B cell receptor signaling down-regulates forkhead box transcription factor class O 1 mRNA expression via phosphatidylinositol 3-kinase and Bruton's tyrosine kinase.
BCR cross-linking promotes mature B cell proliferation and survival. PI3K-mediated down-regulation of proapoptotic and antimitogenic genes such as forkhead box transcription factor class O 1 (FOXO1) is an important component of this process. Previously, BCR-induced phosphorylation of FOXO1 was shown to lead to a block in nuclear localization and subsequent protein degradation. We demonstrate that the BCR also signals through PI3K to down-regulate FOXO1 mRNA expression. Bruton's tyrosine kinase (Btk), a downstream effector of PI3K, signals through B cell linker protein (BLNK) and phospholipase C (PLC)gamma2 to mediate B cell proliferation and survival in response to BCR cross-linking. BCR-induced down-regulation of FOXO1 mRNA was impaired in murine knockouts of Btk, BLNK, and PLCgamma2. Because B cells in these models are predominantly immature, experiments were also performed using mature B cells expressing low levels of Btk and BLNK. Similar results were obtained. Inhibitors of downstream components of the Btk/BLNK/PLCgamma2 pathway were used to define the mechanism by which Btk signaling inhibits FOXO1 expression. The protein kinase Cbeta inhibitor Gö6850 had minimal effects on BCR-mediated FOXO1 mRNA down-regulation. However, cyclosporin A, an inhibitor of the Ca(2+)-dependent phosphatase calcineurin, had similar effects on FOXO1 mRNA expression as the PI3K inhibitor LY294002. Neither Btk deficiency nor cyclosporin A prevented FOXO1 protein phosphorylation, indicating that PI3K down-regulates FOXO1 via two independent pathways. We show that the Btk/BLNK/PLCgamma2 pathway mediates BCR-induced changes in expression of the FOXO1 target gene cyclin G2. These observations support the hypothesis that Btk mediates BCR-induced proliferation and survival in part via inhibition of FOXO expression.
Hinman
Rochelle M
RM
Department of Internal Medicine, Division of Rheumatology, University of Texas Southwestern Medical Center, Dallas, TX 75390, USA.
Bushanam
Jessica N
JN
Nichols
Whitney A
WA
Satterthwaite
Anne B
AB
D000077329:Agammaglobulinaemia Tyrosine Kinase; D056766:Cyclin G1; D056767:Cyclin G2; D016213:Cyclins / Q000235:genetics; D015536:Down-Regulation / Q000235:genetics*; D000071161:Forkhead Box Protein O1; D000071316:Forkhead Box Protein O3; D051858:Forkhead Transcription Factors / Q000235:genetics*; D015870:Gene Expression / Q000235:genetics*; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism*; D010766:Phosphorylation; D011505:Protein-Tyrosine Kinases / Q000378:metabolism*; D012333:RNA, Messenger / Q000235:genetics; D011947:Receptors, Antigen, B-Cell / Q000378:metabolism*; D015398:Signal Transduction
D000077329:Agammaglobulinaemia Tyrosine Kinase; D056766:Cyclin G1; D056767:Cyclin G2; D016213:Cyclins; D000071161:Forkhead Box Protein O1; D000071316:Forkhead Box Protein O3; D051858:Forkhead Transcription Factors; D019869:Phosphatidylinositol 3-Kinases; D011505:Protein-Tyrosine Kinases; D012333:RNA, Messenger; D011947:Receptors, Antigen, B-Cell; C534703:Ccng1 protein, mouse; C474033:FoxO3 protein, mouse; C467200:Foxo1 protein, mouse; C000625951:Btk protein, mouse