Age-dependent FOXO regulation of p27Kip1 expression via a conserved binding motif in rat muscle precursor cells

The CX3CL1 intracellular domain exhibits neuroprotection via insulin receptor/ insulin like growth factor receptor signaling

CX3CL1, also known as fractalkine, is best known for its signaling activity through interactions with its cognate receptor CX3CR1. However, its intrinsic function that is independent of interaction with CX3CR1 remains to be fully understood. We demonstrate that the intracellular domain of CX3CL1 (CX3CL1-ICD), generated upon sequential cleavages by α-/β-secretase and γ-secretase, initiates a back signaling activity, which mediates direct signal transmission to gene expression in the nucleus. To study this, we fused a synthetic peptide derived from CX3CL1-ICD, named Tet34, with a 13-amino acid tetanus sequence at the N-terminus to facilitate translocation into neuronal cells. We show that treatment of mouse neuroblastoma Neuro-2A cells with Tet34, but not its scrambled control (Tet34s), induced cell proliferation, as manifested by changes in protein levels of transcription factors and pro-growth molecules Foxo-1, -3, cyclin D1, PCNA, Sox5, and cdk2. Further biochemical assays reveal elevation of phosphorylated insulin receptor β subunit, insulin-like growth factor-1 (IGF-1) receptor β subunit and insulin receptor substrates as well as activation of proliferation-linked kinase AKT. In addition, transgenic mice overexpressing membrane-anchored C-terminal CX3CL1 (CX3CL1- ct) also exhibited activation of insulin/IGF-1 receptor signaling. Remarkably, we found this Tet34 peptide, but not Tet34s, protected against endoplasmic reticulum stress and cellular apoptosis when Neuro-2A cells were challenged with toxic oligomers of β-amyloid peptide or hydrogen peroxide. Taken together, our results suggest CX3CL1-ICD may have translational potential for neuroprotection in Alzheimer's disease and for disorders resulting from insulin resistance.

miRNA-486-5p: signaling targets and role in non-malignant disease

MicroRNAs (miRNAs) are short non-coding RNAs, highly conserved between species, that are powerful regulators of gene expression. Aberrant expression of miRNAs alters biological processes and pathways linked to human disease. miR-486-5p is a muscle-enriched miRNA localized to the cytoplasm and nucleus, and is highly abundant in human plasma and enriched in small extracellular vesicles. Studies of malignant and non-malignant diseases, including kidney diseases, have found correlations with circulating miR-486-5p levels, supporting its role as a potential biomarker. Pre-clinical studies of non-malignant diseases have identified miR-486-5p targets that regulate major signaling pathways involved in cellular proliferation, migration, angiogenesis, and apoptosis. Validated miR-486-5p targets include phosphatase and tensin homolog (PTEN) and FoXO1, whose suppression activates phosphatidyl inositol-3-kinase (PI3K)/Akt signaling. Targeting of Smad1/2/4 and IGF-1 by miR-486-5p inhibits transforming growth factor (TGF)-β and insulin-like growth factor-1 (IGF-1) signaling, respectively. Other miR-486-5p targets include matrix metalloproteinase-19 (MMP-19), Sp5, histone acetyltransferase 1 (HAT1), and nuclear factor of activated T cells-5 (NFAT5). In this review, we examine the biogenesis, regulation, validated gene targets and biological effects of miR-486-5p in non-malignant diseases.

FOXO1 expression in chondrocytes modulates cartilage production and removal in fracture healing

Fracture healing is a multistage process characterized by inflammation, cartilage formation, bone deposition, and remodeling. Chondrocytes are important in producing cartilage that forms the initial anlagen for the hard callus needed to stabilize the fracture site. We examined the role of FOXO1 by selective ablation of FOXO1 in chondrocytes mediated by Col2α1 driven Cre recombinase. Experimental mice with lineage-specific FOXO1 deletion (Col2α1Cre⁺FOXO1^L/L) and negative control littermates (Col2α1Cre^-FOXO1^L/L) were used for in vivo, closed fracture studies. Unexpectedly, we found that in the early phases of fracture healing, FOXO1 deletion significantly increased the amount of cartilage formed, whereas, in later periods, FOXO1 deletion led to a greater loss of cartilage. FOXO1 was functionally important as its deletion in chondrocytes led to diminished bone formation on day 22. Mechanistically, the early effects of FOXO1 deletion were linked to increased proliferation of chondrocytes through enhanced expression of cell cycle genes that promote proliferation and reduced expression of those that inhibit it and increased expression of cartilage matrix genes. At later time points experimental mice with FOXO1 deletion had greater loss of cartilage, enhanced formation of osteoclasts, increased IL-6 and reduced numbers of M2 macrophages. These results identify FOXO1 as a transcription factor that regulates chondrocyte behavior by limiting the early expansion of cartilage and preventing rapid cartilage loss at later phases.

Coordinated regulation of cell survival and cell cycle pathways by DDR2-dependent SRF transcription factor in cardiac fibroblasts

Relative resistance to apoptosis and the ability to proliferate and produce a collagen-rich scar determine the critical role of cardiac fibroblasts in wound healing and tissue remodeling following myocardial injury. Identification of cardiac fibroblast-specific factors and mechanisms underlying these aspects of cardiac fibroblast function is therefore of considerable scientific and clinical interest. In the present study, gene knockdown and overexpression approaches and promoter binding assays showed that discoidin domain receptor 2 (DDR2), a mesenchymal cell-specific collagen receptor tyrosine kinase localized predominantly in fibroblasts in the heart, acts via ERK1/2 MAPK-activated serum response factor (SRF) transcription factor to enhance the expression of antiapoptotic cIAP2 in cardiac fibroblasts, conferring resistance against oxidative injury. Furthermore, DDR2 was found to act via ERK1/2 MAPK-activated SRF to transcriptionally upregulate Skp2 that in turn facilitated post-translational degradation of p27, the cyclin-dependent kinase inhibitor that causes cell cycle arrest, to promote G₁-S transition, as evidenced by Rb phosphorylation, increased proliferating cell nuclear antigen (PCNA) levels, and flow cytometry. DDR2-dependent ERK1/2 MAPK activation also suppressed forkhead box O 3a (FoxO3a)-mediated transcriptional induction of p27. Inhibition of the binding of collagen type I to DDR2 using WRG-28 indicated the obligate role of collagen type I in the activation of DDR2 and its regulatory role in cell survival and cell cycle protein expression. Notably, DDR2 levels positively correlated with SRF, cIAP2, and PCNA levels in cardiac fibroblasts from spontaneously hypertensive rats. To conclude, DDR2-mediated ERK1/2 MAPK activation facilitates coordinated regulation of cell survival and cell cycle progression in cardiac fibroblasts via SRF.NEW & NOTEWORTHY Relative resistance to apoptosis and the ability to proliferate and produce a collagen-rich scar enable cardiac fibroblasts to play a central role in myocardial response to injury. This study reports novel findings that mitogen-stimulated cardiac fibroblasts exploit a common regulatory mechanism involving collagen receptor (DDR2)-dependent activation of ERK1/2 MAPK and serum response factor to achieve coordinated regulation of apoptosis resistance and cell cycle progression, which could facilitate their survival and function in the injured myocardium.

Ascorbic acid diminishes bone morphogenetic protein 2-induced osteogenic differentiation of muscle precursor cells

INTRODUCTION

Muscle precursor cells (MPC) are integral to the maintenance of skeletal muscle and have recently been implicated in playing a role in bone repair. The primary objective of this study was to understand better the role of oxidative stress during the osteogenic differentiation of MPCs.

METHODS

Muscle precursor cells were treated with various combinations of ascorbic acid (AA), bone morphogenetic protein (BMP)-2, and either a superoxide dismutase analog (4-hydroxy-TEMPO [TEMPOL]) or polyethyleneglycol-conjugated catalase. Muscle precursor cell proliferation and differentiation were determined, and alkaline phosphatase activity was measured as an index of osteogenic differentiation.

RESULTS

After treatment with 200 μM AA, superoxide was increased 1.5-fold, whereas AA in combination with 100 ng/ml BMP-2 did not increase alkaline phosphatase (ALP) activity. When cells were treated with TEMPOL in combination with 100 ng/ml BMP-2 and 200 μM AA, ALP activity significantly increased.

DISCUSSION

These data suggest that increasing oxidative stress with AA induces sublethal oxidative stress that prevents BMP-2-induced osteogenic differentiation of MPCs. Muscle Nerve 59:501-508, 2019.

NSun2 delays replicative senescence by repressing p27 (KIP1) translation and elevating CDK1 translation

A rise in the levels of the cyclin-dependent kinase (CDK) inhibitor p27^KIP1 is important for the growth arrest of senescent cells, but the mechanisms responsible for this increase are poorly understood. Here, we show that the tRNA methyltransferase NSun2 represses the expression of p27 in replicative senescence. NSun2 methylated the 5′-untranslated region (UTR) of p27 mRNA at cytosine C64 in vitro and in cells, thereby repressing the translation of p27. During replicative senescence, increased p27 protein levels were accompanied by decreased NSun2 protein levels. Knockdown of NSun2 in human diploid fibroblasts (HDFs) elevated p27 levels and reduced the expression of CDK1 (encoded by CDK1 mRNA, a previously reported target of NSun2), which in turn further repressed cell proliferation and accelerated replicative senescence, while overexpression of NSun2 exerted the opposite effect. Ectopic overexpression of the p27 5′UTR fragment rescued the effect of NSun2 overexpression in lowering p27, increasing CDK1, promoting cell proliferation, and delaying replicative senescence. Our findings indicate that NSun2-mediated mRNA methylation regulates p27 and CDK1 levels during replicative senescence.

FOXO3a potentiates hTERT gene expression by activating c-MYC and extends the replicative life-span of human fibroblast

In our previous studies, we reported that SIRT1 prevents cellular senescence in human fibroblast, and that SIRT1-induced inhibition of cellular senescence is due to enhanced hTERT gene expression. In this study, we investigate the molecular mechanisms behind SIRT1-induced potentiation of hTERT transcription and show that FOXO3a functions downstream of SIRT1 and prevents the induction of cellular senescence by enhancing hTERT gene expression. Furthermore, we found that FOXO3a-induced potentiation of hTERT gene expression is regulated in a c-MYC/E-box dependent manner. In addition, we found that FOXO3a binds to the novel binding element in the c-MYC promoter, and this interaction activates the transcription of the c-MYC gene. The resulting increase in c-MYC leads to higher levels of c-MYC recruited to the hTERT promoter and, in turn, activates hTERT gene expression. Taken together, this pathway might constitute the molecular basis for the anti-senescence effects of SIRT1 and FOXO3a.

Pro-inflammatory mediation of myoblast proliferation

Skeletal muscle satellite cell function is largely dictated by the surrounding environment following injury. Immune cell infiltration dominates the extracellular space in the injured area, resulting in increased cytokine concentrations. While increased pro-inflammatory cytokine expression has been previously established in the first 3 days following injury, less is known about the time course of cytokine expression and the specific mechanisms of cytokine induced myoblast function. Therefore, the expression of IL-1β and IL-6 at several time points following injury, and their effects on myoblast proliferation, were examined. In order to do this, skeletal muscle was injured using barium chloride in mice and tissue was collected 1, 5, 10, and 28 days following injury. Mechanisms of cytokine induced proliferation were determined in cell culture using both primary and C2C12 myoblasts. It was found that there is a ∼20-fold increase in IL-1β (p≤0.05) and IL-6 (p = 0.06) expression 5 days following injury. IL-1β increased proliferation of both primary and C2C12 cells ∼25%. IL-1β stimulation also resulted in increased NF-κB activity, likely contributing to the increased proliferation. These data demonstrate for the first time that IL-1β alone can increase the mitogenic activity of primary skeletal muscle satellite cells and offer insight into the mechanisms dictating satellite cell function following injury.

Mechanisms in cardiac fibroblast growth: an obligate role for Skp2 and FOXO3a in ERK1/2 MAPK-dependent regulation of p27kip1

Cardiac fibroblast hyperplasia associated with enhanced matrix deposition is a major determinant of tissue remodeling in several disease states of the heart. However, mechanisms controlling cell cycle progression in cardiac fibroblasts remain unexplored. Identification of cell cycle regulatory elements in these cells is important to develop strategies to check adverse cardiac remodeling under pathological conditions. This study sought to probe the mechanisms underlying ERK1/2-mediated p27Kip1 regulation in mitogenically stimulated cardiac fibroblasts. Addition of 10% fetal calf serum to quiescent cultures of adult rat cardiac fibroblasts promoted ERK1/2 activation, as evidenced by its phosphorylation status. Reduction in [3H]thymidine incorporation into DNA increased population doubling time, flow cytometry, and Western blot analysis showing reduced levels of cyclins D and A, p27Kip1 induction, and retinoblastoma protein (Rb) hypophosphorylation in ERK1/2-inhibited cells indicated ERK1/2 dependence of G1-S transition in cardiac fibroblasts. Lack of p27Kip1 protein in serum-stimulated, ERK1/2-active cells was associated with increased levels of Skp2, an E3 ubiquitin ligase for p27Kip1, whose knockdown by RNA interference induced p27Kip1 expression. Further, forced expression of Skp2 in ERK1/2-inhibited cells downregulated p27Kip1. Transcriptional upregulation of p27Kip1 mRNA in ERK1/2-inhibited cells, demonstrated by real-time PCR, correlated with forkhead box O 3a (FOXO3a) transcription factor activation, shown by gel shift assay. FOXO3a knockdown attenuated p27Kip1 mRNA and protein expression in ERK1/2-inhibited cells. We provide evidence for the first time that, in cardiac fibroblasts, activated ERK1/2 regulates p27Kip1 expression transcriptionally and posttranslationally via FOXO3a- and Skp2-dependent mechanisms. Additionally, this study uncovers interesting interactions between critical cell cycle regulatory elements that are only beginning to be understood.

Transcription factor binding site analysis identifies FOXO transcription factors as regulators of the cutaneous wound healing process

The search for significantly overrepresented and co-occurring transcription factor binding sites in the promoter regions of the most differentially expressed genes in microarray data sets could be a powerful approach for finding key regulators of complex biological processes. To test this concept, two previously published independent data sets on wounded human epidermis were re-analyzed. The presence of co-occurring transcription factor binding sites for FOXO1, FOXO3 and FOXO4 in the majority of the promoter regions of the most significantly differentially expressed genes between non-wounded and wounded epidermis implied an important role for FOXO transcription factors during wound healing. Expression levels of FOXO transcription factors during wound healing in vivo in both human and mouse skin were analyzed and a decrease for all FOXOs in human wounded skin was observed, with FOXO3 having the highest expression level in non wounded skin. Impaired re-epithelialization was found in cultures of primary human keratinocytes expressing a constitutively active variant of FOXO3. Conversely knockdown of FOXO3 in keratinocytes had the opposite effect and in an in vivo mouse model with FOXO3 knockout mice we detected significantly accelerated wound healing. This article illustrates that the proposed approach is a viable method for identifying important regulators of complex biological processes using in vivo samples. FOXO3 has not previously been implicated as an important regulator of wound healing and its exact function in this process calls for further investigation.

Endothelial FoxO1 is an intrinsic regulator of thrombospondin 1 expression that restrains angiogenesis in ischemic muscle

Peripheral artery disease (PAD) is characterized by chronic muscle ischemia. Compensatory angiogenesis is minimal within ischemic muscle despite an increase in angiogenic factors. This may occur due to the prevalence of angiostatic factors. Regulatory mechanisms that could evoke an angiostatic environment during ischemia are largely unknown. Forkhead box O (FoxO) transcription factors, known to repress endothelial cell proliferation in vitro, are potential candidates. Our goal was to determine whether FoxO proteins promote an angiostatic phenotype within ischemic muscle. FoxO1 and the angiostatic matrix protein thrombospondin 1 (THBS1) were elevated in ischemic muscle from PAD patients, or from mice post-femoral artery ligation. Mice with conditional endothelial cell-directed deletion of FoxO proteins (Mx1Cre (+), FoxO1,3,4 (L/L) , referred to as FoxOΔ) were used to assess the role of endothelial FoxO proteins within ischemic tissue. FoxO deletion abrogated the elevation of FoxO1 and THBS1 proteins, enhanced hindlimb blood flow recovery and improved neovascularization in murine ischemic muscle. Endothelial cell outgrowth from 3D explant cultures was more robust in muscles derived from FoxOΔ mice. FoxO1 overexpression induced THBS1 production, and a direct interaction of endogenous FoxO1 with the THBS1 promoter was detectable in primary endothelial cells. We provide evidence that FoxO1 directly regulates THBS1 within ischemic muscle. Altogether, these findings bring novel insight into the regulatory mechanisms underlying the repression of angiogenesis within peripheral ischemic tissues.

Impaired autophagy, chaperone expression, and protein synthesis in response to critical illness interventions in porcine skeletal muscle

Critical illness myopathy (CIM) is characterized by a preferential loss of the motor protein myosin, muscle wasting, and impaired muscle function in critically ill intensive care unit (ICU) patients. CIM is associated with severe morbidity and mortality and has a significant negative socioeconomic effect. Neuromuscular blocking agents, corticosteroids, sepsis, mechanical ventilation, and immobilization have been implicated as important risk factors, but the causal relationship between CIM and the risk factors has not been established. A porcine ICU model has been used to determine the immediate molecular and cellular cascades that may contribute to the pathogenesis prior to myosin loss and extensive muscle wasting. Expression profiles have been compared between pigs exposed to the ICU interventions, i.e., mechanically ventilated, sedated, and immobilized for 5 days, with pigs exposed to critical illness interventions, i.e., neuromuscular blocking agents, corticosteroids, and induced sepsis in addition to the ICU interventions for 5 days. Impaired autophagy as well as impaired chaperone expression and protein synthesis were observed in the skeletal muscle in response to critical illness interventions. A novel finding in this study is impaired core autophagy machinery in response to critical illness interventions, which when in concert with downregulated chaperone expression and protein synthesis may collectively affect the proteostasis in skeletal muscle and may exacerbate the disease progression in CIM.

FoxO1 is crucial for sustaining cardiomyocyte metabolism and cell survival

Diabetic cardiomyopathy is a term used to describe cardiac muscle damage-induced heart failure. Multiple structural and biochemical reasons have been suggested to induce this disorder. The most prominent feature of the diabetic myocardium is attenuated insulin signalling that reduces survival kinases (Akt), potentially switching on protein targets like FoxOs, initiators of cell death. FoxO1, a prominent member of the forkhead box family and subfamily O of transcription factors and produced from the FKHR gene, is involved in regulating metabolism, cell proliferation, oxidative stress response, immune homeostasis, pluripotency in embryonic stem cells, and cell death. In this review we describe distinctive functions of FoxOs, specifically FoxO1 under conditions of nutrient excess, insulin resistance and diabetes, and its manipulation to restore metabolic equilibrium to limit cardiac damage due to cell death. Because FoxO1 helps cardiac tissue to combat a variety of stress stimuli, it could be a major determinant in regulating diabetic cardiomyopathy. In this regard, we highlight studies from our group and others who illustrate how cardiac tissue-specific FoxO1 deletion protects the heart against cardiomyopathy and how its down-regulation in endothelial tissue could prevent against atherosclerotic plaques. In addition, we also describe studies that show FoxO1's beneficial qualities by highlighting their role in inducing anti-oxidant, autophagic, and anti-apoptotic genes under stress conditions of ischaemia-reperfusion and myocardial infarction. Thus, the aforementioned FoxO1 traits could be useful in curbing cardiac tissue-specific impairment of function following diabetes.

Forkhead Box O1 is present in quiescent pituitary cells during development and is increased in the absence of p27 Kip1

Congenital pituitary hormone deficiencies have been reported in approximately one in 4,000 live births, however studies reporting mutations in some widely studied transcription factors account for only a fraction of congenital hormone deficiencies in humans. Anterior pituitary hormones are required for development and function of several glands including gonads, adrenals, and thyroid. In order to identify additional factors that contribute to human congenital hormone deficiencies, we are investigating the forkhead transcription factor, FOXO1, which has been implicated in development of several organs including ovary, testis, and brain. We find that FOXO1 is present in the nuclei of non-dividing pituitary cells during embryonic development, consistent with a role in limiting proliferation and/or promoting differentiation. FOXO1 is present in a subset of differentiated cells at e18.5 and in adult with highest level of expression in somatotrope cells. We detected FOXO1 in p27(Kip1)-positive cells at e14.5. In the absence of p27(Kip1) the number of pituitary cells containing FOXO1 is significantly increased at e14.5 suggesting that a feedback loop regulates the interplay between FOXO1 and p27(Kip1).

FoxOs in neural stem cell fate decision

Neural stem cells (NSCs) persist over the lifespan of mammals to give rise to committed progenitors and their differentiated cells in order to maintain the brain homeostasis. To this end, NSCs must be able to self-renew and otherwise maintain their quiescence. Suppression of aberrant proliferation or undesired differentiation is crucial to preclude either malignant growth or precocious depletion of NSCs. The PI3K-Akt-FoxO signaling pathway plays a central role in the regulation of multiple stem cells including one in the mammalian brain. In particular, members of FoxO family transcription factors are highly expressed in these stem cells. As an important downstream effector of growth, differentiation, and stress stimuli, mammalian FoxO transcription factor family controls cellular proliferation, oxidative stress response, homeostasis, and eventual maintenance of long-term repopulating potential. The review will focus on the current understanding of FoxO function in NSCs as well as discuss their biological activities that contribute to determining neural stem cell fate.

Adaptations in muscle metabolic regulation require only a small dose of aerobic-based exercise

This study investigated the hypothesis that the duration of aerobic-based cycle exercise would affect the adaptations in substrate and metabolic regulation that occur in vastus lateralis in response to a short-term (10 day) training program. Healthy active but untrained males (n = 7) with a peak aerobic power ([Formula: see text]) of 44.4 ± 1.4 ml kg(-1) min(-1) participated in two different training programs with order randomly assigned (separated by ≥2 weeks). The training programs included exercising at a single intensity designated as light (L) corresponding to 60 % [Formula: see text], for either 30 or 60 min. In response to a standardized task (60 % [Formula: see text]), administered prior to and following each training program, L attenuated the decrease (P < 0.05) in phosphocreatine and the increase (P < 0.05) in free adenosine diphosphate and free adenosine monophosphate but not lactate. These effects were not altered by daily training duration. In the case of muscle glycogen, training for 60 versus 30 min exaggerated the increase (P < 0.05) that occurred, an effect that extended to both rest and exercise concentrations. No changes were observed in [Formula: see text] measured during progressive exercise to fatigue or in [Formula: see text] and RER during submaximal exercise with either training duration. These findings indicate that reductions in metabolic strain, as indicated by a more protected phosphorylation potential, and higher glycogen reserves, can be induced with a training stimulus of light intensity applied for as little as 30 min over 10 days. Our results also indicate that doubling the duration of daily exercise at L although inducing increased muscle glycogen reserves did not result in a greater metabolic adaptation.

Regulation of DMD pathology by an ankyrin-encoded miRNA

Background

Duchenne muscular dystrophy (DMD) is an X-linked myopathy resulting from the production of a nonfunctional dystrophin protein. MicroRNA (miRNA) are small 21- to 24-nucleotide RNA that can regulate both individual genes and entire cell signaling pathways. Previously, we identified several mRNA, both muscle-enriched and inflammation-induced, that are dysregulated in the skeletal muscles of DMD patients. One particularly muscle-enriched miRNA, miR-486, is significantly downregulated in dystrophin-deficient mouse and human skeletal muscles. miR-486 is embedded within the ANKYRIN1(ANK1) gene locus, which is transcribed as either a long (erythroid-enriched) or a short (heart muscle- and skeletal muscle-enriched) isoform, depending on the cell and tissue types.

Results

Inhibition of miR-486 in normal muscle myoblasts results in inhibited migration and failure to repair a wound in primary myoblast cell cultures. Conversely, overexpression of miR-486 in primary myoblast cell cultures results in increased proliferation with no changes in cellular apoptosis. Using bioinformatics and miRNA reporter assays, we have identified platelet-derived growth factor receptor β, along with several other downstream targets of the phosphatase and tensin homolog deleted on chromosome 10/AKT (PTEN/AKT) pathway, as being modulated by miR-486. The generation of muscle-specific transgenic mice that overexpress miR-486 revealed that miR-486 alters the cell cycle kinetics of regenerated myofibers in vivo, as these mice had impaired muscle regeneration.

Conclusions

These studies demonstrate a link for miR-486 as a regulator of the PTEN/AKT pathway in dystrophin-deficient muscle and an important factor in the regulation of DMD muscle pathology.

Differential and complementary effects of glucose and prolactin on islet DNA synthesis and gene expression

The mechanisms by which lactogenic hormones promote β-cell expansion remain poorly understood. Because prolactin (PRL) up-regulates β-cell glucose transporter 2, glucokinase, and pyruvate dehydrogenase activities, we reasoned that glucose availability might mediate or modulate the effects of PRL on β-cell mass. Here, we used male rat islets to show that PRL and glucose have differential but complementary effects on the expression of cell cyclins, cell cycle inhibitors, and various other genes known to regulate β-cell replication, including insulin receptor substrate 2, IGF-II, menin, forkhead box protein M1, tryptophan hydroxylase 1, and the PRL receptor. Differential effects on gene expression are associated with synergistic effects of glucose and PRL on islet DNA synthesis. The effects of PRL on gene expression are mirrored by β-cell overexpression of signal transducer and activator of transcription 5b and are opposed by dexamethasone. An ad-small interfering RNA specific for cyclin D2 attenuates markedly the effects of PRL on islet DNA synthesis. Our studies suggest a new paradigm for the control of β-cell mass and insulin production by hormones and nutrients. PRL up-regulates β-cell glucose uptake and utilization, whereas glucose increases islet PRL receptor expression and potentiates the effects of PRL on cell cycle gene expression and DNA synthesis. These findings suggest novel targets for prevention of neonatal glucose intolerance and gestational diabetes and may provide new insight into the pathogenesis of β-cell hyperplasia in obese subjects with insulin resistance.

Age-related impairment of T cell-induced skeletal muscle precursor cell function

Sarcopenia is the age-associated loss of skeletal muscle mass and strength. Recent evidence suggests that an age-associated loss of muscle precursor cell (MPC) functionality contributes to sarcopenia. The objectives of the present study were to examine the influence of activated T cells on MPCs and determine whether an age-related defect in this signaling occurs. MPCs were collected from the gastrocnemius and plantaris of 3-mo-old (young) and 32-mo-old (old) animals. Splenic T cells were harvested using anti-CD3 Dynabead isolation. T cells were activated for 48 h with costimulation of 100 IU/ml interleukin-2 (IL-2) and 5 μg/ml of anti-CD28. Costimulation increased 5-bromo-2'-deoxyuridine incorporation of T cells from 13.4 ± 4.6% in control to 64.8 ± 6.0% in costimulated cells. Additionally, T cell cytokines increased proliferation on MPCs isolated from young muscle by 24.0 ± 5.7%, whereas there was no effect on MPCs isolated from aged muscle. T cell cytokines were also found to be a chemoattractant. T cells were able to promote migration of MPCs isolated from young muscle; however, MPCs isolated from aged muscle did not respond to the T cell-released chemokines. Conversely, whereas T cell-released cytokines did not affect myogenesis of MPCs isolated from young animals, there was a decrease in MPCs isolated from old animals. These data suggest that T cells may play a critical role in mediating MPC function. Furthermore, aging may alter T cell-induced MPC function. These findings have implications for developing strategies aimed at increasing MPC migration and proliferation leading to an improved regenerative capacity of aged skeletal muscle.

Insulin-like growth factor-I-forkhead box O transcription factor 3a counteracts high glucose/tumor necrosis factor-α-mediated neuronal damage: implications for human immunodeficiency virus encephalitis

In HIV patients, antiretroviral medications trigger metabolic abnormalities, including insulin resistance. In addition, the inflammatory cytokine tumor necrosis factor-α (TNFα), which is elevated in human immunodeficiency virus encephalitis (HIVE), also induces insulin resistance and inflicts neuronal damage in vitro. In differentiated PC12 cells and rat cortical neurons, high glucose (HG; 25 mM) triggers reactive oxygen species (ROS) accumulation, contributing to the retraction of neuronal processes, with only a minimal involvement of neuronal apoptosis. In the presence of TNFα, HG-treated neurons undergo massive apoptosis. Because mammalian homolog of the Forkhead family of transcription factors, Forkhead box O transcription factor 3a (FOXO3a), controls ROS metabolism, we asked whether FOXO3a could affect the fate of differentiated neurons in the paradigm of HIVE. We observed FOXO3a nuclear translocation in HG-treated neuronal cultures, accompanied by partial loss of mitochondrial potential and gradual retraction of neuronal processes. Addition of TNFα to HG-treated neurons increased expression of the FOXO-dependent proapoptotic gene Bim, which resulted in extensive apoptotic death. Insulin-like growth factor-I (IGF-I) significantly lowered intracellular ROS, which was accompanied by IGF-I-mediated FOXO3a nuclear export and decrease in its transcriptional activity. The clinical relevance of these findings is supported by detection of nuclear FOXO3a in TUNEL-positive cortical neurons from HIVE, especially in brain areas characterized by elevated TNFα.

Proteome analysis and genome-wide regulatory motif prediction identify novel potentially sex-hormone regulated proteins in rat efferent ducts

The efferent ducts are a series of tubules that conduct sperm from the rete testis to the epididymis. They absorb most fluid and proteins originating from the rete testis during concentration of spermatozoa prior to their entry into the epididymis. Proteome analysis of micro-dissected efferent duct samples from adult rats was combined with genome-wide computational prediction of conserved hormone response elements to identify factors likely regulated by oestrogens and androgens. We identified 165 proteins and found subsets of the promoters controlling their corresponding genes to contain androgen- and oestrogen response elements (ARE/EREs) at similar frequencies. Moreover, EREs were significantly enriched among the loci identified compared with their genome-wide occurrence. The expression and localization of Anxa6, Ckb, Krt19, Park7, Pdzk1 and Tpt1 in the efferent ducts and other related hormone controlled tissues was further validated at the RNA or protein level. This study identifies many novel proteins predicted to play roles in sperm maturation and male fertility and provides significant computational evidence that the efferent ducts express genes transcriptionally controlled by sex hormones.

O-GlcNAcylation enhances FOXO4 transcriptional regulation in response to stress

The FOXO4 transcription factor plays an important role in cell survival in response to oxidative stress. The regulation of FOXO4 is orchestrated by post-translational modifications including phosphorylation, acetylation, and ubiquitination. Here, we demonstrate that O-GlcNAcylation also contributes to the FOXO4-dependent oxidative stress response. We show that hydrogen peroxide treatment of HEK293 cells increases FOXO4 association with OGT, the enzyme that adds O-GlcNAc to proteins, causing FOXO4 O-GlcNAcylation and enhanced transcriptional activity under acute oxidative stress. O-GlcNAcylation is known to be protective for cells under stress conditions, including oxidative stress. Our data provide a mechanism of FOXO4 anti-oxidative protection through O-GlcNAcylation.

The PI3K/Akt/FOXO3a/p27Kip1 signaling contributes to anti-inflammatory drug-suppressed proliferation of human osteoblasts

Akt has been reported to suppress p27(Kip1) promoter activity through Forkhead box O (FOXO) in different kinds of cells. Previous studies indicated that anti-inflammatory drugs up-regulated p27(Kip1), and this effect might play an important role in anti-inflammatory drug-induced cell cycle arrest of human osteoblasts (hOBs). In this study, we hypothesized that these drugs might increase p27(Kip1) expression in hOBs by altering the Akt/FOXO signaling. We tested this hypothesis by examining the influences of three anti-inflammatory drugs on the levels and/or activities of Akt, FOXO and p27(Kip1) as well as the relationship between these factors and proliferation of hOBs. We tested the effects of indomethacin (10(-5) and 10(-4)M), celecoxib (10(-6) and 10(-5)M), and dexamethasone (10(-7) and 10(-6)M) using PI3K inhibitor, LY294002 (10(-5)M) as the basis of comparison. The three drugs suppressed the canonical level of phosphorylated Akt in hOBs. This was accompanied by elevated FOXO3a level and increased promoter activity, mRNA expression and protein level of p27(Kip1). Furthermore, the anti-inflammatory drugs suppressed the EGF-induced increases in proliferation, phosphorylation, and nucleus translocation of Akt. Simultaneously, they suppressed EGF-induced decreases of FOXO3a nucleus accumulation and p27(Kip1) mRNA expression. On the other hand, FOXO silencing significantly attenuated the drug-induced up-regulation of p27(Kip1) and suppression of proliferation in hOBs. To the best of our knowledge, this study represents the first to demonstrate that Akt/FOXO3a/p27(Kip1) pathway contributes to suppression of hOB proliferation by anti-inflammatory drugs. We suggest that anti-inflammatory drugs suppress hOB proliferation, at least partly, through inactivating Akt, activating FOXO3a, and eventually up-regulating p27(Kip1) expression.

The role of FOXO in the regulation of metabolism

Forkhead box O (FOXO) transcription factors play an important role in modulating metabolic functions. FOXO is regulated by several modifications, but one of the most critical is phosphorylation and nuclear exclusion by Akt. Given the impact of insulin signaling on Akt-mediated phosphorylation of FOXO and the relatively high expression of Foxo1 in insulin-responsive tissues, this transcription factor is highly poised to regulate energy metabolism. When nutrient and insulin levels are low, Foxo1 promotes expression of gluconeogenic enzymes. Conversely, in the fed state, insulin levels rise and stimulate uptake of glucose primarily into skeletal muscle and other organs, including adipose tissue. Under certain pathophysiologic conditions, including insulin resistance, negative signaling to Foxo1 is compromised. Further clarification of the role of Foxo1 in insulin-responsive tissues will strengthen our understanding and allow us to better combat insulin resistance and diabetes mellitus.

Fibroblast growth factor 2-stimulated proliferation is lower in muscle precursor cells from old rats

In aged skeletal muscle, impairments in regrowth and regeneration may be explained by a decreased responsiveness of muscle precursor cells (MPCs) to environmental cues such as growth factors. We hypothesized that impaired responsiveness to fibroblast growth factor 2 (FGF2) in MPCs from old animals would be explained by impaired FGF2 signalling. We determined that 5-bromo-2'-deoxyuridine (BrdU) incorporation and cell number increase less in MPCs from 32- compared with 3-month-old rats. In the presence of FGF2, we demonstrated that there were age-associated differential expression patterns for FGF receptor 1 and 2 mRNAs. Measurement of downstream signalling revealed that that mitogen-activated protein kinase/ERK kinase 1/2 (MEK1/2)-extracellular signal-regulated kinase 1/2, protein kinase C and p38 were FGF2-driven pathways in MPCs. Uniquely, protein kinase C signalling was shown to play the largest role in FGF2-stimulated proliferation in MPCs. c-Jun N-terminal kinase (JNK) signalling was ruled out as an FGF2-stimulated proliferation pathway in MPCs. Inhibition of JNK had no effect on FGF2 signalling to BrdU incorporation, and FGF2 treatment was associated with increased phosphorylation of p38, which inhibits, rather than stimulates, BrdU incorporation in MPCs. Surprisingly, the commonly used vehicle, dimethyl sulphoxide, rescued proliferation in MPCs from old animals. These findings provide insight for the development of effective treatment strategies that target the age-related impairments of MPC proliferation in old skeletal muscle.