SMAD4 represses FOSL1 expression and pancreatic cancer metastatic colonization

Less than 8% of patients diagnosed with pancreatic ductal adenocarcinomas (PDAC) survive more than five years in part due to late stage diagnosis. Approximately half of PDAC patients exhibit loss of SMAD4, which correlates with increased metastatic disease. Here we demonstrate that SMAD4 is a suppressor of metastatic colonization. Using isogenic human PDAC cell lines expressing or lacking SMAD4, both in vitro and in vivo, we define SMAD4-dependent gene expression changes in metastatic lesions. We performed a pooled in vivo open reading frame (ORF) screen of SMAD4 transcriptional targets and identified genes that, induce tumor seeding and growth at secondary sites. We found that expression of the transcription factor FOSL1 was sufficient to drive metastatic colonization. SMAD4 directly binds and represses the activity of the FOSL1 enhancer. These studies demonstrate a direct role for SMAD4 in regulating metastatic colonization and identify FOSL1 as a SMAD4-regulated gene involved in PDAC progression. In vitro and in vivo manipulation of SMAD4 with and without TGFB