Emergence of the PI3-kinase pathway as a central modulator of normal and aberrant B cell differentiation

Phosphoinositide 3-kinase (PI3K) defines a family of lipid kinases that direct a wide range of cellular processes and cell fate decisions. Since its discovery, and that of its enzymatic antagonist PTEN, much of the focus on PI3K has been on its oncogenic potential. In recent years, studies on PI3K signaling in B lymphocytes have established the importance of this pathway in effecting B cell differentiation and associated molecular events such as V(D)J recombination and class switch recombination. Intriguing new findings also indicate that there is specificity in the PI3K pathway in B cells, including preferential expression or usage of particular PI3K isoforms and counter-regulation by the PTEN and SHIP phosphatases. The role of PI3K adaptor proteins (CD19, BCAP, and TC21) has also undergone revision to reflect both shared and unique properties. The emergence of Foxo1 as a critical PI3K regulatory target for B cell differentiation has united membrane proximal regulatory events orchestrated by PI3K/PTEN/SHIP with key transcriptional targets. Insights into the regulation and impact of PI3K signaling have been brought to bear in new treatments for B cell malignancies, and will also be an important topic of consideration for B cell-dependent autoimmune diseases.

Introduction

Phosphoinositide 3-kinase (PI3K) is a lipid kinase acting on membrane phosphatidylinositol (PtdIns)(4,5)P₂ to produce PtdIns(3,4,5)P₃. Members of the class IA PI3K subset are heterodimeric molecules consisting of a 110 kDa catalytic subunit (p110α, p110β or p110δ) encoded by individual genes (Pik3ca, Pik3cb or Pik3cd) and a smaller regulatory subunit (Table 1). A single gene (Pik3r1) encodes the regulatory isoforms p50α, p55α and p85α, while the Pik3r2 and Pik3r3 genes encode p85β and p55γ, respectively (Table 1). The regulatory subunits prevent degradation of the catalytic subunit while inhibiting its activity. Binding of the tandem SH2 domains in the regulatory subunit to tyrosine-phosphorylated YXXM motifs releases inhibition of the associated catalytic subunit. p110γ is the sole representative of the class IB enzymes and is activated by G-protein-coupled receptors (GPCR) and regulated by the p84/p101 proteins. Of note, p110α and p110β are ubiquitously expressed, while p110δ and p110γ are expressed primarily in hematopoietic cells (Table 1). B cell-specific regulation of the class IA PI3Ks is also conferred by the YXXM-bearing adaptor proteins, which can be transmembrane (e.g. CD19) or cytosolic (e.g. BCAP and TC21) and show receptor-specific activation properties (Figure 1).

All of the catalytic subunits are capable of generating PtdIns(3,4,5)P₃; however, they also possess unique functions, particularly with respect to crosstalk with the Ras pathway. Ras has been noted to bind and activate p110α,β,γ, but this attribute is not shared by p110δ [1]. Consistently, cells relying on p110δ for oncogenic activity are resistant to inhibitors of the MAP kinase pathway, whereas cells expressing p110α, β and γ lose transforming capacity when treated with MAP kinase inhibitors or when Ras-binding is disabled [1, 2]. Moreover, loss of function due to impaired Ras-binding can be compensated for by the provision of a myristoylation signal, suggesting a role for Ras in p110α,β,γ recruitment to the membrane [1, 2]. Importantly, p110δ is the only isoform that exhibits some level of constitutive activity, and becomes oncogenic upon overexpression [2]. The MAP kinase pathway also intersects at points downstream of PtdIns(3,4,5)P₃ generation, as reviewed elsewhere [3].

Counter-regulation of PI3K activity is achieved by the inositol phosphatases and tensin homolog (PTEN) and SH2-containing inositol phosphatase (SHIP) (Figure 1). PtdIns(3,4,5)P₃ and, perhaps, PtdIns(3,4)P₂ are substrates for the phosphoinositide 3-phosphatase PTEN, which has emerged as the key functional antagonist to PI3K [4]. As such, constitutive PTEN activity counters PI3K activity induced by receptor tyrosine kinases, GPCRs and activated Ras. PTEN functions as a tumor suppressor protein and its loss is likely to activate numerous downstream effector pathways initiated by PtdIns(3,4,5)P₃ binding proteins [5]. In mature B cells, the Ser/Thr kinases Akt and PDK1 and the tyrosine kinase BTK are likely the most critical downstream effectors for PI3K (Figure 1). All of these molecules possess a pleckstrin homology (PH) domain specific for PtdIns(3,4,5)P₃, allowing for re-localization to the plasma membrane. Interestingly, although many proteins possess PH domains specific for PtdIns(3,4,5)P₃, an additional level of regulation is likely dictated by the relative affinity for PtdIns(3,4,5)P₃ to confer selective recruitment of PH-domain-containing proteins based upon local PtdIns(3,4,5)P₃ abundance. For example, the PH domain of PDK1 has a 20-fold greater affinity for PtdIns(3,4,5)P₃ than the PH domain of Akt [6]. The inositol phosphatase SHIP appears to act primarily by dephosphorylating PtdIns(3,4,5)P₃ at the 5-position and perhaps to some extent on PtdIns(4,5)P₂. Although PTEN and SHIP act on the same primary substrate, it is important to note that they generate distinct lipid products. In fact, a significant fraction of PtdIns(3,4)P₂ is thought to be produced by SHIP-mediated dephosphorylation of PtdIns(3,4,5)P₃ rather than PI3K phosphorylation of PtdIns(4)P. There are fewer known targets of PtdIns(3,4)P₂ than PtdIns(3,4,5)P₃, but they include the adaptor proteins Bam32/DAPP1 and TAPP2.