Role of the Activin/Follistatin system during angiogenesis and tumor progression

Activin A (AC) is a member of TGF-ß superfamily of growth factors that regulates cell proliferation, differentiation and apoptosis in many tissues. Follistatin (FS) is a functional antagonist of AC. The FS/AC system regulates endothelial cell (EC) proliferation. AC inhibits EC proliferation in an autocrine (constitutively expressed by ECs) and paracrine manner, contributing to maintenance of quiescent endothelium. Upon autocrine or paracrine bFGF stimulation, endothelial cells express FS, which stimulates EC proliferation by neutralizing growth inhibition mediated by constitutive expression of AC. The effect of FS on angiogenesis in vivo has been confirmed in a rabbit cornea assay. The aim of this study was to examine the hypothesis that AC inhibits angiogenesis and in particular to examine the role of AC/FS system in tumor angiogenesis and tumor growth. To address these questions, three different model systems were utilized: (i) 3D in vitro sprouting angiogenesis model, (ii) in vivo Matrigel assay and (iii) xenotransplant tumorigenesis assay. Results confirmed the expectations that AC inhibits bFGF induced angiogenesis in vitro and in vivo (Matrigel assay). By using a neutralizing antibody to FS in 3D in vitro sprouting angiogenesis assay, it was shown that bFGF induced angiogenesis is entirely mediated by FS that neutralizes AC mediated inhibition of angiogenesis. Further investigations revealed the role of AC and its functional antagonist FS in tumor angiogenesis and tumor growth by using FS and AC overexpressing mammary tumor cell line- R30C in a xenotransplant SCID mouse model. FS overexpression leads to stimulation of angiogenesis, indicating also inhibitory role of AC in tumor angiogenesis. Contrary to expectations, tumor growth was impaired in FS overexpressing tumors and stimulated in AC overexpressing tumors compared to MOCK control. Rate limiting importance of angiogenesis for tumor growth and metastases has so far been documented in numerous experiments and in the analysis of human tumors. Correlation of high microvessel density (MVD) with increased tumor growth, tumor aggressiveness and poor patient prognosis has been shown for many tumors, emphasizing the importance of increased angiogenesis in tumor progression. Results of tumorigenesis experiments with FS and AC overexpressing R30C, however, show that there is no stimulation of tumor growth via the stimulation of angiogenesis. In fact quite opposite, FS overexpressing tumors that have increased MVD compared to control tumors were growth retarded compared to control tumors. Likewise, increased tumor growth may happen in the absence of increased angiogenesis since AC overexpressing tumors grow faster even though they exhibit the same MVD as control tumors. Further examination of FS and AC overexpressing R30C tumors and in vitro experiments with FS and AC overexpressing cell lines revealed the cell autonomous role of AC in stimulation of tumor growth due to decreased susceptibility to apoptosis of AC overexpressing R30C cells. Differential effect of AC on the ECs and tumor cells (inhibits proliferation of ECs and decrease of apoptosis rate of tumor cells) leads to dissociation of tumorigenesis and angiogenesis. The fact that angiogenesis and tumorigenesis can be differentially regulated processes in some tumors points out that the use of MVD counting technique for gaining prognostic information and planning of treatment course is not applicable for all tumors.

Author dc:creator

• Krneta, Jelena

Contributors dc:contributor

• Marmé, Dieter

• Angiogenesis
• Tumor
• Transforming Growth Factor beta
• Activin
• Follistatin

Repository record source_url

https://freidok.uni-freiburg.de/data/1374

OAI identifier oai:identifier

oai:freidok.uni-freiburg.de:1374