Extracellular nucleotides inhibit insulin receptor signaling, stimulate autophagy and control lipoprotein secretion

Hyperglycemia is associated with abnormal plasma lipoprotein metabolism and with an elevation in circulating nucleotide levels. We evaluated how extracellular nucleotides may act to perturb hepatic lipoprotein secretion. Adenosine diphosphate (ADP) (>10 µM) acts like a proteasomal inhibitor to stimulate apoB100 secretion and inhibit apoA-I secretion from human liver cells at 4 h and 24 h. ADP blocks apoA-I secretion by stimulating autophagy. The nucleotide increases cellular levels of the autophagosome marker, LC3-II, and increases co-localization of LC3 with apoA-I in punctate autophagosomes. ADP affects autophagy and apoA-I secretion through P2Y(13). Overexpression of P2Y(13) increases cellular LC3-II levels by ~50% and blocks induction of apoA-I secretion. Conversely, a siRNA-induced reduction in P2Y(13) protein expression of 50% causes a similar reduction in cellular LC3-II levels and a 3-fold stimulation in apoA-I secretion. P2Y(13) gene silencing blocks the effects of ADP on autophagy and apoA-I secretion. A reduction in P2Y(13) expression suppresses ERK1/2 phosphorylation, increases the phosphorylation of IR-β and protein kinase B (Akt) >3-fold, and blocks the inhibition of Akt phosphorylation by TNFα and ADP. Conversely, increasing P2Y(13) expression significantly inhibits insulin-induced phosphorylation of insulin receptor (IR-β) and Akt, similar to that observed after treatment with ADP. Nucleotides therefore act through P2Y(13), ERK1/2 and insulin receptor signaling to stimulate autophagy and affect hepatic lipoprotein secretion.