Premium
Activation of S6K1 during myoblast differentiation inhibits the formation of myotubes independent of IRS‐1
Author(s) -
Hamilton D. Lee,
Pravin John,
Baar Keith
Publication year - 2006
Publication title -
the faseb journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.709
H-Index - 277
eISSN - 1530-6860
pISSN - 0892-6638
DOI - 10.1096/fasebj.20.5.a820-c
Subject(s) - myogenesis , p70 s6 kinase 1 , c2c12 , microbiology and biotechnology , myosin , cellular differentiation , myocyte , myosin light chain kinase , kinase , chemistry , pi3k/akt/mtor pathway , biology , signal transduction , biochemistry , gene
The mammalian target of rapamycin (mTOR) is believed to be essential for myoblast differentiation and primary myotube formation, but neither its kinase activity nor the activity of its downstream target, S6K1, are required. The activity of S6K1 may however play a role in secondary myotube formation and increases in cell size. To evaluate the role of S6K1, and its recently discovered downstream target SKAR, in these processes we overexpressed three mutants of S6K1: wild type; constitutively active; and kinase dead, and wild type SKAR in C 2 C 12 cells using the myosin light chain 2 fast promoter (that begins to be expressed during differentiation). Expression of wild type or constitutively active S6K1 at the beginning of differentiation inhibited the formation of myotubes. Expression of wild type SKAR had a similar effect inhibiting myotube formation and the expression of myosin heavy chain (a marker of myoblast differentiation). The effects of SKAR on differentiation occurred in the absence of changes in IRS‐1 levels. In contrast to these results, expression of the kinase dead S6K1 promoted more rapid formation of myotubes. These myotubes, however, had a visibly smaller diameter than controls. Our results suggest that S6K1 activity at the beginning of differentiation inhibits normal myotube formation. The inhibitory effect of S6K1 is likely mediated through its downstream target, SKAR and not through feedback inhibition of IRS‐1 mediated signaling. While S6K1 prevents differentiation, its activation after myotube formation is necessary for the normal increase in cell size following fusion. Work funded by DARPA (Navy) N66001 ‐02‐C8034 and the Wellcome Trust 077426/Z05/Z.