Majority of anaplastic large-cell lymphomas (ALCLs) are associated with the t(2;5)(p23;q35) translocation, fusing the NPM (nucleophosmin) and ALK (anaplastic lymphoma kinase) genes (NPM-ALK). Recent studies demonstrated that ALK may also be involved in variant translocations, namely, t(1;2)(q25;p23), t(2;3)(p23;q21), t(2;17)(p23;q23) and inv(2)(p23q35), which create the TPM3-ALK, TFG-ALK5, CLTC-ALK, and ATIC-ALK fusion genes, respectively. Although overexpression of NPM-ALK has previously been shown to transform fibroblasts, the transforming potential of variant X-ALK proteins has not been precisely investigated. We stably transfected the cDNAs coding for NPM-ALK, TPM3-ALK, TFG-ALK, CLTC-ALK or ATIC-ALK into nonmalignant NIH3T3 cells. All X-ALK variants are tyrosine phosphorylated and their subcellular distribution was in agreement with that observed in tumors. Moreover, our results show that the in vitro transforming capacity of NIH3T3-transfected cells are in relation to the level of X-ALK fusion proteins excepted for TPM3-ALK for which there is an inverse correlation. The differences between the five X-ALK variants with regard to proliferation rate, colony formation in soft agar, invasion, migration through the endothelial barrier and tumorigenicity seem to be due to differential activation of various signaling pathways such as PI3-kinase/AKT. These findings may have clinical implications in the pathogenesis and prognosis of ALKpositive ALCLs.

Relapses following chemotherapy are a major hindrance to patients' survival in acute myeloid leukemia (AML). To investigate the role of the hematopoietic niche in the chemoresistance of leukemic cells, we examined two pathways: one mediated by adhesion molecules/integrins, and the other by soluble factors of the morphogen Wnt pathway. In our study, both the adhesion of leukemic blasts to fibronectin and the addition of Wnt antagonists induced, independently, resistance of AML cells to daunorubicin in a cell survival assay. Using pharmacological inhibitors and siRNA, we showed that both resistance pathways required the activity of the glycogen synthase kinase 3b (GSK3b). Moreover, the AML cell protection downstream of GSK3b was mediated by NF-jB. A link between the adhesion and the Wnt pathway was found, as adhesion of U937 on human osteoblasts, a component of the hematopoietic niche, triggered the secretion of the Wnt antagonist sFRP-1 and supported resistance to daunorubicin. The osteoblast-conditioned medium could also confer chemoresistance to U937 cells cultured in suspension, and this cell protective effect was abrogated after depletion of sFRP-1. In the context of this potential double in vivo resistance, modulators of the common signal GSK3b and of its target NF-jB could represent important novel therapeutic tools.

Anaplastic lymphoma kinase (ALK) is a receptor tyrosine kinase, initially discovered as part of the NPM-ALK fusion protein, resulting from the t(2;5) translocation that is frequently associated with anaplastic large-cell lymphomas. The native ALK protein is normally expressed in the developing and, at a weaker level, adult nervous system. We recently demonstrated that the oncogenic, constitutively kinase-activated NPM-ALK protein was antiapoptotic when expressed in Jurkat lymphoblastic cells treated with cytotoxic drugs. In contrast, we now show that Jurkat cells overexpressing the wild-type ALK receptor are more sensitive to doxorubicin-induced apoptosis than parental cells. Moreover, the ALK protein is cleaved during apoptosis in a caspase-dependent manner. Mutation of aspartic residues to asparagine allowed us to map the caspase cleavage site in the juxtamembrane region of ALK. In order to assess the role of ALK in neural cell-derived tissue, we transiently expressed ALK in the 13.S.1.24 rat neuroblast immortalized cell line. ALK expression led to apoptotic cell death of the neuroblasts. ALK ligation by specific activating antibodies decreased ALKfacilitated apoptosis in both lymphoid and neuronal cell lines. Moreover, ALK transfection reduced the survival of primary cultures of cortical neurons. Thus, ALK has a proapoptotic activity in the absence of ligand, whereas it is antiapoptotic in the presence of its ligand and when the kinase is intrinsically activated. These properties place ALK in the growing family of dependence receptors.