SMAD3 Negatively Regulates Serum Irisin and Skeletal Muscle FNDC5 and Peroxisome Proliferator-activated Receptor γ Coactivator 1-α (PGC-1α) during Exercise

Beige adipose cells are a distinct and inducible type of thermogenic fat cell that express the mitochondrial uncoupling protein-1 and thus represent a powerful target for treating obesity. Mice lacking the TGF-β effector protein SMAD3 are protected against diet-induced obesity because of browning of their white adipose tissue (WAT), leading to increased whole body energy expenditure. However, the role SMAD3 plays in WAT browning is not clearly understood. Irisin is an exercise-induced skeletal muscle hormone that induces WAT browning similar to that observed in SMAD3-deficient mice. Together, these observations suggested that SMAD3 may negatively regulate irisin production and/or secretion from skeletal muscle. To address this question, we used wild-type and SMAD3 knock-out (Smad3^−/−) mice subjected to an exercise regime and C2C12 myotubes treated with TGF-β, a TGF-β receptor 1 pharmacological inhibitor, adenovirus expressing constitutively active SMAD3, or siRNA against SMAD3. We find that in Smad3^−/− mice, exercise increases serum irisin and skeletal muscle FNDC5 (irisin precursor) and its upstream activator peroxisome proliferator-activated receptor γ coactivator 1-α (PGC-1α) to a greater extent than in wild-type mice. In C2C12 myotubes, TGF-β suppresses FNDC5 and PGC-1α mRNA and protein levels via SMAD3 and promotes SMAD3 binding to the FNDC5 and PGC-1α promoters. These data establish that SMAD3 suppresses FNDC5 and PGC-1α in skeletal muscle cells. These findings shed light on the poorly understood regulation of irisin/FNDC5 by demonstrating a novel association between irisin and SMAD3 signaling in skeletal muscle.     

Decorin Induces Mitophagy in Breast Carcinoma Cells via Peroxisome Proliferator-activated Receptor γ Coactivator-1α (PGC-1α) and Mitostatin

Tumor cell mitochondria are key biosynthetic hubs that provide macromolecules for cancer progression and angiogenesis. Soluble decorin protein core, hereafter referred to as decorin, potently attenuated mitochondrial respiratory complexes and mitochondrial DNA (mtDNA) in MDA-MB-231 breast carcinoma cells. We found a rapid and dynamic interplay between peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α) and the decorin-induced tumor suppressor gene, mitostatin. This interaction stabilized mitostatin mRNA with concurrent accumulation of mitostatin protein. In contrast, siRNA-mediated abrogation of PGC-1α-blocked decorin-evoked stabilization of mitostatin. Mechanistically, PGC-1α bound MITOSTATIN mRNA to achieve rapid stabilization. These processes were orchestrated by the decorin/Met axis, as blocking the Met-tyrosine kinase or knockdown of Met abrogated these responses. Furthermore, depletion of mitostatin blocked decorin- or rapamycin-evoked mitophagy, increased vascular endothelial growth factor A (VEGFA) production, and compromised decorin-evoked VEGFA suppression. Collectively, our findings underscore the complexity of PGC-1α-mediated mitochondrial homeostasis and establish mitostatin as a key regulator of tumor cell mitophagy and angiostasis.     

Sphingosine Kinase 1 Is Regulated by Peroxisome Proliferator-activated Receptor α in Response to Free Fatty Acids and Is Essential for Skeletal Muscle Interleukin-6 Production and Signaling in Diet-induced Obesity

We previously demonstrated that sphingosine kinase 1 (Sphk1) expression and activity are up-regulated by exogenous palmitate (PAL) in a skeletal muscle model system and in diet-induced obesity in mice; however, potential functions and in vivo relevance of this have not been addressed. Here, we aimed to determine the mechanism by which PAL regulates SphK1 in muscle, and to determine potential roles for its product, sphingosine-1-phosphate (S1P), in muscle biology in the context of obesity. Cloning and analysis of the mouse Sphk1 promoter revealed a peroxisome proliferator-activated receptor (PPAR) α cis-element that mediated activation of a reporter under control of the Sphk1 promoter; direct interaction of PPARα was demonstrated by chromatin immunoprecipitation. PAL treatment induced the proinflammatory cytokine interleukin (IL)-6 in a manner dependent on SphK1, and this was attenuated by inhibition of the sphingosine-1-phosphate receptor 3 (S1PR3)_. Diet-induced obesity in mice demonstrated that IL-6 expression in muscle, but not adipose tissue, increased in obesity, but this was attenuated in Sphk1^−/− mice. Moreover, plasma IL-6 levels were significantly decreased in obese Sphk1^−/− mice relative to obese wild type mice, and muscle, but not adipose tissue IL-6 signaling was activated. These data indicate that PPARα regulates Sphk1 expression in the context of fatty acid oversupply and links PAL to muscle IL-6 production. Moreover, this function of SphK1 in diet-induced obesity suggests a potential role for SphK1 in obesity-associated pathological outcomes.     

Na+/K+-ATPase α-subunit (nkaα) Isoforms and Their mRNA Expression Levels,Overall Nkaα Protein Abundance,and Kinetic Properties of Nka in the Skeletal Muscle and Three Electric Organs of the Electric Eel,Electrophorus electricus

This study aimed to obtain the coding cDNA sequences of Na⁺/K⁺-ATPase α (nkaα) isoforms from, and to quantify their mRNA expression in, the skeletal muscle (SM), the main electric organ (EO), the Hunter’s EO and the Sach’s EO of the electric eel, Electrophorus electricus. Four nkaα isoforms (nkaα1c1, nkaα1c2, nkaα2 and nkaα3) were obtained from the SM and the EOs of E. electricus. Based on mRNA expression levels, the major nkaα expressed in the SM and the three EOs of juvenile and adult E. electricus were nkaα1c1 and nkaα2, respectively. Molecular characterization of the deduced Nkaα1c1 and Nkaα2 sequences indicates that they probably have different affinities to Na⁺ and K⁺. Western blotting demonstrated that the protein abundance of Nkaα was barely detectable in the SM, but strongly detected in the main and Hunter’s EOs and weakly in the Sach’s EO of juvenile and adult E. electricus. These results corroborate the fact that the main EO and Hunter’s EO have high densities of Na⁺ channels and produce high voltage discharges while the Sach’s EO produces low voltage discharges. More importantly, there were significant differences in kinetic properties of Nka among the three EOs of juvenile E. electricus. The highest and lowest V _max of Nka were detected in the main EO and the Sach’s EO, respectively, with the Hunter’s EO having a V _max value intermediate between the two, indicating that the metabolic costs of EO discharge could be the highest in the main EO. Furthermore, the Nka from the main EO had the lowest K_m (or highest affinity) for Na⁺ and K⁺ among the three EOs, suggesting that the Nka of the main EO was more effective than those of the other two EOs in maintaining intracellular Na⁺ and K⁺ homeostasis and in clearing extracellular K⁺ after EO discharge.