Lysophosphatidic acid-induced proliferation in opossum kidney proximal tubular cells: Role of PI 3-kinase and ERK

Background. Lysophosphatidic acid (LPA) is a mitogenic lipid bound to albumin in the circulation and implicated in the induction of proximal tubular cell (PTC) injury in proteinuric states. In this study, we investigated the effect of LPA on proliferation of opossum kidney (OK) cells and the roles of the p85/p110 phosphatidylinositol 3-kinase (PI3-kinase) and extracellular signal-regulated kinases (ERKs) ERK-1 and ERK-2 in LPA-induced proliferation. Methods. [H-3]-thymidine incorporation was used as an index of OK cell proliferation. PI 3-kinase and ERK activities were measured by in vitro kinase assays of immunoprecipitates from both wild-type OK cells and OK cells expressing a dominant negative p85 (Delta p8.5) subunit of PI 3-kinase in an inducible vector. Results. LPA stimulated a marked increase in [H-3]-thymidine uptake in wild-type and Delta p85 OK cells. OK cell PI 3-kinase activity was stimulated by LPA and was inhibited by expression of Delta p85. LPA-induced proliferation was inhibited by wortmannin and the induction of Delta p85 expression. These data suggest that LPA stimulates PI 3-kinase activity, which is essential for signaling the induction of proliferation. LPA also stimulated ERK activity (peak at 5 min, return to baseline by 60 min) maximally at a dose of 100 mu M LPA. This increase was approximately 600% above basal and was similar to the effects of 10% fetal calf serum. The proliferative effect of LPA was decreased by the ERK-kinase (MEK) inhibitor PD98059 (5 mu M), therefore suggesting that ERK as well as PI 3-kinase activation is important for proliferation. ERK activation by LPA was not affected by pretreatment with wortmannin or by the expression of Delta p85. PI 3-kinase activation by LPA was not affected by pretreatment with PD98059. Conclusions. We conclude that activation of PI 3-kinase is essential for the LPA-induced proliferation of OK cells and that ERK activation is also important. Therefore, they are both vital elements in separate signaling pathways leading to cell proliferation. LPA filtered into the proximal tubule in proteinuric states is likely to have profound effects on PTC growth.