Knockdown of sarcolipin (SLN) impairs substrate utilization in human skeletal muscle cells

Abstract

Background Recent studies have highlighted that uncoupling of sarco-/endoplasmic reticulum Ca2+-ATPase (SERCA) by sarcolipin (SLN) increases ATP consumption and contributes to heat liberation. Exploiting this thermogenic mechanism in skeletal muscle may provide an attractive strategy to counteract obesity and associated metabolic disorders. In the present study, we have investigated the role of SLN on substrate metabolism in human skeletal muscle cells. Methods and results After generation of skeletal muscle cells with stable SLN knockdown (SLN-KD), cell viability, glucose and oleic acid (OA) metabolism, mitochondrial function, as well as gene expressions were determined. Depletion of SLN did not influence cell viability. However, glucose and OA oxidation were diminished in SLN-KD cells compared to control myotubes. Basal respiration measured by respirometry was also observed to be reduced in cells with SLN-KD. The metabolic perturbation in SLN-KD cells was reflected by reduced gene expression levels of peroxisome proliferator-activated receptor γ coactivator 1α (PGC1α) and forkhead box O1 (FOXO1). Furthermore, accumulation of OA was increased in cells with SLN-KD compared to control cells. These effects were accompanied by increased lipid formation and incorporation of OA into complex lipids. Additionally, formation of complex lipids and free fatty acid from de novo lipogenesis with acetate as substrate was enhanced in SLN-KD cells. Detection of lipid droplets using Oil red O staining also showed increased lipid accumulation in SLN-KD cells. Conclusions Overall, our study sheds light on the importance of SLN in maintaining metabolic homeostasis in human skeletal muscle. Findings from the current study suggest that therapeutic strategies involving SLN-mediated futile cycling of SERCA might have significant implications in the treatment of obesity and associated metabolic disorders. Graphical abstract

The effects of neurogranin knockdown on SERCA pump efficiency in soleus muscles of female mice fed a high fat diet

“…The SERCA pumps transport Ca2+ ions into the sarcoplasmic reticulum and are best known for playing a role in regulating muscle relaxation. Based on its structure and binding capacity, SERCA has an optimal coupling ratio of 2 Ca2+ ions pumped into the sarco(endo)plasmic reticulum per 1 ATP hydrolyzed ( 13 , 14 ) and lowering this coupling ratio can increase SERCA’s energy expenditure and combustion of metabolic substrates ( 15 – 17 ). This can be achieved through the binding of SERCA uncoupling proteins, sarcolipin (SLN), and the newly characterized neuronatin (NNAT), that both lower SERCA’s Ca2+ transport efficiency ( 18 – 20 ).…”

Obesity‐induced tissue alterations resist weight loss: A mechanistic review

“…81 In human-derived myocytes lacking sarcolipin, a marked reduction in glucose and lipid oxidation, accompanied by augmented lipogenesis and TGC storage, was observed. 85 Notably, sarcolipin-dependent SERCA activation is markedly reduced in myocytes of animals 81 and individuals with obesity. 86 While the underlying mechanism is not fully elucidated, changes in the phospholipid pattern of the sarcoplasmic reticulum in obesity may compromise the effective interaction between sarcolipin and SERCA, thereby inhibiting the functionality of the complex.…”

“…In obese animals, an inefficient activation of SERCA leads to the incapacity to sustain elevated energy expenditure rates during both thermoneutral conditions and cold exposure 81 . In human‐derived myocytes lacking sarcolipin, a marked reduction in glucose and lipid oxidation, accompanied by augmented lipogenesis and TGC storage, was observed 85 . Notably, sarcolipin‐dependent SERCA activation is markedly reduced in myocytes of animals 81 and individuals with obesity 86 .…”