Decrease of prostaglandin I₂ binding sites in thyroid cancer

Abstract

The properties of specific prostaglandin I2 (prostacyclin, PGI2) binding sites in normal thyroid tissue have been characterised. Tissue samples obtained intraoperatively from patients with 'cold' solitary thyroid nodules (as preoperatively selected by thyroid gland scintigraphy, thyroid gland ultrasonography and Papanicolaou cytology following fine needle aspiration of the nodule area) have been used for thyroid membrane preparation. Employing [3H]iloprost, a chemically stable PGI2-analogue as a radioligand, saturation experiments for comparative binding studies have been attempted. Scatchard analysis of the binding data obtained for normal thyroid parenchyma distant to the nodule area revealed heterogeneity of the [3H]iloprost sites exhibiting a high-affinity binding capacity (Bmax) of 613.2 +/- 130.4 fmol mg-1 membrane protein and a low-affinity binding capacity of 5.1 +/- 1.6 pmol mg-1 membrane protein. The equilibrium dissociation constant (Kd) amounted to 18.9 +/- 8.9 nM and to 131.5 +/- 39.2 nM, respectively. Scatchard analysis of the binding data obtained for benign thyroid adenoma indicated significant lower binding capacities exhibiting a Bmax of 325.8 +/- 110.0 fmol mg-1 membrane protein (Kd: 31.0 +/- 7.5 nM) for the high-affinity sites and of 3.9 +/- 2.5 pmol mg-1 membrane protein (Kd: 364.9 +/- 183.6) for the low affinity sites. In cancer tissue a selective loss of the low affinity sites and a significant diminution of the high-affinity sites was observed: in well differentiated cancer the high-affinity sites showed a Bmax of 299.7 +/- 46.0 fmol mg-1 membrane protein (Kd: 38.9 +/- 7.3 nM), in anaplastic cancer, less differentiated papillar and follicular cancers of 180.6 +/- 25.1 fmol mg-1 membrane protein (Kd: 54.6 +/- 16.7 nM). Well differentiated papillar and follicular cancers did not differ from each other.(ABSTRACT TRUNCATED AT 250 WORDS)