Guanyl nucleotide regulatory proteins in neoplastic and normal human thyroid tissue.

Abstract

AbstractPapillary and follicular thyroid neoplasms have been documented to have thyrotropin (TSH) receptors and an intact receptor‐adenylate cyclase (AC) system. The AC response to TSH in most benign and malignant thyroid neoplasms is greater in the neoplasms than in the adjacent normal thyroid tissue. This increased AC response could be due to abnormal amounts of guanyl nucleotide regulatory (G) proteins, to an altered ratio of stimulating to inhibiting G proteins, or to the coupling of the G proteins—to either the receptor or to the catalytic unit of AC. To study why tumors have a greater AC response to TSH, we first extracted the G proteins from normal and neoplastic human thyroid tissue and placed them in membranes from cyclase minus S49 mouse lymphoma cells that lack a stimulating guanyl nucleotide regulatory protein (Gs). These studies demonstrated that there were comparable amounts of guanyl nucleotide regulatory protein in normal and neoplastic thyroid tissue. We next investigated whether there was an alteration in the ratio of inhibiting (Gi) to stimulating (Gs) guanyl nucleotide regulating proteins in thyroid neoplasms. Pertussis toxin (PT) (10 μg/ml) was used to study the activity of the inhibiting G protein since it stimulates AC activity by inhibiting the inhibitory regulatory protein by adenosine diphosphate ribosylating the alpha subunit of the Gi protein. Cholera toxin (CT) (10 ng/ml) was used because it selectively activates the stimulating G proteins. AC activity was determined by the conversion of [α‐32P]ATP to cAMP in pmol/mg protein per 30 minutes. In 11 patients, PT‐stimulated AC levels [8.55±1.7 SEM, (mean±standard deviation)] in normal tissues were higher than basal levels of AC (5.14±0.9,p< 0.01). However, there were no differences in basal and PT‐stimulated levels in neoplastic tissues (6.43±1.0 and 6.87 ±1.8, respectively). This suggests that there is less Gi protein in the neoplasms since the AC response to PT was greater in normal tissues by 170%, but there was no significant stimulation in neoplastic tissues. In contrast, the AC response to CT (10 μg/ml that directly activates the stimulatory guanyl nucleotide regulatory proteins was greater in neoplastic thyroid tissue (174.3±30.1) than in normal thyroid tissue (78.5±16.3) from the same patients (p<0.01).These experiments demonstrate that most thyroid neoplasms have less Gi and more Gs protein, which probably accounts for the greater AC response to TSH in thyroid neoplasms. The greater stimulation of Gs and the greater AC response in the neoplasms may be one reason for the more rapid growth of benign and malignant thyroid tumors.