Muc4/Sialomucin Complex, the Intramembrane ErbB2 Ligand, Translocates ErbB2 to the Apical Surface in Polarized Epithelial Cells^*

Victoria P. Ramsauer , Coralie A. Carothers Carraway , Pedro J. I. Salas , and Kermit L. Carraway  ¶

Departments of Cell Biology and Anatomy and Biochemistry and Molecular Biology, University of Miami School of Medicine, Miami, Florida 33101

Abstract: Muc4/Sialomucin complex (SMC) acts as an intramembrane ligand for the receptor tyrosine kinase ErbB2, inducing a limited phosphorylation of the receptor. Because Muc4/SMC is found at the apical surfaces of polarized epithelial cells and ErbB2 is often basolateral, the question arises as to whether these components become associated in polarized cells. To address this question, we examined the localization of these proteins in polarized human colon carcinoma CACO-2 cells. Dual color immunofluorescence analysis by confocal microscopy demonstrated the basolateral localization of the ErbB2 in these cells; it is primarily co-localized with E-cadherin at adherens junctions. Expression of apical Muc4/SMC in these cells by transient transfection results in the localization of the ErbB2 at the apical surface. Twocolor confocal microscopy indicated that ErbB2 is colocalized with Muc4/SMC in the transfected cells but not in untransfected cells in the same culture. The change of localization of ErbB2 was confirmed by cell surface biotinylation of apical and basolateral proteins, followed by streptavidin precipitation and the subsequent detection of ErbB2 by immunoblotting. In contrast, Na⁺/K⁺-ATPase maintains its basolateral localization in Muc4/SMC-transfected cells, indicating that the translocation of ErbB2 is not the result of depolarization of the cells. A potential physiological role for the apical localization of ErbB2 is indicated by the fact that ErbB2 phosphorylated at tyrosine 1248 is found predominantly in Muc4/SMC-transfected cells, but not in untransfected cells, and is co-localized with the apical Muc4/SMC. The ability of Muc4/SMC to alter the localization of ErbB2, particularly a phosphorylated form of it, in epithelial cells, suggests that it has an important role in regulating ErbB2 signaling.

Received for publication March 28, 2003. Revision received April 30, 2003.

^* This research was supported in part by National Institutes of Health Grant CA 74072 and the Sylvester Comprehensive Cancer Center of the University of Miami. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

^¶ To whom correspondence should be addressed: Dept. of Cell Biology and Anatomy (R-124), University of Miami School of Medicine, P.O. Box 016960, Miami, FL 33101. Tel.: 305-243-6512; Fax: 305-243-4431; E-mail: kcarrawa{at}med.miami.edu.

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