Mitogenic action of lysophosphatidic acid in proximal tubular epithelial cells obtained from voided human urine

Focal tubular cell multiplication at sites on an injured nephron is a critical event in the recovery phase following acute tubular necrosis. During this process, numerous viable tubular cells exfoliate and are shed into the urine. Lysophosphatidic acid (LPA) is generated in the plasma membrane of injured cells and acts as an intercellular mediator of various biological processes, including inflammation, proliferation and repair. In the present study, exfoliated proximal tubule (PT) cells were isolated from human urine and the mitogenic effects of LPA were investigated as a model of repair and proliferation following renal injury. LPA stimulated a 23.5% increase in DNA synthesis, a 29.4% increase in cell number and an 86.6% decrease in cAMP content. All of these responses were pertussis toxin sensitive, indicating the involvement of G_i-type G-proteins in LPA signalling. Conversely, the LPA-induced DNA synthesis and the decrease in intracellular cAMP content were insensitive to wortmannin, an inhibitor of phosphatidylinositol 3-kinase (PI3K), suggesting a mitogenic response via PI3K-independent mechanisms. Furthermore, we detected specific mRNA transcripts for the recently cloned human LPA-receptors, endothelial differentiation gene (Edg)-2 and Edg-4 (Edg-2 ≫ Edg-4) by reverse transcription-PCR in PT cells. Our data suggest that LPA may behave as a local growth factor in PT cells following tubular injury.