Study identifies new nutrient-based mechanism of cancer proliferation, drug resistance.


Cancer researchers have known for years that tumours have unusual metabolisms, their rapid use of glucose is used as a diagnostic tool for tumours in PET scans.  However, only recently have scientists begun to flesh out the details of this metabolic shift.  Now, a new study from researchers at the Ludwig Cancer Research and University of California, San Diego have shown that glioblastoma tumours can leverage glucose and another nutrient, acetate, to resist targeted therapies directed at specific cellular molecules.

The team state that the findings, demonstrate that nutrients can strongly affect the signaling molecules that drive tumours.  They go on to add that the current study shows that metabolic and nutritional factors might be quite important in cancer development and treatment. The new study also highlights one way that glioblastoma tumours can evade targeted drugs such as erlotinib and gefitinib, inhibitors of a mutant form of the cellular molecule EGFR (epidermal growth factor receptor) that drives the growth of many glioblastomas and other tumour types.  The opensource study is published in the journal Proceedings of the National Academy of Sciences.

Previous studies from the team as well as other groups show that this shift can occur through the activation of a central cellular signal, mTORC2 (mTOR complex 2). mTORC2 is involved in switching cancer cells to a hyperactive metabolic state, for instance prompting the increased influx of glucose and acetate into cancer cells. Glucose and acetate provide fuel and cellular building blocks to perpetuate the rapid growth of tumours.

The current study found that glucose and acetate in turn regulate mTORC2, propelling glioblastoma tumour growth and fending off targeted drugs.  The team explain that this is a two way street as signaling molecules like mTORC2 can change metabolism, and metabolites can change mTORC2.

The findings first emerged from experiments in glioblastoma cells cultivated in a petri dish. In one experiment, the researchers treated the cells with either glucose or acetate and found that at least one of these nutrients was required in order to turn on mTORC2 in response.

The researchers also tested glioblastoma cells with a mutant form of EGFR that turns on mTORC2 and propels  tumour growth. In the absence of glucose and acetate, EGFR inhibitors can switch off mTORC2 signaling. However, when the researchers added glucose and acetate, the drugs did not work, mTORC2 stayed on and the cells thrived. The researchers delved further, showing how acetate and glucose activate mTORC2 through a molecule formed from these metabolites, called acetyl-coA, which is critical for activating a key component of mTORC2.

The results show that glucose or acetate can activate mTORC2 through the production of acetyl-coA, enabling tumours to resist targeted therapies such as EGFR inhibitors. Activated mTORC2 in turn propels tumour growth by regulating metabolism and other cellular processes. The researchers provide evidence that a similar mechanism operates in cells taken directly from glioblastoma patients and in human glioblastoma cells implanted into mice.

The data findings provide a window into the treatment of glioblastoma, which leaves most newly diagnosed patients with less than two years to live. To reduce deadly brain swelling, many glioblastoma patients require treatment with steroids, which are known to raise blood glucose levels. The current study suggests that the drugs, which may be necessary to control brain swelling, could also have the paradoxical effect of propelling tumour growth through activation of mTORC2. The results also suggest that developing drugs to effectively target mTORC2 may be one avenue to shutting down glioblastoma and possibly other types of tumours.

The researchers hypothesize that this may be a general mechanism in cancer and are now planning to investigate the role of glucose and acetate in other types of tumours.  The team also theorise about how to modify diet in mice to affect the production of these and other metabolites.

The lab note that the study does not point to the value of any particular diet for counteracting cancer and state that it is going to take diligent and careful work to determine how lifestyle changes, including diet, can alter tumour cell metabolism. They are actively studying this process and hope that this information can be used to develop more effective prevention and treatment strategies for cancer patients.

The team conclude that the current study suggests that there may be more interplay between genes involved in cancer and the environment than previously thought.

Source:  Ludwig Cancer Research

Persistent Rictor acetylation renders GBM cells resistant to EGFR-, PI3K-, and AKT-targeted therapies.  mTORC2 forms an autoactivation loop (i) by promoting glucose uptake and acetyl-CoA production through its downstream pathways of c-Myc (19) and (ii) by inactivating class IIa HDACs, which deacetylate Rictor and suppress mTORC2.  By these mechanisms, GBM cells with activated mTORC2 are resistant to targeted therapies toward their upstream stimulators including EGFR and PI3K as well as their downstream effector AKT.  Glucose-dependent acetylation of Rictor promotes targeted cancer therapy resistance.  Mischel et al 2015.

Persistent Rictor acetylation renders GBM cells resistant to EGFR-, PI3K-, and AKT-targeted therapies. mTORC2 forms an autoactivation loop (i) by promoting glucose uptake and acetyl-CoA production through its downstream pathways of c-Myc (19) and (ii) by inactivating class IIa HDACs, which deacetylate Rictor and suppress mTORC2. By these mechanisms, GBM cells with activated mTORC2 are resistant to targeted therapies toward their upstream stimulators including EGFR and PI3K as well as their downstream effector AKT. Glucose-dependent acetylation of Rictor promotes targeted cancer therapy resistance. Mischel et al 2015.

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