Elevation of membrane tyrosine phosphatase activity in density-dependent growth-arrested fibroblasts.

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Abstract
Swiss 3T3 cells harvested at high density contain a membrane protein-tyrosine-phosphatase (EC 3.1.3.48) whose specific activity is on average 8-fold higher than that of cells harvested at low or medium densities. Investigation of the conditions affecting this elevation of specific activity suggests that it is associated with density-dependent growth arrest. Fibroblasts in the exponentially doubling phase have a relatively low level of membrane phosphatase specific activity, which rises only as the rate of cell proliferation decreases and is maximal when cell growth is contact inhibited. These observations have been extended to BALB/c 3T3 fibroblasts and normal human diploid fibroblasts. The increase in membrane tyrosine phosphatase activity is coupled to density arrest and not to cellular quiescence in general, as no increase in phosphatase specific activity is detected when non-contact-inhibited cells are induced to arrest their growth through serum deprivation. The observed alterations in specific activity are attributable to a tyrosine phosphatase of Mr 37,000 that was purified and characterized from solubilized membrane fractions of Swiss 3T3 cells. A regulatory mechanism controlling tyrosine phosphatase activity may play a role in cell proliferation and growth arrest caused by cell contact.