Gamma-secretase-dependent proteolysis of CD44 promotes neoplastic transformation of rat fibroblastic cells.

The metalloprotease-dependent extracellular domain cleavage of the adhesion molecule CD44 is frequently observed in human tumors and is thought to promote metastasis. This cleavage is followed by gamma-secretase-dependent release of CD44 intracellular domain (CD44-ICD), which exhibits nuclear signaling activity. Using a reversible Ret-dependent oncogenic conversion model and a restricted proteomic approach, we identified a positive correlation between the neoplastic transformation of Rat-1 cells and the expression of standard CD44. In these transformed cells, CD44 was found to undergo a sequential metalloprotease and gamma-secretase cleavage, resulting in an increase in expression of CD44-ICD. We showed that this proteolytic fragment possesses a transforming activity. In support of this role, a significant and specific reduction in Ret-induced transformation of Rat-1 cells was observed following drug-mediated inhibition of gamma-secretase. Taken together, these findings suggest that the shedding of CD44 may not only modulate metastasis but also affects earlier events in tumorigenesis through the release of CD44-ICD.

Inducible dimerization of RET reveals a specific AKT deregulation in oncogenic signaling.

Dominant-activating mutations in the RET (rearranged during transfection) proto-oncogene, a receptor tyrosine kinase, are causally associated with the development of multiple endocrine neoplasia type 2A (MEN2A) syndrome. Such oncogenic RET mutations induce its ligand-independent constitutive activation, but whether it spreads identical signaling to ligand-induced signaling is uncertain. To address this question, we designed a cellular model in which RET can be activated either by its natural ligand, or alternatively, by controlled dimerization of the protein that mimics MEN2A dimerization. We have shown that controlled dimerization leaves proximal RET signaling intact but impacts substantially on the tuning of the distal AKT kinase activation (delayed and sustained). In marked contrast, distal activation of ERK remained unaffected. We further demonstrated that specific temporal adjustment of ligand-induced AKT activation is dependent upon a lipid-based cholesterol-sensitive environment, and this control step is bypassed by MEN2A RET mutants. Therefore, these studies revealed that MEN2A mutations propagate previously unappreciated subtle differences in signaling pathways and unravel a role for lipid rafts in the temporal regulation of AKT activation.

T-cell receptor-induced phosphorylation of the zeta chain is efficiently promoted by ZAP-70 but not Syk.

Engagement of the T-cell receptor (TCR) results in the activation of Lck/Fyn and ZAP-70/Syk tyrosine kinases. Lck-mediated tyrosine phosphorylation of signaling motifs (ITAMs) in the CD3-zeta subunits of the TCR is an initial step in the transduction of signaling cascades. However, zeta phosphorylation is also promoted by ZAP-70, as TCR-induced zeta phosphorylation is defective in ZAP-70-deficient T cells. We show that this defect is corrected by stable expression of ZAP-70, but not Syk, in primary and transformed T cells. Indeed, these proteins are differentially coupled to the TCR with a 5- to 10-fold higher association of ZAP-70 with zeta as compared to Syk. Low-level Syk-zeta binding is associated with significantly less Lck coupled to the TCR. Moreover, diminished coupling of Lck to zeta correlates with a poor phosphorylation of the positive regulatory tyr352 residue of Syk. Thus, recruitment of Lck into the TCR complex with subsequent zeta chain phosphorylation is promoted by ZAP-70 but not Syk. Importantly, the presence of ZAP-70 positively regulates the TCR-induced tyrosine phosphorylation of Syk. The interplay between Syk and ZAP-70 in thymocytes, certain T cells, and B-chronic lymphocytic leukemia cells, in which they are coexpressed, will therefore modulate the amplitude of antigen-mediated receptor signaling.