PI3K/AKT Signaling in Cancer

Class IA PI3K is a heterodimer of a p85 regulatory subunit (encoded by PIK3R1, PIK3R2 or PIK3R3) and a p110 catalytic subunit (encoded by PIK3CA, PIK3CB or PIK3CD). In the absence of activating signals, the regulatory subunit stabilizes the catalytic subunit while inhibiting its activity. The complex becomes activated when extracellular signals stimulate the phosphorylation of the cytoplasmic domains of transmembrane receptors or receptor-associated proteins. The p85 regulatory subunit binds phosphorylated motifs of activator proteins, which induces a conformational change that relieves p85-mediated inhibition of the p110 catalytic subunit and enables PI3K to phosphorylate PIP2 to form PIP3. The phosphoinositide kinase activity of PI3K is opposed by the phosphoinositide phosphatase activity of PTEN. <br><br>PIP3 acts as a messenger that recruits PDPK1 (PDK1) and AKT (AKT1, AKT2 or AKT3) to the plasma membrane. PDPK1 also possesses a low affinity for PIP2, so small amounts of PDPK1 are always present at the membrane. Binding of AKT to PIP3 induces a conformational change that enables TORC2 complex to phosphorylate AKT at a conserved serine residue (S473 in AKT1). Phosphorylation at the serine residue enables AKT to bind to PDPK1 and exposes a conserved threonine residue (T308) that is phosphorylated by PDPK1. AKT phosphorylated at both serine and threonine residues dissociates from the plasma membrane and acts as a serine/threonine kinase that phosphorylates a number of cytosolic and nuclear targets involved in regulation of cell metabolism, survival and gene expression. For a recent review, please refer to Manning and Cantley, 2007. <br> Signaling by PI3K/AKT is frequently constitutively activated in cancer. This activation can be via gain-of-function mutations in PI3KCA (encoding catalytic subunit p110alpha), PIK3R1 (encoding regulatory subunit p85alpha) and AKT1. The PI3K/AKT pathway can also be constitutively activated by loss-of-function mutations in tumor suppressor genes such as PTEN. <br> Gain-of-function mutations activate PI3K signaling by diverse mechanisms. Mutations affecting the helical domain of PIK3CA and mutations affecting nSH2 and iSH2 domains of PIK3R1 impair inhibitory interactions between these two subunits while preserving their association. Mutations in the catalytic domain of PIK3CA enable the kinase to achieve an active conformation. PI3K complexes with gain-of-function mutations therefore produce PIP3 and activate downstream AKT in the absence of growth factors (Huang et al. 2007, Zhao et al. 2005, Miled et al. 2007, Horn et al. 2008, Sun et al. 2010, Jaiswal et al. 2009, Zhao and Vogt 2010, Urick et al. 2011). While AKT1 gene copy number, expression level and phosphorylation are often increased in cancer, only one low frequency point mutation has been repeatedly reported in cancer and functionally studied. This mutation represents a substitution of a glutamic acid residue with lysine at position 17 of AKT1, and acts by enabling AKT1 to bind PIP2. PIP2-bound AKT1 is phosphorylated by TORC2 complex and by PDPK1 that is always present at the plasma membrane, due to low affinity for PIP2. Therefore, E17K substitution abrogates the need for PI3K in AKT1 activation (Carpten et al. 2007, Landgraf et al. 2008). <br> Loss-of-function mutations affecting the phosphatase domain of PTEN are frequently found in sporadic cancers (Kong et al. 1997, Lee et al. 1999, Han et al. 2000), as well as in PTEN hamartoma tumor syndromes (PHTS) (Marsh et al. 1998). PTEN can also be inactivated by gene deletion or epigenetic silencing, or indirectly by overexpression of microRNAs that target PTEN mRNA (Huse et al. 2009). Cells with deficient PTEN function have increased levels of PIP3, and therefore increased AKT activity. For a recent review, please refer to Hollander et al. 2011.<br> Because of their clear involvement in human cancers, PI3K and AKT are targets of considerable interest in the development of small molecule inhibitors. Although none of the currently available inhibitors display preference for mutant variants of PIK3CA or AKT, several inhibitors targeting the wild-type kinases are undergoing clinical trials. These include dual PI3K/mTOR inhibitors, class I PI3K inhibitors, pan-PI3K inhibitors, and pan-AKT inhibitors. While none have yet been approved for clinical use, these agents show promise for future therapeutics. In addition, isoform-specific PI3K and AKT inhibitors are currently being developed, and may provide more specific treatments along with reduced side-effects. For a recent review, please refer to Liu et al. 2009.

IA型PI3K（磷脂酰肌醇3激酶）由p85调控亚基（由PIK3R1、PIK3R2或PIK3R3编码）与p110催化亚基（由PIK3CA、PIK3CB或PIK3CD编码）组成的异源二聚体。在缺乏激活信号的情况下，调控亚基稳定催化亚基并抑制其活性。当细胞外信号刺激跨膜受体或受体相关蛋白的细胞质结构域磷酸化时，该复合物被激活。p85调控亚基与激活蛋白的磷酸化基序结合，诱导构象变化，从而解除p85介导的对p110催化亚基的抑制，使PI3K能够将PIP2磷酸化形成PIP3。PI3K的磷脂酰肌醇激酶活性与PTEN（磷脂酰肌醇磷酸酶）的磷脂酰肌醇磷酸酶活性相拮抗。<br><br>PIP3作为一种信使分子，招募PDPK1（PDK1）和AKT（AKT1、AKT2或AKT3）到质膜。PDPK1对PIP2也具有低亲和力，因此PDPK1以少量形式始终存在于质膜上。AKT与PIP3的结合诱导构象变化，使TORC2复合物能够在AKT1中的保守丝氨酸残基（S473）处磷酸化AKT。丝氨酸残基的磷酸化使AKT能够结合PDPK1并暴露出由PDPK1磷酸化的保守苏氨酸残基（T308）。同时丝氨酸和苏氨酸残基磷酸化的AKT从质膜上解离，作为丝氨酸/苏氨酸激酶，磷酸化多种参与细胞代谢、存活和基因表达的细胞质和核靶点。关于最近的相关综述，请参阅Manning和Cantley，2007年的文献。<br>PI3K/AKT信号通路在癌症中常常被组成型激活。这种激活可以通过PI3KCA（编码催化亚基p110alpha）、PIK3R1（编码调控亚基p85alpha）和AKT1的获得功能突变来实现。PI3K/AKT通路也可以通过肿瘤抑制基因如PTEN的失活功能突变来组成型激活。<br>获得功能突变通过多种机制激活PI3K信号。影响PIK3CA螺旋域的突变和影响PIK3R1的nSH2和iSH2域的突变损害了这两个亚基之间的抑制性相互作用，同时保持了它们的关联。PIK3CA催化域的突变使激酶能够达到活性构象。因此，具有获得功能突变的PI3K复合物在没有生长因子的情况下产生PIP3并激活下游的AKT（Huang等，2007，Zhao等，2005，Miled等，2007，Horn等，2008，Sun等，2010，Jaiswal等，2009，Zhao和Vogt，2010，Urick等，2011）。<br>尽管AKT1基因拷贝数、表达水平和磷酸化在癌症中通常增加，但只有一种低频率点突变在癌症中反复报道并被功能研究。这种突变代表了AKT1第17位上的谷氨酸残基被赖氨酸所取代，并通过使AKT1能够结合PIP2来发挥作用。由于PIP2的低亲和力，PIP2结合的AKT1被TORC2复合物和始终存在于质膜上的PDPK1磷酸化。因此，E17K替换消除了AKT1激活对PI3K的需求（Carpten等，2007，Landgraf等，2008）。<br>失活功能突变影响PTEN的磷酸酶域，在散发性癌症（Kong等，1997，Lee等，1999，Han等，2000）以及PTEN错构瘤肿瘤综合征（PHTS）（Marsh等，1998）中经常发现。PTEN还可以通过基因删除、表观遗传沉默或通过过表达靶向PTEN mRNA的microRNAs间接失活（Huse等，2009）。PTEN功能缺陷的细胞PIP3水平增加，因此AKT活性增加。关于最近的相关综述，请参阅Hollander等，2011年的文献。<br>由于PI3K和AKT在人类癌症中的明确作用，它们成为开发小分子抑制剂的焦点。尽管目前可用的抑制剂均未表现出对PIK3CA或AKT突变体的偏好，但针对野生型激酶的几种抑制剂正在临床试验中。这些包括双PI3K/mTOR抑制剂、I类PI3K抑制剂、广谱PI3K抑制剂和广谱AKT抑制剂。尽管这些抑制剂尚未获得临床批准，但它们在未来的治疗中显示出希望。此外，目前还在开发针对同种型特异性PI3K和AKT的抑制剂，它们可能提供更特异的治疗并减少副作用。关于最近的相关综述，请参阅Liu等，2009年的文献。