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* International Institute of Genetics and Biophysics, Consiglio Nazionale delle Ricerche, Via Marconi, 10, Napoli, Italy; Serine phosphorylation of human pro-urokinase (pro-uPA) by A431 human carcinoma cells results
in a catalytically active molecule with reduced sensitivity to plasminogen activator inhibitor type 1. We
mapped the phosphorylated seryl residues by analyzing
the in vivo phosphorylation state of engineered prouPA variants carrying a COOH-terminal poly-histidine
tag. Stably transfected A431 cells do not incorporate radioactive phosphate into tagged pro-uPA in which the
serines 138 and 303 have been replaced with glutamic residues, although endogenous nontagged pro-uPA is
32P-labeled on A and B chains. Moreover, the catalyticindependent ability of the mono- and di-substituted
"phosphorylation-like" variants to bind to the GPIanchored urokinase receptor (uPAR) and promote adherence of differentiating U937, HL-60, and THP-1 myelomonocytic cells was examined. We found that
glutamic residues as well as the naturally occurring
phosphoserines at positions 138 and 303 abolish proadhesive ability, although they do not interfere with receptor binding. In addition, pro-uPA carrying Glu138/303
lacks the capability to induce a chemotactic response of
THP-1 cells. The exclusive presence of Glu138 reduces
pro-uPA proadhesive and chemotactic ability by 70-
80%, indicating that a phosphoserine residue at the
same position plays a major inhibitory role of myeloid
cell response to pro-urokinase. The di-substitution does
not affect pro-uPA ability to interact with vitronectin
or to enhance binding of urea-denatured vitronectin to
uPAR. However, unlike wild-type tagged pro-uPA, the
di-substituted variant does not induce receptor polarization in pre-adherent U937 cells. Taken together, the
data support the possibility that pro-uPA phosphorylation on Ser138/303 can modulate uPAR transducing
ability.
Farmitalia Carlo Erba, Nerviano (Milano), Italy; and § Lepetit Spa Research Center, Gerenzano (Varese), Italy
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