Turnover Rates of the Low-Density Lipoprotein Receptor and PCSK9: Added Dimension to the Cholesterol Homeostasis Model

Abstract

Objective: We measured the turnover rates of the LDLR (low-density lipoprotein receptor) and PCSK9 (proprotein convertase subtilisin/kexin type 9) in mice by metabolic labeling with heavy water and mass spectrometry. Approach and Results: In liver of mice fed high-cholesterol diets, LDLR mRNA levels and synthesis rates were markedly lower with complete suppression of cholesterol synthesis and higher cholesterol content, consistent with the Brown-Goldstein model of tissue cholesterol homeostasis. We observed markedly lower PCSK9 mRNA levels and synthesis rates in liver and lower concentrations and synthesis rates in plasma. Hepatic LDLR half-life (t½) was prolonged, consistent with an effect of reduced PCSK9, and resulted in no reduction in hepatic LDLR content despite reduced mRNA levels and LDLR synthesis rates. These changes in PCSK9 synthesis complement and expand the well-established model of tissue cholesterol homeostasis in mouse liver, in that reduced synthesis and levels of PCSK9 counterbalance lower LDLR synthesis by promoting less LDLR catabolism, thereby maintaining uptake of LDL cholesterol into liver despite high intracellular cholesterol concentrations. Conclusions: Lower hepatic synthesis and secretion of PCSK9, an SREBP2 (sterol response element binding protein) target gene, results in longer hepatic LDLR t½ in response to cholesterol feeding in mice in the face of high intracellular cholesterol content. PCSK9 modulation opposes the canonical lowering of LDLR mRNA and synthesis by cholesterol surplus and preserves LDLR levels. The physiological and therapeutic implications of these opposing control mechanisms over liver LDLR are of interest and may reflect subservience of hepatic cholesterol homeostasis to whole body cholesterol needs.

Cold shock domain–containing protein E1 is a posttranscriptional regulator of the LDL receptor

“…Whether this difference between our models reflects hepatocyte physiology within an intact organism or instead a species-specific difference between the HDL-predominant mouse ( 69 , 70 ) and the LDL-predominant human remains to be determined. Our data are consistent with metabolic labeling experiments demonstrating the homeostatic defense of steady-state hepatic LDLR in the mouse in response to cholesterol loading conditions and altered LDLR transcription ( 78 ). Our data are also consistent with mouse experiments that markedly perturb Ldlr mRNA but have comparatively muted effects on LDLR protein ( 66 , 79 ).…”

The LEPIS-HuR-TMOD4 axis regulates hepatic cholesterol homeostasis and accelerates atherosclerosis

“…In this study, the increase in LDL-C in plasma was mainly due to impaired cholesterol uptake in hepatocytes, and the final targets of LEPIS and TMOD4 were PCSK9 and LDLR. In cholesterol homeostasis, LDLR primarily mediates the uptake of cholesterol-containing LDL particles from the blood by peripheral cells (Dandan, Han et al, 2021). The PCSK9-LDLR complex is directed to lysosomes and degraded, blocking LDLR recycling and the endocytosis of LDL particles (Luo et al, 2020).…”

Regulation of cholesterol homeostasis in health and diseases: from mechanisms to targeted therapeutics

“… 137 , 138 LDLR transcription is mainly regulated by SREBP2 and can respond to changes of intracellular cholesterol. 90 PCSK9 reduces LDLR expression in the post-translational manner. It binds to LDLR to induce LDLR entry into cells for lysosomal degradation and inhibits the ability of LDL uptake in the liver.…”

“… 89 Macrophage cholesterol uptake is mainly mediated through multiple scavenger receptors, the molecules lack of SRE, rather than LDLR. 90 Thus, without feedback control mechanisms, macrophage scavenger receptors may uptake cholesterol unlimitedly in patients with hypercholesterolemia. Macrophages scavenger receptors include scavenger receptor A1 (SR-A1), SR-BI, lectin-like oxidized LDL receptor 1 (LOX-1), CD36 and so on.…”