Eukaryotic cells utilize a complex network of signaling pathways to regulate cell growth. This network is known to control aging and its malfunction leads to a wide range of diseases. At the center of the network lies the target of rapamycin complex 1 (TORC1) pathway. In the presence of the appropriate hormones and nutrients, this pathway is active and drives growth by inducing ribosome and protein synthesis and repressing catabolic metabolism. However, when nutrient or hormone levels are low, or cells are exposed to noxious stress, TORC1 signaling is inactivated and growth is slowed. How TORC1 activity is regulated remains largely unknown, and in particular it is uncertain (1) what output the complex branched TOR pathway has in stress, (2) which stress signaling pathways are involved in TOR regulation, and (3) how they impinge on the TOR pathway itself. Here we propose to address these questions in the model organism S. cerevisiae, taking advantage of the fact that the structure and function of the TORC1 pathway is highly conserved from yeast to human. In preliminary studies, we have examined TORC1 signaling in osmotic stress and find that stress alters TORC1 pathway output differently than other stimuli, such as nutrient deprivation and rapamycin. In addition, we find that TORC1 pathway repression depends significantly on the activity of the stress activated MAPK Hog1/p38. Building on these results, we will now construct a quantitative model of TOR pathway regulation and output in stress using a multipronged approach. First, we will use DNA microarray analysis of mutant strains to construct a model of the TOR/Hog1 signaling circuit to explain how stress and Hog1 alter signals transmitted through TORC1 and other growth pathways. We will then use high-throughput screens to identify additional upstream of regulators of TORC1, and then determine how these proteins interact with each other and the TOR pathway, again using microarray analysis. Finally we will add mechanistic detail to our model by studying the protein-protein interactions and post-translational modifications underlying the regulatory events we uncover. The proposed research will provide the first detailed view of the signaling pathways underlying stress dependent growth/TORC1 regulation in eukaryotes. This will not only shed light onto a fundamental and poorly understood regulatory system, but due to the high level of conservation in the TORC1 and stress pathways, our data and model will likely have a profound influence on our understanding of TOR and cell growth related diseases such as cancer, aging, obesity and diabetes.
This research will unravel the mechanisms by which stress influences cell growth. Dissecting these regulatory mechanisms will help us to explain how the circuitry that controls cell growth malfunctions in diseases such as cancer, diabetes and obesity.
