LAUSR

It recently has been reported that partial‐epithelial–mesenchymal transition (p‐EMT) program is associated with metastasis in head and neck squamous cell carcinoma (HNSCC). We previously have identified POSTN (which encodes periostin) as an invasion‐promoting molecule in HNSCC. Interestingly, POSTN expression is frequently observed in cancer cells with higher p‐EMT score by using a previous single‐cell transcriptomic data of HNSCC cases. Although it is known that POSTN has 11 splicing variants, the role of them has not been determined in HNSCC. Here, we found that HNSCC cells with EMT features expressed POSTN isoforms, Iso3 (lacking exon 17 and 21) and Iso5 (lacking exon 17), whereas fibroblast expressed Iso3 and Iso4 (lacking exon 17, 18, and 21). The expression of POSTN Iso3 and Iso4 are known to be widely observed in various cell types including stromal cells. Therefore, we focused on the role of novel cancer cell‐derived POSTN isoform, Iso5, in HNSCC. Single overexpression of POSTN Iso5 as well as Iso3 promoted invasion. Surprisingly, Iso5 synergistically promoted invasion together with Iso3. Notably, Iso5 as well as Iso3 upregulated p‐EMT‐related genes. We suggest that a novel cancer‐specific POSTN isoform lacking exon 17 (Iso5) can be a useful marker for detecting cancer cells undergoing p‐EMT. Moreover, a POSTN Iso5 can be a novel target for diagnosis and therapy in HNSCC.