Phosphatidylinositol-3,4-Bisphosphate and Its Binding Protein Lamellipodin Regulate Chemotaxis of Malignant B Lymphocytes.

Cell migration is controlled by PI3Ks, which generate lipid messengers phosphatidylinositol-3,4,5-trisphosphate and phosphatidylinositol-3,4-bisphosphate [PI(3,4)P2] and consequently recruit pleckstrin homology (PH) domain-containing signaling proteins. PI3K inhibition impairs migration of normal and transformed B cells, an effect thought to partly underlie the therapeutic efficacy of PI3K inhibitors in treatment of B cell malignancies such as chronic lymphocytic leukemia. Although a number of studies have implicated phosphatidylinositol-3,4,5-trisphosphate in cell migration, it remains unknown whether PI(3,4)P2 plays a distinct role. Using the PI(3,4)P2-specific phosphatase inositol polyphosphate 4-phosphatase, we investigate the impact of depleting PI(3,4)P2 on migration behavior of malignant B cells. We find that cells expressing wild-type, but not phosphatase dead, inositol polyphosphate 4-phosphatase show impaired SDF-induced PI(3,4)P2 responses and reduced migration in Transwell chamber assays. Moreover, PI(3,4)P2 depletion in primary chronic lymphocytic leukemia cells significantly impaired their migration capacity. PI(3,4)P2 depletion reduced both overall motility and migration directionality in the presence of a stable chemokine gradient. Within chemotaxing B cells, the PI(3,4)P2-binding cytoskeletal regulator lamellipodin (Lpd) was found to colocalize with PI(3,4)P2 on the plasma membrane via its PH domain. Overexpression and knockdown studies indicated that Lpd levels significantly impact migration capacity. Moreover, the ability of Lpd to promote directional migration of B cells in an SDF-1 gradient was dependent on its PI(3,4)P2-binding PH domain. These results demonstrate that PI(3,4)P2 plays a significant role in cell migration via binding to specific cytoskeletal regulators such as Lpd, and they suggest that impairment of PI(3,4)P2-dependent processes may contribute to the therapeutic efficacy of PI3K inhibitors in B cell malignancies.

p110δ phosphoinositide 3-kinase represses IgE switch by potentiating BCL6 expression.

PI3Ks are key signaling enzymes required for triggering many immunological functions. In B lymphocytes, PI3K signaling is required for Ag-induced proliferation and robust production of most Ab isotypes. Paradoxically, PI3K was found to have a negatively regulatory function regarding Ab class switch recombination, and blockade of PI3K can strongly potentiate IgE switch. In this article, we explore the mechanisms of this unexpected negative regulatory function of PI3K regarding IgE. We demonstrate that p110δ PI3K selectively regulates IgE switch in a B cell-intrinsic manner by controlling germline transcription of the IgE promoter (εGLT). Although p110δ can regulate transcription of activation-induced cytidine deaminase via Akt, repression of εGLT and IgE switch is not dependent on Akt signaling. Inhibition of p110δ, but not Akt, leads to reduced expression of transcriptional repressor B cell lymphoma 6 (BCL6) and concomitant upregulation of εGLT and other BCL6-target genes. p110δ inhibitor treatment strikingly alters the balance between BCL6 and IRF4 (a transcription factor that antagonizes BCL6), leading to increased IRF4 and decreased BCL6 expression levels in germinal center B cells. Ectopic expression of BCL6 can partially overcome the elevated εGLTs and potentiated IgE switching in p110δ-inhibited B cells. To our knowledge, these results provide the first evidence that p110δ PI3K signaling regulates BCL6 expression and indicate that PI3K promotes the germinal center B cell program and selectively represses IgE switch by maintaining sufficient levels of BCL6.