| Elevated circulating free fatty acids (FFA) cause hepatic insulin resistance; however, the mechanisms for this process are incompletely understood. The objective of the studies in the thesis was to examine whether protein kinase C (PKC)-delta (delta), oxidative stress, and the serine kinase IkappaBalpha kinase (IKK) beta are causally involved in FFA-induced hepatic insulin resistance. To test this, we infused rats with lipid with or without inhibitors of the aforementioned factors for 7h, during the last 2h of which a hyperinsulinemic-euglycemic clamp was performed. In Study 1, inhibition of hepatic PKC-delta using antisense oligonucleotide prevented FFA-induced membrane translocation of PKC-delta, which is a marker of its activation, in parallel with prevention of lipid-induced hepatic insulin resistance, without affecting lipid-induced peripheral insulin resistance. These results implicate PKC-delta as a causal mediator of FFA-induced hepatic insulin resistance. In Study 2, the antioxidant N-acetyl-L-cysteine (NAC) prevented lipid-induced hepatic insulin resistance in conjunction with reversal of lipid-induced increase in markers of IKKbeta and c-Jun NH 2-terminal kinase 1 (JNK1) activation, and of impairment of insulin signaling, without affecting PKC-delta membrane translocation and increase in phosphorylated p38 mitogen-activated protein kinase (MAPK) induced by lipid infusion. These findings suggested that oxidative stress is a causal mediator of lipid-induced hepatic insulin resistance upstream of IKKbeta and JNK1, and potentially downstream of PKC-delta and p38 MAPK. In Study 3, sodium salicylate, an IKKbeta inhibitor, prevented FFA-induced hepatic insulin resistance via restoration of hepatic insulin signaling, thus implicating IKKbeta as a causal factor in the process. Together, the results from these studies demonstrate that PKC-delta, oxidative stress, and IKKbeta are causally involved in FFA-induced hepatic insulin resistance and suggest that the sequence for the process is: FFA → PKC-delta → oxidative stress → IKKbeta → impaired hepatic insulin signaling. |
