Metformin-induced Apoptosis Research Articles

PO-074 Metformin-induced apoptosis facilitates degradation of the C-FLIPL protein through a caspase-dependent pathway in human renal cell carcinoma A498 cells

IntroductionGastric cancer (GC) is the fourth most common cancer and the third-leading cause of cancer-related deaths worldwide. Despite certain advances in chemotherapy regimens and targeted therapies, the 5 year survival of patients with advanced GC is disappointing. The positive results of the apatinib phase III trial have cast new light on treatment for patients with advanced gastric cancer (GC) refractory to two or more lines of prior systemic chemotherapies. However, a dose reduction or treatment interruption is required to prevent apatinib toxicity such as hand-foot skin reaction (HFSR), proteinuria, and hypertension. In addition, suitable biomarkers are not accessible for identifying the patients who are most likely to benefit from apatinib. Therefore, predictive biomarkers are urgently needed to reduce toxicity and identify patients that will benefit from apatinib treatment.Material and methodsClonogenic assay, cytotoxicity assay, apoptosis assay, qRT-PCR, western blot were conducted in GC cells to evaluate the effectiveness of apatinib in vitro. Confocal microscopy assay was peformed to examine apatinib-induced autophagic flux in GC cells. Transmission electron microscopy assay was conducted to show the formation of autophagic vesicles. Xenograft models in mice were conducted and six tumour tissues were sent for miRNA-seq and circRNA-seq. Sanger sequencing was applied to determine the full length of the amplification products.Results and discussionsIn this study, we found that apatinib inhibited the growth of GC cells and xenograft tumours. Apatinib promoted autophagy activation via upregulation of ATG7 expression, and autophagy inhibition enhanced apatinib-induced apoptosis. With miRNA-seq and circRNA-seq analyses of GC xenograft models, we demonstrated that circRNA004582 functioned as an endogenous sponge for miR-3657 to inhibit its activity and further upregulate ATG7 expression. Silencing of circRNA004582 inhibited apatinib-induced autophagy, which was rescued by miR-3657. Moreover, knockdown of circRNA004582 promoted apatinib-induced apoptosis in GC cells. These findings provided the first evidence that the circRNA004582-miR-3657-ATG7 axis mediates a regulatory pathway critical for the regulation of autophagy and apatinib sensitivity in GC cells and xenografts.ConclusionIn conclusion, specific blockage of circRNA004582 may be a potential therapeutic target for autophagy inhibition in the context of apatinib treatment in human GC. Prospective clinical studies are warranted to examine these biomarkers.

Metformin-induced apoptosis facilitates degradation of the cellular caspase 8 (FLICE)-like inhibitory protein through a caspase-dependent pathway in human renal cell carcinoma A498 cells.

Renal cell carcinoma (RCC) is one of the most common types of cancer in adults. Previous studies have reported that the survival rate was significantly lower for renal cancer patients with diabetes than for those without diabetes. Metformin is a well-known anti-diabetic agent used for the treatment of type 2 diabetes mellitus (T2DM). It also inhibits cell proliferation and angiogenesis and is known to possess antitumor effects. However, the molecular mechanism for metformin-induced apoptosis in renal cell carcinoma is not understood. In the present study, treatment with metformin induced apoptosis in A498 cells in a dose-dependent manner. It was revealed that degradation of cellular caspase 8 (FLICE)-like inhibitory protein (c-FLIP) and activation of procaspase-8 were associated with metformin-mediated apoptosis. By contrast, treatment with metformin did not affect the mRNA level of c-FLIPL in A498 cells. Treatment with benzyloxycarbonyl-Val-Ala-Asp-fluoromethyl ketone (z-VAD-fmk, a pan-caspase inhibitor) almost completely blocked metformin-induced apoptosis and degradation of c-FLIPL protein. However, N-acetyl-L-cysteine (NAC), a reactive oxygen species (ROS) scavenger, did not inhibit metformin-mediated apoptosis in A498 cells. Taken together, the results of the present study demonstrated that metformin-induced apoptosis involved degradation of the c-FLIPL protein and activation of caspase-8 in human renal cell carcinoma A498 cells and suggested that metformin could be potentially used for the treatment of renal cancer.

Effect of metformin-induced endoplasmic reticulum stress on the apoptosis of thyroid cancer TPC-1 cells

Objective To explore the effect of metformin on the apoptosis of thyroid cancer TPC-1 cells and its mechanism. Methods The proliferation and apoptosis of TPC-1 cells treated with different concentrations of metformin were detected using cell count kit-8 (CCK-8) assay and flow cytometry respectively. Western blot and Real-time PCR were used to detect the expression of endoplasmic reticulum chaperone immunoglobulin heavy chain binding protein (Bip) and endoplasmic reticulum stress associated apoptosis protein CCAAT/enhancer-binding protein homologous protein (CHOP) and cysteinyl aspartate specific proteinase (Caspase-12) on mRNA and protein levels. TPC-1 cells were injected into nude mice to investigate the effect of metformin on the tumorigenesis in vivo. In addition, the expressions of Bip and CHOP were detected by an immunohistochemical method. Results Metformin could decrease proliferation and promote apoptosis of TPC-1 cells. Moreover, metformin increased the expression of Bip and endoplasmic reticulum stress-associated apoptosis protein Caspase-12 and CHOP, both in protein and mRNA levels. 4-phenylbutyrate, the endoplasmic reticulum stress inhibitor, could reverse the metformin-induced apoptosis of TPC-1 cells. By contrast, activating endoplasmic reticulum stress by thapsigargin enhanced the metformin-mediated apoptosis. The average tumor weight and size in metformin group was significantly lower than that in the control group. Meanwhile, the metformin group had higher expression of Bip and CHOP. Conclusion Metformin may induce the apoptosis of thyroid cancer TPC-1 cells through endoplasmic reticulum stress activation. (Chin J Endocrinol Metab, 2018, 34: 233-238) Key words: Thyroid carcinoma; Metformin; Endoplasmic reticulum stress; Apoptosis

SIRT3 aggravates metformin-induced energy stress and apoptosis in ovarian cancer cells

Increasing evidence suggests that mitochondrial respiratory chain complex I participates in carcinogenesis and cancer progression by providing energy and maintaining mitochondrial function. However, the role of complex I in ovarian cancer is largely unknown. In this study we showed that metformin, considered to be an inhibitor of complex I, simultaneously inhibited cell growth and induced mitochondrial-related apoptosis in human ovarian cancer cells. Metformin interrupted cellular energy metabolism mainly by causing damage to complex I that impacted mitochondrial function. Additionally, treatment with metformin increased the activation of sirtuin 3 (SIRT3), a mitochondrial deacetylase. We demonstrated that SIRT3 overexpression aggravated metformin-induced apoptosis, energy stress and mitochondrial dysfunction. Moreover, treatment with metformin or SIRT3 overexpression increased activation of AMP-activated protein kinase (AMPK), a major sensor of cellular energy status. AMPK compensated for energy loss by increasing glycolysis. The impact of this was assessed by reducing glucose levels in the media or by using inhibitors (2-deoxyglucose, Compound C) of glycolysis and AMPK. The combination of these factors with metformin intensified cytotoxicity through further downregulation of ATP. Our study outlines an important role for SIRT3 in the antitumor effect of mitochondrial complex I inhibitors in human ovarian cancer cells. This effect appears to be mediated by induction of energy stress and apoptosis. Strategies that target the mitochondria could be enhanced by modulating glycolysis to further aggravate energy stress that may increase the antitumor effect.

Abstract CT145: A phase I window, dose escalating and safety trial of Metformin in combination with induction chemotherapy (VPLD)in relapsed/refractory acute lymphoblastic leukemia: NCT01324180

Abstract Background: Relapsed Acute Lymphoblastic Leukemia (ALL) remains a major cause of cancer-related deaths in children. We identified the AMP activated protein kinase (AMPK) as a potential target for ALL therapy due to its regulatory effects on the unfolded protein response (UPR), leading to increased vulnerability of ALL cells to endoplasmic reticulum (ER) stress inducers. In vitro, metformin leads to ALL cell death via AMPK-mediated inhibition of the UPR. Methods: Metformin was administered twice daily continuously on a 28 day cycle in addition to the Vincristine, Dexamethasone, PEG-Asparaginase and Doxorubicin (VPLD) systemic regimen and CNS-directed therapy in pediatric patients with relapsed/refractory ALL. Metformin doses were increased in a standard 3+3 phase I design with three dose levels evaluated, 666, 1,000 and 1,333 mg/m2/day. Pharmacokinetic (PK) and pharmacodynamic (PD) evaluation of the AMPK and ER stress/UPR pathways were ascertained on days 1 and 7, and treatment response was assessed on day 29. Results: Fourteen patients were enrolled, 11 evaluable. DL3 was the maximum administered dose with 2 related DLT’s of diarrhea and acidosis. A single DLT of hypoglycemia and acidosis during an episode of sepsis was observed in DL2. Infectious SAE’s occurred in 7 patients. Two patients had posterior reversible encephalopathy syndrome; both died of disease progression within 30 days of coming off study. A single patient had stable disease, 2 had a partial response, and 3 achieved a complete response. PK studies demonstrated levels within the therapeutic range for patients with diabetes, and PD evaluation showed induction of ER stress and inhibition of the UPR. Conclusions: This trial has been completed. We found induction of ER stress with inhibition of UPR consistent with that observed in vitro leading to metformin-induced apoptosis. The chemotherapeutic backbone was tolerable and the combination with metformin yielded responses in a heavily pretreated population. Toxicities attributable to metformin occurred in all dose levels, but DLT’s were only observed in dose levels above the standard dosing for diabetes. Clinical trial information: NCT01324180 Citation Format: Matteo Trucco, Julio Barredo, John Goldberg, Gregory Hale, Jonathan Gill, Bhuvana Setty, Tiffany Smith, Jae Lee, Damon Reed. A phase I window, dose escalating and safety trial of Metformin in combination with induction chemotherapy (VPLD)in relapsed/refractory acute lymphoblastic leukemia: NCT01324180 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr CT145. doi:10.1158/1538-7445.AM2017-CT145

Metformin inhibits proliferation and growth hormone secretion of GH3 pituitary adenoma cells.

Metformin is an anti-hyperglycemic agent used to treat diabetes, and recent evidence suggests it has antitumor efficacy. Because growth hormone-secreting pituitary adenoma (GH-PA) patients have a high incidence of diabetes frequently treated with metformin, we assessed the antitumor effect of metformin on GH-PA. We found that metformin effectively inhibited proliferation and induced apoptosis in the GH-PA cell line GH3. We detected a decrease in mitochondrial membrane potential (MMP), an increase in expression of pro-apoptotic proteins, and a decrease in expression of an anti-apoptotic protein in metformin-treated GH3 cells, which suggests involvement of the mitochondrial-mediated apoptosis pathway. Inhibition of AMPK, which is activated by metformin, failed to reverse the antiproliferative effect. ATF3 was upregulated by metformin, and its knockdown significantly reduced metformin-induced apoptosis. In addition, GH secretion was inhibited by metformin through suppression of STAT3 activity independently of AMPK. Metformin also significantly suppressed cellular proliferation and GH secretion in primary human GH-PA cells. Metformin also significantly inhibited GH3 cell proliferation and GH secretion in vivo. ATF3 upregulation and p-STAT3 downregulation were confirmed in xenografts. These findings suggest metformin is a potentially promising therapeutic agent for the treatment of GH-PA, particularly in patients with diabetes.

Impact of obesity and adiponectin signaling in patients with renal cell carcinoma: A potential mechanism for the obesity paradox.

449 Background: Obesity increases the risk of renal cell carcinoma (RCC); however, obese patients experience longer survival than non-obese patients. The mechanism of this “obesity paradox” is unknown. We examined the impact of obesity, total adiponectin (AD) level, and intratumoral AD receptors expression on RCC aggressiveness and survival, and also investigated the mechanism underlying enhanced cancer aggressiveness in RCC cells with exogenous adiponectin stimulation. Methods: A total of 129 RCC patients treated with surgery were included in the analyses. Preoperative BMI, serum total adiponectin (AD) level, total AD secretion from perinephric adipose tissue, and intratumoral AD receptors mRNA and protein expression were analyzed. Caki-2 and 786- cells were used for in vitro functional analyses. Results: Overweight and obese patients had significantly lower grade cancers than normal patients in all patients and in those without metastasis (p = 0.003, and p = 0.027, respectively). Cancer-specific survival in overweight and obese patients was significantly better than in normal patients in all patients (p = 0.035). A weak inverse correlation existed between serum AD level and BMI in RCC patients (r = −0.344). Tumor size was slightly correlated with serum AD level, and high serum AD was significantly associated with poor overall survival in patients without metastasis (p = 0.035). The AD level of perinephric adipose tissue and intratumoral AdipoR1/R2 expression in tumors did not correlate with RCC aggressiveness and survival. Exogenous AD significantly enhanced proliferation, but not invasion or migration in 786-O and Caki-2 cells. Exogenous AD significantly inhibited starvation- and metformin-induced apoptosis, and up-regulated p-AMPK and Bcl-xL in these cells. Conclusions: Low BMI and high AD level are associated with cancer aggressive and poor survival in RCC patients treated surgically. AD modulates proliferation and apoptosis, which may underlie the “obesity paradox” of RCC.

The impact of obesity and adiponectin signaling in patients with renal cell carcinoma: A potential mechanism for the "obesity paradox".

Although obesity increases the risk of renal cell carcinoma (RCC), obese patients with RCC experience longer survival than non-obese patients. However, the mechanism of this “obesity paradox” is unknown. We examined the impact of preoperative BMI, serum total adiponectin (sAd) level, total adiponectin secretion from perinephric adipose tissue, and intratumoral expression of adiponectin receptors on RCC aggressiveness and survival. We also investigated the mechanism underlying enhanced cancer aggressiveness in RCC cells stimulated with exogenous adiponectin. Overweight and obese patients had significantly lower grade cancers than normal patients in all patients and in those without metastasis (p = 0.003 and p = 0.027, respectively). Cancer-specific survival was significantly longer in overweight and obese patients than in normal patients in all patients (p = 0.035). There was a weak inverse correlation between sAd level and BMI in RCC patients (r = −0.344, p = 0.002). Tumor size was slightly correlated with sAd level, and high sAd was significantly associated with poor overall survival rates in patients with non-metastatic RCC (p = 0.035). Adiponectin levels in perinephric adipose tissue and intratumoral AdipoR1/R2 expression were not correlated with RCC aggressiveness or survival. Proliferation significantly increased in 786-O and Caki-2 cells exposed to exogenous adiponectin, whereas cell invasion and migration were unaffected. In addition, exogenous adiponectin significantly inhibited starvation- and metformin-induced apoptosis, and up-regulated p-AMPK and Bcl-xL levels. In summary, low BMI and high adiponectin levels are associated with aggressive cell behaviors and poor survival in surgically-treated RCC patients. The effects of adiponectin on proliferation and apoptosis might underlie the “obesity paradox” of RCC.

Metformin Inhibits Gemcitabine Induced Apoptosis in Pancreatic Cancer Cell Lines.

Many preclinical and clinical studies are currently evaluating metformin in combination with classical therapeutic agents as anti-cancer therapy. In this study we used three distinct pancreatic cancer cell lines and evaluated cell death by trypan blue assay and Western Blots using antibodies directed against cleaved caspase 3 and PARP. Surprisingly, we observed that 20mM metformin did not enhance, but rather inhibited gemcitabine induced cell death in murine 7265PDA, 6606PDA and 6606l cells. Microenvironmental aspects such as oxygen supply or the pH value did not influence the inhibition of cancer cell apoptosis by metformin. Glucose concentration in the medium, however, had a major effect on the impact of metformin. Medium with 0.5g/L glucose strongly increased metformin induced apoptosis and also prevented the inhibitory effect of metformin on gemcitabine induced cell apoptosis, when compared with medium containing 4.5g/L glucose. We conclude that the combination of metformin with gemcitabine has inappropriate effects for a successful treatment of pancreatic cancer. Thus, it might be more promising to use metformin in combination with other drugs that reduce the uptake or the metabolism of glucose.

Metformin requires 4E-BPs to induce apoptosis and repress translation of Mcl-1 in hepatocellular carcinoma cells.

Metformin inhibits the mammalian target of rapamycin complex 1 (mTORC1) signaling pathway, which is frequently upregulated in hepatocellular carcinoma (HCC). Metformin has also been shown to induce apoptosis in this cancer. Here, we investigate whether metformin-induced apoptosis in HCC is mediated by the downstream mTORC1 effectors eukaryotic initiation factor 4E and (eIF4E)-binding proteins (4E-BPs). Further, we ask whether changes in 4E-BPs activity during metformin treatment negatively regulate translation of the anti-apoptotic myeloid cell leukemia 1 (Mcl-1) mRNA. A genetic HCC mouse model was employed to assess the ability of metformin to reduce tumor formation, induce apoptosis, and control 4E-BP1 activation and Mcl-1 protein expression. In parallel, the HCC cell line Huh7 was transduced with scrambled shRNA (control) or shRNAs targeting 4E-BP1 and 4E-BP2 (4E-BP knock-down (KD)) to measure differences in mRNA translation, apoptosis, and Mcl-1 protein expression after metformin treatment. In addition, immunohistochemical staining of eIF4E and 4E-BP1 protein levels was addressed in a HCC patient tissue microarray. We found that metformin decreased HCC tumor burden, and tumor tissues showed elevated apoptosis with reduced Mcl-1 and phosphorylated 4E-BP1 protein levels. In control but not 4E-BP KD Huh7 cells, metformin induced apoptosis and repressed Mcl-1 mRNA translation and protein levels. Immunostaining of HCC patient tumor tissues revealed a varying ratio of eIF4E/4E-BP1 expression. Our results propose that metformin induces apoptosis in mouse and cellular models of HCC through activation of 4E-BPs, thus tumors with elevated expression of 4E-BPs may display improved clinical chemopreventive benefit of metformin.

Correction: Metformin impairs Rho GTPase signaling to induce apoptosis in neuroblastoma cells and inhibits growth of tumors in the xenograft mouse model of neuroblastoma.

Metformin has been shown to inhibit tumor growth in xenograft rodent models of adult cancers, and various human clinical trials are in progress. However, the precise molecular mechanisms of metformin action are largely unknown. In the present study we examined the anti-tumor activity of metformin against neuroblastoma, and determined the underlying signaling mechanisms. Using human neuroblastoma xenograft mice, we demonstrated that oral administration of metformin (100 and 250 mg/kg body weight) significantly inhibited the growth of tumors. The interference of metformin in spheroid formation further confirmed the anti-tumor activity of metformin. In tumors, the activation of Rac1 (GTP-Rac1) and Cdc42 (GTP-Cdc42) was increased while RhoA activation (GTP-RhoA) was decreased by metformin. It also induced phosphorylation of JNK and inhibited the phosphorylation of ERK1/2 without affecting p38 MAP Kinase. Infection of cells by adenoviruses expressing dominant negative Rac1 (Rac1-N17), Cdc42 (Cdc42-N17) or constitutively active RhoA (RhoA-V14), or incubation of cells with pharmacological inhibitors of Rac1 (NSC23766) or Cdc42 (ML141) significantly protected neuroblastoma cells from metformin-induced apoptosis. Additionally, inhibition of JNK activity along with Rac1 or Cdc42 attenuated cytotoxic effects of metformin. These studies demonstrated that metformin impairs Rho GTPases signaling to induce apoptosis via JNK pathway.

Abstract A56: Metformin targets Rho GTPases to inhibit neuroblastoma cell growth; implications in the treatment of neuroblastoma

Abstract In recent years, oncologists are paying considerable attention to metformin (N', N'-dimethylbiguanide) in the treatment of various types of cancers, as some population-based studies have shown low cancer incidences and mortalities among diabetic-patients treated with metformin. In the present study, using neuroblastoma cells, we explored the anti-tumor activity and the underlying mechanisms of metformin. Neuroblastoma is a malignant cancer of the postganglionic sympathetic nervous system; it develops in the adrenal gland and metastasizes to liver, bone, bone marrow, lymph nodes, neck and chest. It is the most common cancer in infants younger than one and second most common tumors in children. In the United States of America, it accounts for 7% of all childhood cancers, and is responsible for 15% of all cancer deaths in children younger than 15 years. Various genetic and cytological alterations allow cells to develop drug resistance and help neuroblastoma tumors to escape most available therapies. Using human neuroblastoma cells of different genotypes (SH-SY5Y, MYCN-nonamplified; and SK-N-BE(2), MYCN-amplified cells), we generated subcutaneous xenograft mice models, and metformin (50, 100 and 250 mg/kg body weight per mice) was given daily by oral gavage to tumor-bearing mice. Our in vivo results demonstrated that metformin (100 and 250 mg/kg body weight) significantly inhibited the growth of tumors in both mice models. Immunofluorescence and Western blot results indicated reduction in tumor growth was due to the increased DNA fragmentation and apoptotic cell death that occur via activation of caspase-3. Metformin also inhibited the initiation of spheroid formation in 3-D cultures (hanging drop assay). In xenograft tumors, metformin affected the phosphorylation of mitogen-activated protein kinases (MAPKs). Metformin induced the phosphorylation of JNK1/2 and inhibited the phosphorylation of ERK1/2 without affecting p38 MAP Kinase. Since ERK1/2 is involved in cell proliferation and JNK1/2 promotes cell death, our observations clearly indicated that MAP kinases are involved in metformin-induced cell death. Rho-GTPases (RhoA, Rac1 and Cdc42), a family of small G-proteins, act as molecular switches to regulate cellular functions- cell division, motility and cell survival. These small G-proteins exist either as a GTP-bound active form or an inactive GDP-bound form. The aberrant expression and/or activity of Rho-GTPases are associated with the progression of various cancers. Our glutathione-S-transferase (GST)-pull down assays showed that metformin significantly increased activation of Rac1 (GTP-Rac1) and Cdc42 (GTP-Cdc42) while it decreased activation of RhoA (GTP-RhoA) in these tumors. Infection of neuroblastoma cells by adenoviruses expressing dominant negative Rac1, Cdc42 (Rac1-N17 and Cdc42-N17, respectively) and constitutively active RhoA (RhoA-V14), or incubation with pharmacological inhibitors of Rac1 (NSC23766) or Cdc42 (ML141) significantly protected cells from metformin-induced apoptosis. Additionally, inhibition of JNK activity with Rac1 or Cdc42 attenuated cytotoxic effects of metformin. These gain-in and loss-off function studies suggested that metformin impairs Rho-GTPase signaling to initiate apoptosis via JNK pathway. Together, our results reported the inhibitory effects of metformin against neuroblastoma, and demonstrated the role of Rho-GTPases in metformin-mediated apoptotic cell death. The cytotoxic effects of metformin against MYCN-nonamplified and MYCN-amplified multidrug resistant neuroblastoma cells in our studies further signifies that metformin can be a promising drug candidate for neuroblastoma therapy. Citation Format: Ambrish Kumar, Nadia Al-Sammarraie, Donald J. DiPette, Ugra S. Singh. Metformin targets Rho GTPases to inhibit neuroblastoma cell growth; implications in the treatment of neuroblastoma. [abstract]. In: Proceedings of the Thirteenth Annual AACR International Conference on Frontiers in Cancer Prevention Research; 2014 Sep 27-Oct 1; New Orleans, LA. Philadelphia (PA): AACR; Can Prev Res 2015;8(10 Suppl): Abstract nr A56.

Metformin Induces Apoptosis and Downregulates Pyruvate Kinase M2 in Breast Cancer Cells Only When Grown in Nutrient-Poor Conditions.

IntroductionMetformin is proposed as adjuvant therapy in cancer treatment because of its ability to limit cancer incidence by negatively modulating the PI3K/AKT/mTOR pathway. In vitro, in addition to inhibiting cancer cell proliferation, metformin can also induce apoptosis. The molecular mechanism underlying this second effect is still poorly characterized and published data are often contrasting. We investigated how nutrient availability can modulate metformin-induced apoptosis in three breast cancer cell lines.Material and MethodsMCF7, SKBR3 and MDA-MB-231 cells were plated in MEM medium supplemented with increasing glucose concentrations or in DMEM medium and treated with 10 mM metformin. Cell viability was monitored by Trypan Blue assay and treatment effects on Akt/mTOR pathway and on apoptosis were analysed by Western Blot. Moreover, we determined the level of expression of pyruvate kinase M2 (PKM2), a well-known glycolytic enzyme expressed in cancer cells.ResultsOur results showed that metformin can induce apoptosis in breast cancer cells when cultured at physiological glucose concentrations and that the pro-apoptotic effect was completely abolished when cells were grown in high glucose/high amino acid medium. Induction of apoptosis was found to be dependent on AMPK activation but, at least partially, independent of TORC1 inactivation. Finally, we showed that, in nutrient-poor conditions, metformin was able to modulate the intracellular glycolytic equilibrium by downregulating PKM2 expression and that this mechanism was mediated by AMPK activation.ConclusionWe demonstrated that metformin induces breast cancer cell apoptosis and PKM2 downregulation only in nutrient-poor conditions. Not only glucose levels but also amino acid concentration can influence the observed metformin inhibitory effect on the mTOR pathway as well as its pro-apoptotic effect. These data demonstrate that the reduction of nutrient supply in tumors can increase metformin efficacy and that modulation of PKM2 expression/activity could be a promising strategy to boost metformin anti-cancer effect.

Autophagy and protein kinase RNA-like endoplasmic reticulum kinase (PERK)/eukaryotic initiation factor 2 alpha kinase (eIF2α) pathway protect ovarian cancer cells from metformin-induced apoptosis.

Metformin, an oral biguanide for the treatment of type II diabetes, has been shown to have anticancer effects in ovarian cancer. Energy starvation induced by metformin causes endoplasmic reticulum stress-mediated unfolded protein response (UPR) and autophagy. UPR and autophagy act as a survival or death mechanism in cells. In this study, we observed that metformin-induced apoptosis was relieved by autophagy and the PERK/eIF2α pathway in ovarian cancer cells, but not in peripheral blood mononuclear cells (PBMC) or 'normal' ovarian surface epithelial cells (OSE). Increased PARP cleavage and increased LC3B-II with ATG5-ATG12 complex suggested the induction of apoptosis and autophagy, respectively, in metformin-treated ovarian cancer cells. Accumulation of acidic vacuoles in the cytoplasm and downregulation of p62 further supported late-stage autophagy. Interestingly, metformin induced interdependent activation between autophagy and the UPR, especially the PERK/eIF2α pathway. Inhibition of autophagy-induced PERK inhibition, and vice versa, were demonstrated using small molecular inhibitors (PERK inhibitor I, GSK2606414; autophagy inhibitor, 3-MA, and BafA1). Moreover, autophagy and PERK activation protected ovarian cancer cells against metformin-induced apoptosis. Metformin treatment in the presence of inhibitors of PERK and autophagy, however, had no cytotoxic effects on OSE or PBMC. In conclusion, these results suggest that inhibition of autophagy and PERK can enhance the selective anticancer effects of metformin on ovarian cancer cells. © 2015 Wiley Periodicals, Inc.

AMPK/mTOR-mediated inhibition of survivin partly contributes to metformin-induced apoptosis in human gastric cancer cell

Recent studies demonstrated that metformin exerts anti-neoplastic effect in a spectrum of malignancies. However, the mechanism whereby metformin affects various cancers, including gastric cancer, is poorly elucidated. Considering apoptosis plays critical role in tumorigenesis, we, in the present study, investigated the in vitro apoptotic effect of metformin on human gastric cancer cell and the underlying mechanism. Three differently-differentiated gastric cancer cell lines, MKN-28, SGC-7901 and BGC-823, along with one noncancerous gastric cell line GES-1 were used. We found that metformin treatment selectively induces apoptosis in the 3 cancer cell lines, but not the noncancerous one, as confirmed by flow cytometry, Caspase-Glo assay and western blotting against PARP and cleaved caspase 3. Moreover, the apoptotic effect of metformin seems to correlate negatively with the differentiation degree of gastric cancer. Metformin-induced apoptosis may be partially mediated through inhibition of anti-apoptotic survivin. Additionally, AMPK and mTOR, 2 important regulatory molecules responsible for metformin action, were investigated for their possible involvements in metformin-induced apoptosis of gastric cancer cell. AMPK knockdown by siRNA restores metformin-inhibited survivin expression and partially abolishes metformin-induced apoptosis. Similarly, forced overexpression of mTOR downstream effector p70S6K1 relieves metformin-induced inhibition of survivin and partly attenuates metformin-induced apoptosis. More importantly, survivin overexpression alleviates metformin-induced apoptosis. Xenograft nude mouse experiment also confirmed that AMPK/mTOR-mediated decrease of suvivin is in vivo implicated in metformin-induced apoptosis. Taken together, these evidences suggest that AMPK/mTOR-mediated inhibition of survivin may partly contribute to metformin-induced apoptosis of gastric cancer cell.

Metformin impairs Rho GTPase signaling to induce apoptosis in neuroblastoma cells and inhibits growth of tumors in the xenograft mouse model of neuroblastoma.

Metformin has been shown to inhibit tumor growth in xenograft rodent models of adult cancers, and various human clinical trials are in progress. However, the precise molecular mechanisms of metformin action are largely unknown. In the present study we examined the anti-tumor activity of metformin against neuroblastoma, and determined the underlying signaling mechanisms. Using human neuroblastoma xenograft mice, we demonstrated that oral administration of metformin (100 and 250 mg/kg body weight) significantly inhibited the growth of tumors. The interference of metformin in spheroid formation further confirmed the anti-tumor activity of metformin. In tumors, the activation of Rac1 (GTP-Rac1) and Cdc42 (GTP-Cdc42) was increased while RhoA activation (GTP-RhoA) was decreased by metformin. It also induced phosphorylation of JNK and inhibited the phosphorylation of ERK1/2 without affecting p38 MAP Kinase. Infection of cells by adenoviruses expressing dominant negative Rac1 (Rac1-N17), Cdc42 (Cdc42-N17) or constitutively active RhoA (RhoA-V14), or incubation of cells with pharmacological inhibitors of Rac1 (NSC23766) or Cdc42 (ML141) significantly protected neuroblastoma cells from metformin-induced apoptosis. Additionally, inhibition of JNK activity along with Rac1 or Cdc42 attenuated cytotoxic effects of metformin. These studies demonstrated that metformin impairs Rho GTPases signaling to induce apoptosis via JNK pathway.

Metformin-induced killing of triple-negative breast cancer cells is mediated by reduction in fatty acid synthase via miRNA-193b.

The anti-diabetic drug metformin (1,1-dimethylbiguanide hydrochloride) reduces both the incidence and mortality of several types of cancer. Metformin has been shown to selectively kill cancer stem cells, and triple-negative breast cancer (TNBC) cell lines are more sensitive to the effects of metformin as compared to luminal breast cancer. However, the mechanism underlying the enhanced susceptibility of TNBC to metformin has not been elucidated. Expression profiling of metformin-treated TNBC lines revealed fatty acid synthase (FASN) as one of the genes most significantly downregulated following 24 h of treatment, and a decrease in FASN protein was also observed. Since FASN is critical for de novo fatty acid synthesis and is important for the survival of TNBC, we hypothesized that FASN downregulation facilitates metformin-induced apoptosis. Profiling studies also exposed a rapid metformin-induced increase in miR-193 family members, and miR-193b directly targets the FASN 3'UTR. Addition of exogenous miR-193b mimic to untreated TNBC cells decreased FASN protein expression and increased apoptosis of TNBC cells, while spontaneously immortalized, non-transformed breast epithelial cells remained unaffected. Conversely, antagonizing miR-193 activity impaired the ability of metformin to decrease FASN and cause cell death. Further, the metformin-stimulated increase in miR-193 resulted in reduced mammosphere formation by TNBC lines. These studies provide mechanistic insight into metformin-induced killing of TNBC.

Metformin Induces Apoptosis through AMPK-Dependent Inhibition of UPR Signaling in ALL Lymphoblasts

The outcome of patients with resistant phenotypes of acute lymphoblastic leukemia (ALL) or those who relapse remains poor. We investigated the mechanism of cell death induced by metformin in Bp- and T-ALL cell models and primary cells, and show that metformin effectively induces apoptosis in ALL cells. Metformin activated AMPK, down-regulated the unfolded protein response (UPR) demonstrated by significant decrease in the main UPR regulator GRP78, and led to UPR-mediated cell death via up-regulation of the ER stress/UPR cell death mediators IRE1α and CHOP. Using shRNA, we demonstrate that metformin-induced apoptosis is AMPK-dependent since AMPK knock-down rescued ALL cells, which correlated with down-regulation of IRE1α and CHOP and restoration of the UPR/GRP78 function. Additionally rapamycin, a known inhibitor of mTOR-dependent protein synthesis, rescued cells from metformin-induced apoptosis and down-regulated CHOP expression. Finally, metformin induced PIM-2 kinase activity and co-treatment of ALL cells with a PIM-1/2 kinase inhibitor plus metformin synergistically increased cell death, suggesting a buffering role for PIM-2 in metformin’s cytotoxicity. Similar synergism was seen with agents targeting Akt in combination with metformin, supporting our original postulate that AMPK and Akt exert opposite regulatory roles on UPR activity in ALL. Taken together, our data indicate that metformin induces ALL cell death by triggering ER and proteotoxic stress and simultaneously down-regulating the physiologic UPR response responsible for effectively buffering proteotoxic stress. Our findings provide evidence for a role of metformin in ALL therapy and support strategies targeting synthetic lethal interactions with Akt and PIM kinases as suitable for future consideration for clinical translation in ALL.

Mitochondrial energetic and AKT status mediate metabolic effects and apoptosis of metformin in human leukemic cells.

Previous reports demonstrate that metformin, an anti-diabetic drug, can decrease the risk of cancer and inhibit cancer cell growth. However, its mechanism in cancer cells is still unknown. Metformin significantly blocks cell cycle and inhibits cell proliferation and colony formation of leukemic cells. However, the apoptotic response to metformin varies. Furthermore, daily treatment with metformin induces apoptosis and reduces tumor growth in vivo. While metformin induces early and transient activation of AMPK, inhibition of AMPKα1/2 does not abrogate anti-proliferative or pro-apoptotic effects of metformin. Metformin decreases electron transport chain complex I activity, oxygen consumption and mitochondrial ATP synthesis, while stimulating glycolysis for ATP and lactate production, pentose phosphate pathway for purine biosynthesis, fatty acid metabolism, as well as anaplerotic and mitochondrial gene expression. Importantly, leukemic cells with high basal AKT phosphorylation, glucose consumption or glycolysis exhibit a markedly reduced induction of the Pasteur effect in response to metformin and are resistant to metformin-induced apoptosis. Accordingly, glucose starvation or treatment with deoxyglucose or an AKT inhibitor induces sensitivity to metformin. Overall, metformin elicits reprogramming of intermediary metabolism leading to inhibition of cell proliferation in all leukemic cells and apoptosis only in leukemic cells responding to metformin with AKT phosphorylation and a strong Pasteur effect.

Metformin Induces Both Caspase-Dependent and Poly(ADP-ribose) Polymerase-Dependent Cell Death in Breast Cancer Cells

There is substantial evidence that metformin, a drug used to treat type 2 diabetics, is potentially useful as a therapeutic agent for cancer. However, a better understanding of the molecular mechanisms through which metformin promotes cell-cycle arrest and cell death of cancer cells is necessary. It will also be important to understand how the response of tumor cells differs from normal cells and why some tumor cells are resistant to the effects of metformin. We have found that exposure to metformin induces cell death in all but one line, MDA-MB-231, in a panel of breast cancer cell lines. MCF10A nontransformed breast epithelial cells were resistant to the cytotoxic effects of metformin, even after extended exposure to the drug. In sensitive lines, cell death was mediated by both apoptosis and a caspase-independent mechanism. The caspase-independent pathway involves activation of poly(ADP-ribose) polymerase (PARP) and correlates with enhanced synthesis of PARP and nuclear translocation of apoptosis-inducing factor (AIF), which plays an important role in mediating cell death. Metformin-induced, PARP-dependent cell death is associated with a striking enlargement of mitochondria. Mitochondrial enlargement was observed in all sensitive breast cancer cell lines but not in nontransformed cells or resistant MDA-MB-231. Mitochondrial enlargement was prevented by inhibiting PARP activity or expression. A caspase inhibitor blocked metformin-induced apoptosis but did not affect PARP-dependent cell death or mitochondrial enlargement. Thus, metformin has cytotoxic effects on breast cancer cells through 2 independent pathways. These findings will be pertinent to efforts directed at using metformin or related compounds for cancer therapy.