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Department of Oral Growth and Developmental Biology, Hiroshima University Graduate School of Biomedical Sciences, Hiroshima 734-8553, Japan
Myogenic differentiation
in vitro has been usually viewed as being negatively controlled by
serum mitogens. A depletion of critical serum components from medium
has been considered to be essential for permanent withdrawal from the
cell cycle and terminal differentiation of myoblasts. Removal of serum
mitogens induces the expression of insulin-like growth factors (IGFs),
whereas it inhibits that of basic fibroblast growth factor (bFGF) and transforming growth factor (TGF)-
in myoblasts. These responses of
growth factors to medium conditioning seem to be well matched to their
functions in proliferation/differentiation. In the present study, we
showed that C2C12 myoblasts differentiated
actively, even in mitogen-rich medium, and that this medium offered an
advantage over mitogen-poor medium in terms of increasing
differentiation. Our attention focused on endogenous growth factors, as
described above, especially IGFs in mitogen-rich medium. During
differentiation, IGF-I and IGF-II mRNA levels increased, but bFGF and
TGF-
1 mRNAs decreased. Differentiation was commensurable
with IGF mRNA levels and suppressed by antisense oligodeoxynucleotides
and neutralizing monoclonal antibodies against IGFs. These results
suggest that an autocrine/paracrine loop of IGFs, bFGF, and
TGF-
1 is active in proliferating and differentiating
C2C12 cells without a depletion of serum and
that endogenous IGFs actively override the negative control of
differentiation by serum mitogens.
myoblasts; insulin-like growth factor I; insulin-like growth factor II; serum mitogens; differentiation
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