Targeting Pin1 to Overcome Immunosuppressive Tumor Microenvironment in MSS Colorectal Cancer

Jian Wang¹Shuxin Tang²Jinhua Fan³Huihui Xiao⁴Shihua Wang⁵Jianqin Xiang⁴Zhili Liu⁴Hongying Liu⁴Zhigang Pei⁴Dequan Jiang⁴Leiyuan Shuai⁴Han Liu⁴Jian Ye⁴Jianli Xu⁴Chengyuan Qian⁴Siqian Cui^6*Debing Xiang^4*Chunrong Wu^4*

• ¹The Jiang Jin Central Hospital of Chongqing, Chongqing, China
• ²Children's Hospital of Chongqing Medical University, Chongqing, China
• ³Chongqing University Fuling Hospital, Chongqing, China
• ⁴Jiangjin Central Hospital of Chongqing, Chongqing, China
• ⁵Chongqing Medical University, Chongqing, China
• ⁶Chongqing University, Chongqing, China

Background: Management of the immunosuppressive tumor microenvironment (TME) is crucial for microsatellite stability (MSS) colorectal cancer (CRC), which responds poorly to immunotherapy. PIN1, a peptidyl-prolyl cis-trans isomerase that is overexpressed in human malignancies, regulates TME immunosuppression. However, its role in MSS CRC remains insufficiently explored. Methods: We divided 411 CRC patients from the TCGA-COAD database into MSS or microsatellite instability-high (MSI-H) groups and analysed their gene expression profiles. Using smoothed t-statistic SVM, weighted correlation network analysis, and sample clustering, we identified PIN1 as a key biomarker. We assessed Pin1 expression in CRC cell lines and tissues, and conducted functional assays, in vitro co-cultures, and in vivo studies (using a Pin1 inhibitor and anti-PD-1 in CT26 subcutaneous tumor and liver metastasis mouse models) to evaluate its effects and mechanisms. Results: PIN1 was overexpressed in MSS CRC and negatively correlated with CD4+ T and CD8+ T cell infiltration. Knockdown of PIN1 in MSS CRC cells significantly reduced cell proliferation (assessed by CCK-8 assay), impaired migratory capacity (assessed via wound-healing assay), and increased the apoptotic rate (detected by flow cytometry). In CT26 mouse models, combining Pin1 inhibition with PD-1 blockade enhanced immunotherapy efficacy by reducing Treg infiltration, suppressing cancer-associated fibroblast (CAF) activity, and promoting CD8+ T cell recruitment. Mechanistically, PIN1 activated the NF-κB pathway and modulated CCL3-CCR5 signaling, which are critical for Treg migration and CAF activation. Conclusion: Our findings suggest that Pin1 reshapes the immunosuppressive TME in MSS CRC through the NF-κB-CCL3-CCR5 axis, driving CRC progression and immunotherapy resistance. This pathway presents a potential target for overcoming immunotherapy resistance in MSS CRC.