Regan M. Memmott

Metformin prevents tobacco carcinogen-induced lung tumorigenesis

Activation of the mammalian target of rapamycin (mTOR) pathway is an important and early event in tobacco carcinogen–induced lung tumorigenesis, and therapies that target mTOR could be effective in the prevention or treatment of lung cancer. The biguanide metformin, which is widely prescribed for the treatment of type II diabetes, might be a good candidate for lung cancer chemoprevention because it activates AMP-activated protein kinase (AMPK), which can inhibit the mTOR pathway. To test this, A/J mice were treated with oral metformin after exposure to the tobacco carcinogen 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK). Metformin reduced lung tumor burden by up to 53% at steady-state plasma concentrations that are achievable in humans. mTOR was inhibited in lung tumors but only modestly. To test whether intraperitoneal administration of metformin might improve mTOR inhibition, we injected mice and assessed biomarkers in liver and lung tissues. Plasma levels of metformin were significantly higher after injection than oral administration. In liver tissue, metformin activated AMPK and inhibited mTOR. In lung tissue, metformin did not activate AMPK but inhibited phosphorylation of insulin-like growth factor-I receptor/insulin receptor (IGF-1R/IR), Akt, extracellular signal–regulated kinase (ERK), and mTOR. This suggested that metformin indirectly inhibited mTOR in lung tissue by decreasing activation of insulin-like growth factor-I receptor/insulin receptor and Akt upstream of mTOR. Based on these data, we repeated the NNK–induced lung tumorigenesis study using intraperitoneal administration of metformin. Metformin decreased tumor burden by 72%, which correlated with decreased cellular proliferation and marked inhibition of mTOR in tumors. These studies show that metformin prevents tobacco carcinogen–induced lung tumorigenesis and support clinical testing of metformin as a chemopreventive agent. Cancer Prev Res; 3(9); 1066–76. ©2010 AACR.

Akt-dependent and -independent mechanisms of mTOR regulation in cancer.

The protein kinase mTOR (mammalian target of rapamycin) is a critical regulator of cellular metabolism, growth, and proliferation. These processes contribute to tumor formation, and many cancers are characterized by aberrant activation of mTOR. Although activating mutations in mTOR itself have not been identified, deregulation of upstream components that regulate mTOR are prevalent in cancer. The prototypic mechanism of mTOR regulation in cells is through activation of the PI3K/Akt pathway, but mTOR receives input from multiple signaling pathways. This review will discuss Akt-dependent and -independent mechanisms of mTOR regulation in response to mitogenic signals, as well as its regulation in response to energy and nutrient-sensing pathways. Preclinical and clinical studies have demonstrated that tumors bearing genetic alterations that activate mTOR are sensitive to pharmacologic inhibition of mTOR. Elucidation of novel pathways that regulate mTOR may help identify predictive factors for sensitivity to mTOR inhibitors, and could provide new therapeutic targets for inhibiting the mTOR pathway in cancer. This review will also highlight pharmacologic approaches that inhibit mTOR via activation of the AMP-activated protein kinase (AMPK), an important inhibitor of the mTOR pathway and an emerging target in cancer.

Handicapping the Race to Develop Inhibitors of the Phosphoinositide 3-Kinase/Akt/Mammalian Target of Rapamycin Pathway

The phosphoinositide 3-kinase (PI3K)/Akt/mammalian target of rapamycin (mTOR) pathway controls many cellular processes that are important for the formation and progression of cancer, including apoptosis, transcription, translation, metabolism, angiogenesis, and cell cycle progression. Genetic alterations and biochemical activation of the pathway are frequent events in preneoplastic lesions and advanced cancers and often portend a poor prognosis. Thus, inhibition of the PI3K/Akt/mTOR pathway is an attractive concept for cancer prevention and/or therapy. Inhibitors of individual components, such as PI3K, PDK-1, Akt, and mTOR, are being developed at a rapid pace and have promise for improving the care of cancer patients. Here, we review the published data on inhibitors of the pathway and discuss relevant issues, such as the complex regulation of the pathway, the design of clinical trials, and the likelihood of finding a therapeutic index when targeting such a critical signaling pathway.

The Role of the Akt/mTOR Pathway in Tobacco Carcinogen–Induced Lung Tumorigenesis

Lung cancer is the leading cause of cancer-related death in the United States, and 85 to 90% of lung cancer cases are associated with tobacco use. Tobacco components promote lung tumorigenesis through genotoxic effects, as well as through biochemical modulation of signaling pathways such as the Akt/mammalian target of rapamycin (mTOR) pathway that regulates cell proliferation and survival. This review will describe cell surface receptors and other upstream components required for tobacco carcinogen–induced activation of Akt and mTOR. Preclinical studies show that inhibitors of the Akt/mTOR pathway inhibit tumor formation in mouse models of carcinogen-induced lung tumorigenesis. Some of these inhibitors will be highlighted, and their clinical potential for the treatment and prevention of lung cancer will be discussed. Clin Cancer Res; 16(1); 4–10

Phosphatidylinositol Ether Lipid Analogues That Inhibit AKT Also Independently Activate the Stress Kinase, p38α, through MKK3/6-independent and -dependent Mechanisms

Previously, we identified five active phosphatidylinositol ether lipid analogues (PIAs) that target the pleckstrin homology domain of Akt and selectively induce apoptosis in cancer cells with high levels of Akt activity. To examine specificity, PIAs were screened against a panel of 29 purified kinases. No kinase was inhibited, but one isoform of p38, p38α, was uniformly activated 2-fold. Molecular modeling of p38α revealed the presence of two regions that could interact with PIAs, one in the activation loop and a heretofore unappreciated region in the upper lobe that resembles a pleckstrin homology domain. In cells, two phases of activation were observed, an early phase that was independent of the upstream kinase MKK3/6 and inhibited by the p38 inhibitor SB203580 and a latter phase that was coincident with MKK3/6 activation. In short term xenograft experiments that employed immunohistochemistry and immunoblotting, PIA administration increased phosphorylation of p38 but not MKK3/6 in tumors in a statistically significant manner. Although PIAs rapidly activated p38 with similar time and dose dependence as Akt inhibition, p38 activation and Akt inhibition were independent events induced by PIAs. Using SB203580 or p38α-/- cells, we showed that p38α is not required for PIA-induced apoptosis but is required for H2O2- and anisomycin-induced apoptosis. Nonetheless, activation of p38a contributes to PIA-induced apoptosis, because reconstitution of p38a into p38α-/- cells increased apoptosis. These studies indicate that p38α is activated by PIAs through a novel mechanism and show that p38α activation contributes to PIA-induced cell death. Independent modulation of Akt and p38α could account for the profound cytotoxicity of PIAs.

A central role for Foxp3+ regulatory T cells in K-Ras-driven lung tumorigenesis

Background K-Ras mutations are characteristic of human lung adenocarcinomas and occur almost exclusively in smokers. In preclinical models, K-Ras mutations are necessary for tobacco carcinogen-driven lung tumorigenesis and are sufficient to cause lung adenocarcinomas in transgenic mice. Because these mutations confer resistance to commonly used cytotoxic chemotherapies and targeted agents, effective therapies that target K-Ras are needed. Inhibitors of mTOR such as rapamycin can prevent K-Ras-driven lung tumorigenesis and alter the proportion of cytotoxic and Foxp3+ regulatory T cells, suggesting that lung-associated T cells might be important for tumorigenesis. Methods Lung tumorigenesis was studied in three murine models that depend on mutant K-Ras; a tobacco carcinogen-driven model, a syngeneic inoculation model, and a transgenic model. Splenic and lung-associated T cells were studied using flow cytometry and immunohistochemistry. Foxp3+ cells were depleted using rapamycin, an antibody, or genetic ablation. Results Exposure of A/J mice to a tobacco carcinogen tripled lung-associated Foxp3+ cells prior to tumor development. At clinically relevant concentrations, rapamycin prevented this induction and reduced lung tumors by 90%. In A/J mice inoculated with lung adenocarcinoma cells resistant to rapamycin, antibody-mediated depletion of Foxp3+ cells reduced lung tumorigenesis by 80%. Likewise, mutant K-Ras transgenic mice lacking Foxp3+ cells developed 75% fewer lung tumors than littermates with Foxp3+ cells. Conclusions Foxp3+ regulatory T cells are required for K-Ras-mediated lung tumorigenesis in mice. These studies support clinical testing of rapamycin or other agents that target Treg in K-Ras driven human lung cancer.

Phosphatidylinositol Ether Lipid Analogues Induce AMP-Activated Protein Kinase–Dependent Death in LKB1-Mutant Non–Small Cell Lung Cancer Cells

Loss of function of the tumor suppressor LKB1 occurs in 30% to 50% of lung adenocarcinomas. Because LKB1 activates AMP-activated protein kinase (AMPK), which can negatively regulate mTOR, AMPK activation might be desirable for cancer therapy. However, no known compounds activate AMPK independently of LKB1 in vivo, and the usefulness of activating AMPK in LKB1-mutant cancers is unknown. Here, we show that lipid-based Akt inhibitors, phosphatidylinositol ether lipid analogues (PIA), activate AMPK independently of LKB1. PIAs activated AMPK in LKB1-mutant non-small cell lung cancer (NSCLC) cell lines with similar concentration dependence as that required to inhibit Akt. However, AMPK activation was independent of Akt inhibition. AMPK activation was a major mechanism of mTOR inhibition. To assess whether another kinase capable of activating AMPK, CaMKK beta, contributed to PIA-induced AMPK activation, we used an inhibitor of CaMKK, STO-609. STO-609 inhibited PIA-induced AMPK activation in LKB1-mutant NSCLC cells, and delayed AMPK activation in wild-type LKB1 NSCLC cells. In addition, AMPK activation was not observed in NSCLC cells with mutant CaMKK beta, suggesting that CaMKK beta contributes to PIA-induced AMPK activation in cells. AMPK activation promoted PIA-induced cytotoxicity because PIAs were less cytotoxic in AMPKalpha-/- murine embryonic fibroblasts or LKB1-mutant NSCLC cells transfected with mutant AMPK. This mechanism was also relevant in vivo. Treatment of LKB1-mutant NSCLC xenografts with PIA decreased tumor volume by approximately 50% and activated AMPK. These studies show that PIAs recapitulate the activity of two tumor suppressors (PTEN and LKB1) that converge on mTOR. Moreover, they suggest that PIAs might have utility in the treatment of LKB1-mutant lung adenocarcinomas.

The Chemopreventive Agent Myoinositol Inhibits Akt and Extracellular Signal-Regulated Kinase in Bronchial Lesions from Heavy Smokers

Myoinositol is an isomer of glucose that has chemopreventive activity in animal models of cancer. In a recent phase I clinical trial, myoinositol administration correlated with a statistically significant regression of preexisting bronchial dysplastic lesions in heavy smokers. To shed light on the potential mechanisms involved, activation of Akt and extracellular signal-regulated kinase (ERK), two kinases that control cellular proliferation and survival, was assessed in 206 paired bronchial biopsies from 21 patients who participated in this clinical trial. Before myoinositol treatment, strongly positive staining for activation of Akt was detected in 27% of hyperplastic/metaplastic lesions and 58% of dysplastic lesions (P = 0.05, χ2 test). There was also a trend toward increased activation of ERK (28% in regions of hyperplasia/metaplasia to 42% of dysplastic lesions). Following myoinositol treatment, significant decreases in Akt and ERK phosphorylation were observed in dysplastic (P < 0.01 and 0.05, respectively) but not hyperplastic/metaplastic lesions (P > 0.05). In vitro, myoinositol decreased endogenous and tobacco carcinogen–induced activation of Akt and ERK in immortalized human bronchial epithelial cells, which decreased cell proliferation and induced a G1-S cell cycle arrest. These results show that the phenotypic progression of premalignant bronchial lesions from smokers correlates with increased activation of Akt and ERK and that these kinases are targets of myoinositol. Moreover, they suggest that myoinositol might cause regression of bronchial dysplastic lesions through inhibition of active Akt and ERK.

Abstract 614: Efficacy of metformin in two mouse models of Li-Fraumeni syndrome

Li-Fraumeni Syndrome (LFS) is a rare autosomal hereditary disorder that is commonly associated with germline mutations in the TP53 gene. People with LFS are at high risk to develop a wide range of malignancies including breast cancer, brain tumors, acute leukemias, soft tissue sarcomas, bone sarcomas, and adrenal cortical carcinomas. Although no therapies are approved to treat or prevent LFS, preclinical studies have shown that the anti-diabetic drug metformin is preferentially cytotoxic to p53 null cells in the absence of glucose. This led us to hypothesize that metformin has therapeutic potential in LFS. To confirm the p53 dependence of metformin in vitro, isogenic human and murine cancer cell lines were treated with metformin in the absence or presence of glucose. Metformin induced greater toxicity in p53-deficient cells in a dose-dependent manner. Metformin was then tested in two mouse models that recapitulate the increased tumorigenesis and cancer-related mortality in LFS (p53 mutant mice (p53R172H/+) and p53 null mice (p53-/-)). When administered at 5 mg/ml in drinking water, metformin reached steady state plasma concentrations similar to those in diabetic humans. Biomarker studies showed that oral administration of metformin activated AMPK and inhibited the mTOR pathway in liver tissues. To determine the effects of metformin on overall survival, two sets of studies were performed. For each model, treatment with metformin was begun either late in life (for p53R172H/+, 12 mo; for p53-/-, 4 mo) or early in life (for p53R172H/+, 4 mo; for p53-/-, 1 mo). Mice were treated until death. Long-term oral administration of metformin did not cause weight loss or other discernible toxicity. In the late treatment study, metformin significantly prolonged median survival (for p53R172H/+ mice, 20.6 mo vs. 15.8 mo; p=0.0004)(for p53-/- mice, 9.1 mo vs. 5.3 mo; p=0.0006). Likewise, early treatment with metformin also prolonged median survival (for p53R172H/+ mice, median survival 20.5 compared to 13 months; p=0.0025). The early study with p53-/- mice is ongoing, but median survival is undefined in metformin group, compared to 3.6 mo in control group, p=0.0387). These studies show that metformin is well tolerated and highly effective when given early or late in the life span of two mouse models of LFS. Given that metformin is an inexpensive oral drug that is FDA approved, a pilot clinical trial in LFS is planned. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 614. doi:10.1158/1538-7445.AM2011-614

Abstract 4330: A high-fat diet promotes tumorigenesis in two mouse models of K-ras-driven lung cancer

Lung cancer is the leading cause of cancer-related mortality worldwide, and 85% of lung cancer cases are associated with tobacco use. Within this group, activating mutations in K-ras have been identified in ∼25% of lung adenocarcinomas. Using a mouse model of k-ras-driven lung tumorigenesis, we previously demonstrated that deletion of the IGF-1 gene or reduction of systemic IGF-1 levels using the antidiabetic drug metformin markedly reduced tumor burden. Since preclinical and clinical studies suggest that diet composition is the best predictor of IGF-1 levels, we hypothesized that diets high in fat or carbohydrate would promote lung tumorigenesis by increasing systemic IGF-1 levels. To assess the effect of diet on systemic IGF-1 levels, 9 week old C57Bl/6J and A/J mice were fed standard cereal, high-carbohydrate, or high-fat (HFD) diets for 12 weeks. At the conclusion of the study plasma, liver, and lung samples were collected. Compared to the cereal-fed control mice, IGF-1 and insulin levels were increased in both strains of mice only with HFD. Average body weight only increased for the C57Bl/6J group that was fed HFD. We investigated the effect of HFD on lung tumorigenesis using two mouse models of lung cancer. In the first, C57Bl/6LA2 mice, which are genetically modified with a K-ras mutation present in human smokers, were fed either cereal diet or HFD for 10 weeks following weaning. Lung tumor burden in the mice fed HFD was increased 2.7-fold compared to littermates fed cereal diet. In the second model, the tobacco carcinogen 4-(methylnitrosamino)-1-(3-pyridyl)-1 butanone (NNK) was given by IP injection to A/J mice beginning at 6 weeks of age. This carcinogen causes lung tumor formation by inducing K-ras mutations. Following once weekly injections of NNK for 3 weeks, the mice were randomized to cereal diet or HFD. 10 weeks later, mice fed HFD were found to have a 60% increase in lung tumor burden. In both studies, there was no relationship between the final body weight of the mice and tumor burden. These studies show that HFD promotes lung tumor growth resulting from a mutation commonly observed in smokers. Therefore, dietary modification may slow the progression of tumorigenesis resulting from smoking-related genetic changes through IGF-1. Chemopreventative drugs, like metformin, may also have greater efficacy in a HFD model due to the increased insulin and IGF-1 associated with this model. Finally, understanding the molecular mechanisms by which HFD promotes tumor growth may help to identify new targets for cancer prevention. Citation Format: Jeffrey Norris, Krista Pearman, Regan Memmott, Kristin Lastwika, Joell Gills, Phillip Dennis. A high-fat diet promotes tumorigenesis in two mouse models of K-ras-driven lung cancer. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4330.