Sep 5, 2013 - 1) The action of metformin on tumor, depends largely on tumor´s genotype .... non-diabetic women with untreated breast cancer administered ...
METFORMIN AND CANCER: UNDERSTANDING FACTS AND CONTROVERSIES Tomas Koltai Abstract Background: In 2005 Evans et al published the results of an epidemiological research that showed a significant decrease of cancer risk (up to 30%) in diabetic patients treated with metformin. Since then, a considerable amount of investigation in this field was performed. The results were somewhat controversial even within the same disciplines and techniques employed: epidemiology, tumor cell culture, tumor xenografting and risk reduction retrospective studies. These controversies hindered the use of metformin for cancer in the clinical setting. Objective: To understand the reasons of these divergent results and find out the place metformin should have in cancer prevention and treatment. Methods: We reviewed all the papers published and recorded in PUBMED under the headings CANCER, METFORMIN, AMPK and LKB1 starting in 2005 to nowadays. Conclusions: The effects of metformin on cancer is not universal but strongly depend on the tumor type and additional circumstances such as hormonal and metabolic status. 1) The action of metformin on tumor, depends largely on tumor´s genotype and the driving pathways being used by the tumor. This concept may explain the contradictory results between different researches. 2) That the LKB1-AMPK-mTOR is not the only pathway that would explain metformin´s antioncogenic action, nor is the main one. 3) Metformin could be useful in those cases where the main tumor pathway (tumor driver) is PI3K/AKT/mTORC1 with or without PTEN loss. 4) Metformin is not useful in those tumors where the predominant pathway is the RAS/RAF/ERK. 5) Metformin may be useful in tumors that overexpress insulin receptors. 6) Metformin should not be used in those cases where there is a BRAF mutation. 7) Metformin should be used in doses well above those usually employed for diabetes treatment. 8) Thyroid cancer seems to be one of the better suited indications for metformin. 9) Endometrial cancer is also a good candidate for metformin treatment. 10) Many of the controversies arise from the differences in metformin concentration achievable in practice and the concentrations employed in cell cultures.
INTRODUCTION Starting with clinical epidemiology and following with the studies of tumor tissue culture, a clear antitumor action was described for metformin. But it´s use in preventive clinical setting and treatment of human cancer has showed no clear cut results up to now. Clinical epidemiology and a longevity experiment were the two first disciplines that called attention over the benefit that might produce metformin in diabetic patients with cancer, when they were compared with similar patients not receiving metformin. In 2005, Evans et al. (1) published a paper that called attention about the decrease in cancer risk in diabetic patients receiving metformin as part of their treatment. Previous papers showed the importance of the LKB1-AMPK pathway in anti-oncogenesis. As there was evidence of metformin stimulating this pathway the question that naturally aroused was, if metformin stimulates this pathway, and the pathway has antitumor action, may metformin also keep an antitumor action?
The paper of Evans et al (1), based on a case-control study of resident population of Scotland, showed that diabetic patients treated with metformin had a lower relative risk of having cancer (0,912) and those with a long standing treatment showed even a lower risk (0,782). In summary, metformin reduced cancer risk in diabetics and this reduction was dose and duration dependent. In that same year, 2005, a second paper that was less noticed, by Anisimov et al (2 and 3) in transgenic mice Her2/neu that developed spontaneous mammary tumors, showed that mice receiving metformin had a lower incidence and size of mammary adenocarcinomas. Foretz et al (4 and 5) based on these researches suggested that LKB1 may possibly be a target not only in diabetes treatment but also in cancer. With this starting point, many studies at basic research level were performed to confirm metformin´s antitumor action in order to understand the possible mechanism of action. Zakikhani et al (6) confirmed the concepts outlined above and found that metformin had an antitumor action in tissue culture of mammary carcinoma and also confirmed the activation of the LKB1-AMPK pathway and inhibition of mTOR due to metformin. In the last few years, many papers confirming metformin´s antitumor action in the experimental setting has been brought to light. However, there are scientific publications that show that in certain cell types, metformin develops a proangiogenic action (7 and 8). Under hypoxic conditions metformin has produced proangiogenic actions in umbilical cord endothelium (9), and without hypoxemia, in certain prostate carcinomas (10), muscle (11) and human glia cells(12). Contrary to the concept of metformin as an agent in cancer prevention, the LKB1-AMPK pathway promotes cell survival in glucose deprived non tumor cell cultures (13). There are also controversies regarding the association of metformin with chemotherapeutic drugs like cisplatin. Janjetovic et al (14) showed in vitro decreased cytotoxicity by cisplatin when it is associated with metformin in glioma, retinoblastoma, fibrosarcoma and leukemia cells. Rattan et al (15) on the contrary, observed increased cytotoxicity when cisplatin is associated with metformin. At the present moment it is not clear how metformin acts in association with chemotherapeutic drugs (14 and 16: decreased cytotoxicity with cisplatin and 15: increased cytotoxicity). The association of metformin with doxorubicin (17) or paclitaxel (18) increase cytotoxicity. However, the antitumor effect of metformin, which seems so evident in the tissue culture setting, does not fully translate into similar effects in the clinical setting. There is then a conundrum: why a drug that seems so beneficial in the laboratory, does not seem to be equally beneficial in the prevention of cancer risk in the clinical setting? Is it worth to insist with metformin research in cancer? This are the questions we will try to answer in this paper. Starting with the second question, it is necessary to say that the effort is worth, because metformin has shown cytotoxicity on cancer stem cells (17), and this effect may be regarded as a very useful one. We also believe that the information necessary to achieve an answer to the questions stated above, already exist in the medical literature…but these has not been considered in a holistic view. The studies have been segmented between different tumor types and dichotomized between basic and applied sciences, so the full vision of the landscape has been lost and therefore data has not been integrated adequately, scattered as it is. Lets take a deeper look at the scattered and somewhat controversial clinical data on metformin and cancer: ESOPHAGEAL CANCER
1) Metformin does not reduce the risk of esophageal cancer: Becker et al (20) through a case-control study, found no evidence of decreased risk for esophageal cancer in diabetic patients. In Barrett esophagus did not show a decrease in esophageal cancer either (21 and 22). On the other hand Skinner et al (23) found that metformin improved response to chemoradiotherapy in esophageal adenocarcinoma; Feng et al (24) found the same with radiotherapy and metformin ; Kobayashi et al (25) showed that metformin inhibited the growth of three esophageal cancer cell lines with reductions in cyclin D1, Cdk4 and Cdk6; Xu and Lu (26) and Fujihara et al (27) also found growth reduction in esophageal cancer cells; Feng et al (28) found growth inhibition of squamous carcinoma esophageal cells by downregulation of STAT3; on cell culture metformin worked synergistically with 5FU to reduce tumor growth (29) and finally Davelin found that metformin protects squamous cell carcinoma cells from cisplatin (30). Conclusion: many evidences favoring metformin at the laboratory level but no epidemiological proofs to hold that benefit. ENDOMETRIAL CANCER 2) Metformin does not reduce the risk of endometrial cancer (31 and 32), but many of these tumors are found in patients with insulin resistance and the insulin-insuline receptor pathway is overexpressed. There are in vitro and clinical evidences of metformin benefits in this cancer as can be seen in next table 1 TABLE 1: ANTICANCER ACTIVITIES OF METFORMIN IN ENDOMETRIAL CANCER Reference Metformin action Metformin can reduce the secretion of IGF-1 in endometrial cancer cell lines and also the expression of IGF-1R and deactivate downstream signaling 33 involving the PI-3K/Akt/mTOR pathway inhibiting endometrial carcinoma growth. 34
Metformin could inhibit estrogen-mediated proliferation of human endometrial cancer cells.
Metformin could be used as an effective antiestrogenic agent in control of abnormal endometrial proliferative disorders. (Cases analysed were not 35 cancer cases) metformin could induce endometrial atrophy in 21 out of 22 patients) Metformin reduced tumor proliferation in 65% of pre-operative obese 36 patients studied with endometrial carcinoma. Inhibition of the mTOR pathway was observed. Eleven newly diagnosed, untreated, non-diabetic patients with EC received metformin starting at the diagnostic biopsy until surgery. There was reduction 37 of ki-67 and pS6 expression. In the non-treated control group expression was unchanged Metformin-treated cells exhibited significantly lower viability and proliferation and significantly more cell cycle arrest than control cells. These cells also 38 exhibited significantly more apoptosis.Metformin treatment induced autophagy. Inhibition of autophagy, suppressed the anti-proliferative effects of metformin on endometrial cancer cells.
39 Preoperative metformin treatment decreased DNA synthesis in sera and significantly reduced the Ki-67 (44.2% ) and topoisomerase IIα, S6 and ERK1/2. 40
Evidence suggests that insulin resistance plays a central role in endometrial cancer development
41
Metformin inhibited growth of endometrial cancer cells in a dose-dependent manner and Resulted in arrest and induction of apoptosis.
In vitro invasion in ECC-1 cells was significantly attenuated by sera from 42 women with polycystic ovary syndrome after 6 months of metformin treatment compared to matched controls 43
Metformin through interaction with the IGF pathway, induces apoptosis and inhibition of proliferation and migration of USC cell lines.
Epidemiologic studies could not confirm a decrease of risk with metformin in endometrial cancer, but in vitro and in the clinical setting metformin showed encouraging results. PANCREATIC CANCER 3) There is evidence of reduction of pancreatic cancer risk with metformin: Bodmer et al (44) observed a decrease in risk, but limited to female patients. Nair et al (45) proposed that risk lowering was due to decrease in Sp transcription factor due to increased destruction by proteasome. The decrease in Sp has a direct effect on decrease of expression of genes regulated by this transcription factor, such as bcl2, survivin, cycline D1, VEGF and receptor VEGFR1. Schneider et al (46) described a significant risk decrease for pancreatic cancer in hamsters treated with N-nitrosobis-(2oxopropilamine). LUNG CANCER 4) There is no evidence of reduction of lung cancer risk (47) but Quinn et al (48) found molecular evidence signaling a decrease in lung tumorigenesis through a decrease in IGF1/insulin receptor interaction. COLORECTAL CANCER 5) Metformin at low dose does not reduce colorectal cancer risk (49 and 50). Metformin at high dose reduces colorectal cancer mortality: Spillane et al (51) confirm the lack of risk reduction with low dose, but with high dose they found a decrease in mortality. OVARIAN CANCER 6) Metformin reduces ovarian cancer risk: Bodmer et al (52) found a decrease of ovarian cancer risk, however when the statistics in the paper are scrutinized in detail, this decrease is not very convincing due to overlay of results in the confidence interval. PROSTATE CANCER 7) No benefit in prostate cancer: Kaushik et al (53) found no advantage using metformin in prostate cancer. Niraula et al (54) found no improvement in the results of prostate cancer patients refractory to castration treated with docetaxel and metformin.
LARYNX CANCER 8) Increased survival in epidermoid carcinoma of larynx: Sandulache et al (55) found that patients treated with metformin showed lower stages of larynx carcinoma (T1 and T2) and with lower incidence of metastasis. Survival was significantly higher in the metformin group. MELANOMA 9) Controversial effects in melanoma: A) Metformin decreases invasion and metastasis in melanoma in the experimental setting (56) with no confirmation in patients. B) On the other hand melanoma with BRAF V 600 mutation worsens with metformin (57). THYROID CANCER 10) Favorable action of metformin in thyroid cancer (58). In this tumor epidemiology and basic research showed no discrepancy. In Taiwanese diabetic II population the hazard ratio was 0,68 for metformin users. (59). This favorable action has been detected in papillary cancer in vitro and in vivo (60). Chen et al (61) demonstrated growthinhibitory effect on differentiated thyroid cells and undifferentiated thyroid carcinoma cells, including thyroid carcinoma stem cells. Klubo-Gwiezdzinska et al (62) showed anti-tumor activity of metformin in medullary thyroid cancer. We found no controversies regarding the promising adjuvant therapy of metformin in thyroid cancer. Metformin also produces a slight reduction in TSH levels without affecting thyroid hormone levels and decreases iodine uptake (63). This should be considered in case of using radioiodine. BREAST CANCER 11) Controversial effects of metformin on mammary carcinoma. We have already described the epidemiological data that supports a decrease of risk in breast cancer with metformin in diabetic patients. We shall now analyze some complementary information. Aksoy et al (64) describe benefits attributed to metformin intake in diabetes: lower incidence of advanced histological grades, lower incidence of triple negatives, higher incidence of positivity to estrogen and progesterone receptors, increased disease free survival but similar overall survival. Lega et al (65) did not find increased survival with metformin duration of treatment. Metformin decreases aromatase´s action (66). On the other hand Dowling et al (67) in a study performed on non-diabetic women with untreated breast cancer administered metformin for 2 weeks after diagnostic biopsy until surgery. The surgical specimens obtained showed decreased levels of IR expression and Akt phosphorylation in tumours. The authors remark that reduced Akt and ERK1/2 phosphorylation, and decreased insulin and IR levels, suggest that the reduction of the insulin-insulin receptor pathway is important in the clinical setting.
12) Metformin increases 5 hydroxytryptamine secretion in duodenum according to Cubeddu et al (68). From what we summarized above, it is clear that metformin action in cancer and possible benefits are far from being elucidated. In order to bring some light to these contradictory findings, we will try to find an explanation by searching in the medical literature, because we are convinced that the answers already exists.
MATERIAL AND METHODS A search was performed through PUBMED including all those articles published after 2004 that included the key words metformin, cancer, LKB1 and AMPK. RESULTS Since Midle Ages Galega officinalis (the plant that contains metformin) was used for the treatment of patients with some forms of polyuria, up to the beginnings of 1970s. In that moment, biguanides were isolated from Galega officinalis and introduced in Europe for the treatment of diabetes type II (DB2). Biguanides are world widely extended in the treatment of DB2 due to its capacity of lowering blood glucose without producing hypoglycemia. Biguanides, known in Europe since the 50s, have been accompanied by the black legend of lactic acidosis. This was the reason why it took more than 20 years to introduce them in clinical practice. Metformin was approved by FDA in 1995 and is at the present day one of the more prescribed drug for the treatment of DB2 with 40 million prescriptions issued in 2008 in the United States. (69) PUBLICATIONS THAT SHOW POSITIVE ACTION OF METFORMIN IN CANCER The first publication about the possible antitumor effect of biguanides was issued by Dilman and Anisimov in 1979 (2). They found that fenformin increased the antitumor action of cyclophosphamide in xenografts (squamous carcinoma of cervix, hepatoma and Lewis lung tumor). In a further publication, Dilman and Anisimov (3) found that fenformin had a protective effect against cancer. This preventive action was reinforced by the publication of Schneider et al (46) in experiments performed on hamsters treated with pancreatic carcinogens. But the great promoter of metformin´s fame in cancer was the epidemiologic and statistical research performed in Dundee University, Scotland by Evans et al (1) in 2005. In a short article published in the British Medical Journal, Evans et al described their observations performed upon a large population of diabetic patients where they found a significant lower amount of cancer cases in patients receiving metformin as compared with those without this drug. Since that moment, metformin as a reducer of cancer risk has been strongly incorporated to the medical thinking, but not to the oncology practice. This paper by Evans, includes an explanation at the molecular level about the possible mechanism responsible for metformin´s antitumor activity: stimulation of the LKB1-AMPK pathway. Murali Vallipuranathan (70) in response to Evans publication proposes other mechanisms involved in reduction of cancer risk, independent of metformin administration. He mentions the pro-cancer action of insulin (as has been seen in colorectal cancer). So that patients who receive no insulin but instead receive metformin would be less exposed to cancer risk. Other medications involved may act as confounding issues. Malcolm E Kendrick (71), also in response to Evans´ publication suggests that the decrease in insulin secretion due to metformin might be one of the mechanisms involved. In 2006 Zakikhani et al (6) arrived to the conclusion that metformin in breast cancer prevention develops its action through inhibition of epithelial cells growth and through activation of AMPK. They also found that growth inhibition was associated with decreased mTOR and S6 kinase activity which produced a generalized decrease in protein synthesis. In 2007, Isakovic et al (72) working with glioma cells reported that metformin induced arrest of cellular cycle and apoptosis dependent of mitochondria and that this actions were in part mediated by AMPK.
In another research Dowling et al (73) in 2007, confirmed inhibitory action of metformin on mTOR pathway and explained this through activation of AMPK by LKB1 as depicted in figure 1. Specifically this paper showed that metformin inhibits translation of messenger RNA necessary for cellular reproduction in breast cancer cell cultures. Figure N° 1: PROPOSED ANTINEOPLASTIC MECHANISM OF METFORMIN Metformin disrupts mitochondrial complex I decreasing ATP production and increasing AMP/ATP ratio. This activates AMPK phosphorylation by LKB1. AMPK inhibits mTORC1. Inhibition of mTORC1 by AMPK has two mechanisms: one direct on raptor protein and another indirect through activation of TSC2. (This last one is not shown in the figure). Lack of LKB1 atenuates AMPK activation by metformin but does not abrogate it. That is the reason why the figure shows a direct arrow between AMP/ATP increase and AMPK activation without LKB1 intervention. The figure shows also other physiologic mechanisms that may activate AMPK like low glucose, low energy and low oxygen
Figure 1 Jiralerspong et al (74) reported that diabetic patients with breast cancer receiving metformin and adjuvant chemotherapy showed higher remission rates than patients with other treatments for diabetes. But Jiralerspong and his team went a step further and analyzed also patients receiving insulin and found that those women who were on insulin and metformin showed similar results to those on metformin alone. Patients on insulin without metformin had significantly worse results. It must be noted that another observation in this paper shows that
weather patients were on metformin or without metformin, diabetic patients had a lower overall survival rate that non diabetics. Goodwin et al (75) in 2009 after considering Jiralersprong´s publication suggested that the increased mortality could be related with the higher mortality usually found in diabetic populations. But the research of Goodwin et al also postulate a molecular mechanism that may explain metformin`s activity in glucose metabolism and in cancer. Figure 2 shows a modified version of the proposed mechanism. FIGURE 2: PROPOSED MOLECULAR ACTION OF METFORMIN Metformin decreases gluconeogenesis in liver through AMPK activation. Decreased gluconeogenesis means a decrease in circulating insulin and downregulation of the insulin receptors (RI). Decrease in insulin receptor activity downregulates pro-mitotic pathways. Metformin in tumor cells also stimulates AMPK activation which downregulates mTOR. Here we included a direct metformin downregulation of mTOR and the consequence of this activity: increased autophagia.
Figure 2 TABLE 2: EPIDEMIOLOGIC EVIDENCE FAVORING METFORMIN IN CANCER Reference
Findings
Evans (1)
Metformin may reduce the risk of cancer in patients with DB2
Jiralerspong (74)
Diabetic women with breast cancer who received adjuvant therapy and metformin showed better evolution.
Libby (76)
New metformin users show less risk of developing cancer.
Decensi (77)
31% reduction of relative risk in cancer in metformin users.
Landman (78)
Lower mortality due to cancer in diabetic population on metformin.
Murtola (79)
Lower prostate cancer incidence in patients receiving antidiabetic medication of any kind, metformin and insulin included.
Gagnon (80)
Better prognosis in diabetic patients with lung cancer receiving metformin.
Rieken (81)
Bladder cancer: diabetics without metformin exhibit higher risk of recurrence and progression.
TABLE 3: MOLECULAR EVIDENCE FAVORING METFORMIN IN CANCER Reference
Finding
Zakikhani (6)
Metformin inhibits growth of cancer cells in culture. Increases activity of AMPK and downregulates mTOR.
Dowling (73)
Metformin inhibits translation initiation of mRNA under mTOR control in breast cancer cell cultures.
Hirsch (17) Metformin "attacks" cancer stem cells and increase citotoxicity of certain chemotherapeutic drugs in cell culture and xenografts. Alimova (82)
In tissue culture, Metformin showed activity against many breast cancer cell lines and reduced expression of cyclina D1, E2F1 and decreased activity of MAPK, Akt and mTOR.
Liu (83)
In tissue culture and xenograft models of triple negative breast cancer lines, metformin showed growth inhibition, induced apoptosis, decreased EGFR, cyclins D1 and E and patition of PARP.
Iliopoulos (84)
In cancer xenograft models, metformin decreased the necessary chemotherapeutic drugs to prolong remission stage.
dose of
Rocha (85) In mice xenografted with cancer cells, metformin increases AMPK activation and increases cytotoxicity of paclitaxel. Ben Sahra Metformin decreases level of cyclin D1 in vitro and in vivo and decreases growth of (86) prostate cancer cell lines. Buzzai (87) Metformin is active against P53 deficient tumor cells. Zhuang (88)
Metformin arrests growth of breast cancer cell lines activating AMPK, and decreasing cyclina D1.
Zakihkani (89)
Rapamicin and metformin show differences on activity over Akt.
Huang (90)
Established the importance of the LKB1-AMPK pathway in tumorigenesis supression.
Gotlieb (91)
Showed metformin`s anti-tumor activity in ovarian cancer.
Cantrell (92)
Metformin´s anti-tumoral activity on endometrial cancer cell lines in culture. AMPK`s activarion due to metformin.
Younis (93)
Activity against squamous cell carcinoma cells of head and neck. Increase of activity of REDD1.
Vitale(94)
Activity against squamous cell carcinoma cells of head and neck. Descrime AMPK independent tumor activity.
Tan (95)
Human endometrial cancer lines: metformin showed anti-invasion and anti-metastasis activity.
Morizane (96)
Activation of AMPK dicreases MMP9 activity. Activators of AMPK like AICAR and A769662 produced decrease of MMP9 activity, so that is reasonable to expect same activity with metformin.
Ben Sahra Metformin plus 2 deoxiglucose in tissue culture increases citotoxicity on prostate (97) cancer cells. Rattan (98)
Decreased growth in ovarian cancer cell grafted in mice peritoneum and treated with metformin. Metformin showed synergy with cisplatin in this case. Antiangiogenic and antimetastatic activity due to metformin.
Iglesias (99)
Metformin blocks MAPkinase pathway through displacement of RAS from the cellular membrane in endometrial cancer cells.
Lin (100)
Metformin decreases the pro-inflammatory environment in macrophages with PTEN inactivation.Also downregulates Akt y decrease ROS production
TABLE 3:EPIDEMIOLOGIC EVIDENCE AGAINST METFORMIN IN CANCER Reference Becker (20) Becker (31) Bodmer (47) Bodmer (49) Kaushic (101) Niraula (32) Lega (65) Hwang (102)
Findings Does not reduce risk of esophageal cancer. Does not reduce risk of endometrial cancer Does not reduce risk of lung cancer Does not reduce risk of colorectal cancer No benefits in prostate cancer outcome No benefits in prostate cancer associated with docetaxel. Does not improve overall survival in breast cancer Does not improve overall survival in cancer of pancreas.
TABLE 4: MOLECULAR EVIDENCE AGAINST METFORMIN IN CANCER Reference Phoenix (7) Nagata(9)
Lee (10) Ouchi (11)
Findings Pro-angiogenic activity of metformin is described in ERα negative MDA-MB435 breast cancer cells. Endothelial cells of human umbilical cord (HUVEC): under hypoxia there is activation of AMPK and angiogenesis and plays a role in cell migration and tubular structure formation. In normoxia, AMPK activation is not required for angiogenesis. In DU 145 prostate cancer cells AMPK activation is essential for HIF expression. Blocking AMPK decreases expression of HIF-1 and VEGF. In muscle tissue, activation of AMPK produced expression of VEGF and angiogenesis. AMPK activation was induced through hypoxia and AICAR (similar activity as metformin). AICAR also upregulated p38 MAPK.
Neurath (12)
Zwetsloot (103) Li (104)
In human glia hypoxia activated AMPKalha2 fraction and increased production of VEGFR increasing angiogenesis. Inhibition of AMPK activation blocked overexpression of VEGF y de HIF1 AMPK regulates expression of VEGF and muscular capilarization . AMPK induces differentiation of endothelial progenitor cells. AICAR increases angiogenesis and compound C decreases it. (Compound C is an AMPK inhibitor.
It is important to remark an epidemiologic work by Geraldine et al (105) performed in Belgium with a population study of 1,5 million patients-year and they concluded that patients with diabetes had a higher prevalence of cancer, but the interesting point of these conclusions was that decrease in risk was associated not only with metformin but also with the use of other diabetic drugs. Other possible mechanisms of action of metformin in cancer are described in figure 3
Figure 3 ACTIVATE OR DEACTIVATE AMPK ? One of the main mechanisms postulated for the anti-cancer activity of MET is activation of AMPK and consequently inhibit mTOR. But the works by Laderoute et al (106) brings up a substantial doubt: should we activate or inhibit AMPK in cancer treatment? The research by Laderoute et al showed that AMPK is quickly activated in vitro, under normal or pathologic conditions when hypoxia is present. This activation is HIF-1 independent. Tumor microenvironment is almost universally hypoxic, and Laderoute found increased AMPK activation under this conditions in cell culture and in xenographted tumors. In both cases the authors also discovered that inhibition of
AMPK may decrease proliferation in certain tumors. These finding lead them to hypothesize that AMPK may play an important role in cells adaptation to hypoxia and nutrient deprivation usually found in solid tumor microenvironments. At the present time, our initial question cannot be answered. Park et al (107) found that AMPK activation in prostate cancer cells increased growth and survival and that 40% of prostate cancers exhibited an important activation of AMPK (in androgen sensitive tumors like LNCap and hormone-independent tumors like CWR22Rv1). Inhibiting AMPK with compound C suppressed tumor proliferation. We have already mentioned that AMPK activation may increase angiogenesis in endothelial cells (9), breast cancer (7, 8), glioblastoma (12) and may induce HIF-1 transcriptional activity under hypoxic conditions (10). At the present time, our initial question cannot be answered. CONCLUSIONS Controversies about metformin in cancer will continue in the next future, but certain simple conclusions can be reached by analyzing medical literature: 1) The action of metformin on tumor, depends largely on tumor´s genotype and the driving pathways being used by the tumor. This concept may explain the contradictory results between different researches. 2) That the LKB1-AMPK-mTOR is not the only pathway that would explain metformin´s anti-oncogenic action, nor is the main one. 3) Metformin could be useful in those cases where the main tumor pathway (tumor driver) is PI3K/AKT/mTORC1 with or without PTEN loss. 4) Metformin is not useful in those tumors where the predominant pathway is the RAS/RAF/ERK. 5) Metformin may be useful in tumors that overexpress insulin receptors. 6) Metformin should not be used in those cases where there is a BRAF mutation. 7) Metformin should be used in doses well above those usually employed for diabetes treatment. 8) Thyroid cancer seems to be one of the better suited indications for metformin. 9) Endometrial cancer is also a good candidate for metformin treatment. 10) Many of the controversies arise from the differences in metformin concentration achievable in practice and the concentrations employed in cell cultures. REFERENCES 1) Evans JMM, Donelly LA, Emslie_Smith AM, Alessi AR, Morris D. Metformin and reduced risk of cancer in diabetic patients. BMJ. 2005 Jun 4;330(7503):1304-5. Epub 2005 Apr 22. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC558205/ 2) Anisimov VN, Berstein LM, Egormin PA, et al. Effect of metformin on life span and on the development of spontaneous mammary tumors in HER-2/neu transgenic mice. Exp Gerontol. 2005 Aug-Sep;40(8-9):685-93. http://www.ncbi.nlm.nih.gov/pubmed/16125352 3) Anisimov VN, Egormin PA, Bershtein LM, Zabezhinskii MA, Piskunova TS, Popovich IG, Semenchenko AV. Metformin decelerates aging and development of mammary tumors in HER-2/neu transgenic mice. Bull Exp Biol Med. 2005 Jun;139(6):721-3. http://www.ncbi.nlm.nih.gov/pubmed/16224592 4) Foretz M, Taleux N, Guigas B, et al. Regulation of energy metabolism by AMPK: a novel therapeutic approach for the treatment of metabolic and cardiovascular diseases. Med Sci (Paris). 2006 Apr;22(4):381-8.
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