A preferential role for STAT5, not constitutively active STAT3, in promoting survival of a human lymphoid tumor

STATs are believed to play key roles in normal and abnormal cell function. In the present work, we investigated the role of STATs in an IL-2-responsive human lymphoblastic lymphoma-derived cell line, YT. Only STAT3 was found constitutively tyrosine phosphorylated, but not other STATs. Hyperactive STAT3 was not attributable to a pre-existing intermediate affinity IL-2R complex and/or hyperactive Jak activity. Depletion of STAT3 protein expression reduced tumor cell viability with protracted kinetics (72-96 h), while TUNEL assays demonstrated cell death occurred via apoptosis. Interestingly, depletion of STAT5 in this same tumor induced more pronounced cell death compared with STAT3 depletion (24 h). Although IL-2 was able to rescue STAT3-depleted cells from death, it could not compensate for the loss of STAT5. To determine the prosurvival function of STAT3 vs STAT5 within the same tumor model, genes were profiled in STAT3- or STAT5-depleted YT cells by apoptosis-specific microarrays. Several differentially expressed genes were identified. Interestingly, those genes involved in NF-kappaB regulation, such as TNFR-associated factors 2 and 5 and B cell leukemia/lymphoma 10, were readily decreased upon STAT5, but not STAT3, depletion as validated by quantitative RT-PCR. These results suggest that STAT5 and, to a lesser extent, hyperactive STAT3 provide preferential and critical cell survival signals for certain human lymphoid tumors, indicating that nonhyperactive STATs should be considered as therapeutic targets for abrogating tumorigenesis.     

Integrins and EGFR coordinately regulate the pro-apoptotic protein Bim to prevent anoikis

Epithelial cells must adhere to the extracellular matrix (ECM) for survival, as detachment from matrix triggers apoptosis or anoikis. Integrins are major mediators of adhesion between cells and ECM proteins, and transduce signals required for cell survival. Recent evidence suggests that integrin receptors are coupled to growth factor receptors in the regulation of multiple biological functions; however, mechanisms involved in coordinate regulation of cell survival are poorly understood and mediators responsible for anoikis have not been well characterized. Here, we identify the pro-apoptotic protein Bim as a critical mediator of anoikis in epithelial cells. Bim is strongly induced after cell detachment and downregulation of Bim expression by RNA interference (RNAi) inhibits anoikis. Detachment-induced expression of Bim requires a lack of beta(1)-integrin engagement, downregulation of EGF receptor (EGFR) expression and inhibition of Erk signalling. Overexpressed EGFR was uncoupled from integrin regulation, resulting in the maintenance of Erk activation in suspension, and a block in Bim expression and anoikis. Thus, Bim functions as a key sensor of integrin and growth factor signals to the Erk pathway, and loss of such coordinate regulation may contribute to tumour progression.