Evidence of Functional Modulation of the MEKK/JNK/cJun Signaling Cascade by the Low Density Lipoprotein Receptor-related Protein (LRP)

Abstract

Lipoprotein receptors, such as LRP, have been shown to assemble multiprotein complexes containing intracellular signaling molecules; however, in vivo, their signaling function is poorly understood. Using a novel LRP receptor fusion construct, a type I transmembrane protein chimera, termed sIgG-LRP (bearing the intracellular COOH-terminal tail of human LRP as recombinant fusion to a transmembrane region plus the extracellular IgG-F c domain), we here investigated LRP signal transduction specificity in intact cells. First and similar to activated ␣2-macroglobulin as agonist of endogenous LRP, expression of sIgG-LRP demonstrated significant apoptosis protection. Second and similar to ␣2-macroglobulin-induced endogenous LRP, sIgG-LRP is sufficient to negatively modulate mitogen-induced Elk-1 and cJun (but not NF-B) transcriptional activity. Third, expression of sIgG-LRP also impaired cJun transactivation mediated by constitutive active mutants of Rac-1 and MEKK-1. Fourth and unexpectedly, sIgG-LRP expression was found to be associated with a marked enhancement of mitogen-induced cJun amino-terminal kinase (JNK) activation. Fifth, confocal microscopic examination and subcellular fractionation demonstrated that sIgG-LRP and JNK co-localize in transfected cells. Therefore, sIgG-LRP expression was found to significantly impair activation-induced translocation of JNK into the nucleus. Taken together, we here demonstrate that sIgG-LRP protein sequesters activated JNK into the plasma membrane compartment in intact cells, inhibiting nuclear activation of the JNK-dependent transcription factors Elk-1 and cJun. Low density lipoprotein receptor-related protein (LRP)1 is one member of the LDL receptor family that also includes the LDL receptor, the very low density lipoprotein receptor, megalin, LRP5, LRP6, and apoER2 receptor (see Ref. 1 LRP is expressed abundantly in neurons and microglia of the central nervous system (3, 4). Disruption of the LRP gene in mice blocks development of LRP Ϫ/Ϫ embryos around the implantation (5). However, the complex phenotype of the few malformed LRP-deficient embryos that survive until E10 (6), similar to the very low density lipoprotein ApoER2 receptor double knockout phenotype (7), postulated some LRP receptor signaling function(s). Consistently, LRP and several of its ligands, including ␣2-macroglobulin, tissue plasminogen activator (tPA), apoE-containing lipoproteins, and the amyloid precursor protein (APP) (8, 9), have been implicated in various cellular functions including the neuropathogenesis of Alzheimer's disease (see Ref. 10 for review).Based on yeast two-hybrid and co-immunoprecipitation analysis, lipoprotein receptors assemble intracellular multiprotein complexes containing the adapter and scaffold proteins Dab-1 (7), FE65 (11) and Shc (12, 13), the non receptor tyrosine kinases Src and Fyn (14), and the JNK-interacting proteins (JIP-1 & 2) (15, 16), which act as molecular scaffolds for the JNK signaling pathway (see Ref. 17 for review). Quite similarly, such intracellular signali...

The Low-Density Lipoprotein Receptor-Related Protein Is a Pro-Survival Receptor in Schwann Cells: Possible Implications in Peripheral Nerve Injury

“…In neurons, a chimeric receptor that contains the intracytoplasmic tail of LRP-1 promotes cell survival by preventing translocation of activated JNK to the nucleus (Lutz et al, 2002). By a similar mechanism, LRP-1 may control expression of the type-3 collagen gene (Gaultier et al, 2006).…”

“…Binding of Dab-1 to apoER2 and the very low-density lipoprotein (VLDL) receptor, which are related to LRP-1, promotes activation of phosphatidylinositol 3-kinase (PI3K) (Bock et al, 2003). Binding of JIP to a chimeric receptor that includes the cytoplasmic tail of LRP-1 prevents translocation of activated c-Jun N-terminal protein kinase (JNK) to the nucleus (Lutz et al, 2002). By regulating cell signaling, LRP-1 may control cell survival.…”

Aggregated Low-Density Lipoprotein Uptake Induces Membrane Tissue Factor Procoagulant Activity and Microparticle Release in Human Vascular Smooth Muscle Cells

“…For one thing, LRP is not only involved in endocytosis but also in signal transduction 34 and in alterations in intracellular calcium concentrations, which leads to cell activation. 35 Cell activation has been described as one of the mechanisms responsible for MP release. 36 On the other hand, LRP and inactive TF are localized in certain patches of the membrane, named caveolae.…”

Three Complement-like Repeats Compose the Complete α2-Macroglobulin Binding Site in the Second Ligand Binding Cluster of the Low Density Lipoprotein Receptor-related Protein

“…LRP can, thus, discriminate strongly between native and activated ␣ 2 M species. Internalization of complexed ␣ 2 M can result not only in internalization and degradation of the complexed proteinase but also in intracellular signaling responses, making this ligand an especially important one to study (12).…”