Angiopoietin-1 (Ang-1), the main ligand for the endothelial cell (EC)-selective Tie-2 receptors, promotes survival, proliferation, migration and differentiation of these cells. Despite its importance in various aspects of vascular biology, the mechanisms of action of the Ang-1/Tie-2 receptor pathway have not been fully explored. / To identify the downstream modulators of Ang-1, we evaluated changes in the transcriptome of human umbilical vein endothelial cells (HUVECs) treated with Ang-1 protein for four hours by employing the oligonucleotide rnicroarray technology. Eighty-six genes were significantly upregulated by this treatment and forty-nine genes were significantly downregulated. These genes are involved in the regulation of cell cycle, proliferation, apoptosis, transcription and differentiation. Furthermore, we found that the Erk1/2, PI3-Kinase and mTOR pathways are implicated in promoting gene expression in HUVECs in response to Ang-1. Analysis of the microarray data employing the Ingenuity Pathways analysis software to place the regulated genes in the context of biological networks revealed several highly connected nodes including the chemokine Interleukin-8 (IL-8) and the transcription factor Early growth response-1 (Egr-1). Due to the importance of these genes in promoting angiogenesis, we decided to evaluate their roles in Ang-1/Tie-2 receptor signaling and biological effects. / Ang-1 induced IL-8 expression in a time- and dose-dependent manner in ECs through both transcriptional and post-transcriptional mechanisms. To study the functional role of Ang-1-induced IL-8, we generated HUVECs that overexpress Ang-1. In these cells, neutralizing IL-8 significantly reduced EC proliferation and migration. IL-8 promoter activity experiments and gel shift assays revealed the involvement of the transcription factor AP-1 in Ang-1-induced IL-8. Ang-1 stimulated the phosphorylation of c-Jun through activation of Erk1/2, JNK and PI-3 kinase pathways. Similarly, Ang-1 provoked the expression and DNA binding of Egr-1 in HUVECs. Employing siRNA and DNAzyme to specifically knock-down Egr-1, we found that Ang-1-induced Egr-1 also promotes EC proliferation and migration. / We conclude that Ang-1 provokes a coordinated response intended to promote EC survival, proliferation, and angiogenesis and to inhibit EC apoptosis. Ang-1 induces EC proliferation and migration in part through the secretion of the soluble mediator Interleukin-8 and through induction of the transcription factor Egr-1.

The increase in ASMC mass is a major structural change described in airway remodeling in asthma. This increase has been attributed to ASMC hyperplasia and hypertrophy. The distance between ASMC and the epithelium is reduced suggesting expansion of the muscle bundle towards the epithelium. Recent studies have suggested a role of epithelial derived chemokines in ASMC migration toward the epithelium. We hypothesized that chemokines (Eotaxin, RANTES, MIP-1alpha and IL-8) can directly influence ASMC mass by increasing the rate of proliferation or enhancing survival. ASMCs were exposed to different concentrations of eotaxin, RANTES, IL-8 or MIP-1alpha. To test for proliferation, stimulated ASMC were pulsed with 3H-thymidine or stained with BrdU and then analyzed with flow cytometry. Apoptosis was measured using Annexin V and flow cytometry. Expression of phosphorylated p42/p44 and MAPKinases was assessed by Western analysis. In a concentration-dependent manner, chemokines such as Eotaxin, RANTES, IL-8 and MIP-lalpha increased ASMCs 3H-thymidine incorporation and DNA synthesis. Eotaxin, RANTES and IL-8 decreased the number of apoptotic ASMCs compared to the matched controls. A significant increase in phosphorylated p42/p44 MAPKs was seen after treating ASMCs with RANTES and eotaxin. We conclude that chemokines might contribute to airway remodeling by increasing the number of ASMCs.

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