Effects of metformin on cancers in experimental and clinical studies: Focusing on autophagy and AMPK/mTOR signaling pathways

Metformin (MET) is a preferred drug for the treatment of type 2 diabetes mellitus. Recent studies show that apart from its blood glucose-lowering effects, it also inhibits the development of various tumours, by inducing autophagy. Various studies have confirmed the inhibitory effects of MET on cancer cell lines' propagation, migration, and invasion. The objective of the study was to comprehensively review the potential of MET as an anticancer agent, particularly focusing on its ability to induce autophagy and inhibit the development and progression of various tumors. The study aimed to explore the inhibitory effects of MET on cancer cell proliferation, migration, and invasion, and its impact on key signaling pathways such as adenosine monophosphate-activated protein kinase (AMPK), mammalian target of rapamycin (mTOR), and PI3K. This review noted that MET exerts its anticancer effects by regulating key signalling pathways such as phosphoinositide 3-kinase (PI3K), LC3-I and LC3-II, Beclin-1, p53, and the autophagy-related gene (ATG), inhibiting the mTOR protein, downregulating the expression of p62/SQSTM1, and blockage of the cell cycle at the G0/G1. Moreover, MET can stimulate autophagy through pathways associated with the 5' AMPK, thereby inhibiting he development and progression of various human cancers, including hepatocellular carcinoma, prostate cancer, pancreatic cancer, osteosarcoma, myeloma, and non-small cell lung cancer. In summary, this detailed review provides a framework for further investigations that may appraise the autophagy-induced anticancer potential of MET and its repurposing for cancer treatment.

Metformin Intervention—A Panacea for Cancer Treatment?

The molecular mechanism of action and the individual influence of various metabolic pathways related to metformin intervention are under current investigation. The available data suggest that metformin provides many advantages, exhibiting anti-inflammatory, anti-cancer, hepatoprotective, cardioprotective, otoprotective, radioprotective, and radio-sensitizing properties depending on cellular context. This literature review was undertaken to provide novel evidence concerning metformin intervention, with a particular emphasis on cancer treatment and prevention. Undoubtedly, the pleiotropic actions associated with metformin include inhibiting inflammatory processes, increasing antioxidant capacity, and improving glycemic and lipid metabolism. Consequently, these characteristics make metformin an attractive medicament to translate to human trials, the promising results of which were also summarized in this review.

Tumor mechanics and metabolic dysfunction

Desmosplasia is a characteristic of most solid tumors and leads to fibrosis through abnormal extracellular matrix (ECM) deposition, remodeling, and posttranslational modifications. The resulting stiff tumor stroma not only compromises vascular integrity to induce hypoxia and impede drug delivery, but also promotes aggressiveness by potentiating the activity of key growth, invasion, and survival pathways. Intriguingly, many of the protumorigenic signaling pathways that are mechanically activated by ECM stiffness also promote glucose uptake and aerobic glycolysis, and an altered metabolism is a recognized hallmark of cancer. Indeed, emerging evidence suggests that metabolic alterations and an abnormal ECM may cooperatively drive cancer cell aggression and treatment resistance. Accordingly, improved methods to monitor tissue mechanics and metabolism promise to improve diagnostics and treatments to ameliorate ECM stiffening and elevated mechanosignaling may improve patient outcome. Here we discuss the interplay between ECM mechanics and metabolism in tumor biology and suggest that monitoring these processes and targeting their regulatory pathways may improve diagnostics, therapy, and the prevention of malignant transformation.

Obesity and insulin resistance in breast cancer--chemoprevention strategies with a focus on metformin

Obesity and insulin resistance have been associated with breast cancer risk, and breast cancer outcomes. Recent research has focused on insulin as a potential biologic mediator of these effects given frequent expression of insulin/IGF-1 receptors on breast cancer cells which, when activated, can stimulate signaling through PI3K and Ras-Raf signaling pathways to enhance proliferation. Metformin, a commonly used diabetes drug, lowers insulin in non-breast diabetic cancer patients, likely by reducing hepatic gluconeogenesis; it also appears to have potential insulin independent direct effects on tumor cells which are mediated by activation of AMPK with downstream inhibition of mTOR. There is growing epidemiologic, clinical and preclinical (in vitro and in vivo) evidence in keeping with anticancer effects of metformin in breast and other cancers. This has led to the hypothesis that metformin may be effective in breast cancer prevention and treatment. Clinical studies in the neoadjuvant and adjuvant settings are ongoing; additional Phase 2 trials in the metastatic setting and proof of principle studies in the prevention setting are planned.

Targeting metabolism for cancer treatment and prevention: metformin, an old drug with multi-faceted effects

Understanding the complexity of cancer and of the underlying regulatory networks provides a new paradigm that tackles cancer development and treatment through a system biology approach, contemporarily acting on various intersecting pathways. Cancer cell metabolism is an old pathogenetic issue that has recently gained new interest as target for therapeutic approaches. More than 70 years ago, Warburg discovered that malignant cells generally have altered metabolism with high rates of glucose uptake and increased glycolysis, even under aerobic condition. Observational studies have provided evidence that impaired metabolism, obesity, hyperglycemia and hyperinsulinemia may have a role in cancer development, progression and prognosis, and actually diabetic and obese patients have increased cancer risk. On the other hand, caloric restriction has been shown to prolong life span and reduce cancer incidence in several animal models, having an impact on different metabolic pathways. Metformin, an antidiabetic drug widely used for over 40 years, mimics caloric restriction acting on cell metabolism at multiple levels, reducing all energy-consuming processes in the cells, including cell proliferation. By overviewing molecular mechanisms of action, epidemiological evidences, experimental data in tumor models and early clinical study results, this review provides information supporting the promising use of metformin in cancer prevention and treatment.

Diabetes, diabetes therapies and cancer: what’s the link?

Diabetes and cancer are prevalent diseases that have a large global impact on health and healthcare resources. Epidemiological studies suggest that patients with diabetes are at a higher risk of developing certain types of cancers. This may be due to diabetes and cancer sharing common risk factors, but it is likely that any association is intricately linked by biological mechanisms, which are only starting to be unravelled. This is further complicated by emerging data that suggest that some diabetes therapies may predispose or promote development of certain cancers, while metformin appears to have anti-neoplastic effects. This review considers the evidence associating diabetes, diabetes therapies and cancer.

Understanding the benefit of metformin use in cancer treatment

Biguanides have been developed for the treatment of hyperglycemia and type 2 diabetes. Recently, metformin, the most widely prescribed biguanide, has emerged as a potential anticancer agent. Epidemiological, preclinical and clinical evidence supports the use of metformin as a cancer therapeutic. The ability of metformin to lower circulating insulin may be particularly important for the treatment of cancers known to be associated with hyperinsulinemia, such as those of the breast and colon. Moreover, metformin may exhibit direct inhibitory effects on cancer cells by inhibiting mammalian target of rapamycin (mTOR) signaling and protein synthesis. The evidence supporting a role for metformin in cancer therapy and its potential molecular mechanisms of action are discussed.