Desensitization of cyclic adenosine 3,5'-monophosphate response to thyrotropin in normal and primary or metastatic papillary thyroid cancer cells in vitro.

BACKGROUND

Desensitization is an important physiologic process resulting in a decreased functional response after continuous or repeated stimulation. Prior exposure of normal human thyroid tissue to thyrotropin either in vivo or in vitro causes desensitization of adenylate cyclase. Little is known, however, about whether the thyrotropin-adenylate cyclase-cyclic adenosine 3,5'-monophosphate (cAMP) signal transduction system desensitizes in human thyroid carcinomas. Failure to desensitize could result in increased growth or metastases.

METHODS

Cell cultures from Chinese hamster ovary (CHO) cells transfected with normal human thyrotropin receptor (hTSHr) and thyroid neoplasms including one papillary carcinoma and one papillary lymph node metastases were evaluated for desensitization. CHO cells were stably transfected with plasmid DNA containing hTSHr. Cells were incubated with thyrotropin (10 mU/ml) for different periods (1 to 24 hours). A second incubation (30 minutes) was done with and without thyrotropin in medium including 3-isobutyl-1-methylxanthine (1 mmol/L). Intracellular cAMP accumulation was determined by means of radioimmunoassay.

RESULTS

Maximal stimulation and desensitization to thyrotropin were observed at 30 minutes and 4 hours, respectively. The cAMP response to a second incubation with thyrotropin was 52% and 48% lower than the initial response to thyrotropin in CHO-hTSHr and papillary thyroid cancer cells, respectively (p < 0.001). A papillary carcinoma lymph node metastases had an increased basal cAMP level and also an increased cAMP level in response to thyrotropin stimulation (227%) but failed to desensitize (p < 0.001).

CONCLUSIONS

Desensitization of the cAMP response to thyrotropin occurred in a papillary thyroid cancer but not in a metastatic thyroid cancer. Failure to desensitize might play a role in tumor progression.