Leukemia (2003) 17, 590–603 & 2003 Nature Publishing Group All rights reserved 0887-6924/03 $25.00 www.nature.com/leu MOLECULAR TARGETS FOR THERAPY (MTT) Involvement of PI3K/Akt pathway in cell cycle progression, apoptosis, and neoplastic transformation: a target for cancer chemotherapy F Chang1,4, JT Lee1,4, PM Navolanic1, LS Steelman1, JG Shelton1, WL Blalock2, RA Franklin1,3 and JA McCubrey1,3 1Department of Microbiology & Immunology, Brody School of Medicine at East Carolina University, Greenville, NC, USA; 2Shands Cancer Center, University of Florida, Gainesville, FL, USA; and 3Leo Jenkins Cancer Center, Brody School of Medicine at East Carolina University, Greenville, NC, USA The PI3K/Akt signal transduction cascade has been investi- Aberrant PI3K activity and oncogenic transformation gated extensively for its roles in oncogenic transformation. Initial studies implicated both PI3K and Akt in prevention of apoptosis. However, more recent evidence has also associated The relation between dysregulated PI3K activity and cancer is this pathway with regulation of cell cycle progression. Un- well documented. For example, a mutated form of the p85 covering the signaling network spanning from extracellular subunit of PI3K has recently been isolated in a Hodgkin’s environment to the nucleus should illuminate biochemical lymphoma-derived cell line (CO).12 The PI3K/Akt pathway has events contributing to malignant transformation. Here, we been shown to be the predominant growth-factor-activated discuss PI3K/Akt-mediated signal transduction including its pathway in LNCaP human prostate carcinoma cells.13 PI3K mechanisms of activation, signal transducing molecules, and activity has been linked to a variety of human tumors including effects on gene expression that contribute to tumorigenesis. 14 15 16 17 Effects of PI3K/Akt signaling on important proteins controlling breast cancer, lung cancer, melanomas, and leukemia cellular proliferation are emphasized. These targets include among others. Further evidence has shown that Akt (protein cyclins, cyclin-dependent kinases, and cyclin-dependent ki- kinase B, PKB), a downstream kinase of PI3K, is also involved in nase inhibitors. Furthermore, strategies used to inhibit the malignant transformation.18 Inhibition of this pathway is a PI3K/Akt pathway are presented. The potential for cancer promising approach for novel chemotherapeutic agents. treatment with agents inhibiting this pathway is also addressed. Leukemia (2003) 17, 590–603. doi:10.1038/sj.leu.2402824 Keywords: PI3K; PTEN; Akt; cell cycle; apoptosis; targeted Cytokine activation of PI3K/Akt pathway therapy; inhibition Introduction After cytokine activation of a specific receptor, PI3K can be activated by two mechanisms. First, a phosphorylated tyrosine (Y) residue on the receptor serves as a docking site for the p85 An understanding of normal cellular function has been gained 1 from research characterizing various signal transduction path- regulatory subunit of PI3K. This recruits the catalytic subunit of ways. Despite these advances, processes responsible for PI3K, p110, to this complex. Alternatively, upon activation of the receptor by the cognate cytokine, Shc binds the receptor to neoplastic transformation of normal cells remain largely 19–22 undefined. For this reason, many chemotherapeutic regimens enable Grb-2 and Sos to form a complex that activates Ras. currently available are relatively nonspecific and have many Ras can then induce membrane localization and activation of the p110 subunit of PI3K, as determined by crystal structure toxic side effects. The ultimate goal of signal-transduction-based 23 research is to develop chemotherapeutic drugs that target studies. Specifically, this interaction occurs between regions cancerous cells without affecting nontransformed cells. on Ras, known as switch I (residues 33–41) and switch II (residues 63, 64, and 73), which bind to a sequence on PI3K-g Signaling through the Raf/MEK/ERK cascade is relatively well 24 understood1–9 and much progress has been achieved in the referred to as the Ras-binding domain (RBD). It was recently inhibition of this signaling pathway.10,11 In many cases, shown that the PI3K regulatory subunit, p85, contains two however, activation of the Raf/MEK/ERK pathway alone is not additional regulatory domains: a GTPase-responsive domain responsible for oncogenic transformation. Recent evidence has (GRD) that can mediate PI3K activation through binding small suggested that the PI3K/Akt pathway is a viable target for novel GTPases (Ras, Rac1), and an inhibitory domain that can block antineoplastic drugs. This review will describe PI3K/Akt path- these binding events. PI3K inhibition can be lifted when certain tyrosine kinases such as Src, Lck, and Abl phosphorylate the way signaling and its effects on cell cycle progression, 25 prevention of apoptosis, and malignant transformation. Lastly, S668 residue contained within the inhibitory domain. several potential anticancer therapies targeting this cascade are Different Ras isoforms have also been shown to have varying discussed. abilities to activate PI3K. Although Ras molecules are commonly believed to activate both PI3K/Akt and Raf/MEK/ERK cascades, Ha-Ras preferentially activates PI3K and Ki-Ras preferentially Correspondence: JA McCubrey, Department of Microbiology & 26 Immunology, Brody School of Medicine at East Carolina University, activates Raf. Greenville, NC 27858, USA; Fax: 1 252 816 3104 Activated PI3K converts phosphatidylinositol (4,5)-phos- 4 The first two authors contributed equally to this work. phate [PI(4,5)P2] into phosphatidylinositol (3,4,5)-phosphate Received 3 September 2002; accepted 23 January 2002 [PI(3,4,5)P3], which results in membrane localization of P13K/Akt pathway as a target for cancer chemotherapy F Chang et al 591 Figure 1 Activation of the PI3K/Akt pathway. Two mechanisms by which cytokine receptor activation can result in activation of the PI3K/Akt pathway are indicated. Note that significant activation of this pathway occurs in close proximity to the cell membrane. SH2: Src homology-2 domain; PH: pleckstrin homology domain. phosphotidylinositol-dependent kinase-1 (PDK1) via its pleck- active serine/threonine kinase monomers that become activated strin homology (PH) domain. Akt is also recruited to the lipid- when bound to specific cyclins. Cyclins are nuclear proteins that rich plasma membrane by its PH domain and is phosphorylated are transiently expressed in order to activate its cognate Cdk. at residues T308 and S473 by PDK1 and an unidentified kinase, These proteins form complexes that facilitate cell cycle respectively (Figure 1). progression through various cell cycle stages (Figure 3). Cdk Akt is the primary mediator of PI3K-initiated signaling and has activities are inhibited by two families of Cdk inhibitors (CKIs). a number of downstream substrates that may contribute to The first CKI family is composed of inhibitors of Cdk4 (INK4), malignant transformation. Some of these substrates are Bad, which are p16INK4a, p15INK5b, p18INK4c, and p19INK4d. INK4 procaspase-9, I-kB kinase (IKK), CREB, the forkhead family of family CKIs inhibit kinase activity of Cdk4/cyclin D and Cdk6/ transcription factors (FKHR/AFX/FOX), glycogen synthase ki- cyclin D complexes.32 The second family is Cip/Kip CKIs, which nase-3 (GSK-3), p21Cip1, and Raf.18 IKK and CREB are activated includes p21Cip1, p27Kip1, and p57Kip2. Cip/Kip family CKIs by Akt phosphorylation, whereas Raf, Bad, procaspase-9, FKHR, inhibit a broader set of Cdks, including Cdk2, Cdk4, and and GSK-3 are inactivated. Mechanisms by which Akt modu- Cdk6.33,34 The roles that these regulatory proteins play in cell lates activity of these proteins to promote cell survival and cell cycle progression can be modulated by PI3K signaling. cycle progression will be discussed in the following sections. Activity of the PI3K/Akt pathway is negatively regulated by p21Cip1 and PI3K/Akt: cell cycle regulatory effects phosphatases. Phosphatase and tensin homologue deleted on chromosome 10 (PTEN), which is also known as mutated in As previously discussed, the PI3K/Akt pathway is important for multiple advanced cancers-1 (MMAC1), as well as Src the regulation of cell cycle progression.35–40 For instance, the homology 2 (SH2) containing phosphatases 1 and 2 (SHIP-1 PI3K/Akt pathway is crucial for DNA synthesis in mesenchymal 27–31 35 and SHIP-2) remove phosphates from PI(3,4,5)P3. Muta- cells and expression of dominant negative (DN) Akt blocks tions in these phosphatases that eliminate their activity can lead platelet-derived-growth-factor (PDGF)-induced DNA synthesis to tumor progression. Consequently, genes encoding these and cell proliferation.40 p21Cip1 was initially thought to be a CKI phosphatases are referred to as antioncogenes or tumor because it inhibited cell proliferation when overexpressed in suppressor genes. An overview of the PI3K/Akt pathway is human fibroblasts. Evidence gathered by in vitro reconstitution presented in Figure 2. studies showed that p21Cip1 inhibited activity of most Cdk isoforms.41,42 Further investigation has suggested that p21Cip1 has a more complex role in regulating Cdk activity. Under Cell cycle overview physiological conditions, p21Cip1 can bind to Cdk4/cyclin D and Cdk6/cyclin D complexes, and induce their kinase activities 43 Cip1 Uncontrolled cell division can lead to tumorigenesis. Cell from early G1 until the middle of S phase. In contrast, p21 division is coordinated by a number of proteins including can bind