Benzo[a]pyrene inhibits myoblast differentiation through downregulating the Hsp70-K2-p38MAPK complex

Kinesin light chain 1 stabilizes insulin receptor substrate 1 to regulate the IGF-1-AKT signaling pathway during myoblast differentiation

The IGF signaling pathway plays critical role in regulating skeletal myogenesis. We have demonstrated that KIF5B, the heavy chain of kinesin-1 motor, promotes myoblast differentiation through regulating IGF-p38MAPK activation. However, the roles of the kinesin light chain (Klc) in IGF pathway and myoblast differentiation remain elusive. In this study, we found that Klc1 was upregulated during muscle regeneration and downregulated in senescence mouse muscles and dystrophic muscles from mdx (X-linked muscular dystrophic) mice. Gain- and loss-of-function experiments further displayed that Klc1 promotes AKT-mTOR activity and positively regulates myogenic differentiation. We further identified that the expression levels of IRS1, the critical node of IGF-1 signaling, are downregulated in Klc1-depleted myoblasts. Coimmunoprecipitation study revealed that IRS1 interacted with the 88-154 amino acid sequence of Klc1 via its PTB domain. Notably, the reduced Klc1 levels were found in senescence and osteoporosis skeletal muscle samples from both mice and human. Taken together, our findings suggested a crucial role of Klc1 in the regulation of IGF-AKT pathway during myogenesis through stabilizing IRS1, which might ultimately influence the development of muscle-related disorders.

Down-regulated Smyd1 participated in the inhibition of myoblast differentiation induced by cigarette smoke extract

The histone methyltransferase Smyd1 is essential for muscle development; however, its role in smoking-induced skeletal muscle atrophy and dysfunction has not been investigated thus far. In this study, Smyd1 was overexpressed or knocked down in C2C12 myoblasts by an adenovirus vector and cultured in differentiation medium containing 5% cigarette smoke extract (CSE) for 4 days. CSE exposure resulted in inhibition of C2C12 cell differentiation and downregulation of Smyd1 expression, whereas Smyd1 overexpression reduced the degree of inhibition of myotube differentiation caused by CSE exposure. CSE exposure activated P2RX7-mediated apoptosis and pyroptosis, caused increased intracellular reactive oxygen species (ROS) levels, and impaired mitochondrial biogenesis and increased protein degradation by downregulating PGC1α, whereas Smyd1 overexpression partially restored the altered protein levels caused by CSE exposure. Smyd1 knockdown alone produced a phenotype similar to CSE exposure, and Smyd1 knockdown during CSE exposure aggravated the degree of inhibition of myotube differentiation and the degree of activation of P2RX7. CSE exposure suppressed H3K4me2 expression, and chromatin immunoprecipitation confirmed the transcriptional regulation of P2rx7 by H3K4me2 modification. Our findings suggest that CSE exposure mediates C2C12 cell apoptosis and pyroptosis through the Smyd1-H3K4me2-P2RX7 axis, and inhibits PGC1α expression to impair mitochondrial biosynthesis and increase protein degradation by inhibiting Smyd1 expression, ultimately leading to abnormal C2C12 myoblasts differentiation and impaired myotube formation.