PCSK9 in extrahepatic tissues: What can we expect from its inhibition?

Proprotein convertase subtilisin/kexin type 9 (PCSK9) is an enzyme that belongs to the serine protease family and plays a key role in regulating low-density lipoprotein cholesterol (LDL-C) levels in the blood. PCSK9 binds to the LDL receptor (LDLR), targeting it for degradation, resulting in an increase in circulating LDL-C levels. Loss-of-function mutations in the PCSK9 gene are associated with lower LDL-C levels and lower cardiovascular risk; in contrast, gain-of-function mutations are a cause of familial hypercholesterolaemia. The identification of PCSK9 as a pharmacological target led to the development of inhibitors for the treatment of hypercholesterolaemia. To date, the monoclonal antibodies evolocumab and alirocumab (which target plasma PCSK9) and the small-interfering RNA inclisiran (which targets hepatic PCSK9 mRNA) have been approved for the treatment of hypercholesterolaemia. Although hepatic PCSK9 plays a central role in regulating plasma LDL-C levels, this protein is also expressed in other tissues, including the brain, pancreas, heart, kidney, intestine and adipose tissue. In extrahepatic tissues, the functions of PCSK9 are both dependent and independent of LDLR and not necessarily harmful. For this reason, it is essential to uncover any potentially harmful effects of therapies that inhibit PCSK9, beyond their known LDL-C-lowering and CV risk-reducing effects.