Background: Aneurysm dissection is the most dangerous manifestation of aortic disease due to a complex interplay between genetic, local, and environmental factors. The bioactive lipid sphingosine-1-phosphate (S1P) has numerous cardiovascular effects such as blood pressure regulation. It signals through G-protein coupled receptors (S1PR1-5) that are expressed in vasculature. Although S1P regulates vascular smooth muscle cell (VSMC) proliferation and contractility. Its role in the VSMC-driven remodeling resulting during aneurysm formation is largely unknown. The differential expression of S1PRs in human aortic aneurysms (AA) suggests a causal contribution but there is no experimental evidence. We have addressed whether S1P signaling is causally involved in aneurysm and rupture.

Methods: AA formation was induced by AngII administration through osmotic pumps (1 mg/kg/min) in apolipoprotein E (ApoE) - deficient mice or ApoExS1PR3 double mutant mice fed a high cholesterol diet over 28 days. Elevation of endogenous plasma and tissue S1P concentrations was achieved through pharmacological inhibition of the S1P degrading enzyme S1P lyase by 4-deoxypyridoxine (DOP) starting prior to pump implantation. AA incidence, morphological classification, and gene/protein expression were assessed by histopathology, immunostaining, and RT-qPCR/Western blotting, respectively. Blood pressure monitoring and aortic tension studies were performed using a tail cuff system and a small vessel myograph. Rat and murine VSMC for in vitro studies were isolated from whole aortae. Sphingolipidome of murine and human AA samples was measured by LCMS-8050 TQ/MS. Results: Pharmacologically high S1P level increased AA-caused mortality due to deadly rupture (70% in AngII+DOP compared to 25% in AngII after 28 days; p<0.01). Spontaneous deaths were due to dissecting aneurysms (DA) defined by massive blood clots in the thorax with initial origin in a medial dissection. High S1P levels led to intramural hematoma and a 2-fold increase of elastic breaks (in AngII+DOP compared AngII) resulting in more progressive types of AA/DA. Passive tension studies showed reduced elasticity of AngII+DOP aortae making them more prone to rupture. Gene expression of aortae showed a remarkable downregulation of VSMC differentiation and contractile genes associated with human DA (Acta2, Tagln, Myh11). In vitro studies revealed that S1P/S1PR3 signaling was responsible for the downregulation of same contractile genes. Importantly, ApoE x S1PR3 double KO mice were protected against DA occurrence and lethal rupture compared to ApoE mice in the absence and presence of DOP (95% vs. 70% without and 70% versus 40% with DOP); p<0.05). Accordingly, there were less elastic breaks and increased aortic elasticity. Most importantly, expression of all VSMC differentiation and contractile genes was substantially higher in double KO. In a selected group of human AA patients undergoing invasive surgery, we observed lower plasma and higher aneurysm S1P levels compared to controls. Currently, we are investigating if pharmacological inhibition of S1P/S1PR3 signaling could prevent aneurysm formation and rupture.

Conclusion: Tonic S1PR3 signaling may play a role in the pathogenesis and dissection of AA by altering arterial biomechanics activity and expression of VSMC contractility genes. Pharmacological targeting of the S1P/S1PR3 axis may constitute a novel approach to preventing progression and/or stabilizing existing AAs