Variable expressions of ADAM family genes in tissues and cancer cells

Abstract Ectodomain shedding, the proteolytic processing of cell surface proteins is a fast and effective way of regulating cell adhesion, cell signaling, and gene expression. ADAMs have emerged as a major family of cellular proteinases implicated in the ectodomain shedding of growth factors, cytokines, receptors, and cell adhesion proteins. In addition to the catalytic zinc metalloprotease domain, the extracellular part of ADAMs contains characteristic disintegrin and cysteine-rich domains, which mediate cell adhesion and regulate the metalloprotease activity. The subcellular localization and the regulation and targeting of the proteolytic activity of ADAMs are controlled by their cytosolic tail. Furthermore, ADAMs can also mediate intercellular adhesion through integrins or inhibit integrin binding and/or signaling. ADAMs are thus poised to regulate and mediate intercellular adhesion and/or communication important for every cell in multicellular organisms. Many of the molecular functions shown to be mediated by ADAMs in non-neural cells have been implicated in the regulation of neuron-neuron and neuron-glial cell interactions. The regulation of cell adhesion, cell signaling, and gene expression is particularly important for the synaptic plasticity suggested to form the basis of the learning and memory. For example, ADAM-mediated α-processing of APP is required for normal synaptic regulation. On the other hand, misregulation of the ADAM-APP-processing activity has been associated with accumulation of β-amyloid, a neurotoxic APP derivative, implicated in Alzheimer’s disease. Furthermore, misregulated ADAM-activity has been linked to cancer progression. ADAM-mediated growth factor and adhesion protein shedding provide growth and migration signals for cancer cells. Hence, the characterization of the expression patterns of ADAMs and their mechanisms of regulation in the nervous system and in cancer cell lines may provide important information on the molecular interactions essential for normal brain functions and cancer progression. The major goal of this thesis was to elucidate the functional regulation of ADAM genes by characterizing their physiological and pathological expression in normal tissues and in cancer cells. This was the first systematic study of the regional expression of multiple ADAMs in the adult CNS and the results established that ADAMs form a major proteinase and adhesion family in the CNS. The regional expression of ADAM10 implies that it is the most likely physiological APP α-secretase. The concomitant induction of ADAM11 expression with neural development and its wide expression in the adult CNS suggest important neural functions for the ADAM11. The ADAM15 gene was chosen for detailed characterization as a prototype of a functionally important ADAM. The study demonstrated that ADAM15 gene expression is highly and widely regulated in human tissues by alternative splicing. Alternative splicing was found to regulate the ADAM15 cytosolic tail, which is potentially involved in the regulation of ADAM15 activity or cell signaling. Importantly, the alternative splicing was found altered in breast cancer cell lines. Furthermore, the localization of the ADAM15 gene in the chromosomal band 1q21.3 associated with cancers and the gene amplification in breast cancer cell lines suggest involvement of ADAM15 in cancer.