Lysophosphatidic acid : role in CXCL12-mediated vascular repair and atherogenic monocyte recruitment

The CXCL12/CXCR4 axis directs the smooth muscle progenitor cell (SPC) mediated vascular repair following injury. The mechanism that induces CXCL12 following vessel injury leading to neointima formation through SPC recruitment was unknown. It has been shown that incubation of uninjured carotid arteries with unsaturated lysophosphatidic acid (uLPA) results in intimal smooth muscle cell (SMC) accumulation. LPA may activate the CXCL12/CXCR4 axis resulting in SPC-mediated neointima formation. We found that, LPA receptor expression was differentially regulated following vascular injury. LPA1 was reduced, whereas LPA2 and LPA3 were transiently increased. Blocking LPA1 and LPA3 following injury reduced the neointimal SMCs, which express the stem cell marker Sca-1 and macrophage content with no difference in the T-cell content and re-endothelialisation. This suggests that LPA promotes neointima formation by the recruitment of SPCs. Short-term incubation with uLPA induces a sustained upregulation of HIF-1alpha together with CXCL12 in neointimal cells. Knockdown of LPA-induced CXCL12 expression in the vessel wall blocked the SPC mobilisation and neointima formation indicating that LPA activates the HIF-1alpha/CXCL12 axis and thus SPC-mediated vascular repair. The mobilisation of SPCs and the neointima formation following incubation with uLPA was inhibited by blocking LPA1 and LPA3 This suggests that the effect of blocking LPA1 and LPA3 on neointima formation is due to a reduced SPC recruitment. In mice which express a SMC-specific reporter gene only in bone marrow (BM) cells, reporter gene activity was found in neointimal cells following LPA incubation. Blocking LPA1 and LPA3 during LPA incubation inhibited the recruitment of BM-derived cells demonstrating that LPA via LPA1 and/or LPA3 promotes neointima formation by the recruitment of BM-derived SPCs. Silencing of either LPA1 or LPA3 in the vessel wall blocked LPA-induced SPC mobilisation and neointima formation. Therefore, LPA functions independently by either LPA1 or LPA3. Although the LPA-mediated recruitment of SPCs explains the reduced neointimal SMC content after blocking LPA1 and LPA3, it is unclear how LPA is involved in macrophage accumulation. Lesional macrophages are derived from circulating monocytes, which are recruited into the vessel wall by activated endothelial cells (ECs). LDL that is modified in the sub-endothelial space plays a central role in the proatherogenic activation of ECs by inducing CXCL1.LPA, produced during mild oxidation of LDL (moxLDL), induces the adhesion of monocytes to ECs by increasing the expression of endothelial chemokines and adhesion molecules. LPA1 and LPA3 may mediate monocyte recruitment that is induced by modified LDL-derived LPA. We studied the role of LPA in modified LDL-induced monocyte recruitment and atherosclerosis. Ex vivo perfusion experiments of the carotid artery revealed that moxLDL increases monocyte adhesion to the endothelium via CXCL1, which is mediated by endothelial LPA1 and LPA3 and requires the release of autotaxin from ECs. MoxLDL-induced surface expression and secretion of CXCL1 from ECs is mediated by LPA1 and LPA3 indicating that LPA is of crucial importance in moxLDL-induced monocyte adhesion. Treatment with LPA20:4, increased the adhesion of monocytes to the endothelium via CXCL1. In the endothelium of murine carotid arteries, pre-formed CXCL1 was found which was released and immobilised on the cell surface following LPA20:4-stimulation. Unsaturated LPA20:4 but not saturated LPA18:0 increased the CXCL1 mRNA expression in ECs in a LPA1- and LPA3-dependent manner. MoxLDL-derived LPA induces monocyte adhesion to ECs by stimulating the release and transcription of CXCL1. Hyperlipidemia differentially regulates LPA receptor expression in the aorta of ApoE-/- mice characterised by a reduced LPA2, an increased LPA3 and an unchanged LPA1 expression. Immunofluorescence staining demonstrates that both LPA1 and LPA3 are expressed in macrophages and ECs of mouse atherosclerotic plaques. LPA1 and LPA3 are predominantly expressed during atherosclerosis indicating that they may play an important role in atherogenesis. Silencing of either LPA1 or LPA3 in the artery blocked the hyperlipidemia-induced leukocyte adhesion to the endothelium. LPA1 and LPA3 independently play a role in hyperlipidemia-induced leukocyte adhesion. In contrast to LPA18:0, systemic or local treatment of hyperlipidemic mice with LPA20:4 increased the plaque size due to enhanced macrophage accumulation. Pharmacological inhibition of LPA1 and LPA3 by Ki16425 in hyperlipidemic mice reduced the plaque size due to diminished macrophage accumulation. This effect was absent in mice that were treated with a blocking CXCL1 Ab in addition to Ki16425. The expression of CXCL1, but not of CCL2, CCL5 and CX3CL1, was reduced in the plaque after treatment with Ki16425. Endogenous LPA induces macrophage accumulation and atherosclerosis via CXCL1 in hyperlipidemic mice