Identification of the cAMP response element that controls transcriptional activation of the insulin-like growth factor-I gene by prostaglandin E2 in osteoblasts

Anti-Osteogenic Activity of Cadmium in Zebrafish

Among the many anthropogenic chemicals that end up in the aquatic ecosystem, heavy metals, in particular cadmium, are hazardous compounds that have been shown to affect developmental, reproductive, hepatic, hematological, and immunological functions in teleost fish. There is also evidence that cadmium disturbs bone formation and skeletal development, but data is scarce. In this work, zebrafish was used to further characterize the anti-osteogenic/osteotoxic effects of cadmium and gain insights into underlying mechanisms. Upon exposure to cadmium, a reduction of the opercular bone growth was observed in 6-days post-fertilization (dpf) larvae and an increase in the incidence of skeletal deformities was evidenced in 20-dpf post-larvae. The extent and stiffness of newly formed bone was also affected in adult zebrafish exposed to cadmium while regenerating their caudal fin. A pathway reporter assay revealed a possible role of the MTF-1 and cAMP/PKA signaling pathways in mechanisms of cadmium osteotoxicity, while the expression of genes involved in osteoblast differentiation and matrix production was strongly reduced in cadmium-exposed post-larvae. This work not only confirmed cadmium anti-osteogenic activity and identified targeted pathways and genes, but it also suggested that cadmium may affect biomechanical properties of bone.

Insulin-Like Growth Factor I Regulation and Its Actions in Skeletal Muscle

The insulin-like growth factor (IGF) pathway is essential for promoting growth and survival of virtually all tissues. It bears high homology to its related protein insulin, and as such, there is an interplay between these molecules with regard to their anabolic and metabolic functions. Skeletal muscle produces a significant proportion of IGF-1, and is highly responsive to its actions, including increased muscle mass and improved regenerative capacity. In this overview, the regulation of IGF-1 production, stability, and activity in skeletal muscle will be described. Second, the physiological significance of the forms of IGF-1 produced will be discussed. Last, the interaction of IGF-1 with other pathways will be addressed. © 2019 American Physiological Society. Compr Physiol 9:413-438, 2019.

Loss of CD73-mediated extracellular adenosine production exacerbates inflammation and abnormal alveolar development in newborn mice exposed to prolonged hyperoxia

BackgroundHyperoxic lung injury is characterized by cellular damage from high oxygen concentrations that lead to an inflammatory response and it disrupts normal alveolarization in the developing newborn lung. Adenosine is a signaling molecule that is generated extracellularly by ecto-5'-nucleotidase (CD73) in response to injury. Extracellular adenosine signals through cell surface receptors and has been found to have a protective role in acute injury situations; however, chronic elevations have been associated with detrimental changes in chronic lung diseases. We hypothesized that hyperoxia-induced lung injury leads to CD73-mediated increases in extracellular adenosine, which are detrimental to the newborn lung.MethodsC57Bl/6 and CD73-/- mice were exposed to 95% oxygen, 70% oxygen, or room air. Adenosine concentration and markers of pulmonary inflammation and lung development were measured.ResultsExposure to hyperoxia caused pulmonary inflammation and disrupted normal alveolar development in association with increased pulmonary adenosine levels. Loss of CD73-mediated extracellular adenosine production led to decreased survival with exposure to 95% oxygen, and exacerbated pulmonary inflammation and worsened lung development with 70% oxygen exposure.ConclusionExposure to hyperoxia causes lung injury associated with an increase in adenosine concentration, and loss of CD73-mediated adenosine production leads to worsening of hyperoxic lung injury.Pediatric Research advance online publication, 23 August 2017; doi:10.1038/pr.2017.176.

Skeletal effects of growth hormone and insulin-like growth factor-I therapy

The growth hormone/insulin-like growth factor (GH/IGF) axis is critically important for the regulation of bone formation, and deficiencies in this system have been shown to contribute to the development of osteoporosis and other diseases of low bone mass. The GH/IGF axis is regulated by a complex set of hormonal and local factors which can act to regulate this system at the level of the ligands, receptors, IGF binding proteins (IGFBPs), or IGFBP proteases. A combination of in vitro studies, transgenic animal models, and clinical human investigations has provided ample evidence of the importance of the endocrine and local actions of both GH and IGF-I, the two major components of the GH/IGF axis, in skeletal growth and maintenance. GH- and IGF-based therapies provide a useful avenue of approach for the prevention and treatment of diseases such as osteoporosis.

Prostaglandin dependent control of an endogenous estrogen receptor agonist by osteoblasts

Estrogen receptor (ER) activation has complex effects on bone cells, and loss of circulating estradiol adversely affects skeletal status in women. Hormone replacement therapy effectively circumvents bone loss after menopause, but enhances disease risk in other tissues. Here we show that prostaglandin E2 (PGE2) augments the activity of an osteoblast-derived selective ER modulator, ObSERM. The stimulatory effect of PGE2 is replicated in part by either the PG receptor EP3 agonist 17-phenyl trinor PGE2 or by the PG receptor FP agonist PGF2α⋅ Whereas activation of the various PG receptors induces multiple downstream signals, the response to PGE2 was mimicked by activators of protein kinase C, and suppressed by inhibition of protein kinase C but not by inhibition of protein kinase A. Moreover, inhibition of nitric oxide synthesis and activation of the PTH and Wnt pathways increases ObSERM activity. Our studies therefore reveal that ObSERM activity is controlled in distinct ways and revise our understanding of ER activation within bone by agents or events associated with PG expression. They also predict ways to sustain or improve bone formation, fracture repair, and surgical healing without adding the risk of disease in other tissues where ER activation also has important biological functions.

The interaction between microglia and neural stem/precursor cells

In the adult mammalian brain, neurogenesis from neural stem/precursor cell occurs within two regions, the subgranular zone (SGZ) in the dentate gyrus (DG) and the subventricular zone (SVZ) lining the lateral ventricles. The function of neural stem cell is enhanced by external stimuli, such as injury and inflammation. Microglia, as the main immune modulating cells, play important roles in the central nervous system (CNS). Recently, select discoveries reported that microglia might influence the proliferation, differentiation and survival of neural precursor cells (NPCs). Other studies revealed that NPCs might reversibly regulate the function of microglia. Accordingly, in this review we focus on the interaction between microglia and NPCs.

Stratified control of IGF-I expression by hypoxia and stress hormones in osteoblasts

Bone cells respond to the integrated effects of local and systemic regulation. Here we show that hypoxia and the stress hormones PGE2 and glucocorticoid interact in complex ways in osteoblasts, converging on insulin like growth factor I (IGF-I) expression. Whereas hypoxia alone rapidly increased transcription factor HIF activity, it suppressed DNA synthesis, had no significant effects on protein synthesis or alkaline phosphatase activity, and drove discrete changes in a panel of osteoblast mRNAs. Notably, hypoxia increased expression of the acute phase response transcription factor C/EBPδ which can induce IGF-I in response to PGE2, but conversely prevented the stimulatory effect of PGE2 on IGF-I mRNA. However, unlike its effect on C/EBPδ, hypoxia suppressed expression of the obligate osteoblast transcription factor Runx2, which can activate an upstream response element in the IGF-I gene promoter. Hypoxic inhibition of IGF-I and Runx2 were enforced by glucocorticoid, and continued with prolonged exposure. Our studies thus reveal that IGF-I expression is stratified by two critical transcriptional elements in osteoblasts, which are resolved by the individual and combined effects of hypoxic stress and stress hormones. In so doing, hypoxia suppresses Runx2, limits the enhancing influence of PGE2, and interacts with glucocorticoid to reduce IGF-I expression by osteoblasts.

A phosphatase-independent gain-of-function mutation in PTEN triggers aberrant cell growth in astrocytes through an autocrine IGF-1 loop

Loss-of-function mutations in the phosphatase PTEN (phosphatase and tensin homolog deleted on chromosome10) contribute to aberrant cell growth in part through upregulation of the mitogenic IGF-1/PI3K/Akt pathway. In turn, this pathway exerts a homeostatic feedback over PTEN. Using mutagenesis analysis to explore a possible impact of this mutual control on astrocyte growth, we found that truncation of the C-terminal region of PTEN (Δ51) associates with a marked increase in NFκB activity, a transcription factor overactivated in astrocyte tumors. Whereas mutations of PTEN are considered to lead to a loss-of-function, PTENΔ51, a truncation that comprises a region frequently mutated in human gliomas, displayed a neomorphic (gain-of-function) activity that was independent of its phosphatase activity. This gain-of-function of PTENΔ51 includes stimulation of IGF-1 synthesis through protein kinase A activation of the IGF-1 promoter. Increased IGF-1 originates an autocrine loop that activates Akt and NFκB. Constitutive activation of NFκB in PTENΔ51-expressing astrocytes leads to aberrant cell growth; astrocytes expressing this mutant PTEN generate colonies in vitro and tumors in vivo. Mutations converting a tumor suppressor such as PTEN into a tumor promoter through a gain-of-function involving IGF-1 production may further our understanding of the role played by this growth factor in glioma growth and help us define druggable targets for personalized therapy.

Serotonin and the 5-HT7 receptor: the link between hepatocytes, IGF-1 and small intestinal neuroendocrine tumors

Platelet-derived serotonin (5-HT) is involved in liver regeneration. The liver is also the metastatic site for malignant enterochromaffin (EC) cell "carcinoid" (neuroendocrine) neoplasms, the principal cellular source of 5-HT. We hypothesized that 5-HT produced by metastatic EC cells played a role in the hepatic tumor-microenvironment principally via 5-HT₇ receptor-mediated activation of hepatocyte IGF-1 synthesis and secretion. Using isolated rat hepatocytes, we evaluated 5-HT₇ receptor expression (using PCR, sequencing and western blot). ELISA, cell transfection and western blots delineated 5-HT-mediated signaling pathways (pCREB, AKT and ERK). IGF-1 synthesis/secretion was evaluated using QPCR and ELISA. IGF-1 was tested on small intestinal neuroendocrine neoplasm proliferation, while IGF-1 production and 5-HT₇ expression were examined in an in vivo SCID metastasis model. Our results demonstrated evidence for a functional 5-HT₇ receptor. 5-HT activated cAMP/PKA activity, pCREB (130-205%, P < 0.05) and pERK/pAKT (1.2-1.75, P < 0.05). Signaling was reversed by the 5-HT₇ receptor antagonist SB269970. IGF-1 significantly stimulated proliferation of two small intestinal neuroendocrine neoplasm cell lines (EC₅₀: 7-70 pg/mL) and could be reversed by the small molecule inhibitor BMS-754807. IGF-1 and 5-HT were elevated (40-300×) in peri-tumoral hepatic tissue in nude mice, while 5-HT₇ was increased fourfold compared to sham-operated animals. We conclude that hepatocytes express a cAMP-coupled 5-HT₇ receptor, which, at elevated 5-HT concentrations that occur in liver metastases, signals via CREB/AKT and is linked to IGF-1 synthesis and secretion. Because IGF-1 regulates NEN proliferation, identification of a role for 5-HT₇ in the hepatic metastatic tumor microenvironment suggests the potential for novel therapeutic strategies for amine-producing mid-gut tumors.

Insulin-like growth factor 1 and 2 (IGF1, IGF2) expression in human microglia: differential regulation by inflammatory mediators

BACKGROUND

Recent studies in experimental animals show that insulin-like growth factor 1 (IGF1) plays a trophic role during development and tissue injury and that microglia are important sources of IGF1. However, little information is available regarding the expression, regulation, and function of IGF1 and related proteins in human brain cells. In the current study, we examined the expression of IGF1 and IGF2 in human microglia in vivo and in vitro.

METHODS

Expression of IGF1 and IGF2 was examined by immunohistochemistry in post-mortem human brain sections derived from HIV+ and HIV- brains. In primary cultures of human fetal microglia, IGF1 and IGF2 mRNA and protein expression was examined by Q-PCR, ELISA, and Western blot analysis. Additionally, the role of IGF1 and IGF2 in neuroprotection was examined in primary human neuronal glial cultures.

RESULTS

Immunohistochemistry of human brain tissues showed that nonparenchymal cells (vessels and meninges), as well as parenchymal microglia and macrophages were positive for IGF1, in both HIV encephalitis and control brains, while IGF2 was undetectable. Cultured microglia expressed IGF1 mRNA and produced pg/ml levels of IGF1 protein; this was significantly suppressed by proinflammatory mediators, such as lipopolysaccharide (LPS), poly(I:C), and IFNγ. The Th2 cytokines IL-4 and IL-13 had no significant effect, but the cAMP analog (dibutyryl cAMP) significantly increased IGF1 production. In contrast, microglial IGF2 mRNA and protein (determined by Western blot) were upregulated by LPS. IGF1 receptor (IGF1R) immunoreactivity was predominantly expressed by neurons, and both IGF1 and IGF2 significantly protected neurons from cytokine (IL-1/IFNγ) induced death.

CONCLUSIONS

Our study in human brain tissues and cells indicates that microglia are important sources of neurotrophic growth factors IGF1 and IGF2, and that microglial activation phenotypes can influence the growth factor expression. Importantly, our results suggest that chronic neuroinflammation and upregulation of proinflammatory cytokines could lead to neurodegeneration by suppressing the production of microglia-derived neuronal growth factors, such as IGF1.

Estrogen receptor dependent gene expression by osteoblasts - direct, indirect, circumspect, and speculative effects

Hormone activated estrogen receptors (ERs) have long been appreciated as potent mediators of gene expression in female reproductive tissues. These highly targeted responses likely evolved from more elemental roles in lower organisms, in agreement with their widespread effects in the cardiovascular, immunological, central nervous, and skeletal tissue systems. Still, despite intense investigation, the multiple and often perplexing roles of ERs retain significant attention. In the skeleton, this in part derives from apparently opposing effects by ER agonists on bone growth versus bone remodeling, and in younger versus older individuals. The complexity associated with ER activation can also derive from their interactions with other hormone and growth factor systems, and their direct and indirect effects on gene expression. We propose that part of this complexity results from essential interactions between ERs and other transcription factors, each with their own biochemical and molecular intricacies. Solving some of the many questions that persist may help to achieve better, or better directed, use of agents that can drive ER activation in focused and possibly tissue restricted ways.

Mechanism of action of glucagon-like peptide-2 to increase IGF-I mRNA in intestinal subepithelial fibroblasts

IGF-I, a known secretory product of intestinal subepithelial myofibroblasts (ISEMFs), is essential for the intestinotropic effects of glucagon-like peptide-2 (GLP-2). Furthermore, GLP-2 increases IGF-I mRNA transcript levels in vitro in heterogeneous fetal rat intestinal cultures, as well as in vivo in the rodent small intestine. To determine the mechanism underlying the stimulatory effect of GLP-2 on intestinal IGF-I mRNA, murine ISEMF cells were placed into primary culture. Immunocytochemistry showed that the ISEMF cells appropriately expressed α-smooth muscle actin and vimentin but not desmin. The cells also expressed GLP-2 receptor and IGF-I mRNA transcripts. Treatment of ISEMF cells with (Gly2)GLP-2 induced IGF-I mRNA transcripts by up to 5-fold of basal levels after treatment with 10(-8) m GLP-2 for 2 h (P < 0.05) but did not increase transcript levels for other intestinal growth factors, such as ErbB family members. Immunoblot revealed a 1.6-fold increase in phospho (p)-Akt/total-(t)Akt with 10(-8) m GLP-2 treatment (P < 0.05) but no changes in cAMP, cAMP-dependent β-galactosidase expression, pcAMP response element-binding protein/tcAMP response element-binding protein, pErk1/2/tErk1/2, or intracellular calcium. Furthermore, pretreatment of ISEMF cells with the phosphatidylinositol 3 kinase (PI3K) inhibitors, LY294002 and wortmannin, abrogated the IGF-I mRNA response to GLP-2, as did overexpression of kinase-dead Akt. The role of PI3K/Akt in GLP-2-induced IGF-I mRNA levels in the murine jejunum was also confirmed in vivo. These findings implicate the PI3K/Akt pathway in the stimulatory effects of GLP-2 to enhance intestinal IGF-I mRNA transcript levels and provide further evidence in support of a role for IGF-I produced by the ISEMF cells in the intestinotropic effects of GLP-2.

Gene expression patterns in the bone tissue of women with fibrous dysplasia

Fibrous dysplasia is an isolated skeletal disorder caused by a somatic activating mutation of GNAS gene with abnormal unmineralized matrix overproduction and extensive undifferentiated bone cell accumulation in the fibro-osseous lesions. The aim of our investigation was to identify genes that are differently expressed in fibrous versus non-fibrous human bone and to describe the relationships between these genes using multivariate data analysis. Six bone tissue samples from female patients with fibrous dysplastia (FD) and seven bone tissue samples from women without FD (non-FD) were examined. The expression differences of selected 118 genes were analyzed by the TaqMan probe-based quantitative real-time RT-PCR system. The Mann-Whitney U-test indicated marked differences in the expression of 22 genes between FD and non-FD individuals. Nine genes were upregulated in FD women compared to non-FD ones and 18 genes showed a downregulated pattern. These altered genes code for minor collagen molecules, extracellular matrix digesting enzymes, transcription factors, adhesion molecules, growth factors, pro-inflammatory cytokines, and lipid metabolism-affected substrates. Canonical variates analysis demonstrated that FD and non-FD bone tissues can be distinguished by the multiple expression profile analysis of numerous genes controlled via a G-protein coupled pathway and BMP cascade as well as genes coding for extracellular matrix composing molecules. The remarkable changed gene expression profile observed in the fibrous dysplastic human bone tissue may provide further insight into the pathogenetic process of fibrous degeneration of bone.

GATA depletion impacts insulin-like growth factor 1 mRNA and protein levels in luteinizing porcine granulosa cells

GATA4 and GATA6 are zinc-finger transcription factors that regulate specific genes involved in steroidogenesis. Using RNA interference (RNAi)-mediated reduction of GATA4 and/or GATA6 with microarray analysis, we aimed to identify novel GATA target genes in luteinizing porcine granulosa cells under vehicle- and cAMP-treated conditions. Microarray analysis identified IGF1 mRNA to be cAMP- and GATA-responsive, and real-time PCR demonstrated that the cAMP-induced increase in IGF1 mRNA was reduced under conditions of GATA6 depletion and GATA4 plus GATA6 depletion, but not GATA4 depletion. Insulin-like growth factor 1 protein levels in media were also decreased by GATA6 or GATA4 plus GATA6 reduction. IGFBP2 and IGFBP4 mRNAs were increased and IGFBP5 mRNA decreased with vehicle and cAMP treatment under GATA4 plus GATA6 RNAi conditions. GATA6 reduction alone increased basal IGFBP4 and decreased IGFBP5 with both vehicle and cAMP, and GATA4 reduction alone lowered cAMP IGFBP5 levels with cAMP. No changes in IGFBP3 mRNA were observed with GATA reduction relative to the control RNAi condition. Levels of insulin-like growth factor binding proteins 2-5 in media as assessed by Western ligand blotting were not altered by GATA reduction. Electromobility gel shift assays with two GATA-containing oligonucleotides of the IGF1 5'-regulatory region showed GATA4 and GATA6 could bind the more proximal GATA-B site. These studies indicate that although GATA4 and GATA6 can bind the porcine IGF1 5'-region, GATA6 is functionally most important for cAMP-stimulated mRNA levels. Using microarray analysis, we identified other mRNAs that were altered by GATA-reduced conditions, including ALDH1, DIO2, and EDNRB. Our findings further support GATA as a coordinator of endocrine/paracrine/autocrine signals in the ovary.

IGF-1 receptor-mediated ERK/MAPK signaling couples status epilepticus to progenitor cell proliferation in the subgranular layer of the dentate gyrus

Adult progenitor cell proliferation in the subgranular zone (SGZ) of the dentate gyrus is a dynamic process that is modulated by an array of physiological process, including locomotor activity and novel environmental stimuli. In addition, pathophysiological events, such as ischemia and status epilepticus (SE), have been shown to stimulate neurogenesis. Currently, limited information is available regarding the extracellular stimuli, receptors, and downstream intracellular effectors that couple excitotoxic stimulation to progenitor cell proliferation. Here we show that pilocarpine-induced SE triggers a set of signaling events that impinge upon the p42/44 mitogen-activated protein kinase (MAPK) pathway to drive progenitor cell proliferation in the SGZ at 2-days post-SE. Increased proliferation was dependent on insulin-like growth factor-1 (IGF-1), which was localized to activated microglia near the SGZ. Using a combination of techniques, we show that IGF-1 is a CREB-regulated gene and that SE triggered CRE-dependent transcription in microglia at 2-days post-SE. Together, these data identify a potential signaling program that couples SE to progenitor cell proliferation. SE triggers CREB-dependent transcription in reactive microglia. As a CREB-target gene, IGF-1 expression is upregulated, and by 2-days post-SE, IGF-1 triggers MAPK pathway activation in progenitor cells and, in turn, an increase in progenitor cell proliferation.

PI3 kinase/Akt activation mediates estrogen and IGF-1 nigral DA neuronal neuroprotection against a unilateral rat model of Parkinson's disease

Recently, using the medial forebrain bundle (MFB) 6-hydroxydopmaine (6-OHDA) lesion rat model of Parkinson's disease (PD), we have demonstrated that blockade of central IGF-1 receptors (IGF-1R) attenuated estrogen neuroprotection of substantia nigra pars compacta (SNpc) DA neurons, but exacerbated 6-OHDA lesions in IGF-1 only treated rats (Quesada and Micevych [2004]: J Neurosci Res 75:107-116). This suggested that the IGF-1 system is a central mechanism through which estrogen acts to protect the nigrostriatal DA system. Moreover, these results also suggest that IGF-1R-induced intracellular signaling pathways are involved in the estrogen mechanism that promotes neuronal survival. In vitro, two convergent intracellular signaling pathways used by estrogen and IGF-1, the mitogen-activated protein kinase (MAPK/ERK), and phosphatidyl-inositol-3-kinase/Akt (PI3K/Akt), have been demonstrated to be neuroprotective. Continuous central infusions of MAPK/ERK and PI3K/Akt inhibitors were used to test the hypothesis that one or both of these signal transduction pathways mediates estrogen and/or IGF-1 neuroprotection of SNpc DA neurons after a unilateral administration of 6-OHDA into the MFB of rats. Motor behavior tests and tyrosine hydroxylase immunoreactivity revealed that the inhibitor of the PI3K/Akt pathway (LY294002) blocked the survival effects of both estrogen and IGF-1, while an inhibitor of the MAPK/ERK signaling (PD98059) was ineffective. Western blot analyses showed that estrogen and IGF-1 treatments increased PI3K/Akt activation in the SN; however, MAPK/ERK activation was decreased in the SN. Indeed, continuous infusions of inhibitors blocked phosphorylation of PI3K/Akt and MAPK/ERK. These findings indicate that estrogen and IGF-1-mediated SNpc DA neuronal protection is dependent on PI3K/Akt signaling, but not on the MAPK/ERK pathway.

3-ketosteroid reductase activity and expression by fetal rat osteoblasts

In addition to reproductive tissue, sex hormones induce transcriptional events in many connective tissue cells, including osteoblasts. Some sex hormone receptor modulators with bone sparing effects selectively target estrogen or androgen receptors, whereas others appear more promiscuous, in part through enzymatic metabolism. Rat osteoblasts express significant oxidative 3alpha-hydroxysteroid dehydrogenase activity, which can convert precursor substrates to potent androgen receptor agonists. Here we show that they also express 3-ketosteroid reductase activity, exemplified by 7-methyl-17-ethynyl-19-norandrostan-5 (10)en-3-one (tibolone) conversion to potent estrogen receptor alpha agonists. Conversion was rapid and quantitative, with 3alpha-hydroxytibolone as the primary metabolite. Consistently, tibolone induced estrogen receptor alpha-dependent gene promoter activity through cis-acting estrogen response elements, increased the stimulatory effect of TGF-beta on Smad-dependent gene promoter activity, and enhanced prostaglandin E2-induced activity of transcription factor Runx2. Rat osteoblasts express the 3-ketosteroid reductase AKR1C9, an aldo-keto reductase gene family member. Exposure to prostaglandin E2 increased AKR1C9 gene promoter activity and mRNA expression. AKR1C9 promoter activity was also enhanced by overexpression of protein kinase A catalytic subunit or transcription factor C/EBPdelta, and the effect of PGE2 was reduced by dominant negative C/EBPdelta competition or C/EBPdelta antisense expression. Moreover, prostaglandin E2 increased the amount of functional endogenous nuclear C/EBPdelta that could bind specifically to a distinct domain approximately 1.8-kb upstream from the start site of AKR1C9 transcription. In summary, in addition to 3alpha-hydroxysteroid dehydrogenase, rat osteoblasts express significant and regulatable 3-ketosteroid reductase activity. Through these enzymes, they may selectively metabolize precursor compounds into potent steroid receptor agonists locally within bone.

Prostaglandin E2 increases transforming growth factor-beta type III receptor expression through CCAAT enhancer-binding protein delta in osteoblasts

Variations in individual TGF-beta receptors (TbetaRs) may modify TGF-beta activity and significantly alter its effects on connective tissue growth or repair. Differences in the amount of TbetaR type III (TbetaRIII) relative to signal transducing TbetaRI occur on bone cells during differentiation or in response to other growth regulators. Here we investigated prostaglandin (PG) E2, a potent effector during trauma, inflammation, or mechanical load, on TbetaR expression in primary osteoblast-enriched cultures. PGE2 rapidly increased TbetaRIII mRNA and protein expression and enhanced TbetaRIII gene promoter activity through a discrete region within 0.4 kb of the transcription start site. PGE2 alters osteoblast function through multiple signal-inducing pathways. In this regard, protein kinase A (PKA) activators, PGE1 and forskolin, also enhanced gene expression through the TbetaRIII gene promoter, whereas protein kinase C activators, PGF2alpha and phorbol myristate acetate, did not. The stimulatory effect of PGE2 on TbetaRIII promoter activity was suppressed by a dominant negative PKA-regulatory subunit, but not by dominant negative protein kinase C. PGE2 specifically increased nuclear factor CCAAT enhancer-binding protein delta (C/EBPdelta) binding to a half-binding site upstream of the basal TbetaRIII promoter region, and promoter activity was sensitive to C/EBPdelta overexpression and to dominant-negative C/EBPdelta competition. In parallel with their effect on TbetaRIII expression, activators of PKA decreased TGF-beta-induced activity. In summary, high levels of PGE2 that occur with inflammation or trauma may, through PKA-activated C/EBPdelta, preferentially increase TbetaRIII expression and in this way delay TGF-beta-dependent activation of osteoblasts during the early stabilization phase of bone repair.

Developmental regulation of the mouse IGF-I exon 1 promoter region by calcineurin activation of NFAT in skeletal muscle

Skeletal muscle development and growth are regulated through multiple signaling pathways that include insulin-like growth factor I (IGF-I) and calcineurin activation of nuclear factor of activated T cell (NFAT) transcription factors. The developmental regulation and molecular mechanisms that control IGF-I gene expression in murine embryos and in differentiating C2C12 skeletal myocytes were examined. IGF-I is expressed in developing skeletal muscle, and its embryonic expression is significantly reduced in embryos lacking both NFATc3 and NFATc4. During development, the IGF-I exon 1 promoter is active in multiple organ systems, including skeletal muscle, whereas the alternative exon 2 promoter is expressed predominantly in the liver. The IGF-I exon 1 promoter flanking sequence includes two highly conserved regions that contain NFAT consensus binding sequences. One of these conserved regions contains a calcineurin/NFAT-responsive regulatory region that is preferentially activated by NFATc3 in C2C12 skeletal muscle cells and NIH3T3 fibroblasts. This NFAT-responsive region contains three clustered NFAT consensus binding sequences, and mutagenesis experiments demonstrated the requirement for two of these in calcineurin or NFATc3 responsiveness. Chromatin immunoprecipitation analyses demonstrated that endogenous IGF-I genomic sequences containing these conserved NFAT binding sequences interact preferentially with NFATc3 in C2C12 cells. Together, these experiments demonstrated that a NFAT-rich regulatory element in the IGF-I exon 1 promoter flanking region is responsive to calcineurin signaling and NFAT activation in skeletal muscle cells. The identification of a calcineurin/NFAT-responsive element in the IGF-I gene represents a potential mechanism of intersection of these signaling pathways in the control of muscle development and homeostasis.

Primary pigmented nodular adrenocortical disease reveals insulin-like growth factor binding protein-2 regulation by protein kinase A

OBJECTIVE

Primary pigmented nodular adrenocortical disease (PPNAD) can occur as an isolated trait or part of Carney complex, a familial lentiginosis-multiple endocrine neoplasia syndrome frequently caused by mutations in PRKAR1A, which encodes the 1alpha regulatory subunit of protein kinase A (PKA). Because alterations in the insulin-like growth factor (IGF) axis, particularly IGF-II and IGF binding protein (IGFBP)-2 overexpression, have been implicated in sporadic adrenocortical tumors, we sought to examine the IGF axis in PPNAD.

DESIGN

RNA samples and paraffin-embedded sections were procured from adrenalectomy specimens of patients with PPNAD. Changes in expression of IGF axis components were evaluated by real-time quantitative RT-PCR and immunohistochemistry. NCI-H295R cells were used to study PKA and IGF axis signaling in adrenocortical cells in vitro.

RESULTS

IGFBP-2 mRNA level distinguished between the two genetic subtypes of this disease; increased IGFBP-2 expression in PRKAR1A mutation-positive PPNAD tissues was also confirmed by immunohistochemistry. Moreover, PKA inhibitors increased IGFBP-2 expression in NCI-H295R adrenocortical cells, and anti-IGFBP-2 antibody reduced their proliferation.

CONCLUSIONS

IGFBP-2 expression is increased in PPNAD caused by PRKAR1A mutations, and in adrenocortical cancer cells. This is the first evidence for PKA-dependent regulation of IGFBP-2 expression in adrenocortical cells.

Intracellular signaling pathways pave roads to recovery for mood disorders

Mood disorders, including major depression and bipolar disorder, remain a major unmet medical need as current antidepressant and mood stabilizing therapies require chronic treatment for efficacy and are not effective in all patients. Multiple deficits, including cell atrophy and loss, have been observed in limbic and cortical brain regions of patients with mood disorders and in stressed animals. It is thought that antidepressant and mood stabilizing medications restore these deficits by reestablishing proper patterns of gene expression and function. In support of this hypothesis, numerous changes in gene expression and activity have been observed in limbic and cortical brain regions of mood disorder patients, and thymoleptic therapies have been shown to reciprocally regulate many of these changes. These findings have implicated four main signaling pathways in the pathophysiology and/or treatment of mood disorders, namely the cyclic-AMP, phosphoinositol, mitogen-activated protein kinase, and glycogen synthase kinase signaling cascades. Below we review this literature, and discuss potential targets for novel antidepressant and mood stabilizing drug design that are highlighted by these findings.

Insulin-like growth factor-1 and muscle wasting in chronic heart failure

Chronic heart failure is a clinical syndrome of cardiac origin, which affects various organ systems. It is associated with metabolic abnormalities leading to a catabolic syndrome in advanced stages of the disease. As in several other chronic diseases, skeletal muscle dysfunction and structural muscle abnormalities result in progressive muscle wasting and cachexia. These changes are accompanied by increased expression of proinflammatory cytokines, increased rate of apoptosis and activation of the proteolytic ubiquitin-proteasome pathway. Further, reduced expression of the local anabolic insulin-like growth factor-1 has been demonstrated in skeletal muscle of animals and patients with chronic heart failure. This suppression occurs in the presence of normal serum levels of insulin-like growth factor-1. In addition to catabolic effects of proinflammatory cytokines, these recent findings are consistent with reduced anabolism involving altered local insulin-like growth factor-1 levels in progressive muscle atrophy in chronic heart failure. This article describes local effects of insulin-like growth factor-1 on skeletal muscle function and morphology, its role in stem cell recruitment and muscle regeneration as well as its regulation in circumstances of muscle inflammation and wasting.

The insulin-like growth factor-I gene and osteoporosis: a critical appraisal

Osteoporosis, a disorder of skeletal fragility, is common in the elderly, and its prevalence is increasing as more individuals with low bone mineral density (BMD), the strongest predictor of fracture risk, are detected. Previous basic and clinical studies imply there is a significant role for insulin-like growth factor-I (IGF-I) in determining BMD. Recently, polymorphisms upstream of the P1 promoter region of the human IGF-I gene have been found to be associated with serum levels of IGF-I, BMD and fracture risk in various ethnic groups. Multiple quantitative trait loci (QTLs) have been identified that underlie serum IGF-I in a mouse intercross between two inbred strains. The most promising QTL on mouse chromosome 6 has provided clues for unraveling the molecular mechanisms that regulate osteoblast differentiation. Genomic engineering resulting in IGF-I deficient mice, and mice with targeted over-expression of IGF-I reinforce the essential role of IGF-I in bone development at both the embryonic and postnatal stages. Thus, it is apparent that significant new insights into the role of the IGF-I gene in bone remodeling occur through several distinct mechanisms: (1) the skeletal IGF regulatory system; (2) the systemic growth hormone/IGF-I axis; (3) parathyroid hormone signaling; (4) sex steroids; and (5) the OPG/RANKL/RANK cytokine system. Molecular dissection of the IGF regulatory system and its signaling pathway in bone may reveal novel therapeutic targets for the treatment of osteoporosis.

Skeletal hormones and the C/EBP and Runx transcription factors: interactions that integrate and redefine gene expression

Systemic hormones and local growth factors have significant and often complex roles in normal tissue development, growth, remodeling, and repair. Early efforts in skeletal tissue attempted to define active panels of these agents and their direct effects on cell proliferation, matrix production, and secretion of other soluble mediators of differentiated cell function. Initial results resolved many of these questions and began to unveil functional interactions between specific hormones and growth factors. More recent evidence suggests that interactions between individual hormone systems also occur in less anticipated but probably not less meaningful ways. In some cases, these interactions may help to define a spectrum of effects on gene expression by focusing, refocusing, or integrating the activity of previously recognized transcription regulators. Other studies in isolated osteoblasts predict that certain steroid hormones have distinctive effects on specific transcription factors with important roles in bone growth and repair. In this review, we focus on studies that define functional and physical interactions between molecular mediators of hormone activity that could directly effect skeletal growth factor biology.

Interactions between CCAAT enhancer binding protein δ and estrogen receptor α control insulin-like growth factor I (igf1) and estrogen receptor-dependent gene expression in osteoblasts

Although ambient levels of estradiol and synthesis of the osteoblast growth factor IGF-I are inversely related in vivo, estradiol has little or no direct effect on igf1 gene expression in rat osteoblasts in vitro. Rather, estradiol suppresses the effect of hormones that enhance igf1 expression through protein kinase A dependent activation of CCAAT enhancer binding protein (C/EBP) transcription factors. We show here that inhibition of C/EBP activity by estradiol relates to the level of estrogen receptor alpha (ERalpha) expression, and that a complex between hormone-activated ERalpha and C/EBPdelta inhibits transcription by each factor. Protein fragmentation, co-immunoprecipitation, and gene expression studies identified domains for physical and functional interactions between ERalpha and C/EBPdelta. Whereas ERalpha and fragments comprising its various domains associated with C/EBPdelta, only ERalpha fragment A/B alone replicated the suppressive effect of intact ERalpha on endogenous C/EBPdelta activity. Complementary studies showed that several carboxyl regions of C/EBPdelta cooperatively inhibit ERalpha dependent transcription. Therefore, multiple domains of C/EBPdelta and ERalpha can physically interact to alter gene expression in osteoblasts in selective ways that depend on variations in the local hormone environment. Their combined effects on one important gene target, igf1, may help to determine the balance in the overall rates of bone formation.

Phosphatidylinositol 3-kinase and Akt effectors mediate insulin-like growth factor-I neuroprotection in dorsal root ganglia neurons

Insulin-like growth factor-I (IGF-I) protects neurons of the peripheral nervous system from apoptosis, but the underlying signaling pathways are not well understood. We studied IGF-I mediated signaling in embryonic dorsal root ganglia (DRG) neurons. DRG neurons express IGF-I receptors (IGF-IR), and IGF-I activates the phosphatidylinositol 3-kinase (PI3K)/Akt pathway. High glucose exposure induces apoptosis, which is inhibited by IGF-I through the PI3K/Akt pathway. IGF-I stimulation of the PI3K/Akt pathway phosphorylates three known Akt effectors: the survival transcription factor cyclic AMP response element binding protein (CREB) and the pro-apoptotic effector proteins glycogen synthase kinase-3beta (GSK-3beta) and forkhead (FKHR). IGF-I regulates survival at the nuclear level through accumulation of phospho-Akt in DRG neuronal nuclei, increased CREB-mediated transcription, and nuclear exclusion of FKHR. High glucose increases expression of the pro-apoptotic Bcl protein Bim (a transcriptional target of FKHR). However, IGF-I does not regulate Bim or anti-apoptotic Bcl-xL protein expression levels, which suggests that IGF-I neuroprotection is not through regulation of their expression. High glucose also induces loss of the initiator caspase-9 and increases caspase-3 cleavage, effects blocked by IGF-I. These data suggest that IGF-I prevents apoptosis in DRG neurons by regulating PI3K/Akt pathway effectors, including GSK-3beta, CREB, and FKHR, and by blocking caspase activation.

Critical prosurvival roles for C/EBP beta and insulin-like growth factor I in macrophage tumor cells

One of the hallmarks of leukemic cells is their ability to proliferate and survive in the absence of exogenous growth factors (GFs). However, the molecular mechanisms used by myeloid tumor cells to escape apoptosis are not fully understood. Here we report that Myc/Raf-transformed macrophages require the transcription factor C/EBP beta to prevent cell death. In contrast to wild-type cells, C/EBP beta(-/-) macrophages were completely dependent on macrophage colony-stimulating factor or granulocyte-macrophage colony-stimulating factor for survival and displayed impaired tumorigenicity in vivo. Microarray analysis revealed that C/EBP beta-deficient cells expressed significantly reduced levels of the prosurvival factor insulin-like growth factor I (IGF-I). Overexpression of C/EBP beta stimulated transcription from the IGF-I promoter, indicating that IGF-I is a direct transcriptional target of C/EBP beta. Serological neutralization of IGF-I in C/EBP beta(+/+) tumor cell cultures induced apoptosis, showing that IGF-I functions as an autocrine survival factor in these cells. Macrophage tumor cells derived from IGF-I(-/-) mice were GF dependent, similar to C/EBP beta-deficient cells. Forced expression of either C/EBP beta or IGF-I in C/EBP beta(-/-) bone marrow cells restored Myc/Raf-induced transformation and permitted neoplastic growth without exogenous GFs. Thus, our findings demonstrate that C/EBP beta is essential for oncogenic transformation of macrophages and functions at least in part by regulating expression of the survival factor IGF-I.

Proinflammatory cytokines and skeletal muscle

PURPOSE OF REVIEW

Metabolic abnormalities leading to a catabolic syndrome with progressive muscular atrophy are a common final stage of various chronic diseases. Proinflammatory cytokines have been suggested both to induce and mediate local catabolic mechanisms. This review focuses on the role of proinflammatory cytokines in the development of muscular abnormalities resulting in a loss of muscle mass and function. In addition, the underlying molecular signaling pathways, their transcriptional regulation, and the cellular systems contributing to enhanced muscular protein breakdown are discussed.

RECENT FINDINGS

Using transcriptional screening techniques, specific changes in gene expression have been identified that are characteristic of muscular wasting processes. Of particular interest is the atrophy-related and cytokine-inducible expression of a number of E3 ligases (e.g. atrogin-1, muscle ring finger protein 1), highly specific regulators of the ubiquitin-proteasome pathway, which target proteins for proteolytic breakdown by the proteasome. Furthermore, the activity of several transcription factors (e.g. nuclear factor kappa B) has been involved in specific transcriptional mechanisms of local inflammation and muscular atrophy. Finally, proinflammatory cytokines suppress the expression and function of the local anabolic growth factor insulin-like growth factor 1.

SUMMARY

Pronounced catabolic effects of proinflammatory cytokines in various tissues contribute to local catabolism, with progressive atrophic alterations of the skeletal muscle in chronic disease states. Recent developments in this field have explored the underlying pathways of muscular wasting and have already resulted in the description of new molecular targets that might lead to new therapeutic options for the treatment of muscular atrophy.

TNF-alpha suppresses bone sialoprotein (BSP) expression in ROS17/2.8 cells

Tumor necrosis factor-alpha (TNF-alpha) is a major mediator of inflammatory responses in many diseases that inhibits bone formation and stimulates bone resorption. To determine molecular mechanisms involved in the suppression of bone formation we have analyzed the effects of TNF-alpha on BSP gene expression. Bone sialoprotein (BSP) is a mineralized tissue-specific protein that appears to function in the initial mineralization of bone. Previous studies have demonstrated that BSP mRNA expression is essentially restricted to fully-differentiated cells of mineralized connective tissues and that the expression of BSP is developmentally regulated. Treatment of rat osteosarcoma ROS 17/2.8 cells with TNF-alpha (10 ng/ml) for 24 h caused a marked reduction in BSP mRNA levels. The addition of antioxidant N-acetylcysteine (NAC; 20 mM) 30 min prior to stimulation with TNF-alpha attenuated the inhibition of BSP mRNA levels. Transient transfection analyses, using chimeric constructs of the rat BSP gene promoter linked to a luciferase reporter gene, revealed that TNF-alpha (10 ng/ml) suppressed expression in all constructs, including a short construct (pLUC3; nts -116 to +60), transfected into ROS17/2.8 cells. Further deletion analysis of the BSP promoter showed that a region within nts -84 to -60 was targeted by TNF-alpha, the effects which were inhibited by NAC and the tyrosine kinase inhibitor, herbimycin A (HA). Introduction of 2bp mutations in the inverted CCAAT box (ATTGG; nts -50 and -46), a putative cAMP response element (CRE; nts -75 to -68), and a FGF response element (FRE; nts -92 to -85) showed that the TNF-alpha effects were mediated by the CRE. These results were supported by gel mobility shift assays, using a radiolabeled double-stranded CRE oligonucleotide, which revealed decreased binding of a nuclear protein from TNF-alpha-stimulated ROS 17/2.8 cells. Further, the inhibitory effect of TNF-alpha on CRE DNA-protein complex was completely abolished by NAC or HA treatment. These studies, therefore, show that TNF-alpha suppresses BSP gene transcription through a tyrosine kinase-dependent pathway that generates reactive oxygen species and that the TNF-alpha effects are mediated by a CRE element in the proximal BSP gene promoter.

trans-10, cis-12 conjugated linoleic acid reduces insulin-like growth factor-II secretion in HT-29 human colon cancer cells

We previously demonstrated that a mixture of conjugated linoleic acid (CLA) isomers decreases colon cancer incidence in rats treated with 1,2-dimethylhydrazine. Our in vitro studies have also shown that CLA inhibits the growth of HT-29 cells, a human colon cancer cell line. When we compared the individual potencies of the two main isomers found in the mixture of CLA isomers (e.g., cis-9, trans-11 [c9t11] and trans-10, cis-12 [t10c12]), t10c12 CLA decreased viable cell numbers in a dose-dependent manner. By contrast, c9t11 CLA had no effect. Therefore, the present study examined whether the decreased cell growth is related to changes in secretion of insulin-like growth factor (IGF)-II and/or IGF-binding proteins (IGFBPs) that have been shown to regulate HT-29 cell proliferation. Cells were incubated in serum-free medium with various concentrations of the individual CLA isomers, and immunoblot analysis of 24-hour, serum-free, conditioned media using a monoclonal anti-IGF-II antibody was performed. HT-29 cells secreted both mature 7,500 apparent molecular weight (M(r)) and higher-M(r) forms of IGF-II. t10c12 CLA decreased the levels of the higher-M(r) and the mature form of IGF-II in a dose-dependent manner, whereas c9t11 CLA had no effect. Ligand blot analysis of conditioned medium using (125)I-IGF-II revealed that the production of IGFBP-2 and IGFBP-4 was also decreased by t10c12 CLA, whereas c9t11 CLA had no effect. Exogenous IGF-II abrogated the growth inhibition induced by t10c12 CLA. These results indicate that inhibition of HT-29 cell growth by t10c12 CLA may be mediated by decreasing IGF-II secretion in these cells.

Mechanisms of Growth Hormone (GH) Action

Many of the actions of growth hormone (GH) on somatic growth and tissue maintenance are mediated by insulin-like growth factor-I (IGF-I), a secreted protein whose gene expression is rapidly and potently induced by GH by unknown mechanisms. Recent studies implicating Stat5a and Stat5b in the growth response to GH in mice and observations linking Stat5b to control of IGF-I gene transcription in rats have prompted the current investigations into the molecular determinants of a putative regulatory network extending from GH through Stat5b to IGF-I. Here we characterize as critical components of this hormone-activated transcriptional pathway two adjacent Stat5 binding sites in the second intron of the rat IGF-I gene located within a conserved region previously found to undergo acute and reversible changes in chromatin structure after in vivo GH treatment. As assessed by chromatin immunoprecipitation assays, GH rapidly induced binding of Stat5 to this DNA segment in the liver of GH-deficient rats, just prior to the onset of transcription from both major and minor IGF-I gene promoters. Biochemical reconstitution experiments showed that the two intronic Stat5 DNA elements were able to bind Stat5b in vitro after GH treatment could transmit GH- and Stat5b-dependent transcriptional responsiveness to the major IGF-I promoter and to a minimal neutral gene promoter and were required for full stimulation of reporter gene activity by GH. Taken together, these results identify an intronic enhancer as a key mediator of GH-induced IGF-I gene transcription working through Stat5b and provide new insight into the molecular architecture of this transcriptional pathway.

Transcriptional regulation of IGF-I expression in skeletal muscle

The present study investigated the role of transcription in the regulation of insulin-like growth factor (IGF)-I expression in skeletal muscle. RT-PCR was used to determine endogenous expression of IGF-I pre-mRNA and mRNA in control (Con) and functionally overloaded (FO) rat plantaris. The transcriptional activities of five different-length IGF-I promoter fragments controlling transcription of a firefly luciferase (FLuc) reporter gene were tested in vitro by transfection of myoblasts or in vivo during FO by direct gene transfer into the plantaris. Increased endogenous IGF-I gene transcription during 7 days of plantaris FO was evidenced by an approximately 140-160% increase (P < 0.0001) in IGF-I pre-mRNA (a transcriptional marker). IGF-I mRNA expression also increased by approximately 90% (P < 0.0001), and it was correlated (R = 0.93; P < 0.0001) with the pre-mRNA increases. The three longest IGF-I exon 1 promoters induced reporter gene expression in proliferating C2C12 and L6E9 myoblasts. In differentiated L6E9 myotubes, promoter activity increased approximately two- to threefold over myoblasts. Overexpression of calcineurin and MyoD increased the activity of the -852/+192 promoter in C2C12 myotubes by approximately 5- and approximately 18-fold, respectively. However, FO did not induce these exogenous promoter fragments. Nevertheless, the present findings are consistent with the hypothesis that the IGF-I gene is transcriptionally regulated during muscle hypertrophy in vivo as evidenced by the induction of the endogenous IGF-I pre-mRNA during plantaris FO. The exon 1 promoter region of the IGF-I gene is sufficient to direct inducible expression in vitro; however, an in vivo response to FO may require elements outside the -852/+346 region of the exon 1 IGF-I promoter or features inherent to the endogenous IGF-I gene.

Activation domains of CCAAT enhancer binding protein delta: regions required for native activity and prostaglandin E2-dependent transactivation of insulin-like growth factor I gene expression in rat osteoblasts

In osteoblasts, hormones such as prostaglandin E2 that activate protein kinase A increase the translocation of transcription factor CCAAT/enhancer binding protein delta (C/EBPdelta) from the cytoplasm to the nucleus where it rapidly induces IGF-I gene expression. In this study, we identified activation and suppression domains in C/EBPdelta using native and heterologous gene promoter assay systems. We demonstrated functional interactions between C/EBPdelta and trans-gene-expressed cAMP response element binding protein-binding protein, and showed that the ability of C/EBPdelta to promote gene expression was suppressed by viral protein E1A, which blocks the activity of native cAMP response element binding protein-binding protein. Site-directed mutations at serines 62 or 191 within C/EBPdelta reduced its basal transcriptional activity, whereas mutation at serine 191 suppressed the stimulatory effect of prostaglandin E2 on C/EBPdelta function as well as its DNA binding potential. These results are consistent with the location of serine 191 in the DNA binding domain of C/EBPdelta. Our studies provide the first evidence for regions of C/EBPdelta that are important for basal and for hormone-induced transcriptional activity, and for its interactions with other enhancers and suppressers of gene expression.

Mechanisms for the enhancement of fracture healing in rats treated with intermittent low-dose human parathyroid hormone (1-34)

Recent reports have demonstrated that intermittent treatment with parathyroid hormone (1-34) [PTH(1-34)] increases callus formation and mechanical strength in experimental fracture healing. However, little is known about the optimal dose required for enhancement of fracture repair or the molecular mechanisms by which PTH regulates the healing process. In this study, we analyzed the underlying molecular mechanisms by which PTH affects fracture healing and tested the hypothesis that intermittent low-dose treatment with human PTH(1-34) can increase callus formation and mechanical strength. Unilateral femoral fractures were produced and a daily subcutaneous injection of 10 microg/kg of PTH(1-34) was administered during the entire healing period. Control animals were injected with vehicle solution alone. The results showed that on day 28 and day 42 after fracture, bone mineral content (BMC), bone mineral density (BMD), and ultimate load to failure of the calluses were significantly increased in the PTH-treated group compared with controls (day 28, 61, 46, and 32%; day 42, 119, 74, and 55%, respectively). The number of proliferating cell nuclear antigen (PCNA)-positive subperiosteal osteoprogenitor cells was significantly increased in the calluses of the PTH-treated group on day 2, and TRAP+ multinucleated cells were significantly increased in areas of callus cancellous bone on day 7. The levels of expression of type I collagen (COLlA1), osteonectin (ON), ALP, and osteocalcin (OC) mRNA were increased markedly in the PTH-treated group and accompanied by enhanced expression of insulin-like growth factor (IGF)-I mRNA during the early stages of healing (days 4-7). The increased expression of COL1A1, ON, ALP, and OC mRNA continued during the later stages of healing (days 14-21) despite a lack of up-regulation of IGF-I mRNA. These results suggest that treatment of fractures with intermittent low dose PTH(1-34) enhances callus formation by the early stimulation of proliferation and differentiation of osteoprogenitor cells, increases production of bone matrix proteins, and enhances osteoclastogenesis during the phase of callus remodeling. The resultant effect to increase callus mechanical strength supports the concept that clinical investigations on the ability of injectable low-dose PTH(1-34) to enhance fracture healing are indicated.

Trans-10,cis-12-conjugated linoleic acid inhibits Caco-2 colon cancer cell growth

A commercially available mixture of conjugated linoleic acid (CLA) isomers decreases colon cancer cell growth. We compared the individual potencies of the two main isomers in this mixture [cis-9,trans-11 (c9t11) and trans-10,cis-12 (t10c12)] and assessed whether decreased cell growth is related to changes in secretion of insulin-like growth factor II (IGF-II) and/or IGF-binding proteins (IGFBPs), which regulate Caco-2 cell proliferation. Cells were incubated in serum-free medium with different concentrations of the individual CLA isomers. t10c12 CLA dose dependently decreased viable cell number (55 +/- 3% reduction 96 h after adding 5 microM t10c12 CLA). t10c12 CLA induced apoptosis and decreased DNA synthesis, whereas c9t11 CLA had no effect. Immunoblot analysis of 24-h serum-free conditioned medium using a monoclonal anti-IGF-II antibody revealed that Caco-2 cells secreted both a mature 7,500 molecular weight (M(r)) IGF-II and higher M(r) forms of IGF-II. The levels of the higher M(r) and the mature form of IGF-II were decreased 50 +/- 3% and 22 +/- 2%, respectively, by 5 microM t10c12 CLA. c9t11 CLA had no effect. Ligand blot analysis of conditioned medium using 125I-labeled IGF-II revealed that t10c12 CLA slightly decreased IGFBP-2 production; c9t11 CLA had no effect. Exogenous IGF-II reversed t10c12 CLA-induced growth inhibition and apoptosis. These results indicate that CLA-inhibited Caco-2 cell growth is caused by t10c12 CLA and may be mediated by decreasing IGF-II secretion in Caco-2 cells.

Protein kinase C-dependent, CCAAT/enhancer-binding protein beta-mediated expression of insulin-like growth factor I gene

The possible involvement of the protein kinase C (PKC) pathway in transcriptional regulation of the human insulin-like growth factor-I (IGF-I) gene has been suggested. In this study, we sought to determine whether a PKC-dependent pathway is implicated in the transcriptional control, and if it is, how this occurs. Treatment with 12-O-tetradecanoylphorbol 13-acetate (TPA) caused an increase in the activity of the human IGF-I gene major promoter in HepG2 cells. A CCAAT/enhancer-binding protein (C/EBP) binding site located at +22 to +30 was bound by C/EBP beta in a TPA-dependent manner and was solely responsible for the TPA responsiveness. This increase in C/EBP beta activity occurs through transcriptional and posttranslational regulation, and the latter is mediated by activation of p90 ribosomal S6 kinase (RSK): co-expression of dominant negative RSK abolished the TPA-responsive and C/EBP beta-dependent transactivation. Also, TPA-responsive activation of GAL4-C/EBP beta chimera required the Ser residue known as the RSK target. In SK-N-MC cells, which display constitutive, high expression of IGF-I on use of the major promoter, a large amount of C/EBP beta binding was observed with the C/EBP site in the basal state. Treatment with PKC inhibitors substantially reduced the promoter activity and mRNA amounts of IGF-I, with the binding of C/EBP beta to the C/EBP site also being reduced. When the C/EBP site was disrupted, the basal promoter activity was reduced, but the reduction by the PKC inhibitor was no longer observed. These observations suggest that the increase of C/EBP beta binding to the C/EBP site, which is in part mediated via activation of RSK, can primarily explain the TPA responsiveness of the IGF-I gene promoter. The intrinsic PKC activity in SK-N-MC cells should play a major role in the constitutive, high expression of IGF-I and may therefore contribute in part to the maintenance of the tumor phenotype of the cells.

Modulation of CREB activity by the Rho GTPase regulates cell and organism size during mouse embryonic development

Rho GTPases regulate several aspects of tissue morphogenesis during animal development. We found that mice lacking the Rho-inhibitory protein, p190-B RhoGAP, are 30% reduced in size and exhibit developmental defects strikingly similar to those seen in mice lacking the CREB transcription factor. In p190-B RhoGAP-deficient mice, CREB phosphorylation is substantially reduced in embryonic tissues. Embryo-derived cells contain abnormally high levels of active Rho protein, are reduced in size, and exhibit defects in CREB activation upon exposure to insulin or IGF-1. The cell size defect is rescued by expression of constitutively activated CREB, and in wild-type cells, expression of activated Rho or dominant-negative CREB results in reduced cell size. Together, these results suggest that activity of the Rho GTPase modulates a signal from insulin/IGFs to CREB that determines cell size and animal size during embryogenesis.

[Hair growth effect of minoxidil]

The length and size of hair are depend on the anagen term in its hair cycle. It has been reported that the some cell growth factors, such as VEGF, FGF-5S, IGF-1 and KGF, induce the proliferation of cells in the matrix, dermal papilla and dermal papillary vascular system and increase the amount of extra cellular matrix in dermal papilla and then maintain follicles in the anagen phase. On the other hand, negative factors, like FGF-5, thrombospondin, or still unknown ones, terminate the anagen phase. If the negative factors become dominant against cell proliferation factors according to fulfilling some time set by the biological clock for hair follicles, TGF beta induced in the matrix tissues evokes apoptosis of matrix cells and shifts the follicles from anagen to catagen. Androgenetic alopecia is caused by miniaturizing of hair follicles located in the frontal or crown part of scalp and are hereditarily more sensitive to androgen. In their hair cycles, the androgen shortens the anagen phase of follicles and shifts them to the catagen phase earlier than usual. The mode of action of hair growth effect of minoxidil is not completely elucidated, but the most plausible explanation proposed here is that minoxidil works as a sulfonylurea receptor (SUR) activator and prolongs the anagen phase of hair follicles in the following manner: minoxidil (1) induces cell growth factors such as VEGF, HGF, IGF-1 and potentiates HGF and IGF-1 actions by the activation of uncoupled SUR on the plasma membrane of dermal papilla cells, (2) inhibits of TGF beta induced apoptosis of hair matrix cells by opening the Kir 6.0 channel pore coupled with SUR on the mitochondrial inner membrane, and (3) dilates hair follicle arteries and increases blood flow in dermal papilla by opening the Kir 6.0 channel pore coupled with SUR on the plasma membrane of vascular smooth muscle cells.

Regulation of IGF-I mRNA and signal transducers and activators of transcription-3 and -5 (Stat-3 and -5) by GH in C2C12 myoblasts

GH and IGF-I are critical hormones for the regulation of longitudinal growth and the maintenance of lean body mass in humans. The regulation of IGF-I expression by GH in hepatocytes is well documented; however less is known about the regulation of IGF-I in peripheral tissues such as muscle. We have examined the regulation of IGF-I mRNA by GH and IGF-I in C2C12 myoblasts. GH stimulated the accumulation of IGF-I mRNA dose- and time-dependently. An elevation of IGF-I mRNA was observed with GH doses as low as 0.75 ng/ml and after exposure to GH for as little as 1 h, and the increase required ongoing transcription and translation. GH applied in a pulsatile fashion for 10 min followed by an 8-h interpulse interval increased IGF-I mRNA to a greater extent than continuous exposure. GH stimulated tyrosine phosphorylation of the GH receptor, signal transducer and activator of transcription-3 (Stat3), and Stat5. Stat5 was resistant to additional phosphorylation if cells were given a GH pulse within 2 h of a previous GH exposure. The refractory period lasted for 4 h, and cells could be maximally stimulated again after 6 h. Stat3 phosphorylation was also enhanced in cells that were allowed to recover from a previous application of GH. The tyrosine kinase inhibitors, genistein, PP1, and AG-490, and the MAPK kinase inhibitor, PD98059, did not block Stat3 or Stat5 phosphorylation. In contrast, WHI-P154, a Janus kinase-3 inhibitor, dose-dependently prevented Stat3, but not Stat5, phosphorylation. GH-inducible nuclear transport of Stat3 was likewise inhibited by WHI-P154. Most importantly, GH-dependent IGF-I mRNA expression was inhibited by WHI-P154. In contrast, IGF-I mRNA expression was inhibited by IGF-I peptide, and the effect of IGF-I was dominant over that of GH. IGF-I mRNA was regulated by both PI3K and MAPK signal transduction pathways, but IGF-I peptide signaled predominantly through a wortmannin-sensitive pathway to down-regulate its own mRNA. Our data suggest that Janus kinases (Jak2 or Jak3) and their downstream targets (Stat3 and Stat5) may play important roles in the expression of IGF-I mRNA and the myoblast response to GH. In addition, C2C12 cells appear to be a good model system to examine GH regulation of Janus kinase/Stat signaling and the regulation of IGF-I mRNA.

Deletional analysis of the murine IL-12 p35 promoter comparing IFN-gamma and lipopolysaccharide stimulation

IL-12, pivotal to the development of Th1 cells and formed by association of p35 and p40 subunits, is made by macrophages and the macrophage cell line RAW264.7. In this study, the promoter for p35 was cloned and analyzed. The murine IL-12 p35 gene has promoters upstream from each of the first two exons. The exon 1 and exon 2 promoters, cloned into a reporter vector, were responsive to LPS or IFN-gamma/CD40 ligation in transfected RAW264.7 cells. The exon 2 promoter containing bp -809 to +1 has significant homology to the human p35 promoter. Thus, deletion analysis was performed to determine the regions required for responsiveness to LPS, CD40, and/or IFN-gamma. Base pairs -809 to -740 influenced responsiveness to LPS. In contrast, bp -740to -444 and bp -122 to -100 were required for responses to IFN-gamma, IFN-gamma/LPS, or IFN-gamma/CD40 ligation. Removal of bp -444 to -392 increased the response of the exon 2 promoter to each stimulant. IFN regulatory factor (IRF)-1 is involved in the activity of this promoter at bp -108 to -103 because levels of nuclear IRF-1 correlated with exon 2 promoter activity in response to IFN-gamma and IRF-1 overexpression stimulated and enhanced exon 2 promoter activity. Also, site or deletion mutation of the IRF-1 element at bp -108 to -103 reduced the responsiveness of the promoter and IRF-1 bound to an oligonucleotide containing bp -108 to -103. The data suggest that the response of the p35 promoter to IFN-gamma requires a distinct IRF-1 positive regulatory element at bp -108 to -103.

Hormonal control of insulin-like growth factor I gene transcription in human osteoblasts: dual actions of cAMP-dependent protein kinase on CCAAT/enhancer-binding protein delta

Insulin-like growth factor-I (IGF-I) is essential for somatic growth and promotes bone cell replication and differentiation. IGF-I production by rat osteoblasts is stimulated by activation of cAMP-dependent protein kinase (PKA). In this report, we define two interacting PKA-regulated pathways that control IGF-I gene transcription in cultured human osteoblasts. Stimulation of cAMP led to a 12-fold increase in IGF-I mRNA and enhanced IGF-I promoter activity through a DNA response element termed HS3D and the transcription factor CCAAT/enhancer-binding protein delta (C/EBPdelta). Under basal conditions, C/EBPdelta was found in osteoblast nuclei but was transcriptionally silent. Treatment with the PKA inhibitor H-89 caused redistribution of C/EBPdelta to the cytoplasm. After hormone treatment, the catalytic subunit of PKA accumulated in osteoblast nuclei. Inhibition of active PKA with targeted nuclear expression of PKA inhibitor had no effect on the subcellular location of C/EBPdelta but prevented hormone-induced IGF-I gene activation, while cytoplasmic PKA inhibitor additionally caused the removal of C/EBPdelta from the nucleus. These results show that IGF-I gene expression is controlled in human osteoblasts by two PKA-dependent pathways. Cytoplasmic PKA mediates nuclear localization of C/EBPdelta under basal conditions, and nuclear PKA stimulates its transcriptional activity upon hormone treatment. Both mechanisms are indirect, since PKA failed to phosphorylate human C/EBPdelta in vitro.

Regulated nuclear-cytoplasmic localization of CCAAT/enhancer-binding protein delta in osteoblasts

Insulin-like growth factor I (IGF-I) plays a central role in skeletal growth by promoting bone cell replication and differentiation. Prostaglandin E2 (PGE2) and parathyroid hormone enhance cAMP production in cultured rat osteoblasts and stimulate IGF-I expression through a transcriptional mechanism mediated by cAMP-dependent protein kinase (PKA). We previously showed that PGE2 activated the transcription factor CCAAT/enhancer-binding protein delta (C/EBPdelta) in osteoblasts and induced its binding to a DNA element within the IGF-I promoter. We report here that a PKA-dependent pathway stimulates nuclear translocation of C/EBPdelta. Under basal conditions, C/EBPdelta was cytoplasmic but rapidly accumulated in the nucleus after PGE2 treatment (t(1/2) < 30 min). Nuclear translocation occurred without concurrent protein synthesis and was maintained in the presence of hormone. Nuclear localization required PKA and was blocked by a dominant-interfering regulatory subunit of the enzyme, even though C/EBPdelta was not a PKA substrate. Upon removal of hormonal stimulus, C/EBPdelta quickly exited the nucleus (t(1/2) < 12 min) through a pathway blocked by leptomycin B. Mutagenesis studies indicated that the basic domain of C/EBPdelta was necessary for nuclear localization and that the leucine zipper region permitted full nuclear accumulation. We thus define a pathway for PKA-mediated activation of C/EBPdelta through its regulated nuclear import.

Promoter analysis and characteristics of the 5'-untranslated region of the mouse glial cell line-derived neurotrophic factor gene

We have cloned the mouse GDNF cDNA and genomic DNA to study the molecular mechanism of gene expression. Primer extension and RT-PCR analyses indicated that the mouse gene contains 1086 bp of 5'-untranslated region (5'-UTR) [Gene 203 (1997) 149]. In this report, we identified the core promoter region of mouse GDNF and examined the role of the 5'-UTR in gene expression. Promoter deletion analyses indicated that the proximal region (-81 to +28), which includes a TATA-box, is necessary for high-level expression of GDNF. Using reporter constructs encoding luciferase or fusion gene of GDNF to enhanced green fluorescent protein that were transiently transfected to mouse astroglial cell-line TGA-3 cells and rat glioma C6 cells, we investigated effects of the 5'-UTR on promoter activity. Luciferase reporter assay indicated that a region downstream of the transcription initiation site may include a positive regulatory element, while two more distal regions appear to contain negative regulatory elements, which was correlated to the mRNA level based on RNase protection assay. Both negative regulatory elements attenuated promoter activity in a position-dependent manner. Nuclear proteins from C6 glioma cells were shown to interact with several regions (+65/+105, +233/+265, and +554/+582) including each of the regulatory elements, suggesting that regulation of GDNF expression by the 5'-UTR occurred mainly at the transcriptional level.

Runt domain factor (Runx)-dependent effects on CCAAT/ enhancer-binding protein delta expression and activity in osteoblasts

Transcription factor CCAAT/enhancer-binding protein delta (C/EBPdelta) is normally associated with acute-phase gene expression. However, it is expressed constitutively in primary osteoblast cultures where it increases insulin-like growth factor I synthesis in a cAMP-dependent way. Here we show that the 3' proximal region of the C/EBPdelta gene promoter contains a binding sequence for Runt domain factor Runx2, which is essential for osteogenesis. This region of the C/EBPdelta promoter directed high reporter gene expression in osteoblasts, and specifically bound Runx2 in osteoblast-derived nuclear extract. C/EBPdelta gene promoter activity was reduced by mutating the Runx binding sequence or by co-transfecting with Runx2 antisense expression plasmid, and was enhanced by overexpression of Runx-2. Exposure to prostaglandin E(2) increased Runx-dependent gene transactivation independently of Runx2 binding to DNA. Runx2 bound directly to the carboxyl-terminal region of C/EBPdelta itself, and its ability to drive C/EBPdelta expression was suppressed when C/EBPdelta or its carboxyl-terminal fragment was increased by overexpression. Consistent effects also occurred on C/EBPdelta-dependent increases in gene expression driven by synthetic or insulin-like growth factor I gene promoter fragments. These interactions between Runx2 and C/EBPdelta, and their activation by prostaglandin E(2), provide new evidence for their importance during skeletal remodeling, inflammatory bone disease, or fracture repair.

Inhibition of type I and III IP(3)Rs by TGF-beta is associated with impaired calcium release in mesangial cells

Inositol 1,4,5-trisphosphate receptors (IP(3)Rs) mediate cytosolic free calcium concentration ([Ca(2+)](c)) signals in response to a variety of agonists that stimulate mesangial cell contraction and proliferation. In the present study, we demonstrate that mesangial cells express both type I and III IP(3)Rs and that these receptors occupy different cellular locations. Chronic treatment with transforming growth factor-beta1 (TGF-beta1; 10 ng/ml, 24 h) leads to downregulation of both type I and III IP(3)Rs as measured by immunoblot and confocal analysis. TGF-beta1 treatment does not affect IP(3) levels, and downregulation of type I IP(3)R is not due to enhanced degradation of the protein, as the half-life of type I IP(3)R is unchanged in the presence or absence of TGF-beta1. Functional effects of TGF-beta1-induced downregulation of the IP(3)Rs were evaluated by measuring [Ca(2+)](c) changes in response to epidermal growth factor (EGF) in intact cells and sensitivity of [Ca(2+)](c) release to IP(3) in permeabilized cells. TGF-beta1 pretreatment led to a significant decrease of [Ca(2+)](c) release induced by EGF in intact cells and by submaximal IP(3) (400 nm) in permeabilized cells. Total IP(3)-sensitive [Ca(2+)](c) stores were not changed, as assessed by stimulation with maximal doses of IP(3) (10.5 microm) and thapsigargin-mediated calcium release in permeabilized cells. We conclude that prolonged exposure to TGF-beta1 leads to downregulation of both type I and III IP(3)Rs in mesangial cells and this is associated with impaired sensitivity to IP(3).

Time- and dose-related interactions between glucocorticoid and cyclic adenosine 3',5'-monophosphate on CCAAT/enhancer-binding protein-dependent insulin-like growth factor I expression by osteoblasts

Glucocorticoid has complex effects on osteoblasts. Several of these changes appear to be related to steroid concentration, duration of exposure, or specific effects on growth factor expression or activity within bone. One important bone growth factor, insulin-like growth factor I (IGF-I), is induced in osteoblasts by hormones such as PGE2 that increase intracellular cAMP levels. In this way, PGE2 activates transcription factor CCAAT/enhancer-binding protein-delta (C/EBPdelta) and enhances its binding to a specific control element found in exon 1 in the IGF-I gene. Our current studies show that preexposure to glucocorticoid enhanced C/EBPdelta and C/EBPbeta expression by osteoblasts and thereby potentiated IGF-I gene promoter activation in response to PGE2. Importantly, this directly contrasts with inhibitory effects on IGF-I expression that result from sustained or pharmacologically high levels of glucocorticoid exposure. Consistent with the stimulatory effect of IGF-I on bone protein synthesis, pretreatment with glucocorticoid sensitized osteoblasts to PGE2, and in this context significantly enhanced new collagen and noncollagen protein synthesis. Therefore, pharmacological levels of glucocorticoid may reduce IGF-I expression by osteoblasts and cause osteopenic disease, whereas physiological transient increases in glucocorticoid may permit or amplify the effectiveness of hormones that regulate skeletal tissue integrity. These events appear to converge on the important role of C/EBPdelta and C/EBPbeta on IGF-I expression by osteoblasts.

Identification of core sequences involved in metabolism-dependent nuclear protein binding to the rat insulin-like growth factor I gene

In the liver, most insulin-like growth factor I (IGF-I) transcripts originate in exon 1, where important cis-regulatory regions are located downstream from the major transcription initiation sites. Within these regions, we have attempted to identify sequences which are involved in the decrease in IGF-I gene transcription associated with diabetes mellitus. The function of different genomic templates was assessed by in vitro transcription, which revealed a consistent 50-80% decrease in the activity of nuclear extracts from streptozotocin-diabetic as compared with normal rats. The disparity in transcriptional activity between normal and diabetic nuclear extracts was reduced with templates containing 11-bp mutations within DNase I protected regions III or V (+42 and +129 bp, respectively, from the major transcription initiation site), but a mutation between regions IV and V had little effect. Within region III, gel mobility shift analysis and methylation interference studies indicated that DNA-protein interactions involve a GCGC core sequence. In region V, gel mobility shift studies and uracil interference analysis revealed interactions involving a TTAT core. While gel mobility shift analysis and transient transfection studies indicate that the GCGC core sequence in region III recognizes C/EBP, the AT-rich sequence in region V is likely to recognize a protein with homeodomain characteristics. Identification of the nuclear factor(s) interacting with regions III and V, downstream from exon 1 initiation sites, will be important for understanding the mechanism of reduced IGF-I gene transcription due to diabetes mellitus.