The role of phosphatidylinositol 3-kinase and the mitogen-activated protein kinases in insulin-like growth factor-I-mediated effects in vascular endothelial cells

Despite an improved understanding of the molecular mechanisms of insulin-li ke growth factor-I (IGF-I) signaling and the recognition that IGF-I mediate s many effects in endothelial cells, some of which may be important for ath erosclerosis, little is known about the signal transduction pathways that m ediate the effects of IGF-I in endothelial cells. To that end, we examined the signaling pathways activated by IGF-T in endothelial cells and their co ntribution to IGF-I-stimulated endothelial cell migration and nuclear facto r (NF)-kappaB-dependent transcription. Treatment of bovine pulmonary artery endothelial cells (PAEC) with IGF-I activated the mitogen-activated protei n kinases extracellular signal-regulated kinase (ERK)1/2 and ERK5. In contr ast, IGF-I had no effect on either c-Jun amino-terminal kinase or p38 kinas e activity. IGF-I also activated phosphatidylinositol (PI) 3-kinase, as ref lected by increased phosphorylation of Akt. There was no evidence of cross- talk between the ERK and PI 3-kinase pathways in PAEC. In PAEC transiently transfected with pTK81-NF kappaB-Luc, which contained four copies of the NF -kappaB DNA binding site 5 ' to a minimal promoter and the luciferase gene, treatment with 50 ng/ml IGF-I increased luciferase activity 18-fold. Inhib ition of ERK activity using PD98059 and PI S-kinase activity with LY 294002 abrogated the induction of NF-kappaB-dependent transcription by IGF-T, sug gesting that both pathways contribute to the effect of IGF-I on NF-kappaB-d ependent transcription. In contrast to the effect of tumor necrosis factor- alpha on NF-kappaB activation, Western blot analyses demonstrated that TGF- I had no effect on I kappaB phosphorylation and degradation or nuclear tran slocation and DNA binding of NF-kappaB. These data suggest a direct of effe ct of IGF-T on nuclear NF-kappaB. IGF-I also increased endothelial cell mig ration approximately 2-fold, as demonstrated using a Boyden chamber apparat us. IGF-I-induced endothelial cell migration was inhibited, in part, by LY 294002 but not PD98059. Together, these studies demonstrate that IGF-I acti vates multiple signaling pathways in endothelial cells with little evidence for crosstalk between the pathways. Moreover, these pathways appear to med iate both overlapping and distinct effects in that activation of both PI 3- kinase and the ERKs contributed to the stimulation of NF-kappaB-dependent t ranscription by IGF-I, whereas only PI S-kinase mediated IGF-I-stimulated e ndothelial cell migration.