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

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 81; no. 15; pp. 4717 - 4721
• Main Authors: Hirata, F, Matsuda, K, Notsu, Y, Hattori, T, del Carmine, R
• Format: Journal Article
• Language: English
• Published: United States National Acad Sciences 01.08.1984
• Subjects: Animals; Annexins; Calcium-Binding Proteins; Cells, Cultured; Cyclic AMP - pharmacology; Glycoproteins; Lymphocyte Activation; Mice; Mitosis; Phosphoproteins - metabolism; Phosphotyrosine; Protein Kinase C; Protein Kinases - metabolism; Proteins - metabolism; Thymus Gland - metabolism; Tyrosine - analogs & derivatives; Tyrosine - metabolism
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.81.15.4717

When murine thymocytes were stimulated by mitogens such as concanavalin A, the Ca2+ ionophore A23187, or 4 beta-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.