Insulin-like growth factor-I stimulates c-fos and c-jun transcription in PC12 cells

A Systematic Review of Registered Clinical Trials for Peripheral Nerve Injuries

ABSTRACT

Upper extremity peripheral nerve injuries (PNIs) significantly impact daily functionality and necessitate effective treatment strategies. Clinical trials play a crucial role in developing these strategies. However, challenges like retrospective data collection, reporting biases, inconsistent outcome measures, and inadequate data sharing practices hinder effective research and treatment advancements. This review aims to analyze the landscape of reporting, methodological design, outcome measures, and data sharing practices in registered clinical trials concerning upper extremity PNIs. It seeks to guide future research in this vital area by identifying current trends and gaps.A systematic search was conducted on ClinicalTrials.gov and WHO International Clinical Trials Registry Platform up to November 10, 2023, using a combination of MeSH terms and keywords related to upper extremity nerve injury. The PRISMA 2020 guidelines were followed, and the studies were selected based on predefined inclusion and exclusion criteria. A narrative synthesis of findings was performed, with statistical analysis for associations and completion rates.Of 3051 identified studies, 96 met the inclusion criteria. These included 47 randomized controlled trials, 27 nonrandomized trials, and others. Sensory objective measures were the most common primary outcomes. Only 13 studies had a data sharing plan. The analysis revealed varied intervention methods and inconsistencies in outcome measures. There was a significant association between study funding, design, and completion status, but no association between enrollment numbers and completion.This review highlights the need for standardized outcome measures, patient-centered assessments, and improved data sharing in upper extremity PNI trials. The varied nature of interventions and inconsistency in outcome measures indicate the necessity for more rigorous and transparent research practices to strengthen the evidence base for managing these injuries.

Advancing Our Understanding of the Chronically Denervated Schwann Cell: A Potential Therapeutic Target?

Outcomes for patients following major peripheral nerve injury are extremely poor. Despite advanced microsurgical techniques, the recovery of function is limited by an inherently slow rate of axonal regeneration. In particular, a time-dependent deterioration in the ability of the distal stump to support axonal growth is a major determinant to the failure of reinnervation. Schwann cells (SC) are crucial in the orchestration of nerve regeneration; their plasticity permits the adoption of a repair phenotype following nerve injury. The repair SC modulates the initial immune response, directs myelin clearance, provides neurotrophic support and remodels the distal nerve. These functions are critical for regeneration; yet the repair phenotype is unstable in the setting of chronic denervation. This phenotypic instability accounts for the deteriorating regenerative support offered by the distal nerve stump. Over the past 10 years, our understanding of the cellular machinery behind this repair phenotype, in particular the role of c-Jun, has increased exponentially, creating opportunities for therapeutic intervention. This review will cover the activation of the repair phenotype in SC, the effects of chronic denervation on SC and current strategies to 'hack' these cellular pathways toward supporting more prolonged periods of neural regeneration.

Global transcriptome analysis of rat dorsal root ganglia to identify molecular pathways involved in incisional pain

To develop non-opioid therapies for postoperative incisional pain, we must understand its underlying molecular mechanisms. In this study, we assessed global gene expression changes in dorsal root ganglia neurons in a model of incisional pain to identify pertinent molecular pathways. Male, Sprague-Dawley rats underwent infiltration of 1% capsaicin or vehicle into the plantar hind paw (n = 6-9/group) 30 min before plantar incision. Twenty-four hours after incision or sham (control) surgery, lumbar L4-L6 dorsal root ganglias were collected from rats pretreated with vehicle or capsaicin. RNA was isolated and sequenced by next generation sequencing. The genes were then annotated to functional networks using a knowledge-based database, Ingenuity Pathway Analysis. In rats pretreated with vehicle, plantar incision caused robust hyperalgesia, up-regulated 36 genes and downregulated 90 genes in dorsal root ganglias one day after plantar incision. Capsaicin pretreatment attenuated pain behaviors, caused localized denervation of the dermis and epidermis, and prevented the incision-induced changes in 99 of 126 genes. The pathway analyses showed altered gene networks related to increased pro-inflammatory and decreased anti-inflammatory responses in dorsal root ganglias. Insulin-like growth factor signaling was identified as one of the major gene networks involved in the development of incisional pain. Expression of insulin-like growth factor -2 and IGFBP6 in dorsal root ganglia were independently validated with quantitative real-time polymerase chain reaction. We discovered a distinct subset of dorsal root ganglia genes and three key signaling pathways that are altered 24 h after plantar incision but are unchanged when incision was made after capsaicin infiltration in the skin. Further exploration of molecular mechanisms of incisional pain may yield novel therapeutic targets.

Novel Aspects Concerning the Functional Cross-Talk between the Insulin/IGF-I System and Estrogen Signaling in Cancer Cells

The insulin/IGF system plays an important role in cancer progression. Accordingly, elevated levels of circulating insulin have been associated with an increased cancer risk as well as with aggressive and metastatic cancer phenotypes. Numerous studies have documented that estrogens cooperate with the insulin/IGF system in multiple pathophysiological conditions. The biological responses to estrogens are mainly mediated by the estrogen receptors (ER)α and ERβ, which act as transcription factors; however, several studies have recently demonstrated that a member of the G protein-coupled receptors, named GPR30/G-protein estrogen receptor (GPER), is also involved in the estrogen signaling in normal and malignant cells as well as in cancer-associated fibroblasts (CAFs). In this regard, novel mechanisms linking the action of estrogens through GPER with the insulin/IGF system have been recently demonstrated. This review recapitulates the relevant aspects of this functional cross-talk between the insulin/IGF and the estrogenic GPER transduction pathways, which occurs in various cell types and may account for cancer progression.

Hyperplasia and cellularity changes in IGF-1-overexpressing skeletal muscle of crucian carp

The zebrafish skeletal muscle-specific promoter mylz2 was used to cause crucian carp overexpression of the zebrafish IGF-1 cDNA. In stable transgenic germline F1 progenies, a 5-fold increase in the level of IGF-1 in skeletal muscle was observed. Evident skeletal muscle hyperplasia was observed in the transgenic fish through histologic analysis. By analyzing the RNA sequencing transcriptome of the skeletal muscle of IGF-1 transgenic fish and nontransgenic control fish at 15 months of age, 10 966 transcripts with significant expression levels were identified with definite gene descriptions based on the corresponding zebrafish genome information. Based on the results of our RNA sequencing transcriptome profiling analysis and the results of the real-time quantitative PCR analysis performed to confirm the skeletal muscle transcriptomics analysis, several pathways, including IGF-1 signaling, aerobic metabolism, and protein degradation, were found to be activated in the IGF-1-overexpressing transgenic fish. Intriguingly, our transcriptional expression and protein assays indicated that the overexpression of IGF-1 stimulated a significant shift in the myofiber type toward a more oxidative slow muscle type. Although the body weight was surprisingly decreased by IGF-1 transgenic expression, significantly higher oxygen consumption rates were measured in IGF-1-overexpressing transgenic fish compared with their nontransgenic control fish. These results indicate that the sustained overexpression of IGF-1 in crucian carp skeletal muscle promotes myofiber hyperplasia and cellularity changes, which elicit alterations in the body energy metabolism and skeletal muscle growth.

New advances on the functional cross-talk between insulin-like growth factor-I and estrogen signaling in cancer

There is increasing awareness that estrogens may affect cell functions through the integration with a network of signaling pathways. The IGF system is a phylogenetically highly conserved axis that includes the insulin receptor (IR) and the insulin-like growth factor I receptor (IGF-IR) pathways, which are of crucial importance in the regulation of metabolism and cell growth in relationship to nutrient availability. Numerous studies nowadays document that estrogens cooperate with IGF system at multiple levels both in physiology and in disease. Several studies have focused on this bidirectional cross-talk in central nervous system, in mammary gland development and in cancer. Notably, cancer cells show frequent deregulation of the IGF system with overexpression of IR and/or IGF-IR and their ligands as well as frequent upregulation of the classical estrogen receptor (ER)α and the novel ER named GPER. Recent studies have, therefore, unraveled further mechanisms of cross-talk involving membrane initiated estrogen actions and the IGF system in cancer, that converge in the stimulation of pro-tumoral effects. These studies offer hope for new strategies aimed at the treatment of estrogen related cancers in order to prevent an estrogen-independent and more aggressive tumor progression.

The reciprocal regulation of gamma-synuclein and IGF-I receptor expression creates a circuit that modulates IGF-I signaling

Insulin-like growth factor (IGF) system plays important roles in carcinogenesis and maintenance of the malignant phenotype. Signaling through the IGF-I receptor (IGF-IR) has been shown to stimulate the growth and motility of a wide range of cancer cells. γ-synuclein (SNCG) is primarily expressed in peripheral neurons but also overexpressed in various cancer cells. Overexpression of SNCG correlates with tumor progression. In the present study we demonstrated a reciprocal regulation of IGF-I signaling and SNCG expression. IGF-I induced SNCG expression in various cancer cells. IGF-IR knockdown or IGF-IR inhibitor repressed SNCG expression. Both phosphatidylinositol 3-kinase and mitogen-activated protein kinase were involved in IGF-I induction of SNCG expression. Interestingly, SNCG knockdown led to proteasomal degradation of IGF-IR, thereby decreasing the steady-state levels of IGF-IR. Silencing of SNCG resulted in a decrease in ligand-induced phosphorylation of IGF-IR and its downstream signaling components, including insulin receptor substrate (IRS), Akt, and ERK1/2. Strikingly, SNCG physically interacted with IGF-IR and IRS-2. Silencing of IRS-2 impaired the interaction between SNCG and IGF-IR. Finally, SNCG knockdown suppressed IGF-I-induced cell proliferation and migration. These data reveal that SNCG and IGF-IR are mutually regulated by each other. SNCG blockade may suppress IGF-I-induced cell proliferation and migration. Conversely, IGF-IR inhibitors may be of utility in suppressing the aberrant expression of SNCG in cancer cells and thereby block its pro-tumor effects.

Role of cyclic AMP response element-binding protein in insulin-like growth factor-i receptor up-regulation by sex steroids in prostate cancer cells

Insulin-like growth factor-I receptor (IGF-IR) overexpression may play a role in prostate cancer progression. We found previously that, in prostate cancer cells, IGF-IR is up-regulated by both androgens and estrogens via a nongenotropic pathway. We now show that, in prostate cancer cells, stimulation with either androgens or estrogens up-regulates IGF-IR by inducing cyclic AMP response element-binding protein (CREB) activation. Both sex steroids phosphorylated CREB at Ser(133) in a dose-dependent manner in androgen receptor (AR)-positive LNCaP cells, whereas only estrogens phosphorylated CREB in AR-negative PC3 cells. CREB phosphorylation involved c-Src-dependent extracellular signal-regulated kinase 1/2 activation, but not protein kinase A, protein kinase C, or calmodulin-dependent kinase II, and occurred also in cells transfected with AR or estrogen receptor mutants that do not localize into the nucleus. CREB silencing abrogated IGF-IR up-regulation and promoter activation. We also showed that CREB binds to IGF-IR promoter region and identified the relevant CREB-binding site at the 5'-untranslated region fragment of IGF-IR promoter. In conclusion, we describe a novel mechanism of IGF-IR up-regulation and promoter activity by CREB activation, induced by sex steroids, through a nongenotropic signaling.

Oncogenesis and mutagenesis of pituitary tumors

Although pituitary tumors may be present in up to 10% of the population, the pathophysiology of these lesions is not well characterized. Pituitary tumors are composed of monoclonal cell populations with disrupted control of replication pathways. The oncogenes and tumor suppressor genes that are common in other malignancies (i.e. jun, fos, myc, and p53) are rarely involved in the development of these tumors. However, oncogenes, such as gsp, can be present in up to 40% of hormonally active adenomas. The process of pituitary oncogenesis further appears to involve oncogenes such as cyclin E, cyclin D1, and the pituitary tumor transforming gene (PTTG). Finally, the cAMP signaling cascade plays a significant role in generation of both benign and malignant pituitary tumors. In this review, the biology of pituitary adenomas is explored with a special emphasis on potential targets for the development of targeted therapeutics.

Insulin-like growth factor-1 (IGF-1) induces the activation/phosphorylation of Akt kinase and cAMP response element-binding protein (CREB) by activating different signaling pathways in PC12 cells

Background

Insulin-like growth factor-1 (IGF-1) is a polypeptide growth factor with a variety of functions in both neuronal and non-neuronal cells. IGF-1 plays anti-apoptotic and other functions by activating multiple signaling pathways including Akt kinase, a serine/threonine kinase essential for cell survival. The nuclear transcription factor cAMP response element-binding protein (CREB) may also be involved although relationships between these two proteins in IGF-1 receptor signaling and protection is not clear, especially in neuronal cells.

Results

IGF-1, in a concentration- and time-dependent manner, induces the activation/phosphorylation of Akt and CREB in PC12 cells by activating different signaling pathways. IGF-1 induced a sustained phosphorylation of Akt while only a transient one was seen for CREB. The phosphorylation of Akt is mediated by the PI3 kinase pathway while that of CREB is dependent on the activation of both MAPK kinase and p38 MAPK. Moreover, the stimulation of PKC attenuated the phosphorylation of Akt induced by IGF-1 while enhancing that of CREB. Survival assays with various kinase inhibitors suggested that the activation/phosphorylation of both Akt and CREB contributes to IGF-1 mediated cell survival in PC12 cells.

Conclusion

These data suggest that IGF-1 induced the activation of Akt and CREB using distinct pathways in PC12 cells.

Neuroprotective gene expression profiles in ischemic cortical cultures preconditioned with IGF-1 or bFGF

The mechanisms underlying growth factor preconditioning of neurons are only partially elucidated, and no studies have been conducted in this area using a gene profiling approach. We used cDNA microarrays to compare the transcriptional profiles of cells preconditioned either with insulin-like growth factor I (IGF-1) or basic fibroblast growth factor (bFGF), to identify differentially regulated genes that may function in growth factor signaling, response to oxygen-glucose deprivation (OGD), and most importantly, cell survival. Primary rat cortical cultures were treated with bFGF or IGF-1 for 2, 24, or 24 h followed by OGD for 90 min, and compared with cells that were subject to OGD without growth factor pretreatment. Although the majority of surveyed genes were unchanged in all experimental treatments, 175 genes (10% of the cDNAs on the chip) were found to be differentially regulated in at least one of the treatment conditions. Hierarchical clustering of these 175 genes was used to identify four expression clusters: IGF-1 regulated, bFGF regulated, OGD regulated, and putative neuroprotective genes. Further analysis using realtime RT-PCR confirmed that we had identified genes that are regulated by single growth factors, as well as several more that are co-regulated by both IGF-1 and bFGF. These genes can influence neuronal survival by affecting diverse pathways such as growth factor signal transduction (CD44, DTR, DUSP6, EPS8, IGFBP3), DNA repair and transcription (FOXJ1), metabolic homeostasis (RASA1, SHMT2), cytoskeletal stability (MSN, MAPT) and cholesterol biosynthesis (FDFT1, FDPS).

Postreceptor resistance to exogenous growth hormone exists in the jejunal mucosa of parenterally fed rats

Our objective was to determine whether the intestinal mucosa is resistant to the mitogenic effects of exogenous growth hormone (GH) but sensitive to exogenous insulin-like growth factor-I (IGF-I) during total parenteral nutrition (TPN) because of decreased GH receptor (GHR) binding or postreceptor responsiveness to GH. First, only continuous i.v. administration of IGF-I, but neither pulsatile subcutaneous nor continuous i.v. GH, stimulated jejunal mucosal hyperplasia; however, both GH and IGF-I increased serum IGF-I and promoted similar whole-body growth after 8 d of exclusive TPN and 6 d of growth factor treatment in rats. This suggests a tissue-specific resistance to GH action in the intestinal mucosa during TPN. Second, exogenous GH during TPN did not reduce GH-specific binding in jejunum, suggesting that the inability of GH to stimulate mucosal hyperplasia is not due to low levels of the GHR. Third, IGF-I, but not GH, induced acute expression of c-fos (P < 0.009) and c-jun (P = 0.053) mRNAs in jejunum based on Northern analysis and in situ hybridization histochemistry 60 min after a single i.v. bolus of GH or IGF-I. This suggests that IGF-I, but not GH, activates early postreceptor growth-related signaling pathways in jejunum. In summary, the lack of early c-fos and c-jun induction in response to GH in TPN rats indicates that the jejunal mucosa is resistant to exogenous GH between GHR activation and induction of immediate early genes. This may contribute to the inability of mucosal cells to respond to the trophic effects of GH.

Insulin-like growth factor-1 downregulates nuclear factor kappa B activation and upregulates interleukin-8 gene expression induced by tumor necrosis factor alpha

Pretreatment of HT29-D4 epithelial adenocarcinoma colic cells with des-IGF-1 upregulated TNF alpha-mediated activation of IL-8 expression at different levels (protein, mRNA, and hnRNA). RNA transcription but not RNA stabilization was found to be involved. In this cell line, cooperation of NF-kappa B with other factors appeared essential for IL-8 expression. Indeed, TNF alpha-induced NF-kappa B translocation was not sufficient to support enhancement of the transcription and des-IGF-1 did not promote but partly inhibited both the TNF alpha-induced NF-kappa B activation and I kappa B alpha degradation through a PI-3K-dependent pathway. A CCAAT/enhancer binding protein (C/EBP) site located on the IL-8 gene enhancer cooperated with a NF-kappa B binding site and led to the upregulation of IL-8 expression. Binding of C/EBP alpha to this sequence disappeared in IGF-1 treated cells. This event may be important for the cross-talk between IGF-1- and TNF alpha-mediated pathways leading to the control of inflammatory processes and the decision concerning apoptosis or cell survival.

Overexpression of insulin-like growth factor II (IGFII) in ZR-75-1 human breast cancer cells: higher threshold levels of receptor (IGFIR) are required for a proliferative response than for effects on specific gene expression

Previous transfection experiments using a zinc-inducible expression vector have shown that overexpression of insulin-like growth factor II (IGFII) in MCF7 human breast cancer cells can reduce dependence on oestrogen for cell growth in vitro (DALY RJ, HARRIS WH, WANG DY, DARBRE PD. (1991) Cell Growth Differentiation 2, 457-464.). Parallel transfections now performed into another oestrogen-dependent human breast cancer cell line (ZR-75-1) yielded three clones of transfected ZR-75-1 cells that produced levels of zinc-inducible IGFII mRNA and secreted mature IGFII protein similar to those found in the transfected MCF7 cells. However, unlike in MCF7 cells, no resulting effects were found on cell growth in the ZR-75-1 clones, even though the ZR-75-1 clones possessed receptors capable of binding 125I-IGFI and showed a growth response to exogenously added IGFII. Medium conditioned by the ZR-75-1 clones could stimulate growth of untransfected MCF7 cells, indicating that the secreted IGFII protein was bioactive. Furthermore, zinc-induced IGFII was capable of increasing both pS2 mRNA levels and CAT activity from a transiently transfected AP1-CAT gene in the ZR-75-1 clones. Constitutive co-overexpression of the protein processing enzyme PC2 resulted in reduced levels of large forms of zinc-inducible IGFII, but zinc treatment still produced no effect on cell growth rate. Finally, however, constitutive co-overexpression of the type I IGF receptor (IGFIR) did result in zinc-inducible increased basal cell growth and reduced dependence on oestrogen for cell growth. These results demonstrate that while overexpression of IGFII per se was sufficient to deregulate MCF7 cell growth, the ZR-75-1 cells are limited in their proliferative response by their intrinsic receptor levels. However, although the proliferative response was limited, molecular responses (expression of pS2 and AP1-CAT) were not limited, indicating that different cellular responses can have different threshold receptor level requirements.

Regulation of the Akt/Glycogen synthase kinase-3 axis by insulin-like growth factor-II via activation of the human insulin receptor isoform-A

Insulin-like growth factor II (IGF-II) plays a key role in mitogenesis during development and tumorigenesis and is believed to exert its mitogenic functions mainly through the IGF-I receptor. Recently, we identified the insulin receptor isoform A (IR(A)) as an additional high affinity receptor for IGF-II in both fetal and cancer cells. Here we investigated the mitogenic signaling of IGF-II via the Akt/Glycogen synthase kinase 3 (Gsk3) axis employing R-IR(A) cells that are IGF-I receptor null mouse embryonic fibroblasts expressing the human IR(A). IGF-II induced activation of the proto-oncogenic serine kinase Akt, reaching maximal at 5-10 min. IGF-II also caused the rapid and sustained deactivation of glycogen synthase kinase 3-beta (Gsk3beta), reaching maximal at 1-3 min, shortly preceding, therefore, maximal activation of Akt. Under our conditions, IGF-II and insulin induced 70-80% inhibition of Gsk3betaactivity. In these cells IGF-II also deactivated Gsk3alpha although less effectively than Gsk3beta. In parallel experiments, we found that IGF-II induced transient activation of extracellular-signal-regulated kinases (Erk) reaching maximal at 5-10 min and decreasing thereafter. Time courses and potencies of regulation of both mitogenic pathways (Akt/Gsk3beta and Erk) by IGF-II via IR(A) were similar to those of insulin. Furthermore, IGF-II like insulin effectively stimulated cell cycle progression from the G0/G1 to the S and G2/M phases. Interestingly, AP-1-mediated gene expression, that was reported to be negatively regulated by Gsk3beta was only weakly increased after IGF-II stimulation. Our present data suggest that the coordinated activation or deactivation of Akt, Gsk3beta, and Erk may account for IGF-II mitogenic effects and support an active role for IR(A) in IGF-II action.

Prolonged activation of transcription factor AP-1 during NGF-mediated rescue from apoptotic cell death in PC12 cells

Rat pheochromocytoma (PC12) cells exhibit apoptotic cell death when deprived of serum and can be rescued by nerve growth factor (NGF). We characterized AP-1 DNA binding activity in PC12 cells after serum deprivation in the presence or absence of NGF or other neurotrophic agents. There was a decline in AP-1 DNA binding activity concomitant with apoptosis in PC12 cells after serum deprivation. Treatment of serum-deprived PC12 with NGF induced persistent AP-1 binding activity that was blocked by the Trk receptor inhibitor K252a. PC12 cells treated with dibutyryl cyclic AMP or insulin also displayed increased AP-1 DNA binding activity. While NGF somewhat increased c-Fos and c-Jun protein levels transiently, it had a more robust and persistent stimulatory effect on Jun B protein levels. AP-1 transcriptional activity increased after NGF, dibutyryl cAMP, or insulin treatment under serum free conditions. Curcumin, which inhibits AP-1 activity, blocked the NGF-mediated rescue. These results would suggest that the rescue of serum-deprived PC12 cells from apoptosis requires increasing endogenous levels of specific Fos/Jun components of AP-1.

Insulin-like growth factor I-mediated activation of the transcription factor cAMP response element-binding protein in PC12 cells. Involvement of p38 mitogen-activated protein kinase-mediated pathway

IGF-I is known to support growth and to prevent apoptosis in neuronal cells. Activation of the nuclear transcription factor cAMP response element-binding protein (CREB) has emerged as a central determinant in neuronal functions. In the present investigation, we examined the IGF-I-mediated phosphorylation and transcriptional activation of CREB in rat pheochromocytoma (PC12) cells, a cellular model for neuronal differentiation, and defined three distinct postreceptor signaling pathways important for this effect including the p38 mitogen-activated protein kinase (MAPK) pathway. CREB phosphorylation at serine 133 and its transcriptional activation as measured by a CREB-specific Gal4-CREB reporter and the neuroendocrine-specific gene chromogranin A was induced 2-3.3-fold by insulin-like growth factor (IGF)-I. This activation was significantly blocked (p < 0.001) by the dominant negative K-CREB or by mutation of the CRE site. IGF-I stimulated chromogranin A gene expression by Northern blot analysis 3.7-fold. Inhibition of MAPK kinase with PD98059, PI 3-kinase with wortmannin, and p38 MAPK with SB203580 blocked IGF-I-mediated phosphorylation and transcriptional activation of CREB by 30-50% (p < 0.001). Constitutively active and dominant negative regulators of the Ras and PI 3-kinase pathways confirmed the contribution of these pathways for CREB regulation by IGF-I. Cotransfection of PC12 cells with p38beta and constitutively active MAPK kinase 6 resulted in enhanced basal as well as IGF-I-stimulated chromogranin A promoter. IGF-I activated p38 MAPK, which was blocked by the inhibitor SB203580. This is the first description of a p38 MAPK-mediated nuclear signaling pathway for IGF-I leading to CREB-dependent neuronal specific gene expression.

Insulin activates the hepatitis B virus X gene through the activating protein-1 binding site in HepG2 cells

Insulin stimulates cellular oncogenic activators such as c-jun, c-fos, and c-myc; and hepatitis B virus (HBV) X, a viral transactivator, is known to induce liver cancer in transgenic mice. In this respect, the effect of insulin on the expression of HBx protein was investigated in HepG2 cells. Insulin-stimulated transcription from the HBV X promoter in a dose-dependent manner was assessed by chloramphenicol acetyltransferase (CAT) assay. A mutation preventing AP-1 binding to the E element abolished the activation of the HBV X promoter by insulin. In addition, insulin stimulated the minimal thymidine kinase (tk) gene promoter activity through both the HBV E element and the consensus AP-1 binding site in HepG2 cells. An electrophoretic mobility shift assay (EMSA) using insulin-treated HepG2 nuclear extracts showed that insulin actually enhanced the binding of nuclear proteins to the HBV E element as well as to the consensus AP-1 binding site. Both HBV E and AP-1 oligonucleotides were effective competitors for this binding. These results showed that insulin elevated the expression of HBx protein through the AP-1 binding site of HBV EnI. We suggest that insulin can augment the role of HBx in the development of hepatocellular carcinoma (HCC) in HBV-infected liver, probably through interaction with other cellular oncogenes.

Overexpression of IGF-1 exclusively in skeletal muscle prevents age-related decline in the number of dihydropyridine receptors

Excitation-contraction uncoupling has been identified as a mechanism underlying skeletal muscle weakness in aging mammals (sarcopenia). The basic mechanism for excitation-contraction uncoupling is a larger number of ryanodine receptors (RyR1) uncoupled to dihydropyridine receptors (DHPRs) (Delbono, O., O'Rourke, K. S., and Ettinger, W. H. (1995) J. Membr. Biol. 148, 211-222). In the present study, we used transgenic mice overexpressing human insulin-like growth factor-1 exclusively in skeletal muscle to test the hypothesis that a high concentration of IGF-1 prevents age-related decreases in DHPR number and in muscle force. Transgenic mice express 10-20-fold higher IGF-1 concentrations than nontransgenic mice at all ages (1-24 months). The number of DHPRs is 50-100% higher, and the DHPR/RyR1 ratio is 40% higher in transgenic soleus (predominantly type I fiber muscles), extensor digitorum longus (predominantly type II fiber muscles), and the pool of type I and type II fiber muscles than in nontransgenic young (6 months), adult (12 months), and old (24 months) mice. Furthermore, no age-related changes in DHPRs and the DHPR/RyR1 ratio were observed in transgenic muscles. The specific single twitch and tetanic muscle force in old transgenic soleus and extensor digitorum longus muscles are 50% higher than in old nontransgenic muscles. Taken together, these results support the concept that IGF-1- dependent prevention of age-related decline in DHPR expression is associated with stronger muscle contraction in older transgenic mice.

Quantification of Fos immunoreactivity in cortical cultures treated with growth factors

Among the effects mediated by growth factors in vivo are the switch to growth arrest and differentiation during normal development of the nervous system, survival during the period of naturally occurring cell death, and plasticity and repair of neurons in the adult brain. Much interest has focused on the signalling pathways utilised by growth factors with a large proportion of experiments carried out using the phaeochromocytoma cell line. Here we have quantified Fos immunoreactivity following stimulation of primary cultures of rat cortical neurons with a variety of growth factors including neurotrophins and cytokines. Expression of Fos has been quantified in these cultures using an ELISA technique, and immunocytochemistry followed by digital stereology. Treatment of cultures with brain derived neurotrophic factor (BDNF) or neurotrophin-4 (NT-4) causes a dose-dependent increase in Fos expression, while neurotrophin-3 (NT-3) causes an increase but at high concentrations only. A sub-population of cortical neurons within the cultures express Fos in response to fibroblast growth factor-1 or fibroblast growth factor-2 but no cells respond with Fos expression on treatment with insulin-like growth factor-1. We conclude that BDNF and NT-4 cause dose-dependent increases in the number of Fos immunoreactive cells.

Death-signalling cascade in mouse cerebellar granule neurons

Molecular mechanisms of neuronal cell death are still largely unknown. In the present study, the signal transduction pathway of cell death in cerebellar granule neurons was examined by employing various death-preventative agents. When death was induced by the depletion of serum and a depolarizing level of potassium, transient increase in active c-Jun, mitochondrial membrane potential (deltapsi) loss, activation of caspase-3 (-like) proteases, and nuclear condensation and fragmentation were observed. The protein synthesis inhibitor cycloheximide blocked all these phenomena, whereas RNA synthesis inhibitor actinomycin-D, survival factor such as insulin-like growth factor-1, brain-derived neurotrophic factor, high K+ (25 mM) and overproduced antiapoptotic protein Bcl-2, prevented deltapsi, loss, caspase activation, and nuclear change, but not an increase in active c-Jun. The caspase inhibitor z-Asp-CH2-DCB (carbobenzoxy-L-aspartyl-alpha-[(2,6-dichlorobenzoyl) oxy]methane) only inhibited activation of caspases and nuclear change. These results suggest that the death signal in cerebellar granule neurons is sequentially transduced in the order of c-Jun activation, de novo RNA synthesis, mitochondrial deltapsi loss, activation of caspase-3 (-like) proteases and nuclear change.

Molecular regulation of neuronal apoptosis

Apoptosis is a fundamental biological process used by all muticellular organisms to eliminate unwanted or superfluous cells, and is a prominent feature of normal neural development. Developmentally occurring neuronal apoptosis serves to match the number of neurons to the requirements of their synaptic targets and to rid the nervous system of inappropriate connections. While it is generally accepted that apoptosis is a "suicide program" inherent in all cells, the molecular basis of this program is just beginning to be unraveled. Evidence from numerous recent studies indicate that a variety of proteins are involved in the transmission of external signals to the cell-death machinery within the cell. This review describes many of the recent findings of the regulatory pathways and genes that have been implicated in the induction or suppression of apoptosis in neurons.

Activation of Protein Kinase C-ζ and Phosphatidylinositol 3′-Kinase and Promotion of Macrophage Differentiation by Insulin-Like Growth Factor-I

Phosphoinositides that are phosphorylated at the D3 position have been reported to activate an atypical, Ca2-independent protein kinase C (PKC) isoform designated PKC-ζ, and overexpression of this enzyme leads to monocytic differentiation. In this study, we cultured human HL-60 promyeloid cells with vitamin D3 and insulin-like growth factor-I (IGF-I), a 70-amino-acid peptide that activates phosphatidylinositol 3′-kinase (PI 3-kinase) in murine promyeloid cells. Two days later, the proportion of cells differentiating into macrophages in serum-free medium, as assessed by expression of the α-subunit of the β2 integrin CD11b, increased from 5 ± 1% to 25 ± 3%. Addition of IGF-I increased the proportion of cells differentiating into CD11b-positive macrophages to 78 ± 5%. In the absence of vitamin D3, IGF-I did not induce expression of CD11b (6 ± 1%). The IGF-I-promoted macrophage differentiation was blocked specifically by preincubation of HL-60 cells with a mAb (αIR3) directed against the IGF type I receptor. Similarly, pretreatment of cells with either αIR3 or an IGF-binding protein, IGFBP-3, led to a 75% inhibition of CD11b expression when cells were cultured with vitamin D3 in serum-containing medium. IGF-I, but not vitamin D3, caused a sevenfold increase in the enzymatic activity of both PI 3-kinase and atypical PKC-ζ. Inhibition of IGF-I-inducible PI 3-kinase with either wortmannin or LY294002 abrogated the IGF-I-induced activation of PKC-ζ and totally blocked the enhancement in macrophage differentiation caused by IGF-I. These data establish that PKC-ζ is a putative downstream target of PI 3-kinase that is activated during IGF-I-promoted macrophage differentiation.

Pituitary adenylate cyclase-activating polypeptide stimulates proenkephalin gene transcription through AP1- and CREB-dependent mechanisms

The effects of the pituitary adenylase cyclate-activating peptides (PACAP) 27 and 38 on proenkephalin (PENK) gene transcription were examined in PC12 (rat pheochromocytoma) cells using transient transfection assays. Both ligands stimulated PENK gene transcription in a dose-dependent manner, with an apparent ED50 close to 5 x 10(-11) M. Inactivation of cAMP dependent-protein kinase (PKA) with a dominant inhibitory mutant strongly reduced PACAP-stimulated PENK transcription. Using reporter genes driven by either the minimal TPA-responsive element (TRE: TGACTCA) or cAMP-responsive element (CRE: TGACGTCA), we showed that the two PACAPs activate transcription through both regulatory sequences. These effects could result from direct post-translational activation of Jun and CREB, as shown using GAL4-Jun or GAL4-CREB fusion proteins. Expression of a dominant inhibitory mutant of CREB decreased by 60% the response to PACAP, suggesting that CREB is implicated in PENK transactivation. Similarly, expression of c-fos antisense RNA reduced by 80% the stimulatory effects of PACAP. Taken together, these results indicate that PACAP stimulates PENK transcription by members of both the AP1 and the CREB families. However, AP1 by itself is not sufficient to increase PENK transcription, as insulin-like growth factor 1 (IGF1), which stimulates AP1 activity but not cAMP production, is unable to stimulate PENK transcription. These results indicate a cooperative effect of AP1 and CREB on PENK transcription.

Overexpression of hIGF-1 exclusively in skeletal muscle increases the number of dihydropyridine receptors in adult transgenic mice

The number of dihydropyridine receptors (DHPR) and sarcoplasmic reticulum (SR) Ca2+ release channels (RyR1) and their interaction determine the efficacy of the sarcolemmal excitation-SR Ca2+ release-contraction coupling (ECC). Both receptors play a central role in ECC as demonstrated in various animal species and muscle subtypes. In the present work we studied the effect of transgenic overexpression of human insulin-like growth factor 1 (hIGF-1) on the levels of these two Ca2+ channels in extensor digitorum longus (EDL) (fast-twitch), soleus (slow-twitch) and pool of fast- and slow-twitch muscles from adult C57BL/6 mice. Muscles from hIGF-1 transgenic mice showed a significant increase in IGF-1 concentration (20-30-fold) and in the number of DHPR (52% increase) whereas no significant change in RyR1 binding sites was detected. The differential effect on DHPR and RyR1 resulted in a 30% increase in DHPR/RyR1 ratio. Fast- and slow-twitch muscles showed 50 and 70% increase in the number of DHPR and 30 and 80% increase in DHPR/RyR1, respectively. These results support the concept that the increased autocrine/paracrine secretion of hIGF-1 exerts potent stimulatory effects on DHPR alpha1 subunit expression in adult skeletal muscle.

Insulin-like growth factor-I (IGF-I) regulates IGF-binding protein-5 synthesis through transcriptional activation of the gene in aortic smooth muscle cells

Previous studies have shown that porcine aortic smooth muscle cells (SMCs) secrete two insulin-like growth factor-binding proteins (IGFBP), IGFBP-2 and -4, and that these IGFBPs modulate IGF-I-stimulated SMC proliferation and migration. In this study we demonstrate that porcine SMCs express IGFBP-5 mRNA and synthesize and secrete the protein. In this cell type, the biosynthesis of IGFBP-5 is up-regulated by IGF-I. This increase in IGFBP-5 synthesis is accompanied by an increase in the steady-state mRNA levels. The induction of IGFBP-5 mRNA by IGF-I is time- and dose-dependent and requires de novo protein synthesis. IGF-II and insulin also increase IGFBP-5 mRNA levels at high doses. An IGF-I analog with normal affinity for the IGF-I receptor but reduced affinity for IGFBPs evokes a similar increase. Another analog that binds to IGFBPs but not to the receptor has no effect, indicating that this effect of IGF-I is mediated through the IGF-I receptor. The IGF-I-induced IGFBP-5 gene expression is cell type-specific because IGF-I had no such effect in other cell types examined. Nuclear run-on assays revealed that IGF-I increased transcription rate of the IGFBP-5 gene, while IGF-I did not change the IGFBP-5 mRNA stability. Furthermore, the IGFBP-5 promoter was 3.5-fold more active in directing expression of the luciferase reporter gene in IGF-I-treated aortic SMCs as compared to control cells, whereas the luciferase activity remained the same in control- and IGF-I-treated fibroblasts. These results suggest that IGF-I up-regulates IGFBP-5 synthesis by transcriptionally activating the IGFBP-5 gene in aortic SMCs.