The initial identification of the ALK gene, expressed as C-terminal part of the transforming fusion protein NPM-ALK in the t(2;5)(p23;q35) lymphoma-associated chromosomal translocation, revealed a novel receptor tyrosine kinase (RTK). In order to expand the knowledge on ALK expression in the human system, we examined a panel of human cell lines for ALK expression and found that transcription is completely repressed in cell lines of entodermal origin (0/21). Furthermore, full length receptor expression is absent in cell lines of the hematopoietic system with the exception of t(2;5)-associated anaplastic large cell lymphomas lines (ALCL), which are known to express chimeric NPM-ALK mRNA. Cell lines established from solid tumors of ectodermal origin, including melanoma and breast carcinoma, exhibited widespread mRNA expression of the ALK receptor at a broad range (53/64), an association which was found to be strongest in cell lines derived from neuroblastoma (6/6), glioblastoma (8/8) as well as in cell lines established from Ewing sarcoma (4/4) and retinoblastomas (2/2). Because of the reported involvement of neutrophin tyrosine kinase receptors in autocrine differentiation in neuroblastomas, we analyzed cell lines positive for full length or chimeric ALK protein for the presence of phoshotyrosine residues within the intracellular region of ALK. While the constitutive activation of chimeric NPM-ALK molecules could be shown, no evidence was found for induced or constitutively activated ALK receptors in neuroblastoma, melanoma or breast carcinoma cell lines. Although the receptor could be shown to be consistently expressed with exclusive specificity in tissues developed from the ectoderm, our results do not support any involvement of ALK in the stimulation of tumorigenic cell growth or differentiation so far, indicating that ALK expression is a physiologic rather than a pathologic phenomenon.
Copyright 2002 Wiley-Liss, Inc.