Simple SummaryCTHRC1 is a protein with pro-tumoral effects that is highly expressed in tumors. We found that cancer stroma is the major source of CTHRC1 secretion in pancreatic cancer, and CTHRC1 regulates cancer extracellular matrix (ECM) remodeling by activating pancreatic stellate cells (PSCs), a main component of pancreatic cancer stroma. Our results show that CTHRC1-activated PSCs differentiated into myofibroblast-like CAFs (myCAFs). Moreover, the pro-tumoral effects manifested by CTHRC1 were more potent through PSC activation than via autocrine action. Periostin, a stroma-specific molecule, demonstrated an essential role in the CTHRC1–PSCs–cancer metastasis axis. Blocking CTHRC1 reversed PSC activation in vitro and showed significant anti-tumoral effects in animal models. However, long-term CTHRC1 treatment likely caused a shift from myCAFs into inflammatory CAFs. This study elucidates PSC activation as the central mechanism underlying the pro-tumoral effects of CTHRC1 and suggests that future studies on CTHRC1 as a potential therapeutic target for pancreatic cancer are warranted.Abstract[BACKGROUND]Collagen triple helix repeat containing-1 (CTHRC1) is a secreted protein that contributes to the progression of various cancers, including pancreatic cancer. The higher expression of CTHRC1 in tumor tissues is associated with poorer survival outcomes. However, its specific roles in tumor extracellular matrix (ECM) remodeling remain unclear. Our study aims to investigate the influences of CTHRC1 on pancreatic stellate cells (PSCs), a main source of ECM production in pancreatic cancer.[METHODS AND RESULTS]The analyses of the publicly available pancreatic cancer patient data revealed that CTHRC1 is mainly expressed in cancer stroma and highly correlated with ECM-related genes. An in vitro study showed that more than 40% of these genes can be upregulated by CTHRC1. CTHRC1 specifically activated PSC into myofibroblast-like cancer-associated fibroblasts (myCAFs), which are characterized by a significantly upregulated POSTN gene expression. Periostin (coded by the POSTN gene) has a central role in the CTHRC1–PSCs–cancer metastasis axis. Furthermore, CTHRC1 promoted pancreatic cancer cell proliferation through PSC activation to a greater extent than via direct stimulation. Proof-of-concept experiments showed that the long-term (4-week) inhibition of CTHRC1 led to significant tumor suppression and ECM reduction, and also resulted in an unexpected shift in the CAF subtype from myCAFs to inflammatory CAFs (iCAFs).[CONCLUSION]PSC activation was demonstrated to be the key molecular mechanism responsible for the tumor-promoting effects of CTHRC1, and CTHRC1 has a critical role in CAF subtype differentiation and tumor microenvironment (TME) remodeling. The inhibition of CTHRC1 as a therapeutic strategy for the treatment of pancreatic cancer warrants further investigation.