| How pro-apoptotic signaling pathways and pro-survival signaling pathways are integrated is not yet understood. Two TNF receptor family members, the p75 neurotrophin receptor (p75) and Fas, both harbor cytoplasmic death domains and can elicit apoptosis. However, it has not been clearly established whether these receptors are equivalent in their capability to mediate apoptosis. To understand better the proapoptotic characteristics of p75, we transfected chimeric receptors that contain the extracellular portion of Fas and the intracellular portion of p75 into cells responsive to Fas-mediated apoptosis. Whereas expression of Fas induced widespread apoptosis, expression of p75 and the chimeric receptors induced minimal cell death. The magnitudes of Fas- and p75-induced killing substantially differed, suggesting that the death domains of the two receptors do not function in the same manner.; The Akt kinases are important mediators of cellular survival. This family has been suggested to phosphorylate a number of proapoptotic proteins, leading to suppression of death signals. The stress-induced mitogen-activated protein kinases (MAPK), which include the c-Jun N-terminal kinases (JNK) and p38 kinases, have been linked to apoptosis. We therefore explored the possibility that Akt kinases may exert prosurvival effects by antagonizing the activities of the JNK and p38 kinase pathways.; We have identified two molecular strategies that Akt kinases use to inhibit the pro-apoptotic effects of the stress-induced MAPKs. The first is Akt-mediated phosphorylation of a newly identified substrate, Apoptosis signal-regulating kinase 1 (ASK1), a MAPK kinase kinase which lies upstream of the stress-induced MAPKs. Akt decreased ASK1 kinase activity by phosphorylating a consensus Akt site on ASK1 Ser83, resulting in reduced ASK1-mediated downstream signaling and apoptosis. A second, more surprising, strategy to inhibit stress-induced MAPKs involves Akt1 and JNK interacting protein-1 (JIP1), a scaffold that organizes distinct JNK signaling modules. Akt1 binding to JIP1 inhibited JNK activity by decreasing JIP1 interactions with JNK pathway kinases. These results suggest that Akt1 binding to JIP1 acts as a regulatory gate preventing JNK activation. This hypothesis is supported by apoptosis assays using neurons deficient in Akt1 or expressing Akt1 mutants. Collectively, these results identify novel kinase-dependent and independent mechanisms of Akt-mediated survival. |
