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Abstract B cell activating factor from the TNF family (BAFF) is implicated in not only the physiology of normal B cells, but also the pathophysiology of aggressive B cells related to malignant and autoimmune diseases. Autophagy plays a crucial role in balancing the beneficial and detrimental effects of immunity and inflammation. However, little is known about whether and how excessive BAFF mediates autophagy contributing to B-cell proliferation and survival. Here, we show that excessive human soluble BAFF (hsBAFF) inhibited autophagy with a concomitant reduction of LC3-II in normal and B-lymphoid (Raji) cells. Knockdown of LC3 not only potentiated hsBAFF inhibition of autophagy, but also attenuated hsBAFF activation of Akt/mTOR pathway, thereby diminishing hsBAFF-induced B-cell proliferation/viability. Further, we found that hsBAFF inhibition of autophagy was Akt/mTOR-dependent. This is supported by the findings that hsBAFF increased mTORC1-mediated phosphorylation of ULK1 (Ser757); Akt inhibitor X, mTORC1 inhibitor rapamycin, mTORC1/2 inhibitor PP242, expression of dominant negative Akt, or knockdown of mTOR attenuated hsBAFF-induced phosphorylation of ULK1, decrease of LC3-II level, and increase of cell proliferation/viability. Chelating intracellular free Ca2+ ([Ca2+] i) with BAPTA/AM or preventing [Ca2+] i elevation using EGTA or 2-APB profoundly blocked hsBAFF-induced activation of Akt/mTOR, phosphorylation of ULK1 and decrease of LC3-II, as well as increase of cell proliferation/viability. Similar effects were observed in the cells where CaMKII was inhibited by KN93 or knocked down by CaMKII shRNA. Collectively, these results indicate that hsBAFF inhibits autophagy promoting cell proliferation and survival through activating Ca2+-CaMKII-dependent Akt/mTOR signaling pathway in normal and neoplastic B-lymphoid cells. Our findings suggest that manipulation of intracellular Ca2+ level or CaMKII, Akt, or mTOR activity to promote autophagy may be exploited for prevention of excessive BAFF-induced aggressive B lymphocyte disorders and autoimmune diseases. Graphical abstract Unlabelled Image Highlights • hsBAFF promotes B-cell proliferation and survival by inhibiting autophagy. • Downregulation of LC3 protein level is critical for hsBAFF inhibition in B cells. • hsBAFF-activated Akt/mTOR signaling represses autophagy via phosphorylating ULK1 (Ser757) in B cells. • hsBAFF inhibits autophagy by activating Ca2+-CaMKII-dependent Akt/mTOR signaling pathway in B cells. [ABSTRACT FROM AUTHOR]

Abstract: B-cell activating factor (BAFF) is involved in not only the physiology of normal B cells, but also the pathophysiology of aggressive B cells related to malignant and autoimmune diseases. However, how excessive BAFF promotes aggressive B-cell proliferation and survival is not well understood. Here we show that excessive human soluble BAFF (hsBAFF) enhanced cell proliferation and survival in normal and B-lymphoid (Raji) cells, which was associated with suppression of PP2A, resulting in activation of Erk1/2. This is supported by the findings that pretreatment with U0126 or PD98059, expression of dominant negative MKK1, or overexpression of PP2A prevented hsBAFF-induced activation of Erk1/2 and cell proliferation/viability in the cells. It appears that hsBAFF-mediated PP2A-Erk1/2 pathway and B-cell proliferation/viability was Ca2+-dependent, as pretreatment with BAPTA/AM, EGTA or 2-APB significantly attenuated these events. Furthermore, we found that inhibiting CaMKII with KN93 or silencing CaMKII also attenuated hsBAFF-mediated PP2A-Erk1/2 signaling and B-cell proliferation/viability. The results indicate that BAFF activates Erk1/2, in part through Ca2+-CaMKII-dependent inhibition of PP2A, increasing cell proliferation/viability in normal and neoplastic B-lymphoid cells. Our data suggest that inhibitors of CaMKII and Erk1/2, activator of PP2A or manipulation of intracellular Ca2+ may be exploited for prevention of excessive BAFF-induced aggressive B-cell malignancies and autoimmune diseases. [Copyright &y& Elsevier]