Desensitization of the thyroid cyclic AMP response to thyroid stimulating immunoglobulin: comparison with TSH

A bioluminescence assay for thyrotropin receptor antibodies predicts serum thyroid hormone levels in patients with de novo Graves' disease

BACKGROUND

TSH receptor antibodies (TBII) in Graves' disease (GD) may be TSH receptor stimulating (TSAb) and blocking (TBAb) antibodies. In commercially available assays however, only total TBII titres can be measured, without discriminating between TSAb and TBAb.

OBJECTIVE

To design a TBII bioassay to detect of TSAb and to correlate TSAb activity with severity of hyperthyroidism in de novo GD patients.

PATIENTS

Thirty-five patients with de novo GD and 27 controls.

METHODS

The JP-26-26 cell line, which constitutively expresses the TSH receptor (TSHR), was stably transfected with a cyclic adenosine monophosphate responsive element--luciferase construct. The clone B1 exhibited a near linear increase in luminescence from 0.2 mU/l to 50 mU/l bovine TSH and was used as a TBII bioassay. TBII, free T4 and TSH were measured in the sera of all patients and controls.

RESULTS

In the sera of 35 GD patients, TBII titres did not correlate with serum free T4 concentrations. In contrast, a strong and highly significant correlation was found between TSHR stimulating activity (luminescence) as measured with the TBII bioassay and serum free T4 levels (R = 0.80, P < 0.001). Interestingly, the luminescence/TBII ratio had a wide range at low TBII titres, whereas high TBII titres were associated with a low degree of TSHR activation. The TBII bioassay also detected TBAb in GD patients who spontaneously developed hypothyroidism.

CONCLUSIONS

The B1-TBII-bioassay as developed in our laboratory has a high sensitivity for the detection of TSAb in GD and predicts the severity of hyperthyroidism in untreated GD patients. In addition, we found that high TBII titres are associated with weak TSHR activation.

Concentration-dependent regulation of thyrotropin receptor function by thyroid-stimulating antibody

Thyrotropin receptor (TSHR) Ab's of the stimulating variety are the cause of hyperthyroid Graves disease. MS-1 is a hamster mAb with TSHR-stimulating activity. To examine the in vivo biological activity of MS-1, mice were treated with purified MS-1 intraperitoneally and the thyroid response evaluated. MS-1 induced a dose-dependent increase in serum thyroxine (T4), with a maximum effect after 10 proportional, variant g of MS-1 was administered. MS-1-secreting hybridoma cells were then transferred into the peritoneum of nude mice to study chronic thyroid stimulation. Serum MS-1 levels detected after 2 weeks were approximately 10-50 proportional, variant g/ml, and the serum TSH was suppressed in all animals. Serum triiodothyronine levels were elevated, but only in animals with low serum MS-1 concentrations. In addition, there was a negative correlation between serum T4 and the serum MS-1 concentrations. These in vivo studies suggested a partial TSHR inactivation induced by excessive stimulation by MS-1. We confirmed this inactivation by demonstrating MS-1 modulation of TSHR function in vitro as evidenced by downregulation and desensitization of the TSHR at concentrations of MS-1 achieved in the in vivo studies. Thus, inactivation of the TSHR by stimulating TSHR autoantibodies (TSHR-Ab's) in Graves disease patients may provide a functional explanation for the poor correlation between thyroid function and serum TSHR-Ab concentrations.

Kinetics of thyrotropin-stimulating hormone (TSH) and thyroid-stimulating antibody binding and action on the TSH receptor in intact TSH receptor-expressing CHO cells

The kinetics of TSH binding and the effects of TSH and thyroid-stimulating antibody (TSAb) on cAMP accumulation have been measured in TSH receptor-expressing CHO cells (CHO-TSHR cells). The parallel kinetics of TSH binding to its receptor and of cell cAMP concentration after the addition and withdrawal of TSH show that in the case of this receptor, signal generation and concentration are at all times proportional to occupancy. In physiological ionic medium, TSAb, but not TSH, action is slowed and in some cases almost nonexistent. The kinetics of cAMP disappearance after washout of TSAb is also slower. cAMP accumulation is faster for Fabs than for the TSAb from which they derive. Analysis of the data suggest that 1) serum TSAb are oligoclonal antibodies sets, at low concentrations, with a high affinity for the TSH receptor; 2) ionic interactions are involved in the action of TSAb on the TSH receptor; and 3) TSAb activation of the TSH receptor is at least a two-step process. Among others, a possible explanation is that the full activation of the receptor requires the binding of two or more different antibody molecules on different sites of the same TSH receptor. This analysis provides a benchmark for studies of experimentally induced monoclonal antibodies activating the TSH receptor.

Lack of relationship between 3',5'cyclic adenosine monophosphate desensitization and thyrotropin receptor down regulation in the rat thyroid cell line FRTL5

We studied the cAMP response and radioiodinated TSH binding in the FRTL5 rat thyroid cell line after chronic exposure to TSH. In parallel experiments, continuous exposure to TSH induced both a decrease in the cellular cAMP responsiveness to a second acute stimulation by thyrotropin (desensitization) and a reduction in 125I-TSH cellular binding (down-regulation). These two processes appeared to differ in that: a) cAMP desensitization was maximal (45-55% inhibition in comparison to control cells) after 24 h of preexposure to TSH; on the contrary, a loss of the TSH binding to thyroid cells was detected only after 24 h and was maximal (55-65% lower than control cells) after 48 h of preexposure to TSH; b) a different dependence on the TSH dose was observed for cAMP desensitization and TSH down-regulation: the former was maximal at 10 microU/ml of TSH, the latter only at 10 mU/ml of TSH; c) recovery from cAMP desensitization was detected 3 h after TSH withdrawal and was fully achieved at 24 h, whereas the recovery from the TSH receptor down-regulation was not evident for up to 24 h and a full recovery obtained only at 72 h; d) the use of cAMP analogues proved that, unlike desensitization, TSH receptor down-regulation is a cAMP dependent process. These findings support the concept that different patterns of response to prolonged stimulation with TSH, an earlier and cAMP-independent process (desensitization) and a later cAMP-dependent mechanism (down-regulation), occur in the rat thyroid cell.