Functional Toll-like Receptor 4 Overexpression in Papillary Thyroid Cancer by MAPK/ERK-Induced ETS1 Transcriptional Activity

Emerging evidence suggests that unregulated Toll-like receptor (TLR) signaling promotes tumor survival signals, thus favoring tumor progression. Here, the mechanism underlying TLR4 overexpression in papillary thyroid carcinomas (PTC) mainly harboring the BRAF^V600E mutation was studied. TLR4 was overexpressed in PTC compared with nonneoplastic thyroid tissue. Moreover, paired clinical specimens of primary PTC and its lymph node metastasis showed a significant upregulation of TLR4 levels in the metastatic tissues. In agreement, conditional BRAF^V600E expression in normal rat thyroid cells and mouse thyroid tissue upregulated TLR4 expression levels. Furthermore, functional TLR4 expression was demonstrated in PTC cells by increased NF-B transcriptional activity in response to the exogenous TLR4-agonist lipopolysaccharide. Of note, The Cancer Genome Atlas data analysis revealed that BRAF^V600E-positive tumors with high TLR4 expression were associated with shorter disease-free survival. Transcriptomic data analysis indicated a positive correlation between TLR4 expression levels and MAPK/ERK signaling activation. Consistently, chemical blockade of MAPK/ERK signaling abrogated BRAF^V600E-induced TLR4 expression. A detailed study of the TLR4 promoter revealed a critical MAPK/ERK–sensitive Ets-binding site involved in BRAF^V600E responsiveness. Subsequent investigation revealed that the Ets-binding factor ETS1 is critical for BRAF^V600E-induced MAPK/ERK signaling-dependent TLR4 gene expression. Together, these data indicate that functional TLR4 overexpression in PTCs is a consequence of thyroid tumor-oncogenic driver dysregulation of MAPK/ERK/ETS1 signaling.

Implications: Considering the participation of aberrant NF-B signaling activation in the promotion of thyroid tumor growth and the association of high TLR4 expression with more aggressive tumors, this study suggests a prooncogenic potential of TLR4 downstream signaling in thyroid tumorigenesis. Mol Cancer Res; 16(5); 833–45. ©2018 AACR.

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