PROJECT SUMMARY/ABSTRACT The metabolic interactions and coevolution that occur between cancer and stromal cells is an unexplored topic that combines cancer metabolism and tumor microenvironment. The proposed project will investigate this unique aspect of cancer–host crosstalk from the novel perspective of extracellular miRNAs, whose function in transferring cancer-derived signals to various types of niche cells to facilitate cancer growth and metastasis has been recently recognized. MiRNA negatively regulates gene expression through inducing mRNA degradation and/or translation blockade. The goals of this study are to identify the mechanisms by which cancer cell-secreted miRNAs direct a metabolic plasticity in stromal fibroblasts to engage different modes of cancer–stroma interactions under different metabolic conditions, and to examine potential therapeutic interventions targeting this process. In Aim 1, we will determine the acting mechanisms of selected breast cancer (BC)-secreted miRNAs in the metabolic reprogramming of cancer-associated fibroblasts (CAFs), including a mechanism through activation of MYC-directed metabolic program. In Aim 2, we will characterize the metabolic interplays between BC cells and reprogrammed CAFs under different metabolic conditions using stable isotope tracing and cell co-cultures. The specific role of selected miRNAs as well as their target genes will be determined by genetic modifications and pharmacological inhibition. In Aim 3, we will evaluate the in vivo effects of the herein identified miRNA-regulated pathways, as well as their pharmacological inhibition, on tumor growth and progression using models of co-transplanted BC and CAF cells derived from patients. We will also examine the associations among selected miRNAs and metabolic genes/regulators in human BCs, as well as the circulating levels of these miRNAs in the corresponding serum samples. The proposed studies will provide a novel perspective to our understanding of the dynamic communication between cancer and host as well as cancer's response to metabolic therapies, and will establish rationales for novel therapeutic strategies to slow or stop BC progression, which is our long-term objective. Clinical-stage inhibitors will be examined for their effects to block cancer–stroma metabolic interactions and suppress tumor progression, which allows rapid translation into future clinical trials.