Insulin-like growth factor-I regulates cell proliferation in the developing inner ear, activating glycosyl-phosphatidylinositol hydrolysis and Fos expression

Prospective Assessment of Otologic Adverse Events due to Teprotumumab: Preliminary Results

OBJECTIVE

To assess a series of patients receiving teprotumumab therapy and objectively quantify the rates of otologic adverse events.

STUDY DESIGN

A prospective cohort study of adult patients receiving teprotumumab between May 2020 and January 2022.

SETTING

Tertiary referral center.

METHODS

Prior to treatment initiation, an ototoxicity-specific audiometric battery was completed, which included conventional audiometry (frequencies 250-8000 Hz), ultrahigh-frequency audiometry (9000-20,000 Hz), tympanometry, speech discrimination scores, and distortion product otoacoustic emissions (DPOAEs). Testing was then repeated after treatment completion.

RESULTS

In total, 35 patients were recruited, with a median (range) age of 48.5 years (21-74), and 8 (22.8%) were male. The most common subjective symptom reported was a hearing decline (25.7%), followed by aural fullness (17.1%) and tinnitus (14.3%). Fourteen patients had both pre- and posttreatment audiometric data. Among them, 3 patients (21.4%) were found to have changes in standard frequency audiometry, and 10 (71.4%) had changes in high-frequency audiometry, with 2 patients having changes in both. Less than half (n = 5) of the 11 patients with changes in standard or high-frequency pure tone hearing noted subjective hearing decline. Changes in DPOAE were noted in 4 patients out of 13 (30.7%). Two patients discontinued treatment due to hearing decline. Finally, 3 patients (8.6%) were diagnosed with patulous eustachian tube (PET) by an otolaryngologist, and another 3 patients are suspected to have PET based on symptom description during ophthalmologic follow-up.

CONCLUSION

In our cohort, a high incidence of otologic symptoms was found to be associated with teprotumumab usage. Subjective hearing decline, changes in ultrahigh-frequency hearing as well as eustachian tube dysfunction may be encountered and suggest the potential ototoxicity of teprotumumab.

Integrative Analyses of Genes Associated With Otologic Disorders in Turner Syndrome

Background: Loss or partial loss of one X chromosome induces Turner syndrome (TS) in females, causing major medical concerns, including otologic disorders. However, the underlying genetic pathophysiology of otologic disorders in TS is mostly unclear.

Methods: Ear-related genes of TS (TSEs) were identified by analyzing differentially expressed genes (DEGs) in two Gene Expression Omnibus (GEO)-derived expression profiles and ear-genes in the Comparative Toxicogenomic Database (CTD). Subsequently, Gene Ontology (GO), Kyoto Encyclopedia of Genes and Genomes (KEGG), and Disease Ontology (DO) analyses; Gene Set Enrichment Analysis (GSEA); and Gene Set Variation Analysis (GSVA) were adopted to study biological functions. Moreover, hub genes within the TSEs were identified by assessing protein-protein interaction (PPI), gene-microRNA, and gene-transcription factor (TF) networks. Drug-Gene Interaction Database (DGIdb) analysis was performed to predict molecular drugs for TS. Furthermore, three machine-learning analysis outcomes were comprehensively compared to explore optimal biomarkers of otologic disorders in TS. Finally, immune cell infiltration was analyzed.

Results: The TSEs included 30 significantly upregulated genes and 14 significantly downregulated genes. Enrichment analyses suggested that TSEs play crucial roles in inflammatory responses, phospholipid and glycerolipid metabolism, transcriptional processes, and epigenetic processes, such as histone acetylation, and their importance for inner ear development. Subsequently, we described three hub genes in the PPI network and confirmed their involvement in Wnt/β-catenin signaling pathway and immune cell regulation and roles in maintaining normal auditory function. We also constructed gene-microRNA and gene-TF networks. A novel biomarker (SLC25A6) of the pathogenesis of otologic disorders in TS was identified by comprehensive comparisons of three machine-learning analyses with the best predictive performance. Potential therapeutic agents in TS were predicted using the DGIdb. Immune cell infiltration analysis showed that TSEs are related to immune-infiltrating cells.

Conclusion: Overall, our findings have deepened the understanding of the pathophysiology of otologic disorders in TS and made contributions to present a promising biomarker and treatment targets for in-depth research.

Ceramide Kinase Inhibition Blocks IGF-1-Mediated Survival of Otic Neurosensory Progenitors by Impairing AKT Phosphorylation

Sphingolipids are bioactive lipid components of cell membranes with important signal transduction functions in health and disease. Ceramide is the central building block for sphingolipid biosynthesis and is processed to form structurally and functionally distinct sphingolipids. Ceramide can be phosphorylated by ceramide kinase (CERK) to generate ceramide-1-phosphate, a cytoprotective signaling molecule that has been widely studied in multiple tissues and organs, including the developing otocyst. However, little is known about ceramide kinase regulation during inner ear development. Using chicken otocysts, we show that genes for CERK and other enzymes of ceramide metabolism are expressed during the early stages of inner ear development and that CERK is developmentally regulated at the otic vesicle stage. To explore its role in inner ear morphogenesis, we blocked CERK activity in organotypic cultures of otic vesicles with a specific inhibitor. Inhibition of CERK activity impaired proliferation and promoted apoptosis of epithelial otic progenitors. CERK inhibition also compromised neurogenesis of the acoustic-vestibular ganglion. Insulin-like growth factor-1 (IGF-1) is a key factor for proliferation, survival and differentiation in the chicken otocyst. CERK inhibition decreased IGF-1-induced AKT phosphorylation and blocked IGF-1-induced cell survival. Overall, our data suggest that CERK is activated as a central element in the network of anti-apoptotic pro-survival pathways elicited by IGF-1 during early inner ear development.

The Biomedical Uses of Inositols: A Nutraceutical Approach to Metabolic Dysfunction in Aging and Neurodegenerative Diseases

Inositols are sugar-like compounds that are widely distributed in nature and are a part of membrane molecules, participating as second messengers in several cell-signaling processes. Isolation and characterization of inositol phosphoglycans containing myo- or d-chiro-inositol have been milestones for understanding the physiological regulation of insulin signaling. Other functions of inositols have been derived from the existence of multiple stereoisomers, which may confer antioxidant properties. In the brain, fluctuation of inositols in extracellular and intracellular compartments regulates neuronal and glial activity. Myo-inositol imbalance is observed in psychiatric diseases and its use shows efficacy for treatment of depression, anxiety, and compulsive disorders. Epi- and scyllo-inositol isomers are capable of stabilizing non-toxic forms of β-amyloid proteins, which are characteristic of Alzheimer's disease and cognitive dementia in Down's syndrome, both associated with brain insulin resistance. However, uncertainties of the intrinsic mechanisms of inositols regarding their biology are still unsolved. This work presents a critical review of inositol actions on insulin signaling, oxidative stress, and endothelial dysfunction, and its potential for either preventing or delaying cognitive impairment in aging and neurodegenerative diseases. The biomedical uses of inositols may represent a paradigm in the industrial approach perspective, which has generated growing interest for two decades, accompanied by clinical trials for Alzheimer's disease.

Microenvironment Can Induce Development of Auditory Progenitor Cells from Human Gingival Mesenchymal Stem Cells

Sensorineural hearing loss in mammals occurs due to irreversible damage to the sensory epithelia of the inner ear and has very limited treatment options. The ability to regenerate the auditory progenitor cells is a promising approach for the treatment of sensorineural hearing loss; therefore, finding an appropriate and easily accessible stem cell source for restoring the sense of hearing would be of great interest. Here, we proposed a novel easy-to-access source of cells with the ability to recover auditory progenitor cells. In this study, gingival mesenchymal stem cells (GMSCs) were utilized, as these cells have high self-renewal and multipotent differentiation capacity and can be obtained easily from the oral cavity or discarded tissue samples at dental clinics. To manipulate the biophysical properties of the cellular microenvironment for promoting GMSC differentiation toward the target cells, we also tried to propose a candidate biomaterial. GMSCs in combination with an appropriate scaffold material can, therefore, present advantageous therapeutic options for a number of conditions. Here, we report the potential of GMSCs to differentiate into auditory progenitor cells while supporting them with an optimized three-dimensional scaffold and certain growth factors. A hybrid hydrogel scaffold based on peptide modified alginate and Matrigel was used here in addition to the presence of fibroblast growth factor-basic (bFGF), insulin-like growth factor (IGF), and epidermal growth factor (EGF). Our in vitro and in vivo studies confirmed the auditory differentiation potential of GMSCs within the engineered microenvironment.

TGFβ2-induced senescence during early inner ear development

Embryonic development requires the coordinated regulation of apoptosis, survival, autophagy, proliferation and differentiation programs. Senescence has recently joined the cellular processes required to master development, in addition to its well-described roles in cancer and ageing. Here, we show that senescent cells are present in a highly regulated temporal pattern in the developing vertebrate inner ear, first, surrounding the otic pore and, later, in the otocyst at the endolymphatic duct. Cellular senescence is associated with areas of increased apoptosis and reduced proliferation consistent with the induction of the process when the endolymphatic duct is being formed. Modulation of senescence disrupts otic vesicle morphology. Transforming growth factor beta (TGFβ) signaling interacts with signaling pathways elicited by insulin-like growth factor type 1 (IGF-1) to jointly coordinate cellular dynamics required for morphogenesis and differentiation. Taken together, these results show that senescence is a natural occurring process essential for early inner ear development.

Complementary and distinct roles of autophagy, apoptosis and senescence during early inner ear development

The development of the inner ear complex cytoarchitecture and functional geometry requires the exquisite coordination of a variety of cellular processes in a temporal manner. At early stages of inner ear development several rounds of cell proliferation in the otocyst promote the growth of the structure. The apoptotic program is initiated in exceeding cells to adjust cell type numbers. Apoptotic cells are cleared by phagocytic cells that recognize the phosphatidylserine residues exposed in the cell membrane thanks to the energy supplied by autophagy. Specific molecular programs determine hair and supporting cell fate, these populations are responsible for the functions of the adult sensory organ: detection of sound, position and acceleration. The neurons that transmit auditory and balance information to the brain are also born at the otocyst by neurogenesis facilitated by autophagy. Cellular senescence participates in tissue repair, cancer and aging, situations in which cells enter a permanent cell cycle arrest and acquire a highly secretory phenotype that modulates their microenvironment. More recently, senescence has also been proposed to take place during vertebrate development in a limited number of transitory structures and organs; among the later, the endolymphatic duct in the inner ear. Here, we review these cellular processes during the early development of the inner ear, focusing on how the most recently described cellular senescence participates and cooperates with proliferation, apoptosis and autophagy to achieve otic morphogenesis and differentiation.

Clinical update on sensorineural hearing loss in Turner syndrome and the X-chromosome

Hearing loss is one of the major medical concerns in girls and women with Turner syndrome (TS) and has a negative effect on well-being and quality of everyday life. Sensorineural hearing loss is the most common type of hearing loss, affecting more than half of adults with TS. Karyotypes with a loss of the short p-arm on the X-chromosome are more prone to ear and hearing problems. The importance of detecting, investigating, and treating hearing loss with hearing aids cannot be emphasized enough. The pathophysiology of the sensorineural hearing loss in TS is not known, but theories regarding estrogen deficiency, the cell cycle delay hypothesis, IGF-1 deficiency and the possible role of the KDM6A gene are discussed. Due to the diversity of symptoms and conditions within the same karyotype, a combination of genetic factors altered by epigenetic and/or hormonal effects is probable. Further research is needed regarding the pathophysiology of ear and hearing problems in TS to develop new treatment methods.

Directed Differentiation of Human Embryonic Stem Cells Toward Placode-Derived Spiral Ganglion-Like Sensory Neurons

The ability to generate spiral ganglion neurons (SGNs) from stem cells is a necessary prerequisite for development of cell-replacement therapies for sensorineural hearing loss. We present a protocol that directs human embryonic stem cells (hESCs) toward a purified population of otic neuronal progenitors (ONPs) and SGN-like cells. Between 82% and 95% of these cells express SGN molecular markers, they preferentially extend neurites to the cochlear nucleus rather than nonauditory nuclei, and they generate action potentials. The protocol follows an in vitro stepwise recapitulation of developmental events inherent to normal differentiation of hESCs into SGNs, resulting in efficient sequential generation of nonneuronal ectoderm, preplacodal ectoderm, early prosensory ONPs, late ONPs, and cells with cellular and molecular characteristics of human SGNs. We thus describe the sequential signaling pathways that generate the early and later lineage species in the human SGN lineage, thereby better describing key developmental processes. The results indicate that our protocol generates cells that closely replicate the phenotypic characteristics of human SGNs, advancing the process of guiding hESCs to states serving inner-ear cell-replacement therapies and possible next-generation hybrid auditory prostheses. © Stem Cells Translational Medicine 2017;6:923-936.

Application of insulin-like growth factor-1 in the treatment of inner ear disorders

Sensorineural hearing loss (SNHL) is considered an intractable disease, given that hair and supporting cells (HCs and SCs) of the postnatal mammalian cochlea are unable to regenerate. However, with progress in regenerative medicine in the 21st century, several innovative approaches for achieving regeneration of inner ear HCs and SCs have become available. These methods include stem cell transplantation, overexpression of specific genes, and treatment with growth factors. Insulin-like growth factor-1 (IGF-1) is one of the growth factors that are involved in the development of the inner ear. Treatment with IGF-1 maintains HC numbers in the postnatal mammalian cochlea after various types of HC injuries, with activation of two major pathways downstream of IGF-1 signaling. In the aminoglycoside-treated neonatal mouse cochlear explant culture, promotion of the cell-cycle in SCs as well as inhibition of HC apoptosis was observed in the IGF-1-treated group. Activation of downstream molecules was observed in SCs and, in turn, SCs contribute to the maintenance of HC numbers. Using comprehensive analysis of the gene expression, the candidate effector molecules of the IGF-1 signaling pathway in the protection of HCs were identified as Netrin1 and Gap43. Based on these studies, a clinical trial has sought to investigate the effects of IGF-1 on SNHL. Sudden SNHL (SSHL) that was refractory to systemic steroids was treated with IGF-1 in a gelatin hydrogel and the outcome was compared with a historical control of hyperbaric oxygen therapy. The proportion of patients showing hearing improvement was significantly higher in the IGF-1-treatment group at 24 weeks after treatment than in the control group. A randomized clinical trial is ongoing to compare the effect of IGF-1 treatment with that of intra-tympanic steroids for SSHL that is refractory to systemic steroids.

Early otic development depends on autophagy for apoptotic cell clearance and neural differentiation

Autophagy is a highly regulated program of self-degradation of the cytosolic constituents that has key roles during early development and in adult cell growth and homeostasis. To investigate the role of autophagy in otic neurogenesis, we studied the expression of autophagy genes in early stages of chicken (Gallus gallus) inner ear development and the consequences of inhibiting the autophagic pathway in organotypic cultures of explanted chicken otic vesicles (OVs). Here we show the expression of autophagy-related genes (Atg) Beclin-1 (Atg6), Atg5 and LC3B (Atg8) in the otocyst and the presence of autophagic vesicles by using transmission electron microscopy in the otic neurogenic zone. The inhibition of the transcription of LC3B by using antisense morpholinos and of class III phosphatidylinositol 3-kinase with 3-methyladenine causes an aberrant morphology of the OV with accumulation of apoptotic cells. Moreover, inhibition of autophagy provokes the misregulation of the cell cycle in the otic epithelium, impaired neurogenesis and poor axonal outgrowth. Finally, our results indicate that autophagy provides the energy required for the clearing of neuroepithelial dying cells and suggest that it is required for the migration of otic neuronal precursors. Taken together, our results show for the first time that autophagy is an active and essential process during early inner ear development.

The differentiation of mesenchymal stem cells into inner ear hair cell-like cells in vitro

CONCLUSION

Bone marrow mesenchymal stem cells (MSCs) have the ability to differentiate into hair cells, and this method of culturing MSCs provides a useful tool for studies on mammalian cochlear hair cell regeneration.

OBJECTIVE

To investigate a method to induce bone marrow MSCs to differentiate into inner ear hair cells.

METHODS

Rat bone marrow MSCs were isolated from healthy rats and cultured in vitro. To make sure that the cultured cells were bone marrow MSCs, the expression of MSC markers such as SH2, CD31, CD34, and CD44 genes on the cultured cells was assessed by RT-PCR. Adipogenic cells and osteogenic cells were induced by the differentiation of the cultured cells, respectively, suggesting that the cultured cells have the characteristic of pluripotent differentiation. Then they were induced to differentiate into neural stem cells and hair cell progenitor cells. Immunohistochemistry experiments were carried out to detect the expression of molecular markers. Scanning electron microscope samples were prepared for observation of the morphology of the cells.

RESULTS

Rat bone marrow MSCs were successfully isolated, purified, cultured, and identified in vitro. They were also successfully induced to differentiate into neural progenitor cells and then hair cell-like cells that expressed myosin VIIa.

The role of insulin-like growth factor-I in the physiopathology of hearing

Insulin-like growth factor-I (IGF-I) belongs to the family of polypeptides of insulin, which play a central role in embryonic development and adult nervous system homeostasis by endocrine, autocrine, and paracrine mechanisms. IGF-I is fundamental for the regulation of cochlear development, growth, and differentiation, and its mutations are associated with hearing loss in mice and men. Low levels of IGF-I have been shown to correlate with different human syndromes showing hearing loss and with presbyacusis. Animal models are fundamental to understand the genetic, epigenetic, and environmental factors that contribute to human hearing loss. In the mouse, IGF-I serum levels decrease with aging and there is a concomitant hearing loss and retinal degeneration. In the Igf1(-/-) null mouse, hearing loss is due to neuronal loss, poor innervation of the sensory hair cells, and age-related stria vascularis alterations. In the inner ear, IGF-I actions are mediated by intracellular signaling networks, RAF, AKT, and p38 MAPK protein kinases modulate the expression and activity of transcription factors, as AP1, MEF2, FoxM1, and FoxP3, leading to the regulation of cell cycle and metabolism. Therapy with rhIGF-I has been approved in humans for the treatment of poor linear growth and certain neurodegenerative diseases. This review will discuss these findings and their implications in new IGF-I-based treatments for the protection or repair of hearing loss.

RAF kinase activity regulates neuroepithelial cell proliferation and neuronal progenitor cell differentiation during early inner ear development

Background

Early inner ear development requires the strict regulation of cell proliferation, survival, migration and differentiation, coordinated by the concerted action of extrinsic and intrinsic factors. Deregulation of these processes is associated with embryonic malformations and deafness. We have shown that insulin-like growth factor I (IGF-I) plays a key role in embryonic and postnatal otic development by triggering the activation of intracellular lipid and protein kinases. RAF kinases are serine/threonine kinases that regulate the highly conserved RAS-RAF-MEK-ERK signaling cascade involved in transducing the signals from extracellular growth factors to the nucleus. However, the regulation of RAF kinase activity by growth factors during development is complex and still not fully understood.

Methodology/Principal Findings

By using a combination of qRT-PCR, Western blotting, immunohistochemistry and in situ hybridization, we show that C-RAF and B-RAF are expressed during the early development of the chicken inner ear in specific spatiotemporal patterns. Moreover, later in development B-RAF expression is associated to hair cells in the sensory patches. Experiments in ex vivo cultures of otic vesicle explants demonstrate that the influence of IGF-I on proliferation but not survival depends on RAF kinase activating the MEK-ERK phosphorylation cascade. With the specific RAF inhibitor Sorafenib, we show that blocking RAF activity in organotypic cultures increases apoptosis and diminishes the rate of cell proliferation in the otic epithelia, as well as severely impairing neurogenesis of the acoustic-vestibular ganglion (AVG) and neuron maturation.

Conclusions/Significance

We conclude that RAF kinase activity is essential to establish the balance between cell proliferation and death in neuroepithelial otic precursors, and for otic neuron differentiation and axonal growth at the AVG.

Id gene regulation and function in the prosensory domains of the chicken inner ear: a link between Bmp signaling and Atoh1

Bone morphogenetic proteins (Bmps) regulate the expression of the proneural gene Atoh1 and the generation of hair cells in the developing inner ear. The present work explored the role of Inhibitor of Differentiation genes (Id1-3) in this process. The results show that Id genes are expressed in the prosensory domains of the otic vesicle, along with Bmp4 and Bmp7. Those domains exhibit high levels of the phosphorylated form of Bmp-responding R-Smads (P-Smad1,5,8), and of Bmp-dependent Smad transcriptional activity as shown by the BRE-tk-EGFP reporter. Increased Bmp signaling induces the expression of Id1-3 along with the inhibition of Atoh1. Conversely, the Bmp antagonist Noggin or the Bmp-receptor inhibitor Dorsomorphin elicit opposite effects, indicating that Bmp signaling is necessary for Id expression and Atoh1 regulation in the otocyst. The forced expression of Id3 is sufficient to reduce Atoh1 expression and to prevent the expression of hair cell differentiation markers. Together, these results suggest that Ids are part of the machinery that mediates the regulation of hair cell differentiation exerted by Bmps. In agreement with that, during hair cell differentiation Bmp4 expression, P-Smad1,5,8 levels and Id expression are downregulated from hair cells. However, Ids are also downregulated from the supporting cells which contrarily to hair cells exhibit high levels of Bmp4 expression, P-Smad1,5,8, and BRE-tk-EGFP activity, suggesting that in these cells Ids escape from Bmp/Smad signaling. The differential regulation of Ids in time and space may underlie the multiple functions of Bmp signaling during sensory organ development.

Bone marrow mesenchymal stem cells are progenitors in vitro for inner ear hair cells

Stem cells have been demonstrated in the inner ear but they do not spontaneously divide to replace damaged sensory cells. Mesenchymal stem cells (MSC) from bone marrow have been reported to differentiate into multiple lineages including neurons, and we therefore asked whether MSCs could generate sensory cells. Overexpression of the prosensory transcription factor, Math1, in sensory epithelial precursor cells induced expression of myosin VIIa, espin, Brn3c, p27Kip, and jagged2, indicating differentiation to inner ear sensory cells. Some of the cells displayed F-actin positive protrusions in the morphology characteristic of hair cell stereociliary bundles. Hair cell markers were also induced by culture of mouse MSC-derived cells in contact with embryonic chick inner ear cells, and this induction was not due to a cell fusion event, because the chick hair cells could be identified with a chick-specific antibody and chick and mouse antigens were never found in the same cell.

A mesenchyme-free culture system to elucidate the mechanism of otic vesicle morphogenesis

The vertebrate inner ear has been extensively studied as a model system of morphogenesis and differentiation. The interactions between epithelium and surrounding mesenchyme have not previously been studied directly, because an appropriate experimental system had not been established. Here we describe a mesenchyme-free culture system of E11.5 mouse otic vesicle which retains the ability for (1) formation of the cochlear loop, (2) emigration of ganglion cells from the epithelium and (3) invagination of semicircular canal epithelium. E10.5 otic vesicle was maintained using the same method, but morphogenesis was less successful. Culture of the E11.5 cochlear region alone resulted in regeneration of a structure with semicircular canal character from the cut end, indicating that region-specific cell fate within the otic vesicle is not irreversibly determined at this stage. Co-culturing otic vesicle with cochleovestibular ganglion (CVG) resulted in enhanced looping or ectopic diverticulum formation of the cochlear region, suggesting that the CVG provides a morphogenetic signal for cochlear looping. Cochlear looping was specifically blocked by inhibiting actin polymerization by cytochalasin D, while morphogenesis of the semicircular canal region remained intact. Hyaluronidase treatment inhibited semicircular canal morphogenesis, resulting in a cystic form of the otic vesicle. These data validate this culture system as a tool for elucidating the mechanism of morphogenesis of the otic vesicle.

Identification and analysis of genes from the mouse otic vesicle and their association with developmental subprocesses through in situ hybridization

The otic vesicle (otocyst) occupies a pivotal position in inner ear development, bridging the gap between otic placode determination, and morphogenesis of vestibular and auditory compartments. The molecular mechanisms underlying the progressive subdivision of the developing inner ear into different compartments, and the molecular control and execution of the different developmental processes involved, are largely unknown. Since relatively few genes have been implicated in these processes, we have undertaken this study to identify genes involved in these early embryonic stages. We have used cDNA subtractions of mouse otic vesicle against adult liver cDNA, and describe a set of 280 candidate genes. We have also performed otic vesicle RNA hybridizations against DNA chips to not only confirm the efficacy of the library approach, but also to investigate the utility of DNA array alternatives. To begin to dissect potential developmental roles, we investigated the spatial pattern of gene expression for a selected set of 80 genes in developing mouse embryos at mid-gestation by whole-mount in situ hybridization. These data illustrate the compartmentalisation of gene expression in the otic vesicle for the majority of genes tested, and furthermore, implicate many of the genes tested with distinct developmental subprocesses.

Involvement of neuron-derived orphan receptor-1 (NOR-1) in LDL-induced mitogenic stimulus in vascular smooth muscle cells: role of CREB

OBJECTIVE

Low density lipoproteins (LDLs) modulate the expression of key genes involved in atherogenesis. Recently, we have shown that the transcription factor neuron-derived orphan receptor-1 (NOR-1) is involved in vascular smooth muscle cell (VSMC) proliferation. Our aim was to analyze whether NOR-1 is involved in LDL-induced mitogenic effects in VSMC.

METHODS AND RESULTS

LDL induced NOR-1 expression in a time- and dose-dependent manner. Antisense oligonucleotides against NOR-1 inhibit DNA synthesis induced by LDL in VSMCs as efficiently as antisense against the protooncogene c-fos. The upregulation of NOR-1 mRNA levels by LDL involves pertusis-sensitive G protein-coupled receptors, Ca2+ mobilization, protein kinases A (PKA) and C (PKC) activation, and mitogen-activated protein kinase pathways (MAPK) (p44/p42 and p38). LDL promotes cAMP response element binding protein (CREB) activation (phosphorylation in Ser133). In transfection assays a dominant-negative of CREB inhibits NOR-1 promoter activity, while mutation of specific (cAMP response element) CRE sites in the NOR-1 promoter abolishes LDL-induced NOR-1 promoter activity.

CONCLUSIONS

In VSMCs, LDL-induced mitogenesis involves NOR-1 upregulation through a CREB-dependent mechanism. CREB could play a role in the modulation by LDL of key genes (containing CRE sites) involved in atherogenesis.

Cell death in the nervous system: lessons from insulin and insulin-like growth factors

Programmed cell death is an essential process for proper neural development. Cell death, with its similar regulatory and executory mechanisms, also contributes to the origin or progression of many or even all neurodegenerative diseases. An understanding of the mechanisms that regulate cell death during neural development may provide new targets and tools to prevent neurodegeneration. Many studies that have focused mainly on insulin-like growth factor-I (IGF-I), have shown that insulin-related growth factors are widely expressed in the developing and adult nervous system, and positively modulate a number of processes during neural development, as well as in adult neuronal and glial physiology. These factors also show neuroprotective effects following neural damage. Although some specific actions have been demonstrated to be anti-apoptotic, we propose that a broad neuroprotective role is the foundation for many of the observed functions of the insulin-related growth factors, whose therapeutical potential for nervous system disorders may be greater than currently accepted.

Generation of hair cells by stepwise differentiation of embryonic stem cells

The increase in life expectancy is accompanied by the growing burden of chronic diseases. Hearing loss is perhaps the most prevalent of all chronic diseases. In addition to age-related hearing loss, a substantial number of cases of audiological impairment are either congenital in nature or acquired during childhood. The permanence of hearing loss is mainly due to the inability of the cochlear sensory epithelium to replace lost mechanoreceptor cells, or hair cells. Generation of hair cells from a renewable source of progenitors that can be transplanted into damaged inner ears is a principal requirement for potential cell replacement therapy in this organ. Here, we present an experimental protocol that enables us to routinely create inner ear progenitors from murine embryonic stem cells in vitro. These progenitors express a comprehensive set of marker genes that define the developing inner ear, in particular the organ's developing sensory patches. We further demonstrate that cells that express markers characteristic of hair cells differentiate from embryonic stem cell-derived progenitors. Finally, we show that these progenitors integrate into the developing inner ear at sites of epithelial injury and that integrated cells start expressing hair cell markers and display hair bundles when situated in cochlear or vestibular sensory epithelia in vivo.

Insulin-like growth factor 1 is required for survival of transit-amplifying neuroblasts and differentiation of otic neurons

Neurons that connect mechanosensory hair cell receptors to the central nervous system derive from the otic vesicle from where otic neuroblasts delaminate and form the cochleovestibular ganglion (CVG). Local signals interact to promote this process, which is autonomous and intrinsic to the otic vesicle. We have studied the expression and activity of insulin-like growth factor-1 (IGF-1) during the formation of the chick CVG, focusing attention on its role in neurogenesis. IGF-1 and its receptor (IGFR) were detected at the mRNA and protein levels in the otic epithelium and the CVG. The function of IGF-1 was explored in explants of otic vesicle by assessing the formation of the CVG in the presence of anti-IGF-1 antibodies or the receptor competitive antagonist JB1. Interference with IGF-1 activity inhibited CVG formation in growth factor-free media, revealing that endogenous IGF-1 activity is essential for ganglion generation. Analysis of cell proliferation cell death, and expression of the early neuronal antigens Tuj-1, Islet-1/2, and G4 indicated that IGF-1 was required for survival, proliferation, and differentiation of an actively expanding population of otic neuroblasts. IGF-1 blockade, however, did not affect NeuroD within the otic epithelium. Experiments carried out on isolated CVG showed that exogenous IGF-1 induced cell proliferation, neurite outgrowth, and G4 expression. These effects of IGF-1 were blocked by JB1. These findings suggest that IGF-1 is essential for neurogenesis by allowing the expansion of a transit-amplifying neuroblast population and its differentiation into postmitotic neurons. IGF-1 is one of the signals underlying autonomous development of the otic vesicle.

Delayed inner ear maturation and neuronal loss in postnatal Igf-1-deficient mice

Insulin-like growth factor-1 (IGF-1) has been shown to play a key role during embryonic and postnatal development of the CNS, but its effect on a sensory organ has not been studied in vivo. Therefore, we examined cochlear growth, differentiation, and maturation in Igf-1 gene knock-out mice at postnatal days 5 (P5), P8, and P20 by using stereological methods and immunohistochemistry. Mutant mice showed reduction in size of the cochlea and cochlear ganglion. An immature tectorial membrane and a significant decrease in the number and size of auditory neurons were also evident at P20. IGF-1-deficient cochlear neurons showed increased caspase-3-mediated apoptosis, along with aberrant expression of the early neural markers nestin and Islet 1/2. Cochlear ganglion and fibers innervating the sensory cells of the organ of Corti presented decreased levels of neurofilament and myelin P(0) in P20 mouse mutants. In addition, an abnormal synaptophysin expression in the somata of cochlear ganglion neurons and sensory hair cells suggested the persistence of an immature pattern of synapses distribution in the organ of Corti of these animals. These results demonstrate that lack of IGF-1 in mice severely affects postnatal survival, differentiation, and maturation of the cochlear ganglion cells and causes abnormal innervation of the sensory cells in the organ of Corti.

Pattern of expression of the jun family of transcription factors during the early development of the inner ear: implications in apoptosis

Jun transcription factors have been implicated in the regulation of cell proliferation, differentiation and apoptosis. We have investigated the relationship between Jun expression and cell death in the developing chicken inner ear. c-jun and junD transcripts were expressed in the epithelium of the otic placode and otic vesicle. c-jun expression was restricted to the dorsal area of the otic pit (stages 14–17), dorsal otic vesicle and cochleo-vestibular ganglion (stages 18–20). junD expression was transient and occurred in the dorsal and upper medial aspects of the otic pit and otic cup, but it was down-regulated in the otic vesicle. A parallel TUNEL analysis revealed that expression of c-jun co-located within areas of intense apoptosis. Furthermore, phosphorylation of c-Jun at serine-63 by Jun amino-terminal-kinases was detected in the dorsal otic pit, otic vesicle and cochleo-vestibular ganglion. c-Jun protein exhibited DNA binding activity, as assessed by gel mobility shift assays. The association between c-Jun and apoptosis was further demonstrated by studying nerve growth factor-induced apoptosis in cultured otic vesicles. Nerve growth factor-induced cell death and c-Jun phosphorylation that were suppressed by insulin-like growth factor-I and by viral-mediated overexpression of Raf, which had survival effects. In conclusion, the precise regulation of the expression and activity of Jun proteins in the otic primordium suggests that it may operate as a fundamental mechanism during organogenesis.

[The action of insulin in the morphogenesis of Gallus gallus domesticus embryos]

Aspects concerned with morphogenesis of Gallus gallus domesticus, avail studies related to the action of the insulin in the topography and embryonic structures. At the temperature of 37.5 degrees C, eggs were incubated during 24 h, injected with 5 ml of swine insulin in three concentrations and reincubated for more 72 h. The morphological characteristics of 80 embryos were evaluated and, according to the presented organization, classified in 5 morphogenetic levels. It was registered generalized dysmorphism (4th level) in 21 embryos that went through the tests with insulin. Standard morphogenesis (1st level) and located dysmorphism (3rd level) were verified among those from the control experiments. Those individuals concerned with the 4th level, showed reduced dimension of the body and were characterized by anterior-dorsal limits organized in a cephalic projection, and also presented alterations in the posterior-ventral region. These features evidence a pattern of abnormality in the determination of the cephalic-caudal axis and indicate a specific action of the insulin in the embryonic morphogenesis, in the period of 96 hours of incubation.

Strict regulation of c-Raf kinase levels is required for early organogenesis of the vertebrate inner ear

Regulation of organogenesis involves a dynamic balance of the mechanisms regulating cell division, differentiation and death. Here we have investigated the pattern of expression of c-Raf kinase in the inner ear during early developmental stages and the consequences of manipulating c-Raf levels by misexpression of c-raf viral vectors in organotypic cultures of otic vesicle explants. We found that otic vesicles expressed c-Raf and its level remained constant during embryonic days 2 and 3 (E2-E3). c-Raf activity was increased in response to insulin like growth factor-I (IGF-I) and the activation by IGF-I of the c-Raf kinase pathway was a requirement to turn on cell proliferation in the otic vesicle. Overexpression of c-raf in E2.5 explants increased the proliferative response to low serum and IGF-I and blocked differentiation induced by retinoic acid. The increase in c-Raf levels also prevented nerve growth factor (NGF)-dependent induction of programmed cell death. Consistent with these results, the expression of a dominant negative c-Raf mutant potentiated retinoic acid action and decreased the rate of cell proliferation. We conclude that a strict control of c-Raf levels is essential for the co-ordination of the biological processes that operate simultaneously during early inner ear development.

Induction of cell growth by insulin and insulin-like growth factor-I is associated with Jun expression in the otic vesicle

The present report investigates the cellular mechanisms involved in the regulation of cell proliferation by insulin and insulin-like growth factor-I (IGF-I) in the developing inner ear. The results show that insulin and IGF-I stimulate cell proliferation in the otic vesicle. This effect is associated with the induction of the expression of the nuclear proto-oncogene c-jun. The temporal profile of Jun expression coincided with the proliferative period of growth of the otic vesicle. IGF-I promoted the hydrolysis of a membrane glycosyl-phosphatidylinositol, which was characterised as the endogenous precursor for inositol phosphoglycan (IPG). Both purified IPG and a synthetic analogue, 6-O-(2-amino-2-deoxy-alpha-D-glucopyranosyl)-D-myoinositol-1,2-cyclic phosphate (C3), were able to mimic the effects of IGF-I on Jun expression. Anti-IPG antibodies blocked the effects of IGF-I, which were rescued by the addition of IPG or its analogue. These results suggest that the sequence involving the hydrolysis of membrane glycolipids and the expression of c-jun and c-fos proto-oncogenes is part of the mechanism that activates cell division in response to insulin and IGF-I during early organogenesis of the avian inner ear. The implications of these observations for otic development and regeneration are briefly discussed.

Transforming growth factor-alpha with insulin induces proliferation in rat utricular extrasensory epithelia

Hair cell loss in the human inner ear leads to sensorineural hearing loss and vestibular dysfunction. Recent studies suggest that exogenous addition of growth factors, for example, transforming growth factor-alpha with insulin, may stimulate the production of new supporting cells and hair cells in the mature mammalian vestibular sensory epithelium. Before any growth factor can be seriously considered for the treatment of clinical problems related to hair cell loss, its effects on the extrasensory epithelia must also be fully explored. The aim of this study was to determine whether transforming growth factor-alpha and insulin stimulate cell proliferation in rodent vestibular extrasensory epithelia. The cell proliferation marker, tritiated thymidine, was infused along with transforming growth factor-alpha, insulin, or transforming growth factor-alpha plus insulin into the inner ears of adult rats via osmotic pumps. Effects of the test agents were assessed on normal and drug-damaged utricles. Drug damage was produced by delivering gentamicin directly into the inner ear before the infusion of test agent. Animals were killed 4 or 10 days after pump placement. Utricles were sectioned, processed for autoradiography, and examined for labeled cells within the extrasensory epithelia. In normal animals, transforming growth factor-alpha plus insulin stimulated DNA synthesis in all regions of the extrasensory epithelia, suggesting that these agents are mitogenic for these tissues.

Insulin-like growth factor-I modulation of cerebellar cell populations is developmentally stage-dependent and mediated by specific intracellular pathways

Although development of transgenic animals overexpressing insulin-like growth factor-I has allowed the establishment of a role of this trophic factor in brain growth, detailed knowledge of the action of insulin-like growth factor-I on different brain areas is still lacking. We now provide evidence for a pleiotrophic role of this growth factor on cerebellar development. Insulin-like growth factor-I produced by cerebellar cultures is a survival factor for Purkinje cells and a mitogen/differentiation factor for cerebellar glioblasts. Trophic effects of insulin-like growth factor-I were observed only during specific developmental stages. In addition, insulin-like growth factor-I increased intracellular Ca2+ levels in Purkinje cells and c-Fos in dividing glioblasts. Survival-promoting effects of insulin-like growth factor-I on Purkinje cells required activation of protein kinase C, while glioblast division induced by insulin-like growth factor-I depended on phosphatidylinosytol 3-kinase activation. We conclude that insulin-like growth factor-I is a paracrine/autocrine pleiotrophic factor for both glia and neurons in the cerebellum. Its effects are mediated by distinct intracellular signals and appear to be specific to the developmental stage of the target cell. Since development of the different cell populations that compose a specific brain territory is not synchronized, the pleiotrophic action of growth factors such as insulin-like growth factor-I may be essential to ontogenetic processes underlying normal brain growth.

Nerve growth factor and ceramides modulate cell death in the early developing inner ear

Regulation of normal development involves a dynamic balance of the mechanisms regulating cell division, differentiation and death. We have investigated the signalling mechanisms involved in regulation of the balance between cell proliferation and apoptotic cell death in the otic vesicle. The sphingomyelin pathway signals apoptosis for nerve growth factor upon binding to p75 receptors. It is initiated by sphingomyelin hydrolysis to generate the second messenger ceramide. In the present study, we show that nerve growth factor stimulates sphingomyelin hydrolysis and the concomitant ceramide release in organotypic cultures of otic vesicles. Both nerve growth factor and ceramide induce apoptotic responses to a different extent. Ceramide-induced apoptosis was suppressed by insulin-like growth factor-I which is a strong promoter of cell growth and morphogenesis for the developing inner ear. In contrast, ceramide-1-phosphate protected the explants from apoptosis induced by serum withdrawal but did not antagonise ceramide-induced cell death. This study suggests that sphingomyelin-derived second messengers might be key modulators of programmed cell death during development.

The development of the vertebrate inner ear

The inner ear is a complex sensory organ responsible for balance and sound detection in vertebrates. It originates from a transient embryonic structure, the otic vesicle, that contains all of the information to develop autonomously into the mature inner ear. We review here the development of the otic vesicle, bringing together classical embryological experiments and recent genetic and molecular data. The specification of the prospective ectoderm and its commitment to the otic fate are very early events and can be related to the expression of genes with restricted expression domains. A combinatorial gene expression model for placode specification and diversification, based on classical embryological evidence and gene expression patterns, is discussed. The formation of the otic vesicle is dependent on inducing signals from endoderm, mesoderm and neuroectoderm. Ear induction consists of a sequence of discrete instructions from those tissues that confer its final identity on the otic field, rather than a single all-or-none process. The important role of the neural tube in otic development is highlighted by the abnormalities observed in mouse mutants for the Hoxa1, kreisler and fgf3 genes and those reported in retinoic acid-deficient quails. Still, the nature of the relation between the neural tube and otic development remains unclear. Gene targeting experiments in the mouse have provided evidence for genes potentially involved in regional and cell-fate specification in the inner ear. The disruption of the mouse Brn3.1 gene identifies the first mutation affecting sensory hair-cell specification, and mutants for Pax2 and Nkx5.1 genes show their requirement for the development of specific regions of the otic vesicle. Several growth-factors contribute to the patterned cell proliferation of the otic vesicle. Among these, IGF-I and FGF-2 are expressed in the otic vesicle and may act in an autocrine manner. Finally, little is known about early mechanisms involved in guiding ear innervation. However, targeted disruption of genes coding for neurotrophins and Trk receptors have shown that once synaptic contacts are established, they depend on specific trophic interactions that involve these two gene families. The accessibility of new cellular and molecular approaches are opening new perspectives in vertebrate development and are also starting to be applied to ear development. This will allow this classical and attractive model system to see a rapid progress in the near future.

Calcium-dependence of hydrogen peroxide-induced c-fos expression and growth stimulation of multicellular prostate tumor spheroids

Hydrogen peroxide (H2O2) in nanomolar concentrations (20-100 nM) stimulated the growth of small (diameter 100 +/- 30 microm) multicellular prostate cancer spheroids and increased c-fos expression. H2O2 transiently raised [Ca2+]i by Ca2+ release from intracellular stores as the transient persisted in low (10 nM) Ca2+ solution but was abolished when intracellular Ca2+ stores were depleted by thapsigargin or chelation of [Ca2+]i with BAPTA. The H2O2-induced [Ca2+]i transient was furthermore inhibited by the P2-purinoreceptor antagonists suramin and basilen blue, indicating that H2O2 may act via purinergic receptor stimulation. Treatment of spheroids with either suramin, basilen blue or BAPTA inhibited the H2O2-induced growth stimulation and c-fos expression, indicating that the H2O2-mediated growth stimulation of multicellular spheroids is mediated via a Ca2+-dependent pathway.

Isolation and partial characterisation of insulin-mimetic inositol phosphoglycans from human liver

Extracts of human liver were found to contain activities which copurified and coeluted with the two major subtypes of mediators (type A and type P) isolated from insulin-stimulated rat liver. The putative type A mediator from human liver inhibited cAMP-dependent protein kinase from bovine heart, decreased phosphoenolypyruvate carboxykinase mRNA levels in rat hepatoma cells, and stimulated lipogenesis in rat adipocytes. The putative type P mediator stimulated bovine heart pyruvate dehydrogenase phosphatase. Both fractions were able to stimulate proliferation of EGFR T17 fibroblasts and the type A was able to support growth in organotypic cultures of chicken embryo cochleovestibular ganglia. Both activities were resistant to Pronase treatment and the presence of carbohydrates, phosphate, and free-amino groups were confirmed in the two fractions. These properties are consistent with the structure/ function characteristics of the type A and P inositolphosphoglycans (IPG) previously characterized from rat liver. Further, the ability of the human-derived mediators to interact with rat adipocytes and bovine-derived metabolic enzymes suggests similarity in structure between the mediators purified from different species. Galactose oxidase-susceptible membrane-associated glycosylphosphatidylinositols (GPI) have been proposed to be the precursors of IPG. GPI was purified from human liver membranes followed by treatment with galactose oxidase and reduction with NaB3H4. Serial t.l.c. revealed three radiolabeled bands which comigrated with the putative GPI precursors found in rat liver. These galactose-oxidase-reactive lipidic compounds, however, were only partially susceptible to hydrolysis with phosphatidylinositol-specific phospholipase C from Bacillus thuringiensis and were resistant to glycosylphosphatidylinositol-specific phospholipase C from Trypanosoma brucei. These data indicate that IPG molecules with insulin-like biological activities are present in human liver.

Factors modulating supernumerary hair cell production in the postnatal rat cochlea in vitro

It has been shown in the past that extra hair cells or supernumerary cells can be produced when neonatal cochleae are maintained in vitro. In this report, we investigated the effects of the culture methods, molecules and growth factors that are thought to be involved in cell proliferation. Quantitative studies of supernumerary hair cells were made by measuring the cell density over the entire spiral lamina at two postnatal stages: birth and 3 days after birth. With a standard feeding solution without serum, a difference in cell density was observed between the two methods of culture. Cochlear explants in a standard feeding solution supplemented with serum showed an increase of cell density only when the explantation is made at birth. Retinoic acid added to the standard feeding solution did not increase the hair cell density, while insulin induced an increase, especially at 5 micrograms/ml. Several growth factors were tested. Epidermal growth factor (EGF) presented a dose dependent effect with an increase of up to 30% of hair cell density that was observed in the basal region when the explantation was made at birth. Transforming growth factor-alpha did not induce an increase of cell density, whereas transforming growth factor-beta presented an effect on hair cell density, with a dose dependent effect reaching 37.4% for the basal inner hair cells. Interpretation of these results is limited because of the lack of data concerning the presence of specific membrane receptors. One possibility is that insulin stimulates hair cell differentiation from existing undifferentiated cells. Another hypothesis may be related to the EGF and transforming growth factor-beta, where these molecules might induce transdifferentiation of cells by acting on the transmembrane molecules and the extracellular matrix.

Quantitative analysis of long-term survival and neuritogenesis in vitro: cochleovestibular ganglion of the chick embryo in BDNF, NT-3, NT-4/5, and insulin

The dynamics of survival and growth were examined for cochleovestibular ganglion (CVG) cells maintained in long-term cultures. CVG cells were explanted from chick embryos after 90 h of incubation into a defined-medium containing BDNF, NT-3, or NT-4/5 and an insulin, transferrin, selenium, and progesterone supplement. Explant survival and neuritogenesis was measured for 23 to 24 days in vitro. All three neurotrophins prolonged CVG survival in a dose-dependent manner although insulin acted as a cofactor. In 0.872 microM insulin-containing medium the ED50 for BDNF and NT-3 was 100 pg/ml, whereas the ED50 for NT-4/5 was 600-1200 pg/ml. However, at later ages in vitro, survival decreased with concentrations of BDNF greater than 2 ng/ml. In insulin-free medium, concentrations of 5-200 ng/ml of BDNF or 30-200 ng/ml of NT-4/5 maintained the survival of explants at a rate that was equivalent to or less than the survival rate of cultures treated with insulin but not with neurotrophin. In contrast, NT-3-treated explants in insulin-free medium did not survive the duration of the experiment. Dose-dependent effects of BDNF and NT-3 on explant neuritogenesis were reflected as an initial delay in outgrowth, whereas NT-4/5 had no effect. Insulin regulation of neuritogenesis was suggested when outgrowth decreased in the presence of an antibody to the insulin receptor. These data suggest that while all three of these neurotrophins protect the CVG from death the long-term consequences of cofactors and certain dose levels should be considered when treating CVG cells in vivo.

Glycosyl-phosphatidylinositol-phospholipase type D: a possible candidate for the generation of second messengers

Membrane associated glycosyl-phosphatidylinositols have been shown to be the precursors of inositol phosphoglycan second messengers. Extraction of human liver membranes and purification by serial thin layer chromatography revealed three glycolipids which co-migrated with glycosyl-phosphatidylinositol from rat liver. These lipidic fractions were partially sensitive to treatment with nitrous acid and to hydrolysis by glycosyl-phosphatidylinositol-specific phospholipase D from bovine serum. In parallel, glycosyl-phosphatidylinositol isolated from rat liver was found to be a substrate for the enzyme generating a biologically active inositol phosphoglycan species (determined by measuring inhibition of protein kinase A activity and stimulation of cell proliferation within the chicken embryo cochleovestibular ganglion). This molecule was recognised by an anti-inositol phosphoglycan antibody. Hence, we propose that glycosyl-phosphatidylinositol-specific phospholipase D could be implicated in cellular signalling.

Induction of cell proliferation in avian inner ear sensory epithelia by insulin-like growth factor-I and insulin

Postembryonic production of inner-ear hair cells occurs both normally and after insult in lower vertebrates and avians. To determine how this proliferation is controlled, several growth factors were tested for effects on progenitor-cell division in cultured avian vestibular sensory epithelium. Mitogenic effects of bombesin, epidermal growth factor, insulin-like growth factor-I (IGF-I), insulin, and transforming growth factor-alpha were assayed in organo-typic cultures of utricles from the mature, undamaged (normal) chicken inner ear. Tritiated thymidine and autoradiographic techniques and 5-bromo-2'-deoxyuridine (BrdU) immunocytochemistry were used to identify cells synthesizing DNA. IGF-I stimulated DNA synthesis in the vestibular sensory receptor epithelium in a dose-dependent manner. DNA synthesis was also stimulated by insulin. These results suggest that stimulation of the IGF-I receptors by IGF-I or insulin binding stimulates cell proliferation in the mature avian vestibular sensory epithelium.

Insulin second messengers

The molecular pathways for insulin's signal transduction from its cell surface receptor to the cell's interior metabolic machinery remain in many ways uncharted. Lately two molecules have been proposed as second messengers transducing the insulin signal into the target cell. One is a phospho-oligosaccharide/inositolphosphoglycan and the other is diacylglycerol, both deriving from the same plasma membrane glycolipid, which is hydrolysed in response to insulin treatment. The phospho-oligosaccharide appears to mediate many metabolic effects of insulin through control of the phosphorylation state of key regulatory metabolic enzymes. Diacylglycerol may mediate insulin's stimulation of glucose transport over the plasma membrane. The glycolipid precursor of these putative second messengers, as well as the receptor for insulin, appear to be localized in caveolae microdomains of the plasma membrane, and glucose transporters accumulate in caveolae in response to insulin treatment, suggesting a focal role for caveolae in insulin signalling.

Induction of cell proliferation by fibroblast and insulin-like growth factors in pure rat inner ear epithelial cell cultures

Proliferation of supporting cells in the inner ear is the early major event occurring during hair cell regeneration after acoustic trauma or aminoglycoside treatment. In the present study, we examined the possible influence of 30 growth factors on the proliferation of pure rat utricular epithelial cells in culture. Utricular epithelial sheets were separated and partially dissociated from early postnatal rats via a combined enzymatic and mechanical method. The cultured utricular epithelial cells expressed exclusively epithelial cell antigens, but not fibroblast, glial, or neuronal antigens. With tritiated thymidine incorporation assays, we found that several fibroblast growth factor (FGF) family members, insulin-like growth factor-1 (IGF-1), IGF-2, transforming growth factor-alpha (TGF-alpha), and epidermal growth factor (EGF), stimulated proliferation of the utricular epithelial cells. In contrast, neurotrophins and other growth factors did not elicit any detectable mitogenic effects. Among all of the growth factors examined, FGF-2 was the most potent mitogen. When FGF-2 was added in combination with IGF-1 or TGF-alpha to the medium, combined effects were seen. These results were confirmed with BrdU immunocytochemistry. Thus, the present culture system provides a rapid and reliable assay system to screen novel growth factors involved in proliferation of mammalian inner ear supporting cells. Furthermore, immunostainings revealed that the cultured utricular epithelial cells expressed FGF and IGF-1 receptors, and utricular hair cells produced FGF-2 in vivo. The addition of neutralizing antibodies against FGF-2 or IGF-1 to the cultures significantly inhibited the utricular epithelial cell proliferation. This work suggests that FGF-2 and IGF-1 may regulate the proliferation step during hair cell development and regeneration.

Inositol-phosphoglycan inhibits calcium oscillations in hepatocytes by reducing calcium entry

Inositol-phosphoglycan (IPG) is a putative mediator of insulin action that has been shown to affect numerous biochemical processes. IPG, prepared from liver membranes, promptly inhibited phenylephrine- or vasopressin-induced [Ca2+]i oscillations when perfused over Fura-2-dextran injected rat hepatocytes. An antibody to IPG suppressed the inhibitory effect of insulin on the [Ca2+]i oscillations. Measurement of the rate of quench of cytoplasmic Fura-2 by extracellular Mn2+ showed that Ca2+ entry occurred continuously in the unstimulated cell and was not affected by phenylephrine or vasopressin. IPG, specifically, almost completely abolished the Mn2+ quench rate. Elevated extracellular [Ca2+] reversed the inhibitory effect of IPG on [Ca2+]i oscillations. We conclude that IPG inhibits the hepatocyte Ca2+ oscillatory by reducing the continuous Ca2+ influx that is required to sustain oscillations in [Ca2+]i.