Targeting the phosphoinositide 3-kinase pathway in cancer

Abstract

The phosphoinositide 3-kinase (PI3K) pathway is a key signal transduction system that links oncogenes and multiple receptor classes to many essential cellular functions, and is perhaps the most commonly activated signalling pathway in human cancer. This pathway therefore presents both an opportunity and a challenge for cancer therapy. Even as inhibitors that target PI3K isoforms and other major nodes in the pathway, including AKT and mammalian target of rapamycin (mTOR), reach clinical trials, major issues remain. Here, we highlight recent progress that has been made in our understanding of the PI3K pathway and discuss the potential of and challenges for the development of therapeutic agents that target this pathway in cancer.

Figure 1. The Class I phosphoinositide 3-kinase (PI3K) signaling pathway

Upon growth factor stimulation and subsequent activation of receptor tyrosine kinases (RTKs), class IA PI3Ks, consisting of p110α/p85, p110β/p85 and p110δ/p85, are recruited to the membrane via interaction of the p85 subunit to the activated receptors directly (e.g.PDGFR) or to adaptor proteins associated with the receptors (e.g. insulin receptor substrate 1, IRS1). The activated p110 catalytic subunit converts phosphatidylinositol-4,5-bisphosphate (PIP₂) to phosphatidylinositol-3,4,5-triphosphate (PIP₃) at the membrane, providing docking sites for signaling proteins with pleckstrin-homology (PH) domains including the phosphoinositide-dependent kinase 1 (PDK1) and the Ser-Thr kinase AKT. PDK1 phosphorylates and activates AKT (also known as PKB). The activated AKT elicits a broad spectrum of downstream signaling events. Class IB PI3K (p110γ/p101) can be activated directly by G-protein coupled receptors (GPCRs) through interacting with the Gβγ subunit of trimeric G proteins. The p110β and p110δ can also be activated by GPCRs. PTEN (phosphatase and tensin homologue) antagonizes the PI3K action by dephosphorylating PIP₃. G βγ, guanine nucleotide binding protein (G protein), βγ; FKHR, forkhead transcription factor; NFκB, nuclear factor kappa-light-chain-enhancer of activated B cells; BAD, Bcl-2-associated death promoter protein; SGK, Serum and glucocorticoid-inducible kinase; PKC, protein kinase C; GSK3β, glycogen synthase kinase 3 beta; mTOR, mammalian target of rapamycin; Rac1, Ras-related C3 botulinum toxin substrate 1; S6K, ribosomal protein S6 kinase; LPA, lysophosphatidic acid.

Figure 2

Figure 2a. The members of the phosphoinositide 3-kinase (PI3K) family. PI3Ks have been divided into three classes according to their structural characteristics and substrate specificity. Class IA PI3Ks are heterodimers consisting of a p110 catalytic subunit and a p85 regulatory subunit. In mammals, there are three genes, PIK3CA, PIK3CB and PIK3CD, encoding p110 catalytic isoforms: p110α, p110β and p110δ, respectively. While the expression of p110δ is largely restricted to the immune system, p110α and p110β are ubiquitously expressed^, . The p110 catalytic isoforms are highly homologous and share five distinct domains: an N-terminal p85-binding domain (p85BD) that interacts with the p85 regulatory subunit, a Ras-binding domain (RasBD) that mediates activation by members of the Ras family of small GTPases, a putative membrane-binding domain C2, the helical domain, and the C-terminal kinase catalytic domain. There are also three genes, PIK3R1, PIK3R2 and PIK3R3, encoding p85α (and its splicing variants p55α and p50 α), p85β and p55γ regulatory subunits, respectively, collectively called p85. These regulatory subunits share three core domains including a p110-binding domain (denoted as inter-SH2 or iSH2) flanked by two Src-homology 2 (SH2) domains (N-terminal nSH2 and C-terminal cSH2). The two longer isoforms, p85α and p85β, have a Src-homology 3 (SH3) domain and a BCR homology (BH) domain located in their extended N-terminal regions. In the basal state, p85 binds to the N-terminus of the p110 subunit via its iSH2 domain, inhibiting its catalytic activity^, . Class IB PI3K is a heterodimer composed of a catalytic subunit p110γ and a regulatory subunit p101. p110γ is mainly expressed in leukocytes and can be activated directly by GPCRs. Class II PI3Ks are monomers with only a single catalytic subunit.There are three class II PI3K isoforms, PI3KC2α, PI3KC2β and PI3KC2γ, each of which has a divergent N-terminus followed by a Ras binding domain (RasBD), C2 domain, helical domain, and catalytic domain with PX and C2 domains at the C-termini (reviewed in REF^, ). Class III PI3Ks consists of a single catalytic subunit Vps34 (homolog of the yeast vacuolar protein-sorting defective 34). Figure 2b. The level of phosphatidylinositol-3,4,5-triphosphate, PI(3,4,5)P₃, is regulated by Class I phosphoinositide 3-kinase (PI3K) and phosphatase and tensin homologue (PTEN). PI(3,4,5)P₃ is an important lipid second messenger that regulates multiple cellular processes. Class I PI3Ks phosphorylates the inositol ring of phosphatidylinositol-4,5-triphosphate, PI(4,5)P₂ on the 3-position, to generate PI(3,4,5)P₃. PTEN is a lipid phosphatase that removes phosphate on the 3-position of PI(3,4,5)P₃ and converts it back to PI(4,5)P₂.

Figure 2

Figure 2a. The members of the phosphoinositide 3-kinase (PI3K) family. PI3Ks have been divided into three classes according to their structural characteristics and substrate specificity. Class IA PI3Ks are heterodimers consisting of a p110 catalytic subunit and a p85 regulatory subunit. In mammals, there are three genes, PIK3CA, PIK3CB and PIK3CD, encoding p110 catalytic isoforms: p110α, p110β and p110δ, respectively. While the expression of p110δ is largely restricted to the immune system, p110α and p110β are ubiquitously expressed^, . The p110 catalytic isoforms are highly homologous and share five distinct domains: an N-terminal p85-binding domain (p85BD) that interacts with the p85 regulatory subunit, a Ras-binding domain (RasBD) that mediates activation by members of the Ras family of small GTPases, a putative membrane-binding domain C2, the helical domain, and the C-terminal kinase catalytic domain. There are also three genes, PIK3R1, PIK3R2 and PIK3R3, encoding p85α (and its splicing variants p55α and p50 α), p85β and p55γ regulatory subunits, respectively, collectively called p85. These regulatory subunits share three core domains including a p110-binding domain (denoted as inter-SH2 or iSH2) flanked by two Src-homology 2 (SH2) domains (N-terminal nSH2 and C-terminal cSH2). The two longer isoforms, p85α and p85β, have a Src-homology 3 (SH3) domain and a BCR homology (BH) domain located in their extended N-terminal regions. In the basal state, p85 binds to the N-terminus of the p110 subunit via its iSH2 domain, inhibiting its catalytic activity^, . Class IB PI3K is a heterodimer composed of a catalytic subunit p110γ and a regulatory subunit p101. p110γ is mainly expressed in leukocytes and can be activated directly by GPCRs. Class II PI3Ks are monomers with only a single catalytic subunit.There are three class II PI3K isoforms, PI3KC2α, PI3KC2β and PI3KC2γ, each of which has a divergent N-terminus followed by a Ras binding domain (RasBD), C2 domain, helical domain, and catalytic domain with PX and C2 domains at the C-termini (reviewed in REF^, ). Class III PI3Ks consists of a single catalytic subunit Vps34 (homolog of the yeast vacuolar protein-sorting defective 34). Figure 2b. The level of phosphatidylinositol-3,4,5-triphosphate, PI(3,4,5)P₃, is regulated by Class I phosphoinositide 3-kinase (PI3K) and phosphatase and tensin homologue (PTEN). PI(3,4,5)P₃ is an important lipid second messenger that regulates multiple cellular processes. Class I PI3Ks phosphorylates the inositol ring of phosphatidylinositol-4,5-triphosphate, PI(4,5)P₂ on the 3-position, to generate PI(3,4,5)P₃. PTEN is a lipid phosphatase that removes phosphate on the 3-position of PI(3,4,5)P₃ and converts it back to PI(4,5)P₂.

Figure 3

Figure 3a. Targeting the phosphoinositide 3-kinase (PI3K) pathway in cancer. Inhibitors targeting major nodes of the PI3K signaling pathway, including RTKs, PI3K, AKT and mTOR, have reached clinical trials. Dual inhibitors targeting both PI3K and RTK or PI3K and mTOR (as shown in connected solid and dashed lines) may provide more potent therapeutic effects in suppressing the PI3K signaling. Combinations of PI3K and Raf/MAPK inhibitors may achieve more effective clinical results. mTOR, mammalian target of rapamycin; RTK, receptor tyrosine kinase; EGFR, epidermal growth factor receptor; Erbb2, Epidermal growth factor Receptor 2; c-Met, mesenchymal-epithelial transition factor Ras, a small GTPase protein, Raf, a serine/threonine kinase; MEK, mitogen-activated protein kinase kinase; ERK, extracellular signal regulated kinase. Figure 3b. Inhibitors in clinical development that target the PI3K or related pathways. EGFR, epidermal growth factor receptor; Erbb2, Human Epidermal growth factor Receptor 2; VEGFR, vascular endothelial growth factor receptor; RTK, receptor tyrosine kinase; mTOR, mammalian target of rapamycin. *Bevacizumab targets VEGF-A instead of VEGFR directly. **Both AZD8055 and OSI027 are ATP-competitive mTOR inhibitors that targets both mTORC1 and mTORC2.

Figure 3

Figure 3a. Targeting the phosphoinositide 3-kinase (PI3K) pathway in cancer. Inhibitors targeting major nodes of the PI3K signaling pathway, including RTKs, PI3K, AKT and mTOR, have reached clinical trials. Dual inhibitors targeting both PI3K and RTK or PI3K and mTOR (as shown in connected solid and dashed lines) may provide more potent therapeutic effects in suppressing the PI3K signaling. Combinations of PI3K and Raf/MAPK inhibitors may achieve more effective clinical results. mTOR, mammalian target of rapamycin; RTK, receptor tyrosine kinase; EGFR, epidermal growth factor receptor; Erbb2, Epidermal growth factor Receptor 2; c-Met, mesenchymal-epithelial transition factor Ras, a small GTPase protein, Raf, a serine/threonine kinase; MEK, mitogen-activated protein kinase kinase; ERK, extracellular signal regulated kinase. Figure 3b. Inhibitors in clinical development that target the PI3K or related pathways. EGFR, epidermal growth factor receptor; Erbb2, Human Epidermal growth factor Receptor 2; VEGFR, vascular endothelial growth factor receptor; RTK, receptor tyrosine kinase; mTOR, mammalian target of rapamycin. *Bevacizumab targets VEGF-A instead of VEGFR directly. **Both AZD8055 and OSI027 are ATP-competitive mTOR inhibitors that targets both mTORC1 and mTORC2.

Figure 4

Figure 4a. Differential functions of p110α and p110β isoforms. p110α is the major effector downstream of RTKs and p110β is an effector for both RTKs and GPCRs. Their differential roles in many biological functions are indicated. Many of these roles are associated with cancer (solid lines). An association between chronic inflammation and cancer has been indicated (dashed line). RTK, receptor tyrosine kinase; GPCR, G-protein coupled receptor. Figure 4b. Schematic of PI3K isoform-selective inhibition in the treatment of cancers featuring specific oncogenic lesions. Recent studies point to p110β as a primary target for PTEN-deficient cancers. However, in the case oncogenic lesions such as RTK amplification or mutation, Ras mutation, or activating PIK3CA mutations, the PI3K signaling largely depends on p110α, perhaps even in the absence of PTEN. RTK, receptor tyrosine kinase; GPCR, G-protein coupled receptor; PTEN, phosphatase and tensin homologue. PI(3,4,5)P₃, phosphatidylinositol-3,4,5-triphosphate; PI(4,5)P₂, phosphatidylinositol-4,5-bisphosphate.

Figure 4

Figure 4a. Differential functions of p110α and p110β isoforms. p110α is the major effector downstream of RTKs and p110β is an effector for both RTKs and GPCRs. Their differential roles in many biological functions are indicated. Many of these roles are associated with cancer (solid lines). An association between chronic inflammation and cancer has been indicated (dashed line). RTK, receptor tyrosine kinase; GPCR, G-protein coupled receptor. Figure 4b. Schematic of PI3K isoform-selective inhibition in the treatment of cancers featuring specific oncogenic lesions. Recent studies point to p110β as a primary target for PTEN-deficient cancers. However, in the case oncogenic lesions such as RTK amplification or mutation, Ras mutation, or activating PIK3CA mutations, the PI3K signaling largely depends on p110α, perhaps even in the absence of PTEN. RTK, receptor tyrosine kinase; GPCR, G-protein coupled receptor; PTEN, phosphatase and tensin homologue. PI(3,4,5)P₃, phosphatidylinositol-3,4,5-triphosphate; PI(4,5)P₂, phosphatidylinositol-4,5-bisphosphate.