Jayashree Karar

PI3K/AKT/mTOR Pathway in Angiogenesis

The phosphatidylinositol 3-kinase (PI3K)/AKT/mammalian target of rapamycin (mTOR) pathway is activated in the majority of human cancers. This pathway is known to play a key role in numerous cellular functions including proliferation, adhesion, migration, invasion, metabolism, and survival, but in the current review we focus on its role in angiogenesis. PI3K activation may occur via RAS mutation, loss of phosphatase and tensin homolog (PTEN), or by increased expression of growth factor receptors such as epidermal growth factor receptor. There is a connection between the PI3K pathway and angiogenesis. Hypoxia leads to HIF-1α stabilization and is a major stimulus for increased vascular endothelial growth factor (VEGF) production by tumor cells. However, activation of the PI3K/AKT pathway in tumor cells can also increase VEGF secretion, both by hypoxia-inducible factor 1 (HIF-1) dependent and independent mechanisms. The PI3K/AKT pathway also modulates the expression of other angiogenic factors such as nitric oxide and angiopoietins. Numerous inhibitors targeting the PI3K/AKT/mTOR pathway have been developed, and these agents have been shown to decrease VEGF secretion and angiogenesis. The effect of these inhibitors on tumor vasculature can be difficult to predict. The vasculature of tumors is aberrant, leading to sluggish bloodflow and elevated interstitial blood pressure, which can be perpetuated by the high levels of VEGF. Hence, decreasing VEGF expression can paradoxically lead to vascular normalization and improved bloodflow in some tumors. In addition to its importance in cancer, the PI3K pathway also plays an essential role in the formation of normal blood vessels during development. Embryos with kinase-dead p110α catalytic subunit of PI3K develop vascular defects. Stimulation of endothelial cells by VEGF leads to activation of the PI3K pathway within these cells, which is important for cell migration. Sustained endothelial activation of AKT1 has been shown to induce the formation of structurally abnormal blood vessels that recapitulate the aberrations of tumor vessels. Hence, the PI3K pathway plays an important role in regulating angiogenesis both in normal tissues and in cancers.

Modulating the tumor microenvironment to increase radiation responsiveness

Radiosensitivity can be influenced both by factors intrinsic and extrinsic to the cancer cell. One of the factors in the tumor microenvironment (TME) extrinsic to the cancer cell that can affect radiosensitivity is oxygenation. Severely hypoxic cells require a 2-3 fold higher dose of radiation to achieve the same level of cell killing as do well-oxygenated cells. Other elements in the microenvironment that may influence tumor radiosensitivity are the response of stromal cells to radiation and the expression of factors such as vascular endothelial growth factor (VEGF) and hypoxia inducible factor-1a (HIF-1a). There are currently several classes of agents that may increase tumor radiosensitivity by modulating the TME. Pre-clinical evidence indicates that inhibition of VEGF may increase local control after radiation. Several mechanisms have been postulated to explain this including radiosensitization of tumor endothelial cells, prevention of the establishment of new vasculature post-radiation, and increased oxygenation secondary to vascular normalization. Agents targeting HIF-1a also increase local control after radiation in pre-clinical models. This may occur via indirect inhibition of VEGF, which is a downstream target of HIF-1, or by VEGF-independent means. When combined with radiation, the EGFR inhibitor cetuximab improves local control and survival in patients with head and neck cancer. Pre-clinical data indicate that EGFR inhibitors can increase the intrinsic radiosensitivity of cancer cells. They can also improve tumor blood flow and oxygenation, which may increase extrinsic radiosensitivity. One of the pathways downstream of EGFR that may contribute to this effect is the PI3K/Akt pathway. Agents that directly inhibit this pathway improve blood flow and increase tumor oxygenation in pre-clinical models. The challenge remains to obtain clinical data from patients showing that modulation of the TME is an important mechanism by which biological agents can radiosensitize tumors and then to utilize this information to optimize therapy.

Inhibition of autophagy as a strategy to augment radiosensitization by the dual phosphatidylinositol 3-kinase/mammalian target of rapamycin inhibitor NVP-BEZ235

We investigated the effect of 2-methyl-2-{4-[3-methyl-2-oxo-8-(quinolin-3-yl)-2,3-dihydro-1H-imidazo[4,5-c]quinolin-1-yl]phenyl} propanenitrile (NVP-BEZ235) (Novartis, Basel Switzerland), a dual phosphatidylinositol 3-kinase (PI3K)/mammalian target of rapamycin (mTOR) inhibitor currently being tested in phase I clinical trials, in radiosensitization. NVP-BEZ235 radiosensitized a variety of cancer cell lines, including SQ20B head and neck carcinoma cells and U251 glioblastoma cells. NVP-BEZ235 also increased in vivo radiation response in SQ20B xenografts. Knockdown of Akt1, p110α, or mTOR resulted in radiosensitization, but not to the same degree as with NVP-BEZ235. NVP-BEZ235 interfered with DNA damage repair after radiation as measured by the CometAssay and resolution of phosphorylated H2A histone family member X foci. NVP-BEZ235 abrogated the radiation-induced phosphorylation of both DNA-dependent protein kinase catalytic subunit (DNA-PKcs) and ataxia telangiectasia mutated. Knockdown of either p110α or mTOR failed to decrease the phosphorylation of DNA-PKcs, suggesting that the effect of the drug was direct rather than mediated via p110α or mTOR. The treatment of cells with NVP-BEZ235 also promoted autophagy. To assess the importance of this process in radiosensitization, we used the autophagy inhibitors 3-methyladenine and chloroquine and found that either drug increased cell killing after NVP-BEZ235 treatment and radiation. Knocking down the essential autophagy proteins autophagy related 5 (ATG5) and beclin1 increased NVP-BEZ235-mediated radiosensitization. Furthermore, NVP-BEZ235 radiosensitized autophagy-deficient ATG5(−/−) fibroblasts to a greater extent than ATG5(+/+) cells. We conclude that NVP-BEZ235 radiosensitizes cells and induces autophagy by apparently distinct mechanisms. Inhibiting autophagy via pharmacologic or genetic means increases radiation killing after NVP-BEZ235 treatment; hence, autophagy seems to be cytoprotective in this situation. Our data offer a rationale for combining NVP-BEZ235 along with an autophagy inhibitor (i.e., chloroquine) and radiation in future clinical trials.

In vivo profiling of hypoxic gene expression in gliomas using the hypoxia marker EF5 and laser-capture microdissection

Hypoxia is a key determinant of tumor aggressiveness, yet little is known regarding hypoxic global gene regulation in vivo. We used the hypoxia marker EF5 coupled with laser-capture microdissection to isolate RNA from viable hypoxic and normoxic regions of 9L experimental gliomas. Through microarray analysis, we identified several mRNAs (including the HIF targets Vegf, Glut-1, and Hsp27) with increased levels under hypoxia compared with normoxia both in vitro and in vivo. However, we also found striking differences between the global in vitro and in vivo hypoxic mRNA profiles. Intriguingly, the mRNA levels of a substantial number of immunomodulatory and DNA repair proteins including CXCL9, CD3D, and RAD51 were found to be downregulated in hypoxic areas in vivo, consistent with a protumorigenic role of hypoxia in solid tumors. Immunohistochemical staining verified increased HSP27 and decreased RAD51 protein levels in hypoxic versus normoxic tumor regions. Moreover, CD8(+) T cells, which are recruited to tumors upon stimulation by CXCL9 and CXCL10, were largely excluded from viable hypoxic areas in vivo. This is the first study to analyze the influence of hypoxia on mRNA levels in vivo and can be readily adapted to obtain a comprehensive picture of hypoxic regulation of gene expression and its influence on biological functions in solid tumors.

Anti-tumorigenic effects of Type 1 interferon are subdued by integrated stress responses

Viral and pharmacological inducers of protein kinase RNA-activated (PKR)-like ER kinase (PERK) were shown to accelerate the phosphorylation-dependent degradation of the IFNAR1 chain of the Type 1 interferon (IFN) receptor and to limit cell sensitivity to IFN. Here we report that hypoxia can elicit these effects in a PERK-dependent manner. The altered fate of IFNAR1 affected by signaling downstream of PERK depends on phosphorylation of eIF2α (eukaryotic translational initiation factor 2-α) and ensuing activation of p38α kinase. Activators of other eIF2α kinases such as PKR or GCN2 (general control nonrepressed-2) are also capable of eliminating IFNAR1 and blunting IFN responses. Modulation of constitutive PKR activity in human breast cancer cells stabilizes IFNAR1 and sensitizes these cells to IFNAR1-dependent anti-tumorigenic effects. Although downregulation of IFNAR1 and impaired IFNAR1 signaling can be elicited in response to amino-acid deficit, the knockdown of GCN2 in melanoma cells reverses these phenotypes. We propose that, in cancer cells and the tumor microenvironment, activation of diverse eIF2α kinases followed by IFNAR1 downregulation enables multiple cellular components of tumor tissue to evade the direct and indirect anti-tumorigenic effects of Type 1 IFN.

Dual PI3K/mTOR inhibitor NVP-BEZ235 suppresses hypoxia-inducible factor (HIF)-1α expression by blocking protein translation and increases cell death under hypoxia

The PI3K/Akt pathway is activated in many cancers; therefore, we investigated NVP-BEZ235, a dual PI3K/mTOR inhibitor. BEZ235 was more potent than either the mTOR inhibitor rapamycin or the PI3K inhibitor LY294002 in blocking HIF-1α induction. BEZ235 decreases protein translation, and 7-methyl GTP chromatography showed that the drug induced robust recruitment of 4E-BP1 to eIF4E and a near absence of binding of eIF4G. BEZ235 also decreased expression of other proteins known to be regulated by eIF4E including cyclin B1 and D1 and vascular endothelial growth factor (VEGF). BEZ235 also decreased the level of eIF4G but not eIF4E. As HIF-1α has been associated with adaptation to hypoxic stress, we examined the effect of the drug on cell survival in low pO2. BEZ235 increased killing of cells under hypoxia, measured by short-term (MTT) and long-term (clonogenic) assays. To understand the underlying mechanism, we examined BEZ235’s effect on the expression of factors associated with cell survival. Under normoxia, Akt Ser473 phosphorylation decreased within an hour of BEZ235 treatment, but then increased by 24 h. In contrast, under hypoxia, BEZ235 caused prolonged suppression of Akt Ser473 phosphorylation. Furthermore, there was greater PARP cleavage in hypoxic cells than in normoxic cells, consistent with increased apoptosis. BEZ235 increased autophagy as measured by LC3-I to LC3-II conversion under both normoxic and hypoxic conditions, but our data indicate that this is actually a pro-survival mechanism. In conclusion, we have found that BEZ235 blocks HIF-1α induction by decreasing protein translation and increases cell killing under hypoxia, likely by increasing apoptosis.

LIMD2 Is a Small LIM-Only Protein Overexpressed in Metastatic Lesions That Regulates Cell Motility and Tumor Progression by Directly Binding to and Activating the Integrin-Linked Kinase

Proteins that communicate signals from the cytoskeleton to the nucleus are prime targets for effectors of metastasis as they often transduce signals regulating adhesion, motility, and invasiveness. LIM domain proteins shuttle between the cytoplasm and the nucleus, and bind to partners in both compartments, often coupling changes in gene expression to extracellular cues. In this work, we characterize LIMD2, a mechanistically undefined LIM-only protein originally found to be overexpressed in metastatic lesions but absent in the matched primary tumor. LIMD2 levels in fresh and archival tumors positively correlate with cell motility, metastatic potential, and grade, including bladder, melanoma, breast, and thyroid tumors. LIMD2 directly contributes to these cellular phenotypes as shown by overexpression, knockdown, and reconstitution experiments in cell culture models. The solution structure of LIMD2 that was determined using nuclear magnetic resonance revealed a classic LIM-domain structure that was highly related to LIM1 of PINCH1, a core component of the integrin-linked kinase-parvin-pinch complex. Structural and biochemical analyses revealed that LIMD2 bound directly to the kinase domain of integrin-linked kinase (ILK) near the active site and strongly activated ILK kinase activity. Cells that were null for ILK failed to respond to the induction of invasion by LIMD2. This strongly suggests that LIMD2 potentiates its biologic effects through direct interactions with ILK, a signal transduction pathway firmly linked to cell motility and invasion. In summary, LIMD2 is a new component of the signal transduction cascade that links integrin-mediated signaling to cell motility/metastatic behavior and may be a promising target for controlling tumor spread.

PD46-01 BAP1 DE-UBIQUITINASE IS ALLOSTERICALLY REGULATED BY ASXL1/2: SOMATIC BAP1 MUTATION IN RCC AND OTHER TUMORS INACTIVE THIS REGULATION BY TARGETING MULTIPLE INDEPENDENT DOMAINS

INTRODUCTION AND OBJECTIVES: BAP1 is a ubiquitin carboxy-terminal hydrolaseenzyme (UCH)whichalongwith theASXL1/2 protein forms the PR-DUB, chromatin associated complex which removes monoubiquitin from H2A in nucleosomes regulating gene expression.BAP1 is a tumor suppressor genecommonlymutatedgene in clear cell renal cell carcinoma (ccRCC) and other cancers such as uveal melanoma (UM) and mesothelioma. In tumors, BAP1 mutations occur throughout the protein well outside the N-terminal UCH catalytic region while C-terminal BAP1 truncations/mutations in the ULD domain also inactivate BAP. The aim of this study was to investigate the potential mechanisms whereby these highly spatially distinct mutations in BAP1, many of which occur outside the UCH catalytic domain, serve to completely inactivate the enzyme function. We focused on BAP1s obligate partner protein ASXL1/2 which binds the BAP1 ULD domain and may exert a global regulatory function in the context of PR-DUB in chromatin. A mechanistic understanding of these events might provide insight towards restoration of normal BAP1 functions in tumors leading to therapeutic benefit. METHODS: BAP1 mutations derived from multiple tumor types were identified, including UM, Mesothelioma and ccRCC and introduced into expression vectors and the proteins expressed as recombinants in e.coli and purified to homogeneity. BAP1 and ASXL1/2 protein complexes were evaluated for protein interaction and UCH enzyme activity using GST-interaction studies, Western blot and gel filtration analysis. Molecular modeling of the complex was performed by Blast analysis, molecular docking methods and refined using insights from our mutagenic study. RESULTS: This structure-function study revealed that the BAP1 and ASXL1/2 is direct, high affinity and that multiple contact points between BAP1, ASXL1/2 exist that stabilize the native catalytic structure the The domain structure of BAP1 is unique, multi-lobed and seems to be highly allosteric with respect to enzyme activation. ASXL1/ 2 is an obligate partner for BAP1 and the enzyme is dead with out ASXL1/2 binding which occurs in the BAP1 ULD domain: many tumor derived mutants in the BAP1 ULD domain disrupt ASXL1/2 binding and abolish BAP1 activity. We also discovered a highly conserved domain in ASXL1/2 as minimally essential for BAP1 binding and activation. Mutations in multiple tumors at widely scattered locations decreased ubiquitin hydrolase activity in re-constituted protein complexes and destabilized binding of BAP1 and ASXL1/2 upon modeling. CONCLUSIONS: Different mutations derived from multiple tumors occur throughout the sequence of BAP1 and apparently act allosterically to alter the ability of ASXL1/2 to both bind and/or interact unproductively with the BAP1 UCH region which leads to loss of enzymatic activity and loss of tumor suppression. This analysis suggests that small molecule approaches might be used to reactivate the latent UCH activity in some tumors and restore wild type function of BAP1.

Abstract 2804: Radiosensitization by the Dual PI3K/mTOR inhibitor NVP-BEZ235

Proceedings: AACR 103rd Annual Meeting 2012‐‐ Mar 31‐Apr 4, 2012; Chicago, IL The PI3K/Akt pathway has been implicated in cancer progression and in the survival of cancer cells following DNA damage induced by radiation. Hence, we chose to study the radiosensitizing ability of the Novartis compound, NVP-BEZ235, a dual PI3K/mTOR inhibitor. Clonogenic cell survival assays following ionizing radiation, cell cycle analysis, immunofluorescence for γ-H2AX foci, and Western blotting were performed using standard protocols.Treatment of SQ20B head and neck squamous cell carcinoma cells and U251 glioblastoma cells with NVP-BEZ235 led to a decrease in phosphorylation of Akt at serine 473 and of S6 and 4E-BP1, both of which are downstream markers of mTOR activation. Pre-treatment with the drug (25-50 nM) led to radiosensitization of these cell lines with a dose enhancement ratio (DER) ranging from 1.32 - 1.72. Knockdown of Akt1, p110alpha, or mTOR in these cell lines, also resulted in radiosensitization, but not to the same extent as with NVP-BEZ235. Of note, treatment with rapamycin led to minimal radiosensitization of these cells. Reduced survival with NVP-BEZ235 does not result from cell cycle redistribution during the treatment intervals tested, although combining NVP-BEZ235 with radiation enhances the G2/M delay seen after radiation. We examined how cells died after radiation and found no suggestion of increased apoptosis or necrosis. We did find evidence of increased conversion of microtubule-associated protein light chain 3 (LC3)-I to LC3-II, suggestive of autophagy. In split course experiments, in which cells were either irradiated with a single dose of radiation or with two half-doses separated by time, as the interval between doses increased, so did clonogenic survival. This increase in survival was abolished by pre-treatment with NVP-BEZ235, suggesting that the drug interfered with DNA damage repair. This was confirmed by assessing levels of gamma-H2AX after ionizing radiation. Initial post-radiation levels of gamma-H2AX foci were similar in NVP-BEZ235-pretreated and control cells; however, the resolution of these foci was significantly delayed in the BEZ235-treated cells. We then examined DNA-PKcs, a protein important for DNA damage repair, and found that the drug abrogated the increased phosphorylation normally seen after radiation. These results indicate that NVP-BEZ235 is a potent radiosensitizer of HNC cells in vitro. As siRNA against either p110alpha or mTOR leads to radiosensization, both of these targets of NVP-BEZ235 may be critical mediators of its to radiosensitizing ability; however, a second mechanism involved, may be via delayed repair of DNA damage following radiation, via interference of DNA-PKcs phosphorylation. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 2804. doi:1538-7445.AM2012-2804

Abstract 2800: Dual PI3K/mTOR inhibitor NVP BEZ-235 suppresses translation of hypoxia-inducible factor (HIF)-1α and increases cell death under hypoxia

Proceedings: AACR 103rd Annual Meeting 2012‐‐ Mar 31‐Apr 4, 2012; Chicago, IL Our goal was to investigate the effects of the Novartis compound NVP-BEZ235, a dual PI3K/mTOR inhibitor, on hypoxia-inducible factor-1 alpha (HIF-1α) expression. NVP-BEZ235 was much more potent than either the mTOR inhibitor rapamycin or the PI3K inhibitor LY294002 in blocking the hypoxic induction of HIF-1α in vitro, due to decreased protein translation. 7-methyl GTP chromatography showed that NVP-BEZ235 led to a robust recruitment of 4E-BP1 to eIF4E and a near absence of binding of eIF4G, consistent with inhibition of protein translation. NVP-BEZ235 also decreased expression of several proteins regulated by eIF4E including cyclin B1 and D1, survivin, and vascular endothelial growth factor. NVP-BEZ235 specifically decreased eIF4G but not eIF4E expression. As HIF-1α has been associated with adaptation under hypoxia, we examined the effect of NVP-BEZ235 on cell survival in low pO2 conditions. The drug increased killing of cells under hypoxia as measured by both short-term (MTT) and long-term (clonogenic) assays. To understand the mechanism behind this observation, we performed immunoblotting for factors associated with cell survival. In normoxia, Akt S473 phosphorylation decreased within an hour of NVP-BEZ235 treatment, but then increased by 24 hours. In contrast, under hypoxia, NVP-BEZ235 caused a prolonged suppression of Akt phosphorylation. Furthermore, we found a greater increase in PARP cleavage in hypoxic cells treated 1-16 hours after drug treatment than in normoxic cells, consistent with increased apoptosis. This was confirmed using an ELISA assay that measures cytoplasmic oligonucleosomes. We also found that NVP-BEZ235 increases autophagy as measured by (i) LC3-I to LC3-II conversion on immunoblotting, (ii) degradation of p62 and (iii) GFP-LC3 subcellular localization detected by immunofluorescence. Treatment of cells with the drug under hypoxia further increased autophagy, although it is unclear at this point whether this actually contributes to cell death or is a pro-survival mechanism. In conclusion, we have found that NVP-BEZ235 blocks HIF-1α induction and increases cell killing under hypoxia, which may have important implications for the use of this drug in cancer therapy. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 2800. doi:1538-7445.AM2012-2800