Distinct lactate metabolism between hepatocytes and myotubes revealed by live cell imaging with genetically encoded indicators

The process of glycolysis breaks down glycogen stored in muscles, producing lactate through pyruvate to generate energy. Excess lactate is then released into the bloodstream. When lactate reaches the liver, it is converted to glucose, which muscles utilize as a substrate to generate ATP. Although the biochemical study of lactate metabolism in hepatocytes and skeletal muscle cells has been extensive, the spatial and temporal dynamics of this metabolism in live cells are still unknown. We observed the dynamics of metabolism-related molecules in primary cultured hepatocytes and a skeletal muscle cell line upon lactate overload. Our observations revealed an increase in cytoplasmic pyruvate concentration in hepatocytes, which led to glucose release. Skeletal muscle cells exhibited elevated levels of lactate and pyruvate levels in both the cytoplasm and mitochondrial matrix. However, mitochondrial ATP levels remained unaffected, indicating that the increased lactate can be converted to pyruvate but is unlikely to be utilized for ATP production. The findings suggest that excess lactate in skeletal muscle cells is taken up into mitochondria with little contribution to ATP production. Meanwhile, lactate released into the bloodstream can be converted to glucose in hepatocytes for subsequent utilization in skeletal muscle cells.

Ostarine-Induced Myogenic Differentiation in C2C12, L6, and Rat Muscles

Ostarine (also known as enobosarm or Gtx-024) belongs to the selective androgen receptor modulators (SARMs). It is a substance with an aryl-propionamide structure, classified as a non-steroidal compound that is not subjected to the typical steroid transformations of aromatization and reduction by α5 reductase. Despite ongoing research on ostarine, knowledge about it is still limited. Earlier studies indicated that ostarine may affect the metabolism of muscle tissue, but this mechanism has not been yet described. We aimed to investigate the effect of ostarine on the differentiation and metabolism of muscle. Using C2C12 and L6 cells, as well as muscles obtained from rats administered ostarine, we showed that ostarine stimulates C2C12 and L6 proliferation and cell viability and that this effect is mediated by androgen receptor (AR) and ERK1/2 kinase activation (p < 0.01). We also found that ostarine stimulates muscle cell differentiation by increasing myogenin, MyoD, and MyH expression in both types of cells (p < 0.01). Moreover, pharmacological blocking of AR inhibits the stimulatory effect of ostarine. We further demonstrated that 30 days of ostarine administration increases myogenin, MyoD, and MyH expression, as well as muscle mass, in rats (p < 0.01). Based on our research, we conclude that ostarine stimulates muscle tissue proliferation and differentiation via the androgen receptor.

Aloperine Relieves Type 2 Diabetes Mellitus via Enhancing GLUT4 Expression and Translocation

Aloperine (ALO), a quinolizidine alkaloid isolated from Sophora alopecuroides L. used in the traditional Uygur medicine, induced a significant increase in cellular glucose uptake of L6 cells, suggesting it has the potential to relieve hyperglycemia. Therefore, we investigated the effects of ALO on type 2 diabetes mellitus (T2DM) through in vitro and in vivo studies. The translocation of glucose transporter 4 (GLUT4) and changes in intracellular Ca²⁺ levels were real-time monitored in L6 cells using a laser scanning confocal microscope and related protein kinase inhibitors were used to explore the mechanism of action of ALO. Furthermore, high fat diet combined with low-dose streptozotocin (STZ) was used to induce T2DM in rats, and ALO was given to the stomach of T2DM rats for 4 weeks. In vitro results showed that ALO-induced enhancement of GLUT4 expression and translocation were mediated by G protein-PLC-PKC and PI3K/Akt pathways and ALO-enhanced intracellular Ca²⁺ was involved in activating PKC via G protein-PLC-IP₃R-Ca²⁺ pathway, resulting in promoted GLUT4 plasma membrane fusion and subsequent glucose uptake. ALO treatment effectively ameliorated hyperglycemia, glucose intolerance, insulin resistance and dyslipidemia, alleviated hepatic steatosis, protected pancreatic islet function and activated GLUT4 expression in insulin target tissues of T2DM rats. These findings demonstrated that ALO deserves attention as a potential hypoglycemic agent.

Lysophosphatidic Acid Inhibits Simvastatin-Induced Myocytoxicity by Activating LPA Receptor/PKC Pathway

Statins such as simvastatin have many side effects, including muscle damage, which is known to be the most frequent undesirable side effect. Lysophosphatidic acid (LPA), a kind of biolipid, has diverse cellular activities, including cell proliferation, survival, and migration. However, whether LPA affects statin-linked muscle damage has not been reported yet. In the present study, to determine whether LPA might exert potential protective effect on statin-induced myocyotoxicity, the effect of LPA on cytotoxicity in rat L6 myoblasts exposed to simvastatin was explored. Viability and apoptosis of rat L6 myoblasts were detected via 2,3-bis(2-methoxy-4-nitro-5-sulfophenyl)-5- [(phenylamino)carbonyl]-2H-tetrazolium hydroxide (XTT) assay and terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) assay, respectively. Protein expression levels were detected via Western blotting. Simvastatin decreased viability of L6 cells. Such decrease in viability was recovered in the presence of LPA. Treatment with LPA suppressed simvastatin-induced apoptosis in L6 cells. In addition, treatment with LPA receptor inhibitor Ki16425, protein kinase C (PKC) inhibitor GF109203X, or intracellular calcium chelator BAPTA-AM attenuated the recovery effect of LPA on simvastatin-induced L6 cell toxicity. These findings indicate that LPA may inhibit simvastatin-induced toxicity in L6 cells probably by activating the LPA receptor-PKC pathway. Therefore, LPA might have potential as a bioactive molecule to protect muscles against simvastatin-induced myotoxicity.

Sabinene Prevents Skeletal Muscle Atrophy by Inhibiting the MAPK-MuRF-1 Pathway in Rats

Chrysanthemum boreale Makino essential oil (CBMEO) has diverse biological activities including a skin regenerating effect. However, its role in muscle atrophy remains unknown. This study explored the effects of CBMEO and its active ingredients on skeletal muscle atrophy using in vitro and in vivo models of muscle atrophy. CBMEO reversed the size decrease of L6 myoblasts under starvation. Among the eight monoterpene compounds of CBMEO without cytotoxicity for L6 cells, sabinene induced predominant recovery of reductions of myotube diameters under starvation. Sabinene diminished the elevated E3 ubiquitin ligase muscle ring-finger protein-1 (MuRF-1) expression and p38 mitogen-activated protein kinase (MAPK) and extracellular signal-regulated kinase1/2 (ERK1/2) phosphorylations in starved myotubes. Moreover, sabinene decreased the increased level of reactive oxygen species (ROS) in myotubes under starvation. The ROS inhibitor antagonized expression of MuRF-1 and phosphorylation of MAPKs, which were elevated in starved myotubes. In addition, levels of muscle fiber atrophy and MuRF-1 expression in gastrocnemius from fasted rats were reduced after administration of sabinene. These findings demonstrate that sabinene, a bioactive component from CBMEO, may attenuate skeletal muscle atrophy by regulating the activation mechanism of ROS-mediated MAPK/MuRF-1 pathways in starved myotubes, probably leading to the reverse of reduced muscle fiber size in fasted rats.

Neferine Promotes GLUT4 Expression and Fusion With the Plasma Membrane to Induce Glucose Uptake in L6 Cells

Glucose transporter 4 (GLUT4) is involved in regulating glucose uptake in striated muscle, liver, and adipose tissue. Neferine is a dibenzyl isoquinoline alkaloid derived from dietary lotus seeds and has multiple pharmacological effects. Therefore, this study investigated neferine’s role in glucose translocation to cell surface, glucose uptake, and GLUT4 expression. In our study, neferine upregulated GLUT4 expression, induced GLUT4 plasma membrane fusion, increased intracellular Ca²⁺, promoted glucose uptake, and alleviated insulin resistance in L6 cells. Furthermore, neferine significantly activated phosphorylation of AMP-activated protein kinase (AMPK) and protein kinase C (PKC). AMPK and PKC inhibitors blocked neferine-induced GLUT4 expression and increased intracellular Ca²⁺. While neferine-induced GLUT4 expression and intracellular Ca²⁺ were inhibited by G protein and PLC inhibitors, only intracellular Ca²⁺ was inhibited by inositol trisphosphate receptor (IP₃R) inhibitors. Thus, neferine promoted GLUT4 expression via the G protein-PLC-PKC and AMPK pathways, inducing GLUT4 plasma membrane fusion and subsequent glucose uptake and increasing intracellular Ca²⁺ through the G protein-PLC-IP₃-IP₃R pathway. Treatment with 0 mM extracellular Ca²⁺ + Ca²⁺ chelator did not inhibit neferine-induced GLUT4 expression but blocked neferine-induced GLUT4 plasma membrane fusion and glucose uptake, suggesting the latter two are Ca²⁺-dependent. Therefore, we conclude that neferine is a potential treatment for type 2 diabetes.

Growth hormone and Insulin-like growth factor-I (IGF-I) modulate the expression of L-type amino acid transporters in the muscles of spontaneous dwarf rats and L6 and C2C12 myocytes

OBJECTIVE

Branched-chain amino acids (BCAAs) have been reported to inhibit several types of muscle atrophy via the activation of the mechanistic target of rapamycin complex 1 (mTORC1). However, we previously found that BCAA did not activate mTORC1 in growth hormone (GH)-deficient spontaneous dwarf rats (SDRs), and that GH restored the stimulatory effect of BCAAs toward the mTORC1. The objective of this study was to determine whether GH or Insulin-like growth factor-I (IGF-I) stimulated the expression of L-type amino acid transporters (LATs) that delivered BCAAs, and whether LATs were involved in the mTORC1 activation.

DESIGN

After the continuous administration of GH, cross-sectional areas (CSAs) of muscle fibers and LAT mRNA levels in the skeletal muscles of SDRs were compared to those from the SDRs that received normal saline. The effect of GH and IGF-I on LAT mRNA levels were determined in L6 and C2C12 myocytes. The effects of 2-aminobicyclo[2.2.1]heptane-2-carboxylic acid (BCH), a blocker for LATs, and LAT1 siRNA on mTORC1 activation and cell functions were examined in C2C12 cells.

RESULTS

GH increased LAT1 and LAT4 mRNA levels in accordance with the increase in CSAs of muscle fibers in SDRs. IGF-I, and not GH, increased LAT1 mRNA levels in cultured L6 myocytes. IGF-I also increased LAT1 mRNA level in another myocyte line, C2C12. Furthermore, IGF-I reduced LAT3 and LAT4 mRNA levels in both cell lines. GH reduced LAT3 and LAT4 mRNA levels in L6 cells. BCH decreased basal C2C12 cell proliferation and reduced IGF-I-induced phosphorylation of 4E-BP1 and S6K, both of which are mTORC1 targets, but LAT1 siRNA did not affect the phosphorylation. This suggests that BCH may exert its effect via other pathway than LAT1.

CONCLUSIONS

IGF-I increased LAT1 mRNA level in myocytes. However, the role of LAT1 in IGF-I-induced mTORC1 activation and cell functions remains unclear.

In vitro anti-diabetic effect of flavonoids and pheophytins from Allophylus cominia Sw. on the glucose uptake assays by HepG2, L6, 3T3-L1 and fat accumulation in 3T3-L1 adipocytes

BACKGROUND AND PURPOSE

Based on ethno-botanical information collected from diabetic patients in Cuba and firstly reported inhibition of PTP1B and DPPIV enzymes activities, Allophylus cominia (A. cominia) was identified as possible source of new drugs that could be used for the treatment of type 2 diabetes mellitus (T2-DM).

EXPERIMENTAL APPROACH

in this study, the activity of the characterised extracts from A. cominia was tested on the glucose uptake using HepG2 and L6 cells, 3T3-L1 fibroblasts and adipocytes as well as their effect on the fat accumulation using 3T3-L1 adipocytes.

KEY RESULTS

on 2-NBDG glucose uptake assay using HepG2 and L6 cells, extracts from A. cominia enhanced insulin activity by increasing glucose uptake. On HepG2 cells Insulin EC₅₀ of 93 ± 21nM decreased to 13 ± 2nM in the presence of the flavonoids mixture from A.cominia. In L6 cells, insulin also produced a concentration-dependent increase with an EC₅₀ of 28.6 ± 0.7nM; EC₅₀ decreased to 0.08 ± 0.02nM and 5 ± 0.9nM in the presence of 100μg/ml of flavonoids and pheophytins mixtures, respectively. In 3T3-L1 fibroblasts, insulin had an EC₅₀ of >1000nM that decreased to 38 ± 4nM in the presence of the flavonoids extract. However, in adipocytes, insulin produced a significant concentration-dependent increase and an EC₅₀ of 30 ± 8nM was a further confirmation of the insulin responsiveness of the adipocytes to the insulin. At 100µg/ml, flavonoids and pheophytins extracts decreased fat accumulation in 3T3-L1 adipocytes by two folds in comparison to the control differentiated cells (p < 0.05). The crude extract of A. cominia did not show any enhancement of 2-NBDG uptake by 3T3-L1 adipocytes in the presence or absence of 100nM insulin. In addition, in fully differentiated adipocytes, both extracts produced significant decrease in lipid droplets in the cells and no lipid accumulation were seen after withdrawal of the extracts from the cell growth medium. However, there was no effect of both extracts on total protein concentration in cells as well as on Glut-4 transporters.

CONCLUSIONS AND IMPLICATIONS

the pharmacological effects of the extracts from A. cominia observed in experimental diabetic models were shown in this study. A. cominia is potentially a new candidate for the treatment and management of T2-DM.

Palmitate-induced Slc2a4/GLUT4 downregulation in L6 muscle cells: evidence of inflammatory and endoplasmic reticulum stress involvement

Background

Obesity is strongly associated to insulin resistance, inflammation, and elevated plasma free fatty acids, but the mechanisms behind this association are not fully comprehended. Evidences suggest that endoplasmic reticulum (ER) stress may play a role in this complex pathophysiology. The aim of the present study was to investigate the involvement of inflammation and ER stress in the modulation of glucose transporter GLUT4, encoded by Slc2a4 gene, in L6 skeletal muscle cells.

Methods

L6 cells were acutely (2 h) and chronically (6 and 12 h) exposed to palmitate, and the expression of several proteins involved in insulin resistance, ER stress and inflammation were analyzed.

Results

Chronic and acute palmitate exposure significantly reduced GLUT4 protein (~ 39%, P < 0.01) and its mRNA (18%, P < 0.01) expression. Only acute palmitate treatment increased GRP78 (28%, P < 0.05), PERK (98%, P < 0.01), eIF-2A (35%, P < 0.01), IRE1a (60%, P < 0.05) and TRAF2 (23%, P < 0.05) protein content, and PERK phosphorylation (106%, P < 0.001), but did not elicit eIF-2A, IKK phosphorylation or increased XBP1 nuclear content. Additionally, acute and chronic palmitate increased NFKB p65 nuclear content (~ 30%, P < 0.05) and NFKB binding activity to Slc2a4 gene promoter (~ 45%, P < 0.05).

Conclusion

Different pathways are activated in acute and chronic palmitate induced-repression of Slc2a4/GLUT4 expression. This regulation involves activation of initial component of ER stress, such as the formation of a IRE1a-TRAF2-IKK complex, and converges to NFKB-induced repression of Slc2a4/GLUT4. These results link ER stress, inflammation and insulin resistance in L6 cells.

Anti-diabetic activity of stigmasterol from soybean oil by targeting the GLUT4 glucose transporter

The present study investigated the anti-diabetic activity and potential mechanism of stigmasterol (SMR), which is a kind of phytosterols derived from the edible soybean oil in vitro and in vivo. SMR displayed a mild GLUT4 translocation activity by 1.44-fold in L6 cells. L6 cells were treated with different concentration of SMR, showing significant effects on the enhancing glucose uptake. SMR administrated orally to the KK-Ay mice significantly alleviated their insulin resistance and oral glucose tolerance with reducing fasting blood-glucose levels and blood lipid indexes such as triglyceride and cholesterol. Moreover, the GLUT4 expression in L6 cells, skeletal muscle and white adipose tissue had been also enhanced. In this paper we conclude that, stigmasterol seems to have potential beneficial effects on the treatment of type 2 diabetes mellitus with the probable mechanism of targeting GLUT4 glucose transporter included increasing GLUT4 translocation and expression.

Aromatic Constituents from the Stems of Astragalus membranaceus (Fisch.) Bge. var. Mongholicus (Bge.) Hsiao

Four new aromatic constituents, astraflavonoids A (1), B (2), C (3), and astramemoside A (4), along with sixteen known ones 5-20 were obtained from the stems of A. membranaceus (Fisch.) Bge. var. mongholicus (Bge.) Hsiao. Their structures were elucidated by chemical and spectroscopic methods. Among the known isolates, 14 was obtained from the Astragalus genus for the first time, while 7-12, 18-20 were isolated from the species for the first time. The effects of the compounds obtained from the plant on glucose consumption were analyzed in differentiated L6 myotubes in vitro, whereby compounds 1, 2, 3, 7, 8, 10, 11, 14, 15 and 18 displayed significant promoting effects on glucose consumption in L6 myotubes. Among them, the activities of 1, 2 and 7 were comparable to that of insulin, which suggested that these compounds may be involved in glucose metabolism and transport.

Ca2+ effects on glucose transport and fatty acid oxidation in L6 skeletal muscle cell cultures

We examined the effect of Ca2+ on skeletal muscle glucose transport and fatty acid oxidation using L6 cell cultures. Ca2+ stimulation of glucose transport is controversial. We found that caffeine (a Ca2+ secretagogue) stimulation of glucose transport was only evident in a two-part incubation protocol ("post-incubation"). Caffeine was present in the first incubation, the media removed, and labeled glucose added for the second. Caffeine elicited a rise in Ca2+ in the first incubation that was dissipated by the second. This post-incubation procedure was insensitive to glucose concentrations in the first incubation. With a single, direct incubation system (all components present together) caffeine caused a slight inhibition of glucose transport. This was likely due to caffeine induced inhibition of phosphatidylinositol 3-kinase (PI3K), since nanomolar concentrations of wortmannin, a selective PI3K inhibitor, also inhibited glucose transport, and previous investigators have also found this action. We did find a Ca2+ stimulation (using either caffeine or ionomycin) of fatty acid oxidation. This was observed in the absence (but not the presence) of added glucose. We conclude that Ca2+ stimulates fatty acid oxidation at a mitochondrial site, secondary to malonyl CoA inhibition (represented by the presence of glucose in our experiments). In summary, the experiments resolve a controversy on Ca2+ stimulation of glucose transport by skeletal muscle, introduce an important experimental consideration for the measurement of glucose transport, and uncover a new site of action for Ca2+ stimulation of fatty acid oxidation.

DJ-1 protects against undernutrition-induced atrophy through inhibition of the MAPK-ubiquitin ligase pathway in myoblasts

AIMS

The purpose of this study is to explore whether antioxidant DJ-1 protein affects the atrophy of skeletal muscle cell induced by undernutrition.

MAIN METHODS

To determine cell atrophic responses, L6 cell line and skeletal primary cells from mouse hind limbs were cultivated under condition of FBS-free and low glucose. Changes of protein expression were analyzed using Western blot. Overexpression and knockdown of DJ-1 was performed in cells to assess its influence on cell atrophic responses.

KEY FINDINGS

Undernutrition decreased cell size and increased the abundance of oxidized form and total form of DJ-1 protein in L6 myoblasts. The undernourished cells revealed an elevation in the expression of muscle-specific RING finger-1 (MuRF-1) and atrogin-1, and in the phosphorylations of p38 mitogen-activated protein kinase (MAPK) and stress-activated protein kinase/c-Jun N-terminal kinase compared with control groups. Moreover, DJ-1-knockout mice showed a decrease in cell size and an enhancement in the expression of MuRF-1 and atrogin-1, as well as in the phosphorylation of MAPKs in gastrocnemius muscles; these changes were also observed in L6 cells transfected with siRNA of DJ-1. On the other hand, L6 cells overexpressing full-length DJ-1 did not exhibit the alterations in cell size and ubiquitin ligases seen after undernourished states of control cells. Myotubes differentiated from L6 cells also showed elevated expression of MuRF-1 and atrogin-1 in response to undernutrition.

SIGNIFICANCE

These results suggest that DJ-1 protein may contribute to undernutrition-induced atrophy via MAPKs/ubiquitin ligase pathway in skeletal muscle cells.

Bioactive Constituents from the Aerial Parts of Lippia triphylla

Five new compounds, lippianosides A (1), B (2), C (3), D (4), and E (5), along with 26 (6-31) known ones were obtained from the 95% EtOH extract of Lippia triphylla (L. triphylla) aerial parts collected from Rwanda, Africa. Among the known compounds, 11 and 17-30 were isolated from the Lippia genus for the first time. In addition, 12, 13, and 16 were firstly obtained from this species. The structures of them were elucidated by chemical and spectroscopic methods. The antioxidant and triglyceride accumulation inhibition effects of the 31 compounds were examined in L6 cells and HepG2 cells, respectively.

QRFP-26 enhances insulin's effects on glucose uptake in rat skeletal muscle cells

QRFP is expressed in central and peripheral regions important for nutrient intake and metabolism. Central administration of QRFP-26 and QRFP-43 induces a macronutrient specific increase in the intake of high fat diet in male and female rats. Recently, cell culture models have indicated that QRFP-26 and QRFP-43 are involved in glucose and fatty acid uptake in pancreatic islets and adipocytes. Since skeletal muscle is a major consumer of circulating glucose and a primary contributor to whole body metabolism, the current study examined the effects of QRFP-26 and QRFP-43 on insulin-stimulated uptake of glucose in skeletal muscle using L6 myotubes. The current experiments were designed to test the hypothesis that QRFP and its receptors, GPR103a and GPR103b are expressed in L6 myotubes and that QRFP-26 and QRFP-43 affect insulin-stimulated uptake of glucose in L6 myotubes. The results indicate that prepro-QRFP mRNA and GPR103a mRNA are expressed in L6 cells, though GPR103b mRNA was not detected. Using complementary assays, co-incubation with QRFP-26, increased insulin's ability to induce glycogen synthesis and 2-deoxyglucose uptake in L6 cells. These data suggest that QRFP-26, but not QRFP-43, is involved in the metabolic effects of skeletal muscle and may enhance insulin's effects on glucose uptake in skeletal muscle. These data support a role for QRFP as a modulator of nutrient intake in skeletal muscle.

Cytotoxicity of monensin, narasin and salinomycin and their interaction with silybin in HepG2, LMH and L6 cell cultures

The cytotoxic effect of monensin, narasin and salinomycin followed by their co-action with silybin in the cell line cultures of human hepatoma (HepG2), chicken hepatoma (LMH) or rat myoblasts (L6) have been investigated. The effective concentration of the studied ionophoric polyethers has been assessed within two biochemical endpoints: mitochondrial activity (MTT assay) and membrane integrity (LDH assay) after 24h incubation of each compound and farther, the cytotoxicity influenced in course of their interaction with silybin was determined. The most affected endpoints were found for inhibition of mitochondrial activity of the hepatoma cell lines and their viability depended on concentration of the ionophoric polyether, as well as on the cell line tested. The rat myoblasts were more sensitive target for cellular membrane damage when compared to inhibition of mitochondrial activity. An interaction between the ionophoric polyethers and silybin resulted a considerable cytotoxicity decrease within all studied cell lines; the combination index (CI) showed differences of interaction mode and dependence on cell culture, concentration of silybin, as well as the assay used. The obtained results are of interest in respect to recent findings on applicability of salinomycin and monensin for human therapy.