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Thyrotropin Stimulates the Generation of Inositol 1,4,5-Trisphosphate in Human Thyroid Cells

Institute of Interdisciplinary Research (J.V.S., C.M., J.E.D., C.E.), University of Brussels, School of Medicine, 1070 Brussels, Belgium; and Institut Bordet (D.D., P.L.), University of Brussels, 1000 Brussels, Belgium

Address all correspondence and requests for reprints to: Christophe Erneux, Institute of Interdisciplinary Research, Campus Erasme Building C, 808 Route de Lennik, 1070 Brussels, Belgium. E-mail: cerneux{at}ulb.ac.be.

Context: Dual activation by TSH of the phospholipase C and cAMP cascades has been reported in human thyroid cells. In contrast, Singh et al. reported convincing data in FRTL-5 thyrocytes arguing against such an effect in this model. Their data in FRTL-5 cells indicated no increase in inositol 1,4,5-trisphosphate [Ins(1,4,5)P₃] in response to TSH. Therefore, the authors questioned results previously obtained on human cells by cruder methodology.

Objective: We investigated the formation of inositol phosphates by HPLC techniques in human thyroid slices to separate the inositol phosphate isomers.

Results: Ins(1,4,5)P₃, inositol 1,3,4-trisphosphate, and inositol 1,3,4,5-tetrakisphosphate were increased after TSH stimulation. The effect of TSH in human thyroid cells was reproduced by recombinant TSH and prevented by antibodies blocking the TSH receptor. Thyroid-stimulating antibodies at concentrations eliciting a cAMP response equivalent to TSH failed to stimulate inositol phosphate generation.

Conclusions: TSH, but not thyroid-stimulating antibodies, activates both cAMP and the phospholipase C cascade in human thyroid as now demonstrated by an increase in Ins(1,4,5)P₃ and its inositol phosphate metabolites. Therefore, this effect cannot be extrapolated to the FRTL-5 cell line. The apparent discrepancy may be due to a difference between species (human vs. rat) or to the loss of the fresh tissue properties in a cell line. The dual effect of TSH in human cells, through cAMP on secretion of thyroid hormones and through the diacylglycerol, Ins(1,4,5)P₃ Ca²⁺ pathway on thyroid hormone synthesis, implies the possible separation of these effects in thyroid disease.

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