siRNA knockdown of alanine aminopeptidase impairs myoblast proliferation and differentiation

Inhibition of methionine aminopeptidase in C2C12 myoblasts disrupts cell integrity via increasing endoplasmic reticulum stress

Proteasome-dependent protein degradation and the digestion of peptides by aminopeptidases are essential for myogenesis. Methionine aminopeptidases (MetAPs) are uniquely involved in, both, the proteasomal degradation of proteins and in the regulation of translation (via involvement in post-translational modification). Suppressing MetAP1 and MetAP2 expression inhibits the myogenic differentiation of C2C12 myoblasts. However, the molecular mechanism by which inhibiting MetAPs impairs cellular function remains to be elucidated. Here, we provide evidence for our hypothesis that MetAPs regulate proteostasis and that their inhibition increases ER stress by disrupting the post-translational modification, and thereby compromises cell integrity. Thus, using C2C12 myoblasts, we investigate the effect of inhibiting MetAPs on cell proliferation and the molecular mechanisms underpinning its effects. We found that exposure to bengamide B (a MetAP inhibitor) caused C2C12 myoblasts to lose their proliferative abilities via cell cycle arrest. The underlying mechanism involved the accumulation of abnormal proteins (due to the decrease in the N-terminal methionine removal function) which led to increased endoplasmic reticulum stress, decreased protein synthesis, and a protective activation of the autophagy pathway. To identify the MetAP involved in these effects, we use siRNAs to specifically knockdown MetAP1 and MetAP2 expressions. We found that only MetAP2 knockdown mimicked the effects seen with bengamide B treatment. Thus, we suggest that MetAP2, rather than MetAP1, is involved in maintaining the integrity of C2C12 myoblasts. Our results are useful in understanding muscle regeneration, obesity, and overeating disorders. It will help guide new treatment strategies for these disorders.

The aminopeptidase LAP3 suppression accelerates myogenic differentiation via the AKT-TFE3 pathway in C2C12 myoblasts

Skeletal muscle maintenance depends largely on muscle stem cells (satellite cells) that supply myoblasts required for muscle regeneration and growth. The ubiquitin-proteasome system is the major intracellular protein degradation pathway. We previously reported that proteasome dysfunction in skeletal muscle significantly impairs muscle growth and development. Furthermore, the inhibition of aminopeptidase, a proteolytic enzyme that removes amino acids from the termini of peptides derived from proteasomal proteolysis, impairs the proliferation and differentiation ability of C2C12 myoblasts. However, no evidence has been reported on the role of aminopeptidases with different substrate specificities on myogenesis. In this study, therefore, we investigated whether the knockdown of aminopeptidases in differentiating C2C12 myoblasts affects myogenesis. The knockdown of the X-prolyl aminopeptidase 1, aspartyl aminopeptidase, leucyl-cystinyl aminopeptidase, methionyl aminopeptidase 1, methionyl aminopeptidase 2, puromycine-sensitive aminopeptidase, and arginyl aminopeptidase like 1 gene in C2C12 myoblasts resulted in defective myogenic differentiation. Surprisingly, the knockdown of leucine aminopeptidase 3 (LAP3) in C2C12 myoblasts promoted myogenic differentiation. We also found that suppression of LAP3 expression in C2C12 myoblasts resulted in the inhibition of proteasomal proteolysis, decreased intracellular branched-chain amino acid levels, and enhanced mTORC2-mediated AKT phosphorylation (S473). Furthermore, phosphorylated AKT induced the translocation of TFE3 from the nucleus to the cytoplasm, promoting myogenic differentiation through increased expression of myogenin. Overall, our study highlights the association of aminopeptidases with myogenic differentiation.

Puromycin-sensitive aminopeptidase is required for C2C12 myoblast proliferation and differentiation

The ubiquitin-proteasome system is a major protein degradation pathway in the cell. Proteasomes produce several peptides that are rapidly degraded to free amino acids by intracellular aminopeptidases. Our previous studies reported that proteolysis via proteasomes and aminopeptidases is required for myoblast proliferation and differentiation. However, the role of intracellular aminopeptidases in myoblast proliferation and differentiation had not been clarified. In this study, we investigated the effects of puromycin-sensitive aminopeptidase (PSA) on C2C12 myoblast proliferation and differentiation by knocking down PSA. Aminopeptidase enzymatic activity was reduced in PSA-knockdown myoblasts. Knockdown of PSA induced impaired cell cycle progression in C2C12 myoblasts and accumulation of cells at the G2/M phase. Additionally, after the induction of myogenic differentiation in PSA-knockdown myoblasts, multinucleated circular-shaped myotubes with impaired cell polarity were frequently identified. Cell division cycle 42 (CDC42) knockdown in myoblasts resulted in a loss of cell polarity and the formation of multinucleated circular-shaped myotubes, which were similar to PSA-knockdown myoblasts. These data suggest that PSA is required for the proliferation of myoblasts in the growth phase and for the determination of cell polarity and elongation of myotubes in the differentiation phase.