Binding of low density lipoprotein (LDL) to platelets enhances platelet responsiveness to various aggregation-inducing agents. However, the identity of the platelet surface receptor for LDL is unknown. We have previously reported that binding of the LDL component apolipoprotein B100 to platelets induces rapid phosphorylation of p38 mitogen-activated protein kinase (p38MAPK). Here, we show that LDL-dependent activation of this kinase is inhibited by receptor-associated protein (RAP), an inhibitor of members of the LDL receptor family. Confocal microscopy revealed a high degree of co-localization of LDL and a splice variant of the LDL receptor family member apolipoprotein E receptor-2 (apoER2′) at the platelet surface, suggesting that apoER2′ may contribute to LDL-induced platelet signaling. Indeed, LDL was unable to induce p38MAPKactivation in platelets of apoER2-deficient mice. Furthermore, LDL bound efficiently to soluble apoER2′, and the transient LDL-induced activation of p38MAPKwas mimicked by an anti-apoER2 antibody. Association of LDL to platelets resulted in tyrosine phosphorylation of apoER2′, a process that was inhibited in the presence of PP1, an inhibitor of Src-like tyrosine kinases. Moreover, phosphorylated but not native apoER2′ co-precipitated with the Src family member Fgr. This suggests that exposure of platelets to LDL induces association of apoER2′ to Fgr, a kinase that is able to activate p38MAPK. In conclusion, our data indicate that apoER2′ contributes to LDL-dependent sensitization of platelets.
The Putative Tumor Suppressor LRP1B, a Novel Member of the Low Density Lipoprotein (LDL) Receptor Family, Exhibits Both Overlapping and Distinct Properties with the LDL Receptor-related Protein
