Follistatin impacts Tumor Angiogenesis and Outcome in Thymic Epithelial Tumors

Abstract

Tumor angiogenesis is a key factor in the progression of thymic epithelial tumors (TETs). Activin A, a member of the TGFβ family, and its antagonist Follistatin are involved in several human malignancies and angiogenesis. We investigated Activin A and Follistatin in serum and tumor tissue of patients with TETs in relation to microvessel density (MVD), WHO histology classification, tumor stage and outcome. Membranous Activin A expression was detected in all tumor tissues of TETs, while Follistatin staining was found in tumor nuclei and cytoplasm. Patients with TETs presented with significantly higher Activin A and Follistatin serum concentrations compared to healthy volunteers, respectively. Follistatin serum concentrations correlated significantly with tumor stage and decreased to physiologic values after complete tumor resection. Follistatin serum concentrations correlated further with MVD and were associated with significantly worse freedom from recurrence (FFR). Low numbers of immature tumor vessels represented even an independent worse prognostic factor for FFR at multivariable analysis. To conclude, the Activin A - Follistatin axis is involved in the pathogenesis of TETs. Further study of Follistatin and Activin A in TETs is warranted as the molecules may serve as targets to inhibit tumor angiogenesis and tumor progression.

Figure 1

Activin A and Follistatin serum concentrations in patients with thymic epithelial tumors and Myasthenia Gravis Patients without TETs. Plots show preoperative Activin A and Follistatin serum concentrations in patients with thymic carcinomas (TCs), thymomas and healthy volunteers (A,B), and in patients with TETs according to Masaoka - Koga tumor stage (C,D). Further, the impact of tumor resection on postoperative Activin A and Follistatin serum levels is indicated (E,F). In addition, analysis of Activin A and Follistatin serum concentrations are illustrated in myasthenic patients (MG) compared to volunteers (G,H). Scatter dot plots are indicated as mean ± standard error of the mean.

Figure 2

Activin A and Follistatin expression in thymic malignancies. Strong cytoplasmic and membranous Activin A expression was found in all TETs. Representative Activin A staining is shown in a WHO type AB (A), B2 (C), and B3 (E) thymoma. Conversely, nuclear and cytoplasmic Follistatin staining was found in MNT (B), B2 thymoma (D), and TC (F) (magnification × 400; scale bar: 40 μm). WHO, World Health Organization; TC, thymic carcinoma; MNT, micronodular thymoma.

Figure 3

Microvessel Density in thymic epithelial tumors. CD34 staining of a Masaoka - Koga stage I WHO type B2 thymoma (A), a stage II A thymoma (B) and a stage III A thymoma (C) are shown (200 × magnification; scale bar: 80 μm). Preoperative Follistatin serum concentrations showed a significant negative correlation (D) with the number of immature tumor vessels (r: −0.443; p = 0.014), and a significantly positive correlation (E) with the amount of mature tumor vessels (r: 0.549; p = 0.002). While the amount of immature tumor vessels decreased with higher tumor stage (F), the number of mature tumor vessels increased with level of invasiveness (G). Similarly, total numbers of tumor vessels decreased from stage I to stage IV tumors (H). High preoperative Follistatin serum levels were associated with significantly worse FFR (I).