Expression of MAL, an Integral Protein Component of the Machinery for Raft-mediated Apical Transport, in Human Epithelia

Abstract

The MAL protein is the only integral membrane protein identified as being an essential component of the machinery necessary for apical transport in the canine MDCK cell line, a paradigm of polarized epithelial cells. To characterize the range of human epithelia that use MAL-mediated pathways of transport, we performed an immunohistochemical survey of normal tissues using a monoclonal antibody (MAb) specific for the MAL protein. For comparison, different types of carcinoma were also analyzed. MAL, with a characteristic strong supranuclear granular distribution, was detected in specific types of normal epithelial cells throughout the respiratory system, the gastrointestinal and genitourinary tracts, and in exocrine and endocrine glands. Absorptive cells (e.g., enterocytes), and many different types of specialized secretory cells, either organized in discrete clusters (e.g., endocrine cells in the pancreas), gathered together in an endocrine gland (e.g., thyroid), interspersed with other cells in glands (e.g., parietal cells), or dispersed singly among other cells (e.g., type 2 pneumocytes) were positive for MAL. We also analyzed a series of epithelial renal and thyroid tumors and found alterations dependent on the particular histological type of tumor. These results open potential applications of the anti-MAL antibody for the characterization of neoplastic tissue.