PDK1 regulates B cell differentiation and homeostasis

Abstract

Successful B cell differentiation and prevention of cell transformation depends on balanced and fine-tuned activation of cellular signaling pathways. The phosphatidyl inositol-3 kinase (PI3K) signaling pathway has emerged as a major regulator of B lymphocyte homeostasis and function. Phosphoinositide-dependent protein kinase-1 (PDK1) is the pivotal node in the PI3K pathway, regulating the stability and activity of downstream AGC kinases (including Akt, RSK, S6K, SGK, and PKC). Although the importance of PI3K activity in B cell differentiation is well documented, the role of PDK1 and other downstream effectors is underexplored. Here we used inducible and stage-specific gene targeting approaches to elucidate the role of PDK1 in early and peripheral B cell differentiation. PDK1 ablation enhanced cell cycle entry and apoptosis of IL-7-dependent pro-B cells, blocking Ig synthesis and B cell maturation. PDK1 also was essential for the survival and activation of peripheral B cells via regulation of PKC and Akt-dependent downstream effectors, such as GSK3α/β and Foxo1. We found that PDK1 deletion strongly impaired B cell receptor (BCR) signaling, but IL-4 costimulation was sufficient to restore BCR-induced proliferation. IL-4 also normalized PKCβ activation and hexokinase II expression in BCR-stimulated cells, suggesting that this signaling pathway can act independent of PDK1 to support B cell growth. In summary, our results demonstrate that PDK1 is indispensable for B cell survival, proliferation, and growth regulation.A ctivation of the phosphatidyl inositol-3 kinase (PI3K) signaling pathway is critical to early B cell development as well as peripheral B cell survival and activation (1). Although the catalytic p110 subunits of class I PI3K molecules are partially redundant, the combined loss of the p110α and p110δ isoforms results in impaired IL-7R-driven proliferation (2). Conversely, it has been suggested that attenuation of PI3K signaling via IL-7R signaling is required for pre-B cell differentiation into IgM-expressing cells to cease proliferation and promote RAG expression (3).In peripheral B cells, continued survival requires "tonic" signaling via the B cell receptor (BCR), which can be replaced by constitutive PI3K activity (4). Moreover, generation of the marginal zone (MZ) and B-1 B cell subsets, as well as antigendriven differentiation into antibody-producing cells, are dependent on PI3K (1). PI3K activity generates PtdIns(3,4,5)P 3 , which acts as a secondary messenger by binding the pleckstrin homology domains of downstream effector molecules. PtdIns (3,4,5)P 3 is also the substrate for the phosphatases PTEN and SHIP, generating PtdIns(4,5)P 2 and PtdIns(3,4)P 2 , respectively. Unrestrained activation of PI3K signaling in B cells lacking PTEN and SHIP results in lethal B cell lymphoma (5).Phosphoinositide-dependent kinase 1 (PDK1) represents a pivotal downstream effector of PI3K signaling, regulating cellular responses to growth factors, insulin, and numerous other agonists by activating a number of AGC pr...

TRAF3 deficiency promotes metabolic reprogramming in B cells

“…Glut1 is a glucose transporter and HXK2 is an inducible kinase that phosphorylates glucose, sequestering it intracellularly to be metabolized14. Glut1 and HXK2 are induced in B cells in response to antigen receptor stimulation, an important pro-survival signal612. B cells isolated from B- Traf3 −/− mice had increased protein abundance (Fig.…”

“…HXK2 is an inducible kinase that promotes glucose metabolism and cell survival and has been suggested as a therapeutic target in cancer11. HXK2 is upregulated in lymphocytes upon activation or cytokine stimulation1213.…”

Conditional Disruption of Raptor Reveals an Essential Role for mTORC1 in B Cell Development, Survival, and Metabolism

“…Similarly, targeted disruption of Foxo1 (28), a key transcription factor that is negatively regulated by AKT phosphorylation, also results in an early B cell developmental block due to the requirement of Foxo1 to activate transcription of critical pre-B cell genes including Rag1 , Rag2 (39). Recently, disruption of PDK1, which phosphorylates and activates AKT downstream of PI3K, was also shown to result in reduced mTORC1 signaling, reduced IgH protein expression, and a block in B cell development at the pre-B cell stage (29, 40). Here we show that deletion of Raptor recapitulates the block in B cell development seen with disruption of the PI3K/AKT pathway, suggesting that mTORC1 may act downstream of PI3K/AKT signaling to promote immunoglobulin heavy chain expression and pre-B cell development.…”

Untitled

“…Acute organ rejection is the most common, occurring within the first three months following transplantation; however, there may still be a risk of chronic rejection occurring several years later [12,24]. Chronic rejection is characterized by induction of fibroblast proliferation, pleomorphic effects (such as proliferation of smooth muscles) by some cytokines, maturation of B-cells and antibody synthesis by IL-4 [26][27][28]. Moreover, fibrosis formation is very common with chronic rejection due to the activation of the tissue repair process [29,30].…”