Human c-fgr Induces a Monocyte-Specific Enzyme in NIH 3T3 Cells

Abstract

The mutant c-fgr protein (p58^c-fgr/F523) containing Phe-523 instead of Tyr-523 exhibited transforming activity in NIH 3T3 cells like other protein-tyrosine kinases of the src family, but normal p58^c-fgr/F523 (p58^c-fgr/wt) did not. The mutant protein showed tyrosine kinase activity threefold higher than that of the normal protein in vitro. Surprisingly, transfection of the normal c-fgr gene into NIH 3T3 cells resulted in induction of sodium fluoride (NaF)-sensitive α-naphthyl butyrate esterase (α-NBE), a marker enzyme of cells of monocytic origin, which was not induced in v-src, v-fgr, or lyn-transfected NIH 3T3 cells. The NaF-sensitive α-NBE induced in c-fgr transfectants was shown by isoelectric focusing to have a pI of 5.2 to 5.4, a range which was the same as those for thioglycolate-induced murine peritoneal macrophages and lα,25-dihydroxyvitamin D₃-treated WEHI-3B cells. Immunoblotting studies with antiphosphotyrosine antibodies revealed that 58-, 62-, 75-, 120-, 200-, and 230-kDa proteins were commonly phosphorylated at tyrosine residues in NIH 3T3 cells transfected with normal and mutated c-fgr, while 95-kDa protein was significantly phosphorylated at tyrosine residues in cells transfected with the mutated c-fgr. These findings suggest that tyrosine phosphorylation of specific cellular substrate proteins is important in induction of NaF-sensitive α-NBE and cell transformation by p58^c-fgr.