Protein Phosphorylation in Endothelial Cell Membranes: Relationship to Growth Control

Abstract

The activation of various kinase systems during the proliferative response of fibroblasts and certain tumor cell lines is well documented. The intracellular signalling pathway involved in the triggering of endothelial cell (EC) mitogenesis, however, has received little attention. We have employed cultured porcine aortic EC to examine the possible role of protein phosphorylation events in EC growth control. Triton-solubilized membranes from proliferating (sparse) and quiescent (confluent) EC were incubated under standard kinase assay conditions and radiolabeled membrane phosphoproteins were separated by SDS-polyacrylamide gel electrophoresis and visualized by autoradiography. The extent of phosphorylation was dramatically elevated in sparse over confluent samples for four phosphoproteins including two serine phosphoproteins (180 kDa and 100 kDa), a 97 kDa protein phosphorylated on both serine and threonine and a 55 kDa protein which exhibited a specific and dramatic increase in tyrosine phosphorylation. Cofactor studies revealed that Mn²⁺ is the optimum cofactor for the EC membrane kinase(s) and that Mg²⁺ can replace Mn²⁺ only for the kinase acting on the 100 kDa protein. The kinases, acting on all four phosphoproteins, were not dependent on Ca²⁺, cAMP, or cGMP. The observed membrane phosphorylation patterns were similar in porcine and bovine aortic EC but were different from human fibroblasts or 3T3 cells. Sparse, quiescent EC exhibited a phosphoprotein pattern similar to that of confluent cultures, suggesting a growth dependence for the enhanced kinase activity that was observed in membranes from proliferating cells.