p38 MAPK Is an Early Determinant of Promiscuous Smad2/3 Signaling in the Aortas of Fibrillin-1 (Fbn1)-null Mice

Excessive transforming growth factor-β (TGF-β) signaling characterizes the progression of aortic aneurysm in mouse models of Marfan syndrome, a systemic disorder of the connective tissue that is caused by mutations in the gene encoding the extracellular matrix protein fibrillin-1. Fibrillin-1 mutations are believed to promote abnormal Smad2/3 signaling by impairing the sequestration of latent TGF-β complexes into the extracellular matrix. Here we report that promiscuous Smad2/3 signaling is the cell-autonomous phenotype of primary cultures of vascular smooth muscle cells (VSMC) explanted from the thoracic aortas of Fbn1 mutant mice with either neonatal onset or progressively severe aortic aneurysm. This cellular phenotype was characterized in VSMC isolated from Fbn1-null (mgN/mgN) mice, which recapitulate the most severe form of Marfan syndrome. We found that loss of fibrillin-1 deposition promotes the production of intracellular reactive oxygen species and abnormal accumulation of phosphorylated TGF-β-activated kinase 1 and p38 MAPK, in addition to increasing the levels of endogenous phospho-Smad2. We showed that improper Smad2/3 signaling in Fbn1-null VSMC is in part stimulated by phospho-p38 MAPK, which is in turn activated in response to signals other than those mediated by the kinase activity of the ALK5 receptor. Consistent with these cell culture data, in vivo analyses documented that phospho-p38 MAPK accumulates earlier than phospho-Smad2 in the aortic wall of mgN/mgN mice and that systemic inhibition of phospho-p38 MAPK activity lowers the levels of phospho-Smad2 in this tissue. Collectively, these findings indicate that improper activation of p38 MAPK is a precursor of constitutive Smad2/3 signaling in the aortic wall of a mouse model of neonatal lethal Marfan syndrome.