Electronegative LDL strongly induces LRP1 release from human monocytes and macrophages.

Background: Electronegative LDL (LDL(-)) is a circulant modified LDL with inflammatory properties whose proportion raises in ischemic events. The soluble form of LDL receptor related protein 1 (sLRP1) increases in blood in pathological situations, including ischemic stroke. We aimed to evaluate the effect of LDL(-) on sLRP1 release from monocytes and macrophages.

Methods: LDL(-) and native LDL were isolated from total LDL by anion-exchange chromatography. Both fractions were incubated with THP1 monocytes (overexpressing or not CD14) and derived macrophages. Additional conditions were assayed in macrophages: (1) incubation with aggregated LDLs; (2) LDL fractions in the presence/absence of marimastat, a metalloproteinase inhibitor; and (3) presence/absence of HDL from healthy controls and ischemic stroke patients. After incubation, supernatants and cells were collected for sLRP1 determination by ELISA, and for LRP1 expression by real-time PCR, respectively.

Results: LDLs promoted sLRP1 release in monocytes and derived macrophages, regardless of CD14 overexpression. The effect was greater for LDL(-), inducing 6-fold and 3-fold higher sLRP1 release in monocytes and macrophages than native LDL. In macrophages, aggregated LDLs induced greater sLRP1 release than their non-aggregated counterparts. The LDL(-)-induced sLRP1 was not induced by promoting LRP1 expression or cytotoxicity. Otherwise, inhibition of metalloprotease activity and addition of HDL reduced sLRP1 release. However, HDL from ischemic stroke patients showed an impaired ability to decrease sLRP1 secretion.

Conclusions: LDL(-) potently induces sLRP1 in monocytes and macrophages. This action is not mediated by increased LRP1 expression, but may be related to the shedding of the membrane form in macrophages.