Metformin in cancer prevention and therapy

Ann Transl Med. 2014 Jun; 2(6): 57.
doi: 10.3978/j.issn.2305-5839.2014.06.01
PMCID: PMC4200668
PMID: 25333032
Metformin in cancer prevention and therapy
Jacek Kasznicki,corresponding author Agnieszka Sliwinska, and Józef Drzewoski

Abstract
The prevalence of diabetes is dramatically increasing worldwide. The results of numerous epidemiological studies indicate that diabetic population is not only at increased risk of cardiovascular complications, but also at substantially higher risk of many forms of malignancies. The use of metformin, the most commonly prescribed drug for type 2 diabetes, was repeatedly associated with the decreased risk of the occurrence of various types of cancers, especially of pancreas and colon and hepatocellular carcinoma. This observation was also confirmed by the results of numerous meta-analyses. There are however, several unanswered questions regarding the exact mechanism of the anticancer effect of metformin as well as its activity against various types of cancer both in diabetic and nondiabetic populations. In the present work we discuss the proposed mechanism(s) of anticancer effect of metformin and preclinical and clinical data suggesting its anticancer effect in different populations.


The prevalence of diabetes is dramatically increasing worldwide reaching epidemic proportion. Landmark of diabetes, chronic hyperglycemia leads to the development and progression of life-treating complications, predominantly cardiovascular. The results of several studies indicate that people with diabetes (mainly type 2, T2DM) are also at substantially higher risk of cancer of the pancreas, liver, endometrium, breast, colon, rectum and urinary bladder compared to individuals without this chronic disease (1).

However, the incidence of other types of cancer (e.g., lung, kidney, non-Hodgkin lymphomas) does not seem to be strongly associated with diabetes or the evidence is inconclusive (2). Interestingly enough, it has been suggested that diabetes is associated with a lower risk for prostate cancer (2,3). According to the American Diabetes Association and the American Cancer Society consensus report the relative risks imparted by diabetes are greatest (about two fold or higher) for cancers of the liver, pancreas, and endometrium, and lesser (about 1.2-1.5 fold) for cancers of the colon and rectum, breast, and bladder (2).

Clinical observations indicate that the prevalence of diabetes in newly diagnosed cancer patients ranges from 8% to 18%, suggesting bidirectional association between these two disease (4,5). The association of diabetes and cancer was first reported as an incidental finding in 1932 (6). Nowadays, this coexistence is well recognized, however in spite of the intensive studies its mechanism still remains unclear. There is a general agreement that T2DM and cancer share several common potential risk factors (e.g., aging, sex, obesity, physical inactivity, diet, alcohol, and smoking).

In T2DM, insulin resistance and hyperinsulinemia (either endogenous due to insulin resistance or induced by administration of exogenous insulin formulations) are considered to be independent risk factors for cancer development (1,2). In addition, hyperglycemia-related oxidative stress, accumulation of advances glycation end products as well as low-grade inflammation may also enhance the risk of malignant transformation (7,8).

Recent publications have also suggested the link between hypoglycemic medications and cancer (8-11). The results of numerous preclinical, epidemiological and clinical studies suggested that metformin use is associated with inhibition of cancer cell growth and proliferation and reduction in all-cancer incidents in comparison with users of other hypoglycemic drugs. In the present work we discuss the proposed mechanism(s) of anticancer effect of metformin as well as preclinical and clinical data suggesting this beneficial effect.


Molecular action of metformin in cancer cell
The current proposed anticancer molecular action of metformin is mainly associated with the inhibition of the mammalian target of rapamycin complex 1 (mTORC1). The mTOR pathway plays a pivotal role in metabolism, growth and proliferation of cancer cell (12). Metformin is thought to inhibit mTORC1 pathway (Figure 1).

It is believed that systemic effect of metformin manifested by the reduction of circulating level of insulin and insulin-like growth factor 1 (IGF-1) might be associated with anticancer action (13).

Insulin/IGF-1 is involved not only in regulation of glucose uptake but also in carcinogenesis through upregulation of insulin/IGF receptor signaling pathway (14).
The excessive food consumption (insulin) leads to increased liver production of IGF-1 that binds to IGF-1 receptor and insulin receptor.
Then, through insulin receptor substrate (IRS) the signal is transmitted to phosphoinositide 3-kinase (PI3K), and Akt/protein kinase B (PKB) that indirectly activates (not phosphorylates) mTORC1.
Additionally, insulin receptor through growth factor receptor-bound protein 2 (GRB2) propagates signal to Ras/Raf/ERK pathway that drives cell growth. Evidences indicate that these pathways play important role in changes of cellular metabolism that are typical feature of tumor cells (15).
Increased levels of circulating insulin/IGF1 and upregulation of insulin/IGF receptor signaling pathways were demonstrated to be involved in the formation of many types of cancer.
Metformin was found to reduce insulin level, inhibit insulin/IGF signaling pathways, and modify cellular metabolism in normal and cancer cells (16).

Evidences suggest that the inhibition of mTOR pathway by metformin proceeds dependent and independent on AMP-activated protein kinase (AMPK) activation.
AMPK phosphorylates tuberous sclerosis complex protein 2 (TSC2) that inhibits mTORC1 leading to decrease in protein synthesis and cell growth (17).
Among the first studies that showed the participation of AMPK activation in antitumor action of metformin were researches performed on breast cancer cells (18,19). Dowling et al. showed that compound C, an inhibitor of AMPK, reversed inhibition of initiation of translation evoked by metformin (18).
More recently, Mohammed et al. showed reduction of carcinoma spread in pancreas of transgenic mice fed with metformin (20). Additionally, pancreatic tissue of mice fed with metformin revealed a significant inhibition of mTOR, and an increase of phosphorylated AMPK and TSC2 (20). However, Gwinn el al. demonstrated that inhibition of mTOR could be independent on TSC2, since AMPK directly phosphorylates the rotor compartment of mTOR (21).

Several studies identified that liver kinase B1 (LKB1), a major upstream kinase of AMPK, may be involved in anticancer action of metfromin associated with inhibition of mTOR. In vitro and in vivo studies revealed that deletion of LKB1 function accelerated proliferation of tumor cell and sensitized them to activators of AMPK such as biguanide (22-24).
Due to the fact that p53 expression and phosphorylation is regulated by AMPK and p53 is involved in cell metabolism and control of cell cycle its participation in metformin action is discussed.
Growing evidences from in vivo and in vitro studies of various cancers revealed that metformin blocked cell cycle in G0/G1 phase with a significant decrease expression of G1 cyclins (including cyclin D1) without changes in p53 status (25-27).
However, others researches indicated that inhibitory effect on cancer cell growth of metformin was associated with p53 activity (28-31).
Taking together the results of preclinical studies are inconclusive whether antitumor action of metformin is associated with p53. Some investigators hypothesize that the dose of metformin may determine the effect of metformin. Yi et al. demonstrated on hepatoma cells that low concentration of metformin induced p53-dependent senescence, whereas higher doses induced apoptotic cell death (32).

Inhibition of mTOR by metformin independent on AMPK activation was demonstrated by Memmott et al. in mice lung cancer cells (16). Metformin evoked inhibition of mTOR pathway with accompanied decreasing activation of IGF-1/insulin receptor, Akt, extracellular signal-regulated kinase (ERK) without AMPK activation (16).
Kalender et al. demonstrated in Drosophilla cells that inhibition of mTOR signaling induced by metformin occurred in the absence of AMPK. They reveal the existence of an alternative TSC1/2-mTOR AMPK-independent pathway mediated by RAG GTPase (33).
Metformin was found to inhibit breast carcinoma cell growth through decreasing level of epidermal growth factor receptor 2 (HER2). This effect was mediated by inhibition of the mTOR effector, p70S6K1 (34).
p70S6K is responsible for the phosphorylation of S6 ribosomal protein and thereby protein synthesis at the ribosome (35). Antiproliferative action of metformin related to enhancement of DNA-damage-inducible transcript 4 protein (DDIT4, REDD1) expression, a negative regulator of mTOR, was reported in prostate cancer cells by Ben Sahra et al. (36).
This effect of metformin was also independent on AMPK activation (36).

The results of preclinical studies undoubtedly confirm the efficacy of metformin to inhibit cancer cell growth in vitro and to reduce tumor spread in animal models of various cancers. However, it should be stressed that molecular action of metformin is still investigated and seems to be affected by the type of tumor cell line.