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.