Intracellular plasminogen activator activity in growing and quiescent cells

Intracellular plasminogen activator (PA) was examined in 3T3 and transformed 3T3 cells under various growth conditions to determine whether expression of this activity changes with the growth state. During exponential growth, SV40 and benzpyrene (BP) transformed 3T3 cells exhibited 3- to 5-fold more intracellular PA activity than untransformed 3T3 cells. This relationship changed as the cells exhausted serum factors and arrested in G₁. The specific activity of intracellular PA in cells that have retained a serum-sensitive restriction point in G₁ (G₀) (3T3 and BP 3T3) increased 200- and 20-fold, respectively, at this time, while the level in cells that have lost most growth control mechanisms (SV3T3) remained constant. At confluency, 3T3 cells had considerably more PA than either of their transformed counterparts. Sparse cultures of 3T3 and BP3T3 cells arrest at G₁ following serum depravation, and also accumulate high intracellular PA activity. The addition of serum or purified epidermal growth factor to these cultures initiated cell proliferation and resulted in a rapid, actinomycin D-sensitive loss of this activity. Less than 50% of the original activity remained 30 minutes after growth stimulation. This loss of intracellular PA activity did not appear to result from the presence of serum or cellular inhibitors. Intracellular PA activity remained low following growth stimulation. It increased again as the cells traversed through G₁. These findings indicate that intracellular PA activity fluctuates with the growth state of cells, and may be related to the cell cycle. Culture conditions which place cells, whether normal or transformed, in G₁ arrest lead to increased intracellular PA, while factors that initiate growth again result in a rapid loss of this activity. This behavior is lacking in cells not subject to density-dependent inhibition of growth. Like many other correlates of transformation, comparison of intracellular PA in normal and transformed cells must be defined in terms of the growth state of the cells in question.