Involvement of Pax-2 in the action of activin A on tubular cell regeneration

It has been recently shown that in ischemic rat kidneys activin A is induced in tubular cells and inhibits their regeneration. The present study was conducted to further investigate the action of activin A in tubular cells during regeneration. Among genes thought to be critical for kidney development, Pax-2 was upregulated in tubular cells during regeneration after renal ischemia. Pax-2 protein was localized in nuclei of tubular and interstitial cells, some of which co-expressed a mesenchymal cell marker, vimentin, suggesting that a population of Pax-2-positive cells have properties of immature progenitor-like tubular cells. The Pax-2-expressing cells co-expressed a cell proliferation marker, BrdU, activin A, and the type 11 activin receptor. Activin A modulated growth of BrdU/Pax-2 double-positive cells since an administration of follistatin increased; conversely, exogenous activin A de-creased the number of BrdU/Pax-2 double-positive cells after renal ischemia. Activin A also reduced the expression of Pax-2 in cultured metanephroi. A proximal tubular cell line, LLC-PK1 cells, was used to further study the mode of action of activin A. The expression of Pax-2 was not detected in quiescent LLC-PK1 cells, but it was markedly increased when growth was stimulated. Under this condition, activin A significantly inhibited DNA synthesis and reduced the expression of Pax-2 in LLC-PK1 cells. In contrast, blockade of the activin signaling by overexpressing dominantly negative mutant receptor enhanced the expression level of Pax-2 in LLC-PK1 cells and induced an immature phenotype. These results suggest that activin A regulates tubular cell growth and differentiation by modulating the expression of Pax-2 during regeneration.