Phosphorylation at a tyrosine residue of lipomodulin in mitogen-stimulated murine thymocytes

When murine thymocytes were stimulated by mitogens such as concanavalin A, the Ca²⁺ ionophore A23187, or 4β-phorbol 12-myristate 13-acetate, there was a marked increase of ³²P incorporation into immunoprecipitable lipomodulin, a phospholipase inhibitory protein. These compounds enhanced ⁴⁵Ca²⁺ influx into thymocytes, which, in turn, increased protein phosphorylation, probably by Ca²⁺- and phospholipid-dependent protein kinase (protein kinase C). Cyclic 8-bromo-AMP, an inhibitor of lymphocyte mitogenesis, blocked the mitogen-stimulated phosphorylation of lipomodulin, although it stimulated the protein phosphorylation via cyclic AMP-dependent kinase (protein kinase A). On electrophoresis, the hydrolysates of ³²P-labeled lipomodulin showed a single radioactive spot, which comigrated with authentic phosphotyrosine. The partially purified middle-sized tumor antigen was able to phosphorylate lipomodulin after being phosphorylated by protein kinase C but not by the catalytic subunit of protein kinase A. Our findings suggest that the activity of a tyrosine-specific kinase, which phosphorylates lipomodulin in vivo as well as in vitro, is stimulated by protein kinase C and inhibited by protein kinase A.