PI 3-kinase/p110alpha-mediated inactivation of FOXO1 in SMC ameliorates onset and progression of abdominal aortic aneurysms

Background Abdominal aortic aneurysms (AAA) are characterized by the loss of contractile vascular smooth muscle cells (SMCs) as a result of cell death and SMC dedifferentiation. Therefore, therapeutic options should aim to increase SMC numbers. It is crucial to identify signal transduction pathways that induce SMC proliferation without promoting SMC dedifferentiation. The catalytic class IA PI 3-kinase isoform p110α is a key regulator of cell proliferation and survival in numerous cell types. Its role in vascular integrity and AAA development and progression has not yet been investigated. Methods To decipher pathomechanisms of AAA formation and progression, the infrarenal aortic segment of mice lacking p110a or FOXO1 in SMCs (sm-p110α-/-, sm-FOXO1-/- mice) was treated with porcine pancreatic elastase (PPE). Phenotypic changes in aortas and SMCs were analyzed by ultrasound, immunohistochemistry, histochemistry, qPCR, transmission electron microscopy as well as functional and cellular assays. Results PPE treatment of sm-p110α-/- mice resulted in significantly increased abdominal aortic diameter compared to wild-type controls (70.2±32.5% versus 42.4±18.9%, n=9-10, p<0.05). This disease phenotype is promoted by a vascular phenotype that is already expressed under basal conditions: sm-p110α-/- mice exhibited a smaller media area, impaired aortic wall structure (detached SMCs, apoptotic cell death) and reduced endothelium-independent functional reactivity to vasodilators. p110α was also involved in the regeneration processes during AAA progression: While increased medial hypertrophy and cell proliferation were detected in the respective aortic segments of wild-type mice 28 days after PPE intervention, these processes were reduced in sm-p110α-/- aorta. In addition, elastic fibers in sm-p110α-/- aortas were characterized by more frequent fiber breaks. Mechanistic analyses revealed that p110α deficiency impaired SMC proliferation, expression of contractile marker genes and elastin fiber production. This phenotype could be attributed to reduced phosphorylation and inactivation of the transcription factor FOXO1 in p110α-/- SMCs, as specific FOXO1 inhibition fully restored proliferation of p110α-/- SMCs and knockdown of FOXO1 increased the expression of calponin and elastin. Furthermore, FOXO1 deficiency in SMCs reduced the increase in aortic diameter in PPE treated sm-FOXO1-/- mice compared to wild type control mice (43.4±21.8% versus 64.4±19.0%, n=9-10, p<0.05). Conclusions p110α mediated FOXO1 inactivation protected against the development and progression of AAA. This pathway contributes to aortic wall stability under basal conditions and mediates SMC proliferation during AAA progression without promoting SMC dedifferentiation. These data may indicate potential long-term vascular side effects of p110α inhibitors for cancer therapy, while promoting p110α/FOXO1 signaling may arise as a strategy to prevent or treat AAA.