Bcl-x_L Protects From UPR-Associated Apoptosis During Plasma Cell Differentiation

Abstract 3288

Understanding factors that control plasma cell survival is important for the development of therapeutic approaches to diseases including multiple myeloma and autoimmune disorders. As part of the program that allows for B cell differentiation to a plasma cell, a required signal includes the activation of an unfolded protein response (UPR). However unlike stress-induced activation of the UPR, induction of apoptosis does not occur, suggesting that compensatory survival signals are also activated during plasma cell differentiation. The compensatory survival pathways are less defined and require further research. Therefore we employed a model of plasma cell differentiation to better define the survival signaling during this process. The murine B cell lymphoma cell line, Bcl1 can be stimulated to secrete immunoglobulin using IL-5 and LPS. To determine the effects of exogenous ER stress on plasma cell differentiation, we treated the cells with the inhibitor of N-linked glycosylation, tunicamycin, for 5 hours prior to the differentiation signal. The 5 hour pulse of tunicamycin was sufficient to induce significant apoptosis in undifferentiated cells or cells treated with IL-5, resulting in 78% and 74% cell death respectively by 24 hours post treatment. However, if LPS was included in the differentiation stimulus the cells were able to differentiate into IgM-secreting plasma cells with similar kinetics as cells differentiated in the absence of tunicamycin pretreatment. Thus LPS-induced differentiation is sufficient to block ER stress-induced cell death. Since these cells also activate a UPR during differentiation, we hypothesized that part of the differentiation program included protection from UPR-associated cell death. To investigate this effect, we first examined the levels of the antiapoptotic proteins Bcl-2, Bcl-x_L and Mcl-1 during plasma cell differentiation. We found that differentiation induced Bcl-x_L and caused the loss of Mcl-1. From this data we hypothesized that the differentiation of these cells resulted in Bcl-x_L dependence during plasma cell differentiation. To test this we used ABT-737, which selectively blocks the binding pocket of Bcl-x_L and Bcl-2 but not Mcl-1 and kills cells that are dependent on Bcl-2 or Bcl-x_L. Undifferentiated Bcl1 cells were insensitive to ABT-737 with an IC₅₀ > 2μM. However ABT-737 sensitized LPS-treated Bcl1 cells to tunicamycin pretreatment resulting in 89% death in 24 h compared to 23% in untreated cells. These data suggest that the induction of Bcl-x_L is responsible for the survival of cells undergoing ER stress. Most importantly, cells treated with LPS and IL-5 for differentiation became sensitive to ABT-737 with 59% cell death versus 26% in untreated cells, thus demonstrating that during plasma cell differentiation, cells switch to a Bcl-x_L-dependent state. To determine the molecular basis for these findings we investigated the effects of ABT-737 on the expression levels of Bcl-2 proteins as well as the effects of differentiation on their interactions. ABT-737 did not induce changes in the expression of Bcl-2 family proteins. However, co-immunoprecipitation demonstrated a shift in Bim binding from Mcl-1 in untreated cells to Bcl-x_L in differentiating cells. This latter finding is consistent with a shift from Mcl-1 dependence to Bcl-x_L during plasma cell differentiation. To validate these data, primary C57BL/6 splenocytes were isolated, depleted of non-B cells and subsequently stimulated with IL-4 and LPS to differentiate into plasmablasts. Realtime qPCR showed an increase in Bcl-x_L mRNA and loss of Mcl-1 and Bcl-2 mRNA in both the primary B cells and the Bcl1 cell line. Western blotting of primary B cell lysates also showed an increase in Bcl-x_L protein and loss of Bcl-2 and Mcl-1 protein. Together these data indicate that during plasma cell differentiation the cell enters a Bcl-x_L-dependent state that protects against differentiation-induced apoptosis.