Tyrosine phosphorylation of the p85 subunit of phosphatidylinositol 3-kinase correlates with high proliferation rates in sublines derived from the Jurkat leukemia

Activation of PI3K/Akt pathway by G protein-coupled receptor 37 promotes resistance to cisplatin-induced apoptosis in non-small cell lung cancer

OBJECTIVES

Lung cancer is a major public health concern and represents the most common cause of cancer-related death worldwide. Among eukaryotes, the G protein-coupled receptor (GPCR) family stands as the largest group of membrane proteins. Alterations in GPCR gene expression and dysregulation of signal transduction have been recognized as the markers of malignancy. As a member of the GPCR family, G protein-coupled receptor 37 (GPR37) exhibits unknown functions in tumors, particularly in non-small-cell lung cancer (NSCLC) METHODS: We explored the expression and prognosis of GPR37 in NSCLC through TCGA, GTEx, GEO, and GEPIA2. We detected the expression of GPR37 in NSCLC tissues and cell lines. The study explored the influence of GPR37 on tumor cell proliferation. Furthermore, we examined the effects of GPR37 on tumor cell apoptosis and invasion. Most importantly, we investigated whether GPR37 affects cisplatin-induced drug resistance in NSCLC. Furthermore, by conducting animal experiments, we assessed the impact of GPR37 on NSCLC and delved into underlying mechanisms.

RESULTS

(1) In NSCLC, the expression of GPR37 is markedly higher than that in corresponding normal tissues. We found that elevated GPR37 expression predicts an unfavorable prognosis. (2) It was demonstrated that GPR37 positively regulates NSCLC cell invasion, migration, and proliferation, suppresses cell apoptosis, heightens resistance to cisplatin, and promotes tumor formation and growth. Conversely, we observed that GPR37 knockdown suppresses NSCLC cell invasion, migration, and proliferation, promotes cell apoptosis, increases sensitivity to cisplatin, and affects tumor formation and growth. (3) GPR37 activates PI3K/Akt/mTOR signal transduction pathways to mediate epithelial-mesenchymal transition (EMT), thereby promoting the progression of NSCLC.

CONCLUSIONS

It was suggested that GPR37 acts a crucial role in promoting the occurrence and development of NSCLC. Knockdown of GPR37 significantly inhibits the occurrence and development of NSCLC. Therefore, our findings demonstrated that GPR37 may represent a viable therapeutic target for NSCLC.

FAK-mediated phosphorylation at Y464 regulates p85β nuclear translocation to promote tumorigenesis of ccRCC by repressing RB1 expression

PI3K regulatory subunit p85s normally stabilizes and regulates catalytic subunit p110s in the cytoplasm. Recent studies show that p110-free p85s in the nucleus plays important roles in biological processes. However, the mechanisms by which p85s translocate into the nucleus remain elusive. Here, we describe the mechanism by which p85β translocates into the nucleus to promote ccRCC tumorigenesis. Phosphorylation of p85β at the Y464 by FAK facilitates its nuclear translocation in the kidney through enhancing the binding of p85β to KPNA1. PIK3R2/p85β is highly expressed in ccRCC samples and associated with overall survival of ccRCC patients. Nuclear but not cytoplasmic p85β performs oncogenic functions by repressing RB1 expression and regulating the G1/S cell cycle transition. Nuclear p85β represses RB1 expression by stabilizing histone methyltransferase EZH1/EZH2 proteins. Last, the FAK inhibitor defactinib significantly suppresses the tumor growth of ccRCC with high p85β Y464 levels.

Knockdown of USP8 inhibits prostate cancer cell growth, proliferation, and metastasis and promotes docetaxel’s activity by suppressing the NF-kB signaling pathway

Ubiquitin-specific protease 8 (USP8) has been recently reported to be involved in tumorigenesis. Prostate cancer (PCa) is the most diagnosed malignancy among men, but USP8’s role in PCa is not yet investigated comprehensively. Therefore, the PCa cell lines DU145 and PC3 were transfected with USP8 siRNA or overexpressing vector together with or without docetaxel. The silencing USP8 and docetaxel treatment reduced cell viability and migration and promoted apoptosis. In contrast, USP8 knockdown was found to enhance docetaxel antitumor activity. In contrast, increased cell viability and migration were noticed upon USP8 overexpression, thereby decreasing apoptosis and suppressing docetaxel antitumor activity. Notably, although EGFR, PI3K, and NF-kB were found to be increased in both USP8 overexpression and docetaxel treatment, it significantly attenuated the effects in USP8 silencing followed by with or without docetaxel. Although EGFR silencing decreased PI3K and NF-kB activation, overexpression of USP8 was shown to counteract SiEGFR’s effects on NF-kB signaling by increasing PI3K expression. Our findings revealed that USP8 plays an oncogenic role in PCa and can suppress docetaxel activity. Additionally, as EGFR/PI3K/NF-kB was previously reported to develop docetaxel resistance, the combination treatment of USP8 knockdown with docetaxel might be a potential PCa therapeutic.

Signaling Inhibitors Accelerate the Conversion of mouse iPS Cells into Cancer Stem Cells in the Tumor Microenvironment

Cancer stem cells (CSCs) are a class of cancer cells characterized by self-renewal, differentiation and tumorigenic potential. We previously established a model of CSCs by culturing mouse induced pluripotent stem cells (miPSCs) for four weeks in the presence of a conditioned medium (CM) of cancer cell lines, which functioned as the tumor microenvironment. Based on this methodology of developing CSCs from miPSCs, we assessed the risk of 110 non-mutagenic chemical compounds, most of which are known as inhibitors of cytoplasmic signaling pathways, as potential carcinogens. We treated miPSCs with each compound for one week in the presence of a CM of Lewis lung carcinoma (LLC) cells. However, one-week period was too short for the CM to convert miPSCs into CSCs. Consequently, PDO325901 (MEK inhibitor), CHIR99021 (GSK-3β inhibitor) and Dasatinib (Abl, Src and c-Kit inhibitor) were found to confer miPSCs with the CSC phenotype in one week. The tumor cells that survived exhibited stemness markers, spheroid formation and tumorigenesis in Balb/c nude mice. Hence, we concluded that the three signal inhibitors accelerated the conversion of miPSCs into CSCs. Similarly to our previous study, we found that the PI3K-Akt signaling pathway was upregulated in the CSCs. Herein, we focused on the expression of relative genes after the treatment with these three inhibitors. Our results demonstrated an increased expression of pik3ca, pik3cb, pik3r5 and pik3r1 genes indicating class IA PI3K as the responsible signaling pathway. Hence, AKT phosphorylation was found to be up-regulated in the obtained CSCs. Inhibition of Erk1/2, tyrosine kinase, and/or GSK-3β was implied to be involved in the enhancement of the PI3K-AKT signaling pathway in the undifferentiated cells, resulting in the sustained stemness, and subsequent conversion of miPSCs into CSCs in the tumor microenvironment.

Characterization of Apoptosis in a Breast Cancer Cell Line after IL-10 Silencing

Background: Breast cancer is affected by the immune system in that different cytokines play roles in its initiation and progression. Interleukin-10 (IL-10), an anti-inflammatory cytokine, is an immunosuppressive factor involved in tumorigenesis. The present study was conducted to investigate the gene silencing effect of a small interference RNA (siRNA) targeting IL-10 on the apoptotic pathway in breast cancer cell line. Methods: The siRNA targeting IL-10 and a glyceraldehyde 3-phosphate dehydrogenase (GAPDH) clone were introduced into MDA-MB-231 cells. Real-time PCR assays were used to determine IL-10 and GAPDH gene expression levels, in addition to those for protein kinase B (AKT), phosphoinositide 3-kinase (PI3K), B-cell lymphoma 2 (Bcl2), caspase-3 and caspase-9 genes related to apoptosis. Results: Inhibition of IL-10 by the siRNA accelerated apoptosis and was accompanied by significant increase in caspase-3 and caspase-9 and a significant decrease in PI3K, AKT and Bcl2 expression levels compared to the non-transfected case. Conclusions: In conclusion, the production of IL-10 may represent a new escape mechanism by breast cancer cells to evade destruction by the immune system. IL-10 gene silencing causes down regulation of both PI3K/AKT and Bcl2 gene expression and also increases the Bbc3, BAX caspase3, and caspase 3 cleavage expression levels. IL–10 might represent a promising new target for therapeutic strategies.

MicroRNA-294 Promotes Cellular Proliferation and Motility through the PI3K/AKT and JAK/STAT Pathways by Upregulation of NRAS in Bladder Cancer

In our study we examined the role of microRNA-294 (miR-294) in bladder cancer and related mechanisms. Real-time polymerase chain reaction (RT-PCR) was performed to determine the expression level of miR-294. Western blot was used to determine the expression of NRAS, mainly factors in the PI3K/AKT and JAK/STAT pathways. Cell counting kit-8 assay, clonogenic assay, wound-healing assay, transwell and flow cytometry were used to explore, respectively, cell proliferation, survival, migration, invasion, and apoptosis of bladder cancer cell line T24. The expressions of miR-294 in bladder cancer cells including J82, HT1376, T24, and SW780 were significantly increased compared to those in human bladder epithelium cells (both HCV29 and SV-HUC-1). The proliferation rate, surviving fraction, migration, and invasion of T24 cells in miR-294 mimetic transfected group were significantly increased, while they were significantly decreased by miR-294 inhibitor transfection. Moreover, miR-294 suppression could increase the apoptotic rate of T24 cells. In addition, drug resistance of T24 cells to cisplatin was increased in miR-294 mimetic-treated group, while it was decreased by miR-294 inhibitor compared to empty control. Overexpression of miR-294 could upregulate NRAS expression in T24 cells and activate PI3K/AKT and JAK/STAT pathways. We found that miR-294 expression was positively related with proliferation and motility of T24 cells. Moreover, miR-294 suppression could promote the sensitivity of T24 cells to cisplatin. We also found miR-294 could upregulate NRAS and activate the PI3K/AKT and JAK/STAT pathways in T24 cells.

Depletion of the triggering receptor expressed on myeloid cells 2 inhibits progression of renal cell carcinoma via regulating related protein expression and PTEN-PI3K/Akt pathway

The triggering receptor expressed on myeloid cells 2 (TREM-2) is suggested to be involved in the development of certain human malignancies. However, the functions of TREM-2 in renal cell carcinoma (RCC) are still less known. To reveal the effects of TREM-2 on the RCC progression, we examined the TREM-2 expression in RCC tumor tissues. Then, we analyzed the cell proliferation, cell apoptosis, cell cycle and expression of the relative factors in two selected RCC cell lines post RNA interference. We also analyzed the functions of TREM-2 in an in vivo nude mouse model. We found that, the expression of TREM-2 was abnormally elevated in RCC tumor tissues. Silencing TREM-2 inhibited cell growth, induced G1 phase arrest of cell cycle and cell apoptosis in RCC cells. In vivo, the results showed that depletion of TREM-2 significantly inhibited the ACHN tumor growth in the nude mouse model. The analysis of relative protein factors suggested that silencing TREM-2 downregulated the expression levels of Bcl2 and PCNA, and upregulated the expression levels of Bax and caspase-3 in RCC cell lines. Depletion of TREM-2 inactivated PI3K/Akt pathway through increasing the expression of PTEN. Taken together, TREM-2 acts as an oncogene in the development of RCC and can be considered as a novel therapeutic factor in the treatment of RCC.

Blockade of the Ras/Raf/ERK and Ras/PI3K/Akt Pathways by Monacolin K Reduces the Expression of GLO1 and Induces Apoptosis in U937 Cells

Monacolin K, a hydrolytic product of icaritin, is the major active component in the traditional fermented Monascus purpureus. Monacolin K inhibits the proliferation of acute myeloid leukemia (AML), but underlying mechanisms remain to be identified. The present study demonstrates that monacolin K inhibits the proliferation of human AML cell line U937 in a dose-dependent manner. Importantly, morphological, DNA fragmentation, and image cytometry analyses indicated that monacolin K induced U937 cell apoptosis. Monacolin K could inactivate Ras translocation from cytosol to cell membrane. Monacolin K could also reduce the Ras-dependent phosphorylation of ERK and Akt, and the subsequent translocation of nuclear factor kappa B (NF-κB) from cytosol to nucleus in U937 cells. The underlying mechanisms of apoptotic activity of monacolin K were associated with inhibition of the Ras/Raf/ERK and Ras/PI3K/Akt signals and down-regulation of HMG-CoA reductase and glyoxalase 1. On the basis of results obtained using specific inhibitors U0126, LY294002, and JSH-23, the Ras/Raf/ERK/NF-κB/GLO1 and Ras/Akt/NF-κB/GLO1 pathways were proposed for the apoptotic effect of monacolin K in U937 cells.

Proline-rich tyrosine kinase 2 controls PI3-kinase activation downstream of the T cell antigen receptor in human T cells

TCR-induced signaling controls T cell activation that drives adaptive immunity against infections, but it can also induce dysfunctional T cell responses that promote pathologic disease. The PI3K pathway regulates many downstream effector responses after TCR stimulation. However, the molecular mechanisms that induce PI3K function downstream of the TCR are not fully understood. We have previously shown that Pyk2 is activated downstream of the TCR in a PI3K-independent manner. Although Pyk2 controls adhesion, proliferation, and cytokine production in T cells, the mechanisms by which it controls these processes are not known. In this study, we generated Pyk2-deficient human T cells to elucidate further the role that this kinase plays in TCR-induced effector functions and signaling. We observed that Pyk2 localized with the p85 regulatory subunit of PI3K at the LAT complex and that PI3K-dependent signaling was impaired in Pyk2-deficient T cells. Likewise, functions downstream of PI3K, including IFN-γ production and proliferation, were also suppressed in human T cells deficient in Pyk2. Collectively, these data demonstrate that Pyk2 is a critical regulator of PI3K function downstream of the TCR.

A cytotoxic protein (BF-CT1) purified from Bungarus fasciatus venom acts through apoptosis, modulation of PI3K/AKT, MAPKinase pathway and cell cycle regulation

BF-CT1, a 13 kDa protein isolated from Bungarus fasciatus snake venom through CM cellulose ion exchange chromatography at 0.02 M NaCl salt gradient showed cytotoxicity in in vitro and in vivo experimental models. In in vivo Ehrlich ascites carcinoma (EAC) induced BALB/c mice model, BF-CT1 treatment reduced EAC cell count significantly through apoptotic cell death pathway as evidenced by FACS analysis, increased caspase 3, 9 activity and altered pro, antiapoptotic protein expression. BF-CT1 treatment caused cell shrinkage, chromatin condensation and induced apoptosis through increased caspase 3, caspase 9 activity, PARP cleavage and down regulation of heat shock proteins in U937 leukemic cell line. Cytosolic cytochrome C production was increased after BF-CT1 treatment upon U937 cell line. BF-CT1 treated U937 cell showed cell cycle arrest at sub G1 phase through cyclin D and CDK down regulation with up regulation of p15 and p16. It also down regulated PI3K/AKT pathway and MAPkinase pathway and promoted apoptosis and regulated cell proliferation in U937 cells. BF-CT1 prevented angiogenesis in in vitro U937 cell line through decreased VEGF and TGF-β1 production.

Genistein inhibited proliferation and induced apoptosis in acute lymphoblastic leukemia, lymphoma and multiple myeloma cells in vitro

Genistein is one of the major isoflavones in soy products. It has been reported that genistein has apoptotic effects on certain hematological malignancies. However, so far there have been no completely comparative studies of the effect of genistein on malignant hematological diseases, especially multiple myeloma. We investigated genistein's inhibitory effect on the growth of acute lymphoblastic leukemia (RS4;11 and CEM), lymphoma (Toledo and GA10) and multiple myeloma (OPM-2 and U266) cell lines in vitro. We observed that genistein dose- and time-dependently inhibited proliferation of these cells. The cell line sensitivity to genistein treatment based on the 50% inhibitory concentration (IC(50)) values in decreasing order of toxicity was found to be as follows: RS4;11 (4.89 ± 4.28 μM) > GA10 (13.08 ± 3.49 μM) > Toledo (16.94 ± 3.89 μM) > CEM (17.31 ± 0.72 μM) > OPM-2 (46.76 ± 2.26 μM) > U266 (128.82 ± 1.90 μM). The mechanism of growth inhibition was through induction of apoptosis and cell cycle arrest. The concomitant altered expression of apoptosis pathway proteins and cell cycle modulators (caspases 9, 3, 7, PARP [poly(ADP-ribose) polymerase], cIAP1 [inhibitor of apoptosis protein 1], Bcl-2 and cyclin B1) were observed by Western blot and real-time polymerase chain reaction (PCR) analyses. In addition, some malignancy-related embryologic pathway proteins, e.g. Notch1 and Gli1, were modulated by genistein treatment in sensitive cell lines.

NOTCH and phosphatidylinositide 3-kinase/phosphatase and tensin homolog deleted on chromosome ten/AKT/mammalian target of rapamycin (mTOR) signaling in T-cell development and T-cell acute lymphoblastic leukemia

Activating mutations in NOTCH1 consitute the most prominent genetic abnormality in T-cell acute lymphoblastic leukemia (T-ALL). However, most T-ALL cell lines with NOTCH1 mutations are resistant to treatment with γ-secretase inhibitors (GSIs). The spotlight is now shifting to the phosphatidylinositide 3-kinase (PI3K)/phosphatase and tensin homolog deleted on chromosome ten (PTEN)/AKT/mammalian target of rapamycin (mTOR) pathway as another key potential target. These two signaling routes are deregulated in many types of cancer. In this review we discuss these two pathways with respect to their signaling mechanisms, functions during T-cell development, and their mutual roles in the development of T-ALL.

Targeting HER2 in breast cancer: overview of long-term experience

The ability to probe diseases at the genomic level has improved our understanding and enhanced the treatment of breast cancer. One important finding relates to the HER2 oncogene which encodes a novel transmembrane receptor that, when overexpressed, appears to confer growth and survival advantages to breast tumor cells. This fortuitous discovery enabled researchers to develop agents which could inhibit receptor-mediated tumor cell signaling. Numerous clinical trials of such agents have demonstrated improved outcomes in patients with HER2-positive breast cancer. Nonetheless, not all tumors respond to therapy targeting the receptor, while relapses occur after an initial response to treatment. This paper provides a historical and current perspective of the treatment of patients with HER2-positive breast cancer.

Akt as a therapeutic target in cancer

BACKGROUND

The phosphatidylinositol 3-kinase (PI3K)/phosphatase and tensin homolog (PTEN)/v-akt murine thymoma viral oncogene homolog (Akt)/mammalian target of rapamycin (mTOR) pathway is central in the transmission of growth regulatory signals originating from cell surface receptors.

OBJECTIVE

This review discusses how mutations occur that result in elevated expression the PI3K/PTEN/Akt/mTOR pathway and lead to malignant transformation, and how effective targeting of this pathway may result in suppression of abnormal growth of cancer cells.

METHODS

We searched the literature for articles which dealt with altered expression of this pathway in various cancers including: hematopoietic, melanoma, non-small cell lung, pancreatic, endometrial and ovarian, breast, prostate and hepatocellular.

RESULTS/CONCLUSIONS

The PI3K/PTEN/Akt/mTOR pathway is frequently aberrantly regulated in various cancers and targeting this pathway with small molecule inhibitors and may result in novel, more effective anticancer therapies.

The tyrosine phosphatase CD148 interacts with the p85 regulatory subunit of phosphoinositide 3-kinase

CD148 is a transmembrane tyrosine phosphatase that has been implicated in the regulation of cell growth and transformation. However, the signalling mechanisms of CD148 are incompletely understood. To identify the specific intracellular molecules involved in CD148 signalling, we carried out a modified yeast two-hybrid screening assay. Using the substrate-trapping mutant form of CD148 (CD148 D/A) as bait, we recovered the p85 regulatory subunit of PI3K (phosphoinositide 3-kinase). CD148 D/A, but not catalytically active CD148, interacted with p85 in a phosphorylation-dependent manner in vitro and in intact cells. Growth factor receptor and PI3K activity were also trapped by CD148 D/A via p85 from pervanadate-treated cell lysates. CD148 prominently and specifically dephosphorylated p85 in vitro. Co-expression of CD148 reduced p85 phosphorylation induced by active Src, and attenuated the increases in PI3K activity, yet CD148 did not alter the basal PI3K activity. Finally, CD148 knock-down by siRNA (short interfering RNA) increased PI3K activity on serum stimulation. Taken together, these results demonstrate that CD148 may interact with and dephosphorylate p85 when it is phosphorylated and modulate the magnitude of PI3K activity.

PI3K/AKT inhibition induces caspase-dependent apoptosis in HTLV-1-transformed cells

The phosphatidylinositol-3-kinase (PI3K) and AKT (protein kinase B) signaling pathways play an important role in regulating cell cycle progression and cell survival. In previous studies, we demonstrated that AKT is activated in HTLV-1-transformed cells and that Tax activation of AKT is linked to p53 inhibition and cell survival. In the present study, we extend these observations to identify regulatory pathways affected by AKT in HTLV-1-transformed cells. We demonstrate that inhibition of AKT reduces the level of phosphorylated Bad, an important member of the pro-apoptotic family of proteins. Consistent with the decrease of phosphorylated Bad, cytochrome c is released from the mitochondria and caspase-9 is activated. Pretreatment of the cells with caspase-9 specific inhibitor z-LEHD-FMK or pan caspase inhibitor Ac-DEVD-CHO prevented LY294002-induced apoptosis. Of interest, p53 siRNA prevents LY294002-induced apoptosis in HTLV-1-transformed cells, suggesting that p53 reactivation is linked to apoptosis. In conclusion, the AKT pathway is involved in targeting multiple proteins which regulate caspase- and p53-dependent apoptosis in HTLV-1-transformed cells. Since AKT inhibitors simultaneously inhibit NF-kappaB and activate p53, these drugs should be promising candidates for HTLV-1-associated cancer therapy.

Roles of the Raf/MEK/ERK pathway in cell growth, malignant transformation and drug resistance

Growth factors and mitogens use the Ras/Raf/MEK/ERK signaling cascade to transmit signals from their receptors to regulate gene expression and prevent apoptosis. Some components of these pathways are mutated or aberrantly expressed in human cancer (e.g., Ras, B-Raf). Mutations also occur at genes encoding upstream receptors (e.g., EGFR and Flt-3) and chimeric chromosomal translocations (e.g., BCR-ABL) which transmit their signals through these cascades. Even in the absence of obvious genetic mutations, this pathway has been reported to be activated in over 50% of acute myelogenous leukemia and acute lymphocytic leukemia and is also frequently activated in other cancer types (e.g., breast and prostate cancers). Importantly, this increased expression is associated with a poor prognosis. The Ras/Raf/MEK/ERK and Ras/PI3K/PTEN/Akt pathways interact with each other to regulate growth and in some cases tumorigenesis. For example, in some cells, PTEN mutation may contribute to suppression of the Raf/MEK/ERK cascade due to the ability of activated Akt to phosphorylate and inactivate different Rafs. Although both of these pathways are commonly thought to have anti-apoptotic and drug resistance effects on cells, they display different cell lineage specific effects. For example, Raf/MEK/ERK is usually associated with proliferation and drug resistance of hematopoietic cells, while activation of the Raf/MEK/ERK cascade is suppressed in some prostate cancer cell lines which have mutations at PTEN and express high levels of activated Akt. Furthermore the Ras/Raf/MEK/ERK and Ras/PI3K/PTEN/Akt pathways also interact with the p53 pathway. Some of these interactions can result in controlling the activity and subcellular localization of Bim, Bak, Bax, Puma and Noxa. Raf/MEK/ERK may promote cell cycle arrest in prostate cells and this may be regulated by p53 as restoration of wild-type p53 in p53 deficient prostate cancer cells results in their enhanced sensitivity to chemotherapeutic drugs and increased expression of Raf/MEK/ERK pathway. Thus in advanced prostate cancer, it may be advantageous to induce Raf/MEK/ERK expression to promote cell cycle arrest, while in hematopoietic cancers it may be beneficial to inhibit Raf/MEK/ERK induced proliferation and drug resistance. Thus the Raf/MEK/ERK pathway has different effects on growth, prevention of apoptosis, cell cycle arrest and induction of drug resistance in cells of various lineages which may be due to the presence of functional p53 and PTEN and the expression of lineage specific factors.

Newly identified c-KIT receptor tyrosine kinase ITD in childhood AML induces ligand-independent growth and is responsive to a synergistic effect of imatinib and rapamycin

Activating mutations of c-KIT lead to ligand-independent growth. Internal tandem duplications (ITDs) of exon 11, which encodes the juxtamembrane domain (JMD), are constitutively activating mutations found in 7% of gastrointestinal stromal tumors (GISTs) but have not been described in childhood acute myeloid leukemia (AML). DNA and cDNA from 60 children with AML were screened by polymerase chain reaction (PCR) for mutations of the JMD. A complex ITD (kit cITD) involving exon 11 and exon 12 was identified with a relative frequency of 7% (4/60). The human kit cITDs were inserted into the murine c-Kit backbone and expressed in Ba/F3 cells. KIT cITD induced factorindependent growth and apoptosis resistance, and exhibited constitutive autophosphorylation. KIT cITD constitutively activated the PI3K/AKT pathway and phosphorylated STAT1, STAT3, STAT5, and SHP-2. Imatinib (IM) or rapamycin (Rap) led to complete inhibition of growth, with IC50 values at nanomolar levels. IM and Rap synergistically inhibited growth and surmounted KIT cITD-induced apoptosis resistance. IM but not LY294002 inhibited phosphorylation of STAT3 and STAT5, suggesting aberrant cross talk between PI3K- and STAT-activating pathways. The findings presented may have immediate therapeutic impact for a subgroup of childhood AML-expressing c-KIT mutations.

Roles of the RAF/MEK/ERK and PI3K/PTEN/AKT pathways in malignant transformation and drug resistance

The Ras/Raf/MEK/ERK and PI3K/PTEN/AKT signaling cascades play critical roles in the transmission of signals from growth factor receptors to regulate gene expression and prevent apoptosis. Components of these pathways are mutated or aberrantly expressed in human cancer (e.g., Ras, B-Raf, PI3K, PTEN, Akt). Also, mutations occur at genes encoding upstream receptors (e.g., EGFR and Flt-3) and chimeric chromosomal translocations (e.g., BCR-ABL) which transmit their signals through these cascades. These pathways interact with each other to regulate growth and in some cases tumorigenesis. For example, in some cells, PTEN mutation may contribute to suppression of the Raf/MEK/ERK cascade due to the ability of elevated activated Akt levels to phosphorylate and inactivate Raf-1. We have investigated the genetic structures and functional roles of these two signaling pathways in the malignant transformation and drug resistance of hematopoietic, breast and prostate cancer cells. Although both of these pathways are commonly thought to have anti-apoptotic and drug resistance effects on cells, they display different cell-lineage-specific effects. Induced Raf expression can abrogate the cytokine dependence of certain hematopoietic cell lines (FDC-P1 and TF-1), a trait associated with tumorigenesis. In contrast, expression of activated PI3K or Akt does not abrogate the cytokine dependence of these hematopoietic cell lines, but does have positive effects on cell survival. However, activated PI3K and Akt can synergize with activated Raf to abrogate the cytokine dependence of another hematopoietic cell line (FL5.12) which is not transformed by activated Raf expression by itself. Activated Raf and Akt also confer a drug-resistant phenotype to these cells. Raf is more associated with proliferation and the prevention of apoptosis while Akt is more associated with the long-term clonogenicity. In breast cancer cells, activated Raf conferred resistance to the chemotherapeutic drugs doxorubicin and paclitaxel. Raf induced the expression of the drug pump Mdr-1 (a.k.a., Pgp) and the Bcl-2 anti-apoptotic protein. Raf did not appear to induce drug resistance by altering p53/p21Cip-1 expression, whose expression is often linked to regulation of cell cycle progression and drug resistance. Deregulation of the PI3K/PTEN/Akt pathway was associated with resistance to doxorubicin and 4-hydroxyl tamoxifen, a chemotherapeutic drug and estrogen receptor antagonist used in breast cancer therapy. In contrast to the drug-resistant breast cancer cells obtained after overexpression of activated Raf, cells expressing activated Akt displayed altered (decreased) levels of p53/p21Cip-1. Deregulated expression of the central phosphatase in the PI3K/PTEN/Akt pathway led to breast cancer drug resistance. Introduction of mutated forms of PTEN, which lacked lipid phosphatase activity, increased the resistance of the MCF-7 cells to doxorubicin, suggesting that these lipid phosphatase deficient PTEN mutants acted as dominant negative mutants to suppress wild-type PTEN activity. Finally, the PI3K/PTEN/Akt pathway appears to be more prominently involved in prostate cancer drug resistance than the Raf/MEK/ERK pathway. Some advanced prostate cancer cells express elevated levels of activated Akt which may suppress Raf activation. Introduction of activated forms of Akt increased the drug resistance of advanced prostate cancer cells. In contrast, introduction of activated forms of Raf did not increase the drug resistance of the prostate cancer cells. In contrast to the results observed in hematopoietic cells, Raf may normally promote differentiation in prostate cells which is suppressed in advanced prostate cancer due to increased expression of activated Akt arising from PTEN mutation. Thus in advanced prostate cancer it may be advantageous to induce Raf expression to promote differentiation, while in hematopoietic cancers it may be beneficial to inhibit Raf/MEK/ERK-induced proliferation. These signaling and anti-apoptotic pathways can have different effects on growth, prevention of apoptosis and induction of drug resistance in cells of various lineages which may be due to the expression of lineage-specific factors.

Involvement of PI3K/Akt pathway in cell cycle progression, apoptosis, and neoplastic transformation: a target for cancer chemotherapy

The PI3K/Akt signal transduction cascade has been investigated 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 this pathway with regulation of cell cycle progression. Uncovering the signaling network spanning from extracellular environment to the nucleus should illuminate biochemical events contributing to malignant transformation. Here, we discuss PI3K/Akt-mediated signal transduction including its mechanisms of activation, signal transducing molecules, and effects on gene expression that contribute to tumorigenesis. Effects of PI3K/Akt signaling on important proteins controlling cellular proliferation are emphasized. These targets include cyclins, cyclin-dependent kinases, and cyclin-dependent kinase inhibitors. Furthermore, strategies used to inhibit the PI3K/Akt pathway are presented. The potential for cancer treatment with agents inhibiting this pathway is also addressed.

Phosphatidylinositol-3 kinase p85 enhances expression from the myelin basic protein promoter in oligodendrocytes

Phosphatidylinositol-3 kinase (PI3K) is a family of enzymes that phosphorylates the D3 position of phosphoinositides in membranes which can then act as a second messenger and affect many essential cellular processes such as survival, proliferation and differentiation. Class IA PI3K is composed of two subunits: a regulatory subunit, p85, and a catalytic subunit, p110. The p85 subunit is composed of several adapter domains which, upon interaction with the appropriate molecules, transmit the signal to activate p110. We have used the spontaneously immortalized oligodendrocyte cell line, CG4, to examine the role of PI3K in maturation of the oligodendrocyte. We show that overexpression of the p85 subunit enhances expression of myelin basic protein (MBP) upon differentiation of CG4 cells and primary oligodendrocytes. In experiments in CG4 cells, neither cotransfection with the tumor suppressor PTEN, which dephosphorylates the D3 position of phosphoinositides, nor inhibition of PI3K activity with wortmannin mimics this effect. Further, we have shown that this effect is dependent on the coexpression of the two SH2 domains within p85. Thus, the p85-mediated enhancement of MBP promoter activity in oligodendrocytes appears to be independent of PI3K activity and dependent on the adapter functions of the p85 subunit's SH2 domains.

Tyrosine phosphorylation of p85 relieves its inhibitory activity on phosphatidylinositol 3-kinase

Under resting conditions, the p85 regulatory subunit of phosphatidylinositol 3-kinase (PI3K) serves to both stabilize and inactivate the p110 catalytic subunit. The inhibitory activity of p85 is relieved by occupancy of the NH(2)-terminal SH2 domain of p85 by phosphorylated tyrosine. Src family kinases phosphorylate tyrosine 688 in p85, a process that we have shown to be reversed by the activity of the p85-associated SH2 domain-containing phosphatase SHP1. We demonstrate that phosphorylation of the downstream PI3K target Akt is increased in cells lacking SHP1, implicating phosphorylation of p85 in the regulation of PI3K activity. Furthermore, the in vitro specific activity of PI3K associated with tyrosine- phosphorylated p85 is higher than that associated with nonphosphorylated p85. Expression of wild-type p85 inhibits PI3K enzyme activity as indicated by PI3K- dependent Akt phosphorylation. The inhibitory activity of p85 is accentuated by mutation of tyrosine 688 to alanine and reversed by mutation of tyrosine 688 to aspartic acid, changes that block and mimic tyrosine phosphorylation, respectively Strikingly, mutation of tyrosine 688 to aspartic acid completely reverses the inhibitory activity of p85 on cell viability and activation of the downstream targets Akt and NFkappaB, indicative of the physiological relevance of p85 phosphorylation. Tyrosine phosphorylation of Tyr(688) or mutation of tyrosine 688 to aspartic acid is sufficient to allow binding to the NH(2)-terminal SH2 domain of p85. Thus an intramolecular interaction between phosphorylated Tyr(688) and the NH(2)-terminal SH2 domain of p85 can relieve the inhibitory activity of p85 on p110. Taken together, the data indicate that phosphorylation of Tyr(688) in p85 leads to a novel mechanism of PI3K regulation.