Regulation of B-Cell Receptor Signaling and Its Therapeutic Relevance in Aggressive B-Cell Lymphomas

The proliferation and survival signals emanating from the B-cell receptor (BCR) constitute a crucial aspect of mature lymphocyte's life. Dysregulated BCR signaling is considered a potent contributor to tumor survival in different subtypes of B-cell non-Hodgkin lymphomas (B-NHLs). In the last decade, the emergence of BCR-associated kinases as rational therapeutic targets has led to the development and approval of several small molecule inhibitors targeting either Bruton's tyrosine kinase (BTK), spleen tyrosine kinase (SYK), or phosphatidylinositol 3 kinase (PI3K), offering alternative treatment options to standard chemoimmunotherapy, and making some of these drugs valuable assets in the anti-lymphoma armamentarium. Despite their initial effectiveness, these precision medicine strategies are limited by primary resistance in aggressive B-cell lymphoma such as diffuse large B-cell lymphoma (DLBCL) and mantle cell lymphoma (MCL), especially in the case of first generation BTK inhibitors. In these patients, BCR-targeting drugs often fail to produce durable responses, and nearly all cases eventually progress with a dismal outcome, due to secondary resistance. This review will discuss our current understanding of the role of antigen-dependent and antigen-independent BCR signaling in DLBCL and MCL and will cover both approved inhibitors and investigational molecules being evaluated in early preclinical studies. We will discuss how the mechanisms of action of these molecules, and their off/on-target effects can influence their effectiveness and lead to toxicity, and how our actual knowledge supports the development of more specific inhibitors and new, rationally based, combination therapies, for the management of MCL and DLBCL patients.

Dual Targeting by Inhibition of Phosphoinositide-3-Kinase and Mammalian Target of Rapamycin Attenuates the Neuroinflammatory Responses in Murine Hippocampal Cells and Seizures in C57BL/6 Mice

Emerging evidence suggests the association of seizures and inflammation; however, underlying cell signaling mechanisms are still not fully understood. Overactivation of phosphoinositide-3-kinases is associated with both neuroinflammation and seizures. Herein, we speculate the PI3K/Akt/mTOR pathway as a promising therapeutic target for neuroinflammation-mediated seizures and associated neurodegeneration. Firstly, we cultured HT22 cells for detection of the downstream cell signaling events activated in a lipopolysaccharide (LPS)-primed pilocarpine (PILO) model. We then evaluated the effects of 7-day treatment of buparlisib (PI3K inhibitor, 25 mg/kg p.o.), dactolisib (PI3K/mTOR inhibitor, 25 mg/kg p.o.), and rapamycin (mTORC1 inhibitor, 10 mg/kg p.o.) in an LPS-primed PILO model of seizures in C57BL/6 mice. LPS priming resulted in enhanced seizure severity and reduced latency. Buparlisib and dactolisib, but not rapamycin, prolonged latency to seizures and reduced neuronal loss, while all drugs attenuated seizure severity. Buparlisib and dactolisib further reduced cellular redox, mitochondrial membrane potential, cleaved caspase-3 and p53, nuclear integrity, and attenuated NF-κB, IL-1β, IL-6, TNF-α, and TGF-β1 and TGF-β2 signaling both in vitro and in vivo post-PILO and LPS+PILO inductions; however, rapamycin mitigated the same only in the PILO model. Both drugs protected against neuronal cell death demonstrating the contribution of this pathway in the seizure-induced neuronal pyknosis; however, rapamycin showed resistance in a combination model. Furthermore, LPS and PILO exposure enhanced pAkt/Akt and phospho-p70S6/total-p70S6 kinase activity, while buparlisib and dactolisib, but not rapamycin, could reduce it in a combination model. Partial rapamycin resistance was observed possibly due to the reactivation of the pathway by a functionally different complex of mTOR, i.e., mTORC2. Our study substantiated the plausible involvement of PI3K-mediated apoptotic and inflammatory pathways in LPS-primed PILO-induced seizures and provides evidence that its modulation constitutes an anti-inflammatory mechanism by which seizure inhibitory effects are observed. We showed dual inhibition by dactolisib as a promising approach. Targeting this pathway at two nodes at a time may provide new avenues for antiseizure therapies.

PI3Kγ Regulatory Protein p84 Determines Mast Cell Sensitivity to Ras Inhibition-Moving Towards Cell Specific PI3K Targeting?

Mast cells are the major effector cells in immunoglobulin E (IgE)-mediated allergy. The high affinity IgE receptor FcεRI, as well as G protein-coupled receptors (GPCRs) on the mast cell surface signals to phosphoinositide 3-kinase γ (PI3Kγ) to initiate degranulation, cytokine release, and chemotaxis. PI3Kγ is therefore considered as a target for treatment of allergic disorders. However, leukocyte PI3Kγ is key to many functions in innate and adaptive immunity, and attenuation of host defense mechanisms is an expected adverse effect that complicates treatment of chronic illnesses. PI3Kγ operates as a p110γ/p84 or p110γ/p101 complex, where p110γ/p84 requires Ras activation. Here we investigated if modulation of Ras-isoprenylation could target PI3Kγ activity to attenuate PI3Kγ-dependent mast cell responses without impairment of macrophage functions. In murine bone marrow-derived mast cells, GPCR stimulation triggers activation of N-Ras and H-Ras isoforms, which is followed by the phosphorylation of protein kinase B (PKB/Akt) relayed through PI3Kγ. Although K-Ras is normally not activated in Ras wild-type cells, it is able to compensate for genetically deleted N- and H-Ras isoforms. Inhibition of Ras isoprenylation with farnesyltransferase inhibitor FTI-277 leads to a significant reduction of mast cell degranulation, cytokine production, and migration. Complementation experiments expressing PI3Kγ adaptor proteins p84 or p101 demonstrated a differential sensitivity towards Ras-inhibition depending on PI3Kγ complex composition. Mast cell responses are exclusively p84-dependent and were effectively controlled by FTI-277. Similar results were obtained when GTP-Ras was inactivated by overexpression of the GAP-domain of Neurofibromin-1 (NF-1). Unlike mast cells, macrophages express p84 and p101 but are p101-dominated and thus remain functional under treatment with FTI-277. Our work demonstrates that p101 and p84 have distinct physiological roles, and that Ras dependence of PI3Kγ signaling differs between cell types. FTI-277 reduces GPCR-activated PI3Kγ  responses in p84-expressing but not p101-containing bone marrow derived cells. However, prenylation inhibitors have pleiotropic effects beyond Ras and non-tolerable side-effects that disfavor further clinical validation. Statins are, however, clinically well-established drugs that have previously been proposed to block mast cell degranulation by interference with protein prenylation. We show here that Simvastatin inhibits mast cell degranulation, but that this does not occur via Ras-PI3Kγ pathway alterations.