No abstract

NF-B is critical for determining cellular sensitivity to apoptotic stimuli by regulating both mitochondrial and death receptor apoptotic pathways. The endoplasmic reticulum (ER) emerges as a new apoptotic signaling initiator. However, the mechanism by which ER stress activates NF-B and its role in regulation of ER stress-induced cell death are largely unclear. Here, we report that, in response to ER stress, IKK forms a complex with IRE1␣ through the adapter protein TRAF2. ER stress-induced NF-B activation is impaired in IRE1␣ knockdown cells and IRE1␣ ؊/؊ MEFs. We found, however, that inhibiting NF-B significantly decreased ER stress-induced cell death in a caspase-8-dependent manner. Gene expression analysis revealed that ER stress-induced expression of tumor necrosis factor alpha (TNF-␣) was IRE1␣ and NF-B dependent. Blocking TNF receptor 1 signaling significantly inhibited ER stress-induced cell death. Further studies suggest that ER stress induces down-regulation of TRAF2 expression, which impairs TNF-␣-induced activation of NF-B and c-Jun N-terminal kinase and turns TNF-␣ from a weak to a powerful apoptosis inducer. Thus, ER stress induces two signals, namely TNF-␣ induction and TRAF2 down-regulation. They work in concert to amplify ER-initiated apoptotic signaling through the membrane death receptor.

Phospholipase D exists in various forms that differ in their regulation but predominantly hydrolyze phosphatidylcholine. The Ca(2+)-dependent isozymes of protein kinase C regulate phospholipase D in vitro and play a major role in its control by growth factors and G protein-linked agonists in vivo. Recent studies have demonstrated that small G proteins of the ADP-ribosylation factor (ARF) and Rho families activate the enzyme in vitro, and evidence is accumulating that they also are involved in its control in vivo. Both types of G protein play important roles in cellular function, and the possible mechanisms by which they are activated by agonists are discussed. There is also emerging evidence of the control of phospholipase D and Rho proteins by soluble tyrosine kinases and novel serine/threonine kinases. The possible role of these kinases in agonist regulation of phospholipase D is discussed. The function of phospholipase D in cells is still poorly defined. Postulated roles of phosphatidic acid produced by phospholipase D action include the activation of Ca(2+)-independent isoforms of protein kinase C, the regulation of growth and the cytoskeleton in fibroblasts, and control of the respiratory burst in neutrophils. Another important function of phosphatidic acid is to act as a substrate for a specific phospholipase A2 to generate lysophosphatidic acid, which is becoming increasingly recognized as a major intercellular messenger. Finally, it is possible that the phospholipid changes induced in various cellular membranes by phospholipase D may per se play an important role in vesicle trafficking and other membrane-associated events.