The role of the low-affinity neurotrophin receptor (p75NTR) in signal transduction is undefined. Nerve growth factor can activate the sphingomyelin cycle, generating the putative-lipid second messenger ceramide. In T9 glioma cells, addition of a cell-permeable ceramide analog mimicked the effects of nerve growth factor on cell growth inhibition and process formation. This signaling pathway appears to be mediated by p75NTR in T9 cells and NIH 3T3 cells overexpressing p75NTR. Expression of an epidermal growth factor receptor-p75NTR chimera in T9 cells imparted to epidermal growth factor the ability to activate the sphingomyelin cycle. These data demonstrate that p75NTR is capable of signaling independently of the trk neurotrophin receptor (p140trk) and that ceramide may be a mediator in neurotrophin biology.

Tumor necrosis factor-␣ (TNF-␣) binding to its receptors leads to a diversity of biological responses. The actions of TNF are the result of the interaction of cytoplasmic proteins that bind directly to the intracellular domains of the two TNF receptors, p55 and p75. Here we report a novel interaction between the juxtamembrane region of the p55 TNF receptor and a newly discovered 47-kDa isoform of phosphatidylinositol-4-phosphate 5-kinase (PIP5K), a member of the enzyme family that generates the key signaling messenger, phosphatidylinositol 4,5-bisphosphate. The interaction was found to be specific for the p55 TNF receptor and was not observed with the p75 TNF receptor, the Fas antigen, or the p75 neurotrophin receptor, which are other members of the TNF receptor superfamily. In vitro experiments using recombinant fusion proteins verify the authenticity of the interaction between the p55 receptor and PIP5KII␤, a new isoform of PIP5K, but not the previously identified 53-kDa PIP5KII␣. Treatment of HeLa cells with TNF-␣ resulted in an increased PIP5K activity. These results indicate that phosphatidylinositol turnover may be linked to stimulation of the p55 TNF receptor and suggest that a subset of TNF responses may result from the direct association of PIP5KII␤ with the p55 TNF receptor. Tumor necrosis factor-␣ (TNF-␣)1 initiates its proliferative, differentiative, or cytotoxic actions on mammalian cells by binding to two transmembrane molecules, the p55 and p75 TNF receptors (1-3). The p55 receptor is responsible for many of the biological effects of TNF, including programmed cell death, cell differentiation, and cell proliferation (4 -6). A major step in understanding the mechanism of the p55 receptor has been the identification of the interacting protein, TRADD, which accounts for signals leading to apoptosis and increased gene expression through NF-B-mediated events (7). Likewise, the p75 receptor is capable of signal transduction through the association of ring finger proteins, such as TRAF1 and TRAF2, with the cytoplasmic domain of p75 (8, 9).The interaction of TRADD with the p55 TNF receptor has revealed the importance of protein-protein interactions via a region of homology called the "death domain." This sequence has been found in a variety of transmembrane and cytosolic molecules and is usually localized at the C-terminal region of each protein. The functional significance of this domain has been demonstrated in studies with the p55 TNF receptor and the Fas antigen, which contain similar functional death domain sequences (10). The binding of p55 receptors to TRADD, a cytoplasmic protein containing a death domain, and the binding of the Fas antigen to FADD, an analogous protein, have been localized to an 80-amino acid region at the C terminus of both receptors. While overexpression of TRADD or FADD in heterologous cells leads to cell death (7, 11), deletions or mutations in the death domains abolish the ability of these molecules to participate in the initiation of apoptosis (6,12).A link to the interleuki...

A stress-responsive gene highly expressed in brain and reproductive organs (BRE) is down-regulated after UV irradiation, DNA damaging agents, or retinoic acid treatment. The human BRE gene encodes a mRNA of 1.9 kb, which gives rise to a protein of 383 amino acids with a molecular size of 44 kilodaltons. BRE is not homologous to any known gene and its function has not been defined. Here we report that BRE was identified multiple times in a yeast two-hybrid screen of a murine cerebellar cDNA library, using the juxtamembrane domain of the p55 tumor necrosis factor alpha (TNF) receptor. The interaction between the p55 receptor and BRE was verified by an in vitro biochemical assay by using recombinant fusion proteins and by co-immunoprecipitation of transfected mammalian cells. In the yeast two-hybrid assay, BRE specifically interacted with p55 TNF receptor but not with other TNF family members such as the Fas receptor, the p75 TNF receptor, and p75 neurotrophin receptor. Overexpression of BRE inhibited TNF-induced NFkappaB activation, indicating that the interaction of BRE protein with the cytoplasmic region of p55 TNF receptor may modulate signal transduction by TNF-alpha.

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