PKBalpha/Akt1 acts downstream of DNA-PK in the DNA double-strand break response and promotes survival

Inhibiting DNA-PKcs in a non-homologous end-joining pathway in response to DNA double-strand breaks

DNA-dependent protein kinase catalytic subunit (DNA-PKcs) is a distinct factor in the non-homologous end-joining (NHEJ) pathway involved in DNA double-strand break (DSB) repair. We examined the crosstalk between key proteins in the DSB NHEJ repair pathway and cell cycle regulation and found that mouse embryonic fibroblast (MEF) cells deficient in DNA-PKcs or Ku70 were more vulnerable to ionizing radiation (IR) compared with wild-type cells and that DSB repair was delayed. γH2AX was associated with phospho-Ataxia-telangiectasia mutated kinase (Ser1987) and phospho-checkpoint effector kinase 1 (Ser345) foci for the arrest of cell cycle through the G2/M phase. Inhibition of DNA-PKcs prolonged IR-induced G2/M phase arrest because of sequential activation of cell cycle checkpoints. DSBs were introduced, and cell cycle checkpoints were recruited after exposure to IR in nasopharyngeal carcinoma SUNE-1 cells. NU7441 radiosensitized MEF cells and SUNE-1 cells by interfering with DSB repair. Together, these results reveal a mechanism in which coupling of DSB repair with the cell cycle radiosensitizes NHEJ repair-deficient cells, justifying further development of DNA-PK inhibitors in cancer therapy.

AKT2 suppresses pro-survival autophagy triggered by DNA double-strand breaks in colorectal cancer cells

DNA double-strand breaks (DSBs) are critical DNA lesions, which threaten genome stability and cell survival. DSBs are directly induced by ionizing radiation (IR) and radiomimetic agents, including the cytolethal distending toxin (CDT). This bacterial genotoxin harbors a unique DNase-I-like endonuclease activity. Here we studied the role of DSBs induced by CDT and IR as a trigger of autophagy, which is a cellular degradation process involved in cell homeostasis, genome protection and cancer. The regulatory mechanisms of DSB-induced autophagy were analyzed, focusing on the ATM-p53-mediated DNA damage response and AKT signaling in colorectal cancer cells. We show that treatment of cells with CDT or IR increased the levels of the autophagy marker LC3B-II. Consistently, an enhanced formation of autophagosomes and a decrease of the autophagy substrate p62 were observed. Both CDT and IR concomitantly suppressed mTOR signaling and stimulated the autophagic flux. DSBs were demonstrated as the primary trigger of autophagy using a DNase I-defective CDT mutant, which neither induced DSBs nor autophagy. Genetic abrogation of p53 and inhibition of ATM signaling impaired the autophagic flux as revealed by LC3B-II accumulation and reduced formation of autophagic vesicles. Blocking of DSB-induced apoptotic cell death by the pan-caspase inhibitor Z-VAD stimulated autophagy. In line with this, pharmacological inhibition of autophagy increased cell death, while ATG5 knockdown did not affect cell death after DSB induction. Interestingly, both IR and CDT caused AKT activation, which repressed DSB-triggered autophagy independent of the cellular DNA-PK status. Further knockdown and pharmacological inhibitor experiments provided evidence that the negative autophagy regulation was largely attributable to AKT2. Finally, we show that upregulation of CDT-induced autophagy upon AKT inhibition resulted in lower apoptosis and increased cell viability. Collectively, the findings demonstrate that DSBs trigger pro-survival autophagy in an ATM- and p53-dependent manner, which is curtailed by AKT2 signaling.

Lack of Constitutively Active DNA Repair Sensitizes Glioblastomas to Akt Inhibition and Induces Synthetic Lethality with Radiation Treatment in a p53-Dependent Manner

Treatment refractory glioblastoma (GBM) remains a major clinical problem globally, and targeted therapies in GBM have not been promising to date. The Cancer Genome Atlas integrative analysis of GBM reported the striking finding of genetic alterations in the p53 and PI3K pathways in more than 80% of GBMs. Given the role of these pathways in making cell-fate decisions and responding to genotoxic stress, we investigated the reliance of these two pathways in mediating radiation resistance. We selected a panel of GBM cell lines and glioma stem cells (GSC) with wild-type TP53 (p53-wt) and mutant TP53, mutations known to interfere with p53 functionality (p53-mt). Cell lines were treated with a brain permeable inhibitor of P-Akt (ser473), phosphatidylinositol ether lipid analogue (PIA), with and without radiation treatment. Sensitivity to treatment was measured using Annexin-V/PI flow cytometry and Western blot analysis for the markers of apoptotic signaling, alkaline COMET assay. All results were verified in p53 isogenic cell lines. p53-mt cell lines were selectively radiosensitized by PIA. This radiosensitization effect corresponded with an increase in DNA damage and a decrease in DNA-PKcs levels. TP53 silencing in p53-wt cells showed a similar response as the p53-mt cells. In addition, the radiosensitization effects of Akt inhibition were not observed in normal human astrocytes, suggesting that this treatment strategy could have limited off-target effects. We demonstrate that the inhibition of the PI3K/Akt pathway by PIA radiosensitizes p53-mt cells by antagonizing DNA repair. In principle, this strategy could provide a large therapeutic window for the treatment of TP53-mutant tumors. Mol Cancer Ther; 17(2); 336-46. ©2017 AACRSee all articles in this MCT Focus section, "Developmental Therapeutics in Radiation Oncology."

The multifaceted roles of inflammasome proteins in cancer

PURPOSE OF REVIEW

Inflammasomes are major actors of the innate immune system, through their regulation of inflammatory caspases and maturation of IL-1β and IL-18. These multiprotein complexes have been shown to play major roles in inflammatory and metabolic diseases and have more recently been implicated in tumor development and dissemination. In this review, we address these recent findings, focusing particularly on colorectal cancer (CRC) initiation and tumor dissemination.

RECENT FINDINGS

Based mostly on loss-of-function experiments in mouse models, paradoxical results were obtained as both protumoral and antitumoral activities were reported. Moreover, several studies report major inflammasome-independent functions for some of these innate receptor proteins such as absent in melanoma 2, nod-like receptor family pyrin containing 3 (NLRP3) or nod-like receptor family CARD containing 4 (NLRC4), functions exerted in epithelial cells as well as in immune cells.

SUMMARY

The current review summarizes recent findings on the implication of inflammasomes and of absent in melanoma 2, NLRC4 and NLRP3 inflammasome-independent functions in cancer development and dissemination. Although contradictory in certain aspects, these studies highlight a lack of understanding of their mechanistic functions and regulations in cancer and the need for further investigations.

Nuclear Acetyl-CoA Production by ACLY Promotes Homologous Recombination

While maintaining the integrity of the genome and sustaining bioenergetics are both fundamental functions of the cell, potential crosstalk between metabolic and DNA repair pathways is poorly understood. Since histone acetylation plays important roles in DNA repair and is sensitive to the availability of acetyl coenzyme A (acetyl-CoA), we investigated a role for metabolic regulation of histone acetylation during the DNA damage response. In this study, we report that nuclear ATP-citrate lyase (ACLY) is phosphorylated at S455 downstream of ataxia telangiectasia mutated (ATM) and AKT following DNA damage. ACLY facilitates histone acetylation at double-strand break (DSB) sites, impairing 53BP1 localization and enabling BRCA1 recruitment and DNA repair by homologous recombination. ACLY phosphorylation and nuclear localization are necessary for its role in promoting BRCA1 recruitment. Upon PARP inhibition, ACLY silencing promotes genomic instability and cell death. Thus, the spatial and temporal control of acetyl-CoA production by ACLY participates in the mechanism of DNA repair pathway choice.

Long Noncoding RNA GAS5, Which Acts as a Tumor Suppressor via microRNA 21, Regulates Cisplatin Resistance Expression in Cervical Cancer

OBJECTIVES

The aims of this study were to investigate the functions of GAS5 as a tumor suppressor in cervical cancer and explore the mechanism.

METHODS

The expression of GAS5 and microRNA 21 (miR-21) was detected in primary cervical cancer tissue specimens, as well as in cervical cancer cell lines. We identified the interaction of GAS5 and miR-21 by quantitative polymerase chain reaction, Western blot, and dual-luciferase reporter assay. We also studied the functions of GAS5 in proliferation, apoptosis, migration, and invasion in cervical cancer cells in vitro and vivo. Finally, the impact of GAS5 on cisplatin resistance and its mechanism in cervical cancer cells was also identified.

RESULTS

The expression of GAS5 and miR-21 was detected in primary cervical cancer tissue specimens, as well as in cervical cancer cell lines. GAS5, which is a tumor suppressor playing roles in inhibiting the malignancy of cervical cancer cells, including proliferation in vivo and vitro, migration, and invasion, has a low expression in cervical cancer tissue and cervical cancer cell lines, whereas miR-21 expression is high. GAS5 significantly decreased the expression of miR-21, and there is a reciprocal repression of gene expression between GAS5 and miR-21. Besides, most importantly, we found that high expression of GAS5 and low expression of miR-21 can enhance the sensitivity of SiHa/cDDP cancer cells to cisplatin. A further experiment for identifying the mechanism of cisplatin resistance by GAS5 showed that GAS5 can not only regulate phosphatase and tensin homolog through miR-21 but also influence the phosphorylation of Akt.

CONCLUSIONS

Our results indicate that GAS5 is a direct target of miR-21 and can predict the clinical staging of cervical cancer. Most importantly, GAS5 can also influence cisplatin resistance in cervical cancer via regulating the phosphorylation of Akt. All of these suggest that GAS5 may be a novel therapeutic target for treating cervical cancer.

AKT/PKB Signaling: Navigating the Network

The Ser and Thr kinase AKT, also known as protein kinase B (PKB), was discovered 25 years ago and has been the focus of tens of thousands of studies in diverse fields of biology and medicine. There have been many advances in our knowledge of the upstream regulatory inputs into AKT, key multifunctional downstream signaling nodes (GSK3, FoxO, mTORC1), which greatly expand the functional repertoire of AKT, and the complex circuitry of this dynamically branching and looping signaling network that is ubiquitous to nearly every cell in our body. Mouse and human genetic studies have also revealed physiological roles for the AKT network in nearly every organ system. Our comprehension of AKT regulation and functions is particularly important given the consequences of AKT dysfunction in diverse pathological settings, including developmental and overgrowth syndromes, cancer, cardiovascular disease, insulin resistance and type 2 diabetes, inflammatory and autoimmune disorders, and neurological disorders. There has also been much progress in developing AKT-selective small molecule inhibitors. Improved understanding of the molecular wiring of the AKT signaling network continues to make an impact that cuts across most disciplines of the biomedical sciences.

Regulation of PI3K/Akt dependent apoptotic markers during b virus infection of human and macaque fibroblasts

B virus (Macacine herpesvirus 1), a simplex virus endemic in macaques, causes encephalitis, encephalomyelitis, and death in 80% of untreated zoonotically infected humans with delayed or no treatment. Here we report a significant difference in PI3K/Akt-dependent apoptosis between B virus infected human and macaque dermal fibroblasts. Our data show that B virus infection in either human or macaque fibroblasts results in activation of Akt via PI3K and this activation does not require viral de novo protein synthesis. Inhibition of PI3K with LY294002 results in a significant reduction of viral titers in B virus infected macaque and human fibroblasts with only a modest difference in the reduction of virus titers between the two cell types. We, therefore, tested the hypothesis that B virus results in the phosphorylation of Akt (S473), which prevents apoptosis, enhancing virus replication in B virus infected macaque dermal fibroblasts. We observed markers of intrinsic apoptosis when PI3K activation of Akt was inhibited in B virus infected macaque cells, while, these apoptotic markers were absent in B virus infected human fibroblasts under the same conditions. From these data we suggest that PI3K activates Akt in B virus infected macaque and human fibroblasts, but this enhances virus replication in macaque fibroblast cells by blocking apoptosis.

Targeting the AKT pathway: Repositioning HIV protease inhibitors as radiosensitizers

Cellular resistance in tumour cells to different therapeutic approaches has been a limiting factor in the curative treatment of cancer. Resistance to therapeutic radiation is a common phenomenon which significantly reduces treatment options and impacts survival. One of the mechanisms of acquiring resistance to ionizing radiation is the overexpression or activation of various oncogenes like the EGFR (epidermal growth factor receptor), RAS (rat sarcoma) oncogene or loss of PTEN (phosphatase and tensin homologue) which in turn activates the phosphatidyl inositol 3-kinase/protein kinase B (PI3-K)/AKT pathway responsible for radiation resistance in various tumours. Blocking the pathway enhances the radiation response both in vitro and in vivo. Due to the differential activation of this pathway (constitutively activated in tumour cells and not in the normal host cells), it is an excellent candidate target for molecular targeted therapy to enhance radiation sensitivity. In this regard, HIV protease inhibitors (HPIs) known to interfere with PI3-K/AKT signaling in tumour cells, have been shown to sensitize various tumour cells to radiation both in vitro and in vivo. As a result, HPIs are now being investigated as possible radiosensitizers along with various chemotherapeutic drugs. This review describes the mechanisms by which PI3-K/AKT pathway causes radioresistance and the role of HIV protease inhibitors especially nelfinavir as a potential candidate drug to target the AKT pathway for overcoming radioresistance and its use in various clinical trials for different malignancies.

Targeting HDAC3, a new partner protein of AKT in the reversal of chemoresistance in acute myeloid leukemia via DNA damage response

Resistance to cytotoxic chemotherapy drugs remains as the major cause of treatment failure in acute myeloid leukemia. Histone deacetylases (HDAC) are important regulators to maintain chromatin structure and control DNA damage; nevertheless, how each HDAC regulates genome stability remains unclear, especially under genome stress conditions. Here, we identified a mechanism by which HDAC3 regulates DNA damage repair and mediates resistance to chemotherapy drugs. In addition to inducing DNA damage, chemotherapy drugs trigger upregulation of HDAC3 expression in leukemia cells. Using genetic and pharmacological approaches, we show that HDAC3 contributes to chemotherapy resistance by regulating the activation of AKT, a well-documented factor in drug resistance development. HDAC3 binds to AKT and deacetylates it at the site Lys20, thereby promoting the phosphorylation of AKT. Chemotherapy drug exposure enhances the interaction between HDAC3 and AKT, resulting in decrease in AKT acetylation and increase in AKT phosphorylation. Whereas HDAC3 depletion or inhibition abrogates these responses and meanwhile sensitizes leukemia cells to chemotoxicity-induced apoptosis. Importantly, in vivo HDAC3 suppression reduces leukemia progression and sensitizes MLL-AF9⁺ leukemia to chemotherapy. Our findings suggest that combination therapy with HDAC3 inhibitor and genotoxic agents may constitute a successful strategy for overcoming chemotherapy resistance.

Timp1 Promotes Cell Survival by Activating the PDK1 Signaling Pathway in Melanoma

High TIMP1 expression is associated with poor prognosis in melanoma, where it can bind to CD63 and β1 integrin, inducing PI3-kinase pathway and cell survival. Phosphatidylinositol (3,4,5)-trisphosphate (PIP3), generated under phosphatidylinositol-3-kinase (PI3K) activation, enables the recruitment and activation of protein kinase B (PKB/AKT) and phosphoinositide-dependent kinase 1 (PDK1) at the membrane, resulting in the phosphorylation of a host of other proteins. Using a melanoma progression model, we evaluated the impact of Timp1 and AKT silencing, as well as PI3K, PDK1, and protein kinase C (PKC) inhibitors on aggressiveness characteristics. Timp1 downregulation resulted in decreased anoikis resistance, clonogenicity, dacarbazine resistance, and in vivo tumor growth and lung colonization. In metastatic cells, pAKT^Thr308 is highly expressed, contributing to anoikis resistance. We showed that PDK1^Ser241 and PKCβII^Ser660 are activated by Timp1 in different stages of melanoma progression, contributing to colony formation and anoikis resistance. Moreover, simultaneous inhibition of Timp1 and AKT in metastatic cells resulted in more effective anoikis inhibition. Our findings demonstrate that Timp1 promotes cell survival with the participation of PDK1 and PKC in melanoma. In addition, Timp1 and AKT act synergistically to confer anoikis resistance in advanced tumor stages. This study brings new insights about the mechanisms by which Timp1 promotes cell survival in melanoma, and points to novel perspectives for therapeutic approaches.

Maximising the potential of AKT inhibitors as anti-cancer treatments

PI3K/AKT signalling is commonly disrupted in human cancers, with AKT being a central component of the pathway, influencing multiple processes that are directly involved in tumourigenesis. Targeting AKT is therefore a highly attractive anti-cancer strategy with multiple AKT inhibitors now in various stages of clinical development. In this review, we summarise the role and regulation of AKT signalling in normal cellular physiology. We highlight the mechanisms by which AKT signalling can be hyperactivated in cancers and discuss the past, present and future clinical strategies for AKT inhibition in oncology.

Cross-talk between the CK2 and AKT signaling pathways in cancer

CK2 and AKT display a high degree of cross-regulation of their respective functions, both directly, through physical interaction and phosphorylation, and indirectly, through an intense cross-talk of key downstream effectors, ultimately leading to sustained AKT activation. Being CK2 and AKT attractive targets for therapeutic intervention, here we would like to emphasize how AKT and CK2 might influence cell fate through their complex isoform-specific and contextual-dependent cross-talk, to the extent that such functional interplay should be considered when devising therapies that target one or both these key signaling kinases.

Activating Akt1 mutations alter DNA double strand break repair and radiosensitivity

The survival kinase Akt has clinical relevance to radioresistance. However, its contributions to the DNA damage response, DNA double strand break (DSB) repair and apoptosis remain poorly defined and often contradictory. We used a genetic approach to explore the consequences of genetic alterations of Akt1 for the cellular radiation response. While two activation-associated mutants with prominent nuclear access, the phospho-mimicking Akt1-TDSD and the clinically relevant PH-domain mutation Akt1-E17K, accelerated DSB repair and improved survival of irradiated Tramp-C1 murine prostate cancer cells and Akt1-knockout murine embryonic fibroblasts in vitro, the classical constitutively active membrane-targeted myrAkt1 mutant had the opposite effects. Interestingly, DNA-PKcs directly phosphorylated Akt1 at S473 in an in vitro kinase assay but not vice-versa. Pharmacological inhibition of DNA-PKcs or Akt restored radiosensitivity in tumour cells expressing Akt1-E17K or Akt1-TDSD. In conclusion, Akt1-mediated radioresistance depends on its activation state and nuclear localization and is accessible to pharmacologic inhibition.

FANCI is a negative regulator of Akt activation

Akt is a critical mediator of the oncogenic PI3K pathway, and its activation is regulated by kinases and phosphatases acting in opposition. We report here the existence of a novel protein complex that is composed minimally of Akt, PHLPP1, PHLPP2, FANCI, FANCD2, USP1 and UAF1. Our studies show that depletion of FANCI, but not FANCD2 or USP1, results in increased phosphorylation and activation of Akt. This activation is due to a reduction in the interaction between PHLPP1 and Akt in the absence of FANCI. In response to DNA damage or growth factor treatment, the interactions between Akt, PHLPP1 and FANCI are reduced consistent with the known phosphorylation of Akt in response to these stimuli. Furthermore, depletion of FANCI results in reduced apoptosis after DNA damage in accord with its role as a negative regular of Akt. Our findings describe an unexpected function for FANCI in the regulation of Akt and define a previously unrecognized intersection between the PI3K-Akt and FA pathways.

Major Physiological Signaling Pathways in the Regulation of Cell Proliferation and Survival

Multiple signaling pathways regulate cell proliferation and survival and are therefore important for maintaining homeostasis of development. The balance between cell growth and death is achieved through orchestrated signal transduction pathways mediated by complex functional interactions between signaling axes, among which, PI3K/Akt and Ras/MAPK as well as JAK/STAT play a dominant role in promoting cell proliferation, differentiation, and survival. In clinical cancer therapies, drug resistance is the major challenge that occurs in almost all targeted therapeutic strategies. Recent advances in research have suggested that the intrinsic pro-survival signaling crosstalk is the driving force in acquired resistance to a targeted therapy, which may be abolished by interfering with the cross-reacting network.

The involvement of Bcl-2 family proteins in AKT-regulated cell survival in cisplatin resistant epithelial ovarian cancer

Many studies involving patients with cisplatin-resistant ovarian cancer have shown that AKT activation leads to inhibition of apoptosis. The aim of this study was to examine the potential involvement of the Bcl-2 family proteins in AKT-regulated cell survival in response to cisplatin treatment. Cisplatin-sensitive (PEO1) and cisplatin-resistant (PEO4) cells were taken from ascites of patients with ovarian cancer before cisplatin treatment and after development of chemoresistance. It was found that cisplatin treatment activated the AKT signaling pathway and promoted cell proliferation in cisplatin-resistant EOC cells. When AKT was transfected into nucleus of cisplatin-resistant ovarian cancer cells, DNA-PK was phosphorylated at S473. The activated AKT (pAKT-S473) in these cells inhibited the death signal induced by cisplatin thereby inhibiting cisplatin-mediated apoptosis. Results from this study showed that the combination of cisplatin, DNA-PK inhibitor NU7441, and AKT inhibitor TCN can overcome drug resistance, increase apoptosis, and re-sensitize PEO4 cells to cisplatin treatment. A decrease in apoptotic activity was seen in PEO4 cells when Bad was downregulated by siRNA, which indicated that Bad promotes apoptosis in PEO4 cells. Use of the Bcl-2 inhibitor ABT-737 showed that ABT-737 binds to Bcl-2 but not Mcl-1 and releases Bax/Bak which leads to cell apoptosis. The combination of ABT-737 and cisplatin leads to a significant increase in the death of PEO1 and PEO4 cells. All together, these results indicate that Bcl-2 family proteins are regulators of drug resistance. The combination of cisplatin and Bcl-2 family protein inhibitor could be a strategy for the treatment of cisplatin-resistant ovarian cancer.

Judicious Toggling of mTOR Activity to Combat Insulin Resistance and Cancer: Current Evidence and Perspectives

The mechanistic target of rapamycin (mTOR), via its two distinct multiprotein complexes, mTORC1, and mTORC2, plays a central role in the regulation of cellular growth, metabolism, and migration. A dysregulation of the mTOR pathway has in turn been implicated in several pathological conditions including insulin resistance and cancer. Overactivation of mTORC1 and disruption of mTORC2 function have been reported to induce insulin resistance. On the other hand, aberrant mTORC1 and mTORC2 signaling via either genetic alterations or increased expression of proteins regulating mTOR and its downstream targets have contributed to cancer development. These underlined the attractiveness of mTOR as a therapeutic target to overcome both insulin resistance and cancer. This review summarizes the evidence supporting the notion of intermittent, low dose rapamycin for treating insulin resistance. It further highlights recent data on the continuous use of high dose rapamycin analogs and related second generation mTOR inhibitors for cancer eradication, for overcoming chemoresistance and for tumor stem cell suppression. Within these contexts, the potential challenges associated with the use of mTOR inhibitors are also discussed.

mTORC2 activation is regulated by the urokinase receptor (uPAR) in bladder cancer

Mammalian target of rapamycin complex 2 (mTORC2) has been identified as a major regulator of bladder cancer cell migration and invasion. Upstream pathways that mediate mTORC2 activation remain poorly defined. Urokinase-type plasminogen activator receptor (uPAR) is a GPI-anchored membrane protein and known activator of cell-signaling. We identified increased uPAR expression in 94% of invasive human bladder cancers and in 54-71% of non-invasive bladder cancers, depending on grade. Normal urothelium was uPAR-immunonegative. Analysis of publicly available datasets identified uPAR gene amplification or mRNA upregulation in a subset of bladder cancer patients with reduced overall survival. Using biochemical approaches, we showed that uPAR activates mTORC2 in bladder cancer cells. Highly invasive bladder cancer cell lines, including T24, J82 and UM-UC-3 cells, showed increased uPAR mRNA expression and protein levels compared with the less aggressive cell lines, UROtsa and RT4. uPAR gene-silencing significantly reduced phosphorylation of Serine-473 in Akt, an mTORC2 target. uPAR gene-silencing also reduced bladder cancer cell migration and Matrigel invasion. S473 phosphorylation was observed by immunohistochemistry in human bladder cancers only when the tumors expressed high levels of uPAR. S473 phosphorylation was not controlled by uPAR in bladder cancer cell lines that are PTEN-negative; however, this result probably did not reflect altered mTORC2 regulation. Instead, PTEN deficiency de-repressed alternative kinases that phosphorylate S473. Our results suggest that uPAR and mTORC2 are components of a single cell-signaling pathway. Targeting uPAR or mTORC2 may be beneficial in patients with bladder cancer.

Phosphatidylinositide 3-kinase (PI3K) and PI3K-related kinase (PIKK) activity contributes to radioresistance in thyroid carcinomas

Anaplastic (ATC) and certain follicular thyroid-carcinomas (FTCs) are radioresistant. The Phosphatidylinositide 3-kinase (PI3K) pathway is commonly hyperactivated in thyroid-carcinomas. PI3K can modify the PI3K-related kinases (PIKKs) in response to radiation: How PIKKs interact with PI3K and contribute to radioresistance in thyroid-carcinomas is unknown. Further uncertainties exist in how these interactions function under the radioresistant hypoxic microenvironment. Under normoxia/anoxia, ATC (8505c) and FTC (FTC-133) cells were irradiated, with PI3K-inhibition (via GDC-0941 and PTEN-reconstitution into PTEN-null FTC-133s) and effects on PIKK-activation, DNA-damage, clonogenic-survival and cell cycle, assessed. FTC-xenografts were treated with 5 × 2 Gy, ± 50 mg/kg GDC-0941 (twice-daily; orally) for 14 days and PIKK-activation and tumour-growth assessed. PIKK-expression was additionally assessed in 12 human papillary thyroid-carcinomas, 13 FTCs and 12 ATCs. GDC-0941 inhibited radiation-induced activation of Ataxia-telangiectasia mutated (ATM), ATM-and Rad3-related (ATR) and DNA-dependent protein kinase catalytic subunit (DNA-PKcs). Inhibition of ATM and DNA-PKcs was PI3K-dependent, since activation was reduced in PTEN-reconstituted FTC-133s. Inhibition of PIKK-activation was greater under anoxia: Consequently, whilst DNA-damage was increased and prolonged under both normoxia and anoxia, PI3K-inhibition only reduced clonogenic-survival under anoxia. GDC-0941 abrogated radiation-induced cell cycle arrest, an effect most likely linked to the marked inhibition of ATR-activation. Importantly, GDC-0941 inhibited radiation-induced PIKK-activation in FTC-xenografts leading to a significant increase in time taken for tumours to triple in size: 26.5 ± 5 days (radiation-alone) versus 31.5 ± 5 days (dual-treatment). PIKKs were highly expressed across human thyroid-carcinoma classifications, with ATM scoring consistently lower. Interestingly, some loss of ATM and DNA-PKcs was observed. These data provide new insight into the mechanisms of hypoxia-associated radioresistance in thyroid-carcinoma.

Targeting hyperactivated DNA-PKcs by KU0060648 inhibits glioma progression and enhances temozolomide therapy via suppression of AKT signaling

The overall survival remains undesirable in clinical glioma treatment. Inhibition of DNA-PKcs activity by its inhibitors suppresses tumor growth and enhances chemosensitivity of several tumors to chemotherapy. However, whether DNA-PKcs could be a potential target in glioma therapy remains unknown. In this study, we reported that the hyperactivated DNA-PKcs was profoundly correlated with glioma malignancy and observe a significant association between DNA-PKcs activation and survival of the glioma patients. Our data also found that inhibition of DNA-PKcs by its inhibitor KU0060648 sensitized glioma cells to TMZ in vitro. Specifically, we demonstrated that KU0060648 interrupted the formation of DNA-PKcs/AKT complex, leading to suppression of AKT signaling and resultantly enhanced TMZ efficacy. Combination of KU0060648 and TMZ substantially inhibited downstream effectors of AKT. The in vivo results were similar to those obtained in vitro. In conclusion, this study indicated that inhibition of DNA-PKcs activity could suppress glioma malignancies and increase TMZ efficacy, which was mainly through regulation of the of AKT signaling. Therefore, DNA-PKcs/AKT axis may be a promising target for improving current glioma therapy.

Dual Targeting of Akt and mTORC1 Impairs Repair of DNA Double-Strand Breaks and Increases Radiation Sensitivity of Human Tumor Cells

Inhibition of mammalian target of rapamycin-complex 1 (mTORC1) induces activation of Akt. Because Akt activity mediates the repair of ionizing radiation-induced DNA double-strand breaks (DNA-DSBs) and consequently the radioresistance of solid tumors, we investigated whether dual targeting of mTORC1 and Akt impairs DNA-DSB repair and induces radiosensitization. Combining mTORC1 inhibitor rapamycin with ionizing radiation in human non-small cell lung cancer (NSCLC) cells (H661, H460, SK-MES-1, HTB-182, A549) and in the breast cancer cell line MDA-MB-231 resulted in radiosensitization of H661 and H460 cells (responders), whereas only a very slight effect was observed in A549 cells, and no effect was observed in SK-MES-1, HTB-182 or MDA-MB-231 cells (non-responders). In responder cells, rapamycin treatment did not activate Akt1 phosphorylation, whereas in non-responders, rapamycin mediated PI3K-dependent Akt activity. Molecular targeting of Akt by Akt inhibitor MK2206 or knockdown of Akt1 led to a rapamycin-induced radiosensitization of non-responder cells. Compared to the single targeting of Akt, the dual targeting of mTORC1 and Akt1 markedly enhanced the frequency of residual DNA-DSBs by inhibiting the non-homologous end joining repair pathway and increased radiation sensitivity. Together, lack of radiosensitization induced by rapamycin was associated with rapamycin-mediated Akt1 activation. Thus, dual targeting of mTORC1 and Akt1 inhibits repair of DNA-DSB leading to radiosensitization of solid tumor cells.

Integrin-linked kinase as a novel molecular switch of the IL-6-NF-κB signaling loop in breast cancer

Substantial evidence has clearly demonstrated the role of the IL-6-NF-κB signaling loop in promoting aggressive phenotypes in breast cancer. However, the exact mechanism by which this inflammatory loop is regulated remains to be defined. Here, we report that integrin-linked kinase (ILK) acts as a molecular switch for this feedback loop. Specifically, we show that IL-6 induces ILK expression via E2F1 upregulation, which, in turn, activates NF-κB signaling to facilitate IL-6 production. shRNA-mediated knockdown or pharmacological inhibition of ILK disrupted this IL-6-NF-κB signaling loop, and blocked IL-6-induced cancer stem cells in vitro and estrogen-independent tumor growth in vivo Together, these findings establish ILK as an intermediary effector of the IL-6-NF-κB feedback loop and a promising therapeutic target for breast cancer.

NSCLC cells demonstrate differential mode of cell death in response to the combined treatment of radiation and a DNA-PKcs inhibitor

The current standard of care for lung cancer consists of concurrent chemotherapy and radiation. Several studies have shown that the DNA-PKcs inhibitor NU7441 is a highly potent radiosensitizer, however, the mechanism of NU7441's anti-proliferation effect has not been fully elucidated. In this study, the combined effect of NU7441 and ionizing radiation (IR) in a panel of non-small cell lung cancer cell lines (A549, H460 and H1299) has been investigated. We found that NU7441 significantly enhances the effect of IR in all cell lines. The notable findings in response to this combined treatment are (i) prolonged delay in IR-induced DNA DSB repair, (ii) induced robust G2/M checkpoint, (iii) increased aberrant mitosis followed by mitotic catastrophe specifically in H1299, (iv) dramatically induced autophagy in A549 and (v) IR-induced senescence specifically in H460. H1299 cells show greater G2 checkpoint adaptation after combined treatment, which can be attributed to higher expression level of Plk1 compared to A549 and H460. The enhanced autophagy after NU7441 treatment in A549 is possibly due to the higher endogenous expression of pS6K compared to H1299 and H460 cells. In conclusion, choice of cell death pathway is dependent on the mutation status and other genetic factors of the cells treated.

Hydrophobic motif site-phosphorylated protein kinase CβII between mTORC2 and Akt regulates high glucose-induced mesangial cell hypertrophy

PKCβII controls the pathologic features of diabetic nephropathy, including glomerular mesangial cell hypertrophy. PKCβII contains the COOH-terminal hydrophobic motif site Ser-660. Whether this hydrophobic motif phosphorylation contributes to high glucose-induced mesangial cell hypertrophy has not been determined. Here we show that, in mesangial cells, high glucose increased phosphorylation of PKCβII at Ser-660 in a phosphatidylinositol 3-kinase (PI3-kinase)-dependent manner. Using siRNAs to downregulate PKCβII, dominant negative PKCβII, and PKCβII hydrophobic motif phosphorylation-deficient mutant, we found that PKCβII regulates activation of mechanistic target of rapamycin complex 1 (mTORC1) and mesangial cell hypertrophy by high glucose. PKCβII via its phosphorylation at Ser-660 regulated phosphorylation of Akt at both catalytic loop and hydrophobic motif sites, resulting in phosphorylation and inactivation of its substrate PRAS40. Specific inhibition of mTORC2 increased mTORC1 activity and induced mesangial cell hypertrophy. In contrast, inhibition of mTORC2 decreased the phosphorylation of PKCβII and Akt, leading to inhibition of PRAS40 phosphorylation and mTORC1 activity and prevented mesangial cell hypertrophy in response to high glucose; expression of constitutively active Akt or mTORC1 restored mesangial cell hypertrophy. Moreover, constitutively active PKCβII reversed the inhibition of high glucose-stimulated Akt phosphorylation and mesangial cell hypertrophy induced by suppression of mTORC2. Finally, using renal cortexes from type 1 diabetic mice, we found that increased phosphorylation of PKCβII at Ser-660 was associated with enhanced Akt phosphorylation and mTORC1 activation. Collectively, our findings identify a signaling route connecting PI3-kinase to mTORC2 to phosphorylate PKCβII at the hydrophobic motif site necessary for Akt phosphorylation and mTORC1 activation, leading to mesangial cell hypertrophy.

Molecular Targeting of Growth Factor Receptor Signaling in Radiation Oncology

Ionizing radiation has been shown to activate and interact with multiple growth factor receptor pathways that can influence tumor response to therapy. Among these receptor interactions, the epidermal growth factor receptor (EGFR) has been the most extensively studied with mature clinical applications during the last decade. The combination of radiation and EGFR-targeting agents using either monoclonal antibody (mAb) or small-molecule tyrosine kinase inhibitor (TKI) offers a promising approach to improve tumor control compared to radiation alone. Several underlying mechanisms have been identified that contribute to improved anti-tumor capacity after combined treatment. These include effects on cell cycle distribution, apoptosis, tumor cell repopulation, DNA damage/repair, and impact on tumor vasculature. However, as with virtually all cancer drugs, patients who initially respond to EGFR-targeted agents may eventually develop resistance and manifest cancer progression. Several potential mechanisms of resistance have been identified including mutations in EGFR and downstream signaling molecules, and activation of alternative member-bound tyrosine kinase receptors that bypass the inhibition of EGFR signaling. Several strategies to overcome the resistance are currently being explored in preclinical and clinical models, including agents that target the EGFR T790 M resistance mutation or target multiple EGFR family members, as well as agents that target other receptor tyrosine kinase and downstream signaling sites. In this chapter, we focus primarily on the interaction of radiation with anti-EGFR therapies to summarize this promising approach and highlight newly developing opportunities.

Taselisib (GDC-0032), a Potent β-Sparing Small Molecule Inhibitor of PI3K, Radiosensitizes Head and Neck Squamous Carcinomas Containing Activating PIK3CA Alterations

PURPOSE

ActivatingPIK3CAgenomic alterations are frequent in head and neck squamous cell carcinoma (HNSCC), and there is an association between phosphoinositide 3-kinase (PI3K) signaling and radioresistance. Hence, we investigated the therapeutic efficacy of inhibiting PI3K with GDC-0032, a PI3K inhibitor with potent activity against p110α, in combination with radiation in HNSCC.

EXPERIMENTAL DESIGN

The efficacy of GDC-0032 was assessedin vitroin 26 HNSCC cell lines with crystal violet proliferation assays, and changes in PI3K signaling were measured by Western blot analysis. Cytotoxicity and radiosensitization were assessed with Annexin V staining via flow cytometry and clonogenic survival assays, respectively. DNA damage repair was assessed with immunofluorescence for γH2AX foci, and cell cycle analysis was performed with flow cytometry.In vivoefficacy of GDC-0032 and radiation was assessed in xenografts implanted into nude mice.

RESULTS

GDC-0032 inhibited potently PI3K signaling and displayed greater antiproliferative activity in HNSCC cell lines withPIK3CAmutations or amplification, whereas cell lines withPTENalterations were relatively resistant to its effects. Pretreatment with GDC-0032 radiosensitizedPIK3CA-mutant HNSCC cells, enhanced radiation-induced apoptosis, impaired DNA damage repair, and prolonged G2-M arrest following irradiation. Furthermore, combined GDC-0032 and radiation was more effective than either treatment alonein vivoin subcutaneous xenograft models.

CONCLUSIONS

GDC-0032 has increased potency in HNSCC cell lines harboringPIK3CA-activating aberrations. Further, combined GDC-0032 and radiotherapy was more efficacious than either treatment alone inPIK3CA-altered HNSCCin vitroandin vivo This strategy warrants further clinical investigation.

Regulation of the p53 response and its relationship to cancer

p53 has been studied intensively as a major tumour suppressor that detects oncogenic events in cancer cells and eliminates them through senescence (a permanent non-proliferative state) or apoptosis. Consistent with this role, p53 activity is compromised in a high proportion of all cancer types, either through mutation of the TP53 gene (encoding p53) or changes in the status of p53 modulators. p53 has additional roles, which may overlap with its tumour-suppressive capacity, in processes including the DNA damage response, metabolism, aging, stem cell differentiation and fertility. Moreover, many mutant p53 proteins, termed 'gain-of-function' (GOF), acquire new activities that help drive cancer aggression. p53 is regulated mainly through protein turnover and operates within a negative-feedback loop with its transcriptional target, MDM2 (murine double minute 2), an E3 ubiquitin ligase which mediates the ubiquitylation and proteasomal degradation of p53. Induction of p53 is achieved largely through uncoupling the p53-MDM2 interaction, leading to elevated p53 levels. Various stress stimuli acting on p53 (such as hyperproliferation and DNA damage) use different, but overlapping, mechanisms to achieve this. Additionally, p53 activity is regulated through critical context-specific or fine-tuning events, mediated primarily through post-translational mechanisms, particularly multi-site phosphorylation and acetylation. In the present review, I broadly examine these events, highlighting their regulatory contributions, their ability to integrate signals from cellular events towards providing most appropriate response to stress conditions and their importance for tumour suppression. These are fascinating aspects of molecular oncology that hold the key to understanding the molecular pathology of cancer and the routes by which it may be tackled therapeutically.

Therapeutic Implications for Overcoming Radiation Resistance in Cancer Therapy

Ionizing radiation (IR), such as X-rays and gamma (γ)-rays, mediates various forms of cancer cell death such as apoptosis, necrosis, autophagy, mitotic catastrophe, and senescence. Among them, apoptosis and mitotic catastrophe are the main mechanisms of IR action. DNA damage and genomic instability contribute to IR-induced cancer cell death. Although IR therapy may be curative in a number of cancer types, the resistance of cancer cells to radiation remains a major therapeutic problem. In this review, we describe the morphological and molecular aspects of various IR-induced types of cell death. We also discuss cytogenetic variations representative of IR-induced DNA damage and genomic instability. Most importantly, we focus on several pathways and their associated marker proteins responsible for cancer resistance and its therapeutic implications in terms of cancer cell death of various types and characteristics. Finally, we propose radiation-sensitization strategies, such as the modification of fractionation, inflammation, and hypoxia and the combined treatment, that can counteract the resistance of tumors to IR.

Cross talk of tyrosine kinases with the DNA damage signaling pathways

Tyrosine kinases respond to extracellular and intracellular cues by activating specific cellular signaling cascades to regulate cell cycle, growth, proliferation, differentiation and survival. Likewise, DNA damage response proteins (DDR) activated by DNA lesions or chromatin alterations recruit the DNA repair and cell cycle checkpoint machinery to restore genome integrity and cellular homeostasis. Several new examples have been uncovered in recent studies which reveal novel epigenetic and non-epigenetic mechanisms by which tyrosine kinases interact with DDR proteins to dictate cell fate, i.e. survival or apoptosis, following DNA damage. These studies reveal the ability of tyrosine kinases to directly regulate the activity of DNA repair and cell cycle check point proteins by tyrosine phosphorylation. In addition, tyrosine kinases epigenetically regulate DNA damage signaling pathways by modifying the core histones as well as chromatin modifiers at critical tyrosine residues. Thus, deregulated tyrosine kinase driven epigenomic alterations have profound implications in cancer, aging and genetic disorders. Consequently, targeting oncogenic tyrosine kinase induced epigenetic alterations has gained significant traction in overcoming cancer cell resistance to various therapies. This review discusses mechanisms by which tyrosine kinases interact with DDR pathways to regulate processes critical for maintaining genome integrity as well as clinical strategies for targeted cancer therapies.

The critical role of Akt in cardiovascular function

Akt kinase, a member of AGC kinases, is important in many cellular functions including proliferation, migration, cell growth and metabolism. There are three known Akt isoforms which play critical and diverse roles in the cardiovascular system. Akt activity is regulated by its upstream regulatory pathways at transcriptional and post-translational levels. Beta-catenin/Tcf-4, GLI1 and Stat-3 are some of few known transcriptional regulators of AKT gene. Threonine 308 and serine 473 are the two critical phosphorylation sites of Akt1. Translocation of Akt to the cell membrane facilitates PDK1 phosphorylation of the threonine site. The serine site is phosphorylated by mTORC2. Ack1, Src, PTK6, TBK1, IKBKE and IKKε are some of the non-canonical pathways which affect the Akt activity. Protein-protein interactions of Akt to actin and Hsp90 increase the Akt activity while Akt binding to other proteins such as CTMP and TRB3 reduces the Akt activity. The action of Akt on its downstream targets determines its function in cardiovascular processes such as cell survival, growth, proliferation, angiogenesis, vasorelaxation, and cell metabolism. Akt promotes cell survival via caspase-9, YAP, Bcl-2, and Bcl-x activities. Inhibition of FoxO proteins by Akt also increases cell survival by transcriptional mechanisms. Akt stimulates cell growth and proliferation through mTORC1. Akt also increases VEGF secretion and mediates eNOS phosphorylation, vasorelaxation and angiogenesis. Akt can increase cellular metabolism through its downstream targets GSK3 and GLUT4. The alterations of Akt signaling play an important role in many cardiovascular pathological processes such as atherosclerosis, cardiac hypertrophy, and vascular remodeling. Several Akt inhibitors have been developed and tested as anti-tumor agents. They could be potential novel therapeutics for the cardiovascular diseases.

Regulation of MDM2 Stability After DNA Damage

Cells in our body are constantly exposed to various stresses and threats to their genomic integrity. The tumor suppressor protein p53 plays a critical role in successful defense against these threats by inducing apoptotic cell death or cell cycle arrest. In unstressed conditions, p53 levels and activity must be kept low to prevent lethal activation of apoptotic and senescence pathways. However, upon DNA damage or other stressors, p53 is released from its inhibitory state to induce an array of apoptosis and cell cycle genes. Conversely, inactivation of p53 could promote unrestrained tumor proliferation and failure to appropriately undergo apoptotic cell death, which could, in turn, lead to carcinogenesis. The ubiquitin E3 ligase MDM2 is the most critical inhibitor of p53 that determines the cellular response to various p53-activating agents, including DNA damage. MDM2 activity is controlled by post-translational modifications, especially phosphorylation. However, accumulating evidence suggests that MDM2 is also regulated at the level of protein stability, which is controlled by the ubiquitin-proteasome pathway. Here, we discuss how MDM2 can be regulated in response to DNA damage with particular focus on the regulation of MDM2 protein stability.

PtdIns(3,4,5)P3-Dependent Activation of the mTORC2 Kinase Complex

mTOR serves as a central regulator of cell growth and metabolism by forming two distinct complexes, mTORC1 and mTORC2. Although mechanisms of mTORC1 activation by growth factors and amino acids have been extensively studied, the upstream regulatory mechanisms leading to mTORC2 activation remain largely elusive. Here, we report that the pleckstrin homology (PH) domain of SIN1, an essential and unique component of mTORC2, interacts with the mTOR kinase domain to suppress mTOR activity. More importantly, PtdIns(3,4,5)P3, but not other PtdInsPn species, interacts with SIN1-PH to release its inhibition on the mTOR kinase domain, thereby triggering mTORC2 activation. Mutating critical SIN1 residues that mediate PtdIns(3,4,5)P3 interaction inactivates mTORC2, whereas mTORC2 activity is pathologically increased by patient-derived mutations in the SIN1-PH domain, promoting cell growth and tumor formation. Together, our study unravels a PI3K-dependent mechanism for mTORC2 activation, allowing mTORC2 to activate AKT in a manner that is regulated temporally and spatially by PtdIns(3,4,5)P3.

SIGNIFICANCE

The SIN1-PH domain interacts with the mTOR kinase domain to suppress mTOR activity, and PtdIns(3,4,5)P3 binds the SIN1-PH domain to release its inhibition on the mTOR kinase domain, leading to mTORC2 activation. Cancer patient-derived SIN1-PH domain mutations gain oncogenicity by loss of suppressing mTOR activity as a means to facilitate tumorigenesis.

Alternative Activation Mechanisms of Protein Kinase B Trigger Distinct Downstream Signaling Responses

Protein kinase B (PKB/Akt) is an important mediator of signals that control various cellular processes including cell survival, growth, proliferation, and metabolism. PKB promotes these processes by phosphorylating many cellular targets, which trigger distinct downstream signaling events. However, how PKB is able to selectively target its substrates to induce specific cellular functions remains elusive. Here we perform a systematic study to dissect mechanisms that regulate intrinsic kinase activity versus mechanisms that specifically regulate activity toward specific substrates. We demonstrate that activation loop phosphorylation and the C-terminal hydrophobic motif are essential for high PKB activity in general. On the other hand, we identify membrane targeting, which for decades has been regarded as an essential step in PKB activation, as a mechanism mainly affecting substrate selectivity. Further, we show that PKB activity in cells can be triggered independently of PI3K by initial hydrophobic motif phosphorylation, presumably through a mechanism analogous to other AGC kinases. Importantly, different modes of PKB activation result in phosphorylation of distinct downstream targets. Our data indicate that specific mechanisms have evolved for signaling nodes, like PKB, to select between various downstream events. Targeting such mechanisms selectively could facilitate the development of therapeutics that might limit toxic side effects.

Akt promotes tumorigenesis in part through modulating genomic instability via phosphorylating XLF

To maintain genome stability, mammalian cells have developed a delicate, yet efficient, system to sense and repair damaged DNA, including two evolutionarily conserved DNA damage repair (DDR) pathways: homologous recombination (HR) and non-homologous-end-joining (NHEJ). Deregulation in these repair pathways may lead to genomic instability and subsequent human diseases, including cancer. On the other hand, hyper-activation of the oncogenic Akt signaling pathway has been observed in almost all solid tumors. Emerging evidence has begun to reveal a possible role of active Akt in regulating DDR, possibly through suppression of HR. However, whether and how Akt regulates NHEJ remains largely undefined. To this end, we recently reported that Akt impairs NHEJ by phosphorylating XLF at T181, to trigger its dissociation from the functional DNA ligase IV (LIG4)/XRCC4 complex. Here, we provide an additional perspective discussing how Akt is activated upon DNA damage to regulate DNA repair pathways as well as the cellular apoptotic responses.

Akt kinase C-terminal modifications control activation loop dephosphorylation and enhance insulin response

The Akt protein kinase, also known as protein kinase B, plays key roles in insulin receptor signalling and regulates cell growth, survival and metabolism. Recently, we described a mechanism to enhance Akt phosphorylation that restricts access of cellular phosphatases to the Akt activation loop (Thr(308) in Akt1 or protein kinase B isoform alpha) in an ATP-dependent manner. In the present paper, we describe a distinct mechanism to control Thr(308) dephosphorylation and thus Akt deactivation that depends on intramolecular interactions of Akt C-terminal sequences with its kinase domain. Modifications of amino acids surrounding the Akt1 C-terminal mTORC2 (mammalian target of rapamycin complex 2) phosphorylation site (Ser(473)) increased phosphatase resistance of the phosphorylated activation loop (pThr(308)) and amplified Akt phosphorylation. Furthermore, the phosphatase-resistant Akt was refractory to ceramide-dependent dephosphorylation and amplified insulin-dependent Thr(308) phosphorylation in a regulated fashion. Collectively, these results suggest that the Akt C-terminal hydrophobic groove is a target for the development of agents that enhance Akt phosphorylation by insulin.

The PI3K/AKT Pathway and Renal Cell Carcinoma

The phosphatidylinositol 3 kinase (PI3K)/AKT pathway is genetically targeted in more pathway components and in more tumor types than any other growth factor signaling pathway, and thus is frequently activated as a cancer driver. More importantly, the PI3K/AKT pathway is composed of multiple bifurcating and converging kinase cascades, providing many potential targets for cancer therapy. Renal cell carcinoma (RCC) is a high-risk and high-mortality cancer that is notoriously resistant to traditional chemotherapies or radiotherapies. The PI3K/AKT pathway is modestly mutated but highly activated in RCC, representing a promising drug target. Indeed, PI3K pathway inhibitors of the rapalog family are approved for use in RCC. Recent large-scale integrated analyses of a large number of patients have provided a molecular basis for RCC, reiterating the critical role of the PI3K/AKT pathway in this cancer. In this review, we summarize the genetic alterations of the PI3K/AKT pathway in RCC as indicated in the latest large-scale genome sequencing data, as well as treatments for RCC that target the aberrant activated PI3K/AKT pathway.

Phosphatidylinositol 3-kinase/Akt signaling as a key mediator of tumor cell responsiveness to radiation

The phosphatidylinositol 3-kinase (PI3K)/Akt pathway is a key cascade downstream of several protein kinases, especially membrane-bound receptor tyrosine kinases, including epidermal growth factor receptor (EGFR) family members. Hyperactivation of the PI3K/Akt pathway is correlated with tumor development, progression, poor prognosis, and resistance to cancer therapies, such as radiotherapy, in human solid tumors. Akt/PKB (Protein Kinase B) members are the major kinases that act downstream of PI3K, and these are involved in a variety of cellular functions, including growth, proliferation, glucose metabolism, invasion, metastasis, angiogenesis, and survival. Accumulating evidence indicates that activated Akt is one of the major predictive markers for solid tumor responsiveness to chemo/radiotherapy. DNA double-strand breaks (DNA-DSB), are the prime cause of cell death induced by ionizing radiation. Preclinical in vitro and in vivo studies have shown that constitutive activation of Akt and stress-induced activation of the PI3K/Akt pathway accelerate the repair of DNA-DSB and, consequently, lead to therapy resistance. Analyzing dysregulations of Akt, such as point mutations, gene amplification or overexpression, which results in the constitutive activation of Akt, might be of special importance in the context of radiotherapy outcomes. Such studies, as well as studies of the mechanism(s) by which activated Akt1 regulates repair of DNA-DSB, might help to identify combinations using the appropriate molecular targeting strategies with conventional radiotherapy to overcome radioresistance in solid tumors. In this review, we discuss the dysregulation of the components of upstream regulators of Akt as well as specific modifications of Akt isoforms that enhance Akt activity. Likewise, the mechanisms by which Akt interferes with repair of DNA after exposure to ionizing radiation, will be reviewed. Finally, the current status of Akt targeting in combination with radiotherapy will be discussed.

Nuclear basic fibroblast growth factor regulates triple-negative breast cancer chemo-resistance

INTRODUCTION

Chemotherapy remains the only available treatment for triple-negative (TN) breast cancer, and most patients exhibit an incomplete pathologic response. Half of patients exhibiting an incomplete pathologic response die within five years of treatment due to chemo-resistant, recurrent tumor growth. Defining molecules responsible for TN breast cancer chemo-resistance is crucial for developing effective combination therapies blocking tumor recurrence. Historically, chemo-resistance studies have relied on long-term chemotherapy selection models that drive genetic mutations conferring cell survival. Other models suggest that tumors are heterogeneous, being composed of both chemo-sensitive and chemo-resistant tumor cell populations. We previously described a short-term chemotherapy treatment model that enriches for chemo-residual TN tumor cells. In the current work, we use this enrichment strategy to identify a novel determinant of TN breast cancer chemotherapy resistance [a nuclear isoform of basic fibroblast growth factor (bFGF)].

METHODS

Studies are conducted using our in vitro model of chemotherapy resistance. Short-term chemotherapy treatment enriches for a chemo-residual TN subpopulation that over time resumes proliferation. By western blotting and real-time polymerase chain reaction, we show that this chemotherapy-enriched tumor cell subpopulation expresses nuclear bFGF. The importance of bFGF for survival of these chemo-residual cells is interrogated using short hairpin knockdown strategies. DNA repair capability is assessed by comet assay. Immunohistochemistry (IHC) is used to determine nuclear bFGF expression in TN breast cancer cases pre- and post- neoadjuvant chemotherapy.

RESULTS

TN tumor cells surviving short-term chemotherapy treatment express increased nuclear bFGF. bFGF knockdown reduces the number of chemo-residual TN tumor cells. Adding back a nuclear bFGF construct to bFGF knockdown cells restores their chemo-resistance. Nuclear bFGF-mediated chemo-resistance is associated with increased DNA-dependent protein kinase (DNA-PK) expression and accelerated DNA repair. In fifty-six percent of matched TN breast cancer cases, percent nuclear bFGF-positive tumor cells either increases or remains the same post- neoadjuvant chemotherapy treatment (compared to pre-treatment). These data indicate that in a subset of TN breast cancers, chemotherapy enriches for nuclear bFGF-expressing tumor cells.

CONCLUSION

These studies identify nuclear bFGF as a protein in a subset of TN breast cancers that likely contributes to drug resistance following standard chemotherapy treatment.

MERIT40 Is an Akt Substrate that Promotes Resolution of DNA Damage Induced by Chemotherapy

Resistance to cytotoxic chemotherapy drugs, including doxorubicin, is a significant obstacle to the effective treatment of breast cancer. Here, we have identified a mechanism by which the PI3K/Akt pathway mediates resistance to doxorubicin. In addition to inducing DNA damage, doxorubicin triggers sustained activation of Akt signaling in breast cancer cells. We show that Akt contributes to chemotherapy resistance such that PI3K or Akt inhibitors sensitize cells to doxorubicin. We identify MERIT40, a component of the BRCA1-A DNA damage repair complex, as an Akt substrate that is phosphorylated following doxorubicin treatment. MERIT40 phosphorylation facilitates assembly of the BRCA1-A complex in response to DNA damage and contributes to DNA repair and cell survival following doxorubicin treatment. Finally, MERIT40 phosphorylation in human breast cancers is associated with estrogen receptor positivity. Our findings suggest that combination therapy with PI3K or Akt inhibitors and doxorubicin may constitute a successful strategy for overcoming chemotherapy resistance.

The nucleotide excision repair protein XPC is essential for bulky DNA adducts to promote interleukin-6 expression via the activation of p38-SAPK

Polycyclic aromatic hydrocarbons (PAHs) are environmental pollutants, and many are potent carcinogens. Benzo[a]pyrene (B[a]P), one of the best-studied PAHs, is metabolized ultimately to the genotoxin anti-B[a]P-7,8-dihydrodiol-9,10-epoxide (BPDE). BPDE triggers stress responses linked to gene expression, cell death and survival. So far, the underlying mechanisms that initiate these signal transduction cascades are unknown. Here we show that BPDE-induced DNA damage is recognized by DNA damage sensor proteins to induce activation of the stress-activated protein kinase (SAPK) p38. Surprisingly, the classical DNA damage response, which involves the kinases ATM and ATR, is not involved in p38-SAPK activation by BPDE. Moreover, the induction of p38-SAPK phosphorylation also occurs in the absence of DNA strand breaks. Instead, increased phosphorylation of p38-SAPK requires the nucleotide excision repair (NER) and DNA damage sensor proteins XPC and mHR23B. Interestingly, other genotoxins such as cisplatin (CDDP), hydrogen peroxide and ultraviolet radiation also enhance XPC-dependent p38-SAPK phosphorylation. In contrast, anti-benzo[c]phenanthrene-3,4-dihydrodiol-1,2-epoxide, the DNA adducts of which are not properly recognized by NER, does not trigger p38-SAPK activation. As a downstream consequence, expression and secretion of the pro-inflammatory cytokine interleukin-6 is induced by BPDE and CDDP in vitro and by CDDP in the murine lung, and depends on XPC. In conclusion, we describe a novel pathway in which DNA damage recognition by NER proteins specifically leads to activation of p38-SAPK to promote inflammatory gene expression.

AKT phosphorylates H3-threonine 45 to facilitate termination of gene transcription in response to DNA damage

Post-translational modifications of core histones affect various cellular processes, primarily through transcription. However, their relationship with the termination of transcription has remained largely unknown. In this study, we show that DNA damage-activated AKT phosphorylates threonine 45 of core histone H3 (H3-T45). By genome-wide chromatin immunoprecipitation sequencing (ChIP-seq) analysis, H3-T45 phosphorylation was distributed throughout DNA damage-responsive gene loci, particularly immediately after the transcription termination site. H3-T45 phosphorylation pattern showed close-resemblance to that of RNA polymerase II C-terminal domain (CTD) serine 2 phosphorylation, which establishes the transcription termination signal. AKT1 was more effective than AKT2 in phosphorylating H3-T45. Blocking H3-T45 phosphorylation by inhibiting AKT or through amino acid substitution limited RNA decay downstream of mRNA cleavage sites and decreased RNA polymerase II release from chromatin. Our findings suggest that AKT-mediated phosphorylation of H3-T45 regulates the processing of the 3' end of DNA damage-activated genes to facilitate transcriptional termination.

Hyperthermia Sensitizes Glioma Stem-like Cells to Radiation by Inhibiting AKT Signaling

Glioma stem-like cells (GSC) are a subpopulation of cells in tumors that are believed to mediate self-renewal and relapse in glioblastoma (GBM), the most deadly form of primary brain cancer. In radiation oncology, hyperthermia is known to radiosensitize cells, and it is reemerging as a treatment option for patients with GBM. In this study, we investigated the mechanisms of hyperthermic radiosensitization in GSCs by a phospho-kinase array that revealed the survival kinase AKT as a critical sensitization determinant. GSCs treated with radiation alone exhibited increased AKT activation, but the addition of hyperthermia before radiotherapy reduced AKT activation and impaired GSC proliferation. Introduction of constitutively active AKT in GSCs compromised hyperthermic radiosensitization. Pharmacologic inhibition of PI3K further enhanced the radiosensitizing effects of hyperthermia. In a preclinical orthotopic transplant model of human GBM, thermoradiotherapy reduced pS6 levels, delayed tumor growth, and extended animal survival. Together, our results offer a preclinical proof-of-concept for further evaluation of combined hyperthermia and radiation for GBM treatment.

Dendritic cells induce Th2-mediated airway inflammatory responses to house dust mite via DNA-dependent protein kinase

DNA-dependent protein kinase (DNA-PK) mediates double-stranded DNA break repair, V(D)J recombination and immunoglobulin class switch recombination, as well as innate immune and pro-inflammatory responses. However, there is limited information regarding the role of DNA-PK in adaptive immunity mediated by dendritic cells (DCs), which are the primary antigen-presenting cells in allergic asthma. Here we show that house dust mite induces DNA-PK phosphorylation, which is a marker of DNA-PK activation, in DCs via the generation of intracellular reactive oxygen species. We also demonstrate that pharmacological inhibition of DNA-PK, as well as the specific deletion of DNA-PK in DCs, attenuates the induction of allergic sensitization and Th2 immunity via a mechanism that involves the impaired presentation of mite antigens. Furthermore, pharmacological inhibition of DNA-PK following antigen priming similarly reduces the manifestations of mite-induced airway disease. Collectively, these findings suggest that DNA-PK may be a potential target for treatment of allergic asthma.

Akt-mediated phosphorylation of XLF impairs non-homologous end-joining DNA repair

Deficiency in repair of damaged DNA leads to genomic instability and is closely associated with tumorigenesis. Most DNA double-strand-breaks (DSBs) are repaired by two major mechanisms, homologous-recombination (HR) and non-homologous-end-joining (NHEJ). Although Akt has been reported to suppress HR, its role in NHEJ remains elusive. Here, we report that Akt phosphorylates XLF at Thr181 to trigger its dissociation from the DNA ligase IV/XRCC4 complex, and promotes its interaction with 14-3-3β leading to XLF cytoplasmic retention, where cytosolic XLF is subsequently degraded by SCF(β-TRCP) in a CKI-dependent manner. Physiologically, upon DNA damage, XLF-T181E expressing cells display impaired NHEJ and elevated cell death. Whereas a cancer-patient-derived XLF-R178Q mutant, deficient in XLF-T181 phosphorylation, exhibits an elevated tolerance of DNA damage. Together, our results reveal a pivotal role for Akt in suppressing NHEJ and highlight the tight connection between aberrant Akt hyper-activation and deficiency in timely DSB repair, leading to genomic instability and tumorigenesis.

The phosphoinositide 3-kinase pathway and therapy resistance in cancer

The phosphoinositide 3-kinase (PI3K)/Akt/mechanistic target of rapamycin (mTOR) signaling network is a master regulator of processes that contribute to tumorigenesis and tumor maintenance. The PI3K pathway also plays a critical role in driving resistance to diverse anti-cancer therapies. This review article focuses on mechanisms by which the PI3K pathway contributes to therapy resistance in cancer, and highlights potential combination therapy strategies to circumvent resistance driven by PI3K signaling. In addition, resistance mechanisms that limit the clinical efficacy of small molecule inhibitors of the PI3K pathway are discussed.

Dual PI3K/mTOR inhibitors, GSK2126458 and PKI-587, suppress tumor progression and increase radiosensitivity in nasopharyngeal carcinoma

Although combined chemoradiotherapy has provided considerable improvements for nasopharyngeal carcinoma (NPC), recurrence and metastasis are still frequent. The PI3K/Akt/mTOR pathway plays a critical role in tumor formation and tumor cell survival after radiation-induced DNA damage. In the present study, we evaluated whether inhibition of PI3K/mTOR by two novel dual inhibitors, GSK2126458 and PKI-587, could suppress tumor progression and sensitize NPC cells to radiation. Four NPC cell lines (CNE-1, CNE-2, 5-8F, and 6-10B) were used to analyze the effects of GSK216458 and PKI-587 on cell proliferation, migration, invasion, clonogenic survival, amount of residual γ-H2AX foci, cell cycle, and apoptosis after radiation. A 5-8F xenograft model was used to evaluate the in vivo effects of the two compounds in combination with ionizing radiation (IR). Both GSK216458 and PKI-587 effectively inhibited cell proliferation and motility in NPC cells and suppressed phosphorylation of Akt, mTOR, S6, and 4EBP1 proteins in a concentration- and time-dependent manner. Moreover, both compounds sensitized NPC cells to IR by increasing DNA damage, enhancing G2-M cell-cycle delay, and inducing apoptosis. In vivo, the combination of IR with GSK2126458 or PKI-587 significantly inhibited tumor growth. Antitumor effect was correlated with induction of apoptosis and suppression of the phosphorylation of mTOR, Akt, and 4EBP1. These new findings suggest the usefulness of PI3K/mTOR dual inhibition for antitumor and radiosensitizing. The combination of IR with a dual PI3K/mTOR inhibitor, GSK2126458 or PKI-587, might be a promising therapeutic strategy for NPC.

GOLPH3 links the Golgi, DNA damage, and cancer

GOLPH3 is the first example of an oncogene that functions in secretory trafficking at the Golgi. The discovery of GOLPH3's roles in both cancer and Golgi trafficking raises questions about how GOLPH3 and the Golgi contribute to cancer. Our recent investigation of the regulation of GOLPH3 revealed a surprising response by the Golgi upon DNA damage that is mediated by DNA-PK and GOLPH3. These results provide new insight into the DNA damage response with important implications for understanding the cellular response to standard cancer therapeutic agents.