Atherogenic LOX-1 signaling is controlled by SPPL2-mediated intramembrane proteolysis

Abstract

The lectin-like oxidized LDL receptor 1 (LOX-1) is a key player in the development of atherosclerosis. LOX-1 promotes endothelial activation and dysfunction by mediating uptake of oxidized LDL and inducing pro-atherogenic signaling. However, little is known about modulators of LOX-1–mediated responses. Here, we show that the function of LOX-1 is controlled proteolytically. Ectodomain shedding by the metalloprotease ADAM10 and lysosomal degradation generate membrane-bound N-terminal fragments (NTFs), which we identified as novel substrates of the intramembrane proteases signal peptide peptidase–like 2a and b (SPPL2a/b). SPPL2a/b control cellular LOX-1 NTF levels which, following self-association via their transmembrane domain, can activate MAP kinases in a ligand-independent manner. This leads to an up-regulation of several pro-atherogenic and pro-fibrotic targets including ICAM-1 and the connective tissue growth factor CTGF. Consequently, SPPL2a/b-deficient mice, which accumulate LOX-1 NTFs, develop larger and more advanced atherosclerotic plaques than controls. This identifies intramembrane proteolysis by SPPL2a/b as a novel atheroprotective mechanism via negative regulation of LOX-1 signaling.

C-Type Lectin-Like Receptors: Head or Tail in Cell Death Immunity

“…Following uptake of its ligand, LOX-1 induces pro-inflammatory signaling pathways leading to production of ROS, secretion of pro-inflammatory cytokines and induction of apoptosis signals ( Table 1 and Figure 1). Thus, LOX-1 plays a pivotal role in the development of atherosclerosis, by inducing oxLDL uptake, lipidosis, foam cell generation and ultimately atheroma plaque formation (62,63). This OxLDL/LOX-1 axis was also shown to play a role in cartilage degeneration during age-related osteoarthritis progression in the murine knee (64).…”

Physiological functions of SPP/SPPL intramembrane proteases

“…However, it is not clear yet, whether initial cleavage and processivity of SPPL2b depend on different determinants within a substrate. Although a variety of initial cleavage sites within SPPL2b and also SPPL2a substrates have been determined [28,[37][38][39][40], no obvious consensus sequences were identified. In contrast, analysis of cleavage sites in SPPL3 substrates indicates that methionine and tyrosine in the P1 position are favoured [41], but it still remains to be proven by mutational analysis whether a true consensus sequence for SPPL3 exists or if rather secondary structure elements determine cleavability.…”

“…Beyond mediating oxLDL uptake into vascular cells, it triggers various signalling pathways including MAP kinases and NFκB and thereby activates endothelial cells, which can initiate endothelial dysfunction and atherosclerotic plaque formation [70]. Different proteolytic pathways including ectodomain shedding by ADAM10 and cleavage in lysosomes generate membrane-bound LOX-1 NTFs, which are then further processed by SPPL2a/b [39]. In contrast to CD74 processing, both SPPL2a and SPPL2b contribute to LOX-1 proteolysis under endogenous conditions.…”

C‐type lectin receptors of the Dectin‐1 cluster: Physiological roles and involvement in disease

“…Although normally expressed at low levels, in part through regulation by casein kinase 2‐interacting protein‐1 , LOX‐1 is rapidly upregulated by a variety of factors, particularly pro‐atherogenic stimuli including inflammatory cytokines and modified LDL. LOX‐1 can be cleaved by ADAM10 proteases to produce a soluble form (sLOX‐1) than is detectable in the serum, and N‐terminal fragments that remain bound in the membrane . LOX‐1 is able to recognise a broad range of ligands including bacterial products, C‐reactive protein, and advanced glycation end products, but is best known for its ability to bind oxidised LDL (ox‐LDL).…”

“…LOX‐1 can internalize its ligands through a cytoplasmic tripeptide (DDL) motif, and induce intracellular signalling that results in a variety of cellular responses including the production of ROS, chemokines and cytokines, upregulation of adhesion molecules, apoptosis, as well as activation of the NALP3 inflammasome and NF‐ κ B . How LOX‐1 mediates intracellular signalling is unclear, but recent evidence implicates the membrane N‐terminal fragments in these activities, and their regulation by the signal peptide peptidase–like 2a and b (SPPL2a/b) .…”