Increase in receptor-like protein tyrosine phosphatase activity and expression level on density-dependent growth arrest of endothelial cells

Abstract

Protein tyrosine phosphatase (PTPase) activity was examined in two cell lines: human umbilical vein endothelial (HUVE) cells, which display contact inhibition of cell growth, and A427 human adenocarcinoma cells, which have lost this ability. HUVE cells harvested at high density displayed a 10-fold increase in membrane-associated PTPase activity. A427 cells exhibited no such phenomenon. Moreover, modification of HUVE cell growth rate by a stimulating agent such as basic fibroblast growth factor or by blocking compounds such as thymidine or suramin resulted in no change in PTPase activity, suggesting that the observed increase in membrane-associated activity at confluency was specific for cell-cell-contact-directed growth arrest. The expression of various PTPase mRNAs was examined in HUVE and A427 cells. Of the receptor-like PTPases tested, two were exclusively expressed in HUVE cells (PTP gamma and HPTP beta). Only HPTP beta, which is structurally similar in its extracellular region to cell-adhesion receptors of the immunoglobulin superfamily, displayed a pattern of expression related to the increase in PTPase activity. Competitive PCR was used to quantify its expression during cell culture. A 12-fold increase in HPTP beta mRNA expression was detected and it parallelled the time course of PTPase activity. This observation strongly implicates receptor-like PTPases in density-dependent growth arrest.

Confluence of Vascular Endothelial Cells Induces Cell Cycle Exit by Inhibiting p42/p44 Mitogen-Activated Protein Kinase Activity

“…Therefore, we suggest that orthovanadate-sensitive phosphatases participate in the inactivation of p42/p44 MAPK in confluent endothelial cells. In fact, a few reports have highlighted increases in both cytosolic and membrane-associated tyrosine phosphatase activities at high cell densities in osteoblast cells (48), in endothelial cells (17), and in Swiss 3T3 fibroblasts (40). In accord with these results, we have shown that the phosphatase activity sensitive to sodium orthovanadate is increased in confluent mouse endothelial cells compared to sparse cells.…”

Protein-Tyrosine Phosphatases in the Vessel Wall

“…46 PTP␤ itself has been shown to be a substrate for serine phosphorylation by protein kinase C. 47 However, little is known of potential in vivo functions of PTP␤ other than that provided by a report describing a 12-fold increase in total plasma membraneassociated PTPase activity in confluent versus sparse cultures of human umbilical vein endothelial cells, which was attributed to a parallel increase in PTP␤ mRNA expression. 48 These observations and the endothelium-specific expression of PTP␤ mRNA revealed in the present study suggest that PTP␤ may be a specific regulator of endothelial cell adhesion or endothelial permeability.…”

Functional interaction of vascular endothelial-protein-tyrosine phosphatase with the Angiopoietin receptor Tie-2

“…This is the ®rst identi®cation of a receptor-type PTP speci®cally expressed in ECs. Both, Tie-2 (Schlaeger, 1998) and HPTPb, the human homologue of VE-PTP are upregulated with increasing cell density in cultured EC (Gaits et al, 1995), which in the case of HPTPb is thought to play a role in contact inhibition of growth (Schaapveld et al, 1997). In vivo, this is paralleled by the predominant expression of VE-PTP in ECs of more mature vessels.…”