EGFR activates mTORC2 in glioblastoma – a potential role for new therapies
One of the great things about following the American Association for Cancer Research (AACR) on Twitter, is that they regularly share technical open access articles from their journals for scientists to read. Of course, many will have access through their institution subscription, but there are also probably quite a few interested community oncologists and scientists like me that don’t. The idea of sharing some of their really important scientific research with the broader public is a great one – a little bit of goodwill goes a long way and furthers their cause too.
Yesterday, AACR kindly tweeted and shared a fascinating paper (see references below for open access to all the articles) on how EGFR signaling in glioblastoma (an aggressive form of brain cancer) activates the mTOR pathway, specifically mTORC2, and is partially suppressed by PTEN:
Source: Tanaka et al., (2011)
We know that mTOR and it’s upstream relative, PI3K, are frequently dysregulated in cancer and may also lead to resistance to treatment with some therapies, such as aromatase inhibitors in breast and other cancers. This is also true in glioblastoma, where chemotherapies such as temozolamide are often used, as the authors noted:
“mTORC2 signaling promotes GBM growth and survival and activates NF-κB. Importantly, this mTORC2–NF-κB pathway renders GBM cells and tumors resistant to chemotherapy in a manner independent of Akt.”
One of the challenges though, is elucidating the mechanism behind mTOR activation:
“The mechanisms of mTORC2 activation are not well understood. Growth factor signaling through PI3K, potentially through enhanced association with ribosomes, and up-regulation of mTORC2 regulatory subunits have been proposed as mechanisms of mTORC2 activation.”
Recently, Clohessy et al., (2008) observed that mTORC1 inhibition was not sufficient to block GBM growth, so this new research took a different approach and focused on asking the question of whether oncogenic EGFR affects mTORC2. To test this hypothesis, they used GBM derived cell lines that represent the most common genetic events driving GBM i.e. PTEN loss with EGFR overexpression or activating mutation (EGFRvIII) present or absent. It should be noted that a good marker of mTORC2 activity is the phosphorylation of AKT S473, although SGK1 is also turning out to be a good biomarker of response.
What did they find?
The paper (open access) is well worth reading, but to summarise, here are some of the key findings from this well thought out research:
- mTORC2 signaling promotes GBM growth and survival
- EGFRvIII activates NF-kB through mTORC2
- mTORC1 inhibition alone could not suppress NF-κB activation in GBM cells
- mTORC2 mediates EGFRviii-dependent cisplatin resistance through NF-kB, independently of Akt
- mTORC2 inhibition reverses cisplatin resistance in xenograft tumours
- mTORC2 signaling is hyperactivated and associated with NF-kB and phospho-EGFR in the majority of clinical GBM samples
What stood out for me in their series of experiments and comprehensive analysis was that:
“Elevated phosphorylation of EGFR (Y1068) and Akt (S473) was detected in 44% and 77% of GBMs, respectively. These numbers are consistent with the independent findings of EGFR mutation and/or amplification in 45% and PI3K pathway–activating mutations in 87% of GBMs, reported in the Cancer Genome Atlas studies.”
What do these results all mean?
Looking at question regarding the mechanism underlying mTORC2 activation and its relationship with EGFR was poorly understood, this paper clearly showed that mTORC2 activation is a common event in GBM, including tumors harbouring EGFR-activating lesions. But what was particularly interesting was the finding that EGFRvIII was significantly more potent than wild-type EGFR in promoting mTORC2 activity. This is consistent with previous work from Huang et al., (2007), who found that:
“EGFRvIII preferentially activates PI3K signaling despite lower levels of receptor phosphorylation, leading to differential activation of downstream effectors.”
One outstanding question that has puzzled many researchers is what is the mechanism of rapamycin (mTOR) resistance? There are some clues in this research:
“Here we demonstrated that rapamycin (or genetic mTORC1 inhibition by raptor knockdown) promoted Akt S473 and NDRG1 T346 phosphorylation; this feedback activation could be suppressed by mTORC2 inhibition.”
They also looked at a patient sample to determine if there were any hints for further translational research:
“In a clinical sample from a GBM patient analyzed before and 10 days after treatment with rapamycin, mTORC2 signaling was elevated concomitant with significant mTORC1 inhibition, as measured by decreased S6 phosphorylation.”
This is important because to date, based on much of the data that has emerged from mTOR and PI3K inhibitors we have seen that single agent therapy often leads to either stable disease or low response rates, so the question is how can we improve this by understanding the mechanisms of resistance better in order to direct future combination approaches (as opposed to single agent studies) logically:
“These data suggest the possibility that failure to suppress mTORC2 signaling, including NF-κB signaling, may underlie resistance to rapamycin and the poor clinical outcome associated with it in some patients with GBM.”
This is a crucial finding because some early mTOR inhibitors such as rapamycin target mTORC1 effectively, but are weak inhibitors of mTORC2. The new generation of inhibitors may address this issue better and shut down the mTOR pathway more effectively, although that may not be enough on it own.
Clearly, future research studies will be needed to better understand the potential role of mTORC2/NF-κB signaling in mediating resistance to treatment in GBM:
“The results reported here provide a potential mechanism for mutant EGFR-mediated NF-kB activation in GBM and other types of cancer. The results also suggest that EGFR tyrosine kinase inhibitor resistance could also potentially be abrogated by targeting mTORC2-mediated NF-kB activation.”
So far this is a good start, but we still have a long way to go. There are a number of mTOR and PI3K inhibitors in development for the treatment of GBM – I’m looking forward to seeing the results of those trials and learning which combinations and lines of therapy might see the best results with mTOR inhibitors. Hopefully, there might be some early readouts at ASCO next June.
References:
Tanaka, K., Babic, I., Nathanson, D., Akhavan, D., Guo, D., Gini, B., Dang, J., Zhu, S., Yang, H., De Jesus, J., Amzajerdi, A., Zhang, Y., Dibble, C., Dan, H., Rinkenbaugh, A., Yong, W., Vinters, H., Gera, J., Cavenee, W., Cloughesy, T., Manning, B., Baldwin, A., & Mischel, P. (2011). Oncogenic EGFR Signaling Activates an mTORC2-NF- B Pathway That Promotes Chemotherapy Resistance Cancer Discovery, 1 (6), 524-538 DOI: 10.1158/2159-8290.CD-11-0124
Cloughesy TF, Yoshimoto K, Nghiemphu P, Brown K, Dang J, Zhu S, Hsueh T, Chen Y, Wang W, Youngkin D, Liau L, Martin N, Becker D, Bergsneider M, Lai A, Green R, Oglesby T, Koleto M, Trent J, Horvath S, Mischel PS, Mellinghoff IK, & Sawyers CL (2008). Antitumor activity of rapamycin in a Phase I trial for patients with recurrent PTEN-deficient glioblastoma. PLoS medicine, 5 (1) PMID: 18215105
Huang, P., Mukasa, A., Bonavia, R., Flynn, R., Brewer, Z., Cavenee, W., Furnari, F., & White, F. (2007). Quantitative analysis of EGFRvIII cellular signaling networks reveals a combinatorial therapeutic strategy for glioblastoma Proceedings of the National Academy of Sciences, 104 (31), 12867-12872 DOI: 10.1073/pnas.0705158104