Growth factor-I Messenger Ribonucleic Acid Research Articles

Impaired Protein Synthesis Induced by Acute Alcohol Intoxication Is Associated With Changes in eIF4E in Muscle and eIF2B in Liver

Acute alcohol intoxication in rats decreases protein synthesis in skeletal muscle and, to a lesser extent, in liver. The purpose of the present study was to examine potential mechanisms for the inhibitory effect of acute ethanol exposure. Rats were injected intraperitoneally with either ethanol (75 mmol/kg) or saline, and tissues were examined 2.5 hr later. Rates of protein synthesis in vivo were determined by [3H]phenylalanine incorporation into protein, and various eukaryotic initiation factors (eIFs) were quantitated by Western blot analysis to identify possible mechanisms for regulating translation. Protein synthesis in gastrocnemius and liver was decreased (39% and 21%, respectively) after alcohol administration, compared with saline-injected control animals. Alcohol administration did not alter tissue RNA content but diminished translational efficiency in muscle (43%) and liver (24%). Hepatic eIF2B activity was decreased 24% in alcohol-treated rats, and this was associated with a 95% increase in eIF2alpha phosphorylation. However, alcohol did not alter the amount of 4E-binding protein 1 (4E-BP1) bound to eIF4E, cIF4E bound to eIF4G, or the phosphorylation state of either 4E-BP1 or eIF4E. In contrast to liver, neither eIF2B activity nor the phosphorylation of eIF2alpha was affected in muscle of alcohol-treated rats. However, acute alcohol intoxication increased binding of 4E-BP1 to eIF4E (113%), decreased the amount of cIF4E bound to cIF4G (81%), and decreased the amount of 4E-BP1 in the phosphorylated gamma-form (77%). The plasma concentrations of insulin and insulin-like growth factor-I were unchanged by alcohol, but muscle insulin-like growth factor-I messenger ribonucleic acid abundance was decreased 35%. These data suggest that acute alcohol intoxication decreases translation initiation and protein synthesis in liver and muscle via different mechanisms. Changes in eIF2B appear to predominate in liver, whereas alterations in eIF4E availability appear more critical in skeletal muscle for controlling translation initiation.

Cervical ripening in humans: Potential roles of estrogen, progesterone, and insulin-like growth factor-I

OBJECTIVE: During pregnancy in humans a gradual connective tissue remodeling takes place in the cervix. The aim of this study was to examine a possible relationship between the action of gonadal steroids and growth factors and the biochemically identifiable changes in connective tissues during cervical ripening. STUDY DESIGN: Cervical biopsy specimens and serum samples were taken from 20 term pregnant and 20 nonpregnant menstruating women. Estrogen receptors and progesterone receptors were measured with enzyme immunoassays. The messenger ribonucleic acid levels for estrogen receptors, progesterone receptors, and insulin-like growth factor-I were determined by solution hybridization with human complementary deoxyribonucleic acid probes. The concentration of collagen and its solubility by pepsin digestion were measured. Statistical evaluations were done with the Student t test. RESULTS: In term pregnancy the estrogen receptor level decreased to 14% and the progesterone receptor level to 24% of nonpregnant levels ( p < 0.001 and p < 0.01). The insulin-like growth factor-I messenger ribonucleic acid level increased 400% ( p < 0.01), whereas the messenger ribonucleic acid levels for estrogen receptors and progesterone receptors were unchanged. The changes coincided with a twofold decrease in collagen concentration (hydroxyproline) and a twofold increase in collagen solubility. CONCLUSION: Estrogen receptors and progesterone receptors are present in human cervix. A significant down-regulation of estrogen receptors and progesterone receptors and a fourfold increase in the insulin-like growth factor-I messenger ribonucleic acid level were registered in term pregnant cervix. These findings coincided with the remodeling of the cervical connective tissue. (AM J OBSTET GYNECOL 1996;174:1065-71.)

The colony-stimulating factors induce expression of insulin-like growth factor-I messenger ribonucleic acid during hematopoiesis

Murine bone marrow cells cultured in the presence of colony-stimulating factor-1 (CSF-1) showed coordinate induction of insulin-like growth factor-I (IGF-I) messenger RNA (mRNA) during the differentiation process, and these transcripts increased approximately 50- to 75-fold over the virtually negligible levels measured in freshly isolated bone marrow. In contrast, transcripts for the IGF-I receptor were evident in freshly isolated rat bone marrow cells and showed a 50% down-regulation during differentiation. Addition of a variety of single lineage and multilineage CSFs, including CSF-1, interleukin-3, granulocyte-macrophage-CSF, and granulocyte-CSF to mouse bone marrow cultures revealed that induction of IGF-I mRNA is a universal feature of differentiation with these CSFs, although IGF-I transcripts are at least 10- to 20-fold higher in CSF-1- and interleukin-3-differentiated lineages than in other cultures. The IGF-I induced by CSF-1 was biologically active because a natural ligand of IGF-I, IGF-binding protein-3, caused significant down-regulation of cellular proliferation, and this could be reversed by the addition of exogenous IGF-I. In addition, whereas IGF-I mRNA could be detected in resident peritoneal macrophages, these transcripts were increased 6-fold after a local injection of thioglycollate, a stimulus that induces macrophage proliferation and differentiation in vivo. These results show that CSFs induce expression of the growth factor IGF-I during differentiation of hematopoietic cells into multiple myeloid lineages and that this endogenously produced IGF-I is also a growth factor for hematopoietic cells. The induction of IGF-I mRNA during hematopoiesis should provide a new approach to understanding the expression of this gene during development and differentiation.

The colony-stimulating factors induce expression of insulin-like growth factor-I messenger ribonucleic acid during hematopoiesis.

Murine bone marrow cells cultured in the presence of colony-stimulating factor-1 (CSF-1) showed coordinate induction of insulin-like growth factor-I (IGF-I) messenger RNA (mRNA) during the differentiation process, and these transcripts increased approximately 50- to 75-fold over the virtually negligible levels measured in freshly isolated bone marrow. In contrast, transcripts for the IGF-I receptor were evident in freshly isolated rat bone marrow cells and showed a 50% down-regulation during differentiation. Addition of a variety of single lineage and multilineage CSFs, including CSF-1, interleukin-3, granulocyte-macrophage-CSF, and granulocyte-CSF to mouse bone marrow cultures revealed that induction of IGF-I mRNA is a universal feature of differentiation with these CSFs, although IGF-I transcripts are at least 10- to 20-fold higher in CSF-1- and interleukin-3-differentiated lineages than in other cultures. The IGF-I induced by CSF-1 was biologically active because a natural ligand of IGF-I, IGF-binding protein-3, caused significant down-regulation of cellular proliferation, and this could be reversed by the addition of exogenous IGF-I. In addition, whereas IGF-I mRNA could be detected in resident peritoneal macrophages, these transcripts were increased 6-fold after a local injection of thioglycollate, a stimulus that induces macrophage proliferation and differentiation in vivo. These results show that CSFs induce expression of the growth factor IGF-I during differentiation of hematopoietic cells into multiple myeloid lineages and that this endogenously produced IGF-I is also a growth factor for hematopoietic cells. The induction of IGF-I mRNA during hematopoiesis should provide a new approach to understanding the expression of this gene during development and differentiation.

The augmentation of insulin-like growth factor-I messenger ribonucleic acid in cultured rat hepatocytes: activation of protein kinase-A and -C is necessary, but not sufficient.

In previous studies it was shown that bovine GH (bGH) and glucagon, when individually added to primary rat hepatocyte cultures, modestly stimulated IGF-I mRNA levels 1.8- to 2.5-fold, but when combined, synergized to stimulate IGF-I mRNA levels by 10- to 12-fold. In the present study we have explored further the mechanism of this effect in primary rat hepatocyte cultures. Like glucagon, the addition of 3-isobutyl-1-methylxanthine (100 microM) or (Bu)2cAMP (150 microM) augmented IGF-I mRNA levels 1.8- to 2.0-fold, but when combined with bGH (50 ng/ml), they augmented levels up to 12-fold. The half-life of IGF-I mRNA, determined by incubating hepatocytes with actinomycin-D was 12 h. Although bGH did not affect the decay rate, glucagon (100 ng/ml) or (Bu)2cAMP (100 microM) reduced the rate of loss by about 70%. 4 beta-Phorbol 12 beta-myristate 13 alpha-acetate minimally stimulated IGF-I mRNA levels 1.2- to 1.4-fold, but displayed no synergism when added with bGH, glucagon, or (Bu)2cAMP. The stimulatory effect of bGH plus glucagon was inhibited 80% after preincubation with 4 beta-phorbol 12 beta-myristate 13 alpha-acetate (10 microM) for 24 h. The addition of staurosporine, sphingosine, or H-7 [1-(5-isoquinolinyl sulfonyl)2-methyl piperazine] inhibited the stimulatory effect of bGH plus glucagon on hepatocyte IGF-I mRNA by 80%, 90%, and 85%, respectively. Preincubation with cycloheximide (10 micrograms/ml) blocked the synergistic effect of bGH plus either glucagon or (Bu)2cAMP by 65-80%. The effect of glucagon, mediated via the activation of adenylate cyclase, involves in part the posttranscriptional stabilization of IGF-I mRNA levels. The effect of GH, mediated in part by the activation of protein kinase-C, appears to be at the level of transcription. The synergistic augmentation of hepatocyte IGF-I mRNA levels by GH and glucagon involves the activation of PKA and PKC, but also appears to require the synthesis of one or more protein(s).

The augmentation of insulin-like growth factor-I messenger ribonucleic acid in cultured rat hepatocytes: activation of protein kinase-A and -C is necessary, but not sufficient

In previous studies it was shown that bovine GH (bGH) and glucagon, when individually added to primary rat hepatocyte cultures, modestly stimulated IGF-I mRNA levels 1.8- to 2.5-fold, but when combined, synergized to stimulate IGF-I mRNA levels by 10- to 12-fold. In the present study we have explored further the mechanism of this effect in primary rat hepatocyte cultures. Like glucagon, the addition of 3-isobutyl-1-methylxanthine (100 microM) or (Bu)2cAMP (150 microM) augmented IGF-I mRNA levels 1.8- to 2.0-fold, but when combined with bGH (50 ng/ml), they augmented levels up to 12-fold. The half-life of IGF-I mRNA, determined by incubating hepatocytes with actinomycin-D was 12 h. Although bGH did not affect the decay rate, glucagon (100 ng/ml) or (Bu)2cAMP (100 microM) reduced the rate of loss by about 70%. 4 beta-Phorbol 12 beta-myristate 13 alpha-acetate minimally stimulated IGF-I mRNA levels 1.2- to 1.4-fold, but displayed no synergism when added with bGH, glucagon, or (Bu)2cAMP. The stimulatory effect of bGH plus glucagon was inhibited 80% after preincubation with 4 beta-phorbol 12 beta-myristate 13 alpha-acetate (10 microM) for 24 h. The addition of staurosporine, sphingosine, or H-7 [1-(5-isoquinolinyl sulfonyl)2-methyl piperazine] inhibited the stimulatory effect of bGH plus glucagon on hepatocyte IGF-I mRNA by 80%, 90%, and 85%, respectively. Preincubation with cycloheximide (10 micrograms/ml) blocked the synergistic effect of bGH plus either glucagon or (Bu)2cAMP by 65-80%. The effect of glucagon, mediated via the activation of adenylate cyclase, involves in part the posttranscriptional stabilization of IGF-I mRNA levels. The effect of GH, mediated in part by the activation of protein kinase-C, appears to be at the level of transcription. The synergistic augmentation of hepatocyte IGF-I mRNA levels by GH and glucagon involves the activation of PKA and PKC, but also appears to require the synthesis of one or more protein(s).

Intracellular calcium regulates insulin-like growth factor-I messenger ribonucleic acid levels.

Insulin-like growth factor-I (IGF-I) is involved in repair and regeneration in tissues in which non-GH-mediated regulation of its production has been shown to be important. We have investigated the effects of a second messenger signaling pathway, intracellular calcium, on IGF-I mRNA levels in cultured rat dermal fibroblasts using a RNase protection assay. Intracellular calcium concentrations ([Ca2+]i) were increased using either the calcium ionophore A23187 or thapsigargin. The ability of these agents to increase [Ca2+]i was confirmed by spectrofluorimetry, using fluo-3 as the Ca2+ indicator. Treatment of cells in serum-free medium and 0.25% BSA [Minimum Essential Medium (MEM) + BSA] with 500 nM A23187 or 1 micron thapsigargin decreased IGF-I mRNA levels in a time-responsive manner over 4-8 h. A23187 and thapsigargin also decreased IGF-I mRNA levels to 36% and 47% of control levels, respectively, in a dose-responsive fashion. Basic fibroblast growth factor mRNA levels, which were simultaneously determined, were either unchanged or increased in cells treated with thapsigargin or A23187. Consistent with the change in IGF-I mRNA levels, immunoreactive IGF-I levels in medium conditioned for 48 h by A23187 or thapsigargin decreased to 25% and 14%, respectively, of control levels in cells maintained in MEM + BSA. To determine the role of protein synthesis in the effects of A23187 and thapsigargin, cells were treated with these agents in the presence or absence of cycloheximide. Cycloheximide had no effect on the decrease in IGF-I mRNA levels mediated by thapsigargin, but significantly attenuated the response to A23187. Given these differences in the role of protein synthesis in and the time course of the effects of A23187 and thapsigargin on IGF-I mRNA levels, additivity experiments were performed. Treatment of cells with the combination of A23187 and thapsigargin resulted in IGF-I mRNA levels that were approximately 70% of the levels present in cells treated with either agent alone. These data are consistent with a small additive effect, but suggest that the majority of the effect of A23187 and thapsigargin occurs via the same final pathway.(ABSTRACT TRUNCATED AT 400 WORDS)

Ligands that activate protein kinase-C differ in their ability to regulate basic fibroblast growth factor and insulin-like growth factor-I messenger ribonucleic acid levels.

In this study, rat dermal fibroblasts were used as a model system to examine the ability of ligands that are known to activate protein kinase-C to regulate the levels of the mRNAs encoding basic fibroblast growth factor (bFGF) and insulin-like growth factor-I (IGF-I), two growth factors that are thought to be important in processes such as tissue repair and regeneration and wound healing. Treatment of fibroblasts with the phorbol ester phorbol 12-myristate 13-acetate (PMA), thrombin, bradykinin, serotonin, angiotensin-II, or bombesin increased protein kinase-C activity to a similar degree. Treatment of fibroblasts with 1 microM serotonin transiently increased bFGF mRNA levels about 3-fold compared to the level in control cells maintained in serum-free medium with 0.25% BSA and decreased IGF-I mRNA levels by approximately 50% compared to the level in control cells. This is similar to the previously described changes induced by bradykinin in these cells, but different from the more marked and sustained changes induced by thrombin and PMA. In contrast, angiotensin-II and bombesin had no effect on bFGF or IGF-I mRNA levels. The effects of serotonin, bradykinin, and PMA on bFGF and IGF-I mRNA levels were abrogated by preincubation of cells in 250 nM PMA to down-regulate protein kinase-C. In contrast, the effect of thrombin on bFGF mRNA levels was only partially inhibited by down-regulation of protein kinase-C, while its effect on IGF-I mRNA levels was unaffected. The activation of signaling pathways by the different ligands was further investigated to begin to determine the mechanism for the differences in the effects of thrombin vs. serotonin and bradykinin and in the effects of these three ligands vs. angiotensin-II and bombesin. All of the ligands activated phospholipase-D to a similar degree, suggesting that activation of this enzyme was not responsible for the differential effects of the ligands. In contrast, thrombin, serotonin, and bradykinin had marked effects on the hydrolysis of phosphatidylcholine and phosphatidylinositol, whereas bombesin and angiotensin-II had a small effect on phosphatidylcholine hydrolysis and no effect on phosphatidylinositol hydrolysis.(ABSTRACT TRUNCATED AT 400 WORDS)

Discordant, organ-specific regulation of insulin-like growth factor-I messenger ribonucleic acid in insulin-deficient diabetes in rats.

Linear growth retardation is common in uncontrolled insulin-deficient diabetes, but individual organs such as kidney may hypertrophy. To explore whether this heterogeneity of response might be mediated by differential local insulin-like growth factor-I (IGF-I) gene regulation, we injected rats with ip saline, 65, 120, or 175 mg/kg streptozotocin (STZ). Diabetics were untreated or received daily insulin. Animals were killed 24, 48, or 72 h after documentation of diabetes, and liver, kidney, and lung messenger RNA (mRNA) content analyzed by solution hybridization/RNase protection assay. Untreated diabetics had 10- to 100-fold reductions in hepatic IGF-I mRNA apparent as early as 24 h, and the magnitude of these changes varied directly with the severity of diabetes. In contrast, kidney IGF-I mRNA content increased by 400-500% at 24 h in untreated diabetics given 175 mg/kg STZ, and by 100-200% at 48 h in those given 120 mg/kg STZ, with return to control levels by 72 h. Renal IGF-I mRNA levels actually decreased by 250-350% at 24 h in rats injected with 65 mg/kg STZ, returning to supranormal values by 72 h. These results suggest that severity and/or duration of the metabolic abnormality qualitatively and quantitatively affect this response in the kidney. Liver and kidney IGF-I mRNA levels approached normal with insulin therapy and were similar to controls in rats which received STZ but did not develop diabetes. Lung IGF-I mRNA levels were minimally altered in all experimental groups. At the time point and STZ dosage at which liver IGF-I mRNA changes were most dramatic, little change in liver alpha-tubulin mRNA was noted. At the time point and STZ dosages at which kidney IGF-I mRNA induction was most dramatic, renal IGF-I receptor mRNA was only minimally changed, and renal alpha-tubulin mRNA was modestly reduced. In summary: 1) hepatic IGF-I mRNAs are dramatically reduced, and renal IGF-I mRNAs are significantly increased soon after the onset of insulin-deficient diabetes in STZ-treated rats; 2) insulin therapy restores IGF-I mRNA levels toward normal; and 3) these changes in IGF-I mRNA content are specific and are not the result of hepatic or renal STZ toxicity. These data suggest that IGF-I gene expression is regulated in a discordant, organ-specific manner in diabetes, and that metabolic factors in addition to GH may differentially modulate the endocrine and paracrine effects of IGF-I on growth.

Discordant, organ-specific regulation of insulin-like growth factor-I messenger ribonucleic acid in insulin-deficient diabetes in rats

Discordant, organ-specific regulation of insulin-like growth factor-I messenger ribonucleic acid in insulin-deficient diabetes in rats

Regulation of insulin-like growth factor-I messenger ribonucleic acid expression in Leydig cells

In the present study, we evaluated insulin-like growth factor-I (IGF-I) messenger RNA expression in the rat testis. Crude interstitial cells were separated into three distinct bands on 15–60% Percoll density gradients. IGF-I mRNA was mainly localized in the Leydig cell-enriched fraction (band 3), while band 1 and band 2 cells did not contain significant amounts of IGF-I mRNA. Leydig cell IGF-I mRNA consisted of multiple species varying from 0.8 to 7.5 kb and was present in rat Leydig cells all ages examined, from 25 to 55 days old. To further document that IGF-I mRNAs are present in Leydig cells, the method of Klinefelter et al. (Biol. Reprod. (1987) 36, 769–783) was used to isolate highly purified (> 98% pure) Leydig cells. Most of the IGF-I mRNA was localized in these Leydig cells, while there was no detectable IGF-I mRNA in the whole testis or other interstitial cells. Furthermore, IGF-I mRNA in Leydig cells was increased more than 2-fold by growth hormone (GH) administration in vivo. This suggests that IGF-I mRNA in Leydig cells is also GH dependent. Interstitial IGF-I produced in Leydig cells may have both autocrine and paracrine effects in the testis.

Thyroid hormone and growth hormone interact to regulate insulin-like growth factor-I messenger ribonucleic acid and circulating levels in the rat.

Thyroid hormones influence growth in part by altering the secretion and effects of GH. GH, in turn, mediates its effects by regulating the synthesis and secretion of insulin-like growth factor-I (IGF-I). IGF-I is a pleiotropic growth factor that is synthesized by many tissues and acts on many tissues to regulate both cellular replication and differentiated function. We have studied the direct effects of thyroid hormones and the combined effects of thyroid hormones and GH on the regulation of IGF-I synthesis and secretion in hypophysectomized (hypox) rats in vivo. All rats, except normal littermates and a hypox control group, received 100 micrograms hydrocortisone/100 g BW for 10 days. Circulating IGF-I was measured by specific RIA (normal rats, 1 U/ml), and hepatic IGF-I mRNA was measured by Northern blot hybridization with an antisense cRNA probe. 1) Hypox rats treated with hGH (75 micrograms, ip, twice daily) for 10 days gained 17 g BW vs. 70 g for normal littermates. GH markedly increased hepatic IGF-I mRNA and circulating IGF-I (0.52 +/- 0.14 U/ml 12 h after the last GH injection vs. 0.03 +/- 0.02 for hypox controls). 2) T4 (1 micrograms/100 g BW, ip) for 10 days increased neither weight, hepatic IGF-I mRNA, nor circulating IGF-I. 3) Rats treated with T4 for 10 days followed by a single injection of 1 mg GH, ip, increased hepatic IGF-I mRNA and circulating IGF-I levels comparably as in rats receiving acute GH alone (IGF-I, 12 h, 0.31 +/- 0.09 vs. 0.36 +/- 0.06 U/ml). 4) Hypox rats treated with a single injection of T3 (1.5 micrograms/100 g BW, ip) had slightly increased hepatic IGF-I mRNA, but showed no significant change in circulating IGF-I levels. 5) A single injection of T3 plus GH to hypox rats increased IGF-I mRNA levels above those in rats injected with GH alone and increased serum IGF-I levels to 0.48 +/- 0.12 U/ml compared to 0.36 +/- 0.06 U/ml for GH alone. 6) After 10 days of GH treatment, a single injection of T3 lowered both hepatic IGF-I mRNA and circulating IGF-I (0.52 +/- 0.14 to 0.16 +/- 0.06 U/ml, 6 h after T3). These studies demonstrate that thyroid hormones have relatively little direct effect on IGF-I synthesis but can have major effects on GH-stimulated IGF-I synthesis and secretion. The pattern of these effects depends on the integrity of the pituitary gland, prior exposure of the liver to GH and/or thyroid hormones, and the temporal relationship between GH and thyroid hormone administration.

Regulation of pituitary insulin-like growth factor-I messenger ribonucleic acid levels in rats harboring somatomammotropic tumors: implications for growth hormone autoregulation.

Insulin-like growth factor-I (IGF-I) is produced in multiple tissues and is believed to exert its action in a paracrine or autocrine fashion. We have investigated the GH dependency of tissue IGF-I gene expression in rats harboring implanted somatomammotropic tumors. Circulating GH and radioimmunoassayable IGF-I levels were markedly elevated in these animals. Hepatic IGF-I messenger RNA (mRNA) transcripts were induced about 5-fold in the tumor-bearing rats relative to control animals, and about 50-fold relative to the hypophysectomized liver IGF-I mRNA. IGF-I mRNA content was also modestly induced in the heart, muscle, and kidney of the tumor-bearing animals. IGF-I mRNA transcripts of 8.0, 6.2, 3.8, 2.4, 1.3, and 1.0 kilobases were markedly stimulated in the pituitaries of the GH-tumor bearing rats relative to control pituitaries. Pituitary IGF-I mRNA induction was maximal between 2 and 6 weeks after tumor implantation, a time at which circulating GH levels were also increasing sharply. As previously reported, pituitary GH mRNA was inhibited in tumor-bearing rats. The stimulation of pituitary IGF-I gene expression in these animals, therefore, appears to be dependent on circulating, and not local, pituitary GH concentrations. These data support the GH dependency of pituitary IGF-I gene expression. IGF-I is known to decrease GH production in vitro. High circulating levels of IGF-I present in these animals may account for the observed inhibition of GH gene expression. These data raise the alternative possibility that induction of locally produced IGF-I may contribute to this phenomenon.

Insulin-like growth factor-I messenger ribonucleic acid in the developing human placenta and in term placenta of diabetics.

Fetal growth and development are dependent upon the growth and development of the placenta. Control of placental growth and development is little understood. Immunoreactive insulin-like growth factor-I and -II (IGF-I and IGF-II) have been shown to be released by human placental tissue and human placental membranes have been observed to contain specific receptors for these growth factors. Furthermore, we have demonstrated the presence of IGF-II mRNA transcripts in the developing human placenta and at gestational term in placentae of diabetics. Thus, the IGFs may have a regulatory role in the growth and development of the placenta via autocrine and/or paracrine mechanism(s) of action. In this report we demonstrate the presence of four differing size species of placental poly(A)+ RNA which specifically hybridize to an IGF-I probe originally isolated from an adult human liver cDNA library and localize IGF-I and IGF-II mRNA to syncytiotrophoblasts and fibroblasts, respectively, of the placenta by in situ hybridization. The major transcript is 7500 bases in size and the remaining three transcripts are 5000, 1100, and 900 bases in length with no apparent changes from these sizes throughout gestation and at term in diabetics. Quantification by densitometry of placental IGF-I mRNA detected by dot blot hybridization indicated that first and second trimester placentae each express more IGF-I mRNA relative to that expressed in placenta at term. These results suggest that there are developmental changes in the relative amount of IGF-I mRNA expressed in the human placenta. IGF-I is, therefore, most likely important early in gestation as a placental growth factor. This time period is critical for fetal development and growth, when embryonic induction, organogenesis, and rapid cell proliferation occur.