Chronological aging impacts abundance, function and microRNA content of extracellular vesicles produced by human epidermal keratinocytes.

The disturbance of intercellular communication is one of the hallmarks of aging. The goal of this study is to clarify the impact of chronological aging on extracellular vesicles (EVs), a key mode of communication in mammalian tissues. We focused on epidermal keratinocytes, the main cells of the outer protective layer of the skin which is strongly impaired in the skin of elderly. EVs were purified from conditioned medium of primary keratinocytes isolated from infant or aged adult skin. A significant increase of the relative number of EVs released from aged keratinocytes was observed whereas their size distribution was not modified. By small RNA sequencing, we described a specific microRNA (miRNA) signature of aged EVs with an increase abundance of miR-30a, a key regulator of barrier function in human epidermis. EVs from aged keratinocytes were found to be able to reduce the proliferation of young keratinocytes, to impact their organogenesis properties in a reconstructed epidermis model and to slow down the early steps of skin wound healing in mice, three features observed in aged epidermis. This work reveals that intercellular communication mediated by EVs is modulated during aging process in keratinocytes and might be involved in the functional defects observed in aged skin.

Identification of C-C motif chemokine ligand 5 as a heat-dependent myokine.

The skeletal muscle is an endocrine organ that produces proteins and peptides, collectively termed as myokines. The temperature of skeletal muscles varies during exercise and/or with changes in ambient temperature. However, whether myokine secretion is regulated by heat stimulation is unclear. Thus, we aimed to explore the effects of environmental heat stimulation on myokine secretion. We initially investigated the secretome of C2C12 myotubes and identified several novel heat-responsive myokines. The concentration of C-C motif chemokine ligand 5 (CCL5) dramatically decreased by 0.3-fold in response to heat stress. After 3 h heat stimulation of C2C12 cells, the expression of heat shock protein 70 was induced, and the gene expression and secretion of CCL5 was significantly attenuated in C2C12 cells. We then examined the effects of acute heat stress on serum CCL5 levels in mice and Ccl5 gene expression in skeletal muscles. Mice were maintained at 23°C, exposed to 45°C for 30 min, and then returned to the 23°C chamber for recovery. The expression of Ccl5 in the skeletal muscle significantly decreased after 3 h of recovery. Serum CCL5 levels increased by approximately 1.9-fold after 30 min of heat exposure and then significantly decreased by approximately 0.7-fold after 23 h of recovery. This study suggests that heat stimulation decreases CCL5 secretion from the skeletal muscle in vitro and in vivo. Given its fundamental role in inflammation by recruiting several immune cells, CCL5 has a potential role in controlling inflammatory responses in the body after heat stimulation.

Possible involvement of CXC motif chemokine ligand 10 in exercise-induced collagen production of mouse dermal fibroblasts.

Evidence suggests that exercise can regulate skin functions such as promoting wound healing and inhibiting aging. Physical exercise modulates the secretion of proteins and peptides from skeletal muscles, called myokines, which play a role in transmitting exercise signals throughout the body. Therefore, exercise-regulated myokines may play a role in controlling skin functions; however, the precise mechanisms remain elusive. In this study, we focused on the recently identified CXC motif chemokine ligand 10 (CXCL10), an exercise-reduced myokine, and attempted to elucidate its role in regulating collagen synthesis in dermal fibroblasts. Mouse C2C12 myotubes were stimulated with or without electrical pulse stimulation (EPS) to induce contraction for 24 h, and conditioned medium was collected (EPS-CM or Ctrl-CM, respectively). The reduction in CXCL10 concentration by EPS was confirmed using ELISA. Next, mouse dermal fibroblasts were isolated from the dorsal skin of C57BL6/J mice (2 weeks old) and were stimulated with Ctrl-CM or EPS-CM for 24 h. EPS-CM treatment significantly increased collagen production compared to Ctrl-CM treatment. Even in the Ctrl-CM condition, the addition of an antagonist for CXCR3 (CXCL10 receptor) increased collagen production. In contrast, recombinant CXCL10 abolished EPS-CM-dependent collagen induction. Overall, this study raises the possibility that CXCL10 secretion from skeletal muscles may control collagen production in mouse dermal fibroblasts.

Hydrogen peroxide induces progranulin expression to control neurite outgrowth in HT22 cells.

Progranulin (PGRN) is a multifunctional growth factor expressed in central nervous system. Although PGRN expression is regulated by various stressors, its precise role(s) and regulatory mechanism(s) remain elusive. In this study, we used HT22 cells to investigate the physiological implications of oxidative stress-induced PGRN expression and the regulation of PGRN expression by oxidative stress. We observed that p38 MAP kinase was activated upon the addition of H2O2, and a selective p38 MAP kinase inhibitor attenuated PGRN induction by H2O2. To explore the physiological role(s) of the PGRN induction, we first confirmed H2O2-dependent responses of HT22 cells and found that the length and number of neurites were increased by H2O2. Pgrn knockdown experiments suggested that these changes were mediated by H2O2-induced PGRN expression, at least in part. Overall, the results suggested that an increase in oxidative stress in HT22 cells induced PGRN expression via p38 MAP kinase pathway, thereby controlling neurite outgrowth.

The effects of glucose and fatty acids on CXCL10 expression in skeletal muscle cells.

Skeletal muscles produce secretory factors termed as myokines, which alter physiological functions of target tissues. We recently identified C-X-C chemokine ligand 10 (CXCL10) as a novel myokine, which is downregulated in response to exercise. In the present study, we investigated whether the nutritional changes affect CXCL10 expression in mouse skeletal muscle. Expression of CXCL10 was evaluated in mice fed a normal diet or a high fat diet for 10 weeks. In animals fed on HFD, Cxcl10 expression was significantly induced in fast-twitched muscles, and was accompanied by increased blood glucose and free fatty acid levels. In vitro experiments using C2C12 myotubes suggested that the increased levels of glucose and palmitic acids directly enhanced CXCL10 expression. Interestingly, the effect of palmitic acids was attenuated by palmitoleic acids. Considering its potent angiostatic activity, induction of CXCL10 by nutritional changes may contribute to the impairment of microvascular networks in skeletal muscles.

Identification of CCL5/RANTES as a novel contraction-reducible myokine in mouse skeletal muscle.

Skeletal muscle is an endocrine organ that secretes several proteins, which are collectively termed myokines. Although many studies suggest that exercise regulates myokine secretion, the underlying mechanisms remain unclear and all the exercise-dependent myokines have not yet been identified. Therefore, in this study, we attempted to identify novel exercise-dependent myokines by using our recently developed in vitro contractile model. Differentiated C2C12 myotubes were cultured with or without electrical pulse stimulation (EPS) for 24 h to induce cell contraction, and the myokines secreted in conditioned medium were analyzed using a cytokine array. Although most myokine secretions were not affected by EPS, the secretion of Chemokine (C-C motif) ligand 5 (CCL5) (regulated on activation, normal T cell expressed and secreted (RANTES)) was significantly reduced by EPS. This was further confirmed by ELISA and quantitative PCR. Contraction-dependent calcium transients and activation of 5'-AMP activating protein kinase (AMPK) appears to be involved in this decrease, as the chelating Ca2+ by EGTA blocked contraction-dependent CCL5 reduction, whereas the pharmacological activation of AMPK significantly reduced it. However, Ccl5 gene expression was increased by AMPK activation, suggesting that AMPK-dependent CCL5 decrease occurred via post-transcriptional regulation. Finally, mouse experiments revealed that voluntary wheel-running exercise reduced serum CCL5 levels and Ccl5 gene expression in the fast-twitch muscles. Overall, our study provides the first evidence of an exercise-reducible myokine, CCL5, in the mouse skeletal muscle. Although further studies are required to understand the precise roles of the skeletal muscle cell contraction-induced decrease in CCL5, this decrease may explain some exercise-dependent physiological changes such as those in immune responses.

Skeletal muscle cell contraction reduces a novel myokine, chemokine (C-X-C motif) ligand 10 (CXCL10): potential roles in exercise-regulated angiogenesis.

Accumulating evidence indicates that skeletal muscle secrets proteins referred to as myokines and that exercise contributes to their regulation. In this study, we propose that chemokine (C-X-C motif) ligand 10 (CXCL10) functions as a novel myokine. Initially, we stimulated differentiated C2C12 myotubes with or without electrical pulse stimulation (EPS) to identify novel myokines. Cytokine array analysis revealed that CXCL10 secretion was significantly reduced by EPS, which was further confirmed by enzyme-linked immunosorbent assay and quantitative polymerase chain reaction analysis. Treadmill experiments in mice identified significant reduction of Cxcl10 gene expression in the soleus muscle. Additionally, contraction-dependent p38 MAPK activation appeared to be involved in this reduction. Furthermore, C2C12 conditioned medium obtained after applying EPS could induce survival of MSS31, a vascular endothelial cell model, which was partially attenuated by the addition of recombinant CXCL10. Overall, our findings suggest CXCL10 as a novel exercise-reducible myokine, to control endothelial cell viability.

Glucose deprivation regulates the progranulin-sortilin axis in PC12 cells.

Progranulin (PGRN) is a growth factor implicated in several neurodegenerative diseases, such as frontotemporal lobar degeneration. Despite its important role in the central nervous system (CNS), the mechanisms controlling PGRN expression in the CNS are largely unknown. Recent evidence, however, suggested that several stressors, such as hypoxia, acidosis, or oxidative stress, induce PGRN expression. The present study was mainly aimed at determining whether and, if so, how glucose deprivation affects PGRN expression in PC12 cells. Initially, it was found that glucose deprivation gradually induced PGRN gene expression in PC12 cells. To elucidate the underlying molecular mechanisms, several intracellular signalings that were modified in response to glucose deprivation were examined. Both adenosine monophosphate kinase (AMPK) activation and changes in osmotic pressure, which are modified by extracellular glucose concentration, had no effect on PGRN gene expression; on the other hand, p38 activation in response to glucose deprivation played an important role in inducing PGRN gene expression. It was also found that expression of sortilin, a PGRN receptor implicated in PGRN endocytosis, was dramatically reduced by glucose deprivation. In contrast to glucose-dependent regulation of PGRN gene expression, AMPK activation played a central role in reducing sortilin expression. Overall, the present study suggests that the PGRN-sortilin axis is modulated by glucose deprivation via two distinct mechanisms. As PGRN is neuroprotective, this system may represent a new neuroprotective mechanism activated by glucose deprivation in the CNS.

Glucose deprivation attenuates sortilin levels in skeletal muscle cells.

In skeletal muscle, sortilin plays a predominant role in the sorting of glucose transporter 4 (Glut4), thereby controlling glucose uptake. Moreover, our previous study suggested that the sortilin expression levels are also implicated in myogenesis. Despite the importance of sortilin in skeletal muscle, however, the regulation of sortilin expression has not been completely understood. In the present study, we analyzed if the sortilin expression is regulated by glucose in C2C12 myocytes and rat skeletal muscles in vivo. Sortilin protein expression was elevated upon C2C12 cell differentiation and was further enhanced in the presence of a high concentration of glucose. The gene expression and protein degradation of sortilin were not affected by glucose. On the other hand, rapamycin partially reduced sortilin induction by a high concentration of glucose, which suggested that sortilin translation could be regulated by glucose, at least in part. We also examined if the sortilin regulation by glucose was also observed in skeletal muscles that were obtained from fed or fasted rats. Sortilin expression in both gastrocnemius and extensor digitorum longus (EDL) muscle was significantly decreased by 17-18h of starvation. On the other hand, pathological levels of high blood glucose did not alter the sortilin expression in rat skeletal muscle. Overall, the present study suggests that sortilin protein levels are reduced under hypoglycemic conditions by post-transcriptional control in skeletal muscles.

Glutamate levels control HT22 murine hippocampal cell death by regulating biphasic patterns of Erk1/2 activation: role of metabolic glutamate receptor 5.

Extracellular glutamate concentration is a critical determinant of neuronal cell fate. We recently demonstrated that HT22 murine hippocampal cell viability was reduced by exposure to high concentrations of glutamate, whereas low concentrations promoted cell survival. Extracellular signal-regulated kinase (Erk)1/2 activation by glutamate is important for both glutamate-induced cell death and survival. In this study, we investigated the role of glutamate-induced or hydrogen peroxide (H2O2)-induced Erk1/2 activation in HT22 cell fate determination. Glutamate and H2O2 treatment similarly induced early (<1 h) Erk1/2 phosphorylation regardless of concentration. On the other hand, persistent Erk1/2 phosphorylation (16-24 h) was observed only in the presence of excess glutamate. Only the latter contributed to glutamate-induced cell death, which involved metabolic glutamate receptor 5. Our findings suggest that glutamate concentration modulates two distinct phases of Erk1/2 activation, which can explain the glutamate concentration-dependent determination of HT22 cell fate.

Subtoxic levels of hydrogen peroxide induce brain-derived neurotrophic factor expression to protect PC12 cells.

BACKGROUND: Oxidative stress is one of the mechanisms underlying pathogenesis in neurodegenerative diseases such as Alzheimer's disease. Generally, oxidative stress represents cell toxicity; however, we recently found that oxidative stress promotes the expression of growth factor progranulin (PGRN) in HT22 murine hippocampus cells, thereby protecting the HT22 cells. In this study, we attempted to clarify whether a similar system exists in the other neuronal cell model, rat pheochromocytoma (PC12) cells. RESULTS: After confirming that high concentrations of hydrogen peroxide (H2O2; 100-250 µM) initiate PC12 cell death, we analyzed growth factor expressional changes after H2O2 treatment. We found, intriguingly, that gene expression of brain-derived neurotrophic factor (BDNF), but not PGRN was significantly induced by H2O2. Although little expression of the high affinity BDNF receptor tropomyosin-related kinase TrkB was observed in PC12 cells, expression of low affinity neurotrophin receptor, p75NTR, was clearly observed. This BDNF signaling appeared to contribute to PC12 cell protection, since PC12 cell death was significantly attenuated by BDNF treatment. CONCLUSIONS: Based on our results, we conclude that the induction of BDNF by subtoxic levels of H2O2 and its signaling may have roles in PC12 cell protection.

Dual cell protective mechanisms activated by differing levels of oxidative stress in HT22 murine hippocampal cells.

Oxidative stress is recognized as one of the pathogenic mechanisms involved in neurodegenerative disease. However, recent evidence has suggested that regulation of cellular fate in response to oxidative stress appears to be dependent on the stress levels. In this study, using HT22 cells, we attempted to understand how an alteration in the oxidative stress levels would influence neuronal cell fate. HT22 cell viability was reduced with exposure to high levels of oxidative stress, whereas, low levels of oxidative stress promoted cell survival. Erk1/2 activation induced by a low level of oxidative stress played a role in this cell protective effect. Intriguingly, subtoxic level of H2O2 induced expression of a growth factor, progranulin (PGRN), and exogenous PGRN pretreatment attenuated HT22 cell death induced by high concentrations of H2O2 in Erk1/2-dependent manner. Together, our study indicates that two different cell protection mechanisms are activated by differing levels of oxidative stress in HT22 cells.

Lysine suppresses protein degradation through autophagic-lysosomal system in C2C12 myotubes.

Muscle mass is determined between protein synthesis and protein degradation. Reduction of muscle mass leads to bedridden condition and attenuation of resistance to diseases. Moreover, bedridden condition leads to additional muscle loss due to disuse muscle atrophy. In our previous study (Sato et al. 2013), we showed that administered lysine (Lys), one of essential amino acid, suppressed protein degradation in skeletal muscle. In this study, we investigated that the mechanism of the suppressive effects of Lys on skeletal muscle proteolysis in C2C12 cell line. C2C12 myotubes were incubated in the serum-free medium containing 10 mM Lys or 20 mM Lys, and myofibrillar protein degradation was determined by the rates of 3-methylhistidine (MeHis) release from the cells. The mammalian target of rapamycin (mTOR) activity from the phosphorylation levels of p70-ribosormal protein S6 kinase 1 and eIF4E-binding protein 1 and the autophagic-lysosomal system activity from the ratio of LC3-II/I in C2C12 myotubes stimulated by 10 mM Lys for 0-3 h were measured. The rates of MeHis release were markedly reduced by addition of Lys. The autophagic-lysosomal system activity was inhibited upon 30 min of Lys supplementation. The activity of mTOR was significantly increased upon 30 min of Lys supplementation. The suppressive effect of Lys on the proteolysis by the autophagic-lysosomal system was maintained partially when mTOR activity was inhibited by 100 nM rapamycin, suggesting that some regulator other than mTOR signaling, for example, Akt, might also suppress the autophagic-lysosomal system. From these results, we suggested that Lys suppressed the activity of the autophagic-lysosomal system in part through activation of mTOR and reduced myofibrillar protein degradation in C2C12 myotubes.

Potential neuroprotective effects of SIRT1 induced by glucose deprivation in PC12 cells.

Nutrient availability is one of the most important signals regulating cellular fates including cell growth, differentiation, and death. Recent evidence suggests that the NAD(+)-dependent histone deacetylase sirtuin 1 (SIRT1) plays a prominent role in linking changes in nutritional availability with cellular fate regulation. SIRT1 expression is observed in neurons, yet the expressional and functional regulation of this protein is not fully understood. In the present study, we examined whether extracellular glucose concentration affects the expression and localization of SIRT1 in PC12 cells. Further, we examined levels of forkhead box O3a (FoxO3a), which is also controlled by changes in extracellular glucose concentration. We observed the total expression levels of SIRT1 and FoxO3a in PC12 cells were reduced when glucose availability increased via gene expressional control, at least in part. Nuclear localization of SIRT1 and FoxO3a was increased by glucose deprivation. Even though the changes in extracellular glucose concentration regulated SIRT1 and FoxO3a in a similar direction, the effects of nerve growth factor on these two proteins were completely different. Finally, we found the potent SIRT1 inhibitor enhanced glucose deprivation-induced cell death. Therefore, we propose that glucose deprivation-induced SIRT1 expression potentially plays a major role in protecting PC12 cells.

Characterization of contraction-induced IL-6 up-regulation using contractile C2C12 myotubes.

Muscle contractile activity functions as a potent stimulus for acute interleukin (IL)-6 expression in working skeletal muscles. Recently, we established an "in vitro contraction model" using highly-developed contractile C2C12 myotubes by applying electric pulse stimulation (EPS). Herein, we characterize the effects of EPS-evoked contraction on IL-6 expression in contractile C2C12 myotubes. Both secretion and mRNA expression of IL-6 were significantly up-regulated by EPS in a frequency-dependent manner in contracting myotubes during a 24-h period, and the response was blunted by cyclosporine A, a calcineurin inhibitor. Longer time (~12h) was required for the induction of IL-6 after the initiation of EPS as compared to that of other contraction-inducible CXC chemokines such as CXCL1/KC, which were induced in less than 3 hours. Furthermore, these acute inducible CXC chemokines exhibited no autocrine effect on IL-6 expression. Importantly, contraction-dependent IL-6 up-regulation was markedly suppressed in the presence of high levels of glucose along with increased glycogen accumulations. Experimental manipulation of intracellular glycogen contents by modulating available glucose or pyruvate during a certain EPS period further established the suppressive effect of glycogen accumulations on contraction-induced IL-6 up-regulation, which appeared to be independent of calcineurin activity. We also document that EPS-evoked contractile activity improved insulin-responsiveness in terms of intracellular glycogen accumulations. Taken together, these data provide important insights into the regulation of IL-6 expression in response to contractile activity of muscle cells, which is difficult to examine using in vivo experimental techniques. Our present results thus expand the usefulness of our "in vitro contraction model".

Phosphatidylinositol 3-kinase-binding protein, PI3KAP/XB130, is required for cAMP-induced amplification of IGF mitogenic activity in FRTL-5 thyroid cells.

We previously demonstrated that long-term pretreatment of rat FRTL-5 thyroid cells with TSH or cAMP-generating reagents potentiated IGF-I-dependent DNA synthesis. Under these conditions, cAMP treatment increased tyrosine phosphorylation of a 125-kDa protein (p125) and its association with a p85 regulatory subunit of phosphatidylinositol 3-kinase (p85 PI3K), which were suggested to mediate potentiation of DNA synthesis. This study was undertaken to identify p125 and to elucidate its roles in potentiation of DNA synthesis induced by IGF-I. Matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS) analysis revealed p125 to be a rat ortholog of human XB130, which we named PI3K-associated protein (PI3KAP). cAMP treatment elevated PI3KAP/XB130 mRNA and protein levels as well as tyrosine phosphorylation and interaction with p85 PI3K leading to increased PI3K activities associated with PI3KAP/XB130, supporting the role of PI3KAP/XB130 in DNA synthesis potentiation. Importantly, PI3KAP/XB130 knockdown attenuated cAMP-dependent potentiation of IGF-I-induced DNA synthesis. Furthermore, c-Src was associated with PI3KAP/XB130 and was activated in response to cAMP. Addition of Src family kinase inhibitors, PP1 or PP2, during cAMP treatment abolished tyrosine phosphorylation of PI3KAP/XB130 and its interaction with p85 PI3K. Finally, introduction of PI3KAP/XB130 into NIH3T3 fibroblasts lacking endogenous PI3KAP/XB130 enhanced IGF-I-induced DNA synthesis; however, a mutant Y72F incapable of binding to p85 PI3K did not show this response. Together, these data indicate that cAMP-dependent induction of PI3KAP/XB130, which is associated with PI3K, is required for enhancement of IGF mitogenic activities.

Roles of progranulin in sexual differentiation of the developing brain and adult neurogenesis.

Progranulin (PGRN) is a growth modulating factor released by a variety of cells. This molecule has gained the attention of the neuroscience community with recent discoveries of multifunctional roles of PGRN in normal brain and neurodegenerative disorders. We focus on novel roles of PGRN as a sex steroid-responsible gene in the developing and adult rodent brain. While the developing brain is feminine by default, hormone exposure, including androgen and estrogen, induces masculinization during the critical period. We have shown that PGRN is a sex steroid-responsible gene that may be involved in masculinization of the perinatal rat brain. We also found that in adult rats PGRN gene expression was up-regulated by estrogen in the hippocampus, suggesting that PGRN may mediate the mitogenic effects of estrogen in the active area of neurogenesis. Since it has been recently reported that mutations in PGRN gene are responsible for a type of frontotemporal lobar degeneration in humans, PGRN appears to be also involved in modulating neurodegeneration. Together, PGRN gene expression is induced by estrogen in both developing and adult brains, and it may play multifunctional roles in the organization of functional masculinization in the developing brain and the maintenance of adult brain function.

Characterization of contraction-inducible CXC chemokines and their roles in C2C12 myocytes.

Physical exercise triggers the release of several cytokines/chemokines from working skeletal muscles, but the underlying mechanism(s) by which skeletal muscles decipher and respond to highly complex contractile stimuli remains largely unknown. In an effort to investigate the regulatory mechanisms of the expressions of two contraction-inducible CXC chemokines, CXCL1/KC and CXCL5/LIX, in contracting skeletal muscle cells, we took advantage of our in vitro exercise model using highly developed contractile C(2)C(12) myotubes, which acquire properties similar to those of in vivo skeletal muscle via manipulation of Ca(2+) transients with electric pulse stimulation (EPS). Production of these CXC chemokines was immediately augmented by EPS-evoked contractile activity in a manner dependent on the activities of JNK and NF-kappaB, but not p38, ERK1/2, or calcineurin. Intriguingly, exposure of myotubes to cyclic mechanical stretch also induced expression of these CXC chemokines; however, a much longer period of stimulation (approximately 12 h) was required, despite rapid JNK phosphorylation. We also demonstrate herein that CXCL1/KC and CXCL5/LIX have the ability to raise intracellular Ca(2+) concentrations via CXCR2-mediated activation of pertussis toxin-sensitive Galpha(i) proteins in C(2)C(12) myoblasts, an action at least partially responsible for their migration and differentiation. Although we revealed a possible negative feedback regulation of their own production in response to the contractile activity in differentiated myotubes, exogenous administration of these CXC chemokines did not acutely influence either insulin-induced Akt phosphorylation or GLUT4 translocation in C(2)C(12) myotubes. Taken together, these data shed light on the fundamental characteristics of contraction-inducible CXC chemokine production and their potential roles in skeletal muscle cells.

Contractile C2C12 myotube model for studying exercise-inducible responses in skeletal muscle.

Adequate exercise leads to a vast variety of physiological changes in skeletal muscle as well as other tissues/organs and is also responsible for maintaining healthy muscle displaying enhanced insulin-responsive glucose uptake via GLUT4 translocation. We generated highly developed contractile C(2)C(12) myotubes by manipulating intracellular Ca(2+) transients with electric pulse stimulation (EPS) that is endowed with properties similar to those of in vivo skeletal muscle in terms of 1) excitation-induced contractile activity as a result of de novo sarcomere formation, 2) activation of both the AMP kinase and stress-activated MAP kinase cascades, and 3) improved insulin responsiveness as assessed by GLUT4 recycling. Tbc1d1, a Rab-GAP implicated in exercise-induced GLUT4 translocation in skeletal muscle, also appeared to be phosphorylated on Ser(231) after EPS-induced contraction. In addition, a switch in myosin heavy-chain (MHC) expression from "fast type" to "slow type" was observed in the C(2)C(12) myotubes endowed with EPS-induced repetitive contractility. Taking advantage of these highly developed contractile C(2)C(12) myotubes, we identified myotube-derived factors responsive to EPS-evoked contraction, including the CXC chemokines CXCL1/KC and CXCL5/LIX, as well as IL-6, previously reported to be upregulated in contracting muscles in vivo. Importantly, animal treadmill experiments revealed that exercise significantly increased systemic levels of CXCL1/KC, perhaps derived from contracting muscle. Taken together, these results confirm that we have established a specialized muscle cell culture model allowing contraction-inducible cellular responses to be explored. Utilizing this model, we identified contraction-inducible myokines potentially linked to the metabolic alterations, immune responses, and angiogenesis induced by exercise.

Distinct modes of activation of phosphatidylinositol 3-kinase in response to cyclic adenosine 3', 5'-monophosphate or insulin-like growth factor I play different roles in regulation of cyclin D1 and p27Kip1 in FRTL-5 cells.

Bioactivities of IGFs in various cells are often potentiated in the presence of other hormones. In previous studies we showed that pretreatment of rat FRTL-5 thyroid cells with TSH or other cAMP-generating agents markedly potentiated DNA synthesis induced by IGF-I. Under these conditions we found that phosphatidylinositol (PI) 3-kinase was activated in response to either cAMP or IGF stimulus, and both activation modes were indispensable for the potentiation of DNA synthesis. The present studies were undertaken to elucidate how cAMP and/or IGF-I stimulus regulated the G1 cyclin-cyclin dependent kinase (CDK)-inhibitor system, and to determine the roles of PI 3-kinase activation by cAMP or IGF-I stimulus in this system. We found that cAMP pretreatment enhanced IGF-I-dependent increases in cyclin D1, due to synergistic increases in mRNA and elevation of translation rates. Furthermore, cAMP pretreatment enhanced IGF-I-induced protein degradation of the CDK inhibitor, p27(Kip1). These changes well explained an increase in cyclin E, leading to marked activation of G1 CDKs, followed by retinoblastoma protein phosphorylation. Our results using a PI 3-kinase inhibitor showed that cAMP-dependent PI 3-kinase activation plays an important role in the increase in cyclin D1 translation. In contrast, IGF-I-dependent PI 3-kinase activation was required for the increase in cyclin D1 mRNA levels and degradation of p27(Kip1). Together, the present study elucidates the role of cAMP and IGF-I in differentially activating PI 3-kinase as a mediator of multiple molecular events. These events converge in the regulation of cyclin D1 and p27(Kip1), leading to cAMP-dependent potentiation of IGF-I-dependent CDK activation and DNA synthesis.