding proteins, detected by a charcoal adsorption assay. The release of IGF-II and IGF binding proteins into serum-free conditioned medium (1.7 pmol/10(6) cells.24 h and 0.8 pmol binding sites/10(6) cells.24 h for 3 days, respectively) is inhibited 80% by cycloheximide (10 micrograms/ml). The LIM-1839 cells have type I and type II IGF receptors, determined by affinity cross-linking and competition binding studies. These cells proliferated 1.6-fold over 4 days in serum-free medium, with fresh medium changes on days 0 and 2: their growth was inhibited 56% by 40 micrograms/ml Sm 1.2, a monoclonal antibody which recognizes IGF-I and IGF-II. The addition of 20 and 50 ng/ml multiplication stimulating activity (rat IGF-II) caused 1.8- and 1.7-fold increases in cell growth between days 0 and 4 compared to controls, while [Thr59]IGF-I, at 20 and 50 ng/ml, caused 1.6- and 2.0-fold increases. Insulin, at 2 and 10 micrograms/ml, had no significant effect. The stimulatory effects of endogenous and exogenous IGFs on LIM-1839 cell proliferation were inhibited by a monoclonal antibody to the type I IGF receptor, alpha IR-3. These results suggest that the LIM-1839 cells are biologically responsive to endogenously produced IGF-II, and may thereby provide an in vitro model for autocrine regulation of human tumor growth by IGF-II.     

Evidence from monoclonal antibody studies that insulin stimulates deoxyribonucleic acid synthesis through the type I somatomedin receptor

Somatomedin-C/insulin-like growth factor-I (Sm-C/IGF-I) and insulin stimulate DNA synthesis and cell replication in cultured human fibroblasts. It has been postulated that the growth-promoting actions of both peptides are mediated through the type I Sm-C/IGF-I receptor. This study tests this hypothesis using two recently developed monoclonal antibodies. The antibody designated sm 1.2 is directed to Sm-C, whereas the antibody designated alpha IR-3 is directed against the type I receptor for Sm-C/IGF-I. Radiolabeled monoclonal antibody alpha IR-3 was bound to human foreskin fibroblasts in a reversible time-dependent fashion, with 90% of the specific binding complete after 6 h of incubation at 15 C. Binding of [125I]alpha IR-3 was completely inhibited by excess unlabeled antibody, but not by 50 nM Sm-C or 1000 nM insulin. Specific binding of [125I]Sm-C fell to 27% of the control value in the presence of 50 nM alpha IR-3, and this concentration of antibody significantly reduced the mitogenic response to both Sm-C and insulin. Antibody sm 1.2 blocked the mitogenic response to exogenous Sm-C, but did not block the response to insulin; indeed, in some experiments, sm 1.2 enhanced the response to insulin. We postulate that this enhancement is the result of neutralizing endogenously produced Sm-like substances. This study provides further evidence that the growth-promoting effects of insulin in this cell type are the result of interaction with the Sm-C/IGF-I receptor.     

Stimulatory effects of insulin and insulin-like growth factor I on migration and tube formation by vascular endothelial cells.

The effects of insulin and insulin-like growth factor I (IGF-I) on migration, proliferation and tube-forming activity of endothelial cells were investigated, by using bovine carotid artery endothelial cells. Migration was assayed by a filter membrane technique and tube formation was assayed by a quantitative angiogenesis in vitro model which we have recently developed. In this model, endothelial cells are cultured between two layers of type I collagen gel and become organized into tube-like structures which mimic capillaries in vivo ultrastructurally. Insulin (50-1000 microunits/ml) and IGF-I (10-200 ng/ml) significantly stimulated migration of endothelial cells in a dose-dependent manner with a maximal stimulation of 3.0-fold at 1000 microunits/ml for insulin and 3.8-fold at 200 ng/ml for IGF-I (P less than 0.01). Insulin at concentrations up to 1000 microunits/ml and IGF-I up to 100 ng/ml did not affect proliferation of endothelial cells. When insulin or IGF-I was added in culture medium on collagen gels, tube-forming activity of endothelial cells was markedly stimulated. The specific lengths of tubes significantly increased with the increase in insulin concentration from 25 to 100 microunits/ml (P less than 0.01). At 100 microunits/ml, the stimulation was 1.77-fold (P less than 0.01). IGF-I (1-100 ng/ml) also stimulated the elongation of tubes dose-dependently with a maximal stimulation of 1.96-fold at 100 ng/ml (P less than 0.01). Thus, insulin and IGF-I at pathophysiological concentrations stimulate migration and tube-forming activity of endothelial cells, suggesting that these polypeptides may stimulate repair of endothelial injury in cases such as atherosclerosis and may act as a stimulator of angiogenesis.     

Insulin-like growth factors (IGFs) reduce IGF-binding protein-4 (IGFBP-4) concentration and stimulate IGFBP-3 independently of IGF receptors in human fibroblasts and epidermal cells.

Recent studies have provided a consensus that insulin-like growth factor-I (IGF-I) stimulates IGF-binding protein-3 (IGFBP-3) in vivo and in vitro. While it also appears well established that IGFBP-1 is inversely related to insulin concentrations, evidence regarding regulation of other IGFBP is inconclusive. Using immunoprecipitation and Western ligand blot, we have characterized the IGFBPs released into conditioned medium (CM) by cells from the adult human fibroblast cell line N3652 and the human epidermal squamous cell carcinoma line SCL-1. N3652 cells expressed IGFBP-3, IGFBP-2, a 24-kilodalton (kDa) IGFBP presumed to be IGFBP-4, and IGFBPs at 30 and 28 kDa. SCL-1 expressed IGFBP-3 and a putative IGFBP-4, with intermediate bands at 34 and 30 kDa. As determined by ligand blot of CM from confluent cells 72 h after the addition of peptides to serum-free medium, IGF-I and IGF-II potently stimulated IGFBP-3 in both cell lines, but otherwise IGFBP regulation in the two cells diverged. In N3652 cells, IGFBP-3 concentrations in CM increased to 700% and 800% of basal levels in the presence of IGF-I and IGF-II (at 100 ng/ml; n = 5 experiments), respectively. IGFBP-3 was not affected by insulin up to 10 micrograms/ml. In contrast, IGFBP-4 levels were diminished 54% and 73% by 100 ng/ml IGF-I and IGF-II, respectively, with no response to insulin. In SCL-1 cells, IGF-I and IGF-II were virtually identical in stimulating a mean 200% increase in IGFBP-3 (n = 5 experiments). Insulin was less potent, but caused a significant stimulation of IGFBP-3 levels. IGF-I, IGF-II, and insulin all stimulated an approximately 50% increase in IGFBP-4 concentrations. To test the hypothesis that IGF-induced alterations in IGFBP-3 and IGFBP-4 concentrations were regulated via the type 1 IGF receptor, we attempted to block IGFBP changes with type 1 IGF receptor antibody alpha IR-3 and to induce IGFBP changes with an IGF-II analog, [Leu27]IGF-II, with little affinity for the type 1 receptor. alpha IR-3 failed to block either the IGF-induced rise in IGFBP-3 in each cell line or the decline in IGFBP-4 in N3652 CM. [Leu27]IGF-II was as potent as IGF-II or IGF-I in inducing changes in IGFBP-3 and IGFBP-4 concentrations.(ABSTRACT TRUNCATED AT 400 WORDS)     

The role of mitogen-activated protein kinase in insulin and insulin-like growth factor I (IGF-I) signaling cascades for progesterone and IGF-binding protein-1 production in human granulosa cells

Insulin and IGF-I participate in the regulation of ovulation, steroidogenesis, and IGF-binding protein (IGFBP) production in the ovary. Insulin and IGF-I actions in the ovary are closely related. For example, insulin may amplify IGF-I action in the ovary by up-regulating type I IGF receptors and inhibiting IGFBP-1 production, thus increasing the bioavailability of IGF-I. It is hypothesized that ovarian effects of insulin in insulin-resistant states are mediated via an insulin action pathway(s) distinct from those involved in glucose transport. We previously reported that insulin-induced stimulation of progesterone and inhibition of IGFBP-1 production in the human ovary are mediated by signaling pathways that are independent of phosphatidylinositol 3-kinase, the enzyme whose activation is crucial for glucose transport. We now examined whether activation of MAPK is necessary to mediate insulin-induced or IGF-I-induced stimulation of progesterone or inhibition of IGFBP-1 production in human granulosa cells. Human granulosa cells were obtained during in vitro fertilization. Cells (0.5-1 x 10(5)) were incubated for 24 h in the presence of 0, 10, 10(2), or 10(3) ng/ml insulin or 0, 0.5, 1, 2.5, or 5 ng/ml IGF-I and in the presence or absence of 1 micro M PD98059, a specific inhibitor of ERK1/2 MAPK. The progesterone concentration in the tissue culture medium was measured by RIA (Pantex, Santa Monica, CA), and the IGFBP-1 concentration was measured by immunoradiometric assay (DSL-7800, Diagnostic Systems Laboratories, Inc., Webster, TX). MAPK activity was assessed using the MAPK IP-Kinase assay kit (Upstate Biotechnology, Inc., Lake Placid, NY). ANOVA was used to compare mean values of progesterone or IGFBP-1 concentrations. MAPK was stimulated by insulin up to 350% of the baseline value. Progesterone production in human granulosa cells was stimulated by insulin in a dose-related manner to 123% of the control value (P < 0.001), and IGFBP-1 production was inhibited to 25% of the baseline value (P < 0.001). Despite inhibiting MAPK activity by 99%, PD98059 (1 micro M) did not interfere with insulin-induced stimulation of progesterone or inhibition of IGFBP-1 production. MAPK was stimulated by IGF-I to 730% of the baseline value, with maximal stimulation achieved at 0.5 ng/ml IGF-I. Progesterone production in granulosa cells was stimulated by IGF-I to 130% of the control value (P < 0.001), whereas IGFBP-1 production was inhibited to 44% of the control value (P < 0.001). PD98059 (1 micro M) inhibited IGF-I-induced MAPK activity by 94%. In the presence of 1 micro M PD98059, IGF-I-induced stimulation of progesterone production was inhibited by 96% (P < 0.001). The inhibitory effect of IGF-I on IGFBP-1 production was reduced in the presence of 1 micro M PD98059 by 45% at 5 ng/ml IGF-I and was completely abolished in the presence of 1 micro M PD98059 at concentrations of IGF-I ranging from 0.5-2.5 ng/ml (P < 0.001). We conclude that, under conditions of our experiments, insulin-induced stimulation of progesterone or inhibition of IGFBP-1 production in human granulosa cells does not require MAPK activation, whereas similar effects of IGF-I are largely MAPK dependent.     

Insulin-like growth factor I/somatomedin C: a potent inducer of oligodendrocyte development.

Cell cultures established from cerebrum of 1-day-old rats were used to investigate hormonal regulation of the development of oligodendrocytes, which synthesize myelin in the central nervous system. The number of oligodendrocytes that developed was preferentially increased by insulin, or by insulin-like growth factor I (IGF-I), also known as somatomedin C. High concentrations (5 micrograms/ml) of insulin were required for substantial induction of oligodendrocyte development, whereas only 3.3 ng of IGF-I per ml was needed for a 2-fold increase in oligodendrocyte numbers. At an IGF-I concentration of 100 ng/ml, oligodendrocyte numbers were increased 6-fold in cultures grown in the presence of 10% fetal bovine serum, or up to 60-fold in cultures maintained in serum-free medium. IGF-I produced less than a 2-fold increase in the number of nonoligodendroglial cells in the same cultures. Type I IGF receptors were identified on oligodendrocytes and on a putative oligodendrocyte precursor cell population identified by using mouse monoclonal antibody A2B5. These results indicate that IGF-I is a potent inducer of oligodendrocyte development and suggest a possible mechanism based on IGF deficiency for the hypomyelination that results from early postnatal malnutrition.     

Insulin and growth hormone act synergistically to stimulate insulin-like growth factor-I production by cultured chicken hepatocytes

Cultured chicken hepatocytes were used to investigate whether insulin and GH interact to regulate insulin-like growth factor-I (IGF-I) production in vitro. In the first set of experiments hepatocytes were preincubated for 6 h in hormone-free medium, and the effects of various combinations of insulin and GH on IGF-I production over the next 24 h were quantified by radioimmunoassay. Basal IGF-I production was 5.36 pg IGF-I/micrograms DNA and this was increased 1.31 +/- 0.13-fold (mean +/- S.E.M.) by insulin, 1.90 +/- 0.24-fold by GH and 4.46 +/- 0.68-fold by a combination of insulin and GH. These results demonstrate that insulin and GH interact synergistically to stimulate IGF-I production in vitro. The synergism with GH occurred at physiological concentrations of insulin with half-maximal stimulation occurring at an insulin concentration of 6 ng/ml. In hepatocytes which had been exposed to insulin immediately before the start of the experiment, the presence of insulin was no longer required for maximal stimulation of IGF-I production by GH. This in-vitro system will facilitate the study of the molecular basis of the interaction between insulin and GH.     

Production of insulin-like growth factors and their binding proteins by rabbit articular chondrocytes: relationships with cell multiplication

Articular chondrocytes from 2- to 3-month-old rabbits were cultured in serum-free medium supplemented with fibroblast growth factor. The effects were studied of GH, insulin-like growth factors (IGFs), and insulin on the production of IGF-I, IGF-II, and their binding proteins (BPs) and on cell multiplication. In the control culture medium, IGF-I levels were about one fifth those of IGF-II. Western blot analysis of the BPs revealed a predominant 30K form and 24K and 20K forms which appeared inconsistently and in small quantities. Ten to 100 ng/ml human GH had no mitogenic effect, and even had a slightly inhibitory effect. IGF-I at 10 ng/ml stimulated cell multiplication above the control level by 41% and at 50 ng/ml by 74%, whereas the mean increase obtained with IGF-II (10 and 50 ng/ml) was only 19%. At the same doses, insulin had no effect, but at 5 micrograms/ml it stimulated cell multiplication by a mean of 67%. There was a positive correlation between cell number and release into the medium of both IGF-I (r = 0.86) and IGF-II (r = 0.77). Neither IGF-I nor IGF-II production was affected by GH. Insulin (5 micrograms/ml) increased IGF-I production by a factor of 2.6, but increased IGF-II production by a factor of only 1.4. Under the various conditions of culture with different doses of GH and insulin, cell multiplication, relative to the control value was positively correlated to the IGF-I/IGF-II production ratio (r = 0.77). It would, therefore, seem that IGF-I secreted by the chondrocytes may stimulate their own proliferation. When IGFs or insulin were added to the culture medium, changes in the electrophoretic profiles of the BPs included an increase in the 30K form and an increase in or the appearance of the 24K and 20K forms. Ten and 50 ng/ml IGF-I or IGF-II had effects equal to or greater than those induced by 5 micrograms/ml insulin. These results indicate that the syntheses of BPs and IGFs are coordinated and that IGFs may be implicated in the control of the synthesis of their BPs.     

Insulin and insulin-like growth factor I regulate the same biological functions in HEP-G2 cells via their own specific receptors

The receptors for insulin and insulin-like growth factor I (IGF-I) are closely related molecules, with an extracellular binding domain and an intracellular tyrosine kinase domain. The interaction of insulin and IGF-I with their respective receptors activates the receptor kinase domain, leading to the biological actions of the hormones. Since insulin generally regulates metabolic events and IGF-I generally regulates growth events, it is believed that structural differences in the tyrosine kinase domains of the two respective receptors may elicit different biological responses via different transmembrane signaling mechanisms. We studied the regulation of glycogen metabolism and amino acid uptake in human cultured HEP-G2 hepatoma cells, which have distinct receptors for both insulin and IGF-I. The receptor specificity of these responses was probed with specific monoclonal antibodies to both the insulin and IGF-I receptors. Stimulation of both [3H]glucose incorporation into glycogen and alpha-[3H]aminoisobutyric acid uptake by insulin was half-maximal at concentrations of 1-5 nmol/L. These effects were blocked by the insulin receptor monoclonal antibody MA-10, but not by the IGF-I receptor antibody alpha IR-3. Stimulation of both functions by IGF-I was half-maximal at concentrations of 1-5 nmol/L, and these effects were inhibited by alpha IR-3, but not by MA-10. These studies indicate that in HEP-G2 cells both insulin and IGF-I, via their own receptors, stimulate the same biological responses.     

Insulin and insulin-like growth factor-I stimulate the 3 beta-hydroxysteroid dehydrogenase activity of human placental cytotrophoblasts

The placenta is the primary source of progesterone during pregnancy. Because pregnant diabetic women are reported to have higher serum progesterone levels than nondiabetic pregnant women, we studied the roles of insulin and insulin-like growth factor-I (IGF-I) in the regulation of human cytotrophoblastic 3 beta-hydroxysteroid dehydrogenase (3 beta HSD) activity. Incubation of cytotrophoblasts with insulin or IGF-I for 24 h significantly increased the ability of these cells to convert pregnenolone to progesterone by 75.8 +/- 16.5% (+/- SE) and 65.4 +/- 12.7%, respectively. Treatment with either insulin or IGF-I did not alter cytotrophoblastic production of 20 alpha-hydroxypregn-4-en-3-one (the primary metabolite of progesterone), thus demonstrating that these peptides increased progesterone synthesis (i.e. 3 beta HSD activity) rather than decreased progesterone catabolism. Insulin and IGF-I stimulated 3 beta HSD activity at concentrations as low as 50 and 10 ng/ml, respectively. Insulin- and IGF-I-stimulated 3 beta HSD activities were completely inhibited by concurrent treatment with either actinomycin-D or cycloheximide, suggesting that new mRNA and protein synthesis are required for these peptides to exert their effects. Blockade of the IGF-I receptor of cytotrophoblasts with alpha IR-3, a monoclonal anti-IGF-I receptor antibody, prevented the stimulation of 3 beta HSD activity by IGF-I, but did not alter insulin's stimulatory effect. Thus, the two hormones can each stimulate 3 beta HSD activity via activation of their respective receptors. These studies indicate that insulin and IGF-I can regulate human cytotrophoblastic 3 beta HSD activity in vitro. Since pregnant diabetic women manifest peripheral hyperinsulinemia, and IGF-I levels in fetal cord sera from diabetic pregnancies may be elevated, these observations may help explain the elevated serum progesterone levels associated with pregnancy in the diabetic patient.     

Wheat germ agglutinin stimulation of alpha beta heterodimeric insulin receptor beta-subunit autophosphorylation by noncovalent association into an alpha 2 beta 2 heterotetrameric state

The purified human placental alpha 2 beta 2 heterotetrameric insulin-receptor complex was reduced and dissociated into functional alpha beta heterodimers by a combination of alkaline pH and dithiothreitol treatment. Wheat germ agglutinin (WGA) was observed to stimulate the beta-subunit autophosphorylation of both the alpha 2 beta 2 heterotetrameric and alpha beta heterodimeric complexes in the absence of insulin. However, WGA was without effect on the insulin stimulation of beta-subunit autophosphorylation in these insulin-receptor complexes. In contrast, monomeric WGA was unable to stimulate the basal exogenous substrate protein kinase activity in either the alpha 2 beta 2 heterotetrameric or alpha beta heterodimeric insulin receptor complexes. The stimulatory effect of WGA was biphasic, increasing the basal insulin receptor beta-subunit autophosphorylation at low concentrations (250 ng/ml to 2.5 micrograms/ml) and inhibiting at high concentrations (greater than 25 micrograms/ml). Similarly, equilibrium tracer insulin binding was not significantly altered by low concentrations of WGA (less than 1 microgram/ml) but was dramatically reduced at high WGA concentrations (greater than 2.5 micrograms/ml). In contrast to the insulin-induced covalent association of the alpha beta heterodimeric insulin receptors to form a disulfide-linked alpha 2 beta 2 heterotetrameric complex, nonreducing sodium dodecyl sulfate-polyacrylamide gel electrophoresis demonstrated that the WGA stimulation of beta-subunit autophosphorylation in the alpha beta heterodimer preparation occurred in the absence of covalent association. Nondenaturing Bio-Gel A-1.5m gel filtration chromatography and [125I]insulin affinity cross-linking demonstrated that WGA treatment of the alpha beta heterodimeric insulin receptors induced a noncovalent association into an alpha 2 beta 2 heterotetrameric state. These data support the hypothesis that the insulin receptor protein kinase activity, although dependent upon alpha beta heterodimeric subunit interactions, does not necessarily require covalent disulfide bond formation between the individual alpha beta heterodimeric species.     

Distribution and characterization of insulin and insulin-like growth factor I receptors in normal human ovary

Insulin-resistant hyperinsulinemic states are now widely known to be associated with ovarian hyperandrogenism, and this is thought to be due to an action of insulin on the ovary. However, the identity of the receptor that is responsible for insulin action in these patients, whose insulin receptors on classical target tissues are severely impaired, is unclear. We now report the presence of insulin receptors in stromal and follicular compartments as well as in granulosa cells obtained from normal ovaries. After 15-h incubations at 4 C with [125I]insulin and tissue fragments, specific insulin binding was 6-19% and 7-13%/mg protein (n = 8) to stroma and theca, respectively. Granulosa cells obtained in the course of in vitro fertilization were separated from red cells on a Percoll gradient; specific insulin binding ranged from 9-15%/10(6) cells. Insulin binding was characterized by sensitive insulin competition (half-maximal, 10 ng/ml), appropriately shifted proinsulin competition (20 times to the right), and complete inhibition by specific anti-insulin receptor antibodies (B-2). An antibody to the insulin-like growth factor I (IGF-I) receptor (alpha IR-3) that inhibits IGF-I binding to IGF-I receptors in other cell systems had no effect on insulin binding. Further proof that this binding is to classic insulin receptors was obtained from measurement of insulin-stimulated receptor autophosphorylation. When insulin receptors from stroma were extracted with Triton X-100 and incubated with [gamma-32P]ATP and Mn, insulin increased the incorporation of 32P into the beta-subunit of the receptor 5-fold. In parallel studies with [125I-]IGF-I and specific blocking antibodies to its receptor, no detectable IGF-I binding to stroma or follicles was found. We conclude that specific high affinity insulin receptors possessing tyrosine kinase activity are widely distributed in normal human ovary. IGF-I receptors in normal ovary are either absent or present at very low density. Binding of insulin to its own receptor (as opposed to IGF-I receptors) appears to be the most likely first step in the stimulation of ovarian steroidogenesis by insulin in normal ovaries and possibly in insulin-resistant states as well.     

Insulin-like growth factor type I (somatomedin-C) stimulates high density lipoprotein (HDL) metabolism and HDL-supported progesterone biosynthesis by swine granulosa cells in vitro

Type I insulin-like growth factor (IGF-I) stimulated high density lipoprotein (HDL)-promoted progesterone production by swine granulosa cells cultured under serum-free conditions in vitro. In the presence of pure human IGF-I (50 ng/ml), the half-maximally effective concentration of swine HDL was 16 micrograms/ml (67% confidence limits; 15-17 micrograms/ml) after 2 days of exposure to this growth factor, 5.4 (2.6-9.8) micrograms/ml after 4 days, and 3.8 (1.2-4.8) micrograms/ml after 6 days. Maximal progesterone production increased approximately 10-fold in the presence of IGF-I and HDL on day 2, 125-fold on day 4, and 330-fold on day 6. The facilitative action of IGF-I on HDL-supported progesterone biosynthesis was accompanied by time-dependent stimulatory effects of IGF-I on trypsin-releasable HDL, trypsin-resistant cell-associated HDL, and degraded HDL (P less than 0.01). Moreover, incubation of swine granulosa cells with [3H]cholesteryl oleate-labeled HDL demonstrated that IGF-I exerted a time-dependent stimulatory effect on [3H]free cholesterol and [3H]cholesteryl ester accumulation in granulosa cells, and significantly augmented the secretion of [3H]progesterone (separated by two-dimensional TLC). In addition to the ability of IGF-I to amplify the cellular acquisition of radiolabeled sterol, this growth factor also increased the total mass of cellular cholesteryl ester and total cellular cholesterol as measured by microfluorometric assay (P less than 0.01). We conclude that IGF-I facilitates the effective delivery of HDL-derived sterol substrate into the steroidogenic pool of ovarian cells. Such observations offer an additional role for the differentiative actions of this somatomedin in the expression of full steroidogenic potential by granulosa-luteal cells.     

Octreotide inhibits insulin-like growth factor-I hepatic gene expression in the hypophysectomized rat: evidence for a direct and indirect mechanism of action.

Somatostatin and somatostatin analogs are known to interact with the GH-insulin-like growth factor (IGF)-I axis by inhibiting GH secretion and consequently hepatic IGF-I production. Indirect evidence suggests that octreotide, a somatostatin analog, reduces serum IGF-I levels relatively more than expected from GH reduction, implying a GH-independent pathway of action. To study the role of octreotide in the regulation of IGF-I production, independently of endogenous GH, we used the hypophysectomized (hypox) rat to measure hepatic IGF-I expression and also employed cultured rat hepatocytes to examine whether octreotide has any direct effect on the production of IGF-I. Forty male hypox Sprague-Dawley rats were randomized into 4 groups to receive daily injections for 3 days of either saline, human GH (hGH) (100 g), octreotide (100 g twice), or both hGH (100 g) and octreotide (100 g twice). GH stimulated serum IGF-I levels to 104 +/- 10 micrograms/liter as compared to saline (26 +/- 2 micrograms/liter). Octreotide alone had no effect, but combining octreotide and hGH significantly reduced the hGH-induced rise in the IGF-I levels (52 +/- 6 micrograms/liter). The relative expression of hepatic IGF-I in the rats treated with hGH increased by 4-fold compared to that in the saline-treated rats. Octreotide administered simultaneously with hGH potently blocked the hGH-induced IGF-I expression to control levels. In cultured hepatocytes, IGF-I mRNA levels maximally stimulated by combining bGH and glucagon were significantly inhibited in the presence of octreotide at low concentrations (0.3 and 3 ng/ml) by 25% and 45%, respectively. In contrast, high concentrations of octreotide (30 and 300 ng/ml) had no significant effect on IGF-I mRNA abundance. We conclude that: 1) octreotide inhibits IGF-I serum levels and hepatic gene expression in the hypox rat; and 2) octreotide can inhibit partially the direct effects of GH and glucagon on hepatic IGF-I production.     

Transforming growth factor-beta promotes differentiation of ovarian thecal-interstitial cells but inhibits androgen production

Evidence that transforming growth factor-beta (TGF beta) is produced by thecal-interstitial cells (TIC) has suggested the hypothesis that TGF beta may be an autocrine regulator of TIC function. The purpose of these studies is to begin to test this hypothesis. In the present experiments we tested the effects of TGF beta on steroid production by TIC isolated from the ovaries of hypophysectomized immature rats by Percoll gradient centrifugation. When TIC (10(4) viable cells/well) were cultured in serum-free medium (0.2 ml in 96-well plates), low amounts of androsterone (less than 4 ng/ml) were produced at 2, 4, and 6 days. TGF beta (0.01-100 ng/ml) did not change basal androsterone production. Treatment with LH (50 ng/ml) stimulated a 100-fold increase in androsterone at 2 days and 60-fold increases at 4 and 6 days. Concomitant treatment with TGF beta (10 ng/ml) caused a 65% inhibition (ED50 = 2.3 +/- 0.7 ng/ml) of androsterone production at each time period. Analysis of key steroid metabolites demonstrated that androsterone and androstenedione were inhibited equally, while progesterone was significantly increased (ED50 = 1.2 +/- 0.2 ng/ml). Time-course studies revealed that TGF beta alone did not alter progesterone production at 2 days, but markedly increased progesterone (10-fold) above control levels at 4 and 6 days. Dose-response experiments showed that TGF beta did not alter the sensitivity of the TIC to LH stimulation, indicating that LH activation of the intracellular signaling pathway was not blocked by TGF beta. Treatment with insulin-like growth factor-I (IGF-I) together with LH caused a synergistic increase in androsterone production. The synergistic stimulation of LH action by IGF-I could be blocked by TGF beta. Interestingly, TIC were more sensitive to TGF beta in the presence of IGF-I (ED50 = 0.18 +/- 0.04 ng/ml). In contrast, TGF beta enhanced progesterone production only at the highest dose of TGF beta (10 ng/ml). To further elucidate the mechanism of TGF beta action, the effects of TGF beta on the TIC content of 17 alpha-hydroxylase/C17-20 lyase (P450(17)alpha) and cholesterol side-chain cleavage (P450scc) were analyzed by immunoblotting. TGF beta alone or in combination with LH stimulated an increase in P450scc content, but did not alter P450(17 alpha content. These results lead us to conclude that 1) the TIC are targets for TGF beta; 2) IGF-I increases the sensitivity of TIC to TGF beta action; and 3) TGF beta acts directly on TIC to stimulate progesterone while inhibiting androgen production.     

The effects of insulin and insulin-like growth factors-I and -II on estradiol production by granulosa cells of polycystic ovaries

The objective of this work was to examine the effects of insulin-like growth factors (IGFs) on estradiol (E2) production by granulosa cells obtained from ovaries of patients with polycystic ovary disease (PCO). Granulosa cells, isolated from ovaries of three PCO patients, were cultured in serum-free medium containing either androstenedione alone (10(-7) M) or androstenedione plus graded doses of FSH, IGF-I, IGF-II, and/or insulin. At the end of the culture period (2, 4, or 6 days) E2 levels in the medium were measured by RIA. The results from each patient were similar, and therefore, the data were pooled. In the 6-day time-course experiments, the control (untreated) cells produced relatively high levels of E2 at 2 days; however, none was detected thereafter. Treatment with FSH (30 ng/mL) stimulated E2 production 4-fold at 2 days, but the stimulatory effects of FSH were not sustained during culture. IGF-I at 30 ng/mL mimicked the effects of FSH. Concomitant treatment with FSH and IGF-I caused synergistic increases in E2 production (3-, 13-, and 33-fold at 2, 4, and 6 days, respectively). Dose-response studies revealed that FSH and IGF-I stimulated E2 production in a dose-dependent fashion (ED50 of FSH and IGF-I, were 1.1 +/- 0.3 and 7.6 +/- 7.2 ng/mL, respectively). In the presence of a maximally effective dose of FSH (30 ng/mL), the cells appeared to become more responsive to IGF-I (ED50 of IGF-I plus FSH, 1.09 +/- 0.29 ng/mL); however, this effect was not significant (P = 0.086). In the presence of a maximally effective dose of IGF-I (30 ng/mL), the stimulatory effect of FSH on E2 production was dramatically amplified, but the IGF-I did not significantly (P = 0.85) change the potency of FSH (ED50 of FSH plus IGF-I, 1.07 +/- 2.3 ng/mL). Treatment with IGF-II over the concentration range of 0.1-100 ng/mL had no effect on either control or FSH-stimulated E2 production. Treatment with insulin, either alone or together with FSH, increased the levels of E2, but the insulin effects were seen only at the highest doses tested (0.3-10 micrograms/mL). The results in these in vitro experiments with PCO granulosa cells indicate that 1) physiological concentrations of IGF-I are as effective as FSH in stimulating E2 production; 2) IGF-I and FSH act synergistically to control the level of E2 production; and 3) this synergy was not observed with insulin or IGF-II.     

Insulin-like growth factor I and insulin potentiate luteinizing hormone-induced androgen synthesis by rat ovarian thecal-interstitial cells

We tested the hypothesis that insulin-like growth factor I (IGF-I) and insulin play a role in androgen production by rat ovarian thecal-interstitial cells. Collagenase/DNase-dispersed rat ovarian thecal-interstitial cells obtained from immature hypophysectomized Sprague-Dawley rats were cultured at a concentration of 10(6) cells/ml in serum-free medium in the presence of increasing concentrations of LH, IGF-I, or insulin. The medium was replaced every 48 h, and the androsterone concentration in the culture supernatants was used as an index of androgen production. In the absence of added hormones (control) androsterone levels were consistently less than 0.1 ng/ml. Increasing concentrations of LH stimulated androsterone synthesis in a dose-dependent manner. IGF-I, in the absence of LH, did not significantly increase androsterone levels above control values. However, when combined with 10 ng/ml LH, IGF-I increased androsterone synthesis above levels seen with LH alone in a dose-related fashion: for example, the peak androsterone levels seen with LH and 100 ng/ml (13 nM) IGF-I at 96 h of culture were significantly greater than the peak level seen with 10 ng/ml LH alone (302 +/- 71 vs. 17 +/- 7 ng/ml; P less than 0.0125). Similarly, while insulin alone did not increase androsterone synthesis above control values, androsterone concentrations were increased by insulin in combination with 10 ng/ml LH; a peak value of 240 +/- 67.7 ng/ml was observed at 96 h of culture with 100 ng/ml (18 mM) insulin (P less than 0.025 vs. LH alone) Androsterone levels were slightly less with insulin than with IGF-I, but this difference was not significant. The combination of IGF-I and insulin did not increase levels of androsterone synthesis above those observed with each hormone alone. IGF-I bound to a high affinity binding site on ovarian cell monolayer cultures with an apparent binding affinity of 1.3 x 10(-9) M. Insulin also competed for binding with radiolabeled IGF-I in a dose-dependent manner, but the affinity of insulin was approximately 500-fold less; half-maximal inhibition of [125I] IGF-I binding occurred with an insulin concentration of approximately 300 nM (or approximately 1700 ng/ml). Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of thecal-interstitial cell monolayers affinity labeled with radiolabeled IGF-I in the absence and presence of unlabeled hormone revealed proteins with characteristics of type I IGF receptors. Affinity labeling to a protein of a relative molecular mass of approximately 45,000 was also noted, probably representing IGF carrier proteins synthesized by thecal-interstitial cell monolayers.(ABSTRACT TRUNCATED AT 400 WORDS)     

The role of IGF-binding proteins in mediating the effects of recombinant human IGF-I on insulin requirements in type 1 diabetes mellitus

To determine the role of IGF-binding proteins in mediating the direct effects of recombinant human IGF-I on insulin requirements in type 1(insulin-dependent) diabetes mellitus, overnight changes in IGF-I, IGF-II, and IGF-binding protein-1, -2, and -3, collected under euglycemic conditions, were compared in nine subjects after double blind, randomized, sc administration of recombinant human IGF-I (40 microg/kg) or placebo at 1800 h. On both nights a somatostatin analog infusion (300 ng/kg x h) suppressed endogenous GH production, and three timed discrete GH pulses (total, 0.029 IU/kg x night) ensured identical GH levels. After recombinant human IGF-I administration, IGF-I levels and the IGF-I/IGF-binding protein-3 ratio increased [mean +/- SEM:IGF-I, 401 +/- 22 ng/ml; placebo, 256 +/- 20 ng/ml (P = 0.0002); IGF-I, 0.108 +/- 0.006; placebo, 0.074 +/- 0.004 (P = 0.0003), respectively], and insulin requirements decreased (IGF-I, 0.12 +/- 0.03; placebo, 0.23 +/- 0.03 U/kg x min; P = 0.008). The normal within-individual inverse relationships between insulin and IGF-binding protein-1 levels were observed (lag time 2 h: r = -0.34; P < 0.01). Yet despite reduced free insulin levels (8.5 +/- 1.5; placebo, 12.2 +/- 1.2 mU/liter; P = 0.03), IGF-binding protein-1 levels were reduced after recombinant human IGF-I administration (53.7 +/- 6.8; placebo, 82.2 +/- 11.8 ng/ml; P = 0.008). The largest reductions in free insulin levels after recombinant human IGF-I and thus putative improvement in insulin sensitivity occurred in subjects with the smallest increase in the plasma IGF-I/IGF-binding protein-3 ratio (r = 0.7; P = 0.03). Taken together, these data are consistent with the hypothesis that transcapillary movement of IGF-I (perhaps mediated by IGF-binding protein-1), out of the circulation facilitates altered insulin sensitivity. These data have important implications for risk-benefit assessment of recombinant human IGF-I therapy in type 1 diabetes mellitus.