Beyond LDL Cholesterol, a New Role for PCSK9
Elevated low-density lipoprotein cholesterol (LDLC) levels in the plasma is the most important causative factor of atherosclerosis and associated ischemic cardiovascular diseases. The LDL receptor (LDLR) is the preferential pathway through which LDLs are cleared from the circulation. LDLs bound to the LDLR are internalized into clathrin-coated pits and subsequently undergo lysosomal degradation, whereas the LDLR is recycled back to the plasma membrane.
See accompanying article on page 1333
Familial hypercholesterolemia (FH) is an autosomal dominant disorder associated with elevated LDL levels and premature coronary heart disease. FH is caused primarily by mutations of the LDLR or of apolipoprotein B100 (apoB100), the protein component of LDL that interacts with the LDLR. In 2003, “gain of function” mutations on a newly identified gene, proprotein convertase subtilisin/kexin type 9 (PCSK9), were associated with FH. In 2005, a causative association was established between “loss of function” mutations in PCSK9 and low LDLC levels in 2% of the African-American population. The coronary heart disease risk in these individuals was reduced by 88%. As a result of these landmark studies (reviewed in Reference 1), PCSK9 became the subject of intensive research to discover the underlying mechanisms.
PCSK9 is a serine protease mainly expressed in the liver and the intestine. It acts by reducing the amount of LDLR in hepatocytes. This was demonstrated in vitro and in mouse models …