No abstract

Anaplastic lymphoma kinase (ALK) is a receptor tyrosine kinase that is transiently expressed in specific regions of the central and peripheral nervous systems, suggesting a role in its normal development and function. The nature of the cognate ligands of ALK in vertebrate is still a matter of debate. We produced a panel of monoclonal antibodies (mAbs) directed against the extracellular domain of the human receptor. Two major species of ALK (220 and 140 kDa) were identified in transfected cells, and the use of our mAbs established that the 140-kDa species results from a cleavage of the 220-kDa form. Two mAbs, in the nM range, induced the differentiation of PC12 cells transiently transfected with ALK. In human embryonic kidney 293 cells stably expressing ALK, these two mAbs strongly activated the receptor and subsequently the mitogen-activated protein kinase pathway. We further showed for the first time that activation of ALK also resulted in a specific activation of STAT3. In contrast, other mAbs presented the characteristics of blocking antibodies. Finally, in these cell systems, a mitogenic form of pleiotrophin, a proposed ligand of ALK, failed to activate this receptor. Thus, in the absence of clearly established ligand(s) in vertebrates, the availability of mAbs allowing the activation or the inhibition of the receptor will be essential for a better understanding of the biological roles of ALK. Receptors tyrosine kinase (RTKs)1 play essential roles during the development of the nervous system, regulating a wide range of cellular processes such as proliferation, survival, differentiation, and synaptogenesis. Generally, after ligand binding, RTK dimerizes, autophosphorylates, and initiates signal transduction cascades that subsequently lead to cellular responses (for review, see Ref. 1).Anaplastic lymphoma kinase (ALK) was originally identified as a RTK that acquires transforming capability when truncated and fused in the t(2;5) chromosomal rearrangement associated with the non-Hodgkin lymphoma (2). This translocation produces a fusion gene that encodes a soluble chimeric transforming protein comprising the N-terminal portion of the phosphoprotein nucleophosmin (NPM) linked to the cytoplasmic portion of ALK. It has been demonstrated that the NPM portion is responsible for the dimerization of the fusion protein leading to the constitutive activation of the kinase and to the transforming activity. Phospholipase C␥, phosphatidylinositol 3-kinase, STATs, and Src appear to be important downstream targets of NPM-ALK which contribute to its mitogenic and antiapoptotic activities (3-7). ALK is also involved in different variant chromosomal translocations (for review, see Ref. 8), all leading to the expression of fusion proteins exhibiting a constitutive activation of the kinase.Human, mouse, and Drosophila cDNAs encoding full-length ALK have been characterized (9 -11). The deduced amino acid sequences revealed that ALK is a novel RTK having an extracellular domain, a single transmembrane domain, and an intracellular do...

CART1, a novel human gene, encodes a putative protein exhibiting three main structural domains: first, a cysteine-rich domain located at the amino-terminal part of the protein, which corresponds to an unusual RING finger motif; second, an original cysteine-rich domain located at the core of the protein and constituted by three repeats of an HC3HC3 consensus motif that we designated the CART motif, and which might interact with nucleic acid; third, the carboxyl-terminal part of the CART1 protein corresponds to a TRAF domain known to be involved in protein-protein interactions. Similar association of RING, CART, and TRAF domains was observed in the human CD40-binding protein and in the mouse tumor necrosis factor (TNF) receptor-associated factor 2 (TRAF2), both involved in signal transduction mediated by the TNF receptor family and in the developmentally regulated Dictyostelium discoideum DG17 protein. CART1 is specifically expressed by epithelial cells in breast carcinomas and metastases. Moreover, in these malignant cells, the CART1 protein is localized in the nucleus. Altogether, these observations indicate that CART1 may be involved in TNF-related cytokine signal transduction in breast carcinoma.