Hyperinsulinemia contributes to
cardiac hypertrophy and
heart failure in patients with the
metabolic syndrome and
type 2 diabetes. Here, high circulating levels of
tumor necrosis factor (TNF)-α may synergize with
insulin in signaling
inflammation and
cardiac hypertrophy. We tested whether high
insulin affects activation of TNF-α-induced NF-κB and
myocardin/
serum response factor (SRF) to convey
hypertrophy signaling in cardiac myoblasts. In canine cardiac myoblasts, treatment with high
insulin (10(-8) to 10(-7) m) for 0-24 h increased
insulin receptor substrate (IRS)-1 phosphorylation at Ser-307, decreased
protein levels of chaperone-associated
ubiquitin (Ub)
E3 ligase C terminus of
heat shock protein 70-interacting
protein (CHIP), increased SRF activity, as well as β-
myosin heavy chain (MHC) and
myocardin expressions. Here siRNAs to
myocardin or NF-κB, as well as CHIP overexpression prevented (while
siRNA-mediated CHIP disruption potentiated) high
insulin-induced SR
element (SRE) activation and β-MHC expression.
Insulin markedly potentiated TNF-α-induced NF-κB activation. Compared with
insulin alone, insulin+TNF-α increased SRF/SRE binding and β-MHC expression, which was reversed by the NF-κB inhibitor
pyrrolidine dithiocarbamate (
PDTC) and by NF-κB silencing. In the hearts of db/db diabetic mice, in which Akt phosphorylation was decreased, p38MAPK, Akt1, and IRS-1 phosphorylation at Ser-307 were increased, together with
myocardin expression as well as SRE and NF-κB activities. In response to high
insulin, cardiac myoblasts increase the expression or the promyogenic
transcription factors myocardin/SRF in a CHIP-dependent manner.
Insulin potentiates TNF-α in inducing NF-κB and SRF/SRE activities. In hyperinsulinemic states,
myocardin may act as a nuclear effector of
insulin, promoting
cardiac hypertrophy.