Activin A is a potent activator of renal interstitial fibroblasts.

The present study was conducted to examine the involvement of the activin-follistatin system in the fibrotic process of the kidney. Immunoreactive activin A was upregulated in tubular cells in the kidneys with unilateral ureteral obstruction but not in normal and contralateral kidneys. Activin A promoted cell proliferation, enhanced the expression of type I collagen mRNA, and induced the production of alpha-smooth muscle actin in a rat kidney fibroblast cell line (NRK-49F cells) as well as in primary cultured renal interstitial fibroblasts. In contrast, activin A did not affect the expressions of alpha-smooth muscle actin and type I collagen in renal epithelial tubular cell lines LLC-PK1, and MDCK. Follistatin, an antagonist of activin A, significantly inhibited cell proliferation in NRK-49F cells. Blockade of activin signaling by overexpression of truncated type II activin receptor, which lacked the intracellular kinase domain, decreased cell proliferation and reduced the expression level of type I collagen mRNA in NRK-49F cells. The expression of activin A was induced by TGF-beta 1 or activin A itself. Induction of type I collagen expression by TGF-beta 1 was reduced by follistatin or by overexpression of truncated type II activin receptor. These results suggest that activin A produced by tubular cells acts as a paracrine factor that activates renal interstitial fibroblasts during the fibrotic processes of the kidney.