Role of the ubiquitin-proteasome system in the regulation of P2Y13 receptor expression: impact on hepatic HDL uptake

The protective effect of high density lipoproteins (HDL) against atherosclerosis is mainly attributed to their capacity to transport excess cholesterol from peripheral tissues back to the liver for further elimination into the bile, a process called reverse cholesterol transport (RCT). Recently, the importance of the P2Y(13) receptor (P2Y(13)-R) was highlighted in HDL metabolism since HDL uptake by the liver was decreased in P2Y(13)-R deficient mice, which translated into impaired RCT. Here, we investigated for the first time the molecular mechanisms regulating cell surface expression of P2Y(13)-R. When transiently expressed, P2Y(13)-R was mainly detected in the endoplasmic reticulum (ER) and strongly subjected to proteasome degradation while its homologous P2Y(12) receptor (P2Y(12)-R) was efficiently targeted to the plasma membrane. We observed an inverse correlation between cell surface expression and ubiquitination level of P2Y(13)-R in the ER, suggesting a close link between ubiquitination of P2Y(13)-R and its efficient targeting to the plasma membrane. The C-terminus tail exchange between P2Y(13)-R and P2Y(12)-R strongly restored plasma membrane expression of P2Y(13)-R, suggesting the involvement of the intra-cytoplasmic tail of P2Y(13)-R in expression defect. Accordingly, proteasomal inhibition increased plasma membrane expression of functionally active P2Y(13)-R in hepatocytes, and consequently stimulated P2Y(13)-R-mediated HDL endocytosis. Importantly, proteasomal inhibition strongly potentiated HDL hepatic uptake (> 200 %) in wild-type but not in P2Y(13)-R-deficient mice, thus reinforcing the role of P2Y(13)-R expression in regulating HDL metabolism. Therefore, specific inhibition of the ubiquitin-proteasome system might be a novel powerful HDL therapy to enhance P2Y(13)-R expression and consequently promote the overall RCT.