Hepatic lipase release is inhibited by a purinergic induction of autophagy

We have shown that extracellular adenosine diphosphate (ADP) affects lipoprotein secretion from liver cells by stimulating cellular autophagic degradation. In this study, we investigated the effect of ADP and cellular autophagy on hepatic lipase (HL) release from human liver cells.

Depletion of media serum stimulates an autophagic response in liver cells, which parallels an 8-fold increase in the release of ADP into the media and a complete inhibition of HL release. Treatment of cells with exogenous ADP stimulates cellular autophagy and also blocks HL release. Treatment with the autophagic stimulant and proteasomal inhibitor, ALLN (25 µM), reduces cellular HL levels and blocks HL release at 4h. In contrast, treatment with the autophagy inhibitor, 3-methyladenine (3-MA) (5 mM), increases cellular HL levels and stimulates HL release. ADP acts through the G-protein coupled receptor, P2Y13, to stimulate autophagy. siRNA-targeted reduction in P2Y13 protein expression stimulates the release of HL by 5 to 8-fold, while overexpression of P2Y13 blocks HL release. HL release from liver cells is therefore inhibited by a purinergic induction of autophagy. To evaluate the effect of extracellular ADP on the processing of HL, we expressed a V5-epitope tag-labeled HL (HL-V5) and then measured secretion, uptake and degradation. Two isoforms of HL-V5, at 62 and 68 kDa, are released from HepG2 cells, but only the 62 kDa protein undergoes reuptake / internalization. The 62 kDa HL-V5 isoform progressively accumulates in the cell over 24h, with no detectible modification or degradation. Treatment of liver cells with ADP has no effect on HL-V5 internalization or degradation at 30 min and 4h.

These studies show that extracellular nucleotides act to prevent HL accumulation in the media by stimulating cellular autophagic degradation and blocking HL release.