Circulating Proprotein Convertase Subtilisin/Kexin 9 (PCSK9) Regulates VLDLR Protein and Triglyceride Accumulation in Visceral Adipose Tissue

Abstract

Objective-Proprotein convertase subtilisin/kexin 9 (PCSK9) promotes the degradation of the low-density lipoprotein receptor (LDLR), and its gene is the third locus implicated in familial hypercholesterolemia. Herein, we investigated the role of PCSK9 in adipose tissue metabolism. Methods and Results-At 6 months of age, Pcsk9Ϫ/Ϫ mice accumulated Ϸ80% more visceral adipose tissue than wild-type mice. This was associated with adipocyte hypertrophy and increased in vivo fatty acid uptake and ex vivo triglyceride synthesis. Moreover, adipocyte hypertrophy was also observed in Pcsk9 Ϫ/Ϫ Ldlr Ϫ/Ϫ mice, indicating that the LDLR is not implicated. Rather, we show here by immunohistochemistry that Pcsk9 Ϫ/Ϫ males and females exhibit 4-and Ϸ40-fold higher cell surface levels of very-low-density lipoprotein receptor (VLDLR) in perigonadal depots, respectively. Expression of PCSK9 in the liver of Pcsk9 Ϫ/Ϫ females reestablished both circulating PCSK9 and normal VLDLR levels. In contrast, specific inactivation of PCSK9 in the liver of wild-type females led to Ϸ50-fold higher levels of perigonadal VLDLR. Key Words: PCSK9 Ⅲ VLDL receptor Ⅲ adipose tissue metabolism Ⅲ fatty acid uptake Ⅲ proprotein convertase P roprotein convertase subtilisin/kexin 9 (PCSK9) is the ninth member of the family of proprotein convertases (encoded by the genes PCSK1 to PCSK9) that share identity with subtilisin and kexin. 1 The first 8 members of the family cleave protein precursors of hormones, growth factors, receptors, and transmembrane transcription factors that transit through or reside in the secretory pathway. 2 In contrast, PCSK9 has no known substrates but itself. It undergoes an autocatalytic cleavage of its N-terminal prosegment, 1,3 which remains trapped in the catalytic pocket. 4 PCSK9 shortens the half-life of the low-density lipoprotein receptor (LDLR), 5 a process independent of its catalytic activity. 6 Gain-of-function mutations in the PCSK9 gene lead to autosomal dominant hypercholesterolemia, 7 as do mutations in the LDLR and APOB genes. Other PCSK9 mutations or polymorphisms responsible for loss of function result in hypocholesterolemia. 8 PCSK9 is highly expressed in the liver, 1 where it binds the LDLR and promotes its internalization and degradation in endosomal/lysosomal compartments. 5 Thus, mice lacking PCSK9 (Pcsk9 Ϫ/Ϫ ) exhibit a 2-to 3-fold increase of the LDLR protein in liver homogenates, and a substantial accumulation of the receptor at the hepatocyte cell surface. 9,10 This leads to hypocholesterolemia (Ϫ40%), with a Ϸ5-fold drop in low-density lipoprotein (LDL) cholesterol levels. In humans, where 70% of cholesterol is associated with LDLs, versus only 30% in mice, the hypocholesterolemia due to PCSK9 deficiency is even more dramatic. Two women lacking functional PCSK9 exhibited LDL cholesterol levels of Ϸ0.4 mmol/L (15 mg/dL). 11,12 PCSK9 is now considered a promising target to treat hypercholesterolemia and prevent coronary heart disease. Conclusion-InHuman PCSK9 is abundant in plasma, 13 and analysis of mice tha...

Pcsk9

“…For example, in contrast to the LDLR, the GOF D374Y PCSK9 does not degrade these other receptors more efficiently than wild-type PCSK9. 42 The receptors LDLR, VLDLR, and ApoER2 have been confirmed as PCSK9 target proteins in mice 25,26,65,66 and the LDLR in monkeys 67 and human. 68 Recently, we showed that PCSK9 can enhance the degradation of LRP1 in various cells 44 although proof of this activity in vivo is still lacking.…”

“…Finally, CD36, a scavenger receptor with multiple ligands and cellular functions, including facilitating cellular uptake of free fatty acids (FFA), was also suspected to be a PCSK9 target in intestinal epithelial cells 69 and adipose tissue. 65 Recently, this PCSK9 activity on CD36 was elegantly proven in cells and adipocytes. 70 because the structures of CD36 or CD81 (see below) do not exhibit an EGF-A-like domain, their respective sequences that bind PCSK9 either directly or indirectly have yet to be defined.…”

“…18 Interestingly, the liver-specific loss of PCSK9 expression 26 resulted in its complete absence from circulation, demonstrating that hepatocytes are the primary source of plasma PCSK9. 65 Notably, lipidomics analyses of the plasma of Pcsk9-knockout and transgenic mice revealed that Pcsk9-knockout mice result in a marked reduction in the plasma levels of sphingomyelin and ceramides, which are known risk factors for coronary artery disease. 105 These lipid biomarkers should now also be measured in the plasma of patients treated with inhibitors of cholesterol biosynthesis (eg, statins) or anti-PCSK9 drugs to evaluate the efficacy of treatment because it was also shown that similar reductions are seen in the plasma of individuals with the R46L LOF mutation 105 or those treated with simvastatin.…”

Hepatitis C virus and proprotein convertase subtilisin/kexin type 9: a detrimental interaction to increase viral infectivity and disrupt lipid metabolism

“…Also, circulating PCSK9 down‐regulates LDLR expression on the hepatocyte surface, thus decreasing LDL catabolism and increasing plasma LDL‐cholesterol (LDL‐C) levels 18, 19. Although there is evidence that PCSK9 decreases VLDLR expression in adipose tissue 20, whether such a down‐regulation may occur in the liver is not established and the potential impact of PCSK9‐mediated change in VLDLR expression on HCV infection is still unknown. In addition, a significant down‐regulation of SR‐B1 gene was induced by PCSK9 21, 22, although its impact on HCV life cycle and replication needs to be defined.…”

PCSK9 inhibitors for LDL lowering

“…However, decreased production of triglyceride-rich lipoproteins and upregulation of VLDL receptors may also contribute to the triglyceride lowering (7,14). VLDL receptor expression is increased in visceral adipose tissue of PCSK9 null mice, leading to an 80% increase in visceral adiposity (15). Intracellular degradation of apolipoprotein receptor 2 (ApoER2) is increased by PCSK9 (14).…”