ll adhesion initiates the signals leading to the up-regulation of Necl-4 protein. In the process of cell–cell adhesion formation, it was reported that in MDCK cells, Rac1 is transiently activated by nectins during the initiation of their trans-interactions and then inactivated once the trans-interactions are established. Therefore, it is possible that Necl-4 may be involved in the inactivation of Rac1 after the trans-interactions of nectins are established. We also demonstrated here that Necl-5, nectin-2, and nectin-3 were down-regulated, whereas VE-cadherin remained unchanged by confluence. These results were in agreement with the earlier observations that upon cell–cell adhesion Necl-5 is down-regulated by clathrin-dependent endocytosis, VE-cadherin gene expression remains unchanged, and nectin-2 and nectin-3 gene expression is downregulated. Thus, the expression of Necl-4 is regulated in a novel way that is different from other CAMs implicated in contact inhibition. We previously showed that Necl-4 inhibited the phosphorylation of ErbB3 and the ErbB2/ ErbB3 signaling though PTPN13. Here we extended this observation and showed that Necl-4 inhibited the phosphorylation of VEGFR2 and its signaling 
though PTPN13. Similar to the action of the Necl-4 and PTPN13, VE-cadherin interacted with VEGFR2 and inhibited its phosphorylation through density-enhanced protein tyrosine PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/19768259 phosphatase-1, a tyrosine phosphatase. Thus, the Necl-4PTPN13 and VE-cadherin–DEP1 signalings regulate the contact-dependent inhibition of the phosphorylation of VEGFR2. However, these mechanisms may work at different stages. Because the PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/19769484 trans-interaction between Necl-4 is suggested to precede the trans-interaction between VE-cadherin upon cell–cell adhesion, the phosphorylation 14 / 20 Regulation of Contact Inhibition by Necl-4 of VEGFR2 may be first inhibited by the Necl-4PTPN13 signaling, resulting in reduced cell movement and proliferation, and further inhibited by the VE-cadherin–DEP1 signaling, resulting in the complete termination of cell movement and proliferation. Under confluent conditions, the VEGF-induced phosphorylation of VEGFR2 was inhibited despite the decreased interaction with Necl-4. This may be owing to the increased interaction between PTPN13 and VEGFR2. The reason why the interaction between PTPN13 and VEGFR2 was increased despite the down-regulation of PTPN13 under confluent conditions is SKI-II web currently unknown. However, the trans-interaction between Necl-4 might increase the interaction between Necl-4 and PTPN13, leading to the increased interaction between PTPN13 and VEGFR2. Another possibility is that the phosphatase activity of PTPN13 could increase under confluent conditions for some unknown reason. In the present study, Necl-4 stimulated the basal and VEGF-induced cell movement through the PTPN13ROCK–Rac1 pathway. This reveals a novel signaling pathway that regulates the activity of Rac1. The basal activities of ROCK and Rac1 may be responsible for the basal VEGF-independent cell movement. Compared with the activated state, not so much attention has been paid to the basal activity, but recent studies revealed its significance on various cellular functions. For example, in mouse olfactory sensory neurons, basal G-protein-coupled receptor activity determined the receptor-instructed axonal projection to olfactory bulb glomerulus. In rat hippocampal neurons, the basal activity of PKA was essential for maintaining phosphorylation of neuronal L-type