Abstract 623: CLU inhibition using OGX-011 is a new adjuvant therapeutic strategy for HSP90 inhibition in prostate cancer

Introduction and Objective: Prostate cancer (PCa) responds initially to anti-androgen therapies, however, progression to castration resistant disease (CRPC) frequently occurs. Several small molecule inhibitors of HSP90 show promise in CRPC and other cancers. However, most HSP90 inhibitors (17-AAG or PF-04928473 and its prodrug PF-04929113) trigger the elevation of HSPs (HSP90, 70, 27 and clusterin), which lead to tumor cell survival and treatment resistance. We hypothesized that targeting clusterin (CLU) using siRNA or the antisense drug, OGX-011, may enhance HSP90 inhibitors-induced cell death in PCa.

Methods: Inducible and constitutive CLU and other HSP mRNA and protein levels were measured by real-time RT-PCR and immunoblot assays. The combination of OGX-011 with PF-04928473 or 17-AAG was evaluated in vitro on LNCaP and PC3 cells growth and apoptosis. The HSP90 inhibitor PF-04929113 was evaluated in combination with OGX-011 in vivo in athymic mice bearing castration resistant LNCaP xenografts, while the combination of OGX-011 with 17-AAG was evaluated in PC3 xenografts.

Results: In prostate tumor cell lines, PF-04928473 and 17-AAG induced expression of HSPs in a dose and time dependent manner, and especially CLU at RNA and protein level, by increasing HSF-1 nuclear translocation and transcription activity. In vitro, OGX-011 synergistically enhanced the activity of HSP90 inhibitors on cell growth and apoptosis with increased sub-G1 fraction and PARP cleavage. These biologic events were accompanied by decreased expression of HSPs, Akt, AR and PSA, and HSF-1 transcriptional activity. In vivo, OGX-011, administered 3 times a week (IP, 15mg/kg), potentiated the effect of orally administered HSP90 inhibitors (PF-04929113: 25mg/kg; 17-AAG: 50mg/kg), significantly inhibiting tumor growth by 80% and prolonging survival in PC3 and castrate resistant LNCaP xenograft model compared to the HSP90 inhibitors alone.

Conclusions: These results indicate that HSP90 inhibitor-mediated induction of CLU expression can be attenuated by OGX-011, with synergistic effects on delaying progression of CRPC.

This work was supported by the Canadian Institutes of Health Research fellowship and Association pour la Recherche sur le Cancer, France.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 623. doi:10.1158/1538-7445.AM2011-623

Abstract 3578: A novel and selective 3-phosphoinositide-dependent kinase-1 inhibitor, PF-5177624, blocks insulin-like growth factor-1 induced tumorigenesis in breast cancer cells

Phosphatidylinositol 3-kinases (PI3Ks) phosphorylate membrane phosphatidylinositides to generate phosphatidylinositol 3,4,5-triphosphate (PIP3); PIP3 serves as an important secondary messenger recruiting and activating proteins that contain a pleckstrin homology (PH) domain including AKT and 3’-phosphoinositide-dependent kinase-1 (PDK1). PDK1 is a 63-kDa Ser/Thr kinase with a catalytic domain near its N-terminus and a pleckstrin homology domain at its C-terminus. The PH domain is necessary for targeting PDK1 to the plasma membrane to phosphroylate the T-loop sites of numerous substrates such as AKT at the residue threonine-308 (T308). The phosphorylation at both T308 and the serine-473 residue by mTORC2, fully activates AKT to induce the downstream signaling pathways that are important for tumor progression. PDK1 also has been shown to phosphorylate p70S6K, isoforms of PKCs, and many other kinase substrates. The oncogenic activity of aberrant PI3K pathway signaling through PDK1 has been extensively validated. PDK1 is highly expressed in a majority of human breast cancers and cell lines, and elevated phosphorylation of PDK1 was associated with PIK3CA mutations in human breast tumors and cell lines. Therefore, targeting PDK1 in the PI3K/AKT pathway may provide an opportunity for breast cancer treatment.

In this study, we demonstrate that insulin-like growth factor-1 (IGF-1) stimulates PDK1 activity and a specific and potent PDK1 inhibitor, PF-5177624, inhibits IGF-1 stimulated downstream signaling in breast cancer cells, including a reduction in phosphorylation of both AKT (T308) and p70S6K. The inhibition of PDK1 activity through the IGF-1 axis is sufficient to induce in vitro anti-tumor activity in breast cancer cells such that PF-5177624 inhibits cell proliferation and cell transformation in breast cancer cells. Our data suggest that a specific and potent PDK1 inhibitor (PF-5177624) is likely to have antitumor activity in breast cancer cells and can be developed as a cancer drug for breast cancer patients.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 3578. doi:10.1158/1538-7445.AM2011-3578

A novel HSP90 inhibitor delays castrate-resistant prostate cancer without altering serum PSA levels and inhibits osteoclastogenesis

PURPOSE

Prostate cancer responds initially to antiandrogen therapies; however, progression to castration-resistant disease frequently occurs. Therefore, there is an urgent need for novel therapeutic agents that can prevent the emergence of castrate-resistant prostate cancer (CRPC). HSP90 is a molecular chaperone involved in the stability of many client proteins including Akt and androgen receptor (AR). 17-Allylamino-17-demethoxy-geldanamycin (17-AAG) has been reported to inhibit tumor growth in various cancers; however, it induces tumor progression in the bone microenvironment.

METHODS

Cell growth, apoptosis, and AR transactivation were examined by crystal violet assay, flow cytometric, and luciferase assays, respectively. The consequence of HSP90 therapy in vivo was evaluated in LNCaP xenograft model. The consequence of PF-04928473 therapy on bone metastasis was studied using an osteoclastogenesis in vitro assay.

RESULTS

PF-04928473 inhibits cell growth in a panel of prostate cancer cells, induces cell-cycle arrest at sub-G(1), and leads to apoptosis and increased caspase-3 activity. These biological events were accompanied by decreased activation of Akt and Erk as well as decreased expression of Her2, and decreased AR expression and activation in vitro. In contrast to 17-AAG, PF-04928473 abrogates RANKL-induced osteoclast differentiation by affecting NF-κB activation and Src phosphorylation. Finally, PF-04929113 inhibited tumor growth and prolonged survival compared with controls. Surprisingly, PF-04929113 did not reduce serum prostate-specific antigen (PSA) levels in vivo; in parallel, these decrease in tumor volume.

CONCLUSION

These data identify significant anticancer activity of PF-04929113 in CRPC but suggest that serum PSA may not prove useful as pharmacodynamic tool for this drug.

Abstract A50: Targeting 3-phosphoinositide-dependent kinase-1 and mammalian target of rapamycin synergistically inhibits cell proliferation of breast cancer cells

Aberration of receptor tyrosine kinases (RTKs), PTEN (phosphatase and tensin homolog deleted on chromosome 10), and PIK3CA (encodes the p110 subunit of phosphatidylinositol 3-kinase [PI3K]) frequently contribute to tumor progression through their ability to regulate the intracellular level of phosphatidylinositol-3,4,5- triphosphate (PIP3). PIP3 subsequently recruits 3-phosphoinositide- dependent kinase-1 (PDK-1), a serine/threonine kinase, to the plasma membrane and initiates PDK-1 kinase activity to phosphorylate AKT within the activation loop of the catalytic domain at the residue threonine-308 (T308). The mammalian target of rapamycin (mTOR) plays a critical role in the PI3K/AKT pathway. The mTOR kinase is present as two protein complexes, TORC1 and TORC2. The TORC1 complex is activated by PI3K/AKT and phosphorylates p70S6K and 4E-BP1 to modulate protein translation whereas the TORC2 complex phosphorylates AKT in the regulatory domain at residue serine 473 (S473). Phosphorylation of AKT at both T308 and S473 fully activates AKT function through downstream signaling to mediate cell proliferation, survival, migration, angiogenesis and other pathways which are critical for tumor progression. We have developed a potent and selective PDK1 inhibitor (PF-05177624) that is sufficient to inhibit the phosphorylation of AKT atT308 and block cell proliferation of cancer cells in vitro. In this study, we combined this PDK1 inhibitor (PF-05177624) and a selective mTOR kinase inhibitor (WYE-354) to perform the combination studies in breast cancer cell lines. Our data demonstrate that targeting both PDK1 and mTOR simultaneously produces a synergistic inhibitory effect in AKT phosphorylation. Moreover, this results in a synergistic inhibition of cell proliferation in breast cancer cells. Our data suggest that a combination strategy of targeting both PDK-1 and mTOR may have potential improved clinical efficacy in the development of cancer drugs for breast cancer patients.

Citation Information: Clin Cancer Res 2010;16(14 Suppl):A50.

Nek6 mediates human cancer cell transformation and is a potential cancer therapeutic target

We investigated the role of Nek6, a member of the NIMA-related serine/threonine kinase family, in tumorigenesis. Transcript, protein, and kinase activity levels of Nek6 were highly elevated in the malignant tumors and human cancer cell lines compared with normal tissue and fibroblast cells. Expression of exogenous wild-type Nek6 increased anchorage-independent growth of a variety of human cancer cell lines, whereas overexpression of the kinase-dead Nek6 and RNAi knockdown of endogenous Nek6 suppressed cancer cell transformation and induced apoptosis. Additionally, in in vivo xenograft nude mouse model, knockdown of Nek6 in HeLa cells resulted in reduction of tumor size relative to control siRNA tumors. Most importantly, knocking down endogenous Nek6 levels or exogenous expression of the kinase-dead form did not inhibit cell proliferation, nor did it induce apoptosis in normal fibroblast cells. Taken together, our data indicate a pivotal role for Nek6 in tumorigenesis and establish Nek6 as a potential target for treatment of a variety of human cancers.

Abstract 4473: PF-04691502, a potent and selective mTOR/PI3K dual inhibitor, demonstrates in vitro and in vivo antitumor activity in non-small cell lung carcinoma cells

The phosphatidylinositol 3-kinase (PI3K)/AKT/mammalian target of rapamycin (mTOR) pathway plays a critical role in the regulation of cell growth, proliferation and survival in cancer. In non-small cell lung carcinomas cells (NSCLCs), the PI3K/AKT/mTOR pathway is frequently activated through either the amplification/mutation of PIK3CA (which encodes the p110α subunit of PI3K), or the functional loss of phosphatase and tensin homolog deleted on chromosome 10 (PTEN). In addition, the PI3K/AKT/mTOR pathway plays a critical role downstream of epidermal growth factor receptor (EGFR) in a defined NSCLC population; in particular, those harboring the EGFR mutations. Dual inhibition of PI3K and mTOR therefore represents a high therapeutic value for NSCLC patients. PF-04691502 is a dual-specificity inhibitor of PI3K and mTOR which shows potent and selective activity in biochemical, cell, and animal studies. In this study, we show PF-04691502 inhibits phosphorylation of AKT (at both S473 and T308) and S6 ribosomal protein (S6RP) in a variety of NSCLC lines harboring a PIK3CA mutation or EGFR mutation, subsequently resulting in anti-proliferative activity in the NSCLC lines. Furthermore, we demonstrate that PF-04691502 is capable of inducing tumor growth inhibition in four NSCLC xenograft models including an Erlotinib resistant line, NCI-H1975. In these murine xenograft models, decreased phosphorylation of AKT, p70S6K, and S6RP was observed in treated tumors in a manner consistent with the drug levels achieved. Therefore, PF-04691502 exhibits potent inhibition in the PI3K/mTOR axis and leads to both in vitro and in vivo anti-tumor activity in NSCLCs. Moreover, our data suggests that PF-04691502 may have clinical benefit to NSCLC patients including those found to be resistant to Erlotinib treatment.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 4473.

Abstract 4483: Establishing patient-derived colorectal cancer stem cell models with a PIK3CA mutation for the development of inhibitory drugs as targeted therapies

Emerging evidences suggest that cancer stem cells (CSC) may be critically responsible for tumor initiation, progression, metastasis, and drug resistance. It becomes important to ask whether anti-cancer agents are able to target the tumor-initiating subpopulation in relevant CSC models. In this study, we first established xenograft tumors in NOD/SCID mice from a colorectal cancer patient specimen and demonstrated that CD133/EpCAM-expressing CSC population was highly tumorigenic. We then sought to propagate the CSC population under a serum-free condition. In culture, the tumor cells formed non-adherent, 3-dimensional spheroids, a fraction of which retained expression of the CSC markers. When exposed to a serum-containing medium, tumor spheroid cells differentiated into epithelial-like adherent cells with an increase in cell proliferation rate. In comparison with the differentiated progeny, tumor spheroid cells exhibited resistance to the standard-of-care agent oxaliplatin and, in limiting dilution assays in mice, displayed substantially higher tumorigenic potential. In contrast to the tumors originated from the differentiated cells, tumor spheroid cell-derived tumors recapitulated not only the CSC frequency marked by CD133/EpCAM expression, but also the histological characters of the original tumor. Similarly, only were the fragments of spheroid cell-derived xenograft tumors capable of regenerating highly proliferative tumors in secondary transplantation. Thus, the tumor spheroid culture is indeed enriched of drug resistant, self-renewing, and tumor-initiating CSC populations. Mutation profiling of frequently mutated oncogenes using Sequenom OncoCarta™ panel identified a mutation in the kinase domain of PIK3CA (H1047R) in the cultured CSCs. This mutation has been reported present in a large number of colon cancer patients and likely functions as an oncogene (Samuels et al., Science 304:554; 2004). We further demonstrated that a dual mammalian target of rapamycin (mTOR)/phosphoinositide 3-kinase (PI3K) inhibitor (PF-04691502) exhibited a more potent effect on inhibition of in vitro proliferation of the mutated CSCs compared to the chemotoxic agent oxaliplatin. Collectively, our findings suggest that CSC models provide a novel avenue to drug sensitivity and efficacy studies. The well-characterized CSC model systems may assist in the development of more effective therapy against the subpopulation of tumors driven by the CSCs bearing specific mutations.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 4483.

Abstract 4482: Novel and selective small molecule inhibitors of 3-phosphoinositide-dependent kinase-1 inhibit the PDK-1/AKT signaling pathway and cell proliferation

Receptor tyrosine kinases (RTKs), PTEN (phosphatase and tensin homolog deleted on chromosome 10), and PIK3CA (encodes the p110α subunit of phosphatidylinositol 3-kinase [PI3K]) frequently contribute to tumor progression through their ability to regulate the intracellular level of phosphatidylinositol-3,4,5-triphosphate (PIP3). 3-phosphoinositide-dependent kinase-1 (PDK-1), a serine/threonine kinase, activates the catalytic domain of numerous kinases by phosphorylating their T-loop sites. PDK-1 activity is required for activation of AKT, p70S6K, and RSK which lead to cell proliferation and transformation. The interaction of the pleckstrin homology (PH) domain of AKT with the membrane bound PIP3 confers a conformational change in AKT, allowing PDK-1 to phosphorylate AKT at the residue threonine-308 (T308). This T-loop activation at T308, along with the phosphorylation of the serine-473 residue by mTOR, fully activates the AKT pathway. Although the roles of many PDK-1 substrates have yet to be characterized, the oncogenic activity of aberrant PI3K pathway signaling through PDK-1 has been extensively validated. We have developed a series of 3-Carbonyl-4-Amino-Pyrrolopyrimidne (CAP) compounds that are potent inhibitors of human PDK-1 (full length and kinase domain) which demonstrate more than 100-fold selectivity against P70S6K, PI3K, AKT, and mTOR. In this study, representative compounds from the CAP series were used to perform a variety of anti-tumor assays. We demonstrate PDK-1 compounds inhibit the phosphorylation of T308 on AKT as well as downstream molecules of the PI3K pathway, such as S6 ribosomal protein (S6RP) in breast, lung and colon cancer cell lines harboring a PI3KCA mutation. Additionally, blockade of AKT and molecules in the PI3K pathway leads to the inhibition of cell proliferation and cell transformation in cancer cells. Our data suggest that the inhibition of PDK-1 activity is sufficient to induce anti-tumor activity in cancer cells through the PI3K-PDK1-AKT axis, and that a potent and specific PDK-1 inhibitor could potentially be developed as a therapeutic agent against several cancer types.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 4482.

Abstract 4512: The novel HSP90 inhibitor, PF-04929113, inhibits AR activity and osteoclastogenesis and delays castrate-resistant LNCaP prostate cancer tumor growth

Introduction and Objective: Prostate cancer responds initially to anti-androgen therapies, however, progression to castration resistant disease frequently occurs. Consequently there is an urgent need for novel therapeutic agents that can prevent the emergence of the castration resistant form of disease. Heat shock protein (HSP) 90 is a molecular chaperone involved in the maturation and stability of the Androgen Receptor (AR). In this study, we assessed the in vitro and in vivo antitumor properties of a novel synthetic HSP90 inhibitor, PF-04928473 in vitro and PF-04929113 in vivo, in prostate cancer and bone metastasis development.

Methods: The effects of the HSP90 inhibitor were evaluated in vitro on LNCaP, C4-2, PC3 and DU145 cell growth and apoptosis. AR transactivation and other biochemical assays studied AR/HSP90 interaction. The consequence of HSP90 therapy in vivo was evaluated in athymic mice bearing castration resistant LNCaP xenografts. The consequence of PF-04928473 therapy on bone metastasis was studied using a osteoclastogenesis in vitro assay.

Results: In our panel of prostate tumor cell lines, PF-04928473 inhibited cell growth in a dose-dependent manner (IC50 ± 30nM) and induced apoptosis with an increase of sub-G1 fraction and PARP cleavage. These biologic events were accompanied by decreased expression of AR and PSA, AR transactivation, inhibition of Akt and Erk activation, and modulation of HSP expression. When administered orally 3 times/week (50mg/kg) to mice bearing castration resistant LNCaP tumors, PF-04929113 significantly inhibited tumor growth by 160% and prolonged survival compared to controls, but a small reduction in total bodyweight was observed. In contrast to 17-AAG, the novel PF-04928473 compound was not observed to activate src but did block osteoclast differentiation in vitro.

Conclusions: Targeting HSP90 using PF-04928473 and PF-04929113 inhibited castration resistant LNCaP tumor growth in vitro and in vivo in part through a mechanism involving disruption of the AR signaling axis and appears to inhibit bone metastasis by blocking osteoclastogenesis.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 4512.

Abstract 753: Novel, potent and selective small molecule inhibitors of 3-phosphoinositide-dependent kinase (PDK1)

The phosphatidylinositol 3-kinase (PI3K) signaling pathway plays a crucial role in cell growth, proliferation and survival. Genomic aberrations in the PI3K pathway, such as mutational activation of PI3Kα or loss-of-function of the tumor suppressor PTEN, have been closely linked to the development and progression of a wide range of cancers. Inhibition of the key targets in the pathway, PI3K, AKT, mTOR & PDK1, may provide an effective treatment of cancer. In an effort to discover compounds that inhibit PDK1, we have developed a series of 3-Carbonyl-4-Amino-Pyrrolopyrimidine (CAP) compounds that are selective and potent PDK1 inhibitors. Early screening led to a viable starting point, PF-03772304, (4-amino-7-isopropyl-7H-pyrrolo[2,3-d]pyrimidin-5-yl)-(6-methylamino-pyrazin-2-yl)-methanone, which has an IC50 of 94 nM for PDK1 and a ligand efficiency of 0.42. While potent, this lead was not selective against PI3K. Using structure-based drug design, this lead was modified to expand into the selectivity pocket of PDK1 (under the G-Loop), leading to the identification of a potent and pathway-selective compound, PF-05017255 ((4-Amino-7-isopropyl-7H-pyrrolo[2,3-d]pyrimidin-5-yl)-{6-[(3S,4R)-4-(4-fluoro-phenyl)-tetrahydro-furan-3-ylamino]-pyrazin-2-yl}-methanone). PF-05017255 has a Ki of 0.6 nM for PDK1 and is more than 400-fold selective against other PI3K pathway kinases: PI3Kα, AKT, S6K and mTOR. For even greater kinase selectivity, we sought to lower the clogP of our lead (clogP for PF-05017255 is 3.0) to reduce the contribution from the hydrophobic effect. These efforts led to PF-05168899 (1-{(2R,3R)-3-[6-(4-Amino-7-isopropyl-7H-pyrrolo[2,3-d]pyrimidine-5-carbonyl)-pyrazin-2-ylamino]-2-phenyl-pyrrolidin-1-yl}-ethanone) with a Ki of 0.4 nM for PDK1, a clogP of 2.1, and greater than 1000-fold selectivity against PI3Kα, AKT, S6K, mTOR, CDK2, CHK1 and PAK4. PF-05168899 also showed little inhibitory effect (<50% at 1 uM) against 33 of 35 kinases in a broader panel, demonstrating significant inhibition only against CHK2 (94%) and AuroraB (54%). In addition, the most potent analogs (e.g. PF-05168889) inhibited the phosphorylation of AKT at the residue threonine 308 (IC50 40-200 nM) in a variety of cancer cell lines (e.g. H460, A549). The design, synthesis and SAR of this chemical series will be described.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 753.

The serine/threonine kinase Nek6 is required for cell cycle progression through mitosis

The Aspergillus nidulans protein NIMA (never in mitosis, gene A) is a protein kinase required for the initiation of mitosis, whereas its inactivation is necessary for mitotic exit. Here, we demonstrate that human NIMA-related kinase 6 (Nek6) is required for mitotic progression of human cells. Nek6 is phosphorylated and activated during M phase. Inhibition of Nek6 function by either overexpression of an inactive Nek6 mutant or elimination of endogenous Nek6 by siRNA arrests cells in M phase and triggers apoptosis. Time-lapse recording of the cell cycle progression of cells expressing kinase-inactive Nek6 reveals mitotic arrest at the metaphase stage prior to cells entering apoptosis. In contrast to NIMA and the closely related mammalian Nek2 kinase, which regulate centrosome function and separation, our data demonstrate an important function for Nek6 during mitosis and suggest that Nek6 kinase is required for metaphase-anaphase transition.

Stimulation of NFkappa B activity by multiple signaling pathways requires PAK1

The p21-activated kinase (PAK1) is a serine-threonine protein kinase that is activated by binding to the Rho family small G proteins Rac and Cdc42hs. Both Rac and Cdc42hs have been shown to regulate the activity of the transcription factor NFkappaB. Here we show that expression of active Ras, Raf-1, or Rac1 in fibroblasts stimulates NFkappaB in a PAK1-dependent manner and that expression of active PAK1 can stimulate NFkappaB on its own. Similarly, in macrophages activation of NFkappaB as well as transcription from the tumor necrosis factor alpha promoter depends on PAK1. In these cells lipopolysaccharide is a potent activator of PAK1 kinase activity. We also demonstrate that expression of active PAK1 stimulates the nuclear translocation of the p65 subunit of NFkappaB but does not activate the inhibitor of kappaB kinases alpha or beta. These data demonstrate that PAK1 is a crucial signaling molecule involved in NFkappaB activation by multiple stimuli.

IkappaB kinase alpha (IKKalpha) regulation of IKKbeta kinase activity

Two related kinases, IkappaB kinase alpha (IKKalpha) and IKKbeta, phosphorylate the IkappaB proteins, leading to their degradation and the subsequent activation of gene expression by NF-kappaB. IKKbeta has a much higher level of kinase activity for the IkappaB proteins than does IKKalpha and is more critical than IKKalpha in modulating tumor necrosis factor alpha activation of the NF-kappaB pathway. These results indicate an important role for IKKbeta in activating the NF-kappaB pathway but leave open the question of the role of IKKalpha in regulating this pathway. In the current study, we demonstrate that IKKalpha directly phosphorylates IKKbeta. Moreover, IKKalpha either directly or indirectly enhances IKKbeta kinase activity for IkappaBalpha. Finally, transfection studies to analyze NF-kappaB-directed gene expression suggest that IKKalpha is upstream of IKKbeta in activating the NF-kappaB pathway. These results indicate that IKKalpha, in addition to its previously described ability to phosphorylate IkappaBalpha, can increase the ability of IKKbeta to phosphorylate IkappaBalpha.

Sulindac inhibits activation of the NF-kappaB pathway

Sulindac is a non-steroidal anti-inflammatory agent that is related both structurally and pharmacologically to indomethacin. In addition to its anti-inflammatory properties, sulindac has been demonstrated to have a role in the prevention of colon cancer. Both its growth inhibitory and anti-inflammatory properties are due at least in part to its ability to decrease prostaglandin synthesis by inhibiting the activity of cyclooxygenases. Recently, we demonstrated that both aspirin and sodium salicylate, but not indomethacin, inhibited the activity of an IkappaB kinase beta (IKKbeta) that is required to activate the nuclear factor-kappaB (NF-kappaB) pathway. In this study, we show that sulindac and its metabolites sulindac sulfide and sulindac sulfone can also inhibit the NF-kappaB pathway in both colon cancer and other cell lines. Similar to our previous results with aspirin, this inhibition is due to sulindac-mediated decreases in IKKbeta kinase activity. Concentrations of sulindac that inhibit IKKbeta activity also reduce the proliferation of colon cancer cells. These results suggest that the growth inhibitory and anti-inflammatory properties of sulindac may be regulated in part by inhibition of kinases that regulate the NF-kappaB pathway.

The anti-inflammatory agents aspirin and salicylate inhibit the activity of I(kappa)B kinase-beta

NF-kappaB comprises a family of cellular transcription factors that are involved in the inducible expression of a variety of cellular genes that regulate the inflammatory response. NF-kappaB is sequestered in the cytoplasm by inhibitory proteins, I(kappa)B, which are phosphorylated by a cellular kinase complex known as IKK. IKK is made up of two kinases, IKK-alpha and IKK-beta, which phosphorylate I(kappa)B, leading to its degradation and translocation of NF-kappaB to the nucleus. IKK kinase activity is stimulated when cells are exposed to the cytokine TNF-alpha or by overexpression of the cellular kinases MEKK1 and NIK. Here we demonstrate that the anti-inflammatory agents aspirin and sodium salicylate specifically inhibit IKK-beta activity in vitro and in vivo. The mechanism of aspirin and sodium salicylate inhibition is due to binding of these agents to IKK-beta to reduce ATP binding. Our results indicate that the anti-inflammatory properties of aspirin and salicylate are mediated in part by their specific inhibition of IKK-beta, thereby preventing activation by NF-kappaB of genes involved in the pathogenesis of the inflammatory response.

HTLV-I Tax protein binds to MEKK1 to stimulate IkappaB kinase activity and NF-kappaB activation

NF-kappaB, a key regulator of the cellular inflammatory and immune response, is activated by the HTLV-I transforming and transactivating protein Tax. We show that Tax binds to the amino terminus of the protein kinase MEKK1, a component of an IkappaB kinase complex, and stimulates MEKK1 kinase activity. Tax expression increases the activity of IkappaB kinase beta (IKKbeta) to enhance phosphorylation of serine residues in IkappaB alpha that lead to its degradation. Dominant negative mutants of both IKKbeta and MEKK1 prevent Tax activation of the NF-kappaB pathway. Furthermore, recombinant MEKK1 stimulates IKKbeta phosphorylation of IkappaB alpha. Thus, Tax-mediated increases in NF-kappaB nuclear translocation result from direct interactions of Tax and MEKK1 leading to enhanced IKKbeta phosphorylation of IkappaB alpha.

Differential transcriptional activation by human T-cell leukemia virus type 1 Tax mutants is mediated by distinct interactions with CREB binding protein and p300

The human T-cell leukemia virus type 1 Tax protein transforms human T lymphocytes, which can lead to the development of adult T-cell leukemia. Tax transformation is related to its ability to activate gene expression via the ATF/CREB and the NF-kappaB pathways. Transcriptional activation of these pathways is mediated by the actions of the related coactivators CREB binding protein (CBP) and p300. In this study, immunocytochemistry and confocal microscopy were used to localize CBP and p300 in cells expressing wild-type Tax or Tax mutants that are able to selectively activate gene expression from either the NF-kappaB or ATF/CREB pathway. Wild-type Tax colocalized with both CBP and p300 in nuclear bodies which also contained ATF-1 and the RelA subunit of NF-kappaB. However, a Tax mutant that selectively activates gene expression from only the ATF/CREB pathway colocalized with CBP but not p300, while a Tax mutant that selectively activates gene expression from only the NF-kappaB pathway colocalized with p300 but not CBP. In vitro and in vivo protein interaction studies indicated that the integrity of two independent domains of Tax delineated by these mutants was involved in the direct interaction of Tax with either CBP or p300. These studies are consistent with a model in which activation of either the NF-kappaB or the ATF/CREB pathway by specific Tax mutants is mediated by distinct interactions with related coactivator proteins.

HTLV-1 21 bp repeat sequences facilitate stable association between Tax and CREB to increase CREB binding affinity

The human T-cell leukemia virus type I (HTLV-I) transactivator protein Tax is critical for the activation of viral gene expression and the transformation of T-lymphocytes. Tax activation of HTLV-I gene expression is mediated by three highly homologous regulatory elements known as 21 bp repeats which bind the transcription factor CREB. Questions remain about the mechanism by which Tax can stimulate CREB binding, whether Tax alters CREB binding affinity, what specific sequences in the HTLV-I 21 bp repeat mediate ternary complex formation, and if the ternary complex comprised of Tax and CREB can recruit coactivators such as CBP. To address these points, we used immobilized templates containing either the HTLV-I 21 bp repeats or the somatostatin CRE to assay Tax association with ATF/CREB family members. Tax formed a stable ternary complex on each of the 21 bp repeats with the transcription factor CREB but not related ATF/CREB proteins. In contrast, Tax did not form a similar complex on the CREB binding site in the somatostatin promoter. The formation of this complex was dependent on 3' sequences flanking the CREB binding site within each of the 21 bp repeats and resulted in marked increases in CREB association and binding affinity. Tax increased the binding of phosphorylated CREB to the 21 bp repeat resulting in increased association of the coactivator CBP. However, Tax did not form a complex on the somatostatin CRE in the presence of either phosphorylated or non-phosphorylated CREB and it did not stimulate CBP association to this element. These studies extend previous work and demonstrate how specific DNA sequences flanking the CREB binding site regulate the formation of a stable ternary complex that is able to more efficiently recruit the coactivator CBP.

Complex formation between CREB and Tax enhances the binding affinity of CREB for the human T-cell leukemia virus type 1 21-base-pair repeats

The regulation of human T-cell leukemia virus type 1 (HTLV-1) gene expression is dependent on three cis-acting elements, known as the 21-bp repeats, in the long terminal repeat. Each of the 21-bp repeats contains a nonpalindromic cyclic AMP response element (CRE) sequence which is capable of binding members of the ATF/CREB family of transcription factors. The HTLV-1 transactivator protein Tax is able to markedly stimulate the in vitro binding of CREB to the CRE sites present in each of the 21-bp repeats but not to CRE sites present in cellular promoters. The ability to Tax to stimulate CREB binding to different CRE sites correlates with the ability of Tax to activate gene expression from these sites. We wished to determine how sequence differences between the somatostatin CRE and the 21-bp repeat were involved in this different response to Tax. Scatchard analysis indicated that CREB bound to the somatostatin CRE with a single class of high-affinity binding while CREB bound to the 21-bp repeats with a biphasic binding pattern, indicating the presence of both low- and high-affinity binding. Tax increased the affinity of CREB binding but not that of another ATF/CREB protein, CREB2, to the 21-bp repeat. However, Tax did not increase affinity of binding of CREB to the somatostatin CRE. To determine the mechanism by which Tax increased dCREB binding affinity, immobilized oligonucleotides corresponding to either the 21-bp repeat or the somatostatin CRE were used to demonstrate that Tax formed a highly specific complex with CREB on the 21-bp repeat but not on the somatostatin CRE. These results indicate that formation of a complex between Tax and CREB results in specific high-affinity binding of this ternary complex to the HTLV-1 21 bp repeats.

cAMP-response element-binding protein induces directed DNA bending of the HTLV-I 21-base pair repeat

Gene expression from the human T-cell leukemia virus type I (HTLV-I) long terminal repeat (LTR) is mediated by three cis-acting regulatory elements known as 21-base pair (bp) repeats in addition to the transactivator protein Tax. Each of the 21-bp repeats contain nucleotide sequences which are homologous to a cAMP response element (CRE) which bind members of the ATF/CREB family of transcription factors. In this study, we investigated whether CREB alone or in the presence of Tax was able to induce DNA structural changes when bound to CRE sites in the HTLV-I 21 bp, the cellular somatostatin promoter, or a hybrid CRE construct comprised of both the somatostatin and 21-bp repeat sequences. Circular permutation analysis indicated that CREB was able to induce DNA flexure upon binding to each of these elements. However, phasing analysis, which is a more sensitive method to determine the degree and orientation of directed DNA bending, demonstrated that CREB induced DNA bending of the HTLV-I 21-bp repeat and the hybrid CRE but not the somatostatin CRE. The addition of Tax did not change CREB-mediated bending of the 21-bp repeat or the hybrid CRE although it markedly increased the amount of CREB bound to each of these DNA elements. These results indicate that sequence motifs flanking the CRE in the 21-bp repeat are critical for inducing DNA structural changes and that these changes are likely important in mediating Tax activation of the HTLV-I LTR.

Chimeric proteins composed of Jun and CREB define domains required for interaction with the human T-cell leukemia virus type 1 Tax protein

The regulation of human T-cell leukemia virus type 1 (HTLV-1) long terminal repeat gene expression is dependent on three cis-acting elements known as 21-bp repeats and the transactivator protein Tax. Mutagenesis has demonstrated that sequences in each of the 21-bp repeats can be divided into three domains designated A, B, and C. Tax stimulates the binding of CREB to the B domain, which is essential for Tax activation of HTLV-1 gene expression. In this study, we demonstrate that Tax will stimulate the binding of CREB to the HTLV-1 21-bp repeats but does not stimulate CREB binding to the consensus cyclic AMP response element (CRE) element found in the somatostatin promoter. However, Tax stimulates CREB binding to a consensus CRE in the context of the 21-bp repeats, indicating the importance of these sequences in stimulating CREB binding. To determine the mechanism by which Tax stimulates CREB binding and determine potential interactions between Tax and CREB, we used the mammalian two-hybrid system in conjunction with in vitro binding and gel retardation assays. Two-hybrid analysis indicated that mutations in either the basic or leucine zipper region of CREB prevented interactions with Tax. Since several studies have demonstrated that Tax will also stimulate the binding of a variety of different basic region-leucine zipper proteins to their cognate binding sites, we assayed whether chimeric proteins composed of portions of CREB and another basic region-leucine zipper protein, Jun, could be used to map domains required for interactions with Tax. These studies were possible because we did not detect in vivo or in vitro interactions between Tax and Jun. The amino acid sequence of the CREB basic region and a portion of its leucine zipper were required for both in vivo and in vitro interactions with Tax and increased binding of CREB to the 21-bp repeats in response to Tax. These studies define the domains in CREB required for both in vivo and in vitro interactions by the HTLV-1 Tax protein.