Cyclin D1 expression mediated by phosphatidylinositol 3-kinase through mTOR-p70(S6K)-independent signaling in growth factor-stimulated NIH 3T3 fibroblasts

Paradigms of growth control: relation to Cdk activation

The cyclin-dependent kinases (CDKs) play a key role in cell cycle control, and in this review, we focus on the events that regulate their activities. Emphasis is placed on the CDKs that function during the G(1) phase of the cell cycle and on the CDK inhibitor p27(Kip1). We discuss how CDK activation relates to two basic concepts of cell cycle regulation: (i) the need for multiple mitogens for the proliferation of nontransformed cells and (ii) the inhibitory effect of high culture density on proliferative capacity. We also describe how Cdk2 modulates the expression of the alpha subunit of the interleukin-2 receptor in T cells, and address the question of whether p27(Kip1) functions as an activator or inhibitor of the CDKs associated with the D cyclins.

The protein kinase B/Akt signalling pathway in human malignancy

Protein kinase B or Akt (PKB/Akt) is a serine/threonine kinase, which in mammals comprises three highly homologous members known as PKBalpha (Akt1), PKBbeta (Akt2), and PKBgamma (Akt3). PKB/Akt is activated in cells exposed to diverse stimuli such as hormones, growth factors, and extracellular matrix components. The activation mechanism remains to be fully characterised but occurs downstream of phosphoinositide 3-kinase (PI-3K). PI-3K generates phosphatidylinositol-3,4,5-trisphosphate (PIP(3)), a lipid second messenger essential for the translocation of PKB/Akt to the plasma membrane where it is phosphorylated and activated by phosphoinositide-dependent kinase-1 (PDK-1) and possibly other kinases. PKB/Akt phosphorylates and regulates the function of many cellular proteins involved in processes that include metabolism, apoptosis, and proliferation. Recent evidence indicates that PKB/Akt is frequently constitutively active in many types of human cancer. Constitutive PKB/Akt activation can occur due to amplification of PKB/Akt genes or as a result of mutations in components of the signalling pathway that activates PKB/Akt. Although the mechanisms have not yet been fully characterised, constitutive PKB/Akt signalling is believed to promote proliferation and increased cell survival and thereby contributing to cancer progression. This review surveys recent developments in understanding the mechanisms and consequences of PKB/Akt activation in human malignancy.

Requirement for IGF-I in epidermal growth factor-mediated cell cycle progression of mammary epithelial cells

Induction of cyclin proteins is required for progression of cells through the G(1)-S and G(2)-M cell cycle checkpoints and is a primary mechanism by which mitogens regulate cell cycle progression. IGF-I and the epidermal growth factor (EGF)-related ligands are mitogens for mammary epithelial cells in vitro and are essential for growth of the mammary epithelium during development. We report here that IGF-I in combination with EGF or TGFalpha is synergistic in promoting DNA synthesis in mammary epithelial cells in the intact mammary gland cultured in vitro. We further investigated the role of IGF-I and EGF in cyclin expression and cell cycle progression in the mammary gland and demonstrate that IGF-I and EGF induce expression of early G(1) cyclins. However, we show that IGF-I, but not EGF, induces late G(1) and G(2) cyclins and is required for mammary epithelial cells to overcome the G(1)-S checkpoint. These data demonstrate that IGF-I is essential for cell cycle progression in mammary epithelial cells and that it is required for EGF-mediated progression past the G(1)-S checkpoint in these cells.

Synthesis and function of 3-phosphorylated inositol lipids

The 3-phosphorylated inositol lipids fulfill roles as second messengers by interacting with the lipid binding domains of a variety of cellular proteins. Such interactions can affect the subcellular localization and aggregation of target proteins, and through allosteric effects, their activity. Generation of 3-phosphoinositides has been documented to influence diverse cellular pathways and hence alter a spectrum of fundamental cellular activities. This review is focused on the 3-phosphoinositide lipids, the synthesis of which is acutely triggered by extracellular stimuli, the enzymes responsible for their synthesis and metabolism, and their cell biological roles. Much knowledge has recently been gained through structural insights into the lipid kinases, their interaction with inhibitors, and the way their 3-phosphoinositide products interact with protein targets. This field is now moving toward a genetic dissection of 3-phosphoinositide action in a variety of model organisms. Such approaches will reveal the true role of the 3-phosphoinositides at the organismal level in health and disease.

Activation of big mitogen-activated protein kinase-1 regulates smooth muscle cell replication

This study examined the activation of big mitogen-activated protein (MAP) kinase-1 (BMK1) in rat carotid smooth muscle cells (SMCs). Platelet-derived growth factor, fibroblast growth factor-2, sorbitol, and serum all increased the activation of BMK1 in rat carotid SMCs, whereas angiotensin II, phorbol esters, and tumor necrosis factor-alpha had only slight effects. With the exception of tumor necrosis factor-alpha, all these factors phosphorylated extracellular signal-regulated kinase (ERK)1/2. The MAPK kinase inhibitor (MEKI), U0126 (1 micromol/L), blocked ERK1/2 phosphorylation and at higher doses (5 micromol/L) blocked BMK1 phosphorylation. This inhibitor also blocked SMC DNA synthesis in a dose-dependent manner. When SMCs were transfected with an adenoviral construct expressing dominant mutant BMK1 and stimulated with fibroblast growth factor-2, a significantly smaller increase in cyclin D1 and cyclin A expression and in retinoblastoma factor phosphorylation was detected compared with the increase in cells transfected with an adenoviral construct expressing green fluorescent protein (GFP). SMC DNA synthesis was significantly blocked in the cells transfected with the dominant mutant BMK1. These data support the suggestion that BMK1 is important and necessary for mitogen-induced SMC proliferation.

Interferon-gamma inhibits hepatocyte growth factor-stimulated cell proliferation of human bronchial epithelial cells: upregulation of p27(kip1) cyclin-dependent kinase inhibitor

Proliferation of bronchial epithelial cells is an important biologic process in a variety of physiologic and pathologic conditions. In this study, we demonstrate that hepatocyte growth factor (HGF) stimulates proliferation of human bronchial epithelial cells obtained from healthy volunteers. The mitogenic effect of HGF is dependent on costimulation with serum and is completely abrogated by interferon-gamma (IFN-gamma). In the absence of serum, HGF is capable of inducing activation of extracellular signal-regulated kinases (ERK)1 and ERK2, but fails to stimulate proliferation by itself. These effects of HGF and IFN-gamma were reproduced faithfully in BEAS-2B cells, which are an immortalized cell line derived from human bronchial epithelial cells. Further, we investigated the molecular mechanisms underlying the effects of HGF and IFN-gamma in BEAS-2B cells and found that the MEK1 inhibitor PD98059, but not the p38 M-associated protein kinase inhibitor SB203580, abrogates HGF-induced ERK activation and proliferation in response to HGF and serum. In addition, LY294002, which is the specific inhibitor of phosphatidyl inositol 3-kinase, partially inhibited HGF- and serum-stimulated proliferation. We also found that HGF by itself is capable of inducing a G1 cyclin, cyclin D1, but fails to downregulate p27(kip1) cyclin-dependent kinase (CDK) inhibitor, which is a requisite for G1 to S phase cell cycle progression. IFN-gamma does not interfere with the effects of HGF on either ERK activation or cyclin D1 induction; however, it prevents the downregulation of p27(kip1) CDK inhibitor that takes place in response to a combination of HGF and serum. These results indicate that the MEK-ERK signaling pathway is necessary but not sufficient for human bronchial epithelial cell proliferation, and implicate the significance of HGF and IFN-gamma in the repair processes of injured human bronchial epithelial cells.

Cellular function of phosphoinositide 3-kinases: implications for development, homeostasis, and cancer

The phosphoinositide 3-kinase (PI3K) family of enzymes is recruited upon growth factor receptor activation and produces 3' phosphoinositide lipids. The lipid products of PI3K act as second messengers by binding to and activating diverse cellular target proteins. These events constitute the start of a complex signaling cascade, which ultimately results in the mediation of cellular activities such as proliferation, differentiation, chemotaxis, survival, trafficking, and glucose homeostasis. Therefore, PI3Ks play a central role in many cellular functions. The factors that determine which cellular function is mediated are complex and may be partly attributed to the diversity that exists at each level of the PI3K signaling cascade, such as the type of stimulus, the isoform of PI3K, or the nature of the second messenger lipids. Numerous studies have helped to elucidate some of the key factors that determine cell fate in the context of PI3K signaling. For example, the past two years has seen the publication of many transgenic and knockout mouse studies where either PI3K or its signaling components are deregulated. These models have helped to build a picture of the role of PI3K in physiology and indeed there have been a number of surprises. This review uses such models as a framework to build a profile of PI3K function within both the cell and the organism and focuses, in particular, on the role of PI3K in cell regulation, immunity, and development. The evidence for the role of deregulated PI3K signaling in diseases such as cancer and diabetes is reviewed.

Timing of cyclin D1 expression within G1 phase is controlled by Rho

The expression of cyclin D1 in mid-G1 phase is associated with sustained ERK activity, and we show here that Rho is required for the sustained ERK signal. However, we also report that Rho inhibits an alternative Rac/Cdc42-dependent pathway, which results in a strikingly early G1-phase expression of cyclin D1. Thus, cyclin D1 is induced in mid-G1 phase because a Rho switch couples its expression to sustained ERK activity rather than Rac and Cdc42. Our results show that Rho is crucial for maintaining the correct timing of cyclin D1 expression in G1 phase and describe a new role for cytoskeletal integrity in the regulation of cell cycle progression.

Phosphatidylinositol 3-kinase is necessary but not sufficient for thrombopoietin-induced proliferation in engineered Mpl-bearing cell lines as well as in primary megakaryocytic progenitors

Thrombopoietin and its receptor (Mpl) support survival and proliferation in megakaryocyte progenitors and in BaF3 cells engineered to stably express Mpl (BaF3/Mpl). The binding of thrombopoietin to Mpl activates multiple kinase pathways, including the Jak/STAT, Ras/Raf/MAPK, and phosphatidylinositol 3-kinase pathways, but it is not clear how these kinases promote cell cycling. Here, we show that thrombopoietin induces phosphatidylinositol 3-kinase and that phosphatidylinositol 3-kinase is required for thrombopoietin-induced cell cycling in BaF3/Mpl cells and in primary megakaryocyte progenitors. Treatment of BaF3/Mpl cells and megakaryocytes with the phosphatidylinositol 3-kinase inhibitor LY294002 inhibited mitotic and endomitotic cell cycl-ing. BaF3/Mpl cells treated with thrombopoietin and LY294002 were blocked in G(1), whereas megakaryocyte progenitors treated with thrombopoietin and LY294002 showed both a G(1) and a G(2) cell cycle block. Expression of constitutively active Akt in BaF3/Mpl cells restored the ability of thrombopoietin to promote cell cycling in the presence of LY294002. Constitutively active Akt was not sufficient to drive proliferation of BaF3/Mpl cells in the absence of thrombopoietin. We conclude that in BaF3/Mpl cells and megakaryocyte progenitors, thrombopoietin-induced phosphatidylinositol 3-kinase activity is necessary but not sufficient for thrombopoietin-induced cell cycle progression. Phosphatidylinositol 3-kinase activity is likely to be involved in regulating the G(1)/S transition.

Phosphatidylinositol 3-kinase potentiates, but does not trigger, T cell proliferation mediated by the IL-2 receptor

Proliferative signaling by the IL-2R can occur through two distinct pathways, one mediated by Stat5 and one by the adaptor protein Shc. Although Stat5 induces T cell proliferation by serving as a transcription factor, the mechanism of proliferative signaling by Shc is poorly defined. We examined the roles of two major signaling pathways downstream of Shc, the p44/p42 mitogen-activated protein kinase (extracellular signal-related kinase (Erk)) and phosphatidylinositol 3-kinase (PI3K) pathways, in promitogenic gene induction and proliferation in the IL-2-dependent T cell line CTLL-2. Using IL-2R mutants and specific pharmacologic inhibitors, we found that the PI3K, but not Erk, pathway is required for maximal induction of c-myc, cyclin D2, cyclin D3, cyclin E, and bcl-x(L) by Shc. To test whether the PI3K pathway is sufficient for proliferative signaling, a tamoxifen-regulated form of PI3K (mp110*ER) was expressed in CTLL-2 cells. Activation of the PI3K pathway through mp110*ER failed to up-regulate expression of the c-myc, cyclin D2, cyclin D3, cyclin E, bcl-2, or bcl-x(L) genes or down-regulate expression of p27(Kip1), even when coactivated with the Janus kinases (Jak) or the Raf/Erk pathway. Moreover, mp110*ER induced modest levels of thymidine incorporation without subsequent cell division. Although insufficient for mitogenesis, mp110*ER enhanced Stat5-mediated proliferative signaling through a mechanism independent of Stat5 transcriptional activity. Thus, in addition to serving a necessary, but insufficient role in Shc-mediated promitogenic gene expression, the PI3K pathway contributes to T cell proliferation by potentiating mitogenic signaling by Stat5.

PKB/Akt

The serine/threonine protein kinase PKB (also known as Akt) is thought to be a key mediator of signal transduction processes. The identification of PKB substrates and the role PKB phosphorylation plays in regulating these molecules have been a major focus of research in recent years. A recently developed motif-profile scoring algorithm that can be used to scan the genome for potential PKB substrates is therefore a useful tool, although additional considerations, such as the evolutionary conservation of the phosphorylation site, must also be taken into account. Recent evidence indicates that PKB plays a key role in cancer progression by stimulating cell proliferation and inhibiting apoptosis and is also probably a key mediator of insulin signalling. These findings indicate that PKB is likely to be a hot drug target for the treatment of cancer, diabetes and stroke. There are, however, a number of pitfalls of methodologies currently employed to study PKB function, and therefore caution should be used in interpretation of such experiments.

Integrins and cell proliferation

Cell cycle progression in mammalian cells is strictly regulated by both integrin-mediated adhesion to the extracellular matrix and by binding of growth factors to their receptors. This regulation is mediated by G1 phase cyclin-dependent kinases (CDKs), which are downstream of signaling pathways under the integrated control of both integrins and growth factor receptors. Recent advances demonstrate a surprisingly diverse array of integrin-dependent signals that are channeled into the regulation of the G1 phase CDKs. Regulation of cyclin D1 by the ERK pathway may provide a paradigm for understanding how cell adhesion can determine cell cycle progression.

Cyclic AMP-mediated inhibition of cell growth requires the small G protein Rap1

In many normal and transformed cell types, the intracellular second messenger cyclic AMP (cAMP) blocks the effects of growth factors and serum on mitogenesis, proliferation, and cell cycle progression. cAMP exerts these growth-inhibitory effects via inhibition of the mitogen-activated protein (MAP) kinase cascade. Here, using Hek293 and NIH 3T3 cells, we show that cAMP's inhibition of the MAP kinase cascade is mediated by the small G protein Rap1. Activation of Rap1 by cAMP induces the association of Rap1 with Raf-1 and limits Ras-dependent activation of ERK. In NIH 3T3 cells, Rap1 is required not only for cAMP's inhibition of ERK activation but for inhibition of cell proliferation and mitogenesis as well.

Suppression of apoptosis and granulocyte colony-stimulating factor-induced differentiation by an oncogenic form of Cbl

OBJECTIVE

The retroviral oncogene v-Cbl causes pre-B cell lymphomas and myeloid leukemias in mice, and its Drosophila homologue is oncogenic, causing enhanced receptor tyrosine kinase signaling. The human Cbl gene resides at 11q23. The aim of this study is to determine the effect of oncogenic Cbl on growth-regulating responses.

MATERIALS AND METHODS

The oncogenic mutant of Cbl (CblDelta1-357) was transfected into factor-dependent 32Dcl3 myeloid cells. Consequently, cell survival and differentiation were measured. Lyn, Syk, MAP kinase, and phosphatidylinositol 3'(PI3')-kinase activities, protein phosphorylation, Bcl-2 promoter activity, ubiquitination, and levels of Bcl-2, Bax, Bad, and Bcl-x(L) were determined. In addition, the effect of v-Cbl on TF-1 cell survival upon granulocyte-macrophage colony-stimulating factor withdrawal was studied.

RESULTS

32Dcl3 and TF-1 cells expressing v-Cbl showed resistance to apoptosis upon growth factor withdrawal, and 32Dcl3 cells completely failed to respond to granulocyte colony-stimulating factor's induction of differentiation. Basal activities of Lyn, Syk, and PI3'-kinase were elevated in the v-Cbl line. There was neither enhanced tyrosine phosphorylation of cellular protein content, Cbl, or Jak2, nor serine phosphorylation of MAP kinase or Akt. After factor withdrawal, the level of Bcl-2 was greater in v-Cbl cells than in control cells.

CONCLUSIONS

Neither increased Bcl-2 promoter activity nor decreased ubiquitination of Bcl-2 could account for increased Bcl-2 levels. v-Cbl-expressing 32Dcl3 cells were resistant to differentiation. v-Cbl suppresses apoptosis and differentiation, possibly through enhancement of Lyn, Syk, and PI3'-kinase activities and Bcl-2.

Regulation of cell cycle molecules by the Ras effector system

Eukaryotic cell cycle progression is driven by an ordered array of phosphorylation events that are specifically catalyzed by members of CDK (cyclin-dependent kinase) family serine/threonine protein kinases, each consisting of a catalytic subunit CDK and a positive regulatory subunit cyclin. In mammalian somatic cells extracellular cues act mainly during the G1 phase to regulate the activity of D type cyclin-dependent CDKs, which, in turn, serve as key regulators of G1--S phase progression by phosphorylating and functionally inactivating the tumor suppressor retinoblastoma (Rb) protein. The small molecular weight G protein Ras has been implicated as a crucial molecule that transduces extracellular growth stimuli into intracellular signals. Recent studies, including our own, have demonstrated that maintained cellular Ras activity is required until late in the G1 phase for inactivation of the Rb protein and the G1/S transition and mediates both upregulation of cyclin D1 and downregulation of p27kip1 CDK inhibitor.

Coordinate signaling by integrins and receptor tyrosine kinases in the regulation of G1 phase cell-cycle progression

Cell proliferation is dependent upon the activation of receptor tyrosine kinases and integrins by soluble growth factors and extracellular matrix proteins, respectively. It is now apparent that concerted, rather than individual, signaling by these receptors is the critical feature responsible for cell-cycle progression through G1 phase. ERK (extracellular signal-regulated kinase), Rho GTPases and G1-phase cyclin-dependent kinases are all regulated jointly by growth-factor receptors and integrins. Recent studies have begun to reveal how this regulated signaling in the cytoplasm is linked to activation of the G1-phase cyclin-dependent kinases in the nucleus.

Molecular pathways of cyclic nucleotide-induced inhibition of arterial smooth muscle cell proliferation

Cyclic adenosine 3',5'-monophosphate (cAMP) and cyclic guanosine 3',5'-monophosphate (cGMP) are second messengers involved in the intracellular signal transduction of a wide variety of extracellular stimuli. These signals regulate many biological processes including cell proliferation, differentiation, migration, and apoptosis. Recently, significant progress has been achieved in the molecular basis underlying cyclic nucleotide regulation of cell proliferation. This review summarizes our knowledge of the signaling pathways regulated by cyclic nucleotides in arterial smooth muscle cells.

Overexpression of E2F-1 leads to cytokine-independent proliferation and survival in the hematopoietic cell line BaF-B03

Cytokine receptors activate signals that regulate the transcription factor E2F-1, which then coordinates the expression of genes essential for DNA synthesis and cell cycle progression. Overexpression of E2F-1 most often induces S-phase entry followed by apoptosis, but in some cell types it leads to continuous proliferation and transformation. Here, it is shown that constitutive expression of E2F-1 promotes cytokine-independent proliferation in the murine pro-B cell line BaF-B03. There was no enhancement of apoptosis following cytokine withdrawal in these cells, despite the presence of intact p53-dependent apoptotic pathways. Notwithstanding the continuous presence of E2F-1, the cell cycle-dependent expression of cyclin A, cyclin B1, cyclin D1, cyclin E, and proliferating-cell nuclear antigen was restored with a pattern equivalent to that associated with cytokine stimulation. These findings provide evidence that, in the absence of cytokine, constitutive expression of E2F-1 can promote cell cycle progression and prevent apoptosis.

Pike. A nuclear gtpase that enhances PI3kinase activity and is regulated by protein 4.1N

While cytoplasmic PI3Kinase (PI3K) is well characterized, regulation of nuclear PI3K has been obscure. A novel protein, PIKE (PI3Kinase Enhancer), interacts with nuclear PI3K to stimulate its lipid kinase activity. PIKE encodes a 753 amino acid nuclear GTPase. Dominant-negative PIKE prevents the NGF enhancement of PI3K and upregulation of cyclin D1. NGF treatment also leads to PIKE interactions with 4.1N, which has translocated to the nucleus, fitting with the initial identification of PIKE based on its binding 4.1N in a yeast two-hybrid screen. Overexpression of 4.1N abolishes PIKE effects on PI3K. Activation of nuclear PI3K by PIKE is inhibited by the NGF-stimulated 4.1N translocation to the nucleus. Thus, PIKE physiologically modulates the activation by NGF of nuclear PI3K.

Enhanced cell killing by overexpression of dominant-negative phosphatidylinositol 3-kinase subunit, Deltap85, following genotoxic stresses

Phosphatidylinositol 3-kinase (PI3-K) is a heterodimer of a regulatory subunit, p85, and a catalytic subunit, p110. A number of previous reports showed that PI3-K functions in diverse cellular phenomena such as cell proliferation, glucose catabolism, cell adhesion, and vesicle transport. It is also well known that a survival signal from the receptor tyrosine kinases utilizes Akt via PI3-K to protect cells from apoptosis. To examine the role of PI3-K in cellular sensitivity to genotoxic stresses such as cisplatin and ultraviolet (UV), we introduced deletion type p85 (Dp85) into two human glioblastoma cell lines (T98G and A172) and one melanoma cell line (G361). The Deltap85 works in a dominant-negative fashion on PI3-K activity by disrupting its p85 / p110 interaction. In all three transfected cell lines, the overexpression of Deltap85 rendered the cells markedly more sensitive to these DNA-damaging stresses than the cells transfected with the vector alone. Apoptosis was vigorously induced in cells overexpressing Dp85 following the treatment. The present results imply that PI3-K plays a critical role in determining cellular sensitivity to genotoxic stresses in human cancer cells, and that disruption of the p85 / p110 interaction of PI3-K may be a potential molecular target for developing a novel strategy for cancer treatment.

Regulation of G(1) cyclin-dependent kinases in the mammalian cell cycle

Cyclin-dependent kinases are the key regulators of cell-cycle transitions. In mammalian cells, Cdk2, Cdk4, Cdk6 and associated cyclins control the G(1) to S phase transition. Because proper regulation of this transition is critical for an organism's survival, these protein kinases are exquisitely regulated at different mechanistic levels and in response to a large variety of intrinsic and extrinsic signals.

Inhibitory regulation of Rac activation, membrane ruffling, and cell migration by the G protein-coupled sphingosine-1-phosphate receptor EDG5 but not EDG1 or EDG3

Sphingosine-1-phosphate (S1P) is a bioactive lysophospholipid that induces a variety of biological responses in diverse cell types. Many, if not all, of these responses are mediated by members of the EDG (endothelial differentiation gene) family G protein-coupled receptors EDG1, EDG3, and EDG5 (AGR16). Among prominent activities of S1P is the regulation of cell motility; S1P stimulates or inhibits cell motility depending on cell types. In the present study, we provide evidence for EDG subtype-specific, contrasting regulation of cell motility and cellular Rac activity. In CHO cells expressing EDG1 or EDG3 (EDG1 cells or EDG3 cells, respectively) S1P as well as insulin-like growth factor I (IGF I) induced chemotaxis and membrane ruffling in phosphoinositide (PI) 3-kinase- and Rac-dependent manners. Both S1P and IGF I induced a biphasic increase in the amount of the GTP-bound active form of Rac. In CHO cells expressing EDG5 (EDG5 cells), IGF I similarly stimulated cell migration; however, in contrast to what was found for EDG1 and EDG3 cells, S1P did not stimulate migration but totally abolished IGF I-directed chemotaxis and membrane ruffling, in a manner dependent on a concentration gradient of S1P. In EDG5 cells, S1P stimulated PI 3-kinase activity as it did in EDG1 cells but inhibited the basal Rac activity and totally abolished IGF I-induced Rac activation, which involved stimulation of Rac-GTPase-activating protein activity rather than inhibition of Rac-guanine nucleotide exchange activity. S1P induced comparable increases in the amounts of GTP-RhoA in EDG3 and EDG5 cells. Neither S1P nor IGF I increased the amount of GTP-bound Cdc42. However, expression of N(17)-Cdc42, but not N(19)-RhoA, suppressed S1P- and IGF I-directed chemotaxis, suggesting a requirement for basal Cdc42 activity for chemotaxis. Taken together, the present results demonstrate that EDG5 is the first example of a hitherto-unrecognized type of receptors that negatively regulate Rac activity, thereby inhibiting cell migration and membrane ruffling.

Phosphatidylinositol 3-kinase signaling is important for smooth muscle cell replication after arterial injury

The phosphoinositide 3-kinase [PI(3)K] pathway is a key signaling pathway important for replication of mammalian cells. In this study, we examined the role of PI(3)K in smooth muscle cell (SMC) replication after balloon catheter injury of rat carotid arteries. Protein kinase B (PKB), a downstream target of PI(3)K, was phosphorylated at 30 and 60 minutes after injury and to a lesser degree after 6 hours and 1 and 2 days but not after 7 days. Wortmannin (10 microgram per rat), a PI(3)K inhibitor, given to rats 60 and 5 minutes before and 11 hours after balloon injury, reduced the levels of phosphorylated PKB. SMC replication quantified between 24 to 48 hours was significantly reduced compared with control replication, as were the levels of cyclin D(1). Wortmannin was also administered to rats between days 7 and 8 and between days 7 and 9 after balloon catheter injury. A reduction in levels of phosphorylated PKB was detected, but no decrease in the replication of intimal SMCs was observed in either experiment. These data demonstrate that the PI(3)K signal transduction pathway plays an important role in medial but not intimal SMC replication.

An inhibitor of PI3-K differentially affects proliferation and IL-6 protein secretion in normal and leukemic myeloid cells depending on the stage of differentiation

In this study, we examined the involvement of the phosphatidylinositol 3-kinase (PI3-K) and p70S6 kinase signal transduction pathway in the interleukin-1(IL-1)-mediated proliferation and cytokine production by normal and leukemic myeloid cells. Total AML blast populations, early progenitor (CD34(+)/CD36(-)) cells, and more differentiated (CD34(-)/CD36(+)) cells were treated with the PI3-K inhibitor Ly294002 and p70S6K inhibitor rapamycin. The effects on proliferation, IL-6 protein secretion, and intracellular signaling cascades were determined and compared with normal CD34(+) cells and monocytes. The function of the PI3-K pathway was dependent on the differentiation state of the AML cell population. In immature blasts, the IL-1-induced proliferation was strongly inhibited by Ly294002 and rapamycin, without a distinct effect on IL-6 protein production. In contrast, in mature monocytic blast cells inhibition of the PI3-K signaling route had a stimulatory effect on IL-6 protein secretion. Interestingly, these findings were not specifically linked to the malignant counterpart but were also observed with normal CD34(+) sorted cells vs mature monocytes. Evidence is provided that the Ly294002-induced increase in IL-6 protein secretion is linked to the cAMP dependent signaling pathway and not to changes in the phosphorylation of ERK or p38. However, although the enhanced IL-6 protein secretion is cAMP dependent, it was not found to be mediated by protein kinase A (PKA) or by the GTP-ase Rap1. This study indicates that inhibition of the PI3-K signaling pathway has an inhibitory effect on cell proliferation but a stimulatory effect on IL-6 expression mediated by a cAMP-dependent but PKA-independent route.

Role of phosphoinositide 3-kinase and endocytosis in nerve growth factor-induced extracellular signal-regulated kinase activation via Ras and Rap1

Neurotrophins promote multiple actions on neuronal cells including cell survival and differentiation. The best-studied neurotrophin, nerve growth factor (NGF), is a major survival factor in sympathetic and sensory neurons and promotes differentiation in a well-studied model system, PC12 cells. To mediate these actions, NGF binds to the TrkA receptor to trigger intracellular signaling cascades. Two kinases whose activities mediate these processes include the mitogen-activated protein (MAP) kinase (or extracellular signal-regulated kinase [ERK]) and phosphoinositide 3-kinase (PI3-K). To examine potential interactions between the ERK and PI3-K pathways, we studied the requirement of PI3-K for NGF activation of the ERK signaling cascade in dorsal root ganglion cells and PC12 cells. We show that PI3-K is required for TrkA internalization and participates in NGF signaling to ERKs via distinct actions on the small G proteins Ras and Rap1. In PC12 cells, NGF activates Ras and Rap1 to elicit the rapid and sustained activation of ERKs respectively. We show here that Rap1 activation requires both TrkA internalization and PI3-K, whereas Ras activation requires neither TrkA internalization nor PI3-K. Both inhibitors of PI3-K and inhibitors of endocytosis prevent GTP loading of Rap1 and block sustained ERK activation by NGF. PI3-K and endocytosis may also regulate ERK signaling at a second site downstream of Ras, since both rapid ERK activation and the Ras-dependent activation of the MAP kinase kinase kinase B-Raf are blocked by inhibition of either PI3-K or endocytosis. The results of this study suggest that PI3-K may be required for the signals initiated by TrkA internalization and demonstrate that specific endocytic events may distinguish ERK signaling via Rap1 and Ras.

Akt/protein kinase B isoforms are differentially regulated by epidermal growth factor stimulation

Overexpression of epidermal growth factor receptor (EGFR) in certain cancers is well established. There is growing evidence that epidermal growth factor (EGF) activates Akt/protein kinase B (PKB) in a phosphoinositide 3-OH kinase (PI3K)-dependent manner, but it is not yet clear which Akt isoforms are involved in this signal transduction pathway. We investigated the functional regulation of three Akt isoforms, Akt1/PKBalpha, Akt2/PKBbeta, and Akt3/PKBgamma, in esophageal cancer cells where EGFR is frequently overexpressed. Upon EGF simulation, phosphorylation of Akt1 at the Ser-473 residue was remarkably induced. This result was corroborated by in vitro Akt kinase assays using glycogen synthase kinase 3beta as the substrate. PI3K inhibitors, wortmannin or LY294002, significantly blocked the Akt kinase activity induced by EGF. Akt2 activity was evaluated by electrophoretic mobility shift assays. Robust activation of Akt2 by EGF was observed in some cell lines in a PI3K-dependent manner. EGF-induced Akt3 activation was demonstrated by Ser-472 phosphorylation of Akt3 but in a restrictive fashion. In aggregate, EGF-mediated activation of Akt isoforms is overlapping and distinctive. The mechanism by which EGFR recruits the PI3K/Akt pathway was in part differentially regulated at the level of Ras but independent of heterodimerization of EGFR with either ErbB2 or ErbB3 based upon functional dissection of pathways in esophageal cancer cell lines.

15-Deoxy-Delta(12,14)-prostaglandin J(2) induces G(1) arrest and differentiation marker expression in vascular smooth muscle cells

In search of substances useful for the treatment of atherosclerotic vascular diseases, we studied the effects of 15-deoxy-Delta(12, 14)-prostaglandin J(2) (15d-PGJ(2)), a natural ligand for peroxisome proliferator-activated receptor gamma, on the proliferation and differentiation of vascular smooth muscle cells (VSMCs). 15d-PGJ(2) but not WY14643, an agonist for peroxisome proliferator-activated receptor alpha, dose-dependently inhibited VSMC proliferation; the effect was maximal at 12 microM. This compound strongly suppressed the activities of cyclin-dependent kinases (Cdk) 4, 6, and 2, thereby preventing the phosphorylation of the retinoblastoma protein. These Cdks seemed to be inhibited through two mechanisms: the down-regulation of cyclin D1 and the up-regulation of Cdk inhibitor p21(Cip1/Waf1/Sdi1). 15d-PGJ(2) was found to inhibit the phosphatidylinositol 3-kinase/protein kinase B signaling pathway, which mediates cyclin D1 expression. Mitogenic stimulation of quiescent cells decreased the level of mRNA for the smooth muscle-specific myosin heavy-chain SM1, whereas this reduction was prevented by 15d-PGJ(2). A long-term treatment of exponentially growing VSMCs with 15d-PGJ(2) markedly elevated the mRNA level of SM1 and, moreover, induced SM2, another isoform expressed exclusively in mature VSMCs. 15d-PGJ(2) also increased the expression levels of calponin-h1 and smooth muscle alpha-actin. These results suggest that 15d-PGJ(2) induces G(1) arrest by two distinct mechanisms and promotes VSMC differentiation.

Regulation of cyclin D(1) expression and DNA synthesis by phosphatidylinositol 3-kinase in airway smooth muscle cells

We have shown in bovine tracheal myocytes that extracellular signal-regulated kinase (ERK) and Rac1 function as upstream activators of transcription from the cyclin D(1) promoter. We now examine the role of phosphatidylinositol (PI) 3-kinase in this process. PI 3-kinase activity was increased by platelet-derived growth factor (PDGF) and attenuated by the PI 3-kinase inhibitors wortmannin and LY294002. These inhibitors also decreased cyclin D(1) promoter activity, protein abundance, and DNA synthesis. Overexpression of the active catalytic subunit of PI 3-kinase (p110(PI) (3-K)CAAX) was sufficient to activate the cyclin D(1) promoter. Wortmannin and LY294002 failed to attenuate PDGF-induced ERK activation, and overexpression of p110(PI) (3-K)CAAX was insufficient to activate ERK. p110(PI) (3-K)CAAX-induced cyclin D(1) promoter activity was not blocked by PD98059, an inhibitor of mitogen-activated protein kinase/ERK kinase. We next examined whether PI 3-kinase and the 21-kD guanidine triphosphatase Rac1 regulate cyclin D(1) promoter activity by similar mechanisms. p110(PI) (3-K)CAAX-induced cyclin D(1) promoter activity was decreased by two inhibitors of Rac1-mediated signaling, catalase and diphenylene iodonium. Further, PDGF, PI 3-kinase, and Rac1 each activated the cyclin D(1) promoter at the cyclic adenosine monophosphate response element binding protein (CREB)/activating transcription factor (ATF)-2 binding site, as evidenced by expression of a CREB/ATF-2 reporter plasmid. Finally, PI 3-kinase and Rac1-induced CREB/ATF-2 transactivation were each inhibited by catalase. Together, these data suggest that in airway smooth muscle (ASM) cells, PI 3-kinase regulates transcription from the cyclin D(1) promoter and DNA synthesis in an ERK-independent manner. Further, PI 3-kinase and Rac1 regulate ASM cell cycle traversal via a common cis-regulatory element in the cyclin D(1) promoter.

SRF-dependent gene expression is required for PI3-kinase-regulated cell proliferation

Recent evidence indicates that phosphatidylinositol 3-kinase (PI3K) is a central regulator of mitosis, apoptosis and oncogenesis. Nevertheless, the mechanisms by which PI3K regulates proliferation are not well characterized. Mitogens stimulate entry into the cell cycle by inducing the expression of immediate early genes (IEGs) that in turn trigger the expression of G(1) cyclins. Here we describe a novel PI3K- regulated transcriptional cascade that is critical for mitogen regulation of the IEG, c-fos. We show that PI3K activates gene expression by transactivating SRF-dependent transcription independently of the previously described Rho and ETS TCF pathways. PI3K-stimulated cell cycle progression requires transactivation of SRF and expression of dominant- negative PI3K blocks mitogen-stimulated cell cycle progression. Furthermore, dominant-interfering SRF mutants attenuate mitogen-stimulated cell cycle progression, but are without effect on MEK-stimulated cell cycle entry. Moreover, expression of constitutively active SRF is sufficient for cell cycle entry. Thus, we delineate a novel SRF-dependent mitogenic cascade that is critical for PI3K- and growth factor-mediated cell cycle progression.

Integrating the MAP kinase signal into the G1 phase cell cycle machinery

Growth factors and the extracellular matrix provide the environmental cues that control the proliferation of most cell types. The binding of growth factors and matrix proteins to receptor tyrosine kinases and integrins, respectively, regulates several cytoplasmic signal transduction cascades, among which activation of the mitogen-activated protein kinase cascade, ras --> Raf --> MEK --> ERK, is perhaps the best characterized. Curiously, ERK activation has been associated with both stimulation and inhibition of cell proliferation. In this review, we summarize recent studies that connect ERK signaling to G1 phase cell cycle control and suggest that the cellular response to an ERK signal depends on both ERK signal intensity and duration. We also discuss studies showing that receptor tyrosine kinases and integrins differentially regulate the ERK signal in G1 phase.

Stem cell factor/c-kit up-regulates cyclin D3 and promotes cell cycle progression via the phosphoinositide 3-kinase/p70 S6 kinase pathway in spermatogonia

Stem cell factor (SCF)/c-kit plays an important role in the regulation of hematopoiesis, melanogenesis, and spermatogenesis. In the testis, the SCF/c-kit system is believed to regulate germ cell proliferation, meiosis, and apoptosis. Studies with type A spermatogonia in vivo and in vitro have indicated that SCF induces DNA synthesis and proliferation. However, the signaling pathway for this function of SCF/c-kit has not been elucidated. We now demonstrate that SCF activates phosphoinositide 3-kinase (PI3-K) and p70 S6 kinase (p70S6K) and that rapamycin, a FRAP/mammalian target of rapamycin-dependent inhibitor of p70S6K, completely inhibited bromodeoxyuridine incorporation induced by SCF in primary cultures of spermatogonia. SCF induced cyclin D3 expression and phosphorylation of the retinoblastoma protein through a pathway that is sensitive to both wortmannin and rapamycin. Furthermore, AKT, but not protein kinase C-zeta, is used by SCF/c-kit/PI3-K to activate p70S6K. Dominant negative AKT-K179M completely abolished p70S6K phosphorylation induced by the constitutively active PI3-K catalytic subunit p110. Constitutively active v-AKT highly phosphorylated p70S6K, which was totally inhibited by rapamycin. Thus, SCF/c-kit uses a rapamycin-sensitive PI3-K/AKT/p70S6K/cyclin D3 pathway to promote spermatogonial cell proliferation.

Cellular function of p70S6K: a role in regulating cell motility

The 70 kDa ribosomal S6 kinase (p70S6K) is activated by numerous mitogens, growth factors and hormones. Activation of p70S6K occurs through phosphorylation at a number of sites and the primary target of the activated kinase is the 40S ribosomal protein S6, a major component of the machinery involved in protein synthesis in mammalian cells. In addition to its involvement in regulating translation, p70S6K activation has been implicated in cell cycle control and neuronal cell differentiation. Recent data obtained in this laboratory suggests that p70S6K may also function in regulating cell motility, a cellular response that is important in tumour metastases, the immune response and tissue repair. The present paper reviews the regulation and cellular function of p70S6K and proposes a novel function of p70S6K in regulating cell motility.

Activation of phosphatidylinositol 3-kinase is required for transcriptional activity of F-type 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase: assessment of the role of protein kinase B and p70 S6 kinase

Previous studies have demonstrated that the F isoform of<hsp sp=0.5>6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase(6PF2K/Fru-2,6-BPase) is transcriptionally regulated by growth factors. The aim of this study was to investigate the importance of the phosphatidylinositol 3-kinase (PI 3-kinase) pathway in the regulation of 6PF2K/Fru-2,6-BPase gene expression. We have completed studies using chemical inhibitors and expression vectors for the proteins involved in this signalling cascade. Treatment of cells with LY 294002, an inhibitor of PI 3-kinase, blocked the epidermal growth factor (EGF)-dependent stimulation of 6PF2K/Fru-2,6-BPase gene transcription. Transient transfection of a constitutively active PI 3-kinase was sufficient to activate transcription from the F-type 6PF2K/Fru-2,6-BPase promoter. In contrast, co-transfection with a dominant-negative form of PI 3-kinase completely abrogated the stimulation by EGF, and down-regulated the basal promoter activity. In an attempt to determine downstream proteins that lie between PI 3-kinase and 6PF2K/Fru-2,6-BPase gene expression, the overexpression of a constitutively active form of protein kinase B (PKB) was sufficient to activate 6PF2K/Fru-2,6-BPase gene expression, even in the presence of either a dominant-negative form of PI 3-kinase or LY 294002. The over-expression of p70/p85 ribosomal S6 kinase or the treatment with its inhibitor rapamycin did not affect 6PF2K/Fru-2,6-BPase transcription. We conclude that PI 3-kinase is necessary for the transcriptional activity of F-type 6PF2K/Fru-2,6-BPase, and that PKB is a downstream effector of PI 3-kinase directly involved in the regulation of 6PF2K/Fru-2,6-BPase gene expression.

Phosphatidylinositol 3-kinase activity regulates alpha -thrombin-stimulated G1 progression by its effect on cyclin D1 expression and cyclin-dependent kinase 4 activity

In this study, we present evidence that PI 3-kinase is required for alpha-thrombin-stimulated DNA synthesis in Chinese hamster embryonic fibroblasts (IIC9 cells). Previous results from our laboratory demonstrate that the mitogen-activated protein kinase (extracellular signal-regulated kinase (ERK)) pathway controls transit through G(1) phase of the cell cycle by regulating the induction of cyclin D1 mRNA levels and cyclin dependent kinase 4 (CDK4)-cyclin D1 activity. In IIC9 cells, PI 3-kinase activation also is an important controller of the expression of cyclin D1 protein and CDK4-cyclin D1 activity. Pretreatment of IIC9 cells with the selective PI 3-kinase inhibitor, LY294002 blocks the alpha-thrombin-stimulated increase in cyclin D1 protein and CDK4 activity. However, LY294002 does not affect alpha-thrombin-induced cyclin D1 steady state message levels, indicating that PI 3-kinase acts independent of the ERK pathway. Interestingly, expression of a dominant-negative Ras significantly decreased both alpha-thrombin-stimulated ERK and PI 3-kinase activities. These data clearly demonstrate that the alpha-thrombin-induced Ras activation coordinately regulates ERK and PI 3-kinase activities, both of which are required for expression of cyclin D1 protein and progression through G(1).

Reversible G(1) arrest induced by inhibition of the epidermal growth factor receptor tyrosine kinase requires up-regulation of p27(KIP1) independent of MAPK activity

We have used quinazoline inhibitors of the epidermal growth factor receptor (EGFR) tyrosine kinase to study the link between EGFR signaling and G(1) to S traverse. Treatment of A431 and MDA-468 human tumor cells with 0.1-10 microM AG-1478 inhibited basal and ligand-stimulated EGFR phosphorylation without a decrease in receptor content, EGF-binding sites, or binding affinity. Incubation of A431 cells with 0.1-1 microM AG-1517 abrogated (125)I-EGF internalization. Both AG-1478 and AG-1517 markedly inhibited A431 and MDA-468 colony formation in soft agarose at concentrations between 0.01 and 1 microM. Daily injections of AG-1478 at 50 mg/kg delayed A431 tumor formation in athymic nude mice. A transient exposure of A431 cells to AG-1478 resulted in a dose-dependent up-regulation of the cyclin-dependent kinase inhibitor p27, down-regulation of cyclin D1 and of active MAPK, and hypophosphorylation of the retinoblastoma protein (Rb). These changes were temporally associated with recruitment of tumor cells in G(1) phase and a marked reduction of the proportion of cells in S phase. Upon removal of the kinase inhibitor, EGFR and Rb phosphorylation and the levels of cyclin D1 protein were quickly restored, but the cells did not reenter S phase until p27 protein levels were decreased. Phosphorothioate p27 oligonucleotides decreased p27 protein in A431 cells and abrogated the quinazoline-mediated G(1) arrest. Treatment of A431 cells with PD 098509, a synthetic inhibitor of MEK1, inhibited MAPK activity without inducing G(1) arrest or increasing the levels of p27. However, treatment with LY 294002, an inhibitor of phosphatidylinositol 3-kinase (PI3K), inhibited basal Akt activity, up-regulated p27, and recruited cells in G(1). These data suggest that p27 is required for the growth arrest that follows interruption of the EGFR kinase in receptor-overexpressing cells. In addition, the G(1) arrest and up-regulation of p27 resulting from EGFR blockade are not due to the interruption of MAPK, but to the interruption of constitutively active PI3K function.

Integration of positive and negative growth signals during ras pathway activation in vivo

Expression of RAS proteins can have either positive or negative effects on cell growth, differentiation and death. New technologies are being developed for the generation of animal models to address the questions of where, when and how much Ras is expressed during tumorigenesis, and how these disparate signals are integrated during multistage carcinogenesis.

Differentiation-inducing factor-1, a morphogen of dictyostelium, induces G(1) arrest and differentiation of vascular smooth muscle cells

Differentiation-inducing factor-1 (DIF-1) is a morphogen that induces differentiation of DICTYOSTELIUM: Recently, DIF-1 has been shown to inhibit proliferation and induce differentiation in tumor cells, although the underlying mechanisms remain unknown. In this study, we examined the effects of DIF-1 on the proliferation and differentiation of vascular smooth muscle cells, to explore novel therapeutic strategies for atherosclerosis. DIF-1 nearly completely inhibited DNA synthesis and cell division in mitogen-stimulated cells. DIF-1 inhibited the phosphorylation of the retinoblastoma protein and the activities of cyclin-dependent kinase (Cdk) 4, Cdk6, and Cdk2, which phosphorylate the retinoblastoma protein. DIF-1 strongly suppressed the expression of cyclins D1, D2, and D3, as well as those of cyclins E and A, which normally began after that of the D-type cyclins. The mRNAs for the smooth muscle myosin heavy chains SM1 and SM2 were expressed in quiescent cells in primary culture, and these expression levels decreased after mitogenic stimulation. In the presence of DIF-1, the rate of the reduction was significantly decelerated. Moreover, the addition of DIF-1 to dedifferentiated cells induced the expressions of SM1 and SM2, accompanied by a reduction in the level of SMemb, a nonmuscle-type myosin heavy chain. Therefore, DIF-1 seemed to interrupt a very early stage of G(1) probably by suppressing the expressions of the D-type cyclins. Furthermore, this compound may prevent phenotypic modulation and induce differentiation of vascular smooth muscle cells.

Analysis of prolactin-modulated gene expression profiles during the Nb2 cell cycle using differential screening techniques

BACKGROUND

Rat Nb2-11C lymphoma cells are dependent on prolactin for proliferation and are widely used to study prolactin signaling pathways. To investigate the role of this hormone in the transcriptional mechanisms that underlie prolactin-stimulated mitogenesis, five different techniques were used to isolate differentially expressed transcripts: mRNA differential display, representational difference analysis (RDA), subtractive suppressive hybridization (SSH), analysis of weakly expressed candidate genes, and differential screening of an organized library.

RESULTS

About 70 transcripts were found to be modulated in Nb2 cells following prolactin treatment. Of these, approximately 20 represent unknown genes. All cDNAs were characterized by northern blot analysis and categorized on the basis of their expression profiles and the functions of the known genes. We compared our data with other cell-cycle-regulated transcripts and found several new potential signaling molecules that may be involved in Nb2 cell growth. In addition, abnormalities in the expression patterns of several transcripts were detected in Nb2 cells, including the constitutive expression of the immediate-early gene EGR-1. Finally, we compared the differential screening techniques in terms of sensitivity, efficiency and occurrence of false positives.

CONCLUSIONS

Using these techniques to determine which genes are differentially expressed in Nb2 lymphoma cells, we have obtained valuable insight into the potential functions of some of these genes in the cell cycle. Although this information is preliminary, comparison with other eukaryotic models of cell-cycle progression enables identification of expression abnormalities and proteins potentially involved in signal transduction, which could indicate new directions for research.

Inhibition of phosphoinositide 3-kinase impairs pre-commitment cell cycle traverse and prevents differentiation in erythroleukaemia cells

During the early hours after exposure to differentiation inducing agents, Friend erythroleukaemia cells undergo alterations which commit them to cessation of growth and development of the characteristics of differentiation. Our current experiments have compared the expression and activity of phosphoinositide 3-kinase (PI 3-kinase) in control cells with cells undergoing differentiation which has been induced by dimethyl sulfoxide (DMSO). When the cultures were initiated with stationary phase cells and DMSO was added at the time of seeding, PI 3-kinase activity was stimulated in both treated and control cells during the first 3 h from seeding. This event appears to be a rate limiting step in commitment since pretreatment of cells with 10 microM LY294002 or down-regulation of p85 expression prior to adding DMSO completely prevents commitment to erythropoiesis. Accordingly, PI 3-kinase inhibition during the commitment period prevents DNA-binding of the transcription factor GATA-1, essential for erythroid differentiation. However, once cells are committed to differentiate, PI 3-kinase activity and expression dramatically decreases along with the differentiation programme, to become barely detectable after 96 h. Remarkably, LY294002 treatment leads to accumulation of cell in G1 phase and prevents DMSO-dependent cyclin D3 induction. Based on these data, we suggest that PI 3-kinase is rate limiting for the completion of the first round cycle of cell division required for initiation of erythrocytic differentiation. On the other hand, the late decrease of PI 3-kinase associated with the differentiation process seems to be part of the programmed shut off of genes not needed in mature erythrocytes.

PTEN tumour suppressor is linked to the cell cycle control through the retinoblastoma protein

The tumour suppressor PTEN, also named MMAC1 or TEP1, is associated with a number of malignancies in human populations. This protein has a dual protein phosphatase activity, being also capable to dephosphorylate phosphatidylinositol 3,4,5 triphosphate. We have studied the mechanism of growth suppression attributable to PTEN. We observed that PTEN overexpression inhibits cell growth in a variety of normal and transformed, human and murine cells. Bromodeoxyuridine (BrdU) incorporation and TUNEL labelling experiments in transiently transfected cells demonstrate that this inhibition is due to a cell cycle arrest rather than induction of apoptosis. Given that PTEN is unable to cause cell growth arrest in retinoblastoma (Rb)-deficient cell lines, we have explored the possible requirement for pRb in the PTEN-induced inhibition of cell proliferation. We found that the co-expression of SV40 antigen, but not a mutant form (which binds exclusively to p53), and cyclin D1/cdk4 are able to overcome the PTEN-mediated growth suppression. In addition, the reintroduction of a functional pRb, but not its relatives p107 or p130, in Rb-deficient cells restores the sensitivity to PTEN-induced arrest. Finally, the hyperphosphorylation of transfected pRb is inhibited by PTEN co-expression and restored by PI-3K co-expression. Accordingly, PTEN gene is mostly expressed, in parallel to Akt, in mid-late G1 phase during cell cycle progression prior to pRb hyperphosphorylation. Finally, we have studied the signal transduction pathways modulated by PTEN expression. We found that PTEN-induced growth arrest can be rescued by the co-expression of active PI-3K and downstream effectors such as Akt or PDK1, and also certain small GTPases such as Rac1 and Cdc42, but not by active Ha-ras, raf or RhoA. Collectively, our data link the tumour suppressor activities of PTEN to the machinery controlling cell cycle through the modulation of signalling molecules whose final target is the functional inactivation of the retinoblastoma gene product.