The PI3K signaling pathway; from normal lymphopoiesis to lymphoid malignancies

INTRODUCTION

As a vital mechanism of survival, lymphopoiesis requires the collaboration of different signaling molecules to orchestrate each step of cell development and maturation. The PI3K pathway is considerably involved in the maturation of lymphatic cells and therefore, its dysregulation can immensely affect human well-being and cause some of the most prevalent malignancies. As a result, studies that investigate this pathway could pave the way for a better understanding of the lymphopoiesis mechanisms, the undesired changes that lead to cancer progression, and how to design drugs to solve this issue.

AREAS COVERED

The present review addresses the aforementioned aspects of the PI3K pathway and helps pave the way for future therapeutic approaches. In order to access the articles, databases such as Medicine Medline/PubMed, Scopus, Google Scholar, and Science Direct were utilized. The search formula was established by identifying main keywords including PI3K/Akt/mTOR pathway, Lymphopoiesis, Lymphoid malignancies, and inhibitors.

EXPERT OPINION

The PI3K pathway is crucial for lymphocyte development and differentiation, making it a potential target for therapeutic intervention in lymphoid cancers. Studies are focused on developing PI3K inhibitors to impede the progression of hematologic malignancies, highlighting the pathway's significance in lymphoma and lymphoid leukemia.

Immunological Aspects of Richter Syndrome: From Immune Dysfunction to Immunotherapy

Richter Syndrome (RS) is defined as the development of an aggressive lymphoma in patients with a previous or simultaneous diagnosis of chronic lymphocytic leukemia (CLL). Two pathological variants of RS are recognized: diffuse large B-cell lymphoma (DLBCL)-type and Hodgkin lymphoma (HL)-type RS. Different molecular mechanisms may explain the pathogenesis of DLBCL-type RS, including genetic lesions, modifications of immune regulators, and B cell receptor (BCR) pathway hyperactivation. Limited data are available for HL-type RS, and its development has been reported to be similar to de novo HL. In this review, we focus on the immune-related pathogenesis and immune system dysfunction of RS, which are linked to BCR over-reactivity, altered function of the immune system due to the underlying CLL, and specific features of the RS tumor microenvironment. The standard of care of this disease consists in chemoimmunotherapy, eventually followed by stem cell transplantation, but limited possibilities are offered to chemo-resistant patients, who represent the majority of RS cases. In order to address this unmet clinical need, several immunotherapeutic approaches have been developed, namely T cell engagement obtained with bispecific antibodies, PD-1/PD-L1 immune checkpoint blockade by the use of monoclonal antibodies, selective drug delivery with antibody-drug conjugates, and targeting malignant cells with anti-CD19 chimeric antigen receptor-T cells.

The B Cell Antigen Receptor Activates the Akt (Protein Kinase B)/Glycogen Synthase Kinase-3 Signaling Pathway Via Phosphatidylinositol 3-Kinase

We have previously shown that the B cell Ag receptor (BCR) activates phosphatidylinositol (PI) 3-kinase. We now show that a serine/threonine kinase called Akt or protein kinase B is a downstream target of PI 3-kinase in B cells. Akt has been shown to promote cell survival as well as the transcription and translation of proteins involved in cell cycle progression. Using an Ab that specifically recognizes the activated form of Akt that is phosphorylated on serine 473, we show that BCR engagement activates Akt in a PI 3-kinase-dependent manner. These results were confirmed using in vitro kinase assays. Moreover, BCR ligation also induced phosphorylation of Akt of threonine 308, another modification that is required for activation of Akt. In the DT40 chicken B cell line, phosphorylation of Akt on serine 473 was completely dependent on the Lyn tyrosine kinase, while the Syk tyrosine kinase was required for sustained phosphorylation of Akt. Complementary experiments in BCR-expressing AtT20 endocrine cells confirmed that Src kinases are sufficient for BCR-induced Akt phosphorylation, but that Syk is required for sustained phosphorylation of Akt on both serine 473 and threonine 308. In insulin-responsive cells, Akt phosphorylates and inactivates the serine/threonine kinase glycogen synthase kinase-3 (GSK-3). Inactivation of GSK-3 may promote nuclear accumulation of several transcription factors, including NF-ATc. We found that BCR engagement induced GSK-3 phosphorylation and decreased GSK-3 enzyme activity. Thus, BCR ligation initiates a PI 3-kinase/Akt/GSK-3 signaling pathway.

An 11-Amino Acid Sequence in the Cytoplasmic Domain of CD40 Is Sufficient for Activation of c-Jun N-Terminal Kinase, Activation of MAPKAP Kinase-2, Phosphorylation of IκBα, and Protection of WEHI-231 Cells from Anti-IgM-Induced Growth Arrest

We have previously shown that CD40 causes strong activation of the c-Jun N-terminal kinase (JNK), the p38 mitogen-activated protein kinases (MAPK) and MAPKAP kinase-2, a downstream target of p38 MAPK. To identify signaling motifs in the CD40 cytoplasmic domain that are responsible for activation of these kinases, we have created a set of 11 chimeric receptors consisting of the extracellular and transmembrane domains of CD8 fused to portions of the murine CD40 cytoplasmic domain. These chimeric receptors were expressed in WEHI-231 B lymphoma cells. We found that amino acids 35–45 of the CD40 cytoplasmic domain constitute an independent signaling motif that is sufficient for activation of the JNK and p38 MAPK pathways, as well as for induction of IκBα phosphorylation and degradation. Amino acids 35–45 were also sufficient to protect WEHI-231 cells from anti-IgM-induced growth arrest. This is the same region of CD40 required for binding the TNF receptor-associated factor-2 (TRAF2), TRAF3, and TRAF5 adapter proteins. These data support the idea that one or more of these TRAF proteins couple CD40 to the kinase cascades that activate NF-κB, JNK, and p38 MAPK.

CD40-Mediated Signal Transduction in Human Airway Smooth Muscle

CD40 is a member of the TNF receptor family that was initially described on the surface of B cells. Recently, CD40 has also been described on mesenchymal cells, such as endothelial cells and fibroblasts, where engagement by its ligand CD40 ligand can lead to up-regulation of costimulatory and cell adhesion molecules, as well as secretion of proinflammatory cytokines. Since airway inflammation potentially involves cell-cell interactions of T cells and eosinophils (which express CD40 ligand) with airway smooth muscle (ASM) cells, we postulated that ASM may express CD40 and that engagement of ASM CD40 may modulate smooth muscle cell function. We demonstrate that CD40 is expressed on cultured human ASM and that expression can be increased by treatment with TNF-α or IFN-γ. Cross-linking CD40 on ASM resulted in enhanced IL-6 secretion and an increase in intracellular calcium concentrations, which were dependent on calcium influx. We show that CD40-mediated signaling events include protein tyrosine phosphorylation and activation of NF-κB. Pretreatment of ASM with the tyrosine kinase inhibitors genistein or herbimycin inhibited the rapid mobilization of calcium induced via CD40, suggesting that calcium mobilization was coupled to activation of protein tyrosine kinases. In addition, inhibition of calcium influx inhibited both CD40-mediated NF-κB activation and enhancement of IL-6 secretion. These results delineate a potentially important CD40-mediated signal-transduction pathway in ASM, involving protein tyrosine kinase-dependent calcium mobilization, NF-κB activation, and IL-6 production. Together, these results suggest a mechanism whereby T cell/smooth muscle cell interactions may potentiate airway inflammation.

TNF Receptor-Associated Factor-3 Signaling Mediates Activation of p38 and Jun N-Terminal Kinase, Cytokine Secretion, and Ig Production Following Ligation of CD40 on Human B Cells

CD40 engagement induces a variety of functional outcomes following association with adaptor molecules of the TNF receptor-associated factor (TRAF) family. Whereas TRAF2, -5, and -6 initiate NF-κB activation, the outcomes of TRAF3-initiated signaling are less characterized. To delineate CD40-induced TRAF3-dependent events, Ramos B cells stably transfected with a dominant negative TRAF3 were stimulated with membranes expressing recombinant CD154/CD40 ligand. In the absence of TRAF3 signaling, activation of p38 and control of Ig production were abrogated, whereas Jun N-terminal kinase activation and secretion of IL-10, lymphotoxin-α, and TNF-α were partially blocked. By contrast, induction of apoptosis, activation of NF-κB, generation of granulocyte-macrophage CSF, and up-regulation of CD54, MHC class II, and CD95 were unaffected by the TRAF3 dominant negative. Together, these results indicate that TRAF3 initiates independent signaling pathways via p38 and JNK that are associated with specific functional outcomes.

Activation of Protein Kinase C-ζ and Phosphatidylinositol 3′-Kinase and Promotion of Macrophage Differentiation by Insulin-Like Growth Factor-I

Phosphoinositides that are phosphorylated at the D3 position have been reported to activate an atypical, Ca2-independent protein kinase C (PKC) isoform designated PKC-ζ, and overexpression of this enzyme leads to monocytic differentiation. In this study, we cultured human HL-60 promyeloid cells with vitamin D3 and insulin-like growth factor-I (IGF-I), a 70-amino-acid peptide that activates phosphatidylinositol 3′-kinase (PI 3-kinase) in murine promyeloid cells. Two days later, the proportion of cells differentiating into macrophages in serum-free medium, as assessed by expression of the α-subunit of the β2 integrin CD11b, increased from 5 ± 1% to 25 ± 3%. Addition of IGF-I increased the proportion of cells differentiating into CD11b-positive macrophages to 78 ± 5%. In the absence of vitamin D3, IGF-I did not induce expression of CD11b (6 ± 1%). The IGF-I-promoted macrophage differentiation was blocked specifically by preincubation of HL-60 cells with a mAb (αIR3) directed against the IGF type I receptor. Similarly, pretreatment of cells with either αIR3 or an IGF-binding protein, IGFBP-3, led to a 75% inhibition of CD11b expression when cells were cultured with vitamin D3 in serum-containing medium. IGF-I, but not vitamin D3, caused a sevenfold increase in the enzymatic activity of both PI 3-kinase and atypical PKC-ζ. Inhibition of IGF-I-inducible PI 3-kinase with either wortmannin or LY294002 abrogated the IGF-I-induced activation of PKC-ζ and totally blocked the enhancement in macrophage differentiation caused by IGF-I. These data establish that PKC-ζ is a putative downstream target of PI 3-kinase that is activated during IGF-I-promoted macrophage differentiation.