Metformin: a case of divide and conquer

Metformin is a widely prescribed anti-diabetic drug and its use is associated with lower cancer incidence. The mechanisms by which metformin attenuates tumorigenesis are not clearly understood. In a paper published in Proceedings of the National Academy of Sciences of the United States of America, Hirsch and colleagues show that metformin interferes with a signaling pathway, mediated by the transcription factor NF-κB, which drives cell transformation and is required for the maintenance of cancer stem cells.

Treatment of cells expressing ER-Src with tamoxifen elicits a signaling cascade, referred to as the infl ammatory response or infl ammatory feedback loop, that is mediated by the transcription factor NF-κB and its downstream target cytokine IL-6 ( Figure 1) [4,5]. Activation of the infl ammatory response is essential for Src-induced transformation of mammary epithelial cells, and the cells' ability to form tumors is impeded when this pathway is blocked. Further more, Src activation promotes the expansion of the CSC population and CSCs have enhanced activity of the infl ammatory pathway compared with non-CSCs. CSCs are therefore likely to exhibit increased dependence on the infl ammatory feedback loop, and pharmacological interference with this pathway may limit their tumori genic potential.
Metformin is a widely prescribed anti-diabetic drug, and epidemiological studies show that metformin use is associated with lower cancer incidence [6]. Previous work by Hirsch and colleagues showed that metformin selectively kills CSCs over non-CSCs and prolongs tumor remission in mouse xenograft cancer models when combined with chemotherapy agents such as doxorubicin and taxanes [7,8].
In their latest paper in Proceedings of the National Academy of Sciences of the United States of America, Hirsch and colleagues probed the mechanism of metformin's action and found that metformin treatment inhibits transformation by attenuating the infl ammatory feedback loop [9]. In particular, metformin prevented transformation-induced IL-6 expression by inhibiting the translocation of NF-κB to the nucleus. Expression of exogenous Lin28 or treatment with IL-1β (both NF-κB targets) overcomes the anti-transformation eff ect of metformin. Intriguingly, the inhibitory eff ect of metformin on infl ammatory response components was more pronounced in CSCs than non-CSCs, consistent with the observation that NF-κB nuclear translocation and signal transducer and activator of transcription 3 (STAT3) activity were only inhibited in CSCs. Th e authors also provided evidence that the sensitivity of other transformed cell lines to metformin is determined by the degree of infl ammatory pathway activity (IL-6 levels) that they exhibit.
NF-κB plays a central role in immune-cell-mediated tumor infl ammation [10]. Th e fact that infl ammatory

Abstract
Metformin is a widely prescribed anti-diabetic drug and its use is associated with lower cancer incidence. The mechanisms by which metformin attenuates tumorigenesis are not clearly understood. In a paper published in Proceedings of the National Academy of Sciences of the United States of America, Hirsch and colleagues show that metformin interferes with a signaling pathway, mediated by the transcription factor NF-κB, which drives cell transformation and is required for the maintenance of cancer stem cells.
pathway activity correlates with metformin sensitivity in xenografts indicates that the eff ects of metformin are independent of a potential infl uence of the drug on immune cells. IL-6 from CSCs is suffi cient to induce the conversion of non-CSCs to CSCs in a paracrine fashion that establishes a dynamic equilibrium between the two populations [11]. Irrespective of whether metformin can interfere with NF-κB activation in the immune system, IL-6 delivered from immune cells within the tumor is likely to skew the cancer cell population dynamics and this has the potential to impact tumor development [12]. Non-epithelial sources of IL-6 might supersede the role of the epithelial infl ammatory response. Th e infl uence of metformin on CSC population dynamics in autochthonous tumors remains to be seen.
Another lingering question is how metformin inhibits NF-κB activity. A critical observation by Hirsch and colleagues that may shed some light on this is that metformin only inhibits transformation when adminis tered during an early window of up to 3 hours following Src induction, after which metformin is less eff ective [9]. Th is is surprising given that a 5-minute activation of Src suffi ces to transform cells (albeit with slower kinetics) and NF-κB activity is similar at 1 hour and 4 hours after Src activation [4]. Taken together, these observations suggest that some aspect of metformin function sets the scene to inhibit NF-κB indirectly. Multiple lines of evidence indicate that metformin regulates cellular metabolism, in part, through indirect activation of AMPactivated protein kinase [13], and genetic epistasis experiments are warranted to investigate a potential link between AMP-activated protein kinase and NF-κB in CSCs. Sensitivity to metformin is also infl uenced by which nutrients are predominantly utilized for anabolic reactions to fuel proliferation [14], so diff erences in metabolic pathway activity between non-CSCs and CSCs may underlie diff erential sensitivity to the drug. Regardless of these factors, selective depletion of CSCs promises to pave the way towards more eff ective therapies [15] and the insights from Hirsch and colleagues bring us a signifi cant step further towards this goal.  ) activates Src and elicits a transcriptional response, mediated by the transcription factor NF-κB, to drive the expression of Lin28, a miRNA binding protein. Lin28 binds to and attenuates the function of let-7 miRNAs, which normally control, among others, the translation of the cytokine IL-6. IL-6 is a potent activator of NF-κB, thereby providing a positive feedback that further amplifi es the pathway [4]. In parallel, IL-6 activates another transcription factor, signal transducer and activator of transcription 3 (STAT3), to promote the expression of miR-21 and miR-181b-1, which inhibit the translation of the tumor suppressors PTEN and CYLD, respectively, further enhancing NF-κB activity [5]. Because NF-κB, STAT3 and IL-6 are also players in bona fi de infl ammatory responses elicited by immune cells, this signaling cascade is known as the infl ammatory response or infl ammatory feedback loop, alluding to the self-amplifying nature of the pathway.