title: Extracellular calcium influx stimulates metalloproteinase cleavage and secretion of heparin-binding EGF-like growth factor independently of protein kinase C.
abstract: The phorbol ester, tetradecanoyl-phorbol 13-acetate (TPA), stimulates rapid proteolytic processing of the transmembrane, pro- form of heparin-binding epidermal growth factor-like growth factor (HB-EGF) at cell surfaces, suggesting the involvement of protein kinase C (PKC) isoforms in the HB-EGF secretion mechanism. To test this possibility, we expressed a chimeric protein, consisting of proHB-EGF fused to placental alkaline phosphatase (AP) near the amino terminus of processed HB-EGF, in NbMC-2 prostate epithelial cells. The proHB-EGF-AP chimera localized to plasma membranes and functioned as a diphtheria toxin receptor. Secreted HB-EGF-AP bound to heparin and exhibited potent growth factor activity. The presence of the AP moiety allowed highly quantitative measurements of cleavage-secretion responses of proHB-EGF to extracellular stimuli. As expected, rapid secretion of HB-EGF-AP was induced in a time- and dose-dependent manner by TPA. However, this was also observed with the Ca2+ ionophore, ionomycin, suggesting the involvement of extracellular Ca2+ ions in the secretion mechanism. Ionomycin-induced secretion was inhibited by extracellular calcium chelation but not by the PKC inhibitors, GF109203X, staurosporine, or chelerythrine. The TPA-mediated secretion effect was inhibited by staurosporine, GF109203X, and by pretreatment with TPA, but not by calcium chelation. A small secretion response was induced by thapsigargin, which releases Ca2+ from intracellular stores, but this was completely eliminated by extracellular calcium chelation. Ionomycin- and TPA-induced HB-EGF-AP secretion was not dependent on the presence of the proHB-EGF cytoplasmic domain and was specifically inhibited by the metalloproteinase inhibitors 1,10-phenanthroline and tissue inhibitor of metalloproteinase-1 (TIMP-1). These data demonstrate that extracellular Ca2+ influx activates a membrane-associated metalloproteinase to process proHB-EGF by a pathway that does not require PKC.
authors.affiliation: Department of Urology, Children's Hospital, Boston, Massachusetts 02115, USA.
authors.forename: S M
authors.initials: SM
authors.lastname: Dethlefsen
authors.forename: G
authors.initials: G
authors.lastname: Raab
authors.forename: M A
authors.initials: MA
authors.lastname: Moses
authors.forename: R M
authors.initials: RM
authors.lastname: Adam
authors.forename: M
authors.initials: M
authors.lastname: Klagsbrun
authors.forename: M R
authors.initials: MR
authors.lastname: Freeman
mesh_terms: D000469:Alkaline Phosphatase / Q000235:genetics; D000818:Animals; D002118:Calcium / Q000378:metabolism* / Q000502:physiology; D002478:Cells, Cultured; D003593:Cytoplasm / Q000378:metabolism; D004791:Enzyme Inhibitors / Q000494:pharmacology; D004815:Epidermal Growth Factor / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*; D005110:Extracellular Space / Q000201:enzymology*; D006493:Heparin / Q000378:metabolism*; D066257:Heparin-binding EGF-like Growth Factor; D006801:Humans; D036341:Intercellular Signaling Peptides and Proteins; D008297:Male; D008666:Metalloendopeptidases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D011493:Protein Kinase C / Q000378:metabolism*; D011498:Protein Precursors / Q000378:metabolism; D051381:Rats; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology
chemical_list: D004791:Enzyme Inhibitors; C587883:HBEGF protein, human; C587885:Hbegf protein, rat; D066257:Heparin-binding EGF-like Growth Factor; D036341:Intercellular Signaling Peptides and Proteins; D011498:Protein Precursors; D011993:Recombinant Fusion Proteins; D004815:Epidermal Growth Factor; D006493:Heparin; D011493:Protein Kinase C; D000469:Alkaline Phosphatase; D008666:Metalloendopeptidases; D013755:Tetradecanoylphorbol Acetate; D002118:Calcium