Abstract SY01-02: Autophagy and metabolic reprogramming in the tumor stroma
Authors: Moscat Jorge , Linares Juan F. , Duran Angeles , Valencia Tania , Ahn Christopher S. , Metallo Christian , Diaz-Meco Maria T. 
Published: August 2015

Cancer is the result of the stepwise acquisition of genetic alterations within the epithelial compartment. However, recent data supports the relevance of the tumor microenvironment in the control of cancer progression. Central to this process is the nutritional and metabolic state of the epithelial cancer cell and the surrounding stroma. In addition, although relatively little is known about the signaling pathways that mediate the stroma-epithelium crosstalk, inflammation and metabolic reprogramming are two critical factors contributing to the pro-tumorigenic properties of the stroma. Nutrient sensing by the mTORC1 complex is a critical mechanism whereby cancer cells activate the anabolic pathways necessary to create the biomass required for their proliferation. mTORC1 is composed of the kinase mTOR, Raptor and mLST8/GβL, and its activation by nutrients is under the control of the Rag GTPases, which play a critical role in the recruitment of mTORC1 to the lysosomal surface where it is activated. The pentameric, lysosomal-based complex Ragulator is endowed with guanine nucleotide exchange factor activity for RagA/B. Interestingly, mTORC1 not only activates anabolic pathways under conditions of nutrient proficiency but also represses catabolic processes such as autophagy. Under conditions of nutrient deficiency, a situation commonly found in poorly vascularized tumors, mTORC1 is inhibited and autophagy is consequently activated.

We have recently discovered that the signaling adapter p62 is an important activator of mTORC1 in response to nutrients, specifically to branched-chain amino acids (BCAA). p62 binds Raptor and the Rag GTPases, and interacts with TRAF6. Notably, p62, TRAF6, and importantly their interaction are required for the translocation of mTORC1 to the lysosome and its subsequent activation. Moreover, TRAF6 is recruited to and activates the mTORC1 complex through p62 in amino acid-stimulated cells, for which TRAF6 promotes the K63-ubiquitination of mTOR. Interestingly, TRAF6 through its interaction with p62, and activation of mTORC1, modulates autophagy and is an important step in cancer cell proliferation. We show that interfering the interaction of p62 with TRAF6 in vitro and in vivo with cell-permeable peptides represses mTORC1, which results in increased autophagy and cell growth inhibition. The mechanisms whereby p62 and TRAF6 sense the nutritional state of the cell and the subsequent activation of mTORC1 remained to be discovered. Our most recent data allowed us to identify a new signaling cascade that regulates the interaction of p62 with TRAF6 and the subsequent activation of mTORC1 at the lysosomes. This new pathway is selective for BCAA and is not regulated by growth factors or insulin. These new unpublished observations will be presented and their potential relevance as new therapeutic targets in cancer will be discussed along with new chemical biology data recently obtained by our laboratory.

However, mTORC1 inhibitors are showing only limited efficacy in clinical trials. Several causes could account for these relatively disappointed results. These include potential toxicity of mTOR kinase inhibitors that impair not only the activity of the mTORC1 complex but also that of the mTORC2 complex. Also, the activation of the Akt activating “feedback loop” well-known to be triggered by the chronic inhibition of mTORC1 is another source of concern and a possible explanation of mTOR inhibitors poor results in the clinic. To better target mTORC1 in cancer, we must have a ful comprehension of not only the biochemical interactions regulating its activity but also the implication of inhibiting mTORC1 in cells other than the epithelial tumor in the microenvironment. In this regard, our most recent data show that inhibiting mTORC1 in the stroma by ablating p62 results in an inflammatory and protumorigenic microenvironment that could explain the relative inefficiency of mTORC1-targeted cancer therapy. Thus, cancer-associated fibroblasts (CAFs) have been proposed to be key mediators of the crosstalk between malignant tumor cells and their microenvironment. CAFs through the complex set of proteins that they secrete generate an inflammatory microenvironment that promotes the pro-tumorigenic status of the stromal cells. IL-6 has been implicated as a critical component of the pro-inflammatory signature of the prostate cancer (PCa) stroma and has been demonstrated to be an essential pro-tumorigenic molecule in many types of cancer, including PCa. We demonstrate that the inactivation of mTORC1 in p62-deficient stromal fibroblasts promotes a metabolic reprogramming caused by c-Myc inactivation, which leads to increased levels of IL-6. This enhanced inflammatory milieu promotes the synthesis of TGFβ and the acquisition of the CAF reactive phenotype. These p62-deficient, “activated” CAFs promote epithelial cell invasion and proliferation. Therefore, p62 behaves as a tumor suppressor in the tumor stroma. These results contrast with the pro-tumorigenic role of p62 in epithelial cancer cells. This has important implications from a therapeutic perspective since inhibition of p62 and/or mTORC1 may result in opposite effects in the stroma and the epithelium of the tumor, which might explain the poor efficacy of mTORC1-based chemotherapeutic approaches. An additional observation in our studies is that the tumor epithelium effectively reduces the levels of p62 in the associated stroma, establishing a feed-forward loop whereby the tumor epithelium by downregulating p62 in the stroma promotes a protumorigenic reprogramming of CAF metabolism that feeds back to the tumor increasing its aggressive phenotype. The mechanisms whereby p62 is downregulated in the stroma from epithelium-derived signals is a key question to be resolved as therapies aimed at inhibiting p62 downregulation might increase the efficacy of mTOR inhibitors. Results on this important question will be discussed in this presentation.

Citation Format: Jorge Moscat, Juan F. Linares, Angeles Duran, Tania Valencia, Christopher S. Ahn, Christian Metallo, Maria T. Diaz-Meco. Autophagy and metabolic reprogramming in the tumor stroma. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr SY01-02. doi:10.1158/1538-7445.AM2015-SY01-02