AICAR and metformin, but not exercise, increase muscle glucose transport through AMPK-, ERK-, and PDK1-dependent activation of atypical PKC

Activators of 5′- AMP-activated protein kinase( AMPK) 5-aminoimidazole-4-carboxamide-1-β-d-ribofuranoside (AICAR), metformin, and exercise activate atypical protein kinase C (aPKC) and ERK and stimulate glucose transportin muscle by uncertain mechanisms. Here, in cultured L6 myotubes: AICAR- and metformin-induced activation of AMPKwas required for activation of aPKC and ERK; aPKC activation involved and required phosphoinositide-dependent kinase 1(PDK1) phosphorylation of Thr410-PKC-ζ; aPKC Thr410 phosphorylation and activation also required MEK1-dependent ERK; and glucose transporteffects of AICAR and metforminwere inhibited by expressionof dominant-negative AMPK, kinase-inactive PDK1, MEK1 inhibitors, kinase-inactive PKC-ζ, and RNA interference (RNAi)-mediated knockdown of PKC-ζ. In mice, muscle-specific aPKC (PKC-λ) depletion by conditional gene targeting impaired AICAR-stimulated glucose disposal and stimulatory effects of both AICAR and metforminon 2- deoxyglucose/ glucose uptakein muscle in vivo and AICAR stimulation of 2-[3H] deoxyglucoseuptake in isolated extensor digitorum longus muscle; however, AMPKactivation was unimpaired. In marked contrast to AICAR and metformin, treadmill exercise-induced stimulation of 2- deoxyglucose/ glucose uptakewas not inhibited in aPKC-knockout mice. Finally, in intact rodents, AICAR and metforminactivated aPKC in muscle, but not in liver, despite activating AMPKin both tissues. The findings demonstrate that in muscle AICAR and metforminactivate aPKC via sequential activation of AMPK, ERK, and PDK1 and the AMPK/ERK/PDK1/aPKC pathway is required for metformin- and AICAR-stimulated increases in glucose transport. On the other hand, although aPKC is activated by treadmill exercise, this activation is not required for exercise-induced increases in glucose transport, and therefore may be a redundant mechanism.