Expression and localization of PCSK9 in rat hepatic cells

Proprotein convertase subtilisin/kexin type 9 (PCSK9), recently cloned in several laboratories, including ours, causes a third form of autosomal dominant hypercholesterolemia. Its mechanism of action remains unclear. We studied the expression and subcellular localization of PCSK9 in fetal and adult rat tissues associated with cholesterol homeostasis using quantitative reverse transcriptase--PCR, Western blot analysis, subcellular fractionation, and confocal immunofluorescent microscopy. PCSK9 mRNA is most abundant in yolk sac and fetal liver, but the highest expression of the protein was found in differentiated hepatoma FAO-1 cell line, which also shows the highest expression of LDLR. In FAO-1 cells PCSK9 expression is downregulated by cholesterol and 25-hydroxycholesterol and upregulated in the absence of sterols following the same pattern of expression as HMG-CoA reductase, synthase, and LDLR. Subcellular fractionation, combined with Western blotting, showed that PCSK9 is localized in the ER and intermediate vesicular compartment of the cell but not in Golgi cisternae. The mature enzyme is secreted from the liver and hepatoma cells. Double labeling with antibodies to PCSK9 and LDLR or clathrin revealed some colocalization of PCSK9 with clathrin-coated vesicles and LDLR. In conclusion, our results show that PCSK9 is processed in the ER, and the mature convertase is secreted in the plasma.