Enhanced survival of skeletal muscle myoblasts in response to overexpression of cold shock protein RBM3.

Cold-inducible RNA-binding protein (RBM3) is suggested to be involved in the regulation of skeletal muscle mass. Cell death pathways are implicated in the loss of muscle mass and therefore the role of RBM3 in muscle apoptosis in C(2)C(12) myoblasts was investigated in this study. RBM3 overexpression was induced by either cold shock (32°C exposure for 6 h) or transient transfection with a myc-tagged RBM3 expression vector. Cell death was induced by H(2)O(2) (1,000 µM) or staurosporine (StSp, 5 µM), and it was shown that cold shock and RBM3 transfection were associated with attenuation of morphological changes and an increase in cell viability compared with normal temperature or empty vector, respectively. No changes in proliferation were observed with either cold shock or RBM3 transfection. DNA fragmentation was not increased in response to H(2)O(2), and a cell permeability assay indicated that cell death in response to H(2)O(2) is more similar to necrosis than apoptosis. RBM3 overexpression reduced apoptosis and the collapse of the membrane potential in response to StSp. Moreover, the increase in caspase-3, -8, and -9 activities in response to StSp was returned to control levels with RBM3 overexpression. These results indicate that increased RBM3 expression decreases muscle cell necrosis as well as apoptosis and therefore RBM3 could potentially serve as an intervention for the loss of muscle cell viability during muscle atrophy and muscle diseases.

Cell death-resistance of differentiated myotubes is associated with enhanced anti-apoptotic mechanisms compared to myoblasts.

Skeletal muscle atrophy is associated with elevated apoptosis while muscle differentiation results in apoptosis resistance, indicating that the role of apoptosis in skeletal muscle is multifaceted. The objective of this study was to investigate mechanisms underlying apoptosis susceptibility in proliferating myoblasts compared to differentiated myotubes and we hypothesized that cell death-resistance in differentiated myotubes is mediated by enhanced anti-apoptotic pathways. C(2)C(12) myoblasts and myotubes were treated with H(2)O(2) or staurosporine (Stsp) to induce cell death. H(2)O(2) and Stsp induced DNA fragmentation in more than 50% of myoblasts, but in myotubes less than 10% of nuclei showed apoptotic changes. Mitochondrial membrane potential dissipation was detected with H(2)O(2) and Stsp in myoblasts, while this response was greatly diminished in myotubes. Caspase-3 activity was 10-fold higher in myotubes compared to myoblasts, and Stsp caused a significant caspase-3 induction in both. However, exposure to H(2)O(2) did not lead to caspase-3 activation in myoblasts, and only to a modest induction in myotubes. A similar response was observed for caspase-2, -8 and -9. Abundance of caspase-inhibitors (apoptosis repressor with caspase recruitment domain (ARC), and heat shock protein (HSP) 70 and -25 was significantly higher in myotubes compared to myoblasts, and in addition ARC was suppressed in response to Stsp in myotubes. Moreover, increased expression of HSPs in myoblasts attenuated cell death in response to H(2)O(2) and Stsp. Protein abundance of the pro-apoptotic protein endonuclease G (EndoG) and apoptosis-inducing factor (AIF) was higher in myotubes compared to myoblasts. These results show that resistance to apoptosis in myotubes is increased despite high levels of pro-apoptotic signaling mechanisms, and we suggest that this protective effect is mediated by enhanced anti-caspase mechanisms.

Age-related changes of cell death pathways in rat extraocular muscle.

Changes in the structure and function of aging non-locomotor muscles remains understudied, despite their importance for daily living. Extraocular muscles (EOMs) have a high incidence of age-related mitochondrial defects possibly because of the metabolic stress resulting from their fast and constant activity. Apoptosis and autophagy (type I and II cell death, respectively) are linked to defects in mitochondrial function and contribute to sarcopenia in hind limb muscles. Therefore, we hypothesized that apoptosis and autophagy are altered with age in the EOMs. Muscles from 6-, 18-, and 30-month-old male Fisher 344-Brown Norway rats were used to investigate type I cell death, caspase-3, -8, -9, and -12 activity, and type II cell death. Apoptosis, as measured by TUNEL positive nuclei, and mono- and oligo-nucleosomal content, did not change with age. Similarly, caspase-3, -8, -9, and -12 activity was not affected by aging. By contrast, autophagy, as estimated by gene expression of Atg5 and Atg7, and protein abundance of LC3 was lower in EOMs of aged rats. Based on these data, we suggest that the decrease in autophagy with age leads to the accumulation of damaged organelles, particularly mitochondria, which results in the decrease in function observed in EOM with age.