Epstein-Barr Virus Latent Membrane Protein 1 Increases Calcium Influx through Store-operated Channels in B Lymphoid Cells*

Abstract

Ca²⁺ signaling plays an important role in B cell survival and activation and is dependent on Ca²⁺ trapped in the endoplasmic reticulum (ER) and on extracellular Ca²⁺. Epstein-Barr virus (EBV) can immortalize B cells and contributes to lymphomagenesis. Previously, we showed that the ER Ca²⁺ content of Burkitt lymphoma cell lines was increased following infection with immortalization-competent virus expressing the full set of EBV latency genes (B95–8). In contrast, infection with an immortalization-deficient virus (P3HR-1) not expressing LMP-1 is without effect. LMP-1 protein expression was sufficient to increase the ER Ca²⁺ content and to increase the cytosolic Ca²⁺ concentration ([Ca²⁺]_cyt). In this follow-up study, we showed that the resting [Ca²⁺]_cyt of P3HR-1-infected cells was decreased, implying that EBV not only modified the ER homeostasis but also affected the cytosolic Ca²⁺ homeostasis. Furthermore, even if the store-operated calcium entry (SOCE) of these cells was normal, the [Ca²⁺]_cyt increase after thapsigargin + CaCl₂ stimulation was blunted. In contrast, the resting [Ca²⁺]_cyt of B95–8 infected cells was not changed, even if their SOCE was increased significantly. When expressed alone, LMP-1 induced an increase of the SOCE amplitude and the expression of the protein allowing this influx, Orai1, showing the effect of EBV on SOCE of B cells are mediated by LMP-1. However, other hitherto unidentified EBV processes, unmasked in P3HR-1 infected cells, counteract this LMP-1-dependent increase of SOCE amplitude to impair a general and potentially toxic increase of [Ca²⁺]_i. Thus, EBV infection modifies the cellular Ca²⁺ homeostasis by acting on the ER and plasma membrane transporters.