Glyceollin has been shown to have
antidiabetic properties, although its molecular mechanism is not known. Here, we have investigated the metabolic effects of
glyceollin in animal models of
insulin resistance and in endoplasmic reticulum (ER) stress-responsive muscle cells. db/db mice were treated with
glyceollin for 4weeks and
triglycerides, total
cholesterol,
low-density lipoprotein (
LDL) and
high-density lipoprotein (HDL) levels were measured.
Glyceollin reduced serum
insulin and
triglycerides and increased HDL levels in db/db mice. Furthermore,
glyceollin caused a significant improvement in
glucose homeostasis without altering
body weight and food intake in db/db mice. In muscle cells,
glyceollin increased the activity of
AMP-activated protein kinase (AMPK) as well as cellular
glucose uptake.
Fatty acid oxidation was also increased. In parallel, phosphorylation of
acetyl-CoA carboxylase (ACC) at Ser-79 was increased, consistent with decreased ACC activity. An
insulin-resistant state was induced by exposing cells to 5μg/ml of
tunicamycin as indicated by decreased
insulin-mediated
tyrosine phosphorylation of
insulin receptor substrate-1 (IRS-1) and
glucose uptake. Inhibition of
insulin-mediated
tyrosine phosphorylation of IRS-1 and
glucose uptake under ER stress was prevented by
glyceollin. Strikingly,
glyceollin reduced ER stress-induced, c-Jun NH2-terminal
kinase activation and subsequently increased
insulin signaling via stimulation of AMPK activity in L6 myotubes. Pharmacologic inhibition or knockdown of
Ca(2+)/calmodulin-dependent protein kinase kinase blocked
glyceollin-increased AMPK phosphorylation and
insulin sensitivity under ER stress conditions. Taken together, these results indicate that
glyceollin-mediated enhancement of
insulin sensitivity under ER stress conditions is predominantly accomplished by activating AMPK, thereby having beneficial effects on
hyperglycemia and
insulin resistance.