Targeting mTORC2 inhibits colon cancer cell proliferation in vitro and tumor formation in vivo
- Didier Roulin†1,
- Yannick Cerantola†1,
- Anne Dormond-Meuwly1,
- Nicolas Demartines1 and
- Olivier Dormond1Email author
© Roulin et al; licensee BioMed Central Ltd. 2010
Received: 19 November 2009
Accepted: 12 March 2010
Published: 12 March 2010
The mammalian target of rapamycin (mTOR), which exists in two functionally distinct complexes, mTORC1 and mTORC2 plays an important role in tumor growth. Whereas the role of mTORC1 has been well characterized in this process, little is known about the functions of mTORC2 in cancer progression. In this study, we explored the specific role of mTORC2 in colon cancer using a short hairpin RNA expression system to silence the mTORC2-associated protein rictor. We found that downregulation of rictor in HT29 and LS174T colon cancer cells significantly reduced cell proliferation. Knockdown of rictor also resulted in a G1 arrest as observed by cell cycle analysis. We further observed that LS174T cells deficient for rictor failed to form tumors in a nude mice xenograft model. Taken together, these results show that the inhibition of mTORC2 reduces colon cancer cell proliferation in vitro and tumor xenograft formation in vivo. They also suggest that specifically targeting mTORC2 may provide a novel treatment strategy for colorectal cancer.
The mammalian target of rapamycin (mTOR) is a serine-threonine kinase that regulates cell growth and proliferation in response to the availability of growth factors and nutrients [1, 2]. mTOR exists in two functionally distinct complexes: mTORC1 and mTORC2, which have different functions and are differentially sensitive to rapamycin . mTORC1 is composed of mTOR, mLST8, PRAS40, deptor and raptor and is sensitive to rapamycin. mTORC1 enhances cell growth and proliferation through various mechanisms including synthesis of proteins and lipids as well as reduction of autophagy. Part of the functions of mTORC1 are mediated by the phosphorylation and activation of S6K1 and 4E-BP1, two well characterized downstream effectors of mTORC1 . mTORC2 contains mTOR, mLST8, mSIN1, deptor, protor-1 and rictor. Although mTORC2 is classically insensitive to rapamycin, prolonged exposure to rapamycin may also inhibit mTORC2 activity by blocking its assembly in certain cell types . In contrast to mTORC1, relatively little is known about the functions of mTORC2. It has been described that mTORC2 regulates cell survival and cytoskeletal organization through the regulation of Akt and PKCα respectively [5, 6].
Since mTOR plays a major role in cell growth and proliferation, targeting mTOR in cancer therapy has been viewed as a promising approach [3, 7]. Indeed, alterations of mTOR signaling pathway are commonly observed in solid tumors. Whereas the role of mTORC1 in cancer progression has been extensively characterized, the role of mTORC2 is much less documented. So far it has been shown that targeting mTORC2 might be beneficial in tumors harboring high levels of activated Akt, such as gliomas or tumors caused by PTEN deletion [8, 9]. Since the activation of Akt is frequently observed in colorectal cancer , we wish here to analyse the role of mTORC2 in colon cancer.
Several studies have demonstrated that the activity of mTOR is increased in tumors . Particularly in colorectal cancer, mTOR signaling components were highly activated in tumor specimen compared to non cancerous mucosa . Furthermore, targeting mTOR with a specific siRNA to mTOR also reduced SW480 and HCT116 colon cancer cell proliferation and survival in vitro and injection of small interfering RNA to mTOR into HCT116 tumor xenografts also blocked tumor growth in vivo . In addition, the inhibition of mTORC1 suppressed the formation of colonic adenomas and cancers in a mouse model of familial adenomatous polyposis . Therefore, targeting mTOR in colorectal cancer might be a successful strategy. mTOR exists in two distinct complexes, however, no study so far has analyzed the specific role of mTORC2 in colorectal cancer development. Here, we found that mTORC2 plays an important role in colon cancer cell proliferation. Downregulation of mTORC2, by either blocking the expression of mTOR or rictor, reduced the proliferation of HT29 and LS174T colon cancer cells in vitro and inhibited the formation of tumor xenografts in vivo. We therefore propose that targeting mTORC2 in colon cancer could be a promising therapeutical strategy.
Targeting mTOR with rapamycin or its analogs in clinical studies has been less successful than expected . However, as mTORC2 and part of mTORC1 functionality are resistant to rapamycin, one may speculate that therapies that target both complexes will be much more efficient in cancer therapy. Consistent with this hypothesis, we found that the inhibition of mTORC1 reduced colon cancer cell proliferation, however to a lesser extend than the inhibition of both complexes, or mTORC2 alone. Recently, several groups have developed ATP-competitive and selective mTOR inhibitors that target simultaneously both complexes [13–15]. Initial experiments have shown that the antiproliferative efficacy of these inhibitors is superior to rapamycin . Future studies will reveal the efficacy of such inhibitors in cancer therapy; one major concern being that these inhibitors might have considerable toxicity in vivo. However, our results suggest that targeting only mTORC2 might be sufficient to prevent colon cancer progression and thus give a rationale for the development of drugs that specifically target mTORC2.
mammalian target of rapamycin
small hairpin RNA.
We thank David Sabatini for sharing plasmids. This work was supported by a research grant from the Swiss National Science Foundation (SCORE 32323B-123821 to OD).
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