Jl. Chuang et Rr. Schleef, Recombinant semliki forest virus enhanced plasminogen activator inhibitor 1 expression and storage in the megakaryocytic cell line MEG-01, J CELL BIOC, 82(2), 2001, pp. 277-289
Platelet plasminogen activator inhibitor I (PAI-1), a trace alpha -granule
protein, is a key physiological regulator of fibrinolysis. Because informat
ion on the packaging of PAI-1 into alpha -granules during megakaryocytopoie
sis may reveal novel approaches for controlling hemostasis, this study inve
stigated basal, plasmid-mediated, and alphavirus-mediated PAI-1 packaging i
nto alpha -granules-like structures in the megakaryocytic cell line MEG-01.
Differentiation of MEG-01 cells with phorbol myristate acetate (PMA) was o
bserved to result in a four-fold increase in both secreted and cell-associa
ted PAI-1 antigen over a four day period. Subcellular fractionation of PMA-
treated MEG-01 cells on 45% self-forming Percoll gradients was employed to
separate low density membrane and Golgi-rich fractions from a high density
granule-containing region. A subsequent 30-60% pre-formed Percoll gradient
was employed to remove contaminating lysosomes from the PAI-1/glycoprotein
IIbIIIa-containing granules. Electron microscopy showed that these MEG-01 g
ranules share a similar size distribution (350-600 nm) and morphology to pl
atelet alpha -granules. PAI-1 (40 ng/mg protein) in isolated MEG-01 storage
granules was similar to 10% Of the levels present in isolated platelet alp
ha -granules. To elevate PAI-1 production/storage, two expression systems w
ere investigated. Experiments with plasmids encoding PAI-1 and beta -galact
osidase resulted in low transfection efficiency (0.001%). In contrast, Seml
iki Forest virus (SFV)mediated gene transfer increased cellular PAI-1 by 31
-fold (1,200 ng/10(6) cells at 10 MOI) in comparison to mock-infected cells
. Pulse-chase experiments demonstrated that SFV/PAI-1 mediated gene express
ion could enhance PAI-1 storage 6-9-fold, reaching levels present within pl
atelets. To document the ability of PAI-1 to be stored in a rapidly releasa
ble form in MEG-01 cells, we isolated platelet-like particles from the medi
a conditioned by the cells and examined secretagogue-induced release of PAI
-1. Particles from SFV/PAI-1 infected cells display a 5-fold enhanced secre
tion of PAI-1 following treatment with ADP in comparison to particles incub
ated in the absence of secretagogue. These results suggest that SFV mediate
d gene expression in MEG-01 cells provides a useful framework for analyzing
the production and storage of alpha -granule proteins. J. Cell. Biochem. 8
2: 277-289, 2001. (C) 2001 Wiley-Liss, Inc.