Expression of MAL and MAL2, two elements of the protein machinery for raft-mediated transport, in normal and neoplastic human tissue.

Polarized transport of lipids and proteins to the apical and basolateral membrane subdomains is essential for the functioning of epithelial cells. Apical transport is mediated by a direct route from the Golgi and an indirect route, referred to as transcytosis, involving the transport of the protein to the basolateral membrane followed by its internalization and subsequent transcellular transport to the apical subdomain. MAL and MAL2 have been demonstrated to be essential components of the machinery for the direct and indirect routes, respectively. Herein, we review the range of expression of MAL and MAL2 in normal human tissue and compare it with that of neoplastic tissue. Our analysis provides insight into the potential use of MAL- and MAL2-mediated pathways in many types of epithelial cells as well as in nonepithelial cells. In addition, the specific alterations in MAL and/or MAL2 expression observed in specific types of carcinoma provides a basis to understand the loss of the polarized phenotype that frequently accompanies the neoplastic transformation process. This points out potential applications of MAL and MAL2 as markers for tumor characterization.