CRP immunodeposition and proteomic analysis in abdominal aortic aneurysm

Abstract

Objective: The molecular mechanisms of the degeneration of the aortic wall in abdominal aortic aneurysm (AAA) are poorly understood. The monomeric form of C-reactive protein (mCRP) is deposited in damaged cardiovascular organs and aggravates the prognosis; however, it is unknown whether mCRP is deposited in the degenerated aorta of abdominal aortic aneurysm (AAA). We investigated whether mCRP is deposited in AAA and examined the associated pathogenic signaling pathways.

Methods: Twenty-four cases of AAA were analyzed and their histological features were compared according to the level of serum CRP and the degree of mCRP deposition. Proteomic analysis was performed in AAA cases with strong and diffuse CRP immunopositivity (n = 7) and those with weak, focal, and junctional CRP immunopositivity (n = 3).

Results: mCRP was deposited in the aortic specimens of AAA in a characteristic pattern that coincided with the lesion of the diminished elastic layer of the aortic wall. High serum CRP level was associated with stronger mCRP immunopositivity and a larger maximal diameter of aortic aneurysm. Proteomic analysis in AAA showed that multiple proteins were differentially expressed according to mCRP immunopositivity. Also, ingenuity pathway analysis showed that pathways associated with atherosclerosis, acute phase response, complement system, immune system, and coagulation were enriched in AAA cases with high mCRP immunopositivity.

Conclusions: AAA showed a characteristic deposition of mCRP, and multiple potentially pathologic signaling pathways were upregulated in AAA cases with strong CRP immunopositivity. mCRP and the aforementioned pathological pathways may serve as targets for managing the progression of AAA.

Fig 1. CRP immunopositivity with the corresponding histopathologic findings.

(A–D) Aorta specimen from a patient with abdominal aortic aneurysm and mildly elevated serum CRP (0.28 mg/dL). The site of strong and linear CRP (C) and mCRP (D) immunopositivity was found in the interface (arrow) between atherosclerotic plaque (asterisk) and the thinned aneurysmal wall (between the two arrowheads). This immunopositive lesion was correlated with the area of the diminished elastic lamella of the eroded media (B). Atheroma was diffusely immunopositive for both CRP and mCRP. (A, H&E, B, elastic staining, C, anti-CRP antibody, D, anti-mCRP antibody, × 40). (E–H) Aorta specimen from a patient with ascending aortic dissection with mildly elevated serum CRP (0.74 mg/dL). CRP and mCRP were faintly and nonspecifically stained in the smooth muscle cell of tunica media. CRP was not stained at the boundary of the damaged and the torn aortic wall. (E, H&E, F, elastic staining, G, anti-CRP antibody, H, anti-mCRP antibody, × 40).

Fig 2. Representative CRP and mCRP immunostaining according to serum CRP levels.

(A) Low serum CRP (≤0.1 mg/dL). (B) High serum CRP (1.39 mg/dL) (anti-CRP antibody, anti-mCRP antibody, × 40).

Fig 3. CRP and mCRP immunostaining according to serum CRP level.

(A, ≤0.1 mg/dL; B, >0.1 mg/dL) in cases of abdominal aortic aneurysm with atherosclerosis (all magnification × 40).

Fig 4. Representative images of aortic specimens from cases of AAA according to serum CRP levels.

H&E, elastic staining, and the immunostaining patterns of mCRP, CD68, MCP-1, and complement components (C3a, C5a, C5a receptor, C1q) in patients with high serum CRP level (0.58 mg/dL, A) and low serum CRP level (0.1 mg/dL, B). (× 40; MCP-1, monocyte chemoattractant protein-1).

Fig 5. Proteomic profiles and comparison of identified proteomes among the AAA-high mCRP, AAA-low mCRP, and AAD groups.

(A) Two-dimensional principal component analysis plots using quantitative protein information. (B) Venn diagram showing the overlap of differentially expressed proteins among the three groups. (C) Hierarchical clustering of top abundant 1,127 DEPs between AAD, AAA-low mCRP, and AAA-high mCRP groups (ANOVA t-test, permutation-based FDR ≤0.01). Rows represent proteins and columns represent different samples. Darker shades of red and blue each indicate increased and decreased expressions compared with control. This figure was generated using Instant Clue (version 0.9.2 from http://www.instantclue.uni-koeln.de/) (D) Volcano plots illustrating significantly differentially abundant proteins between AAD, AAA-low mCRP, and AAA-high mCRP groups. Each dot indicates a protein and red colors indicate significantly enriched proteins with q values <0.05 and fold changes of more than ± 2-fold change, and permutation-base FDR ≤0.01. AAA-low mCRP indicates aortic aneurysm with weak and focal CRP immunopositivity; AAA-high mCRP, Aortic aneurysm with strong and diffuse CRP; AAD, ascending aortic dissection.

Fig 6. Bioinformatics analysis of 187 proteins that showed higher abundance in both AAA-high mCRP and AAA-low mCRP compared with AAD.

(A) Gene ontology analysis. Top 5 subcategories in biologic process, molecular function, and cellular component. (B) Top canonical pathways identified in core analysis in IPA. The bar graph shows the canonical pathway represented by gene enrichment. The orange line running through the bars shows the threshold for the P-value for the particular pathway’s enrichment. The vertical axis is the–log(p-value) and the horizontal axis shows the given pathways.

Fig 7. Canonical pathway analysis of signaling pathways.

Annotation enrichment analysis. Networks including “engulfment of cells,” “the interaction of leukocytes,” and “migration of phagocytes” were significantly enriched in the AAA-high mCRP group than in the AAA-low mCRP group.