Blockage of retinoic acid signaling via RARγ suppressed the proliferation of pancreatic cancer cells by arresting the cell cycle progression of the G1-S phase

Background: Ours and several studies have reported that, in some cancers, including pancreatic ductal adenocarcinoma (PDAC), the expression of squamous lineage markers, such as esophagus-tissue specific genes, correlated with a poor prognosis. However, the mechanism by which the acquisition of squamous lineage phenotypes leads to a poor prognosis remains unclear. We previously reported that retinoic acid signaling via retinoic acid receptor γ (RARγ signaling) determines the differentiation lineage into the esophageal squamous epithelium. These findings hypothesized that the activation of RARγ signaling contributed to acquiring squamous lineage phenotypes and malignant behavior in PDAC. Methods: This study utilized public databases and immunostaining of surgical specimens to examine RARγ expression in PDAC. We evaluated the function of RARγ signaling by inhibitors and siRNA knockdown using PDAC cell line and patient-derived PDAC organoids. The mechanism of the tumor-suppressive effects by blockage of RARγ signaling was determined by RNA-sequencing and Western blotting. Results: RARγ expression increased via transformation from normal pancreatic duct to pancreatic intraepithelial neoplasia (PanIN) and PDAC, and its expression correlated with a poor patient prognosis. In PDAC cell lines, blockage of RARγ signaling suppressed cell proliferation by inducing the cell cycle arrest in the G1 phase without causing apoptosis. We demonstrated that blockage of RARγ signaling upregulated p21 and p27 and downregulated many cell cycle genes, including cyclin-dependent kinase 2 (CDK2), CDK4 and CDK6. Furthermore, using patient-derived PDAC organoids, we confirmed the tumor-suppressive effect of RARγ inhibition and indicated the synergistic effects of RARγ inhibition with gemcitabine. Conclusions: This study clarified the function of RARγ signaling in PDAC progression and demonstrated the tumor-suppressive effect of selective blockage of RARγ signaling against PDAC. These results suggested that RARγ signaling might be a new therapeutic target for PDAC.