Figure 3.

Testosterone-induced muscle fiber hypertrophy in old mice is associated with stimulation of Notch signaling and suppression of age-specific induction of CDK inhibitor p21. Panel A, Western blots of muscle lysates show increased levels of Notch 1 in muscle lysates from old animals after testosterone (T) treatment when compared with both untreated young and old mice, in which a modest expression of Notch is detected. Activation of Notch signaling after testosterone treatment is associated with increased expression of PCNA in muscle lysates. Compared with young animals, in which little or no expression is detected, a distinct age-related increase in p21 expression is detected in aged mice. Testosterone treatment at both doses effectively up-regulates both Notch 1 and PCNA but attenuates age-specific increase in p21 levels. Panels B–D, Quantification of band intensities shows robust up-regulation of Notch 1 and PCNA and complete restoration of p21 to its young levels in aged mice after testosterone treatment. Values are mean ± sem. Means with unlike letters are significantly (P < 0.05) different.