Atherosclerosis is a chronic inflammatory process where pro-inflammatory cytokines and chemokines mediate the continuous recruitment of inflammatory cells and thereby increase lesion inflammation. The chronic inflammation in atherosclerosis damages the structural components of lesions, such as SMCs and collagen, by increasing degradation and apoptosis, which promotes lesion vulnerability to hemodynamic stress. Hence, treatment strategies to stabilize advanced rupture prone lesions should aim at decreasing the lesion inflammation and increasing the structural components, which would increase the biomechanical strength of the lesions. Treatment with ex vivo expanded SPCs induces stable lesions in mice model of atherosclerosis. Moreover, elevation in plasma CXCL12 level for short duration mobilizes SPCs from BM. Furthermore, LPA-mediated HIF-1alpha expression in injured carotid artery up-regulates CXCL12, which leads to recruitment of BM-derived SPCs to the lesion site via CXCR4. Recruited SPCs differentiate to SMCs and contributes to the neointimal growth. Therefore, the effect of CXCL12 treatment on stabilization of vulnerable lesions was studied. Application of CXCL12 transiently increased SPCs in the circulation by eliminating the physiological CXCL12 gradient between the BM and circulation. Repeated treatment with CXCL12 reduced the lesional macrophage content without affecting the lesion size. Moreover, FC thickness, and the lesional SMC and collagen-I content were also increased by CXCL12. The induction of a stable lesion phenotype by CXCL12 was due to increased recruitment of SPCs from the circulation, which differentiated specifically to lesional SMCs. Recruitment of SPCs was in part due to increased expression of CXCL12 in lesions after CXCL12 treatment presumably mediated by HIF-1alpha. Furthermore, CXCL12 treatment had no effect on inflammatory cell mobilization or recruitment. Of note, the stable lesion phenotype with increased SMCs, collagen and decreased macrophages was sustained even in the absence of further CXCL12 treatment, which subjected application of CXCL12 as a potential therapeutic approach for treating unstable lesions. To study the role of endothelial HIF-1a in atherosclerosis, Apoe-/- mice with an EC specific deletion of Hif1a were studied. Endothelial deletion of HIF-1alpha reduced the lesion size and lesional macrophage content in aortas in diet-atherosclerosis and in disturbed flow induced atherosclerosis in the carotid artery. Interestingly no effect on lesional SMCs and collagen-I was observed, which suggests that endothelial HIF-1alpha is involved primarily in monocyte recruitment to the lesion site. Moreover, HIF-1alpha was up-regulated in ECs by the pro-atherogenic factors moxLDL, unsaturated LPA, and AT-2. Furthermore, stimulation of ECs with moxLDL and LPA20:4 increased transcription and translation of CXCL1 in an HIF- 1alpha- and LPA receptor-dependent manner. Silencing of HIF-1alpha reduced monocyte adhesion to moxLDL- and LPA20:4-stimulated ECs. Thus, these findings suggest that endothelial HIF-1alpha participates in the early recruitment of monocytes by augmenting monocyte adhesion to the endothelium via increased synthesis of CXCL1 and thus drives the inflammatory response in atherosclerosis
