CELL CYCLE-DEPENDENT FLUCTUATION OF UROKINASE-TYPE PLASMINOGEN-ACTIVATOR, ITS RECEPTOR, AND INHIBITORS IN CULTURED BOVINE MAMMARY EPITHELIAL AND MYOEPITHELIAL CELLS

Bovine mammary epithelial (BME-UV1, clone E-T and BME-UV, clone E-T2) and myoepithelial (BMM-UV, clone m-T2) cell lines were used to study t he modulation of cell-associated activity of urokinase-type plasminoge n activator (u-PA), as well as mRNA transcripts of u-PA, its receptor (u-PAR), and inhibitors (PAI-1 and PAI-2) during the cell cycle. After release from a growth arrest accomplished by growth factor deprivatio n, the length of the cell cycle was determined as 19-21 h, with G(1), S, and G(2)+M phases of 6-7, 7-9, and 5-6 h respectively. As the cell cycle progressed, accumulated cell-associated u-PA activity increased. Maximal activity occurred at the S/G(2) boundary and decreased during the G(2)/M phases. All cell lines tested produced plasmin-specific in hibitor(s). Accumulation of u-PA mRNA peaked 3 h after stimulation int o the growth cycle for m-T2 and E-T and during 3-6 h for E-T2 cells. M aximum levels of u-PAR mRNA were observed at 3 h for the E-T cell line , 6-9 h for E-T2 cells, and 3-9 h for m-T2 cells. The cell cycle distr ibution of the PAI-1 mRNA was similar to that of u-PA for both epithel ial cell lines, while for m-T2 cells maximal accumulation of PAI-1 mRN A was detected at 3-9 h after growth initiation. The increase of PAI-2 mRNA transcription for m-T2 and E-T cells was detected at 3-6 h. The PAI-2 mRNA in E-T2 cells was under detectable levels. The data indicat e that the expression of the constituents of the PA system in bovine m ammary epithelial and myoepithelial cells is not cell type-dependent b ut is tightly connected to the phase of the cell cycle. (C) 1998 Elsev ier Science B.V.