Transcription factors related to chondrogenesis in pleomorphic adenoma of the salivary gland: a mechanism of mesenchymal tissue formation.

In salivary gland pleomorphic adenoma, expression of extracellular matrix (ECM) substances indicates that tumor epithelial cells are becoming chondrogenic and will produce cartilage-like mesenchymal tissues. Sox9, the master transcription factor of chondrogenesis, is expressed in mouse salivary gland cells. To clarify the mechanism behind chondrogenesis in tumor epithelial cells, we examined the expression of transcription factors related to chondrogenesis in tumors and salivary glands. Reverse transcriptase-polymerase chain reaction (RT-PCR), quantitative real-time RT-PCR, and immunostaining were performed on pleomorphic adenoma tissues, salivary gland tissues, and human submandibular gland (HSG) cells. The mRNAs of essential transcription factors for chondrogenesis-Sox9, Sox6, and Sox5-were detected in both tumor and salivary gland tissues. The mRNAs of aggrecan and type II collagen-cartilage-specific ECM substances-were detected only in tumors. Sox9 and Sox6 proteins were colocalized in many epithelial cells in tumors and salivary glands. Tumor epithelial cells also possessed aggrecan protein and occasionally type II collagen protein. Moreover, mRNAs for transcription repressors of chondrogenesis δEF1 and AP-2α were detected in both tumors and salivary glands, whereas Twist1 mRNA was detected only in salivary glands and was at significantly low-to-undetectable levels in tumors. Twist1 protein was localized in the Sox9-expressing salivary gland cells. HSG cells expressed Sox9, Sox6, and Twist1, but not aggrecan or type II collagen, and thus were similar to salivary gland cells. Twist1 depletion by Twist1 siRNA led to the upregulation of aggrecan and type II collagen mRNA expression in HSG cells. In contrast, forced expression of Twist1, using Twist1 cDNA, resulted in the downregulation of both these genes. Taken together, these results indicate that salivary gland cells have a potential for chondrogenesis, and Twist1 depletion concomitant with neoplastic transformation, which would permit tumor epithelial cells to produce cartilage-like mesenchymal tissues in salivary gland pleomorphic adenoma.