Hypotonicity stimulates renal epithelial sodium transport by activating JNK via receptor tyrosine kinases

We previously reported that hypotonic stress stimulated transepithelial Na+ transport via a pathway dependent on protein tyrosine kinase (PTK; Niisato N, Van Driessche W, Liu M, Marunaka Y. J Membr Biol 175: 63 - 77, 2000). However, it is still unknown what type of PTK mediates this stimulation. In the present study, we investigated the role of receptor tyrosine kinase (RTK) in the hypotonic stimulation of Na+ transport. In renal epithelial A6 cells, we observed inhibitory effects of AG1478 [an inhibitor of the EGF receptor (EGFR)] and AG1296 [an inhibitor of the PDGF receptor (PDGFR)] on both the hypotonic stress-induced stimulation of Na+ transport and the hypotonic stress-induced ligand-independent activation of EGFR. We further studied whether hypotonic stress activates members of the MAP kinase family, ERK1/2, p38 MAPK, and JNK/SAPK, via an RTK-dependent pathway. The present study indicates that hypotonic stress induced phosphorylation of ERK1/2 and JNK/ SAPK, but not p38 MAPK, that the hypotonic stress- induced phosphorylation of ERK1/2 and JNK/SAPK was diminished by coapplication of AG1478 and AG1296, and that only JNK/ SAPK was involved in the hypotonic stimulation of Na+ transport. A further study using cyclohexamide (a protein synthesis inhibitor) suggests that both RTK and JNK/ SAPK contributed to the protein synthesis-independent early phase in hypotonic stress-induced Na+ transport, but not to the protein synthesis-dependent late phase. The present study also suggests involvement of phosphatidylinositol 3-kinase (PI3-kinase) in RTK-JNK/SAPK cascade-mediated Na+ transport. These observations indicate that 1) hypotonic stress activates JNK/ SAPK via RTKs in a ligand-independent pathway, 2) the RTK-JNK/SAPK cascade acts as a mediator of hypotonic stress for stimulation of Na+ transport, and 3) PI3-kinase is involved in the RTK-JNK/SAPK cascade for the hypotonic stress-induced stimulation of Na+ transport.