Tryptophan-Derived 3-Hydroxyanthranilic Acid Contributes to Angiotensin II-Induced Abdominal Aortic Aneurysm Formation in Mice In Vivo

Background: Abnormal amino acid metabolism is associated with vascular disease. However, the causative link between dysregulated tryptophan metabolism and abdominal aortic aneurysm (AAA) is unknown.

Methods: Indoleamine 2,3-dioxygenase (IDO) is the first and rate-limiting Enzyme in the kynurenine pathway of tryptophan metabolism. Mice with deficiencies in both Apolipoprotein e (apoE) and IDO (apoE^-/-/IDO^-/-) were generated by cross-breeding IDO^-/- mice with apoE^-/- mice.

Results: The acute infusion of angiotensin II markedly increased the incidence of AAA in apoE^-/- mice, but not in apoE^-/-/IDO^-/- mice, which presented decreased elastic lamina degradation and aortic expansion. These features were not altered by the reconstitution of bone marrow cells from IDO^+/+ mice. Moreover, angiotensin II infusion instigated interferon-γ, which induced the expression of IDO and kynureninase and increased 3-hydroxyanthranilic acid (3-HAA) levels in the plasma and aortas of apoE^-/- mice, but not in IDO^-/- mice. Both IDO and kynureninase controlled the production of 3-HAA in vascular smooth muscle cells. 3-HAA upregulated matrix metallopeptidase 2 via transcription factor nuclear factor-κB. Furthermore, kynureninase knockdown in mice restrained 3-HAA, matrix metallopeptidase 2, and resultant AAA formation by angiotensin II infusion. Intraperitoneal injections of 3-HAA into apoE^-/- and apoE^-/-/IDO^-/- mice for 6 weeks increased the expression and activity of matrix metallopeptidase 2 in aortas without affecting metabolic parameters. Finally, human AAA samples had stronger staining with the Antibodies against 3-HAA, IDO, and kynureninase than those in adjacent nonaneurysmal aortic sections of human AAA samples.

Conclusions: These data define a previously undescribed causative role for 3-HAA, which is a product of tryptophan metabolism, in AAA formation. Furthermore, these findings suggest that 3-HAA reduction may be a new target for treating cardiovascular diseases.

3-hydroxyanthranilic acid; angiotensin II; aortic aneurysm, abdominal; indoleamine-pyrrole 2,3-dioxygenase; matrix metallopeptidases; tryptophan.