Abstract A64: Calprotectin suppresses PIWIL1 and tumor growth in oral squamous cell carcinoma

Calprotectin (a heterodimer of S100A8 and S100A9), normally expressed in oral epithelial cells, is down‐regulated in oral squamous cell carcinoma (OSCC). Carcinoma cells over‐expressing calprotectin show reduced tumor growth in vivo and in vitro and are morphologically more differentiated than carcinoma cells negative for calprotectin expression. Phosphorylation of histone H3 at serine 10 (Ser10) is a marker for increased cell division, CD24 is expressed on differentiated cells and PIWIL1 plays a role in cellular self‐renewal. Objective: To determine the molecular pathways involved in calprotectin‐associated suppression of tumor growth and self‐renewal. Methods: To study the regulatory function of calprotectin in tumor growth, a non‐calprotectin expressing human epithelial carcinoma cell line (KB) was transfected with either an empty vector (KB‐EGFP) or vector containing S100A8 and S100A9 genes (KB‐S100A8/A9). To study the loss of function, S100A8 and S100A9 expression was silenced in calprotectin‐positive OSCC cell line (TR146) by small interfering RNA. Genome wide analysis of gene regulation was compared between calprotectin‐negative and calprotectin‐positive KB cells using Affymetrix HG‐U133A microarrays and validated by quantitative real‐time RT‐PCR. Changes in protein expression were examined by immunofluorescence flow cytometry. Results: Calprotectin expression in carcinoma cells increases phosphosphorylation of histone H3 (Ser10) correlating with a larger population at the cell cycle checkpoint G2/M and significant reduction in growths in vitro (KB‐S100A8/A9 and TR146) and in vivo (KB‐S100A8/A9). Among the cellular differentiation‐associated molecules, expression of CD24 is significantly up‐regulated, whereas, the expression of self‐renewal factor PIWIL1 is reduced at both mRNA and protein levels when calprotectin is over‐expressed. Conclusion: Calprotectin may play essential roles in suppressing tumor growth by decreasing self‐renewal capability and maintaining cellular differentiation. Loss of calprotectin expression in OSCC may lead to unregulated growth and de‐differentiation. This study was supported by the NIH/NIDCR grant R01DE11831. Citation Information: Cancer Res 2009;69(23 Suppl):A64.