Palmitic Acid Impairs Myogenesis and Alters Temporal Expression of miR-133a and miR-206 in C2C12 Myoblasts

RNA Sequencing Reveals the Inhibitory Effect of High Levels of Arachidonic Acid and Linoleic Acid on C2C12 Differentiation and Myogenic Biomarkers

Over the past three decades, studies have shown that consuming polyunsaturated fatty acids (PUFAs) can enhance animal and human health and welfare through biological, biochemical, pathological, and pharmacological impacts. Furthermore, omega-6 plays key roles in the cardiopulmonary system, including promoting airway relaxation and inhibiting atherosclerosis and hypertension. However, findings from investigations of the effects of omega-6 fatty acids on molecular and cellular activity and discussions on their influence on biomarkers are still unclear. Therefore, the present study aimed to evaluate omega-6 fatty acids, the arachidonic acid (AA), and linoleic acid (LA) effects on C2C12 proliferation, myogenesis morphology, and relative myogenic biomarker expression through the Wnt pathway. C2C12 cells were cultured with and without 25, 50, 100, and 150 µM of LA and AA and then subjected to CCK8, Giemsa staining, RT qPCR, Western blotting, and RNA Sequencing. The CCK8 Assay results showed that 25, 50, 100, and 150 µM LA significantly decreased the viability after 72 h for 25, 50, 100, and 150 µM concentrations. Also, AA supplementation decreased cell viability after 24 h for 150 µM, 48 h for 150 µM, and 72 h for 50, 100, and 150 µM concentrations. Moreover, the LA and AA inhibitory effects noticed through Gimesa staining were morphological changes during myoblast differentiation. Both LA and AA showed inhibiting IGF1, Cola1, Col6a2, Col6a1, Itga10, Itga11, SFRP2, DAAM2, and NKD2 effects; however, the depressing effect was higher for AA compared to LA. The previous results were confirmed through Western blotting, which showed that 50 µM LA and AA significantly reduced DAAM2 and SFRP2 protein levels compared to the control. Regarding RNA sequencing results, LA and AA increased the number of differentially expressed (DE) Mt-rRNA and snoRNA; however, the numbers of lncRNA detected decreased compared to the control. Our findings demonstrate that high and moderate LA and AA concentrations reduce primary myoblast proliferation and differentiation. Also, they highlight novel biomarkers and regulatory factors to improve our understanding of how the nutrition of fatty acids can control and modulate the myogenesis and differentiation process through different biomarker families.

Palmitate-Induced Inflammation and Myotube Atrophy in C2C12 Cells Are Prevented by the Whey Bioactive Peptide, Glycomacropeptide

BACKGROUND

Metabolic diseases are often associated with muscle atrophy and heightened inflammation. The whey bioactive compound, glycomacropeptide (GMP), has been shown to exhibit anti-inflammatory properties and therefore may have potential therapeutic efficacy in conditions of skeletal muscle inflammation and atrophy.

OBJECTIVES

The purpose of this study was to determine the role of GMP in preventing lipotoxicity-induced myotube atrophy and inflammation.

METHODS

C2C12 myoblasts were differentiated to determine the effect of GMP on atrophy and inflammation and to explore its mechanism of action in evaluating various anabolic and catabolic cellular signaling nodes. We also used a lipidomic analysis to evaluate muscle sphingolipid accumulation with the various treatments. Palmitate (0.75 mM) in the presence and absence of GMP (5 μg/mL) was used to induce myotube atrophy and inflammation and cells were collected over a time course of 6-24 h.

RESULTS

After 24 h of treatment, GMP prevented the palmitate-induced decrease in the myotube area and myogenic index and the increase in the TLR4-mediated inflammatory genes tumor necrosis factor-α and interleukin 1β. Moreover, phosphorylation of Erk1/2, and gene expression of myostatin, and the E3 ubiquitin ligases, FBXO32, and MuRF1 were decreased with GMP treatment. GMP did not alter palmitate-induced ceramide or diacylglycerol accumulation, muscle insulin resistance, or protein synthesis.

CONCLUSIONS

In summary, GMP prevented palmitate-induced inflammation and atrophy in C2C12 myotubes. The GMP protective mechanism of action in muscle cells during lipotoxic stress may be related to targeting catabolic signaling associated with cellular stress and proteolysis but not protein synthesis.

Linoleic acid-induced ANGPTL4 inhibits C2C12 skeletal muscle differentiation by suppressing Wnt/β-catenin

Skeletal muscle differentiation is an essential process in embryonic development as well as regeneration and repair throughout the lifespan. It is well-known that dietary fat intake impacts biological and physiological function in skeletal muscle, however, understanding of the contribution of nutritional factors in skeletal muscle differentiation is limited. Therefore, the objective of the current study was to evaluate the effects of free fatty acids (FFAs) on skeletal muscle differentiation in vitro. We used C2C12 murine myoblasts and treated them with various FFAs, which revealed a unique response of angiopoietin-like protein-4 (ANGPTL4) with linoleic acid (LA) treatment that was associated with reduced differentiation. LA significantly inhibited myotube formation and lowered the protein expression of myogenic regulatory factors, including MyoD and MyoG and increased Pax7 during cell differentiation. Next, recombinant ANGPTL4 protein or siRNA knockdown of ANGPTL4 was employed to examine its role in skeletal muscle differentiation, and we confirmed that ANGPTL4 knockdown at day 2 and -6 of differentiation restored myotube formation in the presence of LA. RNA-sequencing analysis revealed that ANGPTL4-mediated inhibition of skeletal muscle differentiation at day 2 as well as LA at day 2 or -6 led to a reduction in Wnt/β-catenin signaling pathways. We confirmed that LA reduced Wnt11 and Axin2 while increasing expression of the Wnt inhibitor, Dkk2. ANGPTL4 knockdown increased β-catenin protein in the nucleus in response to LA and increased Axin2 and Wnt11 expression. Taken together, these results demonstrate that LA induced ANGPTL4 inhibits C2C12 differentiation by suppressing Wnt/β-catenin signaling.

Integration of Transcriptomics and Non-Targeted Metabolomics Reveals the Underlying Mechanism of Skeletal Muscle Development in Duck during Embryonic Stage

Skeletal muscle is an important economic trait in duck breeding; however, little is known about the molecular mechanisms of its embryonic development. Here, the transcriptomes and metabolomes of breast muscle of Pekin duck from 15 (E15_BM), 21 (E21_BM), and 27 (E27_BM) days of incubation were compared and analyzed. The metabolome results showed that the differentially accumulated metabolites (DAMs), including the up-regulated metabolites, l-glutamic acid, n-acetyl-1-aspartylglutamic acid, l-2-aminoadipic acid, 3-hydroxybutyric acid, bilirubin, and the significantly down-regulated metabolites, palmitic acid, 4-guanidinobutanoate, myristic acid, 3-dehydroxycarnitine, and s-adenosylmethioninamine, were mainly enriched in metabolic pathways, biosynthesis of secondary metabolites, biosynthesis of cofactors, protein digestion and absorption, and histidine metabolism, suggesting that these pathways may play important roles in the muscle development of duck during the embryonic stage. Moreover, a total of 2142 (1552 up-regulated and 590 down-regulated), 4873 (3810 up-regulated and 1063 down-regulated), and 2401 (1606 up-regulated and 795 down-regulated) DEGs were identified from E15_BM vs. E21_BM, E15_BM vs. E27_BM and E21_BM vs. E27_BM in the transcriptome, respectively. The significantly enriched GO terms from biological processes were positive regulation of cell proliferation, regulation of cell cycle, actin filament organization, and regulation of actin cytoskeleton organization, which were associated with muscle or cell growth and development. Seven significant pathways, highly enriched by FYN, PTK2, PXN, CRK, CRKL, PAK, RHOA, ROCK, INSR, PDPK1, and ARHGEF, were focal adhesion, regulation of actin cytoskeleton, wnt signaling pathway, insulin signaling pathway, extracellular matrix (ECM)-receptor interaction, cell cycle, and adherens junction, which participated in regulating the development of skeletal muscle in Pekin duck during the embryonic stage. KEGG pathway analysis of the integrated transcriptome and metabolome indicated that the pathways, including arginine and proline metabolism, protein digestion and absorption, and histidine metabolism, were involved in regulating skeletal muscle development in embryonic Pekin duck. These findings suggested that the candidate genes and metabolites involved in crucial biological pathways may regulate muscle development in the Pekin duck at the embryonic stage, and increased our understanding of the molecular mechanisms underlying the avian muscle development.

Increased Expression of Syncytin-1 in Skeletal Muscle of Humans With Increased Body Mass Index

Obesity negatively impacts skeletal muscle protein metabolism, and also impairs skeletal muscle maintenance and regeneration. We analyzed muscle biopsy samples from humans with increased body mass index (BMI) (i.e. > 30 kg/m²) and controls (i.e., BMI < 25 kg/m²) for expression of syncytin-1, a fusogenic protein regulating skeletal muscle regeneration. When compared to controls, humans with increased BMI and concomitant reduction in muscle protein synthesis had higher expression of syncytin-1 in skeletal muscle (p < 0.05). Across human subjects, muscle protein synthesis correlated inversely (r = −0.51; p = 0.03) with syncytin-1 expression in muscle. Using a C2C12 cell line we found that expression of syncytin-A (i.e, corresponding protein in murine tissue) is increased by insulin, and that this response is impaired in the presence of fatty acids, whose metabolism is altered within the metabolic environment induced by increased BMI. In C2C12 cells, the response of the protein 4E-BP1, which signals increase in protein synthesis in muscle, resembled that of syncytin-A. These findings provide novel insights into the expression of syncytin-1 in skeletal muscle of humans with increased BMI, as well as its basic regulation by insulin and fatty acids in muscle. The findings signify the need for further research into the regulation of syncytin-1 in skeletal muscle of humans with increased BMI, as well as its biological implications for altering muscle protein metabolism and regeneration.

Free Fatty Acid Species Differentially Modulate the Inflammatory Gene Response in Primary Human Skeletal Myoblasts

Simple Summary

Epidemiological studies show that obesity increases the risk of muscle mass loss with age, a syndrome called sarcopenic obesity. Obesity leads to increased free fatty acids (FFAs) and excessive fat deposits, which impair the integrity of skeletal muscles by unknown mechanisms. This report indicates that FFAs directly affect human skeletal muscle cell replication and inflammatory gene expression. The structural characteristics of FFAs play a decisive role in triggering both processes. Thus, the characterization of abundant FFA species in the skeletal muscle of obese individuals may become a useful tool to predict the progression of sarcopenic obesity.

Abstract

Age-related loss of skeletal muscle is associated with obesity and inflammation. In animal models, intramuscular fat deposits compromise muscle integrity; however, the relevant fat components that mediate muscular inflammation are not known. Previously, we hypothesized that free fatty acids (FFAs) may directly induce inflammatory gene expression in skeletal muscle cells of obese rats. Here, we examined this hypothesis in primary human skeletal myoblasts (SkMs) using multiplex expression analysis of 39 inflammatory proteins in response to different FFA species. Multiplex mRNA quantification confirmed that the IL6, IL1RA, IL4, LIF, CXCL8, CXCL1, CXCL12 and CCL2 genes were differentially regulated by saturated and unsaturated C16 or C18 FFAs. Fluorescence staining revealed that only saturated C16 and C18 strongly interfere with myoblast replication independent of desmin expression, mitochondrial abundance and oxidative activity. Furthermore, we addressed the possible implications of 71 human receptor tyrosine kinases (RTKs) in FFA-mediated effects. Phosphorylated EphB6 and TNK2 were associated with impaired myoblast replication by saturated C16 and C18 FFAs. Our data suggest that abundant FFA species in human skeletal muscle tissue may play a decisive role in the progression of sarcopenic obesity by affecting inflammatory signals or myoblast replication.

Palmitic Acid-Induced miR-429-3p Impairs Myoblast Differentiation by Downregulating CFL2

MicroRNAs are known to play a critical role in skeletal myogenesis and maintenance, and cofilin-2 (CFL2) is necessary for actin cytoskeleton dynamics and myogenic differentiation. Nonetheless, target molecules and the modes of action of miRNAs, especially those responsible for the inhibitory mechanism on the myogenesis by saturated fatty acids (SFA) or obesity, still remain unclear. Here, we reported the role played by miR-429-3p on CFL2 expression, actin filament dynamics, myoblast proliferation, and myogenic differentiation in C2C12 cells. Palmitic acid (PA), the most abundant SFA in diet, inhibited the myogenic differentiation of myoblasts, accompanied by CFL2 reduction and miR-429-3p induction. Interestingly, miR-429-3p suppressed the expression of CFL2 by targeting the 3'UTR of CFL2 mRNA directly. Transfection of miR-429-3p mimic in myoblasts increased F-actin formation and augmented nuclear YAP level, thereby promoting cell cycle progression and myoblast proliferation. Moreover, miR-429-3p mimic drastically suppressed the expressions of myogenic factors, such as MyoD, MyoG, and MyHC, and impaired myogenic differentiation of C2C12 cells. Therefore, this study unveiled the crucial role of miR-429-3p in myogenic differentiation through the suppression of CFL2 and provided implications of SFA-induced miRNA in the regulation of actin dynamics and skeletal myogenesis.