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ARMER, Apoptotic Regulator in the Membrane of the Endoplasmic Reticulum, a Novel Inhibitor of Apoptosis
We have identified a novel protein, apoptotic regulator in the membrane of the endoplasmic reticulum (ARMER), which protects HT1080 fibrosarcoma cells from apoptosis induced by various stimuli. We demonstrate that ARMER is an endoplasmic reticulum (ER) integral membrane protein with four predicted transmembrane domains and a COOH-terminal KKXX ER retrieval motif. We used an inducible expression system (pIND) to study the effects of regulated ARMER overexpression. Cells in which ARMER was overexpressed exhibited protection from multiple apoptotic inducers including serum starvation, doxorubicin, UV irradiation, tumor necrosis factor alpha, and the ER stressors brefeldin A, tunicamycin, and thapsigargin. Analysis of the caspase proteolytic cascade reveals that ARMER inhibits proteolysis of the caspase-9-specific fluorogenic substrate LEHD-AFC as well as endogenous substrates downstream of caspase-9; however, it does not inhibit cytochrome c release or cleavage of caspase-9 itself. Apoptotic stimuli cause endogenous levels of ARMER protein and RNA to decrease, leading to cell death; however, sustaining ARMER protein levels through exogenous expression inhibits apoptosis. These data suggest that ARMER is a novel ER integral membrane protein which protects cells by inhibiting caspase-9 activity and reveal a possible role for ARMER in cell survival.