INDUCED PAI-1 MESSENGER-RNA EXPRESSION AND TARGETED PROTEIN ACCUMULATION ARE EARLY G(1) EVENTS IN SERUM-STIMULATED RAT-KIDNEY CELLS

Expression of plasminogen activator inhibitor type-1 (PAI-1), a member of the SERPIN gene family that functions to regulate the plasmin-base d pericellular proteolytic cascade, is growth state-regulated in norma l rat kidney (NRK) cells (Ryan and Higgins, 1990, !. Cell. Physiol., 1 55:376-384; Ryan et al., 1996, Biochem. J., 314:1041-1046). Comparativ e analysis of arrest states induced in NRK cells upon exposure to seru m-deficient (0.5% FBS) or serum-free culture conditions served to defi ne the kinetics of PAI-1 gene expression and fate of de novo-synthesiz ed PAI-1 protein. While cells rendered quiescent in serum-free or seru m-deficient media were equivalent with regard to the time course of PA I-1 mRNA induction, the fever of expressed transcripts (27-fold vs. 12 -fold) and accumulated saponin fraction PAI-1 protein (12-fold vs. 6-f old) were consistently greater in cells recruited into exponential gro wth phase from a serum-free as compared to a serum-deficient arrest co ndition. Relative PAI-I mRNA abundance increased within 1-2 hr post-se rum addition, was maximal at 4 hr, and declined rapidly thereafter; th is time course of expression coupled with placement of entry into DNA synthetic phase at approximately 12 hr after stimulation indicates tha t PAI-1 induction is an early-to-mid G1 phase event. Induced PAI-1 pro tein was evident immunocytochemically within 2 hr of serum stimulation as a peripheral ''rim'' of accumulated protein restricted to the cell ular ventral surface at the plane of the substrate. No PAI-1 was detec ted between individual cells suggesting that this protein may be targe ted directly to the undersurface region. By 6 hr post-stimulation, the rim of PAI-1 deposition increased in intensity and broadened to occup y approximately 30 to 50% of the total undersurface area. Double-label immunocytochemistry indicated that accumulated PAI-1 was deposited in close proximity to, but not actually within, vinculin-containing foca l contact structures. Potential functionality of induced PAI-1 express ion to either the initiation or maintenance of the serum-stimulated ph enotype was assessed using antibodies to PAI-1. The IgG fractions of t wo different antisera which neutralize the ability of PAI-1 to complex with and thereby inhibit the catalytic activity of urokinase plasmino gen activator significantly reduced (by 25-35%) the incidence of cells displaying the serum-stimulated phenotype; antibodies that bind PAI-l but do not block PAI-1 inhibitory activity were without effect In vie w of the vagaries of antibody accessibility and in situ neutralizing a ctivity (particularly in a region as structurally complex as the focal contact), these data may actually underestimate the importance of PAI -1 in maintaining the activated phenotype. (C) 1997 Wiley-Liss, Inc.