PI 3-kinase/AKT/FOXO1 signaling in smooth muscle cells protects against abdominal aortic aneurysm
Authors: Vantler M , Schorscher M , Berghausen E M , Wong D W L , Zhao X , Gnatzy-Feik L , Zierden M , Mehrkens D , Odenthal M , Boor P , Maegdefessel L , Baldus S , Rosenkranz S 
Published: October 2024

Abstract

Background
Abdominal aortic aneurysms (AAA) are characterized by loss of vascular smooth muscle cells (SMCs). Growth factors induce proliferation and cell survival of SMCs. PI 3-kinase (PI3K) is an important downstream mediator in growth factor signaling.

Objective
This project is based on the hypothesis that PI3K-mediated SMC proliferation can compensate for the loss of SMCs in AAA and thus inhibit AAA progression. Therefore, we determined effects of reduced PI3K activity in SMCs on AAA progression, cell proliferation as well as vascular remodeling.

Methods
To investigate the role of PI3K, we analyzed mice and aortic SMCs harboring a smooth muscle-specific deficiency of the PI3K catalytic subunit p110α (SM-p110α-/-). AAA development in SM-p110α-/- mice and wild-type (WT) littermates was induced by perfusion of the infrarenal aortic segment with porcine pancreatic elastase (PPE). AAA diameter was determined by echocardiography. Structural changes in the aorta were identified by (immuno)histochemical staining. SMC proliferation was measured in vitro by BrdU incorporation or real-time cell analysis. p110α induced signal transduction pathways were characterized by Western blot and transcriptome analysis.

Results
PPE treatment led to a massive loss of SMCs in the compromised aortic segment after 3 days in both SM-p110α-/- and WT mice. The aortic diameter was enlarged after 28 days in all PPE-treated mice. However, the increase was significantly higher in SM-p110α-/- mice (70.2±10.8%, n=9) compared to WT animals (42.4±6.0%, n=10) (p&lt;0.05). A similar effect was obtained in aged (untreated) mice: In 24-month-old SM-p110α-/- mice, abdominal aortic lumen diameter was increased by 42% compared to 3-month-old animals, but only by 15% in corresponding WT mice.
PPE-treated SM-p110α-/- mice were characterized by reduced vascular remodeling with less PCNA-positive proliferating cells compared to WT animals. In addition, proliferation of aortic p110α-/- SMCs in serum-containing medium was impaired. Mechanistic analyses showed that PDGF- and insulin-induced activation of AKT as well as AKT-dependent phosphorylation and inactivation of the transcription factors FOXO-1, -3 and -4 were significantly reduced in p110α-/- SMCs compared to WT cells. Further analyses revealed that specific inhibition of FOXO1 by AS1842856 rescued PDGF or serum stimulated proliferation of p110α-/- SMCs. In addition, expression analysis of FOXOs in patients revealed significantly increased FOXO1 expression in AAA compared to aortic samples from healthy individuals.

Conclusion
Deficiency of the catalytic PI3K isoform p110α in SMCs promotes the development and progression of AAA. p110α/AKT-dependent inactivation of FOXO1 induces SMC proliferation, which can ameliorate aortic wall damage. As FOXO1 inhibitors and a recently developed p110α activator are available, the PI3K/AKT/FOXO1 signaling cascade may provide a therapeutic target to influence the stability of the aortic wall in AAA.