Endoplasmic Reticulum Stress and Ca2+ Depletion Differentially Modulate the Sterol Regulatory Protein PCSK9 to Control Lipid Metabolism

Abstract

Edited by Roger J. ColbranAccumulating evidence implicates endoplasmic reticulum (ER) stress as a mediator of impaired lipid metabolism, thereby contributing to fatty liver disease and atherosclerosis. Previous studies demonstrated that ER stress can activate the sterol regulatory element-binding protein-2 (SREBP2), an ER-localized transcription factor that directly up-regulates sterol regulatory genes, including PCSK9. Given that PCSK9 contributes to atherosclerosis by targeting low density lipoprotein (LDL) receptor (LDLR) degradation, this study investigates a novel mechanism by which ER stress plays a role in lipid metabolism by examining its ability to modulate PCSK9 expression. Herein, we demonstrate the existence of two independent effects of ER stress on PCSK9 expression and secretion. In cultured HuH7 and HepG2 cells, agents or conditions that cause ER Ca 2؉ depletion, including thapsigargin, induced SREBP2-dependent up-regulation of PCSK9 expression. In contrast, a significant reduction in the secreted form of PCSK9 protein was observed in the media from both thapsigargin-and tunicamycin (TM)-treated HuH7 cells, mouse primary hepatocytes, and in the plasma of TMtreated C57BL/6 mice. Furthermore, TM significantly increased hepatic LDLR expression and reduced plasma LDL concentrations in mice. Based on these findings, we propose a model in which ER Ca 2؉ depletion promotes the activation of SREBP2 and subsequent transcription of PCSK9. However, conditions that cause ER stress regardless of their ability to dysregulate ER Ca 2؉ inhibit PCSK9 secretion, thereby reducing PCSK9-mediated LDLR degradation and promoting LDLR-dependent hepatic cholesterol uptake. Taken together, our studies provide evidence that the retention of PCSK9 in the ER may serve as a potential strategy for lowering LDL cholesterol levels.

Loss-of-function PCSK9 mutants evade the unfolded protein response sensor GRP78 and fail to induce endoplasmic reticulum stress when retained

“…This form of PCSK9 SV fails to undergo autocatalytic cleavage and secretion as demonstrated previously (22). Our data demonstrate that sterol deprivation, which induces PCSK9 expression (13), was necessary to attain the SSO-induced formation of ER-resident PCSK9 SV (Fig. 3A).…”

“…There is now ample evidence supporting the notion that the retention of PCSK9 in the ER leads to a significant reduction of circulating LDL levels. In addition, we have recently demonstrated that ER stress blocks the secretion of PCSK9 in cultured hepatocytes and mice, and this was associated with increased hepatic LDLR expression and reduced circulating LDL levels (13). Furthermore, the retention of PCSK9 in the ER via GRP94 promotes elevated hepatic LDLR levels (14).…”

“…To investigate the effect of PCSK9 variants on UPR activation, HuH7 cells cotransfected with the ER activated indicator (ERAI) plasmid in addition to plasmids encoding a variety of PCSK9 variants were examined via immunoblotting of GRP78 and CHOP. The ERAI plasmid encodes an ER stressinducible FLAG-XBP1 and was used as an additional tool to characterize UPR activation in our model (13,28). A group of cells were also treated with the ER stress-inducing agent thapsigargin (TG; 100 nM) to serve as a benchmark of UPR activation.…”

Snapshots: Endoplasmic Reticulum Stress in Lipid Metabolism and Cardiovascular Disease

“…PCSK9 can contribute to atherosclerosis by targeting LDL receptor degradation. However, tunicamycin blocks PCSK9 secretion and reduces the LDL cholesterol content in plasma (Lebeau et al, 2017). Altogether, these findings suggest that ERS may interfere with lipid metabolism and that blockade of ERS may ameliorate disturbances in lipid metabolism (Figure 1).…”

4‐Phenylbutyrate protects against atherosclerotic lesion growth by increasing the expression of HSP25 in macrophages and in the circulation of Apoe −/− mice

“…THP-1 or Apoe 2/2 mouse peritoneal macrophages were treated in the absence or presence of either 1 or 10 mM 4-PBA for 24 h followed by immunofluorescence microscopy as previously described by Lebeau et al (27). Briefly, cells were fixed with 4% paraformaldehyde and blocked with 1% bovine serum albumin for 30 min.…”