We have previously shown that mitochondrial activity increases in response to
insulin in differentiating muscle cells. Moreover, the
protein kinase kinase/extracellular-signal-regulated kinase (
MAPKK/ERK-
MEK) inhibitor
PD98059 accelerates
insulin-mediated myogenesis, whereas the
phosphatidylinositol 3-kinase (PI3-K) inhibitor
LY294002 or blockade of mitochondrial respiration abrogates
insulin-mediated myogenesis. Our present study focuses on the mitochondrial transmembrane
protein,
hyperplasia suppressor gene/mitofusin2 (HSG/Mfn2), which regulates both mitochondrial fusion (as demonstrated by perinuclear mitochondria clustering) and
insulin-dependent myogenesis in vitro. Increased mitochondrial length and interconnectivity are not observed after the inhibition of PI3-K activity with
LY294002.
Insulin induces Mfn2 and subunits I and IV of
cytochrome-c oxidase (MTCOI and NCOIV) in L6 myoblasts. Inhibition of the
MEK-dependent signalling pathway elevates the Mfn-2
protein level. The molecular mechanism of this phenomenon is unknown, although immunoprecipitation studies indicate that, during
insulin-mediated myogenesis,
Ras protein (an upstream activator of the MAPK/ERK1/2 cascade) interacts with HSG/Mfn2 in muscle cells. Interaction of Ras with Mfn2 continues unless
insulin is present and is reduced after
PD98059 co-treatment indicating that
insulin-mediated myogenesis is increased by the inhibition of
MEK, most probably by the lack of mitogenic signals opposing muscle differentiation. We conclude that
insulin-mediated myogenesis depends on PI3-K activity, which stimulates mitochondrial activity and the extensive fusion of mitochondria. We further suggest that
insulin stimulates the expression of Mfn2
protein, which in turn binds to Ras and inhibits the
MEK-dependent signalling pathway. At the same time, the PI3-K-dependent signalling pathway is boosted, mitochondrial respiration increases and the rate of myogenesis is accelerated.