The thyrotropin receptor is not involved in the activation of p42/p44 mitogen-activated protein kinases by thyrotropin preparations in Chinese hamster ovary cells expressing the human thyrotropin receptor.

Abstract

We studied whether bovine pituitary thyrotropin (bTSH) or human recombinant thyrotropin (rhTSH) stimulated p42/p44 mitogen-activated protein kinases (MAPKs) in Chinese hamster ovary cells expressing human thyrotropin receptor (CHO-hTSHR cells). We show that p42/p44 MAPK phosphorylation was induced by both TSH preparations at similar levels in CHO-hTSHR cells and in wild-type CHO cells. In contrast, cyclic adenosine monophosphate (cAMP) production was stimulated by TSH only in CHO-hTSHR cells, demonstrating that p42/p44 MAPK stimulation was independent of the TSH receptor. Moreover, similar results were obtained with two other cell lines: the FRTL-5 thyroid cell line and the CCL39 fibroblast cell line. Maximal stimulation of p42/p44 MAPK phosphorylation was observed after a 5- to 10-minute incubation with bTSH and rhTSH preparations. At this time, the phosphorylation of GST-Elk1 was also increased in a time- and concentration-dependent manner by bTSH preparations. The phosphorylation of p42/p44 MAPKs was abolished by PD 98059 and GF 109203X, indicating the involvement of MAPK kinases (MEK 1/2) and protein kinase C. In contrast, the activation of p42/p44 MAPKs was insensitive to H89, to cholera toxin and to pertussis toxin. These data suggest that the protein kinase A pathway was not implicated in p42/p44 MAPK activation by TSH preparations. Moreover, Gs or Gi/Go proteins do not appear to participate in p42/p44 MAPK activation. We also showed that these TSH preparations failed to induce activation of c-Jun NH2 terminal kinase. We therefore conclude that the commercial TSH preparations used in this study contained factor(s) responsible for the specific activation of p42/p44 MAPKs by a TSH receptor-independent mechanism.