Wortmannin-mediated inhibition of phosphatidylinositol 3-kinase activity triggers apoptosis in normal and neoplastic B lymphocytes which are in cell cycle

Abstract

The B cell functional response following ligation of surface (s) IgM is dependent upon the differentiation stage of the population studied: cross-linking sIgM promotes proliferation of resting tonsillar follicular mantle (FM) B lymphocytes but induces apoptosis in the susceptible Epstein-Barr virus genome-negative Burkitt lymphoma (BL) cell line Ramos (Ramos-BL). This study investigates whether phosphatidylinositol-3-kinase (PI3-kinase), which has been reported to be intimately involved in the regulation of cellular growth, plays a role in the regulation of these slg-promoted B cell responses, and uses the selective and irreversible inhibitor of PI3-kinase activity, wortmannin (Wm). In Ramos-BL B cells, at 8 h post-treatment, Wm triggers a transient increase in apoptosis of 16 ± 6.9% with a concomitant cellular loss of 16 ± 6.1% from the G1 phase of cell cycle; [3H]thymidine incorporation also decreases by 33 ± 5.0%, from 37,274 c.p.m. ± 10% to 25,127 c.p.m. ± 4.0%. Moreover, at 72 h culture, Wm inhibits anti-IgM-induced FM B lymphocyte levels of [3H]thymidine incorporation typically by 47% and triggers 80% apoptosis from the G0G1 phase of cell cycle. Ramos-BL B cells exhibit high basal levels of PI3-kinase activity, as determined by immunoprecipitation with antibody to the p85 regulatory subunit of PI3-kinase and 32P incorporation into phosphatidylinositol, which is not significantly affected by anti-IgM stimulation; by contrast, anti-IgM stimulates significant PI3-kinase activity over negligible basal levels in FM B lymphocytes. Pre-treatment with Wm inhibits PI3-kinase activity in both cell types. Taken together these data indicate that in Ramos-BL B cells slgM-triggered growth arrest and apoptosis is PI3-kinase independent, whereas PI3-kinase activity is critical for slgM-triggered mitogenesis of FM B lymphocytes. Thus PI3-kinase plays a pivotal role in the regulation of both normal and neoplastic B lymphocyte progression through the cell cycle, such that if this PI3-kinase-dependent pathway is inhibited these cells default to apoptosis.