Leptin rapidly activates PPARs in C2C12 muscle cells

Promotion of fatty acid metabolism and glucose metabolism in the muscle of sub-adult grass carp (Ctenopharyngodon idella): The role of alpha-linoleic acid/linoleic acid (ALA/LNA) ratios

The n6/n3 ratios improved meat quality of terrestrial animals, but alpha-linolenic acid/linoleic acid (ALA/LNA) ratios were rarely studied in aquatic animals. In this study, sub-adult grass carp (Ctenopharyngodon idella) were fed diets fed diets containing six varying ALA/LNA ratios (0.03, 0.47, 0.92, 1.33, 1.69, and 2.15) for 9 weeks and the total value of n3 + n6 (1.98) was kept constant for all six treatments. The results indicated optimal ALA/LNA ratio improved growth performance, changed fatty acid composition in grass carp muscle, and promoted glucose metabolism. Additionally, optimal ALA/LNA ratio improved chemical attributes by increasing crude protein and lipid contents, and technological attributes by increasing pH24h value and shear force in grass carp muscle. The signaling pathways related to fatty acid metabolism and glucose metabolism (LXRα/SREBP-1, PPARα, PPARγ, AMPK) might be responsible for these changes. Dietary optimal ALA/LNA ratio based on PWG, UFA and glucose contents was 1.03, 0.88 and 0.92, respectively.

DNA fragmentation induced by all-trans retinoic acid and its steroidal analogue EA-4 in C2 C12 mouse and HL-60 human leukemic cells in vitro

We have recently shown that retinoic acid induces micronucleation mainly via chromosome breakage (Alakhras et al. Cancer Lett 2011; 306: 15-26). To further study retinoic acid clastogenicity and evaluate DNA damaging potential we investigated the ability of (a) all-trans retinoic acid and its steroidal analogue EA-4 to induce DNA fragmentation by using Comet assay under alkaline unwinding and neutral condition electrophoresis, and (b) the retinoids under study to induce small (0-1 kb) DNA fragments. Two cell lines, C2C12 mouse cells and HL-60 human leukemic cells were used in this study. We found that all-trans retinoic acid and its steroidal analogue EA-4 (a) provoke DNA migration due to DNA fragmentation as it is shown by the increased values of Comet parameters, and (b) induce significantly small-size fragmented genomic DNA as indicated by the quantification of necrotic/apoptotic small DNA segments in both cell systems. A different response between the two cell lines was observed in relation to retinoid ability to increase the percentage of DNA in the tail as well as break DNA in to small fragments. Our findings confirm the ability of retinoic acid to provoke micronucleation by disrupting DNA into fragments, among which small pieces of double-stranded DNA up to 1 kb are identified.

PPARgamma inhibits NF-kappaB-dependent transcriptional activation in skeletal muscle

Skeletal muscle pathology associated with a chronic inflammatory disease state (e.g., skeletal muscle atrophy and insulin resistance) is a potential consequence of chronic activation of NF-kappaB. It has been demonstrated that peroxisome proliferator-activated receptors (PPARs) can exert anti-inflammatory effects by interfering with transcriptional regulation of inflammatory responses. The goal of the present study, therefore, was to evaluate whether PPAR activation affects cytokine-induced NF-kappaB activity in skeletal muscle. Using C(2)C(12) myotubes as an in vitro model of myofibers, we demonstrate that PPAR, and specifically PPARgamma, activation potently inhibits inflammatory mediator-induced NF-kappaB transcriptional activity in a time- and dose-dependent manner. Furthermore, PPARgamma activation by rosiglitazone strongly suppresses cytokine-induced transcript levels of the NF-kappaB-dependent genes intracellular adhesion molecule 1 (ICAM-1) and CXCL1 (KC), the murine homolog of IL-8, in myotubes. To verify whether muscular NF-kappaB activity in human subjects is suppressed by PPARgamma activation, we examined the effect of 8 wk of rosiglitazone treatment on muscular gene expression of ICAM-1 and IL-8 in type 2 diabetes mellitus patients. In these subjects, we observed a trend toward decreased basal expression of ICAM-1 mRNA levels. Subsequent analyses in cultured myotubes revealed that the anti-inflammatory effect of PPARgamma activation is not due to decreased RelA translocation to the nucleus or reduced RelA DNA binding. These findings demonstrate that muscle-specific inhibition of NF-kappaB activation may be an interesting therapeutic avenue for treatment of several inflammation-associated skeletal muscle abnormalities.

Leptin effects on testis and epididymis in the lizard Podarcis sicula, during summer regression

In this study we assessed the effect of leptin treatment on testicular morphology, spermatogenesis, Peroxisome Proliferator Activated Receptor (PPAR) alpha, 17beta-hydroxysteroide dehydrogenase, 17beta-estradiol and testosterone levels in the testis and blood of the lizard Podarcis sicula at the beginning of summer regression before entering the refractory period, when lizards no longer respond to hormonal and environmental stimuli. Lizards treated with five injections of leptin showed seminiferous tubules with germinal cells at all stages and wider lumina with respect to the controls. After 10 injections, the diameter of the lumina increased compared to the controls and 5 injection-group. After 10 injections plus 20 days before the sacrifice, the seminiferous tubules with open lumina and germinal cells were less abundant than in the 5 and 10 injection-groups. In all groups, the epididymis epithelium was higher than in the controls, with mitosis and binucleated cells. In both the control and treated animals secondary spermatocytes and spermatids were immunoreactive to leptin receptor and PPARalpha. In treated animals the interstitial cells and peritubular fibrocytes were also leptin receptor immunoreactive, while PPARalpha immunoreactivity translocated from the cytoplasm to the nucleus. 17beta-HSD immunoreactivity was present in the spermatids and interstitial cells of control lizards and in secondary spermatocytes and spermatids of treated lizards. Leptin treatment had no statistically significant effect on testicular and circulating 17beta-estradiol and testosterone levels. These observations indicate that leptin brings about a delay in testis summer regression in Podarcis sicula, playing a regulatory role in reproduction in this species as already hypothesized for mammals.

Glucocorticoid-regulated crosstalk between arachidonic acid and endocannabinoid biochemical pathways coordinates cognitive-, neuroimmune-, and energy homeostasis-related adaptations to stress

Arachidonic acid and its derivatives constitute the major group of signaling molecules involved in the innate immune response and its communication with all cellular and systemic aspects involved on homeostasis maintenance. Glucocorticoids spread throughout the organism their influences over key enzymatic steps of the arachidonic acid biochemical pathways, leading, in the central nervous system, to a shift favoring the synthesis of anti-inflammatory endocannabinoids over proinflammatory metabolites, such as prostaglandins. This shift modifies local immune-inflammatory response and neuronal activity to ultimately coordinate cognitive, behavioral, neuroendocrine, neuroimmune, physiological, and metabolic adjustments to basal and stress conditions. In the hypothalamus, a reciprocal feedback between glucocorticoids and arachidonate-containing molecules provides a mechanism for homeostatic control. This neurochemical switch is susceptible to fine-tuning by neuropeptides, cytokines, and hormones, such as leptin and interleukin-1beta, assuring functional integration between energy homeostasis control and the immune/stress response.

Leptin induces macrophage lipid body formation by a phosphatidylinositol 3-kinase- and mammalian target of rapamycin-dependent mechanism

Leptin is an adipocyte-derived hormone/cytokine that links nutritional status with neuroendocrine and immune functions. Lipid bodies (lipid droplets) are emerging as dynamic organelles with roles in lipid metabolism and inflammation. Here we investigated the roles of leptin in signaling pathways involved in cytoplasmic lipid body biogenesis and leukotriene B(4) synthesis in macrophages. Our results demonstrated that leptin directly activated macrophages and induced the formation of adipose differentiation-related protein-enriched lipid bodies. Newly formed lipid bodies were sites of 5-lipoxygenase localization and correlated with an enhanced capacity of leukotriene B(4) production. We demonstrated that leptin-induced macrophage activation was dependent on phosphatidylinositol 3-kinase (PI3K) activity, since the lipid body formation was inhibited by LY294002 and was absent in the PI3K knock-out mice. Leptin induces phosphorylation of p70(S6K) and 4EBP1 key downstream signaling intermediates of the mammalian target of rapamycin (mTOR) pathway in a rapamycin-sensitive mechanism. The mTOR inhibitor, rapamycin, inhibited leptin-induced lipid body formation, both in vivo and in vitro. In addition, rapamycin inhibited leptin-induced adipose differentiation-related protein accumulation in macrophages and lipid body-dependent leukotriene synthesis, demonstrating a key role for mTOR in lipid body biogenesis and function. Our results establish PI3K/mTOR as an important signaling pathway for leptin-induced cytoplasmic lipid body biogenesis and adipose differentiation-related protein accumulation. Furthermore, we demonstrate a previously unrecognized link between intracellular (mTOR) and systemic (leptin) nutrient sensors in macrophage lipid metabolism. Leptin-induced increased formation of cytoplasmic lipid bodies and enhanced inflammatory mediator production in macrophages may have implications for obesity-related cardiovascular diseases.

Acute, but not chronic, leptin treatment induces acyl-CoA oxidase in C2C12 myotubes

BACKGROUND

The product of the obesity gene (ob), leptin, has a well-recognized role in regulating energy homeostasis. During the period of weight maintenance, circulating leptin concentration reflects total body fat mass. On the other hand, overnutrition is accompanied by progressive hyperleptinemia. In overnourished animals, the elevation in circulating fatty acids results in increased uptake and excessive deposition of lipids within muscle cells. Consequently, triglicerydes overload seems to strongly correlate to the impairment of insulin signaling in skeletal muscle, the primary target for insulin stimulated glucose disposal. High levels of leptin in the course of fat storage may protect non-adipose tissues from lipid accumulation.

AIM OF THE STUDY

Here, we aim to evaluate in vitro the relationship between leptin treatment and expression of acyl-CoA oxidase (ACOX), a peroxisomal key enzyme involved in fatty acid catabolism. We also evaluate the adaptive response of cells to a putative oxidative insult, resulting from H(2)O(2) production.

METHODS

The effects of increasing levels of leptin, at different times, were assessed on mouse C2C12 myotubes by semiquantitative PCR. Activation pathway was investigated by using extracellular signal-regulated kinase (ERK) and cytosolic phospholipase A(2) (cPLA(2)) inhibitors. Cellular adaptive response to oxidative stress was evaluated by measuring glutathione concentration, oxidized/reduced glutathione ratio and the main antioxidant enzymatic activities.

RESULTS

A 1.8-fold increase in ACOX mRNA expression was evident at 20 ng/ml leptin, a dose comparable to that found in hyperleptinemic subjects. The induction was dose-dependent, with an increase of 3-fold at 100 ng/ml; the ability of leptin to stimulate ACOX mRNA reached a maximum at 20 min and was lost in myotubes continuously exposed for more than 1 h. ACOX enzymatic activity followed mRNA changes: it was doubled after 1 h treatment and remained elevated for 24 h. ERK and cPLA(2) pathway is involved, since their inhibitors abrogated the ACOX mRNA induction. Myotubes counteract the resulting oxidative insult by catalase and glutathione peroxidase activation, thus removing H(2)O(2) at the expenses of the reduced glutahione pool.

CONCLUSIONS

The present study shows that acute, but not chronic, leptin treatment of C2C12 myotubes induces ACOX expression. Peroxisomal fatty acid oxidation may work together with mitochondrial beta-oxidation to remove excessive lipids from non-adipose tissues, during early stages of overnutrition and before development of leptin resistance.