The oncogene PDGF-B provides a key switch from cell death to survival induced by TNF.

Tumor necrosis factor (TNF) induces both cell death and survival signals. NF-kappaB, a transcription factor activated by TNF, is critical for controlling survival signals through trans-activation of downstream target genes. However, few NF-kappaB target survival genes have been identified with direct roles in oncogenesis. We report that platelet-derived growth factor B (PDGF-B), an oncogene and growth factor, is highly induced by TNF in fibroblasts in an NF-kappaB-dependent manner. PDGF-B can rescue NF-kappaB-deficient fibroblasts from TNF-mediated killing, and inhibition of PDGF-B signaling sensitizes wild-type cells to TNF-induced death. Interestingly, PDGF-B-transformed NIH-3T3 cells are even more highly sensitized to TNF-induced cell death with PDGF-B inhibition. Our results suggest that while normal cells contain multiple TNF-induced survival signals, tumor cells may favor a specific survival gene that is abnormally upregulated in order to evade death signals.

Lck-dependent Fyn activation requires C terminus-dependent targeting of kinase-active Lck to lipid rafts.

Mechanisms regulating the activation and delivery of function of Lck and Fyn are central to the generation of the most proximal signaling events emanating from the T cell antigen receptor (TcR) complex. Recent results demonstrate that lipid rafts (LR) segregate Lck and Fyn and play a fundamental role in the temporal and spatial coordination of their activation. Specifically, TcR-CD4 co-aggregation-induced Lck activation outside LR results in Lck translocation to LR where the activation of LR-resident Fyn ensues. Here we report a structure-function analysis toward characterizing the mechanism supporting Lck partitioning to LR and its capacity to activate co-localized Fyn. Using NIH 3T3 cells ectopically expressing FynT, we demonstrate that only LR-associated, kinase-active (Y505F)Lck reciprocally co-immunoprecipitates with and activates Fyn. Mutational analyses revealed a profound reduction in the formation of Lck-Fyn complexes and Fyn activation, using kinase domain mutants K273R and Y394F of (Y505F)Lck, both of which have profoundly compromised kinase activity. The only kinase-active Lck mutants tested that revealed impaired physical and enzymatic engagement with Fyn were those involving truncation of the C-terminal sequence YQPQP. Remarkably, sequential truncation of YQPQP resulted in an increasing reduction of kinase-active Lck partitioning to LR, in both fibroblasts and T cells. This in turn correlated with an ablation of the capacity of these truncates to enhance TcR-mediated interleukin-2 production. Thus, Lck-dependent Fyn activation is predicated by proximity-mediated transphosphorylation of the Fyn kinase domain, and targeting kinase-active Lck to LR is dependent on the C-terminal sequence QPQP.