The effect of STAT1 expression and the Y701 phosphorylation on primary Undifferentiated Pleomorphic Sarcoma cells

Undifferentiated Pleomorphic Sarcoma (UPS) is a genetically complex type of soft tissue sarcoma (STS) and there are few therapeutic options for advanced disease. Immune checkpoint blockade therapy may be of benefit to a subset of UPS patients, particularly those with elevated tumoral Programmed Cell Death Ligand 1 (PD-L1) expression and an inflamed tumour microenvironment. From transcriptomic analyses we previously identified Signal Transducer and Activator of Transcription 1 (STAT1) expression, a key mediator of interferon-γ (IFN-γ) signaling, as enriched in a cohort of UPS cases with elevated PD-L1 levels that were observed to have an improved clinical outcome. This led us to investigate the role of STAT1 in UPS biology. We examined the effects of STAT1 loss, and overexpression of either wild type or tyrosine phosphorylation-deficient STAT1 in primary human UPS cell lines. We observed that IFN-γ treatment decreased cell counts, increased cell death, and induced the secretion of CXCR3 ligands in a STAT1-dependent manner. Overexpression of wild type STAT1, but not a tyrosine to phenylalanine phospho-mutant (Y701F), restored IFN-γ-mediated cell death and cytokine secretion, highlighting the importance of Y701 phosphorylation in STAT1-associated antitumour functions. STAT1 Y701 phosphorylation status was associated with gene expression changes that were cell line specific. Together, these findings shed light on the central role of STAT1, and specifically the Y701 phosphorylation, in mediating IFN-γ signaling and related phenotypes. This study underscores the need for further exploration of the role of STAT1 phosphorylation in tumor-immune interactions within UPS.