AMPK: A metabolic checkpoint that regulates the growth of EGFR activated glioblastomas

Small Molecule Modulators of AMP-Activated Protein Kinase (AMPK) Activity and Their Potential in Cancer Therapy

AMP-activated protein kinase (AMPK) is a central mediator of cellular metabolism and is activated in direct response to low ATP levels. Activated AMPK inhibits anabolic pathways and promotes catabolic activities that generate ATP through the phosphorylation of multiple target substrates. AMPK is a therapeutic target for activation in several chronic metabolic diseases, and there is increasing interest in targeting AMPK activity in cancer where it can act as a tumor suppressor or conversely it can support cancer cell survival. Small molecule AMPK activators and inhibitors have demonstrated some success in suppressing cancer growth, survival, and drug resistance in preclinical cancer models. In this perspective, we summarize the role of AMPK in cancer and drug resistance, the influence of the tumor microenvironment on AMPK activity, and AMPK activator and inhibitor development. In addition, we discuss the potential importance of isoform-selective targeting of AMPK and approaches for selective AMPK targeting in cancer.

SREBP-1 upregulates lipophagy to maintain cholesterol homeostasis in brain tumor cells

Cholesterol is a structural component of cell membranes. How rapidly growing tumor cells maintain membrane cholesterol homeostasis is poorly understood. Here, we found that glioblastoma (GBM), the most lethal brain tumor, maintains normal levels of membrane cholesterol but with an abundant presence of cholesteryl esters (CEs) in its lipid droplets (LDs). Mechanistically, SREBP-1 (sterol regulatory element-binding protein 1), a master transcription factor that is activated upon cholesterol depletion, upregulates critical autophagic genes, including ATG9B, ATG4A, and LC3B, as well as lysosome cholesterol transporter NPC2. This upregulation promotes LD lipophagy, resulting in the hydrolysis of CEs and the liberation of cholesterol from the lysosomes, thus maintaining plasma membrane cholesterol homeostasis. When this pathway is blocked, GBM cells become quite sensitive to cholesterol deficiency with poor growth in vitro. Our study unravels an SREBP-1-autophagy-LD-CE hydrolysis pathway that plays an important role in maintaining membrane cholesterol homeostasis while providing a potential therapeutic avenue for GBM.

Lipid Metabolism in Glioblastoma: From De Novo Synthesis to Storage

Glioblastoma (GBM) is the most lethal primary brain tumor. With limited therapeutic options, novel therapies are desperately needed. Recent studies have shown that GBM acquires large amounts of lipids for rapid growth through activation of sterol regulatory element-binding protein 1 (SREBP-1), a master transcription factor that regulates fatty acid and cholesterol synthesis, and cholesterol uptake. Interestingly, GBM cells divert substantial quantities of lipids into lipid droplets (LDs), a specific storage organelle for neutral lipids, to prevent lipotoxicity by increasing the expression of diacylglycerol acyltransferase 1 (DGAT1) and sterol-O-acyltransferase 1 (SOAT1), which convert excess fatty acids and cholesterol to triacylglycerol and cholesteryl esters, respectively. In this review, we will summarize recent progress on our understanding of lipid metabolism regulation in GBM to promote tumor growth and discuss novel strategies to specifically induce lipotoxicity to tumor cells through disrupting lipid storage, a promising new avenue for treating GBM.

The Role of Microglia in Glioblastoma

Glioblastoma (GB), the most aggressive malignant glioma, is made up of a large percentage of glioma-associated microglia/macrophages (GAM), suggesting that immune cells play an important role in the pathophysiology of GB. Under physiological conditions, microglia, the phagocytes of the central nervous system (CNS), are involved in various processes such as neurogenesis or axonal growth, and the progression of different conditions such as Alzheimer's disease. Through immunohistochemical studies, markers that enhance GB invasiveness have been shown to be expressed in the peritumoral area of ​​the brain, such as Transforming Growth Factor α (TGF-α), Stromal Sell-Derived Factor 1 (SDF1/CXCL12), Sphingosine-1-Phosphate (S1P) and Neurotrophic Factor Derived from the Glial cell line (GDNF), contributing to the increase in tumor mass. Similarly, it has also been described 17 biomarkers that are present in hypoxic periarteriolar HSC niches in bone marrow and in hypoxic periarteriolar GSC niches in glioblastoma. Interestingly, microglia plays an important role in the microenvironment that supports GB progression, being one of the most important focal points in the study of therapeutic targets for the development of new drugs. In this review, we describe the altered signaling pathways in microglia in the context of GB. We also show how microglia interact with glioblastoma cells and the epigenetic mechanisms involved. Regarding the interactions between microglia and neurogenic niches, some authors indicate that glioblastoma stem cells (GSC) are similar to neural stem cells (NSC), common stem cells in the subventricular zone (SVZ), suggesting that this could be the origin of GB. Understanding the similarities between SVZ and the tumor microenvironment could be important to clarify some mechanisms involved in GB malignancy and to support the discovering of new therapeutic targets for the development of more effective glioblastoma treatments.

Substituted oxindol-3-ylidenes as AMP-activated protein kinase (AMPK) inhibitors

AMP-activated protein kinase (AMPK) is a central metabolic regulator that promotes cancer growth and survival under hypoxia and plays a role in the maintenance of cancer stem cells. A major challenge to interrogating the potential of targeting AMPK in cancer is the lack of potent and selective small molecule inhibitors. Compound C has been widely used as an AMPK inhibitor, but it lacks potency and has a poor selectivity profile. The multi-kinase inhibitor, sunitinib, has demonstrated potent nanomolar inhibition of AMPK activity and has scope for modification. Here, we have designed and synthesized several series of oxindoles to determine the structural requirements for AMPK inhibition and to improve selectivity. We identified two potent, novel oxindole-based AMPK inhibitors that were designed to interact with the DFG motif in the ATP-binding site of AMPK, this key feature evades interaction with the common recptor tyrosine kinase targets of sunitinib. Cellular engagement of AMPK by these oxindoles was confirmed by the inhibition of phosphorylation of acetyl-CoA carboxylase (ACC), a known substrate of AMPK, in myeloid leukemia cells. Interestingly, although AMPK is highly expressed and activated in K562 cells these oxindole-based AMPK inhibitors did not impact cell viability or result in significant cytotoxicity. Our studies serve as a platform for the further development of oxindole-based AMPK inhibitors with therapeutic potential.

Metformin Enhanced in Vitro Radiosensitivity Associates with G2/M Cell Cycle Arrest and Elevated Adenosine-5'-monophosphate-activated Protein Kinase Levels in Glioblastoma

Background

It is hypothesized that metabolism plays a strong role in cancer cell regulation. We have recently demonstrated improved progression-free survival in patients with glioblastoma who received metformin as an antidiabetic substance during chemoradiation. Although metformin is well-established in clinical use the influence of metformin in glioblastoma is far from being understood especially in combination with other treatment modalities such as radiation and temozolomide.

Materials and Methods

In this study, we examined the influence of metformin in combinations with radiation and temozolomide on cell survival (clonogenic survival), cell cycle (routine flow cytometric analysis, FACScan), and phosphorylated Adenosine-5’-monophosphate-activated protein kinase (AMPK) (Phopho-AMPKalpha1 - ELISA) levels in glioblastoma cell lines LN18 and LN229.

Results

Metformin and temozolomide enhanced the effectiveness of photon irradiation in glioblastoma cells. Cell toxicity was more pronounced in O⁶-methylguanine DNA methyltransferase (MGMT) promoter non-methylated LN18 cells. Induction of a G2/M phase cell cycle block through metformin and combined treatments was observed up to 72 h. These findings were associated with elevated levels of activated AMPK levels in LN229 cells but not in LN18 cells after irradiation, metformin, and temozolomide treatment.

Conclusions

Radiosensitizing effects of metformin on glioblastoma cells treated with irradiation and temozolomide in vitro coincided with G2/M arrest and changes in pAMPK levels.

Metabolic Reprogramming in Glioma

Many cancers have long been thought to primarily metabolize glucose for energy production—a phenomenon known as the Warburg Effect, after the classic studies of Otto Warburg in the early twentieth century. Yet cancer cells also utilize other substrates, such as amino acids and fatty acids, to produce raw materials for cellular maintenance and energetic currency to accomplish cellular tasks. The contribution of these substrates is increasingly appreciated in the context of glioma, the most common form of malignant brain tumor. Multiple catabolic pathways are used for energy production within glioma cells, and are linked in many ways to anabolic pathways supporting cellular function. For example: glycolysis both supports energy production and provides carbon skeletons for the synthesis of nucleic acids; meanwhile fatty acids are used both as energetic substrates and as raw materials for lipid membranes. Furthermore, bio-energetic pathways are connected to pro-oncogenic signaling within glioma cells. For example: AMPK signaling links catabolism with cell cycle progression; mTOR signaling contributes to metabolic flexibility and cancer cell survival; the electron transport chain produces ATP and reactive oxygen species (ROS) which act as signaling molecules; Hypoxia Inducible Factors (HIFs) mediate interactions with cells and vasculature within the tumor environment. Mutations in the tumor suppressor p53, and the tricarboxylic acid cycle enzymes Isocitrate Dehydrogenase 1 and 2 have been implicated in oncogenic signaling as well as establishing metabolic phenotypes in genetically-defined subsets of malignant glioma. These pathways critically contribute to tumor biology. The aim of this review is two-fold. Firstly, we present the current state of knowledge regarding the metabolic strategies employed by malignant glioma cells, including aerobic glycolysis; the pentose phosphate pathway; one-carbon metabolism; the tricarboxylic acid cycle, which is central to amino acid metabolism; oxidative phosphorylation; and fatty acid metabolism, which significantly contributes to energy production in glioma cells. Secondly, we highlight processes (including the Randle Effect, AMPK signaling, mTOR activation, etc.) which are understood to link bio-energetic pathways with oncogenic signals, thereby allowing the glioma cell to achieve a pro-malignant state.

microRNA-29 mediates a novel negative feedback loop to regulate SCAP/SREBP-1 and lipid metabolism

The membrane-bound transcription factors, SREBPs (sterol regulatory element-binding proteins), play a central role in regulating lipid metabolism. The transcriptional activation of SREBPs requires the key protein SCAP (SREBP-cleavage activating protein) to translocate their precursors from the endoplasmic reticulum to the Golgi for subsequent proteolytic activation, a process tightly regulated by a cholesterol-mediated negative feedback loop. Our previous work showed that the SCAP/SREBP-1 pathway is significantly upregulated in human glioblastoma (GBM), the most deadly brain cancer, and that glucose-mediated N-glycosylation of SCAP is a prerequisite step for SCAP/SREBP trafficking. More recently, we demonstrated that microRNA-29 (miR-29) mediates a previously unrecognized negative feedback loop in SCAP/SREBP-1 signaling to control lipid metabolism. We found that SREBP-1, functioning as a transcription factor, promotes the expression of the miR-29 family members, miR-29a, -29b and -29c. In turn, the miR-29 isoforms reversely repress the expression of SCAP and SREBP-1. Moreover, treatment with miR-29 mimics effectively suppressed GBM tumor growth by inhibiting SCAP/SREBP-1 and de novo lipid synthesis. These findings, recently published in Cell Reports, strongly suggest that delivery of miR-29 in vivo may be a promising approach to treat cancer and metabolic diseases by suppressing SCAP/SREBP-1-regulated lipid metabolism.

Antiglioma effects of N6-isopentenyladenosine, an endogenous isoprenoid end product, through the downregulation of epidermal growth factor receptor

Malignant gliomas are highly dependent on the isoprenoid pathway for the synthesis of lipid moieties critical for cell proliferation. The isoprenoid derivative N6-isopentenyladenosine (iPA) displays pleiotropic biological effects, including a direct anti-tumor activity in several tumor models. The antiglioma effects of iPA was then explored in U87MG cells both in vitro and grafted in mice and the related molecular mechanism confirmed in primary derived patients' glioma cells. iPA powerfully inhibited tumor cell growth and induced caspase-dependent apoptosis through a mechanism involving a marked accumulation of the pro-apoptotic BIM protein and inhibition of EGFR. Indeed, activating AMPK following conversion into its iPAMP active form, iPA stimulated EGFR phosphorylation and ubiquitination along a proteasome-mediated pathway which was responsible for receptor degradation and its downstream signaling pathways inhibition, including the STAT3, ERK and AKT cascade. The inhibition of AMPK by compound C prevented iPA-mediated phosphorylation of EGFR, known to precede receptor loss. As expected the block of EGFR degradation, by exposure to the proteasome inhibitor MG132, significantly reduced iPA-induced cell death. Given the importance of receptor degradation in iPA-mediated cytotoxicity, we also documented that the EGFR expression levels in a panel of primary glioma cells confers them a high sensitivity to iPA treatment. In conclusion our study provides the first evidence of iPA antiglioma effect. Indeed, as glioma is driven by aberrant signaling of growth factor receptors, particularly the EGFR, iPA, alone or in association with EGFR targeted therapies, might be a promising therapeutic tool to achieve a potent anti-tumoral effect.

Analysis of SCAP N-glycosylation and Trafficking in Human Cells

Elevated lipogenesis is a common characteristic of cancer and metabolic diseases. Sterol regulatory element-binding proteins (SREBPs), a family of membrane-bound transcription factors controlling the expression of genes important for the synthesis of cholesterol, fatty acids and phospholipids, are frequently upregulated in these diseases. In the process of SREBP nuclear translocation, SREBP-cleavage activating protein (SCAP) plays a central role in the trafficking of SREBP from the endoplasmic reticulum (ER) to the Golgi and in subsequent proteolysis activation. Recently, we uncovered that glucose-mediated N-glycosylation of SCAP is a prerequisite condition for the exit of SCAP/SREBP from the ER and movement to the Golgi. N-glycosylation stabilizes SCAP and directs SCAP/SREBP trafficking. Here, we describe a protocol for the isolation of membrane fractions in human cells and for the preparation of the samples for the detection of SCAP N-glycosylation and total protein by using western blot. We further provide a method to monitor SCAP trafficking by using confocal microscopy. This protocol is appropriate for the investigation of SCAP N-glycosylation and trafficking in mammalian cells.

Inhibition of SOAT1 Suppresses Glioblastoma Growth via Blocking SREBP-1-Mediated Lipogenesis

PURPOSE

Elevated lipogenesis regulated by sterol regulatory element-binding protein-1 (SREBP-1), a transcription factor playing a central role in lipid metabolism, is a novel characteristic of glioblastoma (GBM). The aim of this study was to identify effective approaches to suppress GBM growth by inhibition of SREBP-1. As SREBP activation is negatively regulated by endoplasmic reticulum (ER) cholesterol, we sought to determine whether suppression of sterol O-acyltransferase (SOAT), a key enzyme converting ER cholesterol to cholesterol esters (CE) to store in lipid droplets (LDs), effectively suppressed SREBP-1 and blocked GBM growth.

EXPERIMENTAL DESIGN

The presence of LDs in glioma patient tumor tissues was analyzed using immunofluorescence, immunohistochemistry, and electronic microscopy. Western blotting and real-time PCR were performed to analyze protein levels and gene expression of GBM cells, respectively. Intracranial GBM xenografts were used to determine the effects of genetically silencing SOAT1 and SREBP-1 on tumor growth.

RESULTS

Our study unraveled that cholesterol esterification and LD formation are signature of GBM, and human patients with glioma possess elevated LDs that correlate with GBM progression and poor survival. We revealed that SOAT1 is highly expressed in GBM and functions as a key player in controlling the cholesterol esterification and storage in GBM. Targeting SOAT1 suppresses GBM growth and prolongs survival in xenograft models via inhibition of SREBP-1-regulated lipid synthesis.

CONCLUSIONS

Cholesterol esterification and storage in LDs are novel characteristics of GBM, and inhibiting SOAT1 to block cholesterol esterification is a promising therapeutic strategy to treat GBM by suppressing SREBP-1. Clin Cancer Res; 22(21); 5337-48. ©2016 AACR.

SCAP links glucose to lipid metabolism in cancer cells

We recently uncovered that glucose is a critical activator of sterol regulatory element-binding proteins (SREBPs). Glucose promotes SREBP-cleavage activating protein (SCAP)/SREBP complex trafficking from the ER to the Golgi and subsequent SREBP activation via N-glycosylation of SCAP. Our study also demonstrated that SCAP plays a critical role in tumor growth.

The influence of hyperglycemia during radiotherapy on survival in patients with primary glioblastoma

BACKGROUND AND PURPOSE

Metabolism in tumor cells depends mainly on glycolysis and thus hyperglycemia has been shown to influence tumor properties in various tumor entities. In this retrospective study we set out to determine if hyperglycemic serum levels during radiation therapy impact patient survival and progression patterns in primary glioblastoma (GBM).

MATERIAL AND METHODS

We retrospectively analyzed glucose serum levels, survival and progression patterns on magnetic resonance imaging (MRI) in 262 GBM patients receiving radiation therapy. Hyperglycemia was classified as mild (> 180 mg/dL) or excessive (≥ 300 mg/dL), and isolated (one hyperglycemic event) or persistent (≥ 3 hyperglycemic events). The multivariate Cox proportional hazards ratio was used to assess the influence of cofactors on survival.

RESULTS

Persistent mild (HR = 2.23; p < 0.001) and excessive hyperglycemia (HR = 2.51; p < 0.001) were associated with a decrease in overall survival rates, even when considering the covariate corticosteroid therapy. Here metabolic imbalances did not affect the progression-free interval (p = 0.402), the occurrence of distant (p = 0.587) and multifocal progression (p = 0.445).

CONCLUSION

Our findings support the theory that hyperglycemia during radiation therapy in GBM patients is an unfavorable prognostic cofactor for survival and is detrimental to the survival rates independent of corticosteroid therapy. However, no significant effects of hyperglycemic metabolism on the progression-free interval and recurrence patterns were found.

Lipid metabolism emerges as a promising target for malignant glioma therapy

Malignant gliomas are one of the most treatment-refractory cancers. Development of resistance to chemo- and radio-therapies contributes to these tumors' aggressive phenotypes. Elevated lipid levels in gliomas have been reported for the last 50 years. However, the molecular mechanisms of how tumor tissues obtain lipids and utilize them are not well understood. Recently, the oncogenic signaling EGFR/PI3K/Akt pathway has been shown to enhance lipid synthesis and uptake by upregulating SREBP-1, a master transcriptional factor, to control lipid metabolism. This article discusses the analytical chemistry results of lipid components in glioma tissues from different research groups. The molecular mechanisms that link oncogenes with lipid programming, and identification of the key molecular targets and development of effective drugs to inhibit lipid metabolism in malignant gliomas will be discussed.

Targeting SREBP-1-driven lipid metabolism to treat cancer

Metabolic reprogramming is a hallmark of cancer. Oncogenic growth signaling regulates glucose, glutamine and lipid metabolism to meet the bioenergetics and biosynthetic demands of rapidly proliferating tumor cells. Emerging evidence indicates that sterol regulatory element-binding protein 1 (SREBP-1), a master transcription factor that controls lipid metabolism, is a critical link between oncogenic signaling and tumor metabolism. We recently demonstrated that SREBP-1 is required for the survival of mutant EGFR-containing glioblastoma, and that this pro-survival metabolic pathway is mediated, in part, by SREBP-1-dependent upregulation of the fatty acid synthesis and low density lipoprotein (LDL) receptor (LDLR). These results have identified EGFR/PI3K/Akt/SREBP-1 signaling pathway that promotes growth and survival in glioblastoma, and potentially other cancer types. Here, we summarize recent insights in the understanding of cancer lipid metabolism, and discuss the evidence linking SREBP-1 with PI3K/Akt signaling-controlled glycolysis and with Myc-regulated glutaminolysis to lipid metabolism. We also discuss the development of potential drugs targeting the SREBP-1- driven lipid metabolism as anti-cancer agents.

Role of AMP-activated protein kinase activators in antiproliferative multi-drug pituitary tumour therapies: effects of combined treatments with compounds affecting the mTOR-p70S6 kinase axis in cultured pituitary tumour cells

AMP-activated protein kinase (AMPK) is activated under conditions that deplete cellular ATP levels and elevate AMP levels. We have recently shown that AMPK can represent a valid target for improving the medical treatment of growth hormone (GH)-secreting pituitary adenomas and the effects of its activation or inhibition in pituitary tumour cells are worthy of further characterisation. We aimed to determine whether AMPK may have a role in combined antiproliferative therapies based on multiple drugs targeting cell anabolic functions at different levels in pituitary tumour cells to overcome the risk of cell growth escape phenomena. Accordingly, we tried to determine whether a rationale exists in combining compounds activating AMPK with compounds targeting the phosphatidylinositol-3-kinase (PI3K)/Akt/mTOR/p70S6K signalling pathway. AMPK down-regulation by specific small-interfering RNAs confirmed that activated AMPK had a role in restraining growth of GH3 cells. Hence, we compared the effects of compounds directly targeting the mTOR-p70S6K axis, namely the mTOR inhibitor rapamycin and the p70S6K inhibitor PF-4708671, with the effects of the AMPK activator 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR) on cell signalling and cell growth, in rat pituitary GH3 cells. AICAR was able to reduce growth factor-induced p70S6K activity, as shown by the decrease of phospho-p70S6K levels. However, it was far less effective than rapamycin and PF-4708671. We observed significant differences between the growth inhibitory effects of the three compounds in GH3 and GH1 cells. Interestingly, PF-4708671 was devoid of any effect. AICAR was at least as effective as rapamycin and the co-treatment was more effective than single treatments. AICAR induced apoptosis of GH3 cells, whereas rapamycin caused preferentially a decrease of cell proliferation. Finally, AICAR and rapamycin differed in their actions on growth factor-induced extracellular signal regulated kinase 1/2 phosphorylation. In conclusion, the results of the present study suggest the increased efficacy of combined antiproliferative therapies, including rapamycin analogues and AMPK activators in GH-secreting pituitary tumours, as a result of complementary and only partially overlapping mechanisms of action.

Interplay between the intracellular energy sensor AMP-activated protein kinase (AMPK) and the estrogen receptor activities in regulating rat pituitary tumor cell (GH3) growth in vitro

Estrogen receptor α has a role in regulating rat somatolactotroph tumor cell growth (GH3 cells). AMP-activated protein kinase (AMPK) is a metabolic checkpoint which is able to negatively regulate intracellular signaling downstream of growth factors receptors in conditions increasing cellular AMP levels. We have recently reported on the role of AMPK activation in affecting viability and proliferation of GH3 cells. In the present study, we investigated the interplay between ER- and AMPK-pathways. Results can be regarded as relevant to the development of novel multi-targeted pharmacological therapies against pituitary tumors. We confirmed that estradiol (E2) and the ER antagonist fulvestrant exert stimulatory and inhibitory effects, respectively on GH3 cell growth in a competitive manner. The upstream kinase LKB1 is known to phosphorylate and activate AMPK. Here we showed that neither E2 nor fulvestrant caused a downregulation of LKB1 expression and phospho-AMPK levels in GH3 cells. Actually, fulvestrant strongly reduced the phosphorylation of ACC, which is a direct target of AMPK and a known index of AMPK activity. 2-deoxyglucose, a compound reducing glucose utilization, caused an increase in AMPK activity vs baseline and was able to hinder the stimulatory effect of E2 on cell viability, confirming that the exposure of GH3 cells to estrogens does not prevent them from being responsive to the inhibitory activity of compounds activating AMPK. Finally, the AMPK activator AICAR (AMP analog) did not cause further decrease in cell viability in the course of co-treatments with fulvestrant versus fulvestrant alone, in agreement with impaired phospho-AMPK activity in the presence of the anti-estrogen.

Tumor metabolism of malignant gliomas

Constitutively activated oncogenic signaling via genetic mutations such as in the EGFR/PI3K/Akt and Ras/RAF/MEK pathways has been recognized as a major driver for tumorigenesis in most cancers. Recent insights into tumor metabolism have further revealed that oncogenic signaling pathways directly promote metabolic reprogramming to upregulate biosynthesis of lipids, carbohydrates, protein, DNA and RNA, leading to enhanced growth of human tumors. Therefore, targeting cell metabolism has become a novel direction for drug development in oncology. In malignant gliomas, metabolism pathways of glucose, glutamine and lipid are significantly reprogrammed. Moreover, molecular mechanisms causing these metabolic changes are just starting to be unraveled. In this review, we will summarize recent studies revealing critical gene alterations that lead to metabolic changes in malignant gliomas, and also discuss promising therapeutic strategies via targeting the key players in metabolic regulation.

Effects of AMPK activation and combined treatment with AMPK activators and somatostatin on hormone secretion and cell growth in cultured GH-secreting pituitary tumor cells

We investigated the effects of the AMPK activator AICAR as compared to somatostatin-14 on cell viability and GH secretion in human GH-secreting pituitary adenomas in vitro and in rat GH3 cells. Overnight treatment with AICAR increased phospho-(threonine-172) AMPK levels (activated AMPK) in cultured human adenomas. As to the effects on cell viability, four adenomas out of 15 were responsive to AICAR (0.4mM) and five adenomas were responsive to SS-14 (100 nM). One adenoma was responsive to both somatostatin and AICAR. The effects of cotreatment with SS-14 and AICAR were investigated in eight adenomas. In two adenomas, the effects of AICAR+SS-14 did not exceed the effect of AICAR. In two adenomas which were not responsive to either AICAR or SS-14, the cotreatment was able to reduce cell viability versus control. Two adenomas were not responsive to any treatment. As to the effects on GH secretion, nine adenomas out of 15 were responsive to AICAR. Twelve adenomas were responsive to SS-14. Eight adenomas were responsive to both AICAR and SS-14. Cotreatment exceeded the effect of single treatments in 4 out of 10 adenomas. In GH3 cells, AICAR reduced the activity of p70S6 kinase, which plays an important role in cell growth. SS-14 did not affect significantly AMPK phosphorylation and p70S6K activity but it was able to enhance the inhibitory effect of AICAR on phospho-S6 levels. Moreover, AICAR and SS-14 reduced ERK phosphorylation with a different time course. The combined treatment reduced phospho-ERK levels at any time point.

Ectopic NGAL expression can alter sensitivity of breast cancer cells to EGFR, Bcl-2, CaM-K inhibitors and the plant natural product berberine

Neutrophil gelatinase-associated lipocalin (NGAL, a.k.a Lnc2) is a member of the lipocalin family and has diverse roles. NGAL can stabilize matrix metalloproteinase-9 from autodegradation. NGAL is considered as a siderocalin that is important in the transport of iron. NGAL expression has also been associated with certain neoplasias and is implicated in the metastasis of breast cancer. In a previous study, we examined whether ectopic NGAL expression would alter the sensitivity of breast epithelial, breast and colorectal cancer cells to the effects of the chemotherapeutic drug doxorubicin. While abundant NGAL expression was detected in all the cells infected with a retrovirus encoding NGAL, this expression did not alter the sensitivity of these cells to doxorubicin as compared with empty vector-transduced cells. We were also interested in determining the effects of ectopic NGAL expression on the sensitivity to small-molecule inhibitors targeting key signaling molecules. Ectopic NGAL expression increased the sensitivity of MCF-7 breast cancer cells to EGFR, Bcl-2 and calmodulin kinase inhibitors as well as the natural plant product berberine. Furthermore, when suboptimal concentrations of certain inhibitors were combined with doxorubicin, a reduction in the doxorubicin IC 50 was frequently observed. An exception was observed when doxorubicin was combined with rapamycin, as doxorubicin suppressed the sensitivity of the NGAL-transduced MCF-7 cells to rapamycin when compared with the empty vector controls. In contrast, changes in the sensitivities of the NGAL-transduced HT-29 colorectal cancer cell line and the breast epithelial MCF-10A cell line were not detected compared with empty vector-transduced cells. Doxorubicin-resistant MCF-7/Dox (R) cells were examined in these experiments as a control drug-resistant line; it displayed increased sensitivity to EGFR and Bcl-2 inhibitors compared with empty vector transduced MCF-7 cells. These results indicate that NGAL expression can alter the sensitivity of certain cancer cells to small-molecule inhibitors, suggesting that patients whose tumors exhibit elevated NGAL expression or have become drug-resistant may display altered responses to certain small-molecule inhibitors.

Metformin inhibits leptin-induced growth and migration of glioblastoma cells

Metformin, a derivative of biguanide, is a first-line therapy for type 2 diabetes mellitus. Since the drug has been shown to significantly reduce the risk of various cancers and cancer mortality in diabetic patients, it is being considered as a potential anticancer therapeutic or preventive agent. In cellular models, metformin inhibits the growth of many types of cancer cells; however, its effects on glioblastoma multi-forme (GBM) are not well characterized. Here, we analyzed the effects of metformin on the growth and migration of LN18 and LN229 GBM cells cultured under basal conditions or exposed to leptin, a cytokine that has recently been implicated in GBM development. We found that 2-16 mM metformin reduced basal and leptin-stimulated growth of GBM cells in a dose-dependent manner. Furthermore, the drug significantly inhibited the migration of GBM cells. The action of metformin was mediated through the upregulation of its main signaling molecule, the adenosine monophosphate-activated protein kinase (AMPK), as well as through the downregulation of the signal transducer and activator of transcription 3 (STAT3) and the Akt/PKB serine/threonine protein kinase. In leptin-treated cells, the drug reversed the effects of the cytokine on the AMPK and STAT3 pathways, but modulated Akt activity in a cell-dependent manner. Our results suggest that metformin or similar AMPK-targeting agents with optimized blood-brain-barrier penetrability could be developed as potential treatments of GBM and could be used in conjunction with other target drugs such as leptin receptor antagonists.

Mutations and deregulation of Ras/Raf/MEK/ERK and PI3K/PTEN/Akt/mTOR cascades which alter therapy response

The Ras/Raf/MEK/ERK and PI3K/PTEN/Akt/mTOR cascades are often activated by genetic alterations in upstream signaling molecules such as receptor tyrosine kinases (RTK). Certain components of these pathways, RAS, NF1, BRAF, MEK1, DUSP5, PP2A, PIK3CA, PIK3R1, PIK3R4, PIK3R5, IRS4, AKT, NFKB1, MTOR, PTEN, TSC1, and TSC2 may also be activated/inactivated by mutations or epigenetic silencing. Upstream mutations in one signaling pathway or even in downstream components of the same pathway can alter the sensitivity of the cells to certain small molecule inhibitors. These pathways have profound effects on proliferative, apoptotic and differentiation pathways. Dysregulation of components of these cascades can contribute to: resistance to other pathway inhibitors, chemotherapeutic drug resistance, premature aging as well as other diseases. This review will first describe these pathways and discuss how genetic mutations and epigenetic alterations can result in resistance to various inhibitors.

Review: molecular pathology in adult high-grade gliomas: from molecular diagnostics to target therapies

The classification of malignant gliomas is moving from a morphology-based guide to a system built on molecular criteria. The development of a genomic landscape for gliomas and a better understanding of its functional consequences have led to the development of internally consistent molecular classifiers. However, development of a biologically insightful classification to guide therapy is still a work in progress. Response to targeted treatments is based not only on the presence of drugable targets, but rather on the molecular circuitry of the cells. Further, tumours are heterogeneous and change and adapt in response to drugs. Therefore, the challenge of developing molecular classifiers that provide meaningful ways to stratify patients for therapy remains a major challenge for the field. In this review, we examine the potential role of MGMT methylation, IDH1/2 mutations, 1p/19q deletions, aberrant epidermal growth factor receptor and PI3K pathways, abnormal p53/Rb pathways, cancer stem-cell markers and microRNAs as prognostic and predictive molecular markers in the setting of adult high-grade gliomas and we outline the clinically relevant subtypes of glioblastoma with genomic, transcriptomic and proteomic integrated analyses. Furthermore, we describe how these advances, especially in epidermal growth factor receptor/PI3K/mTOR signalling pathway, affect our approaches towards targeted therapy, raising new challenges and identifying new leads.

An LXR agonist promotes glioblastoma cell death through inhibition of an EGFR/AKT/SREBP-1/LDLR-dependent pathway

Glioblastoma (GBM) is the most common malignant primary brain tumor of adults and one of the most lethal of all cancers. Epidermal growth factor receptor (EGFR) mutations (EGFRvIII) and phosphoinositide 3-kinase (PI3K) hyperactivation are common in GBM, promoting tumor growth and survival, including through sterol regulatory element-binding protein 1 (SREBP-1)-dependent lipogenesis. The role of cholesterol metabolism in GBM pathogenesis, its association with EGFR/PI3K signaling, and its potential therapeutic targetability are unknown. In our investigation, studies of GBM cell lines, xenograft models, and GBM clinical samples, including those from patients treated with the EGFR tyrosine kinase inhibitor lapatinib, uncovered an EGFRvIII-activated, PI3K/SREBP-1-dependent tumor survival pathway through the low-density lipoprotein receptor (LDLR). Targeting LDLR with the liver X receptor (LXR) agonist GW3965 caused inducible degrader of LDLR (IDOL)-mediated LDLR degradation and increased expression of the ABCA1 cholesterol efflux transporter, potently promoting tumor cell death in an in vivo GBM model. These results show that EGFRvIII can promote tumor survival through PI3K/SREBP-1-dependent upregulation of LDLR and suggest a role for LXR agonists in the treatment of GBM patients.

mTOR signaling in glioblastoma: lessons learned from bench to bedside

Phosphatidyl-inositol-3 kinases (PI3Ks) constitute a family of intracellular lipid kinases that are frequently hyperactivated in glioblastoma. The PI3K complex links growth factor signaling with cellular proliferation, differentiation, metabolism, and survival. Mammalian target of rapamycin (mTOR) acts both as a downstream effector and upstream regulator of PI3K, thus highlighting its importance in glioblastoma. This review highlights laboratory and clinical evidence of mTOR's role in glioblastoma. Mechanisms of escape from mTOR inhibition are also discussed, as well as future clinical strategies of mTOR inhibition.