Metformin as a potential therapy for malignant astrocytoma

Abstract

Background Glioblastoma Multiforme (GBM) is the most commonly occurring tumour of the central nervous system (CNS). Currently GBM is considered an incurable malignancy with patients experiencing abysmal life expectancies. Lack of progress in the discovery of novel treatments has led to the repurposing of existing licenced medication as a possible alternative option. Metformin is from the biguanide family of drugs and is the most common medication used in the treatment of type 2 diabetes. Clinical studies have reported that, in type 2 diabetic patients, metformin might reduce cancer incidence and severity. Currently, metformin is being assessed in clinical trials as a treatment for a range of cancer types including GBM. The antineoplastic mechanisms utilized by metformin and other biguanides have not been fully elucidated.

Methods The effects of metformin were evaluated, alone and in combination with other agents, on a panel of GBM cell cultures. Functional analysis of metformin mechanism of action was assessed through measurement of apoptosis, depolarisation of the mitochondria membrane, caspase pathway activation, cell cycle progression and the expression levels of micoRNAs.

Results Analysis of fourteen GBM cell cultures showed a cytotoxic response to metformin that was significantly linked to the P53 status (p=0.0024). In combination drug testing, one of the four drugs showed a synergistic pairing with metformin. The kinase inhibitor sorafenib, showed synergism (CI ≤ 1) in eight GBM cell cultures. Flow cytometry of metformin treated GBM cells showed no significant increase (p>0.005) in apoptotic cell populations. Caspase 3/7 levels showed no significant increase post metformin treatment (p>0.005). Metformin caused depolarisation of the mitochondrial membrane in six GBM cell cultures. Four microRNAs were shown to have expression levels changes post-metformin treatment. Upregulation of expression was identified in miR-140, miR-192, let-7c. Downregulation was identified in miR-222.

Conclusions Metformin was shown to have cytotoxic effect on a GBM cell cultures and has potential as GBM therapeutic agent and possible treatment synergy with sorafenib. The significance of P53 status to metformin sensitivity may suggest that its use should be directed to a sub-set of GBM patients. Mechanism for cell death by metformin was shown not to rely on apoptotic pathways but caspase 3/7 independent depolarization of mitochondrial cell membranes and cell cycle arrest. Investigations into autophagy may help to further define the pathways metformin is utilising to promote cell death. The impact of metformin on the expression profile of miR-222, miR-192 and let-7c is in line with clinical studies of other cancer types. This shows possible insight into the cancer independent actions of metformin. The interplay recorded between glucose availability and cell death indicates a possible key factor in the utilisation of metformin as a therapeutic agent. This finding may warrant the addition of dietary control regimes in clinical trials to maximise metformin efficacy. This work highlights the strong potential for biguanides in the development of new drug treatments and in expanding our knowledge of cancer metabolism.