Background: Aortic aneurysm (AA) dissection is a life-threatening condition that results from a complex interplay between genetic, local and environmental factors. Currently, there is no pharmacological treatment to stabilize and prevent AA rupture. Sphingosine-1-phophate (S1P) is a bioactive lipid mediator involved in cardiac and vascular homeostasis via signaling through five G-protein coupled receptors (S1PR1-5). Importantly, its effects in vasculature range from blood pressure regulation and endothelial barrier stabilization to modulation of vascular smooth muscle cell function (VSMCs).

Hypothesis: Deregulated S1P receptor (S1PR) signaling by excess S1P affects AA incidence, progression and rupture.

Methods: AA formation was induced by angiotensin (Ang) II administration through osmotic pumps (1000 ng/kg/min) over 28 days in apolipoprotein E deficient mice fed a high cholesterol diet. Elevation of endogenous plasma and tissue S1P concentrations was generated through pharmacological inhibition of the S1P lyase, the sole enzyme responsible for its irreversible degradation, using 5-deoxypyridoxine (DOP). AA incidence, morphological classification and gene expression were assessed by histopathology, immunostaining and real time PCR. Vasoreactivity and active as well as passive tension studies of thoracic and abdominal aortic segments were performed by using a small vessel myograph. The sphingolipidome of murine and human AA samples as well as plasma was characterized by LC-MS/MS.

Results: DOP treatment dramatically increased AA-caused mortality due to deadly rupture of aortic aneurysms (70% in the Ang II+DOP compared to 35% in the Ang II group after 28 days; p<0.01). Each spontaneous death was due to a dissecting aneurysm (DA) as evidenced by massive blood clots in the thorax or abdomen with their initial origin in a medial dissection. Classification of aneurysms in surviving animals from the DOP group showed an increased incidence of the most progressive types. Histological assessment of the arterial wall revealed intramural hematoma and an increased number of elastic breaks (2-fold higher in Ang II+DOP compared to the Ang II group). Vasoreactivity studies in unaffected thoracic and abdominal segments from the DOP group showed a significantly higher response to phenylephrine. Passive tension studies showed reduced elasticity and increased stiffness of Ang II+DOP aortae making them more prone to rupture. Gene expression studies of whole aortae showed a remarkable downregulation of VSMC differentiation and contractile genes (Acta2, Tagln, Myh11). In vitro studies in isolated rat and mouse VSMC revealed that contractile gene expression is under control of S1P/S1PR2 signaling. Interestingly, S1PR expression was altered in aortic sections of Ang II infused mice (S1PR2 expression 3-fold higher in abdominal and thoracic segments compared to control tissue). LC-MS/MS analysis of the sphingolipidome in patients with aortic aneurysms revealed increased S1P levels in aortic aneurysm tissue.