Abstract: Since its discovery as the elusive tumor suppressor gene at the frequently mutated 10q23 locus, PTEN has been identified as lost or mutated in several sporadic and heritable tumor types. A decade of work has established that PTEN is a nonredundant phosphatase that is essential for regulating the highly oncogenic prosurvival PI3K/AKT signaling pathway. This review discusses emerging modes of PTEN function and regulation, and speculates about how manipulation of PTEN function could be used for cancer therapy. Since its discovery as the elusive tumor suppressor gene at the frequently mutated 10q23 locus, PTEN has been identified as lost or mutated in several sporadic and heritable tumor types. A decade of work has established that PTEN is a nonredundant phosphatase that is essential for regulating the highly oncogenic prosurvival PI3K/AKT signaling pathway. This review discusses emerging modes of PTEN function and regulation, and speculates about how manipulation of PTEN function could be used for cancer therapy. PTEN (phosphatase and tensin homolog deleted on chromosome 10) is one of the most frequently mutated tumor suppressor genes in human cancer. PTEN was first discovered by independent groups and recognized as the frequently lost tumor suppressor gene on human chromosome 10q23, a locus that is highly susceptible to mutation in primary human cancers (Li et al., 1997Li J. Yen C. Liaw D. Podsypanina K. Bose S. Wang S.I. Puc J. Miliaresis C. Rodgers L. McCombie R. et al.PTEN, a putative protein tyrosine phosphatase gene mutated in human brain, breast, and prostate cancer.Science. 1997; 275: 1943-1947Crossref PubMed Scopus (2889) Google Scholar, Steck et al., 1997Steck P.A. Pershouse M.A. Jasser S.A. Yung W.K. Lin H. Ligon A.H. Langford L.A. Baumgard M.L. Hattier T. Davis T. et al.Identification of a candidate tumour suppressor gene, MMAC1, at chromosome 10q23.3 that is mutated in multiple advanced cancers.Nat. Genet. 1997; 15: 356-362Crossref PubMed Scopus (1876) Google Scholar). The frequency of monoallelic mutations at this locus has been estimated at 50%–80% in sporadic tumors (including endometrial carcinoma, glioblastoma, and prostate cancer) and at 30%–50% in breast, colon, and lung tumors. Complete loss of PTEN is observed at highest frequencies in endometrial cancer and glioblastoma and is generally associated with advanced cancers and metastases (Ali et al., 1999Ali I.U. Schriml L.M. Dean M. Mutational spectra of PTEN/MMAC1 gene: a tumor suppressor with lipid phosphatase activity.J. Natl. Cancer Inst. 1999; 91: 1922-1932Crossref PubMed Google Scholar). A recent landmark study reveals that PTEN loss is a common event in breast cancers caused by BRCA1 deficiency (Saal et al., 2008Saal L.H. Gruvberger-Saal S.K. Persson C. Lovgren K. Jumppanen M. Staaf J. Jonsson G. Pires M.M. Maurer M. Holm K. et al.Recurrent gross mutations of the PTEN tumor suppressor gene in breast cancers with deficient DSB repair.Nat. Genet. 2008; 40: 102-107Crossref PubMed Scopus (151) Google Scholar). The importance of PTEN as a tumor suppressor is further supported by the study of PTEN germline mutations in a group of autosomal dominant syndromes characterized by developmental disorders, neurological deficits, multiple hamartomas, and an increased risk of breast, thyroid, and endometrial cancers. Collectively, these are referred to as the PTEN hamartoma tumor syndromes (PHTS), which include Cowden syndrome, Lhermitte-Duclos disease, Bannayan-Riley-Ruvalcaba syndrome, and Proteus and Proteus-like syndromes. Various mouse models in which Pten is deleted also demonstrate the crucial role of PTEN as a tumor suppressor in multiple tumor types (Di Cristofano et al., 1998Di Cristofano A. Pesce B. Cordon-Cardo C. Pandolfi P.P. Pten is essential for embryonic development and tumour suppression.Nat. Genet. 1998; 19: 348-355Crossref PubMed Scopus (883) Google Scholar, Podsypanina et al., 1999Podsypanina K. Ellenson L.H. Nemes A. Gu J. Tamura M. Yamada K.M. Cordon-Cardo C. Catoretti G. Fisher P.E. Parsons R. Mutation of Pten/Mmac1 in mice causes neoplasia in multiple organ systems.Proc. Natl. Acad. Sci. USA. 1999; 96: 1563-1568Crossref PubMed Scopus (600) Google Scholar, Suzuki et al., 1998Suzuki A. de la Pompa J.L. Stambolic V. Elia A.J. Sasaki T. del Barco Barrantes I. Ho A. Wakeham A. Itie A. Khoo W. et al.High cancer susceptibility and embryonic lethality associated with mutation of the PTEN tumor suppressor gene in mice.Curr. Biol. 1998; 8: 1169-1178Abstract Full Text Full Text PDF PubMed Google Scholar, Trotman et al., 2003Trotman L.C. Niki M. Dotan Z.A. Koutcher J.A. Di Cristofano A. Xiao A. Khoo A.S. Roy-Burman P. Greenberg N.M. Van Dyke T. et al.Pten dose dictates cancer progression in the prostate.PLoS Biol. 2003; 1: E59Crossref PubMed Scopus (296) Google Scholar, Wang et al., 2003Wang S. Gao J. Lei Q. Rozengurt N. Pritchard C. Jiao J. Thomas G.V. Li G. Roy-Burman P. Nelson P.S. et al.Prostate-specific deletion of the murine Pten tumor suppressor gene leads to metastatic prostate cancer.Cancer Cell. 2003; 4: 209-221Abstract Full Text Full Text PDF PubMed Scopus (450) Google Scholar). Functionally, PTEN is a nonredundant, plasma-membrane lipid phosphatase that antagonizes the phosphatidylinositol-3-kinase (PI3K) signaling pathway (Maehama and Dixon, 1998Maehama T. Dixon J.E. The tumor suppressor, PTEN/MMAC1, dephosphorylates the lipid second messenger, phosphatidylinositol 3,4,5-trisphosphate.J. Biol. Chem. 1998; 273: 13375-13378Crossref PubMed Scopus (1662) Google Scholar, Stambolic et al., 1998Stambolic V. Suzuki A. de la Pompa J.L. Brothers G.M. Mirtsos C. Sasaki T. Ruland J. Penninger J.M. Siderovski D.P. Mak T.W. Negative regulation of PKB/Akt-dependent cell survival by the tumor suppressor PTEN.Cell. 1998; 95: 29-39Abstract Full Text Full Text PDF PubMed Scopus (1355) Google Scholar). Upon stimulation of cells with growth stimuli, class I PI3K family members catalyze the conversion of phosphatidylinositol 4,5-bisphosphate (PIP2) to phosphatidylinositol 3,4,5-trisphosphate (PIP3), a second messenger that promotes survival, growth, and proliferation. Specifically, PTEN hydrolyzes the 3-phosphate on PIP3 to generate PIP2, and thereby negatively regulates PIP3-mediated downstream signaling. Through its role in phosphatidylinositol homeostasis, PTEN is implicated in cell polarity and migration and thereby provides a potential link between outer membrane phospholipids and pathways that lead to cytoskeleletal reorganization (reviewed in Franca-Koh et al., 2007Franca-Koh J. Kamimura Y. Devreotes P.N. Leading-edge research: PtdIns(3,4,5)P3 and directed migration.Nat. Cell Biol. 2007; 9: 15-17Crossref Scopus (37) Google Scholar; see also the SnapShot by A. Carracedo, L. Salmena, and P.P. Pandolfi on the last page of this issue). Upon PTEN loss, PIP3 accumulates and promotes the recruitment of a subset of proteins that contain a pleckstrin homology domain to cellular membranes, including the serine/threonine kinases AKT1, AKT2, AKT3, and PDK1. Once positioned at cell membranes, AKT isoforms are activated by phosphorylation at two different residues. AKT is thought to be phosphorylated by PDK1 on Thr308 and by the mTOR kinase complex 2 (mTORC2) on Ser473 (as reviewed in Guertin and Sabatini, 2007Guertin D.A. Sabatini D.M. Defining the role of mTOR in cancer.Cancer Cell. 2007; 12: 9-22Abstract Full Text Full Text PDF PubMed Scopus (1082) Google Scholar, Manning and Cantley, 2007Manning B.D. Cantley L.C. AKT/PKB signaling: navigating downstream.Cell. 2007; 129: 1261-1274Abstract Full Text Full Text PDF PubMed Scopus (1680) Google Scholar). Termination of AKT signaling is thought to be elicited by the protein phosphatase PHLIPP, which directly dephosphorylates phospho-Ser473 on AKT (Brognard et al., 2007Brognard J. Sierecki E. Gao T. Newton A.C. PHLPP and a second isoform, PHLPP2, differentially attenuate the amplitude of Akt signaling by regulating distinct Akt isoforms.Mol. Cell. 2007; 25: 917-931Abstract Full Text Full Text PDF PubMed Scopus (198) Google Scholar, Gao et al., 2005Gao T. Furnari F. Newton A.C. PHLPP: a phosphatase that directly dephosphorylates Akt, promotes apoptosis, and suppresses tumor growth.Mol. Cell. 2005; 18: 13-24Abstract Full Text Full Text PDF PubMed Scopus (351) Google Scholar). Activation of AKT kinases promotes cell survival, proliferation, growth, angiogenesis, and cellular metabolism through phosphorylation of myriad cellular substrates including MDM2, GSK3, FOXO, BAD, CASP9, and p27 (Manning and Cantley, 2007Manning B.D. Cantley L.C. AKT/PKB signaling: navigating downstream.Cell. 2007; 129: 1261-1274Abstract Full Text Full Text PDF PubMed Scopus (1680) Google Scholar). AKT activation also leads to activation of the mTOR kinase complex 1 (mTORC1) through an inhibitory phosphorylation of the TSC tumor suppressor complex and consequent activation of RHEB, a Ras-related small GTPase (Guertin and Sabatini, 2007Guertin D.A. Sabatini D.M. Defining the role of mTOR in cancer.Cancer Cell. 2007; 12: 9-22Abstract Full Text Full Text PDF PubMed Scopus (1082) Google Scholar). As a consequence of PTEN inactivation, activation of mTORC1 in turn leads to enhanced translation of mRNA into protein, a hallmark of many cancers (Tee and Blenis, 2005Tee A.R. Blenis J. mTOR, translational control and human disease.Semin. Cell Dev. Biol. 2005; 16: 29-37Crossref Scopus (176) Google Scholar). In addition to being repressed by a number of tumor suppressor genes directly implicated in human cancer, the mTOR arm of the PTEN/PI3K/AKT pathway is emerging as an effective target for anti-cancer agents, especially in tumors in which the activity of the mTOR pathway is elevated (Faivre et al., 2006Faivre S. Kroemer G. Raymond E. Current development of mTOR inhibitors as anticancer agents.Nat. Rev. Drug Discov. 2006; 5: 671-688Crossref PubMed Scopus (461) Google Scholar, Guertin and Sabatini, 2007Guertin D.A. Sabatini D.M. Defining the role of mTOR in cancer.Cancer Cell. 2007; 12: 9-22Abstract Full Text Full Text PDF PubMed Scopus (1082) Google Scholar). Many important publications have expertly reviewed the wide spectrum of mutations found in the PTEN gene and the multitude of tumor types that arise upon loss of PTEN activity. Also, the contribution of PTEN loss to activation of the PI3K/AKT signaling pathway and consequent tumorigenesis is well established. In this review, we present emerging tenets for the regulation of the PTEN gene and PTEN protein, discuss unconventional downstream effectors and pathways of PTEN function, and introduce new potential strategies for PTEN-associated cancer therapies. The classical premise of tumor suppression asserts that both copies of a given tumor suppressor gene must be lost for cancer to arise (Knudson, 1971Knudson A.G. Mutation and cancer: statistical study of retinoblastoma.Proc. Natl. Acad. Sci. USA. 1971; 68: 820-823Crossref PubMed Google Scholar). Although this is definitely the case in certain circumstances and in particular tissue types (such as in a large fraction of retinoblastoma [RB] lesions that experience homozygous loss of the RB gene), heterozygous loss of other tumor suppressor genes can have critical biological consequences toward cancer initiation and progression. Loss of only one allele of Pten in mice has been shown to promote the progression of a lethal polyclonal autoimmune disorder with high penetrance (Di Cristofano et al., 1999Di Cristofano A. Kotsi P. Peng Y.F. Cordon-Cardo C. Elkon K.B. Pandolfi P.P. Impaired Fas response and autoimmunity in Pten+/− mice.Science. 1999; 285: 2122-2125Crossref PubMed Scopus (371) Google Scholar), thereby suggesting that Pten is functionally haploinsufficient (that is that one functional allele is not enough to sustain a wild-type condition). Similarly, Pten heterozygosity appears to be the driving force for epithelial cancers, such as prostate cancer, in mouse models of Pten loss (Di Cristofano et al., 2001Di Cristofano A. De Acetis M. Koff A. Cordon-Cardo C. Pandolfi P.P. Pten and p27KIP1 cooperate in prostate cancer tumor suppression in the mouse.Nat. Genet. 2001; 27: 222-224Crossref PubMed Scopus (291) Google Scholar). Moreover, our group has demonstrated that cellular levels of Pten protein inversely correlate with the occurrence of invasive prostate cancer (Figure 1). This was established by generating a so-called “hypomorphic Pten allelic series” in the mouse where Pten dose is progressively decreased below heterozygous levels (Trotman et al., 2003Trotman L.C. Niki M. Dotan Z.A. Koutcher J.A. Di Cristofano A. Xiao A. Khoo A.S. Roy-Burman P. Greenberg N.M. Van Dyke T. et al.Pten dose dictates cancer progression in the prostate.PLoS Biol. 2003; 1: E59Crossref PubMed Scopus (296) Google Scholar). This suggests that Pten is a haploinsufficient tumor suppressor gene in specific mouse tissues. Despite evidence from mouse models, whether PTEN is a haploinsufficient tumor suppressor gene in humans remains to be determined. To date, there are only a handful of reports that provide support for this notion. By definition, PTEN is haploinsufficient for the development of PTEN hamartoma tumor syndrome, given that heterozygosity leads to characteristic phenotypes, including various developmental disorders and benign polyps. With respect to its role as a tumor suppressor, the increased susceptibility of patients with PTEN hamartoma tumor syndrome to develop tumors may be consistent with haploinsufficiency. Further support for haploinsufficiency is provided by the observation that some tumors derived from patients with Cowden syndrome do not have detectable biallelic mutation of the PTEN gene (Dahia, 2000Dahia P.L. PTEN, a unique tumor suppressor gene.Endocr. Relat. Cancer. 2000; 7: 115-129Crossref PubMed Google Scholar, Marsh et al., 1998Marsh D.J. Dahia P.L. Coulon V. Zheng Z. Dorion-Bonnet F. Call K.M. Little R. Lin A.Y. Eeles R.A. Goldstein A.M. et al.Allelic imbalance, including deletion of PTEN/MMACI, at the Cowden disease locus on 10q22–23, in hamartomas from patients with Cowden syndrome and germline PTEN mutation.Genes Chromosomes Cancer. 1998; 21: 61-69Crossref PubMed Scopus (77) Google Scholar). Moreover, primary prostate tumors often show loss or alteration of one PTEN allele at presentation (as in 70%–80% of cases of primary prostate cancer [Gray et al., 1998Gray I.C. Stewart L.M. Phillips S.M. Hamilton J.A. Gray N.E. Watson G.J. Spurr N.K. Snary D. Mutation and expression analysis of the putative prostate tumour-suppressor gene PTEN.Br. J. Cancer. 1998; 78: 1296-1300Crossref PubMed Google Scholar, Whang et al., 1998Whang Y.E. Wu X. Suzuki H. Reiter R.E. Tran C. Vessella R.L. Said J.W. Isaacs W.B. Sawyers C.L. Inactivation of the tumor suppressor PTEN/MMAC1 in advanced human prostate cancer through loss of expression.Proc. Natl. Acad. Sci. USA. 1998; 95: 5246-5250Crossref PubMed Scopus (455) Google Scholar]) whereas homozygous inactivation is observed at much lower frequencies. Similarly in breast cancer, there is a lack of concordance between the occurrence of monoallelic mutation of PTEN (30%–40%) and the occurrence of biallelic loss (5%) (Ali et al., 1999Ali I.U. Schriml L.M. Dean M. Mutational spectra of PTEN/MMAC1 gene: a tumor suppressor with lipid phosphatase activity.J. Natl. Cancer Inst. 1999; 91: 1922-1932Crossref PubMed Google Scholar, Bose et al., 1998Bose S. Wang S.I. Terry M.B. Hibshoosh H. Parsons R. Allelic loss of chromosome 10q23 is associated with tumor progression in breast carcinomas.Oncogene. 1998; 17: 123-127Crossref Scopus (97) Google Scholar, Feilotter et al., 1999Feilotter H.E. Coulon V. McVeigh J.L. Boag A.H. Dorion-Bonnet F. Duboue B. Latham W.C. Eng C. Mulligan L.M. Longy M. Analysis of the 10q23 chromosomal region and the PTEN gene in human sporadic breast carcinoma.Br. J. Cancer. 1999; 79: 718-723Crossref PubMed Scopus (86) Google Scholar). Indeed, complete loss of PTEN is observed in many advanced cancers. However, the observation that monoallelic mutation of PTEN without loss or mutation of the second allele is prevalent in breast and prostate cancer lesions is consistent with the notion that monoallelic loss of PTEN is sufficient for tumor initiation and progression. Overall, the question of PTEN haploinsufficiency remains an important one, and the analysis of more tumor samples is required to clarify whether, and in which types of human tissue, PTEN haploinsufficiency is critical. In line with the notion that PTEN haploinsufficiency contributes to tumor progression, there are indications that even a minor impairment in PTEN function may lead to the development of cancer. This is illustrated by the identification of Cowden syndrome and tumor-derived PTEN mutations that preserve partial or even full PTEN lipid phosphatase function (Waite and Eng, 2002Waite K.A. Eng C. Protean PTEN: form and function.Am. J. Hum. Genet. 2002; 70: 829-844Abstract Full Text Full Text PDF PubMed Scopus (259) Google Scholar). For example, some C2-domain mutations in PTEN identified in Cowden syndrome as well as other somatic mutations that produce C-terminal truncations retain phosphatase activity in biochemical assays (Han et al., 2000Han S.Y. Kato H. Kato S. Suzuki T. Shibata H. Ishii S. Shiiba K. Matsuno S. Kanamaru R. Ishioka C. Functional evaluation of PTEN missense mutations using in vitro phosphoinositide phosphatase assay.Cancer Res. 2000; 60: 3147-3151PubMed Google Scholar, Waite and Eng, 2002Waite K.A. Eng C. Protean PTEN: form and function.Am. J. Hum. Genet. 2002; 70: 829-844Abstract Full Text Full Text PDF PubMed Scopus (259) Google Scholar). These truncations ultimately affect PTEN phosphorylation, stability and protein-protein interactions by deletion of phosphorylated residues and the PDZ domain. Moreover, N-terminal mutants are thought to influence PTEN stability yet maintain catalytic activity (Han et al., 2000Han S.Y. Kato H. Kato S. Suzuki T. Shibata H. Ishii S. Shiiba K. Matsuno S. Kanamaru R. Ishioka C. Functional evaluation of PTEN missense mutations using in vitro phosphoinositide phosphatase assay.Cancer Res. 2000; 60: 3147-3151PubMed Google Scholar), and point mutations of the central C2 domain have been demonstrated to impact proper PTEN localization (Trotman et al., 2007Trotman L.C. Wang X. Alimonti A. Chen Z. Teruya-Feldstein J. Yang H. Pavletich N.P. Carver B.S. Cordon-Cardo C. Erdjument-Bromage H. et al.Ubiquitination regulates PTEN nuclear import and tumor suppression.Cell. 2007; 128: 141-156Abstract Full Text Full Text PDF PubMed Scopus (241) Google Scholar). Nevertheless, it must be noted that even though partially functional mutants do indeed exist, the majority of PTEN mutations are believed to profoundly inhibit catalytic activity. In conclusion, that a partially functional allele of PTEN exists in heterozygosity in disorders that confer increased cancer susceptibility (as observed in PTEN hamartoma tumor syndrome) may point toward a critical role for slight and selective functional impairment of PTEN in tumor susceptibility. This notion is timely and particularly relevant given that mechanisms for PTEN control at the transcriptional and posttranslational levels may be altered in disease. Moreover, mutants of PTEN that maintain partial function may have a selective advantage over mutants that confer a complete loss of function, because complete loss of PTEN induces the activation of a p53-dependent cellular senescence response, as discussed below (Chen et al., 2005Chen Z. Trotman L.C. Shaffer D. Lin H.K. Dotan Z.A. Niki M. Koutcher J.A. Scher H.I. Ludwig T. Gerald W. et al.Crucial role of p53-dependent cellular senescence in suppression of Pten-deficient tumorigenesis.Nature. 2005; 436: 725-730Crossref PubMed Scopus (760) Google Scholar). While studying the relationship between Pten dose and tumor progression in mouse models of prostate specific loss of Pten, Chen et al., 2005Chen Z. Trotman L.C. Shaffer D. Lin H.K. Dotan Z.A. Niki M. Koutcher J.A. Scher H.I. Ludwig T. Gerald W. et al.Crucial role of p53-dependent cellular senescence in suppression of Pten-deficient tumorigenesis.Nature. 2005; 436: 725-730Crossref PubMed Scopus (760) Google Scholar observed an unexpected and intriguing phenomenon: complete acute loss of Pten did not provide a proliferative advantage as would be expected, but instead promoted a strong senescence response that opposed tumor progression. Senescence, a cellular program that triggers an irreversible growth arrest and limits the replicative life span of cultured primary cells, has been proposed to function as an anti-tumor mechanism set off by tumor suppressor genes in response to triggers including DNA damage and oncogene activation (Campisi and d'Adda di Fagagna, 2007Campisi J. d'Adda di Fagagna F. Cellular senescence: when bad things happen to good cells.Nat. Rev. Mol. Cell Biol. 2007; 8: 729-740Crossref PubMed Scopus (820) Google Scholar). Mice with conditional inactivation of Pten in the prostate develop invasive cancer; however early-stage tumor development has been associated with slow growth, increased p53 levels, and cellular senescence. As predicted from these findings, combined inactivation of Pten and Trp53 leads to unconstrained tumor growth as demonstrated by the generation of massive invasive prostate tumors. This implies that complete ablation of PTEN can be detrimental to tumor growth in the absence of other mutations and highlights the importance of haploinsufficiency or partial PTEN impairment in tumor progression (Figure 1). Clinically, these findings provide an explanation as to why complete PTEN loss is not frequently observed at cancer presentation and, importantly, imply that PTEN-deficient prostate cancer may benefit from drugs that can promote p53 activation and enhancement of p53-dependent cellular senescence. Genetic loss or mutation of tumor suppressor genes is a frequent event initiating and/or promoting tumorigenesis (Vogelstein and Kinzler, 2004Vogelstein B. Kinzler K.W. Cancer genes and the pathways they control.Nat. Med. 2004; 10: 789-799Crossref PubMed Scopus (1847) Google Scholar). Tumour suppressor genes are also subject to countless regulatory mechanisms including epigenetic effects, transcriptional modulation, and posttranscriptional and posttranslational modifications that ultimately govern protein levels, activity, localization, binding partners, and function. Disrupted tumor suppressor regulation by one or more of these mechanisms may also have catastrophic consequences for a cell. In addition to mutations that partially or fully inactivate a given PTEN allele, emerging evidence shows that complete or partial loss of PTEN protein expression, through as-yet-unidentified mechanisms, can impact tumor suppression (Figure 2A). For instance, spontaneous cancers that harbor monoallelic mutations of PTEN possess at least one functional wild-type PTEN allele, yet they further or completely lose PTEN protein immunoreactivity in the absence of detectable mutations of the remaining PTEN allele (Leupin et al., 2003Leupin N. Cenni B. Novak U. Hugli B. Graber H.U. Tobler A. Fey M.F. Disparate expression of the PTEN gene: a novel finding in B-cell chronic lymphocytic leukaemia (B-CLL).Br. J. Haematol. 2003; 121: 97-100Crossref Scopus (22) Google Scholar, Mutter et al., 2000Mutter G.L. Lin M.C. Fitzgerald J.T. Kum J.B. Baak J.P. Lees J.A. Weng L.P. Eng C. Altered PTEN expression as a diagnostic marker for the earliest endometrial precancers.J. Natl. Cancer Inst. 2000; 92: 924-930Crossref PubMed Google Scholar, Shi et al., 2003Shi W. Zhang X. Pintilie M. Ma N. Miller N. Banerjee D. Tsao M.S. Mak T. Fyles A. Liu F.F. Dysregulated PTEN-PKB and negative receptor status in human breast cancer.Int. J. Cancer. 2003; 104: 195-203Crossref Scopus (63) Google Scholar, Zhou et al., 2002Zhou X.P. Loukola A. Salovaara R. Nystrom-Lahti M. Peltomaki P. de la Chapelle A. Aaltonen L.A. Eng C. PTEN mutational spectra, expression levels, and subcellular localization in microsatellite stable and unstable colorectal cancers.Am. J. Pathol. 2002; 161: 439-447Abstract Full Text Full Text PDF PubMed Google Scholar). These observations imply that epigenetic silencing by aberrant promoter methylation, deregulated transcription, increased degradation, and/or mislocalization of the PTEN protein may disrupt function and promote tumorigenesis. Examples of these consequences are highlighted below. Due to its robust expression levels and long cellular half-life, PTEN is thought to be constitutively expressed and minimally regulated in normal tissues. This notion is in fact misleading given that the exact pattern of regulation of PTEN during development and adult life is unclear. Furthermore, the stability of PTEN may be altered dramatically in pathological settings (Figure 2B). PTEN was originally cloned as a gene transcriptionally regulated by transforming growth factor β (TGFβ) Li and Sun, 1997Li D.M. Sun H. TEP1, encoded by a candidate tumor suppressor locus, is a novel protein tyrosine phosphatase regulated by transforming growth factor beta.Cancer Res. 1997; 57: 2124-2129PubMed Google Scholar). Since this discovery, numerous factors have been demonstrated to upregulate PTEN transcription including the peroxisome proliferation-activated receptor γ (PPARγ) (Patel et al., 2001Patel L. Pass I. Coxon P. Downes C.P. Smith S.A. Macphee C.H. Tumor suppressor and anti-inflammatory actions of PPARgamma agonists are mediated via upregulation of PTEN.Curr. Biol. 2001; 11: 764-768Abstract Full Text Full Text PDF PubMed Scopus (207) Google Scholar), and the early growth-regulated transcription factor-1 (EGR-1) (Virolle et al., 2001Virolle T. Adamson E.D. Baron V. Birle D. Mercola D. Mustelin T. de Belle I. The Egr-1 transcription factor directly activates PTEN during irradiation-induced signalling.Nat. Cell Biol. 2001; 3: 1124-1128Crossref PubMed Scopus (233) Google Scholar), which functions downstream of insulin-like growth factor 2 (IGF-2) (Moorehead et al., 2003Moorehead R.A. Hojilla C.V. De Belle I. Wood G.A. Fata J.E. Adamson E.D. Watson K.L. Edwards D.R. Khokha R. Insulin-like growth factor-II regulates PTEN expression in the mammary gland.J. Biol. Chem. 2003; 278: 50422-50427Crossref PubMed Scopus (40) Google Scholar). Additionally, Stambolic et al., 2001Stambolic V. MacPherson D. Sas D. Lin Y. Snow B. Jang Y. Benchimol S. Mak T.W. Regulation of PTEN transcription by p53.Mol. Cell. 2001; 8: 317-325Abstract Full Text Full Text PDF PubMed Scopus (449) Google Scholar identified a putative p53-binding element in the promoter sequence of PTEN and characterized a p53-mediated cellular survival mechanism that functions through the activation of PTEN transcription. Induction of PTEN may be an important mechanism by which its presence is ensured when it is required to perform its tumor suppressive function. Unlike the tumor suppressor p53, which is acutely and rapidly upregulated in response to potentially oncogenic stresses, PTEN expression is constitutive and essential at all times. As previously mentioned, suppression of PTEN transcription may have an important and underestimated role in cancer (Figure 2B). Indeed, recent studies have demonstrated a link between the oncogenic RAS-MAPK pathway and aberrant transcriptional downregulation of PTEN in both fibroblast and epithelial cell types and in human cancer cells. Chow et al., 2007Chow J.Y. Quach K.T. Cabrera B.L. Cabral J.A. Beck S.E. Carethers J.M. RAS/ERK modulates TGFbeta-regulated PTEN expression in human pancreatic adenocarcinoma cells.Carcinogenesis. 2007; 28: 2321-2327Crossref PubMed Scopus (48) Google Scholar attribute RAS-mediated PTEN suppression to a TGFβ-dependent mechanism in pancreatic adenocarcinoma and Vasudevan et al., 2007Vasudevan K.M. Burikhanov R. Goswami A. Rangnekar V.M. Suppression of PTEN expression is essential for antiapoptosis and cellular transformation by oncogenic Ras.Cancer Res. 2007; 67: 10343-10350Crossref PubMed Scopus (31) Google Scholar demonstrate that the oncogenic RAS-RAF-MEK-ERK pathway suppresses PTEN levels through the transcriptional factor, c-Jun (Hettinger et al., 2007Hettinger K. Vikhanskaya F. Poh M.K. Lee M.K. de Belle I. Zhang J.T. Reddy S.A. Sabapathy K. c-Jun promotes cellular survival by suppression of PTEN.Cell Death Differ. 2007; 14: 218-229Crossref PubMed Scopus (52) Google Scholar). Moreover, other stress kinase pathways including MEKK4 and JNK promote resistance to apoptosis by suppressing PTEN transcription via direct binding of NFκB to the PTEN promoter (Xia et al., 2007Xia D. Srinivas H. Ahn Y.H. Sethi G. Sheng X. Yung W.K. Xia Q. Chiao P.J. Kim H. Brown P.H. et al.Mitogen-activated protein kinase kinase-4 promotes cell survival by decreasing PTEN expression through an NF kappa B-dependent pathway.J. Biol. Chem. 2007; 282: 3507-3519Crossref Scopus (47) Google Scholar). Conversely, PTEN was demonstrated to oppose the JNK pathway (Vivanco et al., 2007Vivanco I. Palaskas N. Tran C. Finn S.P. Getz G. Kennedy N.J. Jiao J. Rose J. Xie W. Loda M. et al.Identification of the JNK signaling pathway as a functional target of the tumor suppressor PTEN.Cancer Cell. 2007; 11: 555-569Abstract Full Text Full Text PDF PubMed Scopus (104) Google Scholar). This finding suggests that pathways that are negatively regulated by PTEN can in turn regulate PTEN transcription, thereby identifying a new and potentially important feedback loop. Finally, recent studies have identified new pathways for PTEN regulation occurring downstream of NOTCH1. Active NOTCH1 has been reported to increase PTEN transcription through mechanisms involving the CBF-1 transcription factor (Chappell et al., 2005Chappell W.H. Green T.D. Spengeman J.D. McCubrey J.A. Akula S.M. Bertrand F.E. Increased protein expression of the PTEN tumor suppressor in the presence of constitutively active Notch-1.Cell Cycle. 2005; 4: 1389-1395Cross