Follistatin-like protein 1 plays a tumor suppressor role in clear-cell renal cell carcinoma

Background: We previously showed that the expression of follistatin-like protein 1 (FSTL1) was significantly downregulated in metastatic clear-cell renal cell carcinoma (ccRCC). In this study, we aimed to characterize the role of FSTL1 in the development of ccRCC. Methods: The effects of FSTL1 on cell activity and cell cycle were investigated in ccRCC cell lines with altered FSTL1 expression. Gene expression microarray assays were performed to identify the major signaling pathways affected by FSTL1 knockdown. The expression of FSTL1 in ccRCC and its effect on postoperative prognosis were estimated in a cohort with 89 patients. Results: FSTL1 knockdown promoted anchorage-independent growth, migration, invasion, and cell cycle of ccRCC cell lines, whereas FSTL1 overexpression attenuated cell migration. FSTL1 knockdown up-regulated nuclear factor-kappa B (NF-kappa B) and hypoxia-inducible factor (HIF) signaling pathways, increased epithelial-to-mesenchymal transition, upregulated interleukin-6 expression, and promoted tumor necrosis factor-a-induced degradation of NF-kappa B inhibitor (I kappa Ba) in ccRCC cell lines. FSTL1 immunostaining was selectively positive in epithelial cytoplasm in the loop of Henle, and positive rate of FSTL1 was significantly lower in ccRCC tissues than in adjacent renal tissues (P < 0.001). The multivariate Cox regression analysis showed that the intratumoral FSTL1 expression conferred a favorable independent prognosis with a hazard ratio of 0.325 (95% confidence interval 0.118-0.894). HIF-2 alpha expression was negatively correlated with FSTL1 expression in ccRCC specimens (r = -0.229, P = 0.044). Intratumoral expression of HIF-2 alpha, rather than HIF-1 alpha, significantly predicted an unfavorable prognosis in ccRCC (log-rank, P = 0.038). Conclusions: FSTL1 plays a tumor suppression role possibly via repressing the NF-kappa B and HIF-2 alpha signaling pathways. To increase FSTL1 expression might be a candidate therapeutic strategy for metastatic ccRCC.