Metformin as anticancer agent and adjuvant in cancer combination therapy: Current progress and future prospect

Metformin, as the preferred antihyperglycemic drug for type 2 diabetes, has been found to have a significant effect in inhibiting tumor growth in recent years. However, metformin alone in cancer treatment has the disadvantages of high dose concentrations and few targeted cancer types. Increasing studies have confirmed that metformin can be used in combination with conventional anticancer therapy to obtain more promising clinical benefits, which is expected to be rapidly transformed and applied in clinic. Some combination therapy strategies including metformin combined with chemotherapy, radiotherapy, targeted therapy and immunotherapy have been proven to have more significant antitumor effects and longer survival time than monotherapy. In this review, we summarize the synergistic antitumor effects and mechanisms of metformin in combination with other current conventional anticancer therapies. In addition, we update the research progress and the latest prospect of the metformin-combined application in the cancer treatment. This work could provide more evidence and future direction for the clinical application of metformin in antitumor.

Metformin accelerates bone fracture healing by promoting type H vessel formation through inhibition of YAP1/TAZ expression

Due to increasing morbidity worldwide, fractures are becoming an emerging public health concern. This study aimed to investigate the effect of metformin on the healing of osteoporotic as well as normal fractures. Type H vessels have recently been identified as a bone-specific vascular subtype that supports osteogenesis. Here, we show that metformin accelerated fracture healing in both osteoporotic and normal mice. Moreover, metformin promoted angiogenesis in vitro under hypoxia as well as type H vessel formation throughout fracture healing. Mechanistically, metformin increased the expression of HIF-1α, an important positive regulator of type H vessel formation, by inhibiting the expression of YAP1/TAZ in calluses and hypoxia-cultured human microvascular endothelial cells (HMECs). The results of HIF-1α or YAP1/TAZ interference in hypoxia-cultured HMECs using siRNA further suggested that the enhancement of HIF-1α and its target genes by metformin is primarily through YAP1/TAZ inhibition. Finally, overexpression of YAP1/TAZ partially counteracted the effect of metformin in promoting type H vessel-induced angiogenesis-osteogenesis coupling during fracture repair. In summary, our findings suggest that metformin has the potential to be a therapeutic agent for fractures by promoting type H vessel formation through YAP1/TAZ inhibition.

Murine Breast Cancer Radiosensitization Using Oxygen Microbubbles and Metformin: Vessels Are the Key

Radiotherapy is a cornerstone of cancer treatment, but tumor hypoxia and resistance to radiation remain significant challenges. Vascular normalization has emerged as a strategy to improve oxygenation and enhance therapeutic outcomes. In this study, we examine the radiosensitization potential of vascular normalization using metformin, a widely used anti-diabetic drug, and oxygen microbubbles (OMBs). We investigated the synergistic action of metformin and OMBs and the impact of this therapeutic combination on the vasculature, oxygenation, invasiveness, and radiosensitivity of murine 4T1 breast cancer. We employed in vivo Doppler ultrasonographic imaging for vasculature analysis, electron paramagnetic resonance oximetry, and immunohistochemical assessment of microvessels, perfusion, and invasiveness markers. Our findings demonstrate that both two-week metformin therapy and oxygen microbubble treatment normalize abnormal cancer vasculature. The combination of metformin and OMB yielded more pronounced and sustained effects than either treatment alone. The investigated therapy protocols led to nearly twice the radiosensitivity of 4T1 tumors; however, no significant differences in radiosensitivity were observed between the various treatment groups. Despite these improvements, resistance to treatment inevitably emerged, leading to the recurrence of hypoxia and an increased incidence of metastasis.

Mechanisms and efficacy of metformin-mediated suppression of established experimental abdominal aortic aneurysms

Objective

Metformin treatment attenuates experimental abdominal aortic aneurysm (AAA) formation, as well as reduces clinical AAA diameter enlargement in patients with diabetes. The mechanisms of metformin-mediated aneurysm suppression, and its efficacy in suppressing established experimental aneurysms, remain uncertain.

Methods

Experimental AAAs were created in male C57BL/6J mice via intra-aortic infusion of porcine pancreatic elastase. Metformin alone (250 mg/kg), or metformin combined with the 5' AMP-activated protein kinase (AMPK) antagonist Compound C (10 mg/kg), were administered to respective mouse cohorts daily beginning 4 days following AAA induction. Further AAA cohorts received either the AMPK agonist AICA riboside (500 mg/kg) as positive, or vehicle (saline) as negative, controls. AAA progression in all groups was assessed via serial in vivo ultrasonography and histopathology at sacrifice. Cytokine-producing T cells and myeloid cellularity were determined by flow cytometric analyses.

Results

Metformin limited established experimental AAA progression at 3 (-85%) and 10 (-68%) days following treatment initiation compared with saline control. Concurrent Compound C treatment reduced this effect by approximately 50%. In metformin-treated mice, reduced AAA progression was associated with relative elastin preservation, smooth muscle cell preservation, and reduced mural leukocyte infiltration and neoangiogenesis compared with vehicle control group. Metformin also resulted in reduced interferon-γ-, but not interleukin-10 or -17, producing splenic T cells in aneurysmal mice. Additionally, metformin therapy increased circulating and splenic inflammatory monocytes (CD11b⁺Ly-6C^high), but not neutrophils (CD11b⁺Ly-6G⁺), with no effect on respective bone marrow cell populations.

Conclusions

Metformin treatment suppresses existing experimental AAA progression in part via AMPK agonist activity, limiting interferon-γ-producing T cell differentiation while enhancing circulating and splenic inflammatory monocyte retention.

Inhibiting the Priming for Cancer in Li-Fraumeni Syndrome

Simple Summary

Li-Fraumeni Syndrome (LFS) is a rare cancer pre-disposition syndrome associated with a germline mutation in the TP53 tumour suppressor gene. People with LFS have a 90% chance of suffering one or more cancers in their lifetime. No treatments exist to reduce this cancer risk. This paper reviews the evidence for how cancers start in people with LFS and proposes that a series of commonly used non-cancer drugs, including metformin and aspirin, can help reduce that lifetime risk of cancer.

Abstract

The concept of the pre-cancerous niche applies the ‘seed and soil’ theory of metastasis to the initial process of carcinogenesis. TP53 is at the nexus of this process and, in the context of Li-Fraumeni Syndrome (LFS), is a key determinant of the conditions in which cancers are formed and progress. Important factors in the creation of the pre-cancerous niche include disrupted tissue homeostasis, cellular metabolism and chronic inflammation. While druggability of TP53 remains a challenge, there is evidence that drug re-purposing may be able to address aspects of pre-cancerous niche formation and thereby reduce the risk of cancer in individuals with LFS.

Frontiers in Anti-Cancer Drug Discovery: Challenges and Perspectives of Metformin as Anti-Angiogenic Add-On Therapy in Glioblastoma

Simple Summary

Glioblastoma is the most aggressive primary brain tumor, with the highest incidence and the worst prognosis. Life expectancy from diagnosis remains dismal, at around 15 months, despite surgical resection and treatment with radiotherapy and chemotherapy. Given the aggressiveness of the tumor and the inefficiency of the treatments adopted to date, the scientific research investigates innovative therapeutic approaches. Importantly, angiogenesis represents one of the main features of glioblastoma, becoming in the last few years a major candidate for target therapy. Metformin, a well-established therapy for type 2 diabetes, offered excellent results in preventing and fighting tumor progression, particularly against angiogenic mechanisms. Therefore, the purpose of this review is to summarize and discuss experimental evidence of metformin anti-cancer efficacy, with the aim of proposing this totally safe and tolerable drug as add-on therapy against glioblastoma.

Abstract

Glioblastoma is the most common primitive tumor in adult central nervous system (CNS), classified as grade IV according to WHO 2016 classification. Glioblastoma shows a poor prognosis with an average survival of approximately 15 months, representing an extreme therapeutic challenge. One of its distinctive and aggressive features is aberrant angiogenesis, which drives tumor neovascularization, representing a promising candidate for molecular target therapy. Although several pre-clinical studies and clinical trials have shown promising results, anti-angiogenic drugs have not led to a significant improvement in overall survival (OS), suggesting the necessity of identifying novel therapeutic strategies. Metformin, an anti-hyperglycemic drug of the Biguanides family, used as first line treatment in Type 2 Diabetes Mellitus (T2DM), has demonstrated in vitro and in vivo antitumoral efficacy in many different tumors, including glioblastoma. From this evidence, a process of repurposing of the drug has begun, leading to the demonstration of inhibition of various oncopromoter mechanisms and, consequently, to the identification of the molecular pathways involved. Here, we review and discuss metformin’s potential antitumoral effects on glioblastoma, inspecting if it could properly act as an anti-angiogenic compound to be considered as a safely add-on therapy in the treatment and management of glioblastoma patients.

Inhibiting Autophagy in Renal Cell Cancer and the Associated Tumor Endothelium

The clear cell subtype of kidney cancer encompasses most renal cell carcinoma cases and is associated with the loss of von Hippel-Lindau gene function or expression. Subsequent loss or mutation of the other allele influences cellular stress responses involving nutrient and hypoxia sensing. Autophagy is an important regulatory process promoting the disposal of unnecessary or degraded cellular components, tightly linked to almost all cellular processes. Organelles and proteins that become damaged or that are no longer needed in the cell are sequestered and digested in autophagosomes upon fusing with lysosomes, or alternatively, released via vesicular exocytosis. Tumor development tends to disrupt the regulation of the balance between this process and apoptosis, permitting prolonged cell survival and increased replication. Completed trials of autophagic inhibitors using hydroxychloroquine in combination with other anticancer agents including rapalogues and high-dose interleukin 2 have now been reported. The complex nature of autophagy and the unique biology of clear cell renal cell carcinoma warrant further understanding to better develop the next generation of relevant anticancer agents.

Metformin has a direct effect on ovarian cells that is dependent on organic cation transporters

Metformin (MET) is the most widely prescribed hypoglycemic drug in type 2 diabetes and Polycystic Ovary Syndrome. Besides its effects on glucose metabolism, MET exerts beneficial effects on these patients' fertility. However, the exact mechanisms of action of MET on female fertility are still unclear. In this work, we analyzed a possible direct effect of MET on ovarian cells. We found expression of the organic cation transporters OCT1, OCT2 and OCT3, responsible for MET uptake into the cells, in rat granulosa cells and human cumulus cells. Furthermore, MET increased pAMPK and decreased VEGF levels both in vivo and in rat granulosa cells in culture. These last effects were reversed when OCTs were inhibited. Our results suggest that MET acts directly on ovarian cells regulating cell metabolism and VEGF expression. Our findings are relevant to optimize PCOS fertility treatment and to explore ovarian MET actions in other female pathologies.

Myeloid Derived Suppressor Cells Interactions With Natural Killer Cells and Pro-angiogenic Activities: Roles in Tumor Progression

Myeloid-derived suppressor cells (MDSCs) contribute to the induction of an immune suppressive/anergic, tumor permissive environment. MDSCs act as immunosuppression orchestrators also by interacting with several components of both innate and adaptive immunity. Natural killer (NK) cells are innate lymphoid cells functioning as primary effector of immunity, against tumors and virus-infected cells. Apart from the previously described anergy and hypo-functionality of NK cells in different tumors, NK cells in cancer patients show pro-angiogenic phenotype and functions, similar to decidual NK cells. We termed the pro-angiogenic NK cells in the tumor microenvironment "tumor infiltrating NK" (TINKs), and peripheral blood NK cells in cancer patients "tumor associated NK" (TANKs). The contribution of MDSCs in regulating NK cell functions in tumor-bearing host, still represent a poorly explored topic, and even less is known on NK cell regulation of MDSCs. Here, we review whether the crosstalk between MDSCs and NK cells can impact on tumor onset, angiogenesis and progression, focusing on key cellular and molecular interactions. We also propose that the similarity of the properties of tumor associated/tumor infiltrating NK and MDSC with those of decidual NK and decidual MDSCs during pregnancy could hint to a possible onco-fetal origin of these pro-angiogenic leukocytes.

Metformin suppresses tumor angiogenesis and enhances the chemosensitivity of gemcitabine in a genetically engineered mouse model of pancreatic cancer

AIMS

Pancreatic ductal adenocarcinoma (PDAC) is one of the most malignant diseases and has few effective and reliable therapeutic strategies. The anti-tumor effect of metformin is widely known, however, there is only limited evidence regarding the anti-angiogenesis effect and chemosensitization of metformin and its underlying mechanisms in PDAC.

MAIN METHODS

In the present study, we adopted a spontaneous PDAC mouse model named LSL‑Kras^G12D/+; Trp53^fl/+; Pdx1‑Cre (KPC) mice to explore the mechanism of the modulation of tumor angiogenesis and chemosensitization of metformin by treating KPC mice with metformin, gemcitabine or a combination of the two. H&E staining, Masson staining and immunohistochemical staining were adopted to describe the histopathology and biomarkers of the KPC in different groups.

KEY FINDINGS

Metformin plus gemcitabine reduced tumorigenic potential of PDAC. Specifically, metformin showed an anti-pancreatic stellate cells (PSCs) effect via decreasing the expression of sonic hedgehog (SHH) and then sparked some downstream effects, for example, inhibiting the production of vascular endothelial growth factor (VEGF) in the tumor microenvironment, reducing the formation of tumor neovascularization, attenuating the desmoplastic reaction and enhancing the antitumor effect of gemcitabine.

SIGNIFICANCE

We concluded that metformin suppressed tumor angiogenesis and enhanced the chemosensitivity of gemcitabine via inactivating PSCs in PDAC of KPC mice.

Metformin inhibits ALK1-mediated angiogenesis via activation of AMPK

Anti-VEGF therapy has been proven to be effective in the treatment of pathological angiogenesis. However, therapy resistance often occurs, leading to development of alternative approaches. The present study examines if AMPK negatively regulates ALK1-mediated signaling events and associated angiogenesis. Thus, we treated human umbilical vein endothelial cells with metformin as well as other pharmacological AMPK activators and showed that activation of AMPK inhibited Smad1/5 phosphorylation and tube formation induced by BMP9. This event was mimicked by expression of the active mutant of AMPKα1 and prevented by the dominant negative AMPKα1. Metformin inhibition of BMP9 signaling is possibly mediated by upregulation of Smurf1, leading to degradation of ALK1. Furthermore, metformin suppressed BMP9-induced angiogenesis in mouse matrigel plug. In addition, laser photocoagulation was employed to evaluate the effect of metformin. The data revealed that metformin significantly reduced choroidal neovascularization to a level comparable to LDN212854, an ALK1 specific inhibitor. In conjunction, metformin diminished expression of ALK1 in endothelium of the lesion area. Collectively, our study for the first time demonstrates that AMPK inhibits ALK1 and associated angiogenesis/neovascularization. This may offer us a new avenue for the treatment of related diseases using clinically used pharmacological AMPK activators like metformin in combination with other strategies to enhance the treatment efficacy or in the case of anti-VEGF resistance.

Metformin suppresses retinal angiogenesis and inflammation in vitro and in vivo

The oral anti-diabetic drug metformin has been found to reduce cardiovascular complications independent of glycemic control in diabetic patients. However, its role in diabetic retinal microvascular complications is not clear. This study is to investigate the effects of metformin on retinal vascular endothelium and its possible mechanisms, regarding two major pathogenic features of diabetic retinopathy: angiogenesis and inflammation. In human retinal vascular endothelial cell culture, metformin inhibited various steps of angiogenesis including endothelial cell proliferation, migration, and tube formation in a dose-dependent manner. Its anti-angiogenic activity was confirmed in vivo that metformin significantly reduced spontaneous intraretinal neovascularization in a very-low-density lipoprotein receptor knockout mutant mouse (p<0.05). Several inflammatory molecules upregulated by tumor necrosis factor-α in human retinal vascular endothelial cells were markedly reduced by metformin, including nuclear factor kappa B p65 (NFκB p65), intercellular adhesion molecule-1 (ICAM-1), monocyte chemotactic protein-1 (MCP-1), and interleukin-8 (IL-8). Further, metformin significantly decreased retinal leukocyte adhesion (p<0.05) in streptozotocin-induced diabetic mice. Activation of AMP-activated protein kinase was found to play a partial role in the suppression of ICAM-1 and MCP-1 by metformin, but not in those of NFκB p65 and IL-8. Our findings support the notion that metformin has considerable anti-angiogenic and anti-inflammatory effects on retinal vasculature. Metformin could be potentially used for the purpose of treating diabetic retinopathy in addition to blood glucose control in diabetic patients.

Metformin inhibits esophageal squamous cell carcinoma-induced angiogenesis by suppressing JAK/STAT3 signaling pathway

Although it has been known that the tumor microenvironment affects angiogenesis, the precise mechanism remains unclear. In this study, we simulated the microenvironment of human esophageal squamous cell carcinoma (ESCC) by tumor conditioned medium (TCM) to assess the influence on normal endothelial cells (NECs). We found that the TCM-induced NECs showed enhanced angiogenic properties, such as migration, invasion and tube formation. Moreover, the TCM-induced NECs expressed tumor endothelial cells (TECs) markers at higher levels, which indicated that TCM probably promoted tumor angiogenesis by coercing NECs to change toward TECs. The microarray gene expression analysis indicated that TCM induced great changes in the genome of NECs and altered many regulatory networks, especially c-MYC and JAK/STAT3 signaling pathway. More importantly, we investigated the anti-angiogenic effect of metformin, and found that metformin abrogated the ESCC microenvironment-induced transition of NECs toward TECs by inhibiting JAK/STAT3/c-MYC signaling pathway. Furthermore, we verified the anti-angiogenic activity of metformin in vivo by a human ESCC patient-derived xenograft (PDX) mouse model for the first time. Taken together, our research provides a novel mechanism for the anti-angiogenic effect of metformin, and sets an experimental basis for the development of new anti-angiogenic drugs by blocking the transition of NECs toward TECs, which possibly open new avenues for targeted treatment of cancer.