Expression of VACM-1/cul5 mutant in endothelial cells induces MAPK phosphorylation and maspin degradation and converts cells to the angiogenic phenotype

Vasopressin-activated calcium mobilizing receptor (VACM-1) is a member of the cullin gene family involved in ubiquitin-proteosome dependent regulation of cellular functions. Expression of VACM-1 cDNA in cos-1 cells in vitro decreases basal cAMP levels and inhibits growth. The expression of S730AVACM-1 mutant cDNA, which removes PKA-dependent phosphorylation site in the VACM-1 cDNA sequence, reverses this phenotype. Since the expression of VACM-1 protein in vivo localizes largely to the vascular endothelial cells, in this study, we examined the effects of S730AVACM-1 cDNA expression on cellular signaling in the rat endothelial cell line RAMEC. Our results indicate that expression of S730AVACM-1 cDNA in RAMEC promotes cellular proliferation and induces angiogenic growth patterns. Western blot analyses indicate that S730AVACM-1 cDNA transfected cells express increased levels of Nedd8 modified VACM-1 and have higher levels of phosphorylated MAPK protein when compared to controls. Furthermore, expression of S730AVACM-1 cDNA induces translocation of the endogenous early response gene, egr-1, to the nucleus and leads to morphological changes that involve actin rearrangement. Finally, expression of S730AVACM-1 cDNA in RAMEC decreases concentration of maspin, a putative anti-angiogenic factor with a tumor suppressor activity. These results show that VACM-1 protein regulates endothelial cell growth and may modulate angiogenesis by a mechanism that involves MAPK phosphorylation, nuclear localization of egr-1, maspin expression, and actin polymerization.