title: Thioridazine enhances lysosomal accumulation of epidermal growth factor and toxicity of conjugates of epidermal growth factor with Pseudomonas exotoxin.
abstract: Thioridazine, a phenothiazine calmodulin inhibitor, aggravated the cytotoxic effect of a conjugate (EGF-PE) of epidermal growth factor (EGF) coupled with Pseudomonas exotoxin against cultured HeLa cells. Other phenothiazine calmodulin inhibitors, trifluoperazine and chlorpromazine, also intensified the cytotoxic effect of EGF-PE, whereas N-(6-aminohexyl)-5-chloro-1-naphthalene sulfonamide (W7) had no such effect. By using iodinated epidermal growth factor ( [125I]EGF), the effect of thioridazine on intracellular transport of EGF was examined. The release of radioactivity associated with [125I]EGF into medium was slow in the presence of thioridazine. The Percoll gradient centrifugation pattern showed that thioridazine delayed both the appearance of [125I]EGF in lysosomes and the disappearance of [125I]EGF from the lysosomes. The pH value in lysosomes was 5.28 in thioridazine-treated HeLa cells, while that in untreated cells was 5.15. Thioridazine was found to inhibit lysosomal enzyme activities of cathepsin B and acid phosphatase, but not beta-hexosaminidase when cell extracts were treated with the drug. Electron microscopy showed an increased number of electron-dense bodies, possibly autophagosomes/lysosomes in HeLa cells grown for 48 h with 3 micrograms/ml thioridazine. The potentiating action of EGF-PE by thioridazine is discussed in relation to the altered lysosomal function in treated cells.
mesh_terms: D036002:ADP Ribose Transferases; D001427:Bacterial Toxins; D001692:Biological Transport / Q000187:drug effects; D002147:Calmodulin / Q000037:antagonists & inhibitors*; D002451:Cell Compartmentation / Q000187:drug effects; D004357:Drug Synergism; D004815:Epidermal Growth Factor / Q000378:metabolism*; D005098:Exotoxins / Q000008:administration & dosage / Q000633:toxicity*; D005260:Female; D006367:HeLa Cells / Q000648:ultrastructure; D006801:Humans; D006863:Hydrogen-Ion Concentration; D008247:Lysosomes / Q000187:drug effects* / Q000378:metabolism; D008854:Microscopy, Electron; D013881:Thioridazine / Q000494:pharmacology*; D037521:Virulence Factors; D000097668:Pseudomonas aeruginosa Exotoxin A
chemical_list: D036002:ADP Ribose Transferases; D001427:Bacterial Toxins; D002147:Calmodulin; D004815:Epidermal Growth Factor; D005098:Exotoxins; D013881:Thioridazine; D037521:Virulence Factors; D000097668:Pseudomonas aeruginosa Exotoxin A