AMP-activated protein kinase and adipogenesis in sheep fetal skeletal muscle and 3T3-L1 cells

Tauroursodeoxycholic acid attenuates lipid accumulation in endoplasmic reticulum-stressed macrophages

BACKGROUND/AIM

Recent evidence suggests that endoplasmic reticulum (ER) stress provoked under diabetic conditions augments the expression of scavenger receptors on macrophages, promoting the uptake of oxidized low-density lipoprotein uptake and atherogenesis. The aim of the present study was to test the hypothesis that the chemical chaperone tauroursodeoxycholic acid (TUDCA) attenuates lipid accumulation in macrophages subjected to ER stress.

METHODS

Cultured human macrophages were subjected to ER stress by treating them with tunicamycin. Lipid uptake by macrophages subjected to ER stress in the presence or absence of TUDCA was assessed by oil red O staining and by assessing the cellular uptake of Dil-oxidized low-density lipoprotein by fluorescence measurement. Protein levels and phosphorylation status of ER stress markers, insulin-signaling molecules, and scavenger receptor were assessed by Western blotting.

RESULTS

Treatment of cultured human macrophages with the ER stressor tunicamycin caused an increase in the protein levels of cluster of differentiation 36 (CD-36) and augmentation of lipid uptake both of which were inhibited by TUDCA. TUDCA treatment inhibited tunicamycin-induced ER stress as evidenced by the attenuation of phosphorylation of eukaryotic translation initiation factor-2a and glucose reactive protein-78. In addition, TUDCA improved insulin signaling in macrophages by augmenting Akt phosphorylation and blunting c-Jun N-terminal kinase activity.

CONCLUSIONS

Inhibition of macrophage ER stress may represent a potential strategy in preventing atherogenesis under diabetic conditions.

AMP-activated protein kinase (AMPK) cross-talks with canonical Wnt signaling via phosphorylation of beta-catenin at Ser 552

AMP-activated protein kinase (AMPK) is a key regulator of energy metabolism; its activity is regulated by a plethora of physiological conditions, exercises and many anti-diabetic drugs. Recent studies show that AMPK involves in cell differentiation but the underlying mechanism remains undefined. Wingless Int-1 (Wnt)/beta-catenin signaling pathway regulates the differentiation of mesenchymal stem cells through enhancing beta-catenin/T-cell transcription factor 1 (TCF) mediated transcription. The objective of this study was to determine whether AMPK cross-talks with Wnt/beta-catenin signaling through phosphorylation of beta-catenin. C3H10T1/2 mesenchymal cells were used. Chemical inhibition of AMPK and the expression of a dominant negative AMPK decreased phosphorylation of beta-catenin at Ser 552. The beta-catenin/TCF mediated transcription was correlated with AMPK activity. In vitro, pure AMPK phosphorylated beta-catenin at Ser 552 and the mutation of Ser 552 to Ala prevented such phosphorylation, which was further confirmed using [gamma-(32)P]ATP autoradiography. In conclusion, AMPK phosphorylates beta-catenin at Ser 552, which stabilizes beta-catenin, enhances beta-catenin/TCF mediated transcription, expanding AMPK from regulation of energy metabolism to cell differentiation and development via cross-talking with the Wnt/beta-catenin signaling pathway.

Overnutrition and maternal obesity in sheep pregnancy alter the JNK-IRS-1 signaling cascades and cardiac function in the fetal heart

Maternal obesity in pregnancy predisposes offspring to insulin resistance and associated cardiovascular disease. Here, we used a well-established sheep model to investigate the effects of maternal obesity on cardiac functions. Multiparous ewes were assigned to a control (CON) diet [100% of National Research Council (NRC) recommendations] or an obesogenic (OB) diet (150% of NRC recommendations) from 60 d before conception to necropsy on d 135 of pregnancy. Fetal blood glucose and insulin were increased (P<0.01, n=8) in OB (35.09+/-2.03 mg/dl and 3.40+/-1.43 microU/ml, respectively) vs. CON ewes (23.80+/-1.38 mg/dl and 0.769+/-0.256 microU/ml). Phosphorylation of AMP-activated protein kinase (AMPK), a cardioprotective signaling pathway, was reduced (P<0.05), while the stress signaling pathway, p38 MAPK, was up-regulated (P<0.05) in OB maternal and fetal hearts. Phosphorylation of c-Jun N-terminal kinase (JNK) and insulin receptor substrate-1 (IRS-1) at Ser-307 were increased (P<0.05) in OB fetal heart associated with lower downstream PI3K-Akt activity (P<0.05), indicating impaired cardiac insulin signaling. Although OB fetal hearts exhibited a normal contractile function vs. CON fetal hearts during basal perfusion, they developed an impaired heart-rate-left-ventricular-developed pressure product in response to high workload stress. Taken together, fetuses of OB mothers demonstrate alterations in cardiac PI3K-Akt, AMPK, and JNK-IRS-1 signaling pathways that would predispose them to insulin resistance and cardiac dysfunction.

Up-regulation of Toll-like receptor 4/nuclear factor-kappaB signaling is associated with enhanced adipogenesis and insulin resistance in fetal skeletal muscle of obese sheep at late gestation

Maternal obesity is increasing at an alarming rate. We previously showed that maternal obesity induces an inflammatory response and enhances adipogenesis in fetal skeletal muscle at midgestation. The objective of this study was to evaluate effects of maternal obesity on adipogenesis, inflammatory signaling, and insulin pathways at late gestation when ovine fetal skeletal muscle matures. Nonpregnant ewes were assigned to a control diet (Con, fed 100% of National Research Council nutrient recommendations, n = 6) or obesogenic diet (OB, fed 150% of National Research Council recommendations, n = 6) from 60 d before to 135 d after conception (term 148 d) when the fetal semitendenosus skeletal muscle was sampled. Expression of the adipogenic marker, peroxisome proliferator-activated receptor-gamma, was increased in OB compared with Con fetal semitendenosus muscle, indicating up-regulation of adipogenesis. More intramuscular adipocytes were observed in OB muscle. Phosphorylation of inhibitor-kappaB kinase-alpha/beta and nuclear factor-kappaB RelA/p65 were both increased in OB fetal muscle, indicating activation of nuclear factor-kappaB pathway. Phosphorylation of c-Jun N-terminal kinase and c-Jun (at Ser 63 and Ser 73) was also elevated. Toll-like receptor 4 expression was higher in OB than Con fetal muscle. Moreover, despite higher insulin concentrations in OB vs. Con fetal plasma (2.89 +/- 0.53 vs. 1.06 +/- 0.52 ng/ml; P < 0.05), phosphorylation of protein kinase B at Ser 473 was reduced, indicating insulin resistance. In conclusion, our data show maternal obesity-induced inflammatory signaling in late gestation fetal muscle, which correlates with increased im adipogenesis and insulin resistance, which may predispose offspring to later-life obesity and diabetes.

Osteoblast differentiation is functionally associated with decreased AMP kinase activity

Osteoblasts, originating from mesenchymal stem cells, play a pivotal role in bone formation and mineralization. Several transcription factors including runt-related transcription factor 2 (Runx2) have been reported to be essential for osteoblast differentiation, whereas the cytoplasmic signal transduction pathways controlling the differentiation process have not been fully elucidated. AMP-activated protein kinase (AMPK) is a serine-threonine kinase generally regarded as a key regulator of cellular energy homeostasis, polarity, and division. Recent lines of evidence have indicated that the activity of the catalytic alpha subunit of AMPK is regulated through its phosphorylation by upstream AMPK kinases (AMPKKs) including LKB1. Here, we explored the role of AMPK in osteoblast differentiation using in vitro culture models. Phosphorylation of AMPKalpha was significantly decreased during osteoblastic differentiation in both primary osteoblasts and MC3T3-E1, a mouse osteoblastic cell line. Conversely, the terminal differentiation of primary osteoblasts and MC3T3-E1 cells, represented by matrix mineralization, was significantly inhibited by glucose restriction and stimulation with metformin, both of which are known activators of AMPK. Matrix mineralization of MC3T3-E1 cells was also inhibited by the forced expression of a constitutively active form of AMPKalpha. Metformin significantly inhibited gene expression of Runx2 along with osteoblast differentiation markers including osteocalcin (Ocn), bone sialo protein (Bsp), and osteopontin (Opn). Thus, our present data indicate that differentiation of osteoblasts is functionally associated with decreased AMPK activity.

Fetal programming of skeletal muscle development in ruminant animals

Enhancing skeletal muscle growth is crucial for animal agriculture because skeletal muscle provides meat for human consumption. An increasing body of evidence shows that the level of maternal nutrition alters fetal skeletal muscle development, with long-term effects on offspring growth and performance. Fetal skeletal muscle development mainly involves myogenesis (i.e., muscle cell development), but also involves adipogenesis (i.e., adipocyte development) and fibrogenesis (i.e., fibroblast development). These tissues in fetal muscle are mainly derived from mesenchymal stem cells (MSC). Shifting the commitment of MSC from myogenesis to adipogenesis increases intramuscular fat (i.e., marbling), improving the quality grade of meats. Strong experimental evidence indicates that Wingless and Int (Wnt)/beta-catenin signaling regulates MSC differentiation. Upregulation of Wnt/beta-catenin promotes myogenesis, and downregulation enhances adipogenesis. A lack of nutrients in early to midgestation reduces the formation of secondary muscle fibers in ruminant animals. Nutrient deficiency during mid- to late gestation decreases the number of intramuscular adipocytes and muscle fiber sizes. Knowledge of this regulatory mechanism will allow the development of strategies to enhance muscle growth and marbling in offspring, especially in the setting of nutrient deficiency.

Maternal obesity, inflammation, and fetal skeletal muscle development

Maternal obesity coupled with Western-style high-energy diets represents a special problem that can result in poor fetal development, leading to harmful, persistent effects on offspring, including predisposition to obesity and type 2 diabetes. Mechanisms linking maternal obesity to the increased incidence of obesity and other metabolic diseases in offspring remain poorly defined. Because skeletal muscle is the principal site for glucose and fatty acid utilization and composes 40%-50% of total body mass, changes in the properties of offspring skeletal muscle and its mass resulting from maternal obesity may be responsible for the increase in type 2 diabetes and obesity. Fetal stage is crucial for skeletal muscle development because there is no net increase in the muscle fiber number after birth. Fetal skeletal muscle development involves myogenesis, adipogenesis, and fibrogenesis, which are all derived from mesenchymal stem cells (MSCs). Shifting commitment of MSCs from myogenesis to adipogenesis and fibrogenesis will result in increased intramuscular fat and connective tissue, as well as reduced numbers of muscle fiber and/or diameter, all of which have lasting negative effects on offspring muscle function and properties. Maternal obesity leads to low-grade inflammation, which changes the commitment of MSCs in fetal muscle through several possible mechanisms: 1) inflammation downregulates wingless and int (WNT) signaling, which attenuates myogenesis; 2) inflammation inhibits AMP-activated protein kinase, which promotes adipogenesis; and 3) inflammation may induce epigenetic modification through polycomb group proteins. More studies are needed to further explore the underlying mechanisms associated with maternal obesity, inflammation, and the commitment of MSCs.