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.
Taruno
Akiyuki
A
Dept. of Molecular Cell Physiology, Kyoto Prefectural University of Medicine, Kyoto 602-8566, Japan.
Niisato
Naomi
N
Marunaka
Yoshinori
Y
D000818:Animals; D001693:Biological Transport, Active; D015153:Blotting, Western; D002478:Cells, Cultured; D004789:Enzyme Activation; D004791:Enzyme Inhibitors / Q000494:pharmacology; D004847:Epithelial Cells / Q000201:enzymology / Q000378:metabolism; D004848:Epithelium / Q000378:metabolism; D066246:ErbB Receptors / Q000502:physiology; D048031:JNK Mitogen-Activated Protein Kinases / Q000502:physiology*; D007668:Kidney / Q000378:metabolism*; D008024:Ligands; D009994:Osmolar Concentration; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism; D010766:Phosphorylation; D020794:Receptor Protein-Tyrosine Kinases / Q000037:antagonists & inhibitors / Q000502:physiology*; D015398:Signal Transduction / Q000502:physiology; D013696:Temperature; D014882:Water-Electrolyte Balance / Q000502:physiology*; D014982:Xenopus laevis
D004791:Enzyme Inhibitors; D008024:Ligands; D019869:Phosphatidylinositol 3-Kinases; D066246:ErbB Receptors; D020794:Receptor Protein-Tyrosine Kinases; D048031:JNK Mitogen-Activated Protein Kinases