Fibroblast STAT3 activation drives organ-specific premetastatic niche formation

Pancreatic cancer is associated with a high rate of metastasis and poor prognosis. The formation of a premetastatic niche (PMN) facilitates cancer cell spread and contributes to cancer mortality. Using murine pancreatic cancer models based on expression of oncogenic KRAS in the pancreas epithelium, we discovered that remodeling of the lung microenvironment occurred in mice bearing pancreatic precursor lesions prior to cancer formation. This early lesion premetastatic niche (EL-PMN) resembled the PMN in cancer-bearing mice, and both feature characteristics of overt metastasis, such as transcriptional reprogramming, activation of fibroblast STAT3 signaling, and infiltration of immunosuppressive ARG1+ macrophages. Both pancreatic cancer patients and mouse models demonstrated elevated serum IL6. Inactivating oncogenic KRAS reduced serum IL6 and reverted fibroblast STAT3 phosphorylation in mouse lungs; loss of lung fibroblast STAT3 phosphorylation was similarly observed when mice were treated with the pan-RAS inhibitor RMC7977. While ARG1+ macrophage infiltration was dispensable for fibroblast STAT3 activation, IL6 blockade inhibited lung fibroblast STAT3 activation. Functionally, fibroblast STAT3 activation was necessary for lung metastasis establishment and growth. Interestingly, activation of STAT3 in the PMN was present in the lungs but not in the liver, where fibroblast reprogramming occurred only in overt metastasis, pointing to organ-specific PMN formation. In human metastasis samples, phosphorylated STAT3 in fibroblasts was similarly more abundant in the lungs than liver. Together, these data point to organ-specific mechanisms driving formation of the PMN and indicate that reprogramming of the microenvironment prior to metastasis might support early dissemination of pancreatic cancer.