Overexpression of caveolin-3-enhanced protein synthesis rather than proteolysis inhibition in C2C12 myoblasts: relationship with myostatin activity.

Caveolin-3 (cav-3), which is involved in the regulation of signal transduction and vesicular trafficking, could interact with activin receptor IIB to inhibit myostatin (MSTN) activity and may therefore play a role in muscle development and hypertrophy. MSTN is a member of the transforming growth factor-β family, identified as a negative regulator of skeletal muscle mass. The expression of MSTN is fiber-type specific and the greatest amount of MSTN is present in fiber, which is composed of myosin heavy chain (MHC) type IIb. MSTN acts through the activin receptor IIB to activate smad2/3 which leads to an increase in gene transcription involved in muscle atrophy. Muscle hypertrophy is a consequence of two mechanisms: (1) the inhibition of proteolysis such as the calcium-dependent proteolytic system calpains and calpastatin and (2) an increase in protein synthesis through the Akt/mTOR/p70s6K pathway. In order to determine which of the two processes predominates in inhibition of MSTN activity in a cav-3 context, we transfected a C2C12 cell line with plasmids containing mstn or cav-3 wild genes. The results reported in this study demonstrate that inhibition of MSTN activity by overexpression of cav-3 induces an activation of protein synthesis rather than an inhibition of proteolysis through the calcium proteolytic system. The inhibition of phosphorylation of smad-3 due to overexpression of cav-3 causes an increase in the phosphorylation of the ribosomal protein S6, promoting the synthesis of MHC type II, probably through activation of Akt/mTOR/p70s6K. These data highlight the role of protein synthesis as the predominant mechanism in muscle hypertrophy observed when the expression of MSTN is altered and confirm the value of studying the physiological role of MSTN in the growing processes of skeletal muscle.