PROJECT SUMMARY / ABSTRACT Small cell lung cancer (SCLC) is a recalcitrant cancer that causes 250,000 deaths worldwide each year. Patients with SCLC initially respond to cytotoxic therapies but almost invariably relapse with therapy resistant disease. Moreover, when diagnosed, many patients have metastases in distant organs including the liver, which represents a major impediment to successful therapy. The metastatic cascade involves cancer cell interactions with many normal cell types, which are thought to provide by pro- and anti-metastatic signals. Recent studies have shown that endothelial cells (ECs) play important and diverse roles in metastasis of different cancer types. However, our understanding of the molecular and cellular interactions between SCLC cells and ECs in liver metastasis remains incomplete and no therapies exist to prevent or stop these interactions, which marks great therapeutic possibilities for target treatment for SCLC patients in clinic. In this proposed study, I hypothesize that SCLC-EC interactions alter the cell state of both cell types and these changes are critical for SCLC liver metastasis. To systemically study the reciprocal interactions between SCLC cells and ECs, I have developed an innovative method to qualitatively record physical interactions between various cell types (GFP- based Touching Nexus, G-baToN). Meanwhile, my mentors' labs have generated somatic CRISPR genome editing mouse models and high-throughput tumor barcode sequencing platforms which I will employ to deconvolute metastatic burden, metastatic seeding, and clonal expansion in SCLC liver metastasis. By leveraging all these cutting-edge techniques, this project aims to characterize the role of SCLC-activated ECs in SCLC liver metastasis through studying the function of the CXCLs-CXCR2 axis (Aim 1) and other cell-cell interaction related EC genes (Aim 2). On the other hand, this project will also quantitatively assess the impact of EC-induced SCLC alternations in liver metastasis (Aim 3). Taken together, this project will uncover new mechanistic insights in SCLC metastasis, establish new strategies for investigating cancer- stromal interactions in vitro and in vivo, and identify new therapeutic possibilities to better treat SCLC patients.