Chemokine signalling: pivoting around multiple phosphoinositide 3-kinases

Abstract

The role of chemokines in mediating directional cell migration is well established, but more recently it has become evident that chemokines are able to couple to distinct signalling pathways that are involved in not only chemotaxis, but also cell growth and transcriptional activation. The signalling pathway controlled by the phosphoinositide 3-kinase (PI3K) family of lipid kinases has been the focus of much attention with respect to their role in chemokine-mediated functional responses. Indeed, there now exists convincing biochemical, pharmacological and genetic evidence that both CC and CXC chemokines stimulate PI3K-dependent chemotaxis of inflammatory cells such as eosinophils, macrophages, neutrophils and T lymphocytes. This review considers the role of individual PI3Ks (e.g. the p85/p110 heterodimer, PI3Kgamma and PI3KC2alpha) as well their downstream effector targets in mediating chemokine-stimulated cell migration.

Figure 1

Classification of phosphoinositide 3-kinase (PI3K) family members and synthetic pathways for PI lipids. (a) PI3Ks have been divided into three classes, based on primary structure, substrate specificity and regulatory mechanisms; class 1 is further subdivided according to the associated adapter (regulatory) subunit. The protein domains are as follows: BCR, breakpoint-cluster region; C2, C2 domain; P, proline-rich motif; PIK, phosphatidylinositol kinase domain; Ras-BD; Ras-binding domain; SH2, src-homology domain 2; SH3, src-homology domain 3. (b) Routes of synthesis of known PI lipids: PtdIns 4-kinase and PtdIns(4)P 5-kinase mediate formation of PtdIns(4)P and PtdIns(4,5)P₂ from PtdIns, and all three lipids can potentially serve as substrates for different PI3Ks. The broad substrate specificity depicted is that of the p85/p110 heterodimer and PI3Kγ. PI3K phosphorylates only PtdIns, whilst PI3K-C2α specificity in vitro is restricted primarily to PtdIns and PtdIns(4)P. The thick arrows reflect the major route of accumulation of PtdIns(3,4,5)P₃ from PtdIns(4,5)P₂ upon receptor stimulation and its subsequent conversion to PtdIns(3,4)P₂ by a selective 5-phosphatase. Other selective 3′- and 5′-phosphatases also help regulate the state of phosphorylation of the PI lipids. Representative proteins with specificities for particular 3′-phosphorylated PI lipids, as well as some downstream effectors of PtdIns(3,4,5)P₃-binding proteins, are shown. PTEN, 3′-phosphatase and tensin homologue deleted on chromosome 10 protein; ARF, ADP ribosylation factor; DAPP-1, dual adaptor for phosphotyrosine and 3- phosphoinositides; GEF, guanine nucleotide exchange factor; FYVE, Fab1, YOTB, Vac1 and EEA1 domain; PLC, phospholipase C; PX, phox homology; PKC, protein kinase C; TAPP, tandem PH-domain-containing protein.

Figure 2

Potential role of lipid and protein kinase activity of phosphoinositide 3-kinases (PI3Ks). Proposed routes are shown by which PI3Kγ and p85/p110 PI3K may contribute to chemokine-stimulated functional events via 3′-phosphoinositide lipid dependent and/or protein serine kinase-dependent activity. PDK-1, 3′-phosphoinositide-dependent protein kinase-1; PKB, protein kinase B; GEF, guanine nucleotide exchange factor; PH, pleckstrin homology; PTK, protein tyrosine kinase.

Figure 3

Schematic representation of phosphoinositide 3-kinase (PI3K)-dependent signalling pathways involved in chemokine-mediated functional responses. Following chemokine receptor engagement, both the Gα_i and Gβγ subunits activate distinct subsets of signalling molecules. Activation of PI3Kγ by βγ subunits has been established, but direct activation of p85/p110 PI3Ks by βγ subunits is a contentious issue. GTP-bound Gα_i can directly activate Src family protein tyrosine kinases (PTKs), leading to tyrosine phosphorylation of substrates and the recruitment and activation of Src homology 2 (SH2) domain-containing proteins. This potentially leads to the activation of numerous signalling molecules, including p85/p110 PI3Ks and Ras, which can activate both class IA and IB PI3Ks. Red arrows represent the lipid kinase activity of PI3K, but the relative contribution of individual PI3K isoforms in the activation of particular effectors has yet to be defined. As well as Ras-dependent activation of extracellular signal-regulated kinase (ERK)1/2, Ras-independent activation of ERK1/2 can occur via protein kinase Cξ (PKCξ)-mediated activation of mitogen/ERK kinase-1 (MEK), or by the activation of the mitogen-activated protein kinase (MAPK) cascade by the serine kinase activity of PI3Kγ (dashed red arrow). ARF, ADP-ribosylation factor; GEF, guanine exchange factor; PDK-1 3′-phosphoinositide-dependent protein kinase-1; PKB, protein kinase B; SOS, Son of Seven less homologue.