The small heat shock protein alpha B-crystallin negatively regulates apoptosis during myogenic differentiation by inhibiting caspase-3 activation.
Myoblasts respond to growth factor deprivation either by differentiating into multinucleated myotubes or by undergoing apoptosis; hence, the acquisition of apoptosis resistance by myogenic precursors is essential for their development. Here we demonstrate that the expression of the small heat shock protein alpha B-crystallin is selectively induced in C2C12 myoblasts that are resistant to differentiation-induced apoptosis, and we show that this induction occurs at an early stage in their differentiation in vitro. In contrast, the expression of several known anti-apoptotic proteins (FLIP, XIAP, Bcl-x(L)) was not altered during myogenesis. We also demonstrate that ectopic expression of alpha B-crystallin, but not the closely related small heat shock protein Hsp27, renders C2C12 myoblasts resistant to differentiation-induced apoptosis. Furthermore, we show that the myopathy-causing R120G alpha B-crystallin mutant is partly impaired in its cytoprotective function, whereas a pseudophosphorylation alpha B-crystallin mutant that mimics stress-induced phosphorylation is completely devoid of anti-apoptotic activity. Finally, we demonstrate that alpha B-crystallin negatively regulates apoptosis during myogenesis by inhibiting the proteolytic activation of caspase-3, whereas the R120G and pseudophosphorylation mutants are defective in this function. Taken together, our findings indicate that alpha B-crystallin is a novel negative regulator of myogenic apoptosis that directly links the differentiation program to apoptosis resistance.
Kamradt
Merideth C
MC
Center for Endocrinology, Metabolism, and Molecular Medicine, Department of Medicine, Feinberg School of Medicine, Northwestern University, Chicago, Illinois 60611, USA.
Chen
Feng
F
Sam
Susan
S
Cryns
Vincent L
VL
D000818:Animals; D017209:Apoptosis / Q000502:physiology*; D053148:Caspase 3; D061945:Caspase Inhibitors; D020169:Caspases / Q000378:metabolism*; D002454:Cell Differentiation / Q000502:physiology*; D002460:Cell Line; D004789:Enzyme Activation; D004791:Enzyme Inhibitors / Q000378:metabolism; D006801:Humans; D051379:Mice; D018482:Muscle, Skeletal / Q000166:cytology / Q000502:physiology; D032446:Myoblasts / Q000502:physiology*; D038203:alpha-Crystallin B Chain / Q000235:genetics / Q000378:metabolism*
D061945:Caspase Inhibitors; D004791:Enzyme Inhibitors; D038203:alpha-Crystallin B Chain; C505407:CASP3 protein, human; C505409:Casp3 protein, mouse; D053148:Caspase 3; D020169:Caspases