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Expression and Subcellular Localization of a 35-kDa Carbonic Anhydrase IV in a Human Pancreatic Ductal Cell Line (Capan-1)

Marjorie Fanjul, Laetitia Alvarez and Etienne Hollande

Laboratoire de Biologie Cellulaire et Moléculaire des Epithéliums, Université Paul Sabatier, Toulouse, France (LA,EH), and INSERM U.858/I2MR, Equipe 16, CHU Rangueil, Toulouse, France (MF)

Correspondence to: Prof. E. Hollande, Laboratoire de Biologie Cellulaire et Moléculaire des Epithéliums, Université Paul Sabatier, 38 rue des 36 Ponts, 31400 Toulouse, France. E-mail: hollande{at}cict.fr

The high intraluminal concentrations of HCO₃^– in the human pancreatic ducts have suggested the existence of a membrane protein supplying the Cl^–/HCO₃^– exchanger. Membrane-bound carbonic anhydrase IV (CA IV) is one of the potential candidates for this protein. The difficulties in isolating human pancreatic ducts have led the authors to study the molecular mechanisms of HCO₃^– secretion in cancerous cell lines. In this work, we have characterized the CA IV expressed in Capan-1 cells. A 35-kDa CA IV was detected in cell homogenates and purified plasma membranes. Treatment of purified plasma membranes with phosphatidylinositol-phospholipase-C indicated that this CA IV was not anchored by a glycosylphosphatidylinositol (GPI). In contrast, its detection on purified plasma membranes by an antibody specifically directed against the carboxyl terminus of human immature GPI-anchored CA IV indicated that it was anchored by a C-terminal hydrophobic segment. Immunoelectron microscopy and double-labeling immunofluorescence revealed that this CA IV was present on apical plasma membranes, and in the rough endoplasmic reticulum, the endoplasmic reticulum-Golgi intermediate compartment, the Golgi complex, and secretory granules, suggesting its transport via the classical biosynthesis/secretory pathway. The expression in Capan-1 cells of a 35-kDa CA IV anchored in the apical plasma membrane through a hydrophobic segment, as is the case in the healthy human pancreas, should make the study of its role in pancreatic HCO₃^– secretion easier. (J Histochem Cytochem 55:783–794, 2007)

Key Words: human pancreas • ductal cells • carbonic anhydrase IV • HCO₃^– secretion

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