Objective: The molecular mechanisms underlying ACTH-secreting tumour formation remain unknown. Transmembrane signalling pathways play an important role in several endocrine disorders including pituitary tumours. To investigate the role of the pituitary vasopressin (V3) receptor (R) in ACTH-secreting tumours we have qualitatively and quantitatively analysed its mRNA.
Design: RT-PCR, denaturing gradient gel electrophoresis and S1 nuclease protection experiments were used to analyse V3 mRNA structure in ACTH-secreting tumours. We also developed a competitive RT-PCR system to compare the levels of expression of POMC, V3 and CRH-R genes. This system used as competitor a single mutant template (termed multi-mutant) containing primers for the three genes flanking an unrelated core sequence allowing multiple quantifications from the same cDNA preparations. We analysed 12 normal pituitaries, 15 corticotroph pituitary adenomas and 6 ACTH-secreting bronchial carcinoids.
Results: The V3 mRNA structure and sequence were found to be identical in normal and tumoural pituitary indicating that the tumoural Vs mRNA codes for a normal receptor. POMC RT-PCR signals in the pituitary tumour group were approximately 7-fold higher than in the normal pituitary group. Similarly, V3 and CRH-R signal were increased in pituitary tumors (mean +/- SEM: 5.87 x 10(-6) +/- 1.73 x 10(-6), and 2.33 x 10(-4) +/- 1.4 x 10(-4), respectively), when compared to normal pituitaries (1.19 x 10(-7) +/- 2.39 x 10(-8), and 1.7 x 10(-6) +/- 4.65 x 10(-7), respectively) suggesting that these two genes are expressed at very high levels in corticotroph tumours. When expressed relative to the corresponding POMC signals, increases in V3 and CRH-R signals reached 49-fold and 137-fold, respectively, in pituitary tumours. In ACTH-secreting bronchial carcinoids V3 gene expression level was also higher than in normal pituitary, whereas CRH-R signals were detected in only 4 of the 6 tumours with wide variations.
Conclusion: Our results show that both vasopressin and CRH receptor genes are overexpressed in ACTH-secreting pituitary tumours. They suggest that overexpression of G protein-coupled receptors may be an additional mechanism through which membrane receptors may play a role in human tumours.