Genetics and molecular biology: proprotein convertase subtilisin/kexin type 9 and LDL receptor--an intriguing story.

Genetic studies in humans and mice have demonstrated that proprotein convertase subtilisin/kexin type 9 (PCSK9) protein is involved in the post-translational regulation of LDL-receptor (LDL-R) protein levels in hepatocytes [1,2]. Several lines of evidence indicate that mature PCSK9 is secreted by hepatocytes and interacts with LDL-R on the cell surface. Secreted PCSK9 (or recombinant PCSK9), when added to cultured hepatocytes, readily reduces LDL-R protein levels in a time-dependent and concentration-dependent manner. PCSK9 is endocytosed and exhibits endosomal/lysosomal localization. Endocytosis of PCSK9 is mediated by LDL-R, because it is not observed in LDL-R-deficient hepatocytes or in hepatocytes deficient in autosomal recessive hypercholesterolaemia (ARH) protein [3,4•] – an adaptor protein that is required for LDL-R internalization. When infused into mice, human recombinant PCSK9 was able to reduce hepatic LDL-R protein levels and elevate plasma LDL [3]. Thus, the idea has progressively emerged that, at least in hepatocytes, secreted PCSK9 interacts directly with the extracellular domain of the LDL-R and undergoes LDL-R/ARH-mediated endocytosis, leading to intracellular degradation of the LDL-R [3,4•]. A key question underlying the interaction between PCSK9 and LDL-R concerns the site in the extracellular domain of LDL-R that binds to PCSK9. A recent study [5••] showed that PCSK9 interacts with the first epidermal growth factor repeat (EGF-A) in the EGF-precursor homology domain of LDL-R. This interaction is greatly increased with reduction in pH from 7 to 5.2, suggesting that PCSK9 binds to LDL-R at the cell surface at neutral pH and remains strongly associated with it in the acidic environment of the endosomes, where the binding affinity to LDL-R is increased by more than 100-fold. In endosomes PCSK9 would lock LDL-R in a specific conformation that favours degradation of the receptor instead of its recycling back to the plasma membrane [5••,6•]. This finding raises the question of whether naturally occurring mutations in the EGF-A domain of LDL-R increase or, vice versa, decrease the affinity of the receptor for PCSK9. In the first case, the increased PCSK9-dependent LDL-R degradation would result in hypercholesterolaemia, whereas in the second case the reduced LDL-R degradation would result in hypocholesterolaemia. The site of PCSK9 involved in binding to LDL-R remains to be defined. It is probable that functional and structural studies of PCSK9 mutants will assist with mapping this site [7••,8]. Two other questions have been addressed, namely whether the binding of LDL to LDL-R affects the interaction between PCSK9 and LDL-R, and whether PCSK9 is directly responsible for the proteolytic degradation of LDL-R. According to Fisher et al. [6•], binding of LDL diminishes PCSK9 binding to LDL-R in vitro, and partially inhibits the effect of secreted PCSK9 on LDL-R degradation within the cell. Deletion of the LDL-binding domain of the LDL-R does not affect binding of PCSK9, however, thus indicating that PCSK9 can bind to LDL-R independently of lipoproteins [5••]. With regard to the second question, two studies [9••,10] showed that catalytic activity is not required for secreted PCSK9 to induce LDL-R degradation. Catalytic inactive wild-type or mutant PCSK9 (harbouring the gain-of-function mutation D374Y) exhibited no loss of binding capacity to LDL-R, suggesting that the role played by PCSK9 is that of a molecular chaperone directing LDL-R to lysosomal degradation [9••]. Catalytic activity is, however, required for the intracellular autocatalytic cleavage of the prodomain and the secretion of PCSK9 from the cells. There are, however, gain-of-function mutations that prevent autocatalytic cleavage or secretion of PCSK9, but that nevertheless induce LDL-R degradation, suggesting that PCSK9 may also function intracellularly [11]. The story of PCSK9 continues and is full of surprises.