Caveolae-mediated effects of TNF-α on human skeletal muscle cells

Acute strength exercise training impacts differently the HERV-W expression and inflammatory biomarkers in resistance exercise training individuals

BACKGROUND

Human Endogenous Retroviruses (HERVs) are fossil viruses that composes 8% of the human genome and plays several important roles in human physiology, including muscle repair/myogenesis. It is believed that inflammation may also regulate HERV expression, and therefore may contribute in the muscle repair, especially after training exercise. Hence, this study aimed to assess the level of HERVs expression and inflammation profile in practitioners' resistance exercises after an acute strength training session.

METHODS

Healthy volunteers were separated in regular practitioners of resistance exercise training group (REG, n = 27) and non-trained individuals (Control Group, n = 20). All individuals performed a strength exercise section. Blood samples were collected before the exercise (T0) and 45 minutes after the training session (T1). HERV-K (HML1-10) and W were relatively quantified, cytokine concentration and circulating microparticles were assessed.

RESULTS

REG presented higher level of HERV-W expression (~2.5 fold change) than CG at T1 (p<0.01). No difference was observed in the levels of HERV-K expression between the groups as well as the time points. Higher serum TNF-α and IL-10 levels were verified post-training session in REG and CG (p<0.01), and in REG was found a positive correlation between the levels of TNF-α at T1 and IL-10 at T0 (p = 0.01). Finally, a lower endothelial microparticle percentage was observed in REG at T1 than in T0 (p = 0.04).

CONCLUSION

REG individuals exhibited a significant upregulation of HERV-W and modulation of inflammatory markers when compared to CG. This combined effect could potentially support the process of skeletal muscle repair in the exercised individuals.

Regulation of Death Receptor Signaling by S-Palmitoylation and Detergent-Resistant Membrane Micro Domains-Greasing the Gears of Extrinsic Cell Death Induction, Survival, and Inflammation

Death-receptor-mediated signaling results in either cell death or survival. Such opposite signaling cascades emanate from receptor-associated signaling complexes, which are often formed in different subcellular locations. The proteins involved are frequently post-translationally modified (PTM) by ubiquitination, phosphorylation, or glycosylation to allow proper spatio-temporal regulation/recruitment of these signaling complexes in a defined cellular compartment. During the last couple of years, increasing attention has been paid to the reversible cysteine-centered PTM S-palmitoylation. This PTM regulates the hydrophobicity of soluble and membrane proteins and modulates protein:protein interaction and their interaction with distinct membrane micro-domains (i.e., lipid rafts). We conclude with which functional and mechanistic roles for S-palmitoylation as well as different forms of membrane micro-domains in death-receptor-mediated signal transduction were unraveled in the last two decades.

Phagocytosis mediated by scavenger receptor class BI promotes macrophage transition during skeletal muscle regeneration

Macrophages play an essential role in skeletal muscle regeneration. The phagocytosis of muscle cell debris induces a switch of pro-inflammatory macrophages into an anti-inflammatory phenotype, but the cellular receptors mediating this phagocytosis are still unclear. In this paper, we report novel roles for SRB1 (scavenger receptor class BI) in regulating macrophage phagocytosis and macrophage phenotypic transitions for skeletal muscle regeneration. In a mouse model of cardiotoxin-induced muscle injury/regeneration, infiltrated macrophages expressed a high level of SRB1. Using SRB1 knockout mice, we observed the impairment of muscle regeneration along with decreased myogenin expression and increased matrix deposit. Bone marrow transplantation experiments indicated that SRB1 deficiency in bone marrow cells was responsible for impaired muscle regeneration. Compared with WT mice, SRB1 deficiency increased pro-inflammatory macrophage number and pro-inflammatory gene expression and decreased anti-inflammatory macrophage number and anti-inflammatory gene expression in injured muscle. In vitro, SRB1 deficiency led to a strong decrease in macrophage phagocytic activity on myoblast debris. SRB1-deficient macrophages easily acquired an M1 phenotype and failed to acquire an M2 phenotype in lipopolysaccharide/myoblast debris activation. Furthermore, SRB1 deficiency promoted activation of ERK1/2 MAPK signaling in macrophages stimulated with lipopolysaccharide/myoblast debris. Taken together, SRB1 in macrophages regulates phagocytosis and promotes M1 switch into M2 macrophages, contributing to muscle regeneration.

Palmitoylation is required for TNF-R1 signaling

BACKGROUND

Binding of tumor necrosis factor (TNF) to TNF-receptor 1 (TNF-R1) can induce either cell survival or cell death. The selection between these diametrically opposed effects depends on the subcellular location of TNF-R1: plasma membrane retention leads to survival, while endocytosis leads to cell death. How the respective TNF-R1 associated signaling complexes are recruited to the distinct subcellular location is not known. Here, we identify palmitoylation of TNF-R1 as a molecular mechanism to achieve signal diversification.

METHODS

Human monocytic U937 cells were analyzed. Palmitoylated proteins were enriched by acyl resin assisted capture (AcylRAC) and analyzed by western blot and mass spectrometry. Palmitoylation of TNF-R1 was validated by metabolic labeling. TNF induced depalmitoylation and involvement of APT2 was analyzed by enzyme activity assays, pharmacological inhibition and shRNA mediated knock-down. TNF-R1 palmitoylation site analysis was done by mutated TNF-R1 expression in TNF-R1 knock-out cells. Apoptosis (nuclear DNA fragmentation, caspase 3 assays), NF-κB activation and TNF-R1 internalization were used as biological readouts.

RESULTS

We identify dynamic S-palmitoylation as a new mechanism that controls selective TNF signaling. TNF-R1 itself is constitutively palmitoylated and depalmitoylated upon ligand binding. We identified the palmitoyl thioesterase APT2 to be involved in TNF-R1 depalmitoylation and TNF induced NF-κB activation. Mutation of the putative palmitoylation site C248 interferes with TNF-R1 localization to the plasma membrane and thus, proper signal transduction.

CONCLUSIONS

Our results introduce palmitoylation as a new layer of dynamic regulation of TNF-R1 induced signal transduction at a very early step of the TNF induced signaling cascade. Understanding the underlying mechanism may allow novel therapeutic options for disease treatment in future.