Abstract
Background: To prevent severe inflammation during infection, the patient must quickly clear bacterial endotoxins from the circulation before they accumulate and are able to interact with immune cells and vascular endothelium, and induce inflammatory organ failure. Bacterial endotoxins are carried within lipoprotein particles. Thus, one mechanism of action for sepsis treatments could be acceleration of lipoprotein clearance by adipocytes and hepatocytes. Proprotein convertase subtilisin/kexin type 9 (PCSK9) decreases the rate of lipoprotein clearance. We have recently reported that reduced function of PCSK9 improves outcome and prevents cardiovascular complications associated with sepsis. Hypothesis: PCSK9 inhibits LDL associated LPS clearance through hepatic LDLR and VLDL associated LPS clearance through adipose VLDLR. Methods and Results: Using siRNA against the LDLR in HepG2 hepatocytes decreased uptake of fluorescently labeled LPS (fLPS) after 48 hours by 1.50±0.10 fold (n=3, p<0.05). Addition of recombinant PCSK9 in the absence of LDLR did not alter uptake of LPS. We confirmed that hepatic uptake of LPS is exclusively via the LDLR by fluorescent microscopy of ex vivo LPS treated primary hepatocytes isolated from LDLR -/- mice. To address the importance of the LDLR upon clearance of LPS from plasma, we injected fLPS into the portal vein of LDLR-/-, PCSK9-/- and wild type mice (WT). Compared to WT, LDLR-/- mice had 36±13% (n=9, p<0.001) increase in plasma LPS after 1 hour, whereas PCSK9-/- show a significant decrease (28±4%, n=9, p<0.001) in plasma LPS. LDLR-/-, but not PCSK9-/- mice showed 46±7% decrease (n=10, p<0.05) in hepatic uptake. On the other hand, compared to the WT PCSK9-/- mice had 200±35% (n=8, p<0.001) increase in LPS uptake by visceral adipose tissue whereas LDLR-/- had no effect compared to WT mice. To further investigate LPS uptake by adipose tissue we injected flLPS into the tail vein of VLDLR-/- and WT mice. VLDLR-/- mice had 33±6% (n=10, p<0.001) decrease in visceral adipose tissue uptake, with no significant change in hepatic uptake. Conclusions: Expression of hepatic LDLR and adipose VLDLR is mainly regulated by PCSK9 and both play important role in clearing LPS from circulation.
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