Identification of Novel piR-2158 Isoforms and Their Distinct Antitumor Effects on Triple-Negative Breast Cancer

Background: The diversity of RNA isoforms plays a critical role in regulating the development and progression of human cancers. Our previous work has demonstrated that piR-2158 exerts antitumor activity in breast cancer by repressing IL11-STAT3 signaling. Methods: The isoforms of piR-2158 were analyzed using Sanger sequencing, quantitative real-time PCR (qRT-PCR), and Gene Expression Omnibus (GEO) dataset. Cell proliferation capacity was assessed using Cell Counting Kit-8 (CCK-8) assays and Ki67 immunofluorescence staining. Cell migration was evaluated using wound healing assays. Cancer cell stemness was analyzed using mammosphere formation assay, stemness marker detection, and ALDH activity assay. Results: We identified two types of piR-2158 isoforms in mammary tissues: a 31 nt long isopiR (designated as iso-piR-2158-L) and a 28 nt short isopiR (designated as iso-piR-2158-S). Predominant expression of iso-piR-2158-L was observed in normal mammary epithelial cells and adjacent non-tumor breast tissues, whereas it was significantly downregulated in TNBC cell lines and primary tumor tissues. We experimentally demonstrated that iso-piR-2158-L and iso-piR-2158-S exert distinct effects on TNBC cell proliferation, migration, and stemness, with iso-piR-2158-L showing significantly stronger antitumor effects than iso-piR-2158-S. Subsequent mechanistic studies revealed that iso-piR-2158-L suppresses IL11 expression more effectively, compared to iso-piR-2158-S. Conclusions: Our findings reveal a piRNA isoform-based regulatory pathway that may be involved in regulating pathological transformation, tumor initiation, and progression in TNBC.