Podoplanin‐positive cells located in the basal layer of Bowen disease include tumor cells with cancer stem cell properties

Abstract

Podoplanin (PDPN) is widely used as a marker of lymphatic endothelial cells. PDPN is also involved in tumor progression, and upregulated PDPN expression is often found in various cancers. In this study, we first immunohistochemically examined PDPN expression in 87 cases of Bowen disease. Positive expression was detected in 64.4% of Bowen disease specimens, and the positive cells were exclusively located in the basal layer and corresponded to palisaded basal cells (PBCs). PBCs have been considered to be residual normal keratinocytes so far, but PDPN expression in cancers is generally associated with poor clinical outcomes. We also examined PDPN expression in 27 cases of Bowen carcinoma. Diffuse and strong PDPN expression was detected in 22.2% of Bowen carcinoma specimens, and another 22.2% showed PDPN expression at the leading edges of tumor nests. These results prompted us to determine whether PDPN‐positive cells are more tumorigenic than PDPN‐negative cells. We cultured Bowen disease cells using a three‐dimensional (3D) cell culture system and examined PDPN expression. In the cultured Bowen disease tissue, PDPN expression was again detected in the basal layer. Then, we isolated 1.2 × 10⁵ PDPN‐positive and ‐negative cells from the 3D organotypic culture of Bowen disease by fluorescence‐activated cell sorting analysis and compared their tumorigenicity using 3D culture. The PDPN‐positive tumor cells were able to regenerate Bowen disease tissue, but the PDPN‐negative tumor cells were not. In addition, the regenerated Bowen disease tissue derived from the PDPN‐positive cells exhibited PDPN expression in its basal layer, as the parental Bowen disease tissue did. These results indicate that PDPN‐positive cells include tumor cells with cancer stem cell properties. Although the precise mechanism through which PDPN expression is involved in the pathogenesis of Bowen disease needs to be determined, PDPN may be a novel druggable target for Bowen disease.