This study investigated in a pancreatic alpha-cell line the effects of chronic exposure to
palmitate on the
insulin and
IGF-I receptor (IGF-IR) and intracellular
insulin pathways. alpha-TC1-6 cells were cultured in the presence or absence of
palmitate (0.5 mmol/liter) up to 48 h.
Glucagon secretion, insulin and IGF-IR autophosphorylation, and
insulin receptor substrate (IRS)-1, IRS-2,
phosphatidylinositol kinase (PI3K) (p85 alpha), and
serine-threonine protein kinase (Akt) phosphorylated (active) forms were measured. Erk 44/42 and p38 phosphorylation (P) (MAPK pathway markers) were also measured. Because MAPK can regulate Pax6, a
transcription factor that controls
glucagon expression, paired box gene 6 (Pax6) and
glucagon gene and
protein expression were also measured. Basal
glucagon secretion was increased and the inhibitory effect of acute
insulin exposure reduced in alpha-TC1 cells cultured with
palmitate.
Insulin-stimulated
insulin receptor phosphorylation was greatly reduced by exposure to
palmitate. Similar results were observed with IRS-1-P, PI3K (p85 alpha), and Akt-P. In contrast, with IGF-IR and IRS-2-P, the basal levels (i.e. in the absence of
insulin stimulation) were higher in cells cultured with
palmitate. Similar data were obtained with Erk 44/42-P and p-38-P. Pax6 and
glucagon gene and
protein expression were higher in cells cultured with
palmitate. In these cells cultured, specifics MAPKs inhibitors were able to reduce both Pax6 and
glucagon gene and
protein expression. These results indicate that alpha-cells exposed to
palmitate show
insulin resistance of the IRS-1/PI3K/Akt pathway that likely controls
glucagon secretion. In contrast, the IRS-2/MAPKs pathway is stimulated, through an activation of the IGF-IR, leading to increased Pax6 and
glucagon expression. Our data support the hypothesis that the chronic elevation of
fatty acids contribute to alpha-cell dysregulation frequently observed in
type 2 diabetes.