HOP Regulates Cell Migration Figure 1. HOP promotes endothelial tube formation and sprouting. A. HUVECs transfected with HOP or control siRNAs were plated onto matrigel, and photographs were taken 4 and 16 hours later. B. Immunoblotting analysis of HOP and b-actin expression in HUVECs transfected with HOP or control siRNAs. C. Experiments were performed as in A, and the cumulative tube length was measured. *, p,0.05 versus control. D. HUVECs transfected with GFP, GFP-HOP, GFP-HOP plus HOP shRNA, or GFP-mHOP (shRNA-resistant mutant) plus HOP shRNA were plated onto matrigel, and photographs were taken 1, 3, and 6 hours later. E. Experiments were performed as in D, and the cumulative tube length was measured. *, p,0.05. F. Capillary-like sprout formation from spheroids generated from HUVECs transfected with HOP or control siRNAs. G. Experiments were performed as in F, and the cumulative sprout length was measured. *, p,0.05 versus control. doi:10.1371/journal.pone.0036389.g001 mutant HOP in tube formation (Figure 1D and 1E), validating the specificity of the interference experiments. We then performed capillary sprouting assay to further examine the role of HOP in angiogenesis. We found that the ability of endothelial cell spheroids to form capillary-like sprouts was significantly impaired by transfection with HOP siRNAs (Figure 1F). Calculation of sprout length showed that the #1 and #2 HOP siRNAs decreased sprout length by 71% and 74%, respectively (Figure 1G). These data indicate that HOP is important for angiogenesis in vitro. PLoS ONE | www.plosone.org 3 April 2012 | Volume 7 | Issue 4 | e36389