Supermaterial Joins the War
(March 10th, 2015) So far only known to be valuable to physicists and material scientists, graphene slowly finds its way into biomedical labs, too. Manchester scientists discovered that graphene oxide specifically targets and kills cancer cells.
It is most definitely a war - one that engages personnel in the tens, if not hundreds of thousands with a budget in the billions. The villain is the manipulative, heterogeneous disease called cancer and so far success comes only in small steps with no clear victor. The long fight has revealed signalling pathways we never knew existed and proposed proteins as markers to be therapeutically targeted, but so far, a cure remains elusive. A recent publication from the University of Manchester has thrown another potential treatment into the ring: graphene oxide (GO). In the latest issue of Oncotarget, senior authors Aravind Vijayaraghavan and Michael Lisanti from the university’s School of Materials and National Graphene Institute, and the Institute of Cancer Sciences, respectively, showed this water dispersible, one atom-thin, chicken wire structured, flake-like derivative of graphene to be selective for killing the ‘unkillable’ cancer stem cells (CSCs).
If cancer cells were weeds, CSCs would be the pesky roots that must be pulled out to ensure death. Many scientists believe CSCs to be the root of recurrence and metastasis – most often the cause of death from cancer. Some also believe that not all cells have the capability of becoming tumour cells, but rather all cancer cells arise from CSCs, hence their alternative name tumour-initiating cells (TICs). Therefore, if one immediately targets the root the rest will follow. However, most current cancer treatments attempt to inhibit growth and shrink tumours composed of already differentiated cancer cells, leaving the small fraction of CSCs largely unaffected. As a result, much effort is being channelled into understanding CSCs, which resemble their normal stem cell cousins, while simultaneously developing drugs that selectively target them.
Normal, non-CSC cells, when left in suspension, die by a process called anoikis due to lack of attachment. In contrast, CSCs thrive in suspension and begin a process known as anchorage-independent growth. A single CSC can proliferate and eventually form spheroid structures called tumour- or onco-spheres enriched in CSCs. For Vijayaraghavan and Lisanti, GO initially was not a ‘drug’ of choice. They chose it as carrier for cancer drugs due to its ability to attach to surfaces and its stability in water. Imagine their surprise when GO itself caused death of the 3D cultures.
In their paper, tumour-spheres created from cell lines representing seven types of cancer were used to show that GO selectively targeted the CSCs. Firstly, the authors discerned that both large (5-20 µM) and small (0.2–2 µM) GO flakes caused a dose-dependent inhibition of anchorage-dependent growth of MCF7 (breast cancer) CSCs; whereas, no effect was observed in MCF7 monolayers. In line with the breast cancer CSCs, CSCs from ovarian, prostate, pancreatic, lung and brain (glioblastoma) cancer cells also exhibited decreased viability when treated with GO, with no effect seen on the 2D monolayer cultures. This to the authors suggested a commonality among the CSCs targeted by GO regardless of cancer origin. Normal skin fibroblasts also remained unaffected, suggesting selectivity to cancer cells, which is always a desired feature of any chemotherapeutic.
How does GO target the CSCs? This question was answered by investigating specific signal transduction pathways influenced by GO in MCF7 cells. The authors could link all identified pathways inhibited by GO – WNT, Notch, STAT1/3 and NRF2 – to ‘stemness’. Thus, in their final experiment they attempted to further ascertain how GO decreased stemness. For this, MCF7 cells grown in monolayer were pre-treated with or without GO for 2 days, grown on low attachment plates to induce anoikis of the non-CSCs, followed by analysis of stem cell markers on the surface. In the non-treated cells, a large percentage of cells were CD44(+)CD24-/low, representing CSCs. CD24 expression increased after treating with GO, thus reducing the CSC population, which for the authors indicated that GO causes the breast cancer stem cells to differentiate, so they are no longer able to form tumour-spheres.
In their discussion, the authors speculated that GO could be used as a stand-alone therapeutic which is delivered intravenously or orally, or as a lavage wash after removal of the tumour to get rid of residual CSCs. In reality, there is still much work to be done to show the effectiveness of GO in cancer therapy in patients. However, GO’s specificity toward CSCs, no effect to normal cells, stability in aqueous solutions and its ability to induce CSC differentiation guarantees that it has a fighting chance.
Image: Thomas Szkopek/McGill University