Abstract B014: TLR signaling coordinates SAA-dependent liver metastasis in pancreatic cancer

Abstract Metastasis remains the primary cause of mortality in cancer. This is especially true in pancreatic ductal adenocarcinoma (PDAC) - where the liver is the most common site of metastasis and the 5-year overall survival rate for these patients has remained <3% for the past 20 years. Accumulating evidence shows a critical role for a niche microenvironment in the liver, termed the pro-metastatic niche, that supports metastatic seeding and outgrowth. Our previous work demonstrated that hepatocytes orchestrate the formation of this pro-metastatic niche during early tumorigenesis through the release of Serum Amyloid A1 and 2 (SAA). However, the mechanisms by which SAA directs formation of a pro-metastatic niche in the liver, and the impact of this niche on the fate of disseminating tumor cells (DTCs) seeding the liver, remain ill-defined. To begin to identify the cellular determinants of niche formation, we first analyzed the expression of SAA receptors within the normal liver and found predominant expression of receptors on the resident F4/80+ macrophages. This is in line with current published work as SAA is well characterized to control liver biology through toll-like receptors (TLRs) expressed by liver resident immune cells and hepatic stellate cells. Consistent with this, we also found that SAA is a specific, potent agonist of TLR2 in vitro. In tumor bearing mice we then found increased expression of TLR2 transcripts as well as an accumulation of TLR2+ inflammatory monocytes within the niche. Together, these findings suggested that SAA might form a pro-metastatic niche in the liver withough a TLR2-dependent mechanism. To test this, we next examined the livers of TLR2-/- tumor bearing mice and found a decrease in the expression of surrogate markers of the pro-metastatic niche. We also sought to determine a role for TLR2 in metastatic outgrowth. Using TLR2-/- mice in a model of PDAC with direct metastasis to the liver, we found significantly decreased metastatic burden and outgrowth in the liver. Analysis of micro-metastatic lesions in TLR2-/- tumor bearing mice also revealed that DTCs demonstrated molecular markers of cellular dormancy. Altogether, our findings suggest the evolution of an inflammatory mechanism within the niche wherein SAA may continue to signal through TLR2+ inflammatory monocytes, which then act to coordinate metastatic outgrowth. Future work aims to further elucidate the role of TLR2+ resident macrophages and monocytes in the formation of a pro-metastatic niche in the liver and in regulating metastatic outgrowth. Citation Format: Jesse Lee, Meredith L. Stone, Hana Choi, Gregory L. Beatty. TLR signaling coordinates SAA-dependent liver metastasis in pancreatic cancer [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer; 2022 Sep 13-16; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2022;82(22 Suppl):Abstract nr B014.