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Structural differences between dopamine D2 receptors present in a rat pituitary adenoma and in transplantable rat
C Bouvier1, G Lagacé, M Potier
1Research Unit on Reproductive and Developmental Biology, Hôpital Sainte-Justine, Montréal, Québec, Canada.
Abstract:
We have investigated the structure of dopamine (DA) D2 receptors present in an estrone-induced, prolactin (PRL)-secreting, DA-sensitive adenoma and in two PRL-secreting and DA-insensitive transplantable tumors 7315a and MtTW15, in order to identify better the anomalies present in DA-resistant lactotrophs. D2 receptors were found in both a high- and a low-affinity state in adenomatous lactotrophs as shown by displacement studies with the agonist N-propylnorapomorphine (NPA), but only in the low-affinity state in the two DA-resistant tumors. Treatment with the alkylating agent N-ethylmaleimide induced a disappearance of the high-affinity state of the D2 receptor in the adenoma and a reduction in receptor concentration, but did not have any effect on the affinity of receptors present in DA-resistant tumors. Moreover, target size analysis and radiation inactivation studies of D2 receptors, using membranes preincubated with NPA and [3H]spiperone as ligand or using [3H]NPA as ligand on membranes preparations, have shown the presence of distinct structural differences between adenomatous and tumoral D2 receptors and between the two tumoral receptors themselves; these results suggest that the normal functional unit of the D2 receptor is a dimer associated with a guanine nucleotide-binding protein (G protein) subunit and that tumoral D2 receptors may exist in various polymeric forms unassociated with G proteins. The anomalies found to be present in tumoral D2 receptor complexes may be responsible for the insensitivity of these tumors to dopaminergic agonists' inhibitory activity on PRL release and tumor growth.
Insights
Dopamine D2 receptors in DA-resistant tumors show structural anomalies, existing in polymeric forms unassociated with G proteins. These structural differences in dopamine receptors may explain tumor insensitivity to DA agonists.
Area of Science:
- Endocrinology
- Molecular Pharmacology
- Oncology
Background:
- Prolactin (PRL)-secreting adenomas are often sensitive to dopamine (DA) agonists.
- DA resistance in lactotroph tumors presents a therapeutic challenge.
- Understanding D2 receptor structure is crucial for DA resistance mechanisms.
Purpose of the Study:
- To investigate structural anomalies of dopamine (DA) D2 receptors in DA-resistant lactotroph tumors.
- To compare D2 receptor structure between DA-sensitive adenomas and DA-insensitive tumors.
- To elucidate the molecular basis of DA resistance in PRL-secreting tumors.
Main Methods:
- Displacement studies using N-propylnorapomorphine (NPA) to assess receptor affinity states.
- Treatment with N-ethylmaleimide to probe receptor structure and function.
- Target size analysis and radiation inactivation studies to determine receptor molecular weight and structure.
- Ligand binding assays using [3H]spiperone and [3H]NPA.
Main Results:
- DA D2 receptors in adenomas exhibited both high- and low-affinity states, while resistant tumors only showed low-affinity states.
- N-ethylmaleimide treatment affected high-affinity D2 receptors in adenomas but not in resistant tumors.
- Structural analysis revealed distinct differences in D2 receptors between adenomas and tumors, and among tumors themselves.
- Evidence suggests tumoral D2 receptors may exist as various polymeric forms, unassociated with G proteins.
Conclusions:
- DA-resistant lactotroph tumors possess structurally anomalous D2 receptors.
- These anomalies, including altered receptor forms and lack of G protein association, likely cause insensitivity to DA agonists.
- The findings provide insight into the molecular mechanisms underlying DA resistance in pituitary tumors.