Project Summary: PGC-1 family proteins (PGC-1α, PGC-1β, and PPRC1) are transcriptional coactivators that act as central hubs to coordinate diverse cellular inputs to promote the transcription of genes that regulate metabolism to maintain homeostasis. Transcriptional coactivator Peroxisome Proliferator-Activated Receptor Gamma Coactivator 1 β (PGC-1β) is over-expressed in colorectal cancers (CRC) with K-Ras mutations and promotes the survival of tumor cells. PGC-1 family proteins lack intrinsic enzymatic activity and function by facilitating interactions between transcription factors, epigenetic modifiers, and transcription initiation machinery. To identify the protein-protein interactions required by PGC-1β to promote gene expression we immunoprecipitated PGC- 1β and identified binding partners by mass spectrometry. Our data reveal that Estrogen-Related Receptor α (ERRα) and Host Cell Factor 2 (HCF2) are PGC-1β binding proteins that we propose are required for PGC-1β to regulate the transcription of genes that promote CRC survival. Our long-term goal is to inhibit CRC growth by blocking PGC-1β signaling and we propose that can be achieved by preventing PGC-1β from signaling through either ERRα or HCF2. In Aim 1, we will determine the mechanism by which phosphatase PGAM5 modifies PGC-1β to increase ERRα transcription. In Aim 2, we will identify PGC-1β-HCF2 regulated genes and determine the requirements for HCF2 signaling. In Aim 3, we will test the loss of the PGC-1β, the loss of the PGC-1β-ERRα interaction, the loss of the PGC-1β-HCF2 interaction, and the loss of both interactions in vivo using orthotopic submucosal injections in immunodeficient mice receiving Crispr-Cas9-modified patient-derived tumor organoids establish from liver metastases.