Regulation of human satellite cells in vitro via inflammatory cytokines and myogenic transcription factors across proliferation and differentiation

Skeletal muscle is a primary contributor to body mass and whole-body energy metabolism. It is an adaptive tissue with the ability to fluctuate in size and mechanical properties in response to stimulus. Health conditions involving chronic elevated inflammation levels often feature metabolic inflexibility and losses in skeletal muscle mass. Mononuclear stem cells, termed satellite cells, are mitotic and serve to donate nuclei to muscle fibers to enable skeletal muscle adaptation. Despite the well-characterized nature of satellite cell activation, proliferation, and differentiation, the underlying mechanisms regulating this process is not fully understood. Recent characterization of cytokines secreted by skeletal muscle in an endocrine type fashion has led to discoveries of inflammatory cytokines influencing satellite cell function. However, how the autocrine production and secretion of these cytokines during proliferation and differentiation in humans and their correlation with myogenic transcription factors is not well understood. Our study used satellite cells cultured from the vastus lateralis of 12 male human research subjects, and ELISA analysis to measure levels of TNF-α and IL-6 across proliferation, early differentiation, and late differentiation. Additionally, mRNA levels of Pax7, MyoD, myogenin, IL-6, TNF-α, and TGF-β were assessed in satellite cells cultured from a subset of two endurance trained and two sedentary individuals from the larger group of 12 human subjects. The novelty of our study is the large number of human research subjects and simultaneous analysis of inflammatory cytokine secretion, mRNA inflammatory cytokine expression, and myogenic transcription factor mRNA expression. Results showed an 83% decrease in IL-6 protein secretion 24 hours after exposure to differentiation media (p-value <0.05) before increasing 50-fold after 7 day of exposure to differentiation media (p-value < 0.05). Myogenin and TGF-β mRNA expression levels were positively correlated (R2 = 0.5814, p-value < 0.0001). A negative correlation was found between IL-6 and MyoD (R2 = 0.2473, p – value = 0.0257). After 1 day of exposure to differentiation media, satellite cells from endurance trained subjects exhibited higher levels of TGF-β mRNA expression compared to sedentary satellite cells of sedentary subjects of the same age and levels of adiposity (p-value < 0.05). Results support a potential relationship in humans satellite cells between myogenic transcription factors and inflammatory cytokines, however, further study is necessary in order to investigate the underlying mechanisms behind the correlations.