Ectodomain shedding of pro-TGF-α is required for COX-2 induction and cell survival in renal medullary cells exposed to osmotic stress

In the renal medulla, cyclooxygenase (COX)-2 is induced by osmotic stress as present in this kidney region during antidiuresis. Increasing evidence suggests that EGF receptor (EGFR) signaling is involved in this process. The aim of the present study was to examine the mechanisms responsible for COX-2 expression and PGE2 production during hypertonic conditions and to identify potential autocrine/paracrine EGFR ligands. Immunohistochemisty and Western blot analysis revealed abundant expression of the pro-EGFR ligand pro-transforming growth factor (TGF)-α in renal medullary cells in vivo and in cultured Madin-Darby canine kidney cells. In Madin-Darby canine kidney cells, hypertonicity rapidly increased TNF-α converting enzyme (TACE)-dependent ectodomain shedding of pro-TGF-α; phosphorylation of EGFR, p38, and ERK1/2; expression of COX-2; and production of PGE2. Conversely, TACE inhibition prevented TGF-α release; EGFR, p38, and ERK1/2 activation; and COX-2 expression. Furthermore, cell survival was reduced substantially, a response that could be reversed by the addition of PGE2. Simultaneous addition of recombinant TGF-α during TACE inhibition restored EGFR and MAPK phosphorylation, COX-2 expression, PGE2 production, and cell survival during osmotic stress. These results indicate that hypertonicity induces TACE-mediated ectodomain shedding of pro-TGF-α, which subsequently activates COX-2 expression in an autocrine/paracrine fashion, via EGFR and MAPKs. We conclude that tonicity-induced TGF-α release is required for COX-2 expression, PGE2 synthesis, and survival of renal medullary cells during osmotic stress.