Neuronal nitric oxide synthase mediates insulin- and oxidative stress-induced glucose uptake in skeletal muscle myotubes

Previously published studies strongly suggested that insulin- and exercise-induced skeletal muscle glucose uptake require nitric oxide (NO) production. However, the signal transduction mechanisms by which insulin and contraction regulated NO production and subsequent glucose transport are not known. In the present study, we utilized the myotube cell lines treated with insulin or hydrogen peroxide, the latter to mimic contraction-induced oxidative stress, to characterize these mechanisms. We found that insulin stimulation of neuronal nitric oxide synthase (nNOS) phosphorylation, NO production, and GLUT4 translocation were all significantly reduced by inhibition of either nNOS or Akt2. Hydrogen peroxide (H₂O₂) induced phosphorylation of nNOS at the same residue as did insulin, and also stimulated NO production and GLUT4 translocation. nNOS inhibition prevented H₂O₂-induced GLUT4 translocation. AMP activated protein kinase (AMPK) inhibition prevented H₂O₂ activation and phosphorylation of nNOS, leading to reduced NO production and significantly attenuated GLUT4 translocation. We conclude that nNOS phosphorylation and subsequently increased NO production are required for both insulin- and H₂O₂-stimulated glucose transport. Although the two stimuli result in phosphorylation of the same residue on nNOS, they do so through distinct protein kinases. Thus, insulin and H₂O₂-activated signaling pathways converge on nNOS, which is a common mediator of glucose uptake in both pathways. However, the fact that different kinases are utilized provides a basis for the use of exercise to activate glucose transport in the face of insulin resistance.

IGF-1 Receptor Insufficiency Leads to Age-Dependent Attenuation of Osteoblast Differentiation

In the current study, we determined the effects of IGF-1 receptor haploinsufficiency on osteoblast differentiation and bone formation throughout the lifespan. Bone mineral density was significantly decreased in femurs of male and female Igf1r(+/-) mice compared with wild-type mice. mRNA expression of osteoblast differentiation markers was significantly decreased in femurs and calvariae from Igf1r(+/-) mice compared with cells from wild-type mice. Bone morphogenetic protein-7-induced ectopic bone in Igf1r(+/-) mice was significantly smaller with fewer osteoblasts but more lipid droplets and had reduced expression of osteoblast differentiation markers compared with wild-type mice. In bone marrow cells from middle-aged and old wild-type and Igf1r(+/-) male mice, palmitate inhibited osteoblast markers expression. In cells from young wild-type male mice, palmitate did not inhibit marker expression, but in cells from young male Igf1r(+/-) mice, palmitate inhibited bone sialoprotein and osterix but not osteocalcin or type I collagen (TIC). In female wild-type mice, palmitate inhibited osteoblast markers expression in cells from young, middle-aged, and old mice except TIC in cells from middle-aged mice. Palmitate inhibited bone sialoprotein expression in cells from middle-aged and old female Igf1r(+/-) mice and osteocalcin, osterix, and TIC expression in young and middle-aged female Igf1r(+/-) mice but stimulated expression in cells from old female Igf1r(+/-) mice. We conclude that IGF-1 receptor haploinsufficiency results in a prolipid accrual phenotype in bone in association with inhibition of growth factor-induced osteoblast differentiation, a situation which may phenocopy age-related decreases in bone formation.

Palmitate attenuates osteoblast differentiation of fetal rat calvarial cells

Aging is associated with the accumulation of ectopic lipid resulting in the inhibition of normal organ function, a phenomenon known as lipotoxicity. Within the bone marrow microenvironment, elevation in fatty acid levels may produce an increase in osteoclast activity and a decrease in osteoblast number and function, thus contributing to age-related osteoporosis. However, little is known about lipotoxic mechanisms in intramembraneous bone. Previously we reported that the long chain saturated fatty acid palmitate inhibited the expression of the osteogenic markers RUNX2 and osteocalcin in fetal rat calvarial cell (FRC) cultures. Moreover, the acetyl CoA carboxylase inhibitor TOFA blocked the inhibitory effect of palmitate on expression of these two markers. In the current study we have extended these observations to show that palmitate inhibits spontaneous mineralized bone formation in FRC cultures in association with reduced mRNA expression of RUNX2, alkaline phosphatase, osteocalcin, and bone sialoprotein and reduced alkaline phosphatase activity. The effects of palmitate on osteogenic marker expression were inhibited by TOFA. Palmitate also inhibited the mRNA expression of fatty acid synthase and PPARγ in FRC cultures, and as with osteogenic markers, this effect was inhibited by TOFA. Palmitate had no effect on FRC cell proliferation or apoptosis, but inhibited BMP-7-induced alkaline phosphatase activity. We conclude that palmitate accumulation may lead to lipotoxic effects on osteoblast differentiation and mineralization and that increases in fatty acid oxidation may help to prevent these lipotoxic effects.

Reductions in serum IGF-1 during aging impair health span

In lower or simple species, such as worms and flies, disruption of the insulin-like growth factor (IGF)-1 and the insulin signaling pathways has been shown to increase lifespan. In rodents, however, growth hormone (GH) regulates IGF-1 levels in serum and tissues and can modulate lifespan via/or independent of IGF-1. Rodent models, where the GH/IGF-1 axis was ablated congenitally, show increased lifespan. However, in contrast to rodents where serum IGF-1 levels are high throughout life, in humans, serum IGF-1 peaks during puberty and declines thereafter during aging. Thus, animal models with congenital disruption of the GH/IGF-1 axis are unable to clearly distinguish between developmental and age-related effects of GH/IGF-1 on health. To overcome this caveat, we developed an inducible liver IGF-1-deficient (iLID) mouse that allows temporal control of serum IGF-1. Deletion of liver Igf -1 gene at one year of age reduced serum IGF-1 by 70% and dramatically impaired health span of the iLID mice. Reductions in serum IGF-1 were coupled with increased GH levels and increased basal STAT5B phosphorylation in livers of iLID mice. These changes were associated with increased liver weight, increased liver inflammation, increased oxidative stress in liver and muscle, and increased incidence of hepatic tumors. Lastly, despite elevations in serum GH, low levels of serum IGF-1 from 1 year of age compromised skeletal integrity and accelerated bone loss. We conclude that an intact GH/IGF-1 axis is essential to maintain health span and that elevated GH, even late in life, associates with increased pathology.

Protein kinase D1 is essential for bone acquisition during pubertal growth

Bone formation and maintenance represents the summation of the balance of local and endocrine hormonal stimuli within a complex organ. Protein kinase D (PKD) is a member of the Ca(2+)/calmodulin-dependent kinase superfamily of serine/threonine kinases and has been described as the crossroads for the bone morphogenetic protein (BMP)-IGF-I signaling axis, which plays a major role in bone formation. The current study exploits the PKD1-deficient mouse model to examine the role of PKD in vivo in the skeleton. Dual-energy x-ray absorptiometry scan analysis of male and female pubescent mice demonstrated significantly decreased bone mineral density in the whole body and femoral bone compartments of PKD1 (+/-) mice, compared with their wild-type littermates. The body weight, nasal-anal length, and percentage body fat of the mice were not significantly different from their wild-type littermates. Cultured bone marrow stromal cells from PKD1 (+/-) mice demonstrated lower alkaline phosphatase activity in early differentiating osteoblasts and decreased mineralized nodule formation in mature osteoblasts. Quantitative RT-PCR analysis of osteoblast differentiation markers and osteoclast markers exhibited lower levels of expression in PKD1 (+/-) male mice than wild type. In female mice, however, only markers of osteoblast differentiation were reduced. PKD1 (+/-) mice also demonstrated a profound reduction in mRNA expression levels of BMP type II receptor and IGF-I receptor and in BMP-7 responsiveness in vitro. Together these data suggest that in mice, PKD1 action contributes to the regulation of osteoblastogenesis by altering gene expression with gender-specific effects on osteoclastogenesis, subsequently affecting skeletal matrix acquisition during puberty.

Neuronal nitric oxide synthase is phosphorylated in response to insulin stimulation in skeletal muscle

Type 2 Diabetes (T2DM) is the seventh leading cause of death in the United States, and is quickly becoming a global pandemic. T2DM results from reduced insulin sensitivity coupled with a relative failure of insulin secretion. Reduced insulin sensitivity has been associated with reduced nitric oxide synthase (NOS) activity and impaired glucose uptake in T2DM skeletal muscle. Upon insulin stimulation, NO synthesis increases in normal adult skeletal muscle, whereas no such increase is observed in T2DM adults. Endothelial NOS is activated by phosphorylation in the C-terminal tail in response to insulin. Neuronal NOS (nNOS), the primary NOS isoform in skeletal muscle, contains a homologous phosphorylation site, raising the possibility that nNOS, too, may undergo an activating phosphorylation event upon insulin treatment. Yet it remains unknown if or how nNOS is regulated by insulin in skeletal muscle. Data shown herein indicate that nNOS is phosphorylated in response to insulin in skeletal muscle and that this phosphorylation event occurs rapidly in C2C12 myotubes, resulting in increased NO production. In vivo phosphorylation of nNOS was also observed in response to insulin in mouse skeletal muscle. These results indicate, for the first time, that nNOS is phosphorylated in skeletal muscle in response to insulin and in association with increased NO production.

Deficiency of insulin-like growth factor-1 receptor confers resistance to oxidative stress in C2C12 myoblasts

IGF-1 receptor (IGF-1R) signaling regulates cell growth, transformation and survival. Haploinsufficiency of the IGF-1R is reported to paradoxically confer resistance to oxidative stress in vivo and in cells cultured from Igf1r^+/− mice. In order to determine whether IGF-1R deficiency directly confers resistance to oxidative stress in specific cell types, an siRNA-mediated approach was applied to reduce IGF-1R in C2C12 myoblasts, NIH3T3 fibroblasts and MC3T3-E1 osteoblasts. Treating the IGF-1R deficient myoblasts with H₂O₂ resulted in significantly higher phosphorylation of Akt as compared to cells having normal expression of IGF-1R. Similar results were obtained with UV treatment, another inducer of oxidative stress. This enhanced activation of Akt was associated with reduced level of cleaved caspase-3 and PARP. Moreover, in the IGF-1R knockdown myoblasts, phosphorylation of the Akt substrate Bad was enhanced after peroxide treatment. However, in NIH-3T3 fibroblasts and MC3T3-E1 osteoblasts, the loss of IGF-1R by siRNA directed knockdown was associated with reduced levels of phosphorylated Akt on treatment with H₂O₂ or UV as compared to control cells and these cells showed more apoptosis. These results suggest a novel mechanism of cell type specific differential regulation of resistance to oxidative stress induced apoptosis by reduced levels of IGF-1R.

Insulin-like growth factor 1 physiology: lessons from mouse models

Insulin-like growth factor 1 (IGF-1) is a pleiotropic polypeptide. Its expression is tightly regulated and it plays significant roles during early development, maturation, and adulthood. This article discusses the roles of IGF-1 in determination of body size, skeletal acquisition, muscle growth, carbohydrate metabolism, and longevity, as learned from mouse models.

Does reduced IGF-1R signaling in Igf1r+/- mice alter aging?

Mutations in insulin/IGF-1 signaling pathway have been shown to lead to increased longevity in various invertebrate models. Therefore, the effect of the haplo- insufficiency of the IGF-1 receptor (Igf1r^+/−) on longevity/aging was evaluated in C57Bl/6 mice using rigorous criteria where lifespan and end-of-life pathology were measured under optimal husbandry conditions using large sample sizes. Igf1r^+/− mice exhibited reductions in IGF-1 receptor levels and the activation of Akt by IGF-1, with no compensatory increases in serum IGF-1 or tissue IGF-1 mRNA levels, indicating that the Igf1r^+/− mice show reduced IGF-1 signaling. Aged male, but not female Igf1r^+/− mice were glucose intolerant, and both genders developed insulin resistance as they aged. Female, but not male Igf1r^+/− mice survived longer than wild type mice after lethal paraquat and diquat exposure, and female Igf1r^+/− mice also exhibited less diquat-induced liver damage. However, no significant difference between the lifespans of the male Igf1r^+/− and wild type mice was observed; and the mean lifespan of the Igf1r^+/− females was increased only slightly (less than 5%) compared to wild type mice. A comprehensive pathological analysis showed no significant difference in end-of-life pathological lesions between the Igf1r^+/− and wild type mice. These data show that the Igf1r^+/− mouse is not a model of increased longevity and delayed aging as predicted by invertebrate models with mutations in the insulin/IGF-1 signaling pathway.

High fat diet induced insulin resistance and glucose intolerance are gender-specific in IGF-1R heterozygous mice

Interactions between genes and environment play a critical role in the pathogenesis of type 2 diabetes. Low birth weight, due to genetic and environmental variables affecting fetal growth, is associated with increased susceptibility to the development of type 2 diabetes and metabolic disorders in adulthood. Clinical studies have shown that polymorphisms in the Insulin-like growth factor 1 (IGF-1) gene or heterozygous mutations in IGF-1 and IGF-1 receptor (IGF-1R) genes, resulting in reduced IGF-1 action, are associated with low birth weight and post-natal growth. Mice lacking one of the IGF-1R alleles (Igf1r(+/-)) exhibit a 10% reduction in post-natal growth, and develop glucose intolerance (males) and insulin resistance (males and females) as they age. To investigate whether adverse environmental factors could accelerate the onset of the metabolic syndrome, we conducted a short duration intervention of high fat diet (HFD) feeding in male and female Igf1r(+/-) and wild-type (WT) control mice. The HFD resulted in insulin resistance, hyperglycemia, and impaired glucose tolerance in males of both genotypes whereas in females exacerbated diabetes was observed only in the Igf1r(+/-) genotype, thus suggesting a sexual dimorphism in the influence of obesity on the genetic predisposition to diabetes caused by reduced IGF-1 action.

Increased serum IGF-1 levels protect the musculoskeletal system but are associated with elevated oxidative stress markers and increased mortality independent of tissue igf1 gene expression

Although the literature suggests a protective (anabolic) effect of insulin-like growth factor-1 (IGF-1) on the musculoskeletal system during growth and aging, there is evidence that reductions in IGF-1 signaling are advantageous for promoting an increase in life span through reduction in oxidative stress-induced tissue damage. To better understand this paradox, we utilized the hepatocyte-specific IGF-1 transgenic (HIT) mice, which exhibit 3-fold increases in serum IGF-1, with normal IGF-1 expression in other tissues, and mice with an IGF-1 null background that exclusively express IGF-1 in the liver, which thereby deliver IGF-1 by the endocrine route only (KO-HIT mice). We found that in the total absence of tissue igf1 gene expression (KO-HIT), increases in serum IGF-1 levels were associated with increased levels of lipid peroxidation products in serum and increased mortality rate at 18 months of age in both genders. Surprisingly, however, we found that in female mice, tissue IGF-1 plays an important role in preserving trabecular bone architecture as KO-HIT mice show bone loss in the femoral distal metaphysis. Additionally, in male KO-HIT mice, increases in serum IGF-1 levels were insufficient to protect against age-related muscle loss.

Impairment of IGF-I expression and anabolic signaling following ischemia/reperfusion in skeletal muscle of old mice

With the advancement of age, skeletal muscle undergoes a progressive decline in mass, function, and regenerative capacity. Previously, our laboratory has reported an age-reduction in recovery and local induction of IGF-I gene expression with age following tourniquet (TK)-induced skeletal muscle ischemia/reperfusion (I/R). In this study, young (6 mo) and old (24-28 mo) mice were subjected to 2h of TK-induced ischemia of the hindlimb followed by 1, 3, 5, or 7 days of reperfusion. Real time-PCR analysis revealed clear age-related reductions and temporal alterations in the expression of IGF-I and individual IGF-I Ea and Eb splice variants. ELISA verified a reduction of IGF-I peptide with age following 7 day recovery from TK. Western blotting showed that the phosphorylation of Akt, mTOR, and FoxO3, all indicators of anabolic activity, were reduced in the muscles of old mice. These data indicate that an age-related impairment of IGF-I expression and intracellular signaling does exist following injury, and potentially has a role in the impaired recovery of skeletal muscle with age.

Nocturnin suppresses igf1 expression in bone by targeting the 3' untranslated region of igf1 mRNA

IGF-I is an anabolic factor that mediates GH and PTH actions in bone. Expression of skeletal Igf1 differs for inbred strains of mice, and Igf expression levels correlate directly with bone mass. Previously we reported that peroxisome proliferator-activated receptor-γ2 activation in bone marrow suppressed Igf1 expression and that peroxisome proliferator-activated receptor-γ2 activation-induced Nocturnin (Noc) expression, a circadian gene with peak expression at light offset, which functions as a deadenylase. In 24-h studies we found that Igf1 mRNA exhibited a circadian rhythm in femur with the lowest Igf1 transcript levels at night when Noc transcripts were highest. Immunoprecipitation/RT-PCR analysis revealed a physical interaction between Noc protein and Igf1 transcripts. To clarify which portions of the Igf1 3' untranslated region (UTR) were necessary for regulation by Noc, we generated luciferase constructs containing various lengths of the Igf1 3'UTR. Noc did not affect the 170-bp short-form 3'UTR, but suppressed luciferase activity in constructs bearing the longer-form 3'UTR, which contains a number of potential regulatory motifs involved in mRNA degradation. C57BL/6J mice have low skeletal Igf1 mRNA compared with C3H/HeJ mice, and the Igf1 3' UTR is polymorphic between these strains. Interestingly, the activity of luciferase constructs bearing the long-form 3'UTR from C57BL/6J mice were repressed by Noc overexpression, whereas those bearing the corresponding region from C3H/HeJ were not. In summary, Noc interacts with Igf1 in a strain- and tissue-specific manner and reduces Igf1 expression by targeting the longer form of the Igf1 3'UTR. Posttranscriptional regulation of Igf1 may be critically important during skeletal acquisition and maintenance.

Protein kinase D mediates the synergistic effects of BMP-7 and IGF-I on osteoblastic cell differentiation

We previously showed that exogenous insulin-like growth factor-I (IGF-I) and bone morphogenetic protein-7 (BMP-7) synergistically stimulated osteoblast differentiation in fetal rat calvaria (FRC) cells. We have now shown that BMP-7 alone and the BMP-7 and IGF-I combination synergistically stimulated protein kinase D (PKD) phosphorylation at Ser744/748 and Ser916. Transfection of FRC cells with a constitutively active PKD stimulated marker expression, while transfection with a catalytically inactive PKD did not. Moreover, Gö6976, which inhibits protein kinase C (PKC) α and β1, blocked PKD phosphorylation and the synergistic action of the BMP-7 and IGF-I combination on osteoblast differentiation, whereas Gö6983, which inhibits PKCα, β, γ, δ, and ζ, did not. Our results suggest that the FRC cell differentiation induced by BMP-7 and the BMP-7 and IGF-I combination requires stimulation of PKD activity. Our results are consistent with a novel mechanism in which combined BMP-7 and IGF-I signaling activates upstream novel PKC(s), which then phosphorylates and activates PKD, leading to enhanced osteoblast differentiation.

Minireview: Mechano-growth factor: a putative product of IGF-I gene expression involved in tissue repair and regeneration

The discovery that IGF-I mRNAs encoding isoforms of the pro-IGF-I molecule are differentially regulated in response to mechanical stress in skeletal muscle has been the impetus for a number of studies designed to demonstrate that alternative splicing of IGF-I pre-mRNA involving exons 4, 5, and 6 gives rise to a unique peptide derived from pro-IGF-I that plays a novel role in myoblast proliferation. Research suggests that after injury to skeletal muscle, the IGF-IEb mRNA splice variant is up-regulated initially, followed by up-regulation of the IGF-IEa splice variant at later time points. Up-regulation of IGF-IEb mRNA correlates with markers of satellite cell and myoblast proliferation, whereas up-regulation of IGF-IEa mRNA is correlated with differentiation to mature myofibers. Due to the apparent role of IGF-IEb up-regulation in muscle remodeling, IGF-IEb mRNA was also named mechano-growth factor (MGF). A synthetically manufactured peptide (also termed MGF) corresponding to the 24 most C-terminal residues of IGF-IEb has been shown to promote cellular proliferation and survival. However, no analogous peptide product of the Igf1 gene has been identified in or isolated from cultured cells, their conditioned medium, or in vivo animal tissues or biological fluids. This review will discuss the relationship of the Igf1 gene to MGF and will differentiate actions of synthetic MGF from any known product of Igf1. Additionally, the role of MGF in satellite cell activation, aging, neuroprotection, and signaling will be discussed. A survey of outstanding questions relating to MGF will also be provided.

Role of Akt isoforms in IGF-I-mediated signaling and survival in myoblasts

Oxidative stress has been shown to induce apoptosis in a variety of tissues, while insulin-like growth factor-I (IGF-I) can oppose this effect. We found that H(2)O(2) promoted cell death and apoptosis in C2C12 myoblasts, an effect that was completely prevented by exogenous IGF-I. One downstream mediator of IGF-I survival signaling is the serine/threonine kinase Akt, of which three isoforms have been identified in mammals. We found that Akt1 and Akt3 act on pro-apoptotic target molecules in an isoform-specific manner. Both Akt1 and Akt3 were responsible for phosphorylating FoxO3a at S253 and FoxO1 at T24, while Akt1 alone phosphorylated Bad at S136 and FoxO3a at T32. Our results provide evidence for IGF-I-stimulated isoform-specific actions of Akt on molecules involved in promoting apoptosis.

Current perspectives on Akt Akt-ivation and Akt-ions

The serine/threonine kinase Akt is an effector of PI3K-generated phosphatidylinositol (3,4,5)-trisphosphate [PI(3,4,5)P3] and is a principle mediator of growth factor-induced signal transduction. Akt is activated through phosphorylation by specific kinases, and its activity is reduced directly by phosphorylation-site-specific phosphatases. In addition, Akt activity is effectively reduced by the action of phosphatases which dephosphorylate PI(3,4,5)P3, thereby reducing the levels of the essential lipid activators of PDK1 and Akt. The functions of Akt are pleiotropic and include regulation of cellular proliferation, differentiation, protein synthesis, and survival. Akt stimulates protein synthesis through actions on mTOR/p70S6K, and promotes survival by phosphorylating and inactivating pro-apoptotic molecules such as Ask1, Bad, Bax, and FoxO3a. Furthermore, loss of Akt decreases the intracellular ATP:AMP ratio, thus establishing a role for Akt in energy regulation. Three isoforms of Akt have been identified, and although redundant functions between isoforms exist, recent investigations have enumerated unique functions for each. Therefore, targeting specific Akt isozymes in a tissue- and context-specific fashion may lead to a greater understanding of Akt-mediated processes.

Serum IGF-I-Deficiency Does Not Prevent Compensatory Skeletal Muscle Hypertrophy in Resistance Exercise

The involvement of circulating insulin-like growth factor-I (IGF-I) in the skeletal muscle response to resistance exercise is currently unclear. To address this, we utilized the liver IGF-I-deficient (LID) mouse model, in which the igf1 gene has been disrupted in the hepatocytes, resulting in ~80% reduction in serum IGF-I. Twelve- to 13-month-old male LID and control (L/L) mice were subjected to 16 weeks of resistance training. Resistance exercise resulted in equal strength gains in both L/L and LID mice. Basal IGF-I mRNA levels were greater in LID muscles than in L/L, and exercise increased IGF-I mRNA in quadriceps, gastrocnemius, and plantaris muscles. LID mice had elevated tyrosine phosphorylation of IGF-IR and Stat5b, the latter possibly reflective of increased serum GH. Tyrosine phosphorylation of IGF-IR was increased, while phospho-Stat5b was reduced after resistance training of both wild-type and LID mice. These data suggest that: 1) performance and recovery in response to resistance training is normal even when there is severe deficiency of circulating IGF-I; and 2) upregulation of local IGF-I may be involved in the compensatory growth of muscle that occurs in response to resistance training. Decreased levels of p-Stat5b in exercised mice suggests that the upregulation of local IGF-I gene expression in response to exercise may be GH-independent.

Deletion of the G protein-coupled receptor 30 impairs glucose tolerance, reduces bone growth, increases blood pressure, and eliminates estradiol-stimulated insulin release in female mice

In vitro studies suggest that the G protein-coupled receptor (GPR) 30 is a functional estrogen receptor. However, the physiological role of GPR30 in vivo is unknown, and it remains to be determined whether GPR30 is an estrogen receptor also in vivo. To this end, we studied the effects of disrupting the GPR30 gene in female and male mice. Female GPR30((-/-)) mice had hyperglycemia and impaired glucose tolerance, reduced body growth, increased blood pressure, and reduced serum IGF-I levels. The reduced growth correlated with a proportional decrease in skeletal development. The elevated blood pressure was associated with an increased vascular resistance manifested as an increased media to lumen ratio of the resistance arteries. The hyperglycemia and impaired glucose tolerance in vivo were associated with decreased insulin expression and release in vivo and in vitro in isolated pancreatic islets. GPR30 is expressed in islets, and GPR30 deletion abolished estradiol-stimulated insulin release both in vivo in ovariectomized adult mice and in vitro in isolated islets. Our findings show that GPR30 is important for several metabolic functions in female mice, including estradiol-stimulated insulin release.

Functional deficits and insulin-like growth factor-I gene expression following tourniquet-induced injury of skeletal muscle in young and old rats

This study investigated the effect of age on recovery of skeletal muscle from an ischemia-reperfusion (I/R)-induced injury. Young (6 mo old) and old (24-27 mo old) Sprague-Dawley rats underwent a 2-h bout of hindlimb ischemia induced by a pneumatic tourniquet (TK). The TK was released to allow reperfusion of the affected limb, and animals were divided into 7- and 14-day recovery groups. Maximum plantar flexor force production was assessed in both 7- and 14-day recovery groups of both ages, followed by histological evaluation. Subsequent analysis of IGF-I gene expression and intracellular signaling in 7-day recovery muscles was performed by RT-PCR and Western blotting, respectively. Old rats had significantly greater deficits in force production and exhibited more evidence of histological pathology than young at both 7 and 14 days postinjury. In addition, old rats demonstrated an attenuated upregulation of IGF-I mRNA and induction of proanabolic signaling compared with young in response to injury. We conclude that aged skeletal muscle exhibits more damage and/or defective regeneration following I/R and identify an age-associated decrease in local IGF-I responsiveness as a potential mechanism for this phenomenon.

Activation of peroxisome proliferator-activated receptor gamma (PPARgamma) by rosiglitazone suppresses components of the insulin-like growth factor regulatory system in vitro and in vivo

Rosiglitazone (Rosi) belongs to the class of thiazolidinediones (TZDs) that are ligands for peroxisome proliferator-activated receptor gamma (PPARgamma). Stimulation of PPARgamma suppresses bone formation and enhances marrow adipogenesis. We hypothesized that activation of PPARgamma down-regulates components of the IGF regulatory system, leading to impaired osteoblast function. Rosi treatment (1 microm) of a marrow stromal cell line (UAMS-33) transfected with empty vector (U-33/c) or with PPARgamma2 (U-33/gamma2) were analyzed by microarray. Rosi reduced IGF-I, IGF-II, IGFBP-4, and the type I and II IGF receptor (IGF1R and IGF2R) expression at 72 h in U-33/gamma2 compared with U-33/c cells (P < 0.01); these findings were confirmed by RT-PCR. Rosi reduced secreted IGF-I from U-33/gamma2 cells by 75% (P < 0.05). Primary marrow stromal cells (MSCs) extracted from adult (8 months) and old (24 months) C57BL/6J (B6) mice were treated with Rosi (1 microm) for 48 h. IGF-I, IGFBP-4, and IGF1R transcripts were reduced in Rosi-treated MSCs compared with vehicle (P < 0.01) and secreted IGF-I was also suppressed (P < 0.05). B6 mice treated with Rosi (20 mg/kg.d) for short duration (i.e. 4 d), and long term (i.e. 7 wk) had reduced serum IGF-I; this was accompanied by markedly suppressed IGF-I transcripts in the liver and peripheral fat of treated animals. To determine whether Rosi affected circulating IGF-I in humans, we measured serum IGF-I, IGFBP-2, and IGFBP-3 at four time points in 50 postmenopausal women randomized to either Rosi (8 mg/d) or placebo. Rosi-treated subjects had significantly lower IGF-I at 8 wk than baseline (-25%, P < 0.05), and at 16 wk their levels were reduced 14% vs. placebo (P = 0.15). We conclude that Rosi suppresses IGF-I expression in bone and liver; these changes could affect skeletal acquisition through endocrine and paracrine pathways.

Congenic mice provide in vivo evidence for a genetic locus that modulates serum insulin-like growth factor-I and bone acquisition

We identified quantitative trait loci (QTL) that determined the genetic variance in serum IGF-I through genome-wide scanning of mice derived from C57BL/6J(B6) x C3H/HeJ(C3H) intercrosses. One QTL (Igf1s2), on mouse chromosome 10 (Chr10), produces a 15% increase in serum IGF-I in B6C3 F2 mice carrying c3 alleles at that position. We constructed a congenic mouse, B6.C3H-10 (10T), by backcrossing c3 alleles from this 57-Mb region into B6 for 10 generations. 10T mice have higher serum and skeletal IGF-I, greater trabecular bone volume fraction, more trabeculae, and a higher number of osteoclasts at 16 wk, compared with B6 (P < 0.05). Nested congenic sublines generated from further backcrossing of 10T allowed for recombination and produced four smaller sublines with significantly increased serum IGF-I at 16 wk (i.e. 10-4, 10-7, 10-10, and 10-13), compared with B6 (P < 0.0003), and three smaller sublines that showed no differences in IGF-I vs. age- and gender-matched B6 mice. Like 10T, the 10-4 nested sublines at 16 wk had higher femoral mineral (P < 0.0001) and greater trabecular connectivity density with significantly more trabeculae than B6 (P < 0.01). Thus, by comprehensive phenotyping, we were able to narrow the QTL to an 18.3-Mb region containing approximately 148 genes, including Igf1 and Elk-3(ETS domain protein). Allelic differences in the Igf1s2 QTL produce a phenotype characterized by increased serum IGF-I and greater peak bone density. Congenic mice establish proof of concept of shared genetic determinants for both circulating IGF-I and bone acquisition.

Resistance training, and IGF involvement in the maintenance of muscle mass during the aging process

Sarcopenia is the decline of muscle mass and strength with age. Sarcopenia leads to significant impairment in the ability to carry out normal daily function and thus there is a great need for interventions that will lead to muscle regeneration and repair in the aging population. Age-related sarcopenia in humans, characterized by loss of type I and type II muscle fibers and a decrease in fiber cross-sectional area primarily in type II fibers, can be attenuated by mechanical load on the muscle, which increases cross-sectional area of the remaining fibers, but does not restore fiber numbers characteristic of young muscle. Considerable evidence also implicates age-related declines in muscle insulin-like growth factor action in sarcopenia. IGF-I promotes myoblast proliferation, differentiation, and protein accretion in muscle through multiple signaling mechanisms, including the PI3-kinase, MAP kinase and calcineurin pathways. Exercise and injury induce increases in IGF-I, IGF-I receptors and IGF-I-activated signaling pathways. Although there is evidence that aging muscle retains the ability to synthesize IGF-I, there is also evidence that aging may be associated with attenuation of the ability of exercise to induce an isoform of IGF-I that promotes satellite cell proliferation. Moreover, aging muscle may be resistant to IGF-I, an effect that is reversed by exercise. However, it is clear that over-expression of IGF-I in muscle can protect against age-related sarcopenia.

Genetic increase in serum insulin-like growth factor-I (IGF-I) in C3H/HeJ compared with C57BL/6J mice is associated with increased transcription from the IGF-I exon 2 promoter

C3H/HeJ (C3H) mice exhibit 30-40% higher serum IGF-I than do C57BL/6J (B6) mice, in association with increased bone mineral density and strength. These differences are inherited and thus provide a model for determining molecular mechanisms for genetic variation of serum IGF-I and downstream actions. We now report that increased serum IGF-I in C3H mice is associated with increased transcription from the minor exon 2 promoter in liver from female and male mice. The increase in hepatic IGF-I gene expression caused by increased abundance of IGF-I mRNA transcribed from the exon 2 promoter can quantitatively account for the increased serum IGF-I in C3H mice. Also, levels of both Ea and Eb IGF-I mRNAs are increased in livers of male C3H mice. Fasting lowered serum IGF-I and liver IGF-I mRNA levels in female mice of both strains. However, serum IGF-I and liver IGF-I mRNA levels remained higher in fasted C3H mice compared with fasted B6 mice. Levels of IGF-I transcripts initiated from exon 2 are also significantly increased in skeletal muscle, fat, ovaries, and kidneys of C3H mice. IGF binding protein (IGFBP)-5 mRNA levels are significantly higher in muscle and fat of C3H mice than in B6 mice. Levels of exon 1-containing transcripts are increased in whole femurs of male and female C3H mice. We conclude that increased transcription of the IGF-I gene occurs in a promoter- and tissue-specific manner in C3H mice. The increased IGF binding protein-5 mRNA levels in fat and muscle suggest that IGF-I signaling is increased in these tissues in C3H mice.

Hypoenergetic high-carbohydrate or high-fat parenteral nutrition induces a similar metabolic response with differential effects on hepatic IGF-I mRNA in dexamethasone-treated rats

The optimal level of energy for critically ill patients who require parenteral nutrition (PN) is unclear. Our objective was to determine whether 50% energy (50%E) restriction due to a reduction in carbohydrate or fat, with provision of adequate protein and micronutrients, ameliorates the detrimental effects of dexamethasone (Dex) on body protein catabolism, insulin resistance, and insulin-like growth factor-I (IGF-I) responses in rats administered PN. The experiment included 6 PN groups, adequate energy (AE) +/- Dex, 50% AE with high carbohydrate (50%E CHO) +/- Dex and 50% AE with high fat (50%E FAT) +/- Dex. There was a significant interaction between energy level and Dex such that the increase in body catabolism due to 50%E from CHO or FAT was reduced by approximately 50%, although the amount of body weight and nitrogen lost over 7 d was significantly greater with 50%E than with AE. AE+Dex induced a 60% increase in liver mass, whereas 50%E+Dex reduced the increase to 26%. AE+Dex induced a 5-fold increase in serum insulin level, whereas 50%E+Dex normalized the insulin to glucose ratio. Serum IGF-I levels were reduced 14-18% by Dex and 30% by 50%E. Hepatic immunoreactive IGF-I was significantly correlated with serum IGF-I and nitrogen balance. 50%E CHO and 50%E FAT had differential effects on hepatic IGF-I mRNA with a 40% decrease in IGF-I mRNA due to 50%E FAT+Dex. In summary,CHO or FAT hypoenergetic PN with adequate protein had similar effects in normalizing hyperinsulinemia, attenuating hepatomegaly, and reducing the increment, but not the total amount of body protein catabolism, induced by glucocorticoid excess.

Congenic mice with low serum IGF-I have increased body fat, reduced bone mineral density, and an altered osteoblast differentiation program

Targeted gene studies have demonstrated the importance of insulin-like growth factor-I (IGF-I) for osteoblast (OB) differentiation and the acquisition of peak bone mineral density (BMD). The skeletal response to allelic differences in IGF-I expression can also be measured in vivo, using congenic mice. We created a congenic strain with reduced (approximately 20%) circulating IGF-I (C3H.B6-6T [6T]) by backcrossing a small genomic region (30 cM) of Chromosome 6 (Chr6) from C3H/HeJ (C3H) onto a C57Bl/6J (B6) background. 6T female mice have lower serum IGF-I (P<0.001 vs. B6) but similar growth hormone (GH) and serum IGF binding protein (IGFBP) concentrations as B6. At 16 weeks of age, congenics have greater body fat (P<0.02 vs. B6) despite less total body weight, and exhibit smaller femoral cross-sectional size (P=0.001), reduced cortical thickness (P<0.001) and lower trabecular BV/TV (P<0.05) than B6. 6T mice also have suppressed serum leptin (P<0.01), but compared to B6 have similar markers of bone resorption (i.e., urine CTx and serum TRAP 5B). At 8 weeks of age, skeletal IGF-I mRNA from long bones was reduced by 40% (P<0.05) as were liver mRNA transcripts (i.e., 50%, P<0.01). Osteoblast progenitors from the bone marrow of 6T mice formed less colony forming unit fibroblasts by crystal violet staining than B6 (P<0.007) and had significantly reduced alkaline phosphatase-positive colonies than B6(P<0.0001). In addition, staining of bone marrow with oil red O revealed greater numbers of adipocytes in 6T than B6. Several candidate genes in the Chr6 QTL were excluded by lack of strain-related expression differences in bone, but genes positively regulating adipocyte differentiation including Alox 5 and PPAR-gamma require further study as either "pathway" or candidate genes. In summary, allelic differences in a QTL on Chr6 result in altered IGF-I gene expression, changes in OB lineage allocation, and reduced peak bone mass. Congenic mice are useful models not only for mapping genes related to bone mass but also for elucidating the biology underlying various skeletal phenotypes associated with more subtle manipulation of the mouse genome.

The role of insulin and insulin-like growth factor-I in mammalian ageing

Research with invertebrates over the past 10 years has suggested that alterations in insulin and/or insulin-like growth factor I (IGF-I) signalling result in increased lifespan and retard ageing. In this chapter, we describe the current research in mammalian systems with respect to the role of insulin or IGF-I in ageing. Using rodent models of caloric restriction and genetic mouse models, e.g. the Ames and Snell dwarf mice, fat-specific insulin receptor knockout mice (FIRKO) and mice that are heterozygous for the IGF-I receptor (Igf1r+/-), investigators have shown that a reduction in plasma levels of insulin and/or IGF-I or reductions in insulin/IGF-I signalling appear to be correlated with increased longevity and retarded ageing.

Double-stranded ribonucleic acid decreases c6 rat glioma cell proliferation in part by activating protein kinase R and decreasing insulin-like growth factor I levels

We previously reported that reduction of autocrine IGF-I by polyinosinic-polycytidylic acid [poly(IC)] was permissive for the poly(IC)-mediated decrease in C6 rat glioma cell number. We now report that poly(IC) caused a block in G(1) to S transition in confluent C6 cultures, whereas in subconfluent cultures, poly(IC) decreased the percentage of cells in the G(2)/M phase. Addition of IGF-I to poly(IC)-treated cells decreased the percentage of cells in G(0)/G(1) phase and increased the percentage of cells in G(2)/M phase in confluent and subconfluent C6 cultures, indicating the reversal of cell cycle blocks. Inhibition of protein kinase R (PKR) activation partially prevented the poly(IC)-mediated cytostasis of C6 cells. Poly(IC) induced interferon-alpha in C6 cells. Both IGF-I and a blocking antibody against type I interferon (IFN) prevented the increase in PKR levels and the decrease in cell proliferation caused by poly(IC). We conclude that poly(IC) induces IFN, which mediates the cytostatic effect of poly(IC) on C6 cells at least in part through PKR. IGF-I prevents IFN from inducing PKR, thus explaining the ability of IGF-I to reverse the cell cycle blocks and the decreased C6 proliferation caused by poly(IC).

Generation of a new congenic mouse strain to test the relationships among serum insulin-like growth factor I, bone mineral density, and skeletal morphology in vivo

Insulin-like growth factor (IGF) I is a critical peptide for skeletal growth and consolidation. However, its regulation is complex and, in part, heritable. We previously indicated that changes in both serum and skeletal IGF-I were related to strain-specific differences in total femoral bone mineral density (BMD) in mice. In addition, we defined four quantitative trait loci (QTLs) that contribute to the heritable determinants of the serum IGF-I phenotype in F2 mice derived from progenitor crosses between C3H/HeJ (C3H; high total femoral BMD and high IGF-I) and C57BL/6J (B6; low total femoral BMD and low IGF-I) strains. The strongest QTL, IGF-I serum level 1 (Igflsl-1; log10 of the odds ratio [LOD] score, approximately 9.0), is located on the middle portion of chromosome (Chr) 6. For this locus, C3H alleles are associated with a significant reduction in serum IGF-I. To test the effect of this QTL in vivo, we generated a new congenic strain (B6.C3H-6T [6T]) by placing the Chr 6 QTL region (D6Mit93 to D6Mit150) from C3H onto the B6 background. We then compared serum and skeletal IGF-I levels, body weight, and several skeletal phenotypes from the N9 generation of 6T congenic mice against B6 control mice. Female 6T congenic mice had 11-21% lower serum IGF-I levels at 6, 8, and 16 weeks of age compared with B6 (p < 0.05 for all). In males, serum IGF-I levels were similar in 6T congenics and B6 controls at 6 weeks and 8 weeks but were lower in 6T congenic mice at 16 weeks (p < 0.02). In vitro, there was a 40% reduction in secreted IGF-I in the conditioned media (CMs) from 6T calvaria osteoblasts compared with B6 cells (p < 0.01). Total femoral BMD as measured by peripheral quantitative computed tomography (pQCT) was lower in both 6T male (-4.8%, p < 0.01) and 6T female (-2.3%, p = 0.06) congenic mice. Geometric features of middiaphyseal cortical bone were reduced in 6T congenic mice compared with control mice. Femoral cancellous bone volume (BV) density and trabecular number (Tb.N) were 50% lower, whereas trabecular separation (Tb.Sp) was 90% higher in 8-week-old female 6T congenic mice compared with B6 control mice (p < 0.01 for all). Similarly, vertebral cancellous BV density and Tb.N were lower (-29% and -19%, respectively), whereas Tb.Sp was higher (+29%) in 16-week-old female 6T congenic mice compared with B6 control mice (p < 0.001 for all). Histomorphometric evaluation of the proximal tibia indicated that 6T congenics had reduced BV fraction, labeled surface, and bone formation rates compared with B6 congenic mice. In summary, we have developed a new congenic mouse strain that confirms the Chr 6 QTL as a major genetic regulatory determinant for serum IGF-I. This locus also influences bone density and morphology, with more dramatic effects in cancellous bone than in cortical bone.

Double-stranded RNA decreases IGF-I gene expression in a protein kinase R-dependent, but type I interferon-independent, mechanism in C6 rat glioma cells

We previously demonstrated that Poly (IC) decreased the growth of C6 cultures in association with reduced IGF-I synthesis and secretion. In this study we characterized the mechanism(s) by which Poly (IC) decreased IGF-I mRNA in C6 cells. Both Poly (IC) and type I interferon (IFN) decreased IGF-I mRNA. Cycloheximide and a blocking antibody against IFN did not alter the Poly (IC)-mediated inhibition of IGF-I mRNA, but prevented IFN from reducing IGF-I mRNA. Poly (IC) did not alter the stability of IGF-I mRNA. Poly (IC) decreased the abundance of IGF-I pre-mRNA in C6 nuclei, but did not inhibit proximal IGF-I exon 1 promoter/luciferase fusion constructs in transient transfection assays. Poly (IC) activated double-stranded RNA-activated protein kinase (PKR) at 5 min and increased PKR protein levels at 48 and 72 h. Exogenous IGF-I did not prevent Poly (IC) from activating PKR, but inhibited the Poly (IC)-mediated increase in PKR protein levels. The PKR inhibitor 2-aminopurine prevented the Poly (IC) stimulation of eIF2-alpha phosphorylation and the Poly (IC)-mediated decrease in IGF-I mRNA. We conclude that Poly (IC) decreases IGF-I gene transcription in a mechanism that requires the activation of preexisting PKR, but not the induction of IFN or PKR proteins in C6 cells.

Dexamethasone decreases serum and liver IGF-I and maintains liver IGF-I mRNA in parenterally fed rats

Insulin-like growth factor-I (IGF-I) gene expression is regulated by nutritional and hormonal factors. High-dose glucocorticoids decrease food intake, and this confounds studies addressing glucocorticoid effects on IGF-I gene regulation. We investigated alterations in the hepatic IGF-I endocrine system induced by a catabolic dose of dexamethasone (Dex) in rats given adequate nutrition by continuous infusion of total parenteral nutrition (TPN) solution with or without IGF-I administration. The four TPN groups included control, +Dex, +IGF-I, and +IGF-I + Dex (n = 9-11/group). Dex induced a 12% loss of body weight in association with a 50% decrease in hepatic immunoreactive IGF-I, a 10% decrease in serum IGF-I, and no change in steady-state liver IGF-I mRNA or growth hormone (GH) receptor binding. Exogenous IGF-I increased serum IGF-I, attenuated Dex-induced catabolism, and did not reduce hepatic levels of IGF-I and IGF-I mRNA despite decreased serum GH. These data suggest that Dex-induced catabolism is associated with downregulation of the hepatic IGF-I endocrine system at the translational or posttranslational level when adequate nutrition is provided.

Aberrant expression and activation of insulin-like growth factor-1 receptor (IGF-1R) are mediated by an induction of IGF-1R promoter activity and stabilization of IGF-1R mRNA and contributes to growth factor independence and increased survival of the pancreatic cancer cell line MIA PaCa-2

In the present study we investigated the mechanisms responsible for and the biological consequences of the constitutive activation of the insulin-like growth factor-1 receptor (IGF-1R) in the MIA PaCa-2 cells. An aberrant increase in the expression and activation of the IGF-1R was observed during the transition of growth states from exponential to quiescent. The increase in IGF-1R expression is preceded by an increase in IGF-1R mRNA transcript and is associated with an increase in the IGF-1R promoter activity. Inhibition of de novo transcription by actinomycin D increased the stability of IGF-1R mRNA in exponentially growing cells, thereby increasing the expression of IGF-1R to a level similar to that seen in quiescent cells. Increased IGF-1R signaling mediated the growth factor independence of quiescent MIA PaCa-2 cells through the constitutive activation of mitogen-activated protein kinase (MAPK). Exogenous IGF-1 increased cell proliferation and activated MAPK and AKT signaling pathways. The resistance of cells to apoptosis by IGF-1R signaling was mediated through MAPK and phosphatidylinositol 3-kinase (PI3K) pathways and a yet unidentified pathway(s). Thus, aberrant regulation of IGF-1R signaling is required for resistance to apoptosis and growth factor independence of MIA PaCa-2 cells. This likely protects cells from unfavorable conditions and allows cells to rapidly re-enter the cell cycle when conditions are favorable.

Cyclic AMP inhibits extracellular signal-regulated kinase and phosphatidylinositol 3-kinase/Akt pathways by inhibiting Rap1

Cyclic AMP inhibited both ERK and Akt activities in rat C6 glioma cells. A constitutively active form of phosphatidylinositol 3-kinase (PI3K) prevented cAMP from inhibiting Akt, suggesting that the inactivation of Akt by cAMP is a consequence of PI3K inhibition. Neither protein kinase A nor Epac (Exchange protein directly activated by cAMP), two known direct effectors of cAMP, mediated the cAMP-induced inhibition of ERK and Akt phosphorylation. Cyclic AMP inhibited Rap1 activation in C6 cells. Moreover, inhibition of Rap1 by a Rap1 GTPase-activating protein-1 also resulted in a decrease in ERK and Akt phosphorylation, which was not further decreased by cAMP, suggesting that cAMP inhibits ERK and Akt by inhibiting Rap1. The role of Rap1 in ERK and Akt activity was further demonstrated by our observation that an active form of Epac, which activated Rap1 in the absence of cAMP, increased ERK and Akt phosphorylation. Inhibition of ERK and/or PI3K pathways mediated the inhibitory effects of cAMP on insulin-like growth factor-I (IGF-I) and IGF-binding protein-3 gene expression. Moreover, cAMP, as well as ERK and PI3K inhibitors produced equivalent stimulation and inhibition, respectively, of p27(Kip1) and cyclin D2 protein levels, potentially explaining the observation that cAMP prevented C6 cells from entering S phase.

Differential regulation of IGF-binding protein gene expression by cAMP in rat C6 glioma cells

We previously reported that cAMP inhibits autocrine IGF-I gene expression in rat C6 glioma cells. In this study we examined the influence of cAMP on IGF-binding protein gene expression in C6 cells. cAMP potently inhibited IGF-binding protein-3 mRNA and, to a lesser extent, IGF-binding protein-4 mRNA and transiently stimulated IGF-binding protein-5 mRNA. The changes in secreted IGF-binding proteins whose molecular weights were consistent with IGF-binding protein-3 and -5 correlated with those of mRNA levels. cAMP decreased the IGF-binding protein-3 mRNA half-life, but did not alter IGF-binding protein-4 and -5 mRNA half-lives. An IGF-binding protein-5 promoter/luciferase fusion construct containing 888 bp of 5'-flanking sequence and the first 114 bp of exon 1 sequence was stimulated by cAMP after 24 h by approximately 2-fold in transient transfection assays. 5'- or 3'-deletion to -33 or +10 (the transcription start site was designated as +1), respectively, did not alter the increase caused by cAMP. Site-directed mutagenesis of the region from -14 to -5 led to a loss of the ability of the IGF-binding protein-5 promoter to respond to cAMP. H89, a cell-permeable protein kinase A inhibitor, did not alter the regulation of IGF-binding protein mRNAs in response to cAMP.

Cyclic adenosine 3',5'-monophosphate inhibits insulin-like growth factor I gene expression in rat glioma cell lines: evidence for regulation of transcription and messenger ribonucleic acid stability

cAMP inhibits growth and stimulates differentiation in glioma cells. We examined the effect of cAMP on insulin-like growth factor I (IGF-I) gene expression in the C6 cell line, a rat glioma cell line previously reported to grow in response to autocrine IGF-I. cAMP potently inhibited IGF-I messenger RNA (mRNA) and peptide secretion in C6 cells, associated with an attenuation of DNA synthesis. Exogenous IGF-I peptide at least partially prevented the inhibition of DNA synthesis, suggesting that the reduction in IGF-I biosynthesis may contribute to the inhibitory effect of cAMP on C6 cell growth. cAMP also inhibited IGF-I mRNA in rat RG2 glioma cells, but not in three other nonglioma tumor cell lines. The nuclear IGF-I pre-mRNA level and the half-life of mature IGF-I mRNA were both reduced by cAMP in C6 cells, suggesting effects on gene transcription and mRNA stability. However, cAMP had no effect on the activities of IGF-I exon 1 promoter-luciferase constructs. Protein synthesis inhibition partially reduced the inhibition of IGF-I mRNA by cAMP. Inhibition of cAMP-activated protein kinase A activity by H89 did not alter the inhibition of IGF-I gene expression in response to cAMP, suggesting that protein kinase A does not mediate the cAMP inhibitory effect on IGF-I gene expression.

Double-stranded ribonucleic acid decreases C6 rat glioma cell numbers: effects on insulin-like growth factor I gene expression and action

Poly(IC), a synthetic double-stranded RNA copolymer of inosinic and cytidilic acids, decreases the growth of normal and tumorigenic cells. We tested the hypothesis that Poly(IC) decreases C6 glioma cell growth by disrupting an autocrine insulin-like growth factor I (IGF-I) growth loop. Addition of Poly(IC) decreased C6 cell number in confluent and sparse cultures in a dose-dependent manner. Addition of exogenous IGF-I partially compensated for the decrease in cell number caused by Poly(IC) in confluent and subconfluent cultures of C6 cells, suggesting that one mechanism of Poly(IC) action is through down-regulation of IGF-I gene expression and/or action. Treatment of confluent C6 cells with 10 and 200 microg/ml Poly(IC) for 24 h decreased IGF-I messenger RNA (mRNA) levels to 50% and 25% of the control value, respectively. Treatment of C6 cells with 200 microg/ml Poly(IC) for 24 h reduced IGF-I receptor mRNA levels to 50% of the control level. IGF-binding protein-1 (IGFBP-1), -2, and -6 mRNAs were not expressed in the C6 cells used in this study. Treatment of C6 cells with 200 microg/ml Poly(IC) for 24 h reduced IGFBP-4 mRNA and IGFBP-5 mRNA levels to 26% and 29% of the control level, respectively. There was no significant change in IGFBP-3, insulin receptor, or actin mRNA levels with Poly(IC) treatment. Treatment of confluent C6 cells with 200 microg/ml Poly(IC) for 24 h decreased levels of immunoreactive IGF-I in conditioned medium (CM) to 55% of the control value, decreased IGF-I receptor beta-subunit levels to 28% of the control value, and decreased levels of IGFBP-3, IGFBP-4, and IGFBP-5 protein in CM to 45%, 50%, and 30% of the control values, respectively. There was no significant change in actin and tubulin protein levels with Poly(IC) treatment. These results suggest that IGF-I gene expression is down-regulated by Poly(IC) treatment and that IGF-I bioavailability and action in C6 cells are also altered due to decreases in IGF-I receptor and binding protein levels.

Enterotrophic effect of insulin-like growth factor-I but not growth hormone and localized expression of insulin-like growth factor-I, insulin-like growth factor binding protein-3 and -5 mRNAs in jejunum of parenterally fed rats

BACKGROUND

Administration of insulin-like growth factor (IGF)-I, but not growth hormone (GH), stimulates mucosal hyperplasia in surgically stressed rats with intestinal atrophy induced by hypocaloric total parenteral nutrition (TPN). Our aim was to characterize the basis for this disparity in enterotrophic action by assessing the relationships between stimulation of intestinal growth, nutritional adequacy, and localization of expression of IGF-I, insulin-like growth factor binding protein (IGFBP)-3 and IGFBP-5 mRNAs in jejunum.

METHODS

Rats were maintained with TPN for 8 days and treated with IGF-I or GH and adequate nutrition for 5 days after recovery from surgery. Jejunal mass, morphology, and sucrase activity were assessed. Localization of expression of IGF-I, IGFBP-3, and IGFBP-5 mRNAs in jejunum was accomplished by in situ hybridization.

RESULTS

Serum IGF-I and body weight gain were significantly increased by IGF-I or GH. Jejunal mucosal dry mass, morphology, and sucrase activity were improved with IGF-I but not GH. There were no differences in IGF-I mRNA. IGFBP-3 mRNA was localized in the lamina propria of the villi. IGF-I or GH stimulated IGFBP-3 expression. IGF-I strongly stimulated IGFBP-5 expression in the lamina propria and the muscularis and induced a twofold increase in IGFBP-5 mRNA based on RNase protection assay of intact jejunum total RNA. GH induced a modest increase in IGFBP-5 expression in the muscularis with no effect on intact jejunum total RNA.

CONCLUSIONS

The GH resistance observed in the jejunal mucosa of TPN rats cannot be fully explained by inadequate nutrition. The expression of IGFBP-5 in the lamina propria suggests it may modulate the enterotrophic action of exogeneous IGF-I.

Cell density influences insulin-like growth factor I gene expression in a cell type-specific manner

The effect of cellular density on insulin-like growth factor I (IGF-I) gene expression was characterized in several tumor-derived cell lines. IGF-I messenger RNA (mRNA) transcripts increased more than 200-fold when C6 glioma cells grew to postconfluence. IGF-I receptor and beta-actin mRNAs were induced by 6- and 2-fold, respectively, as a function of confluence. IGF-I mRNA transcripts in GH3 and SK-N-MC cells increased about 4- to 5-fold in confluent cultures compared with sparse cultures. In OVCAR-3 cells, the IGF-I mRNA level remained constant as the cell density increased. Transient transfection experiments were performed with IGF-I exon 1 promoter/luciferase fusion constructs in C6 cells. The luciferase activity of a construct containing exon 1 sequence between +75 and +282 (the most 5' transcription initiation site was designated +1) was stimulated by 2.5-fold in dense cultures compared with that in sparse cultures of C6 cells. Luciferase activities of other constructs containing at least 282 bp of exon 1 sequence were also stimulated about 2- to 4-fold by cell density. However, 3' deletion to +192 led to loss of the cell density stimulatory effect. In contrast, luciferase activities of IGF-I promoter constructs were not altered by cell density in SK-N-MC cells. When the conditioned medium of low density C6 cultures was exchanged with that of high density cultures, the IGF-I mRNA level remained the same. In summary, cell density has a cell type- and gene type-specific effect on IGF-I gene expression. A cell density response element(s) may be located between +192 and +282 of the exon 1 promoter region in C6 cells.

Glucose starvation reduces IGF-I mRNA in tumor cells: evidence for an effect on mRNA stability

The purpose of this study was to characterize the mechanisms by which glucose regulates IGF-I gene expression in rat C6 glioma cells and in rat GH3 pituitary adenoma cells. Glucose starvation for periods of 12 to 48 h decreased IGF-I mRNA levels. In contrast, there was no stimulation of IGF-I mRNA by medium glucose between 1 and 25 mM over a 24-h period. Studies with hexoses and glycolytic metabolites suggested that glucose metabolism was required to maintain IGF-I mRNA. Glucose starvation lowered IGF-I mRNA half-life in both C6 and GH3 cells. Protein synthesis inhibition lowered IGF-I mRNA by about 20% in glucose-fed C6 and GH3 cells, while potently increasing IGF-I mRNA in glucose-starved C6 cells and not altering IGF-I mRNA in glucose-starved GH3 cells. Our results suggest that in these tumor cells, IGF-I mRNA stability is reduced by glucose starvation, secondary to a deficiency in intracellular glucose metabolism. Ongoing protein synthesis is not required for this mRNA de-stabilizing effect in GH3 cells. Rather, in glucose-starved C6 cells, decreased IGF-I mRNA stability may result from the action of a labile protein.

Two putative GATA motifs in the proximal exon 1 promoter of the rat insulin-like growth factor I gene regulate basal promoter activity

The insulin-like growth factor I gene is transcribed from two promoters, which direct synthesis of alternative first exons (exon 1 and exon 2) in insulin-like growth factor I messenger RNAs (mRNAs). An exon 1 promoter construct extending from +75 to +192 (the most upstream exon 1 start site was designated as +1) showed significant promoter activity in C6, OVCAR-3, and SK-N-MC cells. Within the +75 to +192 region, there are two perfect matches to the consensus binding site for GATA transcription factor, at +108 (GATA-A) and at +183 (GATA-B). Mutations of the GATA-A or GATA-B sequences resulted in slight (or no) effect on exon 1 promoter activity in both C6 and OVCAR-3 cells. However, mutation of the GATA-A sequence stimulated exon 1 promoter activity by 68% in SK-N-MC cells. Mutation of the GATA-B sequence inhibited exon 1 promoter activity by 4.4-fold in SK-N-MC cells. Electrophoretic mobility shift assays showed that there were nuclear proteins in SK-N-MC cells capable of specifically binding to the GATA-A and GATA-B elements and that this binding was GATA-sequence specific. GATA-2, GATA-3, and GATA-4 are the only GATA proteins that have been reported to be expressed in neurons. None of the antibodies against these three GATA proteins were capable of inhibiting or supershifting the bands formed by the nuclear proteins and oligonucleotides containing GATA-A or GATA-B elements. A GATA-1 expression vector was used to perform cotransfection experiments. The GATA-A mutation abolished the stimulatory effect of the GATA-1 factor on promoter activity. In contrast, the GATA-B mutation enhanced the stimulatory effect of GATA-1 protein. Anti-GATA-1 antibody was also incapable of inhibiting or supershifting the bands formed by the nuclear proteins and oligonucleotides containing the GATA-A or GATA-B elements. In conclusion, the GATA-A element seems to bind an inhibitory endogenous factor(s) in SK-N-MC cells, whereas the GATA-B element may bind a stimulatory factor(s). These factors seem to be related to GATA transcription factors but are immunologically distinct from GATA-2, GATA-3, or GATA-4. GATA-1 has the potential to transactivate the exon 1 promoter through the GATA-A element but is unlikely to be the endogenous protein binding to the GATA-A or the GATA-B motifs in SK-N-MC cells.

Investigation of insulin-like growth factor (IGF)-I and insulin receptor binding and expression in jejunum of parenterally fed rats treated with IGF-I or growth hormone

To investigate the ability of insulin-like growth factor-I (IGF-I), but not GH, to stimulate jejunal growth, we compared indices of IGF-I and insulin receptor expression in jejunal membranes from rats maintained with total parenteral nutrition (TPN) and treated with rhIGF-I and/or rhGH. TPN without growth factor treatment (TPN control) induced jejunal atrophy, reduced serum IGF-I, increased serum insulin concentrations, and increased IGF-I receptor number, IGF-I receptor messenger RNA, and insulin-specific binding to 133% to 170% of the orally fed reference values, P < 0.01. Compared with TPN control, IGF-I or IGF-I + GH stimulated jejunal mucosal hyperplasia; IGF-I treatment increased serum IGF-I by 2- to 3-fold and decreased serum insulin concentrations by 60%, decreased IGF-I receptor number by 50% (P < 0.001), and increased insulin receptor affinity and insulin receptor protein content. Treatment with GH alone increased serum IGF-I concentration, did not alter TPN-induced jejunal atrophy, and decreased insulin-specific binding and insulin receptor protein content (39% and 59%, respectively, of the TPN control values, P < 0.01). We conclude that: 1) jejunal IGF-I receptor content reflects circulating concentration of ligand and is not limiting for jejunal growth; and 2) increased circulating concentration of IGF-I may promote jejunal growth via interaction with jejunal insulin or IGF-I receptors.

Growth hormone-mediated regulation of insulin-like growth factor I promoter activity in C6 glioma cells

The molecular mechanisms by which GH regulates insulin-like growth factor (IGF-I) gene expression remain obscure. One difficulty has been the lack of established GH-responsive cell lines that express the IGF-I gene. To develop such a cell line, we used rat C6 glioma cells which, as determined by RNase protection assay, express the IGF-I gene but not the GH receptor gene. To confer GH responsiveness, C6 cells were cotransfected with vectors that express the GH receptor (pRc/CMV WTrGHR) and Jak2 (pRc/CMV Jak2). GH responsiveness was demonstrated using luciferase reporter genes containing either the Sis-inducible element from the c-fos gene (pTK81-SIE-Luc) or 6 copies of the GH-responsive GAS-like element (GLE) from the rat spi2.1 gene (pSpi-GLE-Luc). The SIE is activated by binding of STAT1 and 3, whereas the GLE binds STAT5. In cells cotransfected with pRc/CMV WTrGHR, pRc/CMV Jak2, and either pTK81-SIE-Luc or pSpi GLE-Luc, treatment with 500 ng/ml GH for 24 h stimulated a 3.1- and 1.7-fold increase in luciferase activity, respectively. These data suggest that in C6 cells cotransfected with pRc/CMV WTrGHR and pRc/CMV Jak2, GH activates STAT1, 3, and 5. To determine whether GH-responsive IGF-I promoter activity could be demonstrated, C6 cells were cotransfected with pRc/CMV WTrGHR, pRc/ CMV Jak2, and an IGF-I-luciferase fusion gene that contained a fragment of the rat IGF-I gene that extended from -412 in the 5'-flanking region of exon 1 to the Met-22 in exon 3. GH stimulated a modest, but reproducible, 1.7-fold increase in luciferase activity in these cells, suggesting that a GH-responsive element is present in this region of the IGF-I gene. To better localize the GH-responsive element, cells were cotransfected with pRc/CMV WTrGHR, pRc/CMV Jak2 plus one of several IGF-I-luciferase fusion genes containing either fragments of one of the two promoters in the IGF-I gene or a fragment of intron 2 that includes a GH-responsive DNase I hypersensitivity site. For all constructs, treatment with GH for 24 h did not stimulate a significant increase in luciferase activity, suggesting that GH-responsive sequences are not located in these specific regions of the IGF-I gene or that GH-directed transcription of the IGF-I gene is mediated via several different regions of the IGF-I gene and the effect of any one of these regions in isolation was not sufficiently robust to be detected in this model system. In summary, transient expression of the GH receptor and Jak2 in C6 cells creates a GH-responsive system that activates STAT1, 3, and 5. Moreover, a fragment of the IGF-I gene that contains exons 1 and 2, a fragment of exon 3, and introns 1 and 2 is GH responsive using this model system.

Osteogenic protein-1 regulates insulin-like growth factor-I (IGF-I), IGF-II, and IGF-binding protein-5 (IGFBP-5) gene expression in fetal rat calvaria cells by different mechanisms

Osteogenic protein-1 (OP-1 or BMP-7) stimulates new bone formation in vivo and induces cell proliferation and differentiation of osteoblasts in vitro. Previous studies from our laboratory revealed that OP-1 led to a two- to threefold increase in steady-state insulin-like growth factor-I (IGF-I) and IGF-II mRNA levels and a fivefold decrease in IGF-binding protein-5 (IGFBP-5) mRNA levels in primary cultures of fetal rat calvaria (FRC) cells. In the present study, we determined whether the effects of OP-1 were at the transcriptional or posttranscriptional level. OP-1 increased the half-life of the IGF-I mRNA from 6 to 17 h without changing the level of IGF-I nuclear pre-mRNA. In transiently transfected FRC cells, the luciferase activity driven by the -1122/+362 or the -133/+362 IGF-I exon 1 promoter fragment was not changed by OP-1. Similar results were observed using the -1500/+44 or -362/+44 IGF-I exon 2 promoter constructs. Effects of OP-1 on IGF-I mRNA were independent of cell division, as they remained elevated in the presence of hydroxyurea. Cycloheximide inhibited moderately the OP-1-induced increase in IGF-I mRNA, suggesting partial dependency on protein synthesis. On the other hand, the IGF-II nuclear pre-mRNA levels were increased by OP-1 but the half-life of the mature IGF-II mRNA was not affected. Effects of OP-1 on IGF-II mRNA were also independent of cell division, but were dependent on protein synthesis. OP-1 caused a 43-50% reduction in the level of IGFBP-5 nuclear pre-mRNA transcripts and a 40% decrease in the IGFBP-5 promoter activity in FRC cells transfected with the -1278/+1 IGFBP-5 promoter fragment. The half-life of the mature IGFBP-5 mRNA was not affected by OP-1. Hydroxyurea did not prevent the OP-1-induced reduction in IGFBP-5 mRNA. The level of IGFBP-5 mRNA was barely detectable in the presence of cycloheximide, and further suppressive effect of OP-1 on IGFBP-5 mRNA could not be determined. In conclusion, OP-1 regulates IGF-I gene expression at the posttranscriptional level, but regulates IGF-II and IGFBP-5 gene expression at the transcriptional level.

A CACCC box in the proximal exon 2 promoter of the rat insulin-like growth factor I gene is required for basal promoter activity

The insulin-like growth factor I gene is transcribed from two promoter regions, resulting in alternative first exons in insulin-like growth factor I messenger RNAs. A previous study showed that the sequence from -73 to +44 (where +1 is the first nucleotide in the exon 2 transcription initiation cluster) contained an active exon 2 promoter, and that sequences between -73 and -36 were required for promoter activity. In the current study, the roles of two putative cis-acting elements within the -73 to +44 region in basal exon 2 promoter activity were evaluated using mutagenesis and nuclear protein-DNA binding assays. Mutation of the CCCCACCC sequence at position -53 to GAAATCCC resulted in a complete loss of promoter activity in transient transfection assays in GH3, OVCAR-3, C6, and Chinese hamster ovary (CHO) cells. A -73/+24 exon 2 promoter-luciferase construct had partial promoter activity. Mutation of a putative initiator motif surrounding the major exon 2 start site did not alter the activity of this construct. In electrophoretic mobility shift assays, a 32P-labeled oligomer extending from -73 to +44 in the exon 2 promoter was specifically bound by GH3 cell nuclear extracts. A 32P-labeled oligomer which extended from -63 to -37 in the exon 2 promoter was specifically bound by GH3 and OVCAR-3 cell nuclear extracts. These unlabeled oligomers inhibited the binding of a labeled -236/+44 exon 2 promoter fragment to OVCAR-3 nuclear extracts. Mutation of the CCCCACCC sequence prevented the unlabeled -73/+44 oligomer from inhibiting the binding of the -236/+44 fragment. An unlabeled oligomer containing a consensus activating protein-2 (AP-2)-binding site inhibited labeled -236/+44, -73/+44, and -63/-37 exon 2 promoter binding with a much lower affinity than did the respective unlabeled oligomers. Purified AP-2 protein did not bind to the exon 2 promoter fragment, nor did anti-AP-2 antibody alter the binding. Cotransfection of AP-2 expression vector did not significantly increase exon 2 promoter activity. On the other hand, an oligomer containing a consensus Sp1-binding site inhibited labeled -63/-37 exon 2 promoter binding by GH3 cell nuclear extracts with an affinity similar to that of the unlabeled -63/-37 oligomer. A mutation in the Sp1-binding site in this same oligomer resulted in a complete loss of binding affinity. Purified Sp1 bound to the -63/-37 exon 2 promoter oligomer. Addition of polyclonal antibody to Sp1 resulted in a partial supershift of the complex formed between GH3 cell and OVCAR-3 cell nuclear extracts and the labeled -63/-37 oligomer. However, in Drosophila Schneider cells, which are an experimental model for studying the ability of Sp1 to activate transcription, the -73/+44 exon 2 promoter construct was not stimulated by cotransfection with an Sp1 expression plasmid. UV cross-linking studies indicated that proteins of approximate molecular mass 125, 76, 47, and 38 kDa are bound to the proximal (-236/+44) exon 2 promoter region. It is concluded that the CCCCACCC sequence at -53 is required for exon 2 promoter activity. Moreover, specific binding of nuclear proteins to the proximal exon 2 promoter region requires the CCCCACCC sequence. Sequences downstream of the exon 2 initiation site from +24 to +44 are required for full promoter activity. However, the putative initiator surrounding the major transcription start site at +1 does not appear to be important for the strength of the proximal promoter. The CCCCACCC sequence at -53 appears to be a CACCC box, which binds zinc finger transcription factors of the Kruppel family such as Sp1, although protein factors in addition to Sp1 are required to activate exon 2 transcription through the -73/+44 proximal promoter region.

Stimulation of intestinal growth is associated with increased insulin-like growth factor-binding protein 5 mRNA in the jejunal mucosa of insulin-like growth factor-I-treated parenterally fed rats

Surgically stressed rats maintained with total parenteral nutrition (TPN) exhibit jejunal atrophy, which can be attenuated by insulin-like growth factor-I (IGF-I) but not by growth hormone (GH) treatment. In order to understand the basis for the selective action of IGF-I, the levels of mRNAs encoding IGF-I, IGF-binding proteins (IGFBPs), IGF-I receptor, and GH receptor/binding protein (GHR/GHBP) were determined in rats given TPN and treated with GH, IGF-I, or GH + IGF-I. GH treatment significantly stimulated hepatic IGF-I mRNA. IGF-I treatment did not alter liver IGF-I mRNA, nor was there any evidence for interaction between GH and IGF-I. Jejunal mucosa IGF-I mRNA was extremely low and was not altered by TPN or by any of the hormonal treatments. The inability of GH to stimulate jejunal growth was not associated with a deficiency in GHR/GHBP mRNA. In jejunal mucosa, IGF-I and GH treatment independently and synergistically stimulated IGFBP-3 mRNA. IGF-I stimulated jejunal IGFBP-5 mRNA, but GH had no effect on IGFBP-5 mRNA. The levels of IGF-I receptor and IGFBP-1, 2, 4, and 6 mRNAs were extremely low and/or were not altered by any of the treatments. These results suggest that the ability of exogenous IGF-I, but not GH, to induce IGFBP-5 mRNA in jejunal mucosa may lead to the selective growth-promoting effect of IGF-I. Jejunal mucosa IGFBP-3 mRNA levels were not correlated with altered growth. We postulate that IGFBP-5 positively modulates the anabolic effects induced by exogenous IGF-I in the jejunum.

Osteogenic protein-1 and insulin-like growth factor I synergistically stimulate rat osteoblastic cell differentiation and proliferation

Previous studies have shown that osteogenic protein-1 (OP-1; also known as BMP-7) alters the steady state levels of messenger RNA (mRNA) encoding insulin-like growth factor I (IGF-I), IGF-II, and IGF-binding proteins (IGFBPs) in primary cultures of fetal rat calvaria (FRC) cells. In the present study, the effects of exogenous IGF-I on bone cell differentiation and mineralized bone nodule formation induced by OP-1 were examined. Exogenous IGF-I synergistically and dose dependently enhanced OP-1 action in stimulating [3H]thymidine incorporation, alkaline phosphatase activity, PTH-dependent cAMP level, and bone nodule formation. Maximal synergism between OP-1 and IGF-I was observed when both factors were added simultaneously. Synergism was not observed when FRC cells were pretreated with IGF-I for 24 h, followed by OP-1 treatment. These findings suggest that IGF-I acted on OP-1-sensitized FRC cells. To examine the mechanism(s) by which this sensitization may occur, levels of mRNA encoding OP-1 receptor, IGF-I receptor, and IGFBPs were measured. The mRNA levels of both type I and II OP-1 receptors were elevated by OP-1, but were not changed further by combined OP-1 and IGF-I treatment. IGF-I receptor gene expression was not changed by OP-1, IGF-I, or a combination of both factors. OP-1 alone or together with IGF-I increased the steady state IGFBP-3 mRNA level and reduced the steady state mRNA levels of IGFBP-4, -5, and -6. IGF-I alone did not change the steady state mRNA levels of IGFBP-3, -4, and -6, but elevated that of IGFBP-5. Des(1-3)-IGF-I, which has a lower affinity for IGFBPs, was more effective than the full-length IGF-I in enhancing the OP-1-induced alkaline phosphatase activity. Exogenous IGFBP-5 inhibited the OP-1-induced alkaline phosphatase activity and reduced the synergistic stimulatory effect of IGF-I and OP-1. These findings strongly suggest that the OP-1-induced down-regulation of IGFBPs, especially that of IGFBP-5, is an important mechanism by which OP-1 and IGF-I synergize to stimulate FRC cell differentiation.

Characterization of the rat insulin-like growth factor I gene promoters and identification of a minimal exon 2 promoter

Insulin-like growth factor I (IGF-I) promoter activity was characterized in C6, GH3, OVCAR-3, and Chinese hamster ovary (CHO) cells. Maximal exon 1 promoter activity was present in the region extending from -133 to +362 (where +1 is the first transcription start site). Promoter activity was higher in the +75/+362 fragment, which contains exon 1 transcription start sites 3 and 4, than in the -133/+74 fragment, which contains exon 1 transcription start sites 1 and 2. Promoter activity was also observed in constructs containing sequences from -133 to +192, which includes start sites 1, 2, and 3. Inclusion of sequences upstream of -133 inhibited exon 1 proximal promoter activity in a cell type-specific manner. Exon 2 promoter activity was observed in all cell lines with a construct containing 73 bp of 5'-flanking sequence and 44 bp of exon 2. Exon 2 promoter activity was abolished when only 36 bp of 5'-flanking sequence and 44 bp of exon 2 were present, suggesting that an essential minimal promoter element(s) is contained within the -73 to -36 region. A putative CACCC box was observed within this region at -53. Upstream sequence regulated exon 2 promoter activity in a cell type-specific manner. Electrophoretic mobility shift assays revealed a single specifically bound band when the +75/+362 fragment of the exon 1 promoter was used with nuclear extracts from C6 and GH3 cells. Multiple specifically bound bands with slower mobility were observed when the -236/+44 exon 2 promoter fragment was incubated with C6, GH3, CHO, and OVCAR-3 cell nuclear extracts. The exon 1 and exon 2 promoter regions were able to inhibit each other's binding in electrophoretic mobility shift assay using GH3 cell and OVCAR-3 cell nuclear extracts, respectively. Oligonucleotides containing consensus activating protein-1 (AP-1) and AP-3 sequences inhibited exon 1 promoter binding by GH3 cell nuclear extracts. AP-2 and AP-3 sites inhibited exon 2 promoter binding. Our data suggest that the sequence surrounding and including start site 3 in exon 1 functions as a minimal independent promoter. The minimal exon 2 promoter is contained within the 73 bp upstream and 44 bp downstream of the transcription start site cluster. These minimal promoters contain similar and distinct elements that are important for basal transcription. Upstream sequences may contain cell type-specific silencer elements.

Osteogenic protein-1-mediated insulin-like growth factor gene expression in primary cultures of rat osteoblastic cells

Osteogenic protein-1 (OP-1) is a member of the bone morphogenetic protein family and has been shown to induce new bone formation in vivo. In the present study, we determined whether the expression of the IGF system, a significant growth factor system in bone, was altered by OP-1 in primary cultures of fetal rat calvarial cells. Levels of messenger RNA (mRNA) encoding insulin-like growth factor I (IGF-I), IGF-II, IGF-I receptor, and IGF-binding proteins (IGFBP-1 to -6) were determined after OP-1 treatment. The level of total IGF-I mRNA was elevated in an OP-1 concentration (0-1000 ng/ml)-dependent manner, with maximal stimulation of IGF-I mRNA of 2- to 3-fold apparent 24 h after treatment. The increase in the IGF-I mRNA level involved a preferential stimulation of transcripts initiated at start site 2 in the exon 1 promoter. The level of IGF-II mRNA also increased by approximately 2-fold in OP-1 treated cells in a concentration-dependent manner. The level of IGF-I receptor mRNA was not altered by treatment. Whereas IGFBP-1 mRNA was not detected in these cells, IGFBP-2 mRNA was expressed, but the expression was not changed after treatment for 48 h in the concentration range (0-1000 ng/ml) tested. The IGFBP-3 mRNA level was increased slightly 48 h after OP-1 treatment in a concentration-dependent manner. The IGFBP-4, -5, and -6 mRNA levels decreased dramatically in an OP-1 concentration-dependent manner. In addition, coincubation of antisense oligonucleotides corresponding to IGF-I or -II mRNA sequence with OP-1 reduced the OP-1 induced elevation in alkaline phosphatase activity. The present results suggest that the differentiation of rat osteoblastic cells in response to OP-1 is mediated in part by increased IGF-I -II gene expression and alterations in the gene expression of different IGFBPs.