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Class IA Phosphatidylinositol 3-Kinase Signaling in Non-small Cell Lung Cancer

  • Benjamin Solomon
    Correspondence
    Benjamin Solomon, MBBS, PhD, FRACP, Department of Haematology and Medical Oncology, Peter MacCallum Cancer Centre, Locked Bag 1, A'Beckett Street, Victoria 8006, Australia
    Affiliations
    Department of Haematology and Medical Oncology, St. Andrews Place, East Melbourne

    Research Division, Peter MacCallum Cancer Centre, St. Andrews Place, East Melbourne
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  • Richard B. Pearson
    Affiliations
    Research Division, Peter MacCallum Cancer Centre, St. Andrews Place, East Melbourne

    Department of Biochemistry and Molecular Biology, University of Melbourne, Parkville, Victoria, Australia
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      Keywords

      Class IA phosphatidylinositol-3 kinases (PI3Ks) together with AKT and mammalian target of rapamycin (mTOR) comprise the central axis of a complex, interconnected signaling network that integrates signals from growth factors, insulin, nutrients and oxygen to play a critical role in controlling cell growth, proliferation, metabolism, survival, and tumor angiogenesis. De-regulation of these processes is a required hallmark of cancer
      • Hanahan D
      • Weinberg RA
      The hallmarks of cancer.
      and aberrant activation of the class IA PI3K signaling occurs frequently in many malignancies including non-small cell lung cancer (NSCLC).

      The PI3K Family and Downstream Signaling

      PI3Ks are lipid kinases that phosphorylate the 3′-hydoxyl group in phosphatidylinositol and phosphoinositides.
      • Engelman JA
      • Luo J
      • Cantley LC
      The evolution of phosphatidylinositol 3-kinases as regulators of growth and metabolism.
      They are grouped into three classes based on structure and substrate specificity: Class I PI3Ks convert phosphatidylinositol-4,5-bisphosphate (PtdIns(4,5)P2) to phosphatidylinositol-3,4,5-trisphosphate (PtdIns(3,4,5)P3). Class I PI3Ks are subdivided into class IA PI3Ks which are predominantly activated by growth factor receptor tyrosine kinases and class IB PI3Ks which are activated by G protein coupled receptors. Many studies have established a link between aberrant activation of class IA PI3Ks and oncogenesis.
      • Samuels Y
      • Diaz Jr, LA
      • Schmidt-Kittler O
      • et al.
      Mutant PIK3CA promotes cell growth and invasion of human cancer cells.
      • Kang S
      • Bader AG
      • Vogt PK
      Phosphatidylinositol 3-kinase mutations identified in human cancer are oncogenic.
      • Isakoff SJ
      • Engelman JA
      • Irie HY
      • et al.
      Breast cancer-associated PIK3CA mutations are oncogenic in mammary epithelial cells.
      • Bader AG
      • Kang S
      • Vogt PK
      Cancer-specific mutations in PIK3CA are oncogenic in vivo.
      • Zhao JJ
      • Liu Z
      • Wang L
      • Shin E
      • Loda MF
      • Roberts TM
      The oncogenic properties of mutant p110alpha and p110beta phosphatidylinositol 3-kinases in human mammary epithelial cells.
      Class II PI3Ks catalyze the production of PtdIns(3)P and PtdIns(3,4)P2 and are important for clathrin mediated membrane trafficking.
      • Gaidarov I
      • Smith ME
      • Domin J
      • Keen JH
      The class II phosphoinositide 3-kinase C2alpha is activated by clathrin and regulates clathrin-mediated membrane trafficking.
      Class III consists of a single member, hVPS34 which produces PtdIns(3)P and is involved in the regulation of vesicle trafficking, activation of mTOR by amino acids and autophagy.
      • Nobukuni T
      • Kozma SC
      • Thomas G
      Hvps34, an ancient player, enters a growing game: mTOR Complex1/S6K1 signaling.
      • Gulati P
      • Gaspers LD
      • Dann SG
      • et al.
      Amino acids activate mTOR complex 1 via Ca2+/CaM signaling to hVps34.

      Class IA PI3K, Subunits and Activation

      Class IA PI3Ks are heterodimeric proteins that consist of a p110 catalytic subunit and a p85 regulatory subunit. There are three isoforms of the p110 catalytic subunit (p110α, p110β, and p110Δ) encoded by three genes PIK3CA, PIK3CB, and PIK3CD. p110α and p110β are ubiquitously expressed in normal and tumor tissue, while expression of p110Δ is restricted to leukocytes. p110α is important for growth factor and insulin signaling as well as for vascular development and integrity.
      • Foukas LC
      • Claret M
      • Pearce W
      • et al.
      Critical role for the p110[alpha] phosphoinositide-3-OH kinase in growth and metabolic regulation.
      • Knight ZA
      • Gonzalez B
      • Feldman ME
      • et al.
      A Pharmacological Map of the PI3-K Family Defines a Role for p110[alpha] in Insulin Signaling.
      • Yuan TL
      • Choi HS
      • Matsui A
      • et al.
      Class 1A PI3K regulates vessel integrity during development and tumorigenesis.
      To date, only mutations in PIK3CA have been commonly described in cancer.
      • Samuels Y
      • Wang Z
      • Bardelli A
      • et al.
      High Frequency of Mutations of the PIK3CA Gene in Human Cancers.
      • Campbell IG
      • Russell SE
      • Choong DYH
      • et al.
      Mutation of the PIK3CA Gene in Ovarian and Breast Cancer.
      p110β (which may also be activated by G-protein coupled receptors) is important for glucose metabolism, cell proliferation and drives oncogenic transformation in the context of PTEN loss.
      • Guillermet-Guibert J
      • Bjorklof K
      • Salpekar A
      • et al.
      The p110beta isoform of phosphoinositide 3-kinase signals downstream of G protein-coupled receptors and is functionally redundant with p110gamma.
      • Jia S
      • Liu Z
      • Zhang S
      • et al.
      Essential roles of PI(3)K-p110[bgr] in cell growth, metabolism and tumorigenesis.
      The p85 regulatory subunit (present in several isoforms: p85α, p85β, p55α, p55γ, and p50α) is crucial for mediating the activation of class IA PI3Ks by receptor tyrosine kinases. It binds phosphorylated receptor tyrosine kinases directly or via adapter proteins (such as IRS-1 or Gab1) recruiting PI3K to its site of action at the cell membrane and allowing activation of the p110 catalytic subunit and rapid synthesis of PtsIns(3,4,5)P3. Activating mutations in the gene encoding p85, PIK3R1, have been reported in colon cancer and glioblastoma.
      • Philp AJ
      • Campbell IG
      • Leet C
      • et al.
      The Phosphatidylinositol 3′-kinase p85{alpha} Gene is an oncogene in human ovarian and colon tumors.
      • Parsons DW
      • Jones S
      • Zhang X
      • et al.
      An integrated genomic analysis of human glioblastoma multiforme.
      Comprehensive genomic characterization defines human glioblastoma genes and core pathways.
      Class IA PI3Ks bind and may be directly activated by RAS.
      • Rodriguez-Viciana P
      • Warne PH
      • Dhand R
      • et al.
      Phosphatidylinositol-3-OH kinase as a direct target of Ras.
      • Gupta S
      • Ramjaun AR
      • Haiko P
      • et al.
      Binding of ras to phosphoinositide 3-kinase p110alpha is required for ras-driven tumorigenesis in mice.

      Inactivation of PI3K IA Signaling

      The reaction catalyzed by class IA PI3Ks is directly antagonized by the lipid phosphatase PTEN which dephosphorylates the 3′ position of PtdIns(3,4,5)P3 to produce PtdInsI(4,5)P2 or by the Src-homology-2 containing phosphatases SHIP1 and SHIP2 which dephosphorylate PtdIns(3,4,5)P3 to produce PtdIns(3,4)P2.
      • Keniry M
      • Parsons R
      The role of PTEN signaling perturbations in cancer and in targeted therapy.

      The PI3K/AKT/mTOR Pathway

      Generation of PtdIns(3,4,5)P3 within the cell membrane by class IA PI3Ks initiates a signaling cascade (Figure 1) that activates AKT and mTOR (the latter of which is present in two multiprotein complexes mTORC1 and mTORC2, described below). PtdIns(3,4,5)P3 binds the plekstrin homology domain of the AKT (a family of highly homologous serine/thronine kinases, AKT1, AKT2, and AKT3) localizing AKT to the cell membrane. AKT1 is then phosphorylated at Thr308 (Thr309 in AKT2 and Thr305 in AKT3) in the catalytic domain by PDK1, another plekstrin homology domain containing colocalized kinase
      • Bellacosa A
      • Chan TO
      • Ahmed NN
      • et al.
      Akt activation by growth factors is a multiple-step process: the role of the PH domain.
      and at Ser473 in the C-terminal hydrophobic motif (or Ser474 in AKT2 and Ser472 in AKT3) by the mTORC2 complex.
      • Sarbassov DD
      • Guertin DA
      • Ali SM
      • Sabatini DM
      Phosphorylation and regulation of Akt/PKB by the rictor-mTOR complex.
      Activated AKT is released from the plasma membrane and phosphorylates multiple nuclear and cytoplasmic targets (over 100 putative AKT substrates have been reported [www.phosphosite.org]) resulting in pleiotopic effects on cellular homeostasis (Figure 1 and reviewed by Manning and Cantley
      • Manning BD
      • Cantley LC
      AKT/PKB signaling: navigating downstream.
      ). The phosphatase PHLPP inactivates AKT by dephosphorylating AKT at Ser473.
      • Gao T
      • Furnari F
      • Newton AC
      PHLPP: a phosphatase that directly dephosphorylates Akt, promotes apoptosis, and suppresses tumor growth.
      Figure thumbnail gr1
      FIGURE 1Signaling through the class IA phosphatidylinisitol-3 kinases/AKT/mammalian target of rapamycin (PI3K/AKT/mTOR) axis integrates extracellular signals from insulin and growth factors together with energy status, oxygenation and nutrient availability to modulate processes including cell growth, survival, proliferation, glucose metabolism, and angiogenesis. The topology of this signaling network, genetic alterations of components of the PI3K pathway in non-small cell lung cancer (NSCLC) and presence of feedback loops (indicated by the broken red arrows) are described in the text.
      The effects of AKT on cell growth and metabolism are mediated largely by activation of the mTORC1 complex which acts as a master regulator of protein synthesis in response to multiple inputs.
      • Guertin DA
      • Sabatini DM
      Defining the role of mTOR in cancer.
      • Sun SY
      • Fu H
      • Khuri FR
      Targeting mTOR signaling for lung cancer therapy.
      mTORC1 contains mTOR complexed with Raptor, PRAS40, and mLST8 and is sensitive to inhibition by rapamycin and its analogues including RAD001 and CCI-779.
      • Guertin DA
      • Sabatini DM
      Defining the role of mTOR in cancer.
      mTOR also exists in a distinct complex termed mTORC2 where it is combined with Rictor, mSIN1, Protor and mLST8.
      • Guertin DA
      • Sabatini DM
      Defining the role of mTOR in cancer.
      • Pearce LR
      • Huang X
      • Boudeau J
      • et al.
      Identification of Protor as a novel Rictor-binding component of mTOR complex-2.
      This complex is resistant to rapamycin, lies upstream of AKT
      • Sarbassov DD
      • Guertin DA
      • Ali SM
      • Sabatini DM
      Phosphorylation and regulation of Akt/PKB by the rictor-mTOR complex.
      and has other less well-characterized effects including regulation of actin organization.
      • Wullschleger S
      • Loewith R
      • Hall MN
      TOR signaling in growth and metabolism.
      Activation of mTORC1 by AKT involves phosphorylation of TSC2 (which exists in a complex with TSC1) leading to accumulation of the GTP bound form of the GTPase RheB which activates mTORC1. AKT also phosphorylates and inactivates PRAS40, an inhibitory component of the mTORC1 complex.
      • Haar EV
      • Lee SI
      • Bandhakavi S
      • Griffin TJ
      • Kim DH
      Insulin signalling to mTOR mediated by the Akt/PKB substrate PRAS40.
      mTOR may be modulated by energy status via LKB1/STK11 and AMPK,
      • Shaw RJ
      • Bardeesy N
      • Manning BD
      • et al.
      The LKB1 tumor suppressor negatively regulates mTOR signaling.
      hypoxia through REDD1 signaling
      • Sofer A
      • Lei K
      • Johannessen CM
      • Ellisen LW
      Regulation of mTOR and cell growth in response to energy stress by REDD1.
      and nutrient availability by hVPS34
      • Gulati P
      • Gaspers LD
      • Dann SG
      • et al.
      Amino acids activate mTOR complex 1 via Ca2+/CaM signaling to hVps34.
      (Figure 1). mTORC1 acts on substrates including p70 S6 kinase and eIF4E-BP1 to influence translation of mRNA encoding key proteins such as cyclin D1, MYC, and HIF-1α. It also stimulates ribosome biogenesis and inhibits autophagy.
      • Hannan KM
      • Brandenburger Y
      • Jenkins A
      • et al.
      mTOR-dependent regulation of ribosomal gene transcription requires S6K1 and is mediated by phosphorylation of the carboxy-terminal activation domain of the nucleolar transcription factor UBF.
      • Easton JB
      • Houghton PJ
      mTOR and cancer therapy.

      Role of Cross-Talk and Feedback

      The class IA PI3K/AKT/mTOR axis is a critical hub for cross talk with co-operating pathways and negative feedback loops within a complex and interconnected signaling environment. It is intimately linked with the RAS/RAF/MAPK pathway at its apex where RAS binds to and directly activates class IA PI3K
      • Rodriguez-Viciana P
      • Warne PH
      • Dhand R
      • et al.
      Phosphatidylinositol-3-OH kinase as a direct target of Ras.
      and through interactions between many downstream components of each pathway.
      • Carracedo A
      • Pandolfi PP
      The PTEN-PI3K pathway: of feedbacks and cross-talks.
      Indeed, the p110α isoform is critical for RAS driven tumorigenesis and inhibition of both PI3K and MEK seems critical for targeting of KRAS driven lung tumors.
      • Gupta S
      • Ramjaun AR
      • Haiko P
      • et al.
      Binding of ras to phosphoinositide 3-kinase p110alpha is required for ras-driven tumorigenesis in mice.
      • Engelman JA
      • Chen L
      • Tan X
      • et al.
      Effective use of PI3K and MEK inhibitors to treat mutant Kras G12D and PIK3CA H1047R murine lung cancers.
      There is also significant cross talk with other pathways including the JNK and LKB1/AMPK pathways.
      • Vivanco I
      • Palaskas N
      • Tran C
      • et al.
      Identification of the JNK signaling pathway as a functional target of the tumor suppressor PTEN.
      • Hezel AF
      • Bardeesy N
      LKB1; linking cell structure and tumor suppression.
      Adding further complexity is an extensive and context dependent network of feedback loops (reviewed in Carracedo et al.
      • Carracedo A
      • Ma L
      • Teruya-Feldstein J
      • et al.
      Inhibition of mTORC1 leads to MAPK pathway activation through a PI3K-dependent feedback loop in human cancer.
      ) that act as rheostats to maintain exquisite control of the pathway in normal cells.

      Genetic Alterations of the PI3K Pathway in NSCLC

      Phosphorylation of AKT is evident in 50 to 70% of NSCLCs indicating that activation of class IA PI3K/AKT signaling is a frequent event in this malignancy.
      • Balsara BR
      • Pei J
      • Mitsuuchi Y
      • et al.
      Frequent activation of AKT in non-small cell lung carcinomas and preneoplastic bronchial lesions.
      • Cappuzzo F
      • Ligorio C
      • Janne PA
      • et al.
      Prospective study of gefitinib in epidermal growth factor receptor fluorescence in situ hybridization-positive/phospho-Akt-positive or never smoker patients with advanced non-small-cell lung cancer: the ONCOBELL trial.
      • Tsurutani J
      • Fukuoka J
      • Tsurutani H
      • et al.
      Evaluation of two phosphorylation sites improves the prognostic significance of Akt activation in non-small-cell lung cancer tumors.
      Constitutive PI3K activation may occur as a consequence of genetic changes in upstream signaling components (e.g., EGFR mutations or copy number gain and KRAS mutations), mutations or amplification of PIK3CA, PTEN loss or activation of downstream elements of the pathway (summarized in Table 1).
      TABLE 1Genetic Alterations of the PI3K Pathway in Non-small Cell Lung Cancer
      Genetic Change in PI3K Pathway ComponentFrequency in NSCLCReference
      EGFR mutation26% (in about 10% of Caucasian patients; 20–40% of Asian patients)80–82
      Increased EGFR copy number30–40%81, 83
      KRAS mutation18% (20–30% of adenocarcinomas)80, 82
      MET mutation2%84, 85
      MET amplification2–7% (nb. higher frequency in patients with acquired resistance to EGFR TKIs)84, 86–88
      EML4-ALK rearrangements2–8%89–93
      PIK3CA mutation3% of NSCLC14, 46–49
      PIK3CA amplification15–20% (7% in adenocarcinomas; 35% in SCC)48–51
      PTEN lossPromoter hypermethylation in 26–35% of NSCLC55, 56
      AKT1 mutation (E17K (c.49G>A))1% (4/363)58–60
      LKB1 mutation11% of NSCLC61–66
      Genetic changes in components of the PI3K pathway in NSCLC. Figures in bold are from the catalogue of somatic mutations in cancer (http://www.sanger.ac.uk/genetics/CGP/cosmic/).
      NSCLC, non-small cell lung cancer; PI3K, phosphatidylinisitol-3 kinases; TKI, tyrosine kinase inhibitor, EGFR, epidermal growth factor receptor.
      Although activating mutations in PIK3CA have been described in many human cancers
      • Zhao L
      • Vogt PK
      Class I PI3K in oncogenic cellular transformation.
      and occur at high frequency in breast (27%), colorectal (15%), and endometrial cancer (24%),
      • Samuels Y
      • Wang Z
      • Bardelli A
      • et al.
      High Frequency of Mutations of the PIK3CA Gene in Human Cancers.
      • Campbell IG
      • Russell SE
      • Choong DYH
      • et al.
      Mutation of the PIK3CA Gene in Ovarian and Breast Cancer.
      PIK3CA mutations occur in only about 3% of NSCLC.
      • Samuels Y
      • Wang Z
      • Bardelli A
      • et al.
      High Frequency of Mutations of the PIK3CA Gene in Human Cancers.
      • Endoh H
      • Yatabe Y
      • Kosaka T
      • Kuwano H
      • Mitsudomi T
      PTEN and PIK3CA expression is associated with prolonged survival after gefitinib treatment in EGFR-mutated lung cancer patients.
      • Kawano O
      • Sasaki H
      • Endo K
      • et al.
      PIK3CA mutation status in Japanese lung cancer patients.
      • Okudela K
      • Suzuki M
      • Kageyama S
      • et al.
      PIK3CA mutation and amplification in human lung cancer.
      • Yamamoto H
      • Shigematsu H
      • Nomura M
      • et al.
      PIK3CA mutations and copy number gains in human lung cancers.
      However, increased copy number of the PIK3CA gene is a more frequent event in NSCLC being observed in 33% (46 of 139) of squamous cell carcinomas
      • Yamamoto H
      • Shigematsu H
      • Nomura M
      • et al.
      PIK3CA mutations and copy number gains in human lung cancers.
      and 5.9% of adenocarcinomas (12 of 195)
      • Yamamoto H
      • Shigematsu H
      • Nomura M
      • et al.
      PIK3CA mutations and copy number gains in human lung cancers.
      with high-level copy number gains (>fivefold) seen exclusively in squamous cell carcinomas in multiple studies.
      • Okudela K
      • Suzuki M
      • Kageyama S
      • et al.
      PIK3CA mutation and amplification in human lung cancer.
      • Yamamoto H
      • Shigematsu H
      • Nomura M
      • et al.
      PIK3CA mutations and copy number gains in human lung cancers.
      • Kawano O
      • Sasaki H
      • Okuda K
      • et al.
      PIK3CA gene amplification in Japanese non-small cell lung cancer.
      • Angulo B
      • Suarez-Gauthier A
      • Lopez-Rios F
      • et al.
      Expression signatures in lung cancer reveal a profile for EGFR-mutant tumours and identify selective PIK3CA overexpression by gene amplification.
      Inactivating mutations or deletions of PTEN are uncommon in NSCLC (<5%).
      • Forgacs E
      • Biesterveld EJ
      • Sekido Y
      • et al.
      Mutation analysis of the PTEN/MMAC1 gene in lung cancer.
      • Kohno T
      • Takahashi M
      • Manda R
      • Yokota J
      Inactivation of the PTEN/MMAC1/TEP1 gene in human lung cancers.
      • Yokomizo A
      • Tindall DJ
      • Drabkin H
      • et al.
      PTEN/MMAC1 mutations identified in small cell, but not in non-small cell lung cancers.
      However, reduced or absent PTEN protein expression is frequent
      • Marsit CJ
      • Zheng S
      • Aldape K
      • et al.
      PTEN expression in non-small-cell lung cancer: evaluating its relation to tumor characteristics, allelic loss, and epigenetic alteration.
      and may be explained by promoter hypermethylation which is reported to occur in 25 to 40% of cases.
      • Marsit CJ
      • Zheng S
      • Aldape K
      • et al.
      PTEN expression in non-small-cell lung cancer: evaluating its relation to tumor characteristics, allelic loss, and epigenetic alteration.
      • Soria J-C
      • Lee H-Y
      • Lee JI
      • et al.
      Lack of PTEN expression in non-small cell lung cancer could be related to promoter methylation.
      Mutations in two downstream components of the class IA PI3K/AKT/mTOR pathway have been described in NSCLC. An uncommon activating mutation in the pleckstrin homology domain of AKT1 (E17K), initially identified in breast, ovarian, and colorectal cancers,
      • Carpten JD
      • Faber AL
      • Horn C
      • et al.
      A transforming mutation in the pleckstrin homology domain of AKT1 in cancer.
      has been described in about 1% of NSCLC (with reported cases demonstrating squamous histology).
      • Do H
      • Solomon B
      • Mitchell PL
      • Fox SB
      • Dobrovic A
      Detection of the transforming AKT1 mutation E17K in non-small cell lung cancer by high resolution melting.
      • Malanga D
      • Scrima M
      • De Marco C
      • et al.
      Activating E17K mutation in the gene encoding the protein kinase AKT1 in a subset of squamous cell carcinoma of the lung.
      • Bleeker FE
      • Felicioni L
      • Buttitta F
      • et al.
      AKT1(E17K) in human solid tumours.
      More common inactivating mutations in LKB1/STK11 are seen in approximately 11% of NSCLC. These mutations are more frequent in tumors from Caucasians than Asians, adenocarcinomas and large cell carcinomas than squamous cell carcinomas and are associated with smoking history and with KRAS mutations.
      • Sanchez-Cespedes M
      • Parrella P
      • Esteller M
      • et al.
      Inactivation of LKB1/STK11 is a common event in adenocarcinomas of the lung.
      • Carretero J
      • Medina PP
      • Pio R
      • Montuenga LM
      • Sanchez-Cespedes M
      Novel and natural knockout lung cancer cell lines for the LKB1/STK11 tumor suppressor gene.
      • Matsumoto S
      • Iwakawa R
      • Takahashi K
      • et al.
      Prevalence and specificity of LKB1 genetic alterations in lung cancers.
      • Koivunen JP
      • Kim J
      • Lee J
      • et al.
      Mutations in the LKB1 tumour suppressor are frequently detected in tumours from Caucasian but not Asian lung cancer patients.
      • Zhong D
      • Guo L
      • de Aguirre I
      • et al.
      LKB1 mutation in large cell carcinoma of the lung.
      • Onozato R
      • Kosaka T
      • Achiwa H
      • et al.
      LKB1 gene mutations in Japanese lung cancer patients.
      Loss of LKB1 has been shown to potently synergize with KRAS mutations in lung tumorigenesis.
      • Ji H
      • Ramsey MR
      • Hayes DN
      • et al.
      LKB1 modulates lung cancer differentiation and metastasis.

      Challenges in Targeting PI3K Signaling in Lung Cancer

      The first generation of PI3K inhibitors, wortmannin and LY2994002, had preclinical antitumor activity, but suffered from poor specificity,
      • Izzard RA
      • Jackson SP
      • Smith GCM
      Competitive and Noncompetitive inhibition of the DNA-dependent protein kinase.
      • Gharbi SI
      • Zvelebil MJ
      • Shuttleworth SJ
      • et al.
      Exploring the specificity of the PI3K family inhibitor LY294002.
      poor pharmacological properties and toxicities that precluded their clinical use. Novel PI3K inhibitors have been developed
      • Garcia-Echeverria C
      • Sellers WR
      Drug discovery approaches targeting the PI3K/Akt pathway in cancer.
      • Ihle NT
      • Powis G
      Take your PIK: phosphatidylinositol 3-kinase inhibitors race through the clinic and toward cancer therapy.
      and join mTOR inhibitors in the armamentarium of agents to target the PI3K pathway. Clinical trials with SF-1126 (a prodrug of LY294002); PX-866 (a pegylated wortmannin derivative); XL147, a selective PI3K inhibitor; as well as BEZ235, BGT226, and XL765 (dual inhibitors of PI3K and mTOR) have been initiated ( www.clinicaltrials.gov ). Of significance, the presence of feedback and crosstalk within the circuitry of class IA PI3K/AKT/mTOR signaling imply that maximal therapeutic effect is likely require pathway inhibition at multiple levels and/or inhibition of multiple pathways simultaneously. For example, inhibition of mTORC1 by rapamycin analogues leads to increased AKT activation demonstrable in some tumors
      • O'Reilly KE
      • Rojo F
      • She QB
      • et al.
      mTOR inhibition induces upstream receptor tyrosine kinase signaling and activates Akt.
      • Tabernero J
      • Rojo F
      • Calvo E
      • et al.
      Dose- and schedule-dependent inhibition of the mammalian target of rapamycin pathway with everolimus: a phase I tumor pharmacodynamic study in patients with advanced solid tumors.
      • Cloughesy TF
      • Yoshimoto K
      • Nghiemphu P
      • et al.
      Antitumor activity of rapamycin in a Phase I trial for patients with recurrent PTEN-deficient glioblastoma.
      due to a feedback loop where S6K1 phosphorylates and transcriptionally represses IRS-1.
      • Harrington LS
      • Findlay GM
      • Gray A
      • et al.
      The TSC1–2 tumor suppressor controls insulin-PI3K signaling via regulation of IRS proteins.
      Inhibition of mTORC1 may also lead to activation of the ERK-MAPK pathway via a feedback loop involving S6K-IRS-1-PI3K-RAS.
      • Carracedo A
      • Ma L
      • Teruya-Feldstein J
      • et al.
      Inhibition of mTORC1 leads to MAPK pathway activation through a PI3K-dependent feedback loop in human cancer.
      Indeed agents that inhibit both PI3K and mTOR
      • Fan QW
      • Knight ZA
      • Goldenberg DD
      • et al.
      A dual PI3 kinase/mTOR inhibitor reveals emergent efficacy in glioma.
      • Serra V
      • Markman B
      • Scaltriti M
      • et al.
      NVP-BEZ235, a dual PI3K/mTOR inhibitor, prevents PI3K signaling and inhibits the growth of cancer cells with activating PI3K mutations.
      and combinations of P13K inhibitors with MEK inhibitors
      • Engelman JA
      • Chen L
      • Tan X
      • et al.
      Effective use of PI3K and MEK inhibitors to treat mutant Kras G12D and PIK3CA H1047R murine lung cancers.
      • Legrier ME
      • Yang CP
      • Yan HG
      • et al.
      Targeting protein translation in human non small cell lung cancer via combined MEK and mammalian target of rapamycin suppression.
      • Kinkade CW
      • Castillo-Martin M
      • Puzio-Kuter A
      • et al.
      Targeting AKT/mTOR and ERK MAPK signaling inhibits hormone-refractory prostate cancer in a preclinical mouse model.
      have proved to be promising therapeutic strategies in preclinical models. Appreciation of the intricacies of this complex and interconnected signaling network is required to develop clinical strategies that effectively target class IA PI3K signaling in NSCLC.

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