PCSK9 deficiency unmasks a sex- and tissue-specific subcellular distribution of the LDL and VLDL receptors in mice

 In 2015, monoclonal antibodies (mAbs) directed against PCSK9, and preventing its interaction with the LDLR, were made available as Praluent (alirocumab, Sanofi & Regeneron) or Repatha (evolocumab, Amgen). They are subcutaneously injected every two weeks and lower LDLc to levels never achieved before (by ~60%), making them the biggest weapon against cardiovascular disease since the advent of statins. In 2003, we identified the proprotein convertase subtilisin-kexin 9 (PCSK9) and established that its gene is the 3rd one known to be involved in FH after those encoding the LDL receptor and apoB. Indeed, PCSK9 enhanced degradation of the LDLR in endosomes/lysosomes, with ensuing high LDLc. In agreement, healthy individuals lacking functional PCSK9 exhibited a spectacular ~80% drop in their LDLc (~0.4 mM), thus reinforcing the validity of PCSK9 as an excellent target to treat hypercholesterolemia.

The absence of PCSK9 leads to a sex- and tissue-specific subcellular distribution of the LDLR
Like humans, mice lacking PCSK9 exhibit very low levels of cholesterol. In these mice, the fraction of the LDL receptor present at the cell surface of hepatocytes seems to be regulated in a sex-dependent manner. In the liver of male and female mice lacking PCSK9, we observed a similar increase in the total amount of LDL receptor. However, its subcellular distribution differed in a sex-dependent manner: cell surface levels of the LDL receptor were dramatically increased in males, but not females.

Estrogens are responsible for the sex-dependent subcellular distribution of the LDLR
Ovariectomized PCSK9-deficient females treated with placebo exhibited typical male patterns, with an accumulation of the LDLR at the hepatocyte cell surface. In contrast, those receiving 17β-estradiol (E2) maintained female patterns. Our working model is that the “eraser” effect of PCSK9 on cell surface LDL receptor is dominant, and that its presence masks the mechanism by which E2 regulates the surface levels of these two receptors.

Does a similar mechanism exist in women?
If this E2-mediated regulation has its counterpart in humans and leads to a lower LDLR activity, the LDL uptake by the liver may differ in hypercholesterolemic men and women who receive therapeutic PCSK9 mAbs. Could this treatment have a lower efficacy in pre-menopausal women (with high E2 levels) than in men?

Although >100 publications reported on clinical trials that led to the approval of PCSK9 mAbs to treat hypercholesterolemia, none of them compared the sex-dependent efficacy of these mAbs. However, the Clinical Briefing Document for alirocumab (FDA) revealed that, in a long term study, 1,438 men and 872 women (predominantly post-menopausal) responded by 65.5% and 53.4% reductions in LDLc, respectively, thus showing a 22% higher efficacy in men. Moreover, a subgroup analysis revealed that post-menopausal women responded with a 16% higher efficacy than pre-menopausal women (54.8% versus 47.3%). Thus, men seem to respond with a 38.5% higher efficacy than pre-menopausal women (65.5% versus 47.3%). Independently, in the Amgen Clinical Briefing Document, a graph comparing the efficacy of evolocumab injection every two weeks versus that of ezetimibe in 46 women and 52 men also indicates a higher efficacy in men.

A thorough analysis of the clinical data accumulated in phase II and III trials concerning the possible sex-dependent efficacy of PCSK9 monoclonal antibodies will hopefully be soon available. If confirmed, the elucidation of the mechanism implicated will certainly be valuable.