ag-213 and Skin-Neoplasms

Epidermal growth factor (EGF) induces rapid actin filament assembly in the membrane skeleton of A431 cells, leading to a approximately 30% rise in cellular filamentous actin levels. EGF-induced actin polymerization depends upon EGF receptor (EGFR) tyrosine kinase activity, since the selective tyrosine kinase inhibitor AG213 abolishes EGF-induced actin polymerization. In accordance, confocal laser scanning microscopy shows that newly assembled actin filaments localize selectively to the tyrosine-phosphorylated EGFR in the plasma membrane, since actin polymerization is not observed at the internalized tyrosine-phosphorylated EGFR. Actin binding proteins (ABP's) are generally believed to regulate actin filament assembly. Ca2+ is known as one of the important regulatory factors for the activity of ABP's in vitro [15]. Therefore, we investigated the importance of the EGF-induced transient rise in [Ca2+]i for the regulation of actin polymerization in vivo. Continuous high [Ca2+]i in the millimolar range induces a prominent rise in cellular filamentous actin levels to approximately 50% over control cells. However, actin polymerization is unimpaired under conditions which effectively block the EGF-induced [Ca2+]i transient. These data demonstrate that EGF-induced actin polymerization localizes to the activated EGFR in the membrane skeleton, independent of the cytosolic free calcium transient.

Topics: Actins; Adenosine Triphosphate; Calcimycin; Calcium; Carcinoma, Squamous Cell; Catechols; Cell Line; Cell Membrane; Cytosol; Egtazic Acid; Epidermal Growth Factor; ErbB Receptors; Fluorescent Antibody Technique; Humans; Kinetics; Macromolecular Substances; Microscopy, Confocal; Nitriles; Phosphorylation; Skin Neoplasms; Time Factors; Tumor Cells, Cultured; Tyrphostins

The present study was designed to further investigate the role of the epidermal growth factor receptor (EGFr) in mouse skin tumor promotion by evaluating the status of the EGFr in tumor promoter-treated mouse epidermis and in mouse skin tumors. Female SENCAR mice received three topical treatments of either the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) or the nonphorbol esters okadaic acid and chrysarobin. Membrane proteins from SENCAR mouse epidermis were isolated 6 h after the last treatment, and the phosphotyrosine content of the EGFr and several potential substrates were examined by Western blot analysis. The results indicated that multiple applications of all three tumor promoters led to an increase in the phosphotyrosine content of the EGFr and also of several lower molecular weight proteins (M(r) approximately 80,000-85,000). Phosphorylation of PLC gamma 1 on tyrosine residues could not be detected in tumor promoter-treated mouse epidermis when the phosphotyrosine content of the EGFr was elevated or in cultured keratinocytes exposed to exogenous EGF. When two tyrosine kinase inhibitors (tyrphostins RG50864 and RG13022) were incorporated into the treatment regimens, the TPA-induced epidermal hyperplasia and cell proliferation were effectively blocked, and the TPA-stimulated EGFr tyrosine phosphorylation was significantly reduced. Examination of the phosphotyrosine content of epidermal membrane proteins isolated from skin papillomas revealed that the EGFr also had elevated phosphotyrosine levels. These results demonstrate that multiple topical treatments with both phorbol ester and nonphorbol ester tumor promoters lead to activation of the EGFr tyrosine kinase in mouse epidermis. In addition, these data suggest that signaling through the EGFr pathway plays an important role in the tumor promotion stage of multistage carcinogenesis in mouse skin.

Topics: Administration, Topical; Animals; Blotting, Western; Carcinogens; Catechols; Enzyme Inhibitors; Epidermal Cells; ErbB Receptors; Female; Hyperplasia; Isoenzymes; Keratinocytes; Mice; Mice, Inbred SENCAR; Nitriles; Phospholipase C gamma; Phosphorylation; Phosphotyrosine; Pyridines; Skin Neoplasms; Tetradecanoylphorbol Acetate; Type C Phospholipases; Tyrphostins