Fat-induced hepatic
insulin resistance plays a key role in the pathogenesis of
type 2 diabetes in obese individuals. Although PKC and inflammatory pathways have been implicated in fat-induced hepatic
insulin resistance, the sequence of events leading to impaired
insulin signaling is unknown. We used Wistar rats to investigate whether PKCδ and oxidative stress play causal roles in this process and whether this occurs via IKKβ- and JNK-dependent pathways. Rats received a 7-h infusion of
Intralipid plus
heparin (IH) to elevate circulating
free fatty acids (FFA). During the last 2 h of the infusion, a hyperinsulinemic-euglycemic clamp with tracer was performed to assess hepatic and peripheral
insulin sensitivity. An
antioxidant,
N-acetyl-L-cysteine (NAC), prevented IH-induced hepatic
insulin resistance in parallel with prevention of decreased IκBα content, increased JNK phosphorylation (markers of IKKβ and JNK activation, respectively), increased
serine phosphorylation of IRS-1 and IRS-2, and impaired
insulin signaling in the liver without affecting IH-induced hepatic PKCδ activation. Furthermore, an
antisense oligonucleotide against PKCδ prevented IH-induced phosphorylation of p47(
phox) (marker of
NADPH oxidase activation) and hepatic
insulin resistance.
Apocynin, an
NADPH oxidase inhibitor, prevented IH-induced hepatic and peripheral
insulin resistance similarly to NAC. These results demonstrate that PKCδ,
NADPH oxidase, and oxidative stress play a causal role in FFA-induced hepatic
insulin resistance in vivo and suggest that the pathway of FFA-induced hepatic
insulin resistance is FFA → PKCδ →
NADPH oxidase and oxidative stress → IKKβ/JNK → impaired hepatic
insulin signaling.