Temperature sensitivity of phospho-Ser473-PKB/AKT

Novel Ser74 of NF-κB/IκBα phosphorylated by MAPK/ERK regulates temperature adaptation in oysters

Phosphorylation of Ser32 and Ser36 controls the degradation of IκBα is the conserved cascade mechanisms of immune core signaling pathway, NF-κB pathway in metazoans, but it's response to abiotic stress and the presence of novel phosphorylation mechanisms in other species remain unclear. Herein, we reported a novel heat-induced phosphorylation site (Ser74) at oysters' major IκBα, which independently regulated ubiquitination-proteasome degradation without the requirement of phosphorylation at S32 and S36. And this site was phosphorylated by ERK/MAPK pathway, which then promoted REL nuclear translocation to activate cell survival related genes to defend heat-stress. The MAPK-NF-κB cascade exhibited divergent thermal responses and adaptation patterns between two congeneric oyster species with differential habitat temperatures, indicating its involvement in shaping temperature adaptation. This study demonstrated that the existence of complex and unique phosphorylation-mediated signaling transduction mechanism in marine invertebrates, and expanded our understanding of the evolution and function of established classical pathway crosstalk mechanisms.

Role of mTORC2 in biphasic regulation of brown fat metabolism in response to mild and severe cold

Nonshivering thermogenesis is essential for mammals to maintain body temperature. According to the canonical view, temperature is sensed by cutaneous thermoreceptors and nerve impulses transmitted to the hypothalamus, which generates sympathetic signals to ß-adrenergic receptors in brown adipocytes. The energy for heat generation is primarily provided by the oxidation of fatty acids derived from triglyceride hydrolysis and cellular uptake. Fatty acids also activate the uncoupling protein, UCP1, which creates a proton leak that uncouples mitochondrial oxidative phosphorylation from ATP production, resulting in energy dissipation as heat. Recent evidence supports the idea that in response to mild cold, ß-adrenergic signals stimulate not only lipolysis and fatty acid oxidation, but also act through the mTORC2-Akt signaling module to stimulate de novo lipogenesis. This opposing anabolic effect is thought to maintain lipid fuel stores during increased catabolism. We show here, using brown fat-specific Gs-alpha knockout mice and cultured adipocytes that, unlike mild cold, severe cold directly cools brown fat and bypasses ß-adrenergic signaling to inhibit mTORC2. This cell-autonomous effect both inhibits lipogenesis and augments UCP1 expression to enhance thermogenesis. These findings suggest a novel mechanism for overriding ß-adrenergic-stimulated anabolic activities while augmenting catabolic activities to resolve the homeostatic crisis presented by severe cold.

Body temperature elevation during exercise is essential for activating the Akt signaling pathway in the skeletal muscle of type 2 diabetic rats

This study examined the effect of changes in body temperature during exercise on signal transduction-related glucose uptake in the skeletal muscle of type 2 diabetic rats. Otsuka Long-Evans Tokushima Fatty rats (25 weeks of age), which have type 2 diabetes, were divided into the following four weight-matched groups; control (CON, n = 6), exercised under warm temperature (WEx, n = 8), exercised under cold temperature (CEx, n = 8), and heat treatment (HT, n = 6). WEx and CEx animals were subjected to running on a treadmill at 20 m/min for 30 min under warm (25°C) or cold (4°C) temperature. HT animals were exposed to single heat treatment (40–41°C for 30 min) in a heat chamber. Rectal and muscle temperatures were measured immediately after exercise and heat treatment, and the gastrocnemius muscle was sampled under anesthesia. Rectal and muscle temperatures increased significantly in rats in the WEx and HT, but not the CEx, groups. The phosphorylation levels of Akt, AS160, and TBC1D1 (Thr590) were significantly higher in the WEx and HT groups than the CON group (p < 0.05). In contrast, the phosphorylation levels of AMP-activated protein kinase, ACC, and TBC1D1 (Ser660) were significantly higher in rats in the WEx and CEx groups than the CON group (p < 0.05) but did not differ significantly between rats in the WEx and CEx groups. Body temperature elevation by heat treatment did not activate the AMPK signaling. Our data suggest that body temperature elevation during exercise is essential for activating the Akt signaling pathway in the skeletal muscle of rats with type 2 diabetic rats.

Short-term heat stress altered metabolism and insulin signaling in skeletal muscle

Heat-related complications continue to be a major health concern for humans and animals and lead to potentially life-threatening conditions. Heat stress (HS) alters metabolic parameters and may alter glucose metabolism and insulin signaling. Therefore, the purpose of this investigation was to determine the extent to which 12 h of HS-altered energetic metabolism in oxidative skeletal muscle. To address this, crossbred gilts (n = 8/group) were assigned to one of three environmental treatments for 12 h: thermoneutral (TN; 21 °C), HS (37 °C), or pair-fed to HS counterparts but housed in TN conditions (PFTN). Following treatment, animals were euthanized and the semitendinosus red (STR) was recovered. Despite increased relative protein abundance of the insulin receptor, insulin receptor substrate (IRS1) phosphorylation was increased (P = 0.0005) at S307, an inhibitory site, and phosphorylated protein kinase B (AKT) (S473) was decreased (P = 0.03) likely serving to impair insulin signaling following 12 h of HS. Further, HS increased phosphorylated protein kinase C (PKC) ζ/λ (P = 0.02) and phosphorylated PKCδ/θ protein abundance (P = 0.02), which are known to regulate inhibitory serine phosphorylation of IRS1 (S307). Sarcolemmal glucose transporter 4 (Glut4) was decreased (P = 0.04) in the membrane fraction of HS skeletal muscle suggesting diminished glucose uptake capacity. HS-mediated increases (P = 0.04) in mechanistic target of rapamycin (mTOR) were not accompanied by phosphorylation of eukaryotic translation initiation factor 4E-binding protein 1 (4EBP1). HS decreased (P = 0.0006) glycogen synthase (GS) and increased (P = 0.02) phosphorylated GS suggesting impaired glycogen synthesis. In addition, HS altered fatty acid metabolic signaling by increasing (P = 0.02) Acetyl-CoA carboxylase (ACC), decreasing (P = 0.005) phosphorylated ATP-citrate lyase (pATPCL) and fatty acid synthase (P = 0.01) (FAS). These data suggest that 12 h of HS blunted insulin signaling, decreased protein synthesis, and altered glycogen and fatty acid metabolism.

Heat stress-induced phosphorylation of FoxO3a signalling in rat skeletal muscle

AIM

A recent study demonstrated that FoxO3a was directly induced by the overexpression of Hsp72 in rat soleus muscle. However, whether heat stress treatment induces FoxO3a phosphorylation in rat skeletal muscle remains unclear. This study examined the effects of heat stress on the regulation of the FoxO3a signalling pathway in rat skeletal muscle.

METHODS

Thirty-two male Wistar rats (15 weeks old) were randomly assigned into two groups; sedentary control group (Sed, n = 8) and experimental group (n = 24). After an overnight fast, one leg of each rat (HS leg) in the experimental group was immersed in hot water (43 °C) for 30 min, and the soleus and plantaris muscles in both legs were removed immediately (0 min), 30 min, 60 min, or 24 h after the heat stress (n = 6 each group). The contralateral, non-heated leg in the experimental group served as an internal control (CT leg).

RESULTS

Heat stress treatment resulted in a significant increase in FoxO3a phosphorylation (Ser253) in the soleus and plantaris muscles of heat-stressed legs after 24 h. Hsp72 expression in heat-stressed legs was significantly higher at 60 min and 24 h in these muscles. Activation of the PTEN/Akt and MEK/ERK pathways was also observed in these muscles immediately after stress, but not at 24 h. There were no differences in FoxO1 and AMPKα phosphorylation in either muscle.

CONCLUSION

Heat stress in rat skeletal muscle induces phosphorylation of FoxO3a signalling, and it may be related to Hsp72 upregulation, and the activation of the PTEN/Akt and MEK/ERK pathways.

Tissue bioanalysis of biotherapeutics and drug targets to support PK/PD

Contemporary drug discovery leverages quantitative modeling and simulation with increasing emphasis, both to gain deeper knowledge of drug targets and mechanisms as well as improve predictions between preclinical models and clinical applications, such as first-in-human dose projections. Proliferation of novel biotherapeutic modalities increases the need for applied PK/PD modeling as a quantitative tool to advance new therapies. Of particular relevance is the understanding of exposure, target binding and associated pharmacology at the target site of interest. Bioanalytical methods are key to informing PK/PD models and require assessment of both PK and PD end points. Where targets are sequestered in tissues (noncirculating), the ability to quantitatively measure drug or biomarker in tissue compartments becomes particularly important. This perspective provides an overview of contemporary applications of quantitative bioanalysis in tissue compartments as applied to PK and PD assessments associated with novel biotherapeutics. Case studies and key references are provided.

Elevation of muscle temperature stimulates muscle glucose uptake in vivo and in vitro

The purpose of this study was to examine whether elevation of muscle temperature per se might be a stimulatory factor to increase muscle glucose uptake. Heat stimulation to rat hindlimbs increased glucose uptake measured in vivo in the extensor digitorum longus (EDL) and soleus muscles with a significant increase in muscle temperature. This thermal effect was observed again when glucose uptake was measured in vitro in both isolated muscles immediately after the heat stimulation in vivo. When heat stimulation was imposed on isolated EDL muscles, glucose uptake was facilitated in proportion to the increase in muscle temperature. The heat stimulation led to a significant amplification in the phosphorylation of AMP-activated protein kinase (AMPK) and Akt, and treatment with compound C, wortmannin, or LY294002 partially blocked the thermal effect on muscle glucose uptake. We provide evidence that elevation of muscle temperature per se can directly stimulate muscle glucose uptake and that this thermal effect is compound C-, wortmannin-, and LY294002-inhibitable.

Akt signaling and freezing survival in the wood frog, Rana sylvatica

BACKGROUND

The wood frog (Rana sylvatica) exhibits well-developed natural freeze tolerance supported by multiple mechanisms of biochemical adaptation. The present study investigated the role and regulation of the Akt signaling pathway in wood frog tissues (with a focus on liver) responding to freezing stress.

METHODS

Immunoblotting was used to assess total and phospho-Akt levels, total and phospho-PDK1, PTEN protein level, as well as total and phospho-FOXO1 levels. RT-PCR was used to investigate transcript levels of PTEN and microRNAs.

RESULTS

Akt was inhibited in skeletal muscle, kidney and heart after 24h freezing exposure with a reversal after thawing. The responses of the main kinase (PDK-1) and phosphatase (PTEN) that regulate Akt were consistent with freeze activation of Akt in liver; freezing exposure activated PDK-1 via enhanced Ser-241 phosphorylation whereas PTEN protein levels were reduced. Levels of three microRNAs (miR-26a, miR-126 and miR-217) that regulate pten expression were elevated in liver during freezing. One well-known role of Akt is in anti-apoptosis, mediated in part by Akt phosphorylation of Ser-256 on FOXO1. Freezing triggered an increase in liver phospho-FOXO1 Ser-256 content, suggesting that an important action of Akt may be apoptosis inhibition.

CONCLUSIONS

Akt activation in wood frog is stress and tissue specific, with multi-facet regulations (posttranslational and posttranscriptional) involved in supporting this specific signal transduction response.

GENERAL SIGNIFICANCE

This study implicates the Akt pathway in the metabolic reorganization of cellular metabolism in support of freezing survival.

Automated capillary electrophoresis system for fast single-cell analysis

Capillary electrophoresis (CE) is a promising technique for single-cell analysis, but its use in biological studies has been limited by low throughput. This paper presents an automated platform employing microfabricated cell traps and a three-channel system for rapid buffer exchange for fast single-cell CE. Cells loaded with fluorescein and Oregon green were analyzed at a throughput of 3.5 cells/min with a resolution of 2.3 ± 0.6 for the fluorescein and Oregon green. Cellular protein kinase B (PKB) activity, as measured by immunofluorescence staining of phospho-PKB, was not altered, suggesting that this stress-activated kinase was not upregulated during the CE experiments and that basal cell physiology was not perturbed prior to cell lysis. The activity of sphingosine kinase (SK), which is often upregulated in cancer, was measured in leukemic cells by loading a sphingosine-fluorescein substrate into cells. Sphingosine fluorescein (SF), sphingosine-1-phosphate fluorescein (S1PF), and a third fluorescent species were identified in single cells. A single-cell throughput of 2.1 cells/min was achieved for 219 total cells. Eighty-eight percent of cells possessed upregulated SK activity, although subpopulations of cells with markedly different SK activity relative to that of the population average were readily identified. This system was capable of stable and reproducible separations of biological compounds in hundreds of adherent and nonadherent cells, enabling measurements of previously uncharacterized biological phenomena.

Heat stress activates the Akt/mTOR signalling pathway in rat skeletal muscle

AIM

It is well known that various stimuli, such as mechanical stress and nutrients, induce muscle hypertrophy thorough the Akt/mTOR signalling pathway, which is a key mediator of protein synthesis and hypertrophy in skeletal muscle. It was recently reported that heat stress also induces an increase in muscle weight and muscle protein content. In addition, heat stress enhances Akt/mTOR signalling after one bout of resistance exercise. However, it remains unclear whether increased temperature itself stimulates the Akt/mTOR signalling pathway.

METHODS

Forty-two male Wistar rats (279.5 ± 1.2 g) were divided into a control group (CON) or one of five thermal stress groups at 37, 38, 39, 40 or 41 °C (n = 7 each group). After overnight fasting, both legs were immersed in different temperatures of hot water for 30 min under sodium pentobarbital anaesthesia. The soleus and plantaris muscles were immediately removed from both legs after the thermal stress.

RESULTS

The phosphorylation of mTOR or 4E-BP1 and heat shock protein (HSP) expression levels were similar among groups in both the soleus and plantaris muscles. However, Akt and p70S6K phosphorylation significantly increased at 41 °C in the soleus and plantaris muscles. Moreover, we observed a temperature-dependent increase in Akt and p70S6K phosphorylation in both muscles.

CONCLUSION

Our data indicate that the altered temperature increased phosphorylation in a temperature-dependent manner in rat skeletal muscle and may itself be a key stimulator of Akt/mTOR signalling.

Effects of temperature on the nitric oxide-dependent modulation of the Frank-Starling mechanism: the fish heart as a case study

The Frank-Starling law is a fundamental property of the vertebrate myocardium which allows, when the end-diastolic volume increases, that the consequent stretch of the myocardial fibers generates a more forceful contraction. It has been shown that in the eel (Anguilla anguilla) heart, nitric oxide (NO) exerts a direct myocardial relaxant effect, increasing the sensitivity of the Frank-Starling response (Garofalo et al., 2009). With the use of isolated working heart preparations, this study investigated the relationship between NO modulation of Frank-Starling response and temperature challenges in the eel. The results showed that while, in long-term acclimated fish (spring animals perfused at 20 °C and winter animals perfused at 10 °C) the inhibition of NO production by L-N5 (1-iminoethyl)ornithine (L-NIO) significantly reduced the Frank-Starling response, under thermal shock conditions (spring animals perfused at 10 or 15 °C and winter animals perfused at 15 or 20 °C) L-NIO treatment resulted without effect. Western blotting analysis revealed a decrease of peNOS and pAkt expressions in samples subjected to thermal shock. Moreover, an increase in Hsp90 protein levels was observed under heat thermal stress. Together, these data suggest that the NO synthase/NO-dependent modulation of the Frank-Starling mechanism in fish is sensitive to thermal stress.

Acute heat stress prior to downhill running may enhance skeletal muscle remodeling

Heat shock proteins (HSPs) are chaperones that are known to have important roles in facilitating protein synthesis, protein assembly and cellular protection. While HSPs are known to be induced by damaging exercise, little is known about how HSPs actually mediate skeletal muscle adaption to exercise. The purpose of this study was to determine the effects of a heat shock pretreatment and the ensuing increase in HSP expression on early remodeling and signaling (2 and 48 h) events of the soleus (Sol) muscle following a bout of downhill running. Male Wistar rats (10 weeks old) were randomly assigned to control, eccentric exercise (EE; downhill running) or heat shock + eccentric exercise (HS; 41°C for 20 min, 48 h prior to exercise) groups. Markers of muscle damage, muscle regeneration and intracellular signaling were assessed. The phosphorylation (p) of HSP25, Akt, p70s6k, ERK1/2 and JNK proteins was also performed. As expected, following exercise the EE group had increased creatine kinase (CK; 2 h) and mononuclear cell infiltration (48 h) compared to controls. The EE group had an increase in p-HSP25, but there was no change in HSP72 expression, total protein concentration, or neonatal MHC content. Additionally, the EE group had increased p-p70s6k, p-ERK1/2, and p-JNK (2 h) compared to controls; however no changes in p-Akt were seen. In contrast, the HS group had reduced CK (2 h) and mononuclear cell infiltration (48 h) compared to EE. Moreover, the HS group had increased HSP72 content (2 and 48 h), total protein concentration (48 h), neonatal MHC content (2 and 48 h), p-HSP25 and p-p70s6k (2 h). Lastly, the HS group had reduced p-Akt (48 h) and p-ERK1/2 (2 h). These data suggest that heat shock pretreatment and/or the ensuing HSP72 response may protect against muscle damage, and enhance increases in total protein and neonatal MHC content following exercise. These changes appear to be independent of Akt and MAPK signaling pathways.

Cooling-increased phospho-β-arrestin-1 and β-arrestin-1 expression levels in 3T3-L1 adipocytes

Cooling induces several responses that are modulated by molecular inhibitors and activators and receptor signaling. Information regarding potential targets involved in cold response mechanisms is still insufficient. We examined levels of the receptor-signaling mediator β-arrestin-1 and phospho-Ser-412 β-arrestin-1 in 3T3-L1 adipocytes exposed to 4-37 °C or treated with some molecular agents at 37°C. We also cooled cells with or without modification and signal-modulating agents. These conditions did not decrease cell viability, and western blot analysis revealed that exposure to 4 °C for 1.5h and to 28 and 32 °C for 24 and 48 h increased phospho-β-arrestin-1 and β-arrestin-1 levels and that exposure to 4 and 18 °C for 3 and 4.5h increased β-arrestin-1 level. Serum removal and rewarming abolished β-arrestin-1 alterations induced by cooling. Mithramycin A (a transcription inhibitor) treatment for 4 and 24h increased the level of β-arrestin-1 but not that of phospho-β-arrestin-1. The level of phospho-β-arrestin-1 was increased by okadaic acid (a phosphatase inhibitor), decreased by epinephrine and aluminum fluoride (receptor-signaling modulators), and unaffected by N-ethylmaleimide (an alkylating agent) at 37 °C. N-Ethylmaleimide and the receptor-signaling modulators did not alter β-arrestin-1 expression at 37 °C but impaired the induction of phospho-β-arrestin-1 at 28 and 32 °C without affecting the induction of β-arrestin-1. We show that cold-induced β-arrestin-1 alterations are partially mimicked by molecular agents and that the responsive machinery for β-arrestin-1 requires serum factors and N-ethylmaleimide-sensitive sites and is linked to rewarming- and receptor signaling-responsive machinery. Our findings provide helpful information for clarifying the cold-responsive machinery for β-arrestin-1 and elucidating low-temperature responses.

Heat stress enhances mTOR signaling after resistance exercise in human skeletal muscle

This study investigated the effect of heat stress (HS) on mammalian target of rapamycin (mTOR) signaling involved in translation initiation after resistance exercise in human skeletal muscle. Eight young male subjects performed four sets of six maximal repetitions of knee extension exercises, with or without HS, in a randomized crossover design. HS was applied to the belly of the vastus lateralis by using a microwave therapy unit prior to and during exercise. Muscle biopsies were taken from the vastus lateralis before, immediately after, and 1 h after exercise. HS significantly increased the phosphorylation of Akt/PKB, mTOR, and ribosomal protein S6 at 1 h after exercise (P < 0.05), and the 4E-BP1 phosphorylation level, which had initially decreased with exercise, had recovered by 1 h after exercise with HS. In addition, the phosphorylation of ribosomal S6 kinase 1 was significantly increased immediately after exercise with HS (P < 0.05). These results indicate that HS enhances mTOR signaling after resistance exercise in human skeletal muscle.

MDM2-related responses in 3T3-L1 adipocytes exposed to cooling and subsequent rewarming

Insulin-like growth factor-I and insulin induce the production of phospho-Ser-166 MDM2, a target of Akt, and influence the formation of the MDM2 complex. The glycolipid hormone insulin differentially activates phosphatidylinositol 3-kinase (PI3K)/Akt pathways in 3T3-L1 (L1) adipocytes incubated at 19 °C. Responses of L1 adipocytes to different temperature changes and their regulatory mechanisms are poorly understood. We exposed L1 adipocytes to cooling and subsequent rewarming in the presence or absence of wortmannin, a PI3K inhibitor, or mithramycin A, a transcription inhibitor, and examined the induction of phospho-Ser-166 MDM2 and MDM2 and the subcellular formation of the MDM2 complex using western blot analysis. Exposure to 28 and 18 °C induced phospho-MDM2 in cells and increased the level of MDM2 in the plasma membrane of cells. These temperatures did not affect the total MDM2 level. Similar results were obtained when the cells were treated with insulin. Exposure to 4 °C increased the total MDM2 level and did not induce phospho-MDM2, which was induced by rewarming at 37 °C after cooling at 4°C without any alteration in the protein level. Mithramycin A (10 μM) did not alter the increase in protein level induced at 4 °C. The induction of phospho-molecules at 28 and 18 °C was impaired slightly by 1 μM of wortmannin but not by 0.1 μM of wortmannin. This low concentration of wortmannin completely blocked the induction of phospho-MDM2 by rewarming. Our results indicate that temperature changes induce MDM2-related responses, including those that are stimulated by receptor responses and dependent on a kinase inhibitor, in L1 adipocytes.