Annexin A2 Is a Natural Extrahepatic Inhibitor of the PCSK9-Induced LDL Receptor Degradation

Abstract

Proprotein convertase subtilisin/kexin-9 (PCSK9) enhances the degradation of hepatic low-density lipoprotein receptor (LDLR). Deletion of PCSK9, and loss-of-function mutants in humans result in lower levels of circulating LDL-cholesterol and a strong protection against coronary heart disease. Accordingly, the quest for PCSK9 inhibitors has major clinical implications. We have previously identified annexin A2 (AnxA2) as an endogenous binding partner and functional inhibitor of PCSK9. Herein, we studied the relevance of AnxA2 in PCSK9 inhibition and lipid metabolism in vivo. Plasma analyses of AnxA2−/− mice revealed: i) a ∼1.4-fold increase in LDL-cholesterol without significant changes in VLDLs or HDLs, and ii) a ∼2-fold increase in circulating PCSK9 levels. Western blotting and immunohistochemistry of AnxA2−/− tissues revealed that the LDLR was decreased by ∼50% in extrahepatic tissues, such as adrenals and colon. We also show that AnxA2-derived synthetic peptides block the PCSK9≡LDLR interaction in vitro, and adenoviral overexpression of AnxA2 in mouse liver increases LDLR protein levels in vivo. These results suggest that AnxA2 acts as an endogenous regulator of LDLR degradation, mostly in extrahepatic tissues. Finally, we identified an AnxA2 coding polymorphism, V98L, that correlates with lower circulating levels of PCSK9 thereby extending our results on the physiological role of AnxA2 in humans.

The Epigenetic Drug 5-Azacytidine Interferes with Cholesterol and Lipid Metabolism

“…CLEC810, InterScience). Circulating mouse Pcsk9 was immunoprecipitated and analyzed by Western blotting, as described previously (24). Total SREBP-2 was immunoprecipitated from 1 mg of protein (1:500; provided by Dr. Park) together with 50 l of protein A/G PLUS-agarose (catalog no.…”

Pcsk9

“…It was found that annexin A2 is a natural extrahepatic inhibitor of PCSK9 110 and that in Anxa2-knockout mice, plasma PCSK9 doubles and LDLR decreases by ≈50% in some extrahepatic tissues, such as adrenals, small intestine, and colon but not in liver. 111 In addition, transgenic mice expressing human PCSK9 mainly in the kidney resulted in LDLR degradation mostly in the liver but not in adrenals. 112,113 Thus, it is probable that the high levels of annexin A2 in adrenals are responsible for the refractoriness of the LDLR in this tissue to the action of PCSK9.…”

“…112,113 Thus, it is probable that the high levels of annexin A2 in adrenals are responsible for the refractoriness of the LDLR in this tissue to the action of PCSK9. 110,111 Another, not mutually exclusive, possibility is that in refractory tissues, such as the adrenals, the complex [PCSK9≡LDLR] enters endosomes but does not traffic to lysosomes and may actually recycle back to the cell surface. Accordingly, a possible explanation for the tissue-specific activity of PCSK9 could be that extrahepatic tissues that do not respond to PCSK9 are unable to sort the [PCSK9≡LDLR] complex to lysosomes, likely because of unfavorable cellular response or absence of functional specific protein(s) that control lysosomal targeting of the complex (Figure 2).…”

“…114 Alternatively, some tissues, such as adrenals, are enriched in an endogenous inhibitor of PCSK9 (eg, annexin A2), that prevent its function on the LDLR. 111 The physiological role of PCSK9 in kidney is still not clear, especially because its expression therein shifts from the cortex in the embryo to the lumen in adult rodents. 12 It is also possible that PCSK9 acts locally in some extrahepatic tissues that express this protein (eg, small intestine, pancreas, and kidney), and that its circulating levels would not affect the LDLR in these tissues.…”

“…111 None of the published therapeutic anti-PCSK9 approaches in humans used a small-molecule inhibitor, possibly because of the relative flatness of the surface of interaction of the [PCSK9≡LDLR] complex, and thus the absence of a targetable groove that could accommodate a small molecule inhibitor. 30 However, we will have to await the…”