Functional Toll-Like Receptor 4 Conferring Lipopolysaccharide Responsiveness Is Expressed in Thyroid Cells

Abstract

Lipopolysaccharide (LPS), a glycolipid found in the cell wall of Gram-negative bacteria, exerts pleiotropic biological effects in different cell types. LPS is mainly recognized by the Toll-like receptor (TLR) 4/MD2/Cluster of differentiation 14 complex (CD14). We previously demonstrated that LPS produced a direct action on thyroid cells, including up-regulation of thyroglobulin gene expression. This work aimed to study further the effect of LPS on thyroid function and to elucidate the mechanism by which LPS is recognized by the thyroid cell. We could detect the transcript and protein expression of TLR4, MD2, and CD14 in thyroid cells, and that these proteins are localized at the plasma membrane. The sodium iodide symporter (NIS) is the transporter involved in the iodide uptake, the first step in thyroid hormonogenesis. We demonstrated that LPS increases the TSH-induced iodide uptake and NIS protein expression. The LPS agonist lipid A reproduced LPS effect, whereas the LPS antagonist, polymyxin B, abrogated it. By the use of anti-TLR4 blocking antibodies and the transient expression of TLR4 dominant-negative forms, we evidenced the involvement of TLR4 in the LPS action. The enrichment of TLR4 expressing Fisher rat thyroid cell line-5 (FRTL-5) cells confirmed that TLR4 confers LPS responsiveness to thyroid cells. In conclusion, we revealed for the first time that all the components of the LPS receptor complex are expressed in thyroid cells. Evidence that the effects of LPS on rodent thyroid function involve TLR4-induced signaling was obtained. The fact that thyroid cells are able to recognize and respond to LPS supports a role of the endotoxin as a potential modifier of thyroid function.

Demonstration of Innate Immune Responses in the Thyroid Gland: Potential to Sense Danger and a Possible Trigger for Autoimmune Reactions

“…dsRNA and LPS increased NFjB activity (Fig. 2B), which is consistent with the previous reports showing the functional expression of TLR3 and TLR4 in FRTL-5 cells (19)(20)(21)(22). dsDNA also stimulated these two reporter genes as previously described (26).…”

“…Although mRNA from human thyroid tissue could contain mRNAs from other types of cells (fibroblasts, endothelial cells, and infiltrated leukocytes), FRTL-5 cells are pure thyroid cells without contamination by other cell types. Therefore, the results indicate that thyroid cells actually express TLRs as previously demonstrated for TLR3 and TLR4 expression in human and rat thyroid cells (19)(20)(21)(22). …”

“…We have also shown that TLR ligands such as LPS, dsRNA, CpG ODN, and PGN functionally activate the IFN-b promoter and/or NFjB activity in FRTL-5 cells. It was previously demonstrated that dsRNA is recognized by functional TLR3 in the human thyroid (19,20), that LPS is recognized by functional TLR4 in FRTL-5 cells (22), and that genomic dsDNA fragments are recognized by a cytosolic nucleic acid sensor, histone H2B, in FRTL-5 (26). Furthermore, it was clearly demonstrated that recognition of LPS by TLR4 results in activation of NFjB, thereby inducing the expression of the sodium iodide symporter (NIS; Slc5a5) in FRTL-5 cells (21).…”

“…Also, it was shown that TLR4, which mediates responses to LPS, is expressed in FRTL-5 cells (21,22). Moreover, the expression of TLR4 and accessory molecules with a basolateral localization has been described in normal rat thyroid cells (22). This evidence indicates that thyroid cells have the capacity to activate innate immune responses.…”

“…It was demonstrated that rat and human thyroid cells express functional TLR3, which recognizes dsRNA (19,20). Also, it was shown that TLR4, which mediates responses to LPS, is expressed in FRTL-5 cells (21,22). Moreover, the expression of TLR4 and accessory molecules with a basolateral localization has been described in normal rat thyroid cells (22).…”