hsBAFF-upregulated intracellular free Ca2+ homeostasis regulates ERK1/2 activity and cell proliferation in B cells in vitro

We studied hsBAFF activity in in vitro mouse splenic B cells.hsBAFF effects on intracellular free Ca2+ concentration ([Ca2+]i)were assayed, using a laser scanning confocal microscope withfluorescent probe, Fluo-3/AM. We showed that treatment of Bcells with 0.5-5 μg/ml hsBAFF resulted in significantly higher[Ca2+]i levels in a dose-dependent fashion at 12 and 24 h,respectively (p<0.05 or p<0.01 vs. control). Furthermore, wenoticed that 2.5 μg/ml hsBAFF-treated cells were significantlyresistant to decrease of cellular viability induced by thapsigargin(Tg), an endoplasmic reticulum (ER) Ca2+-ATPase inhibitor(p<0.05 hsBAFF plus Tg group vs. Tg group). Thus hsBAFF maypromote B cell survival by direct upregulation of [Ca2+]iphysiological homeostasis contributing to prevention of [Ca2+]idysfunction. Using immunocytochemistry and Western blotanalysis, we found that the activation of ERK1/2 due to hsBAFFwas triggered by a [Ca2+]i-dependent pathway, leading toelevation of B cell proliferation. This is supported by the findingsthat intracellular Ca2+ chelator BAPTA/AM attenuatedphosphorylated ERK1/2 expression and cell proliferation inhsBAFF-stimulated B cells. hsBAFF-stimulated B cell proliferationwas obviously reduced by mitogen extracellular kinase 1/2(MEK1/2, upstream of ERK1/2) inhibitor U0126. Taken together,the main finding of this study is that hsBAFF elicits higher buthomeostatic [Ca2+]i levels, which regulates ERK1/2 activity andcell proliferation in in vitro B cells.