Dopamine-agonist cabergoline (CB) reduces prolactin (PRL) secretion and tumor size in 80% of patients with prolactin-secreting adenomas (PRL-omas) by binding type 2 dopamine receptor (DRD2). The mechanisms responsible for resistance to CB remain largely unknown. To assess the association of DRD2 with sensitivity to CB, TaqI-A1/A2, TaqI-B1/B2, HphI-G/T and NcoI-C/T genotypes were determined in a cross-sectional retrospective study, including 203 patients with PRL-oma. DRD2 alleles frequencies did not differ between patients and 212 healthy subjects. Conversely, NcoI-T allele frequency was higher in resistant rather than responsive patients, considering both PRL normalization (56.6 vs 45.3%, P ¼ 0.038) and tumor shrinkage (70.4 vs 41.4%, P ¼ 0.006). Finally, [TaqI A1À/TaqI B1À/HphI TÀ/NcoI TÀ] haplotype was found in 34.5% of patients normalizing PRL with p3 mg/ week of CB vs 11.3% of resistants (P ¼ 0.021). In conclusion, resistance to CB was associated with DRD2 NcoI-T þ allele, consistent with evidence suggesting that this variant may lead to reduction and instability of DRD2 mRNA or protein.

It has been reported that retinoids may affect hypothalamic-pituitary-thyroid axis, causing central hypothyroidism. In the present study, we evaluated pituitary function in 11 male psoriatic patients at baseline and after 1 and 3 months of treatment with acitretin (all-trans retinoic acid, 35 mg/day). Serum LH, FSH, testosterone, cortisol, GH and IGF-I levels were not affected by the treatment. By contrast, we observed a significant decrease in TSH levels (from 0.92 +/- 0.3 to 0.80 +/- 0.3 mU/I, p < 0.05) at 1 month, that reverted to baseline after 3 months. No change in free T4 (FT4) levels occurred, while free T3 (FT3) levels were reduced at 1 and 3 months (from 6.7 +/- 0.5 to 6.2 +/- 0.3 and 6.1 +/- 0.6 pmol/l; p < 0.05, respectively). Moreover, acitretin treatment induced a significant reduction of PRL levels after 3 months (from 182 +/- 70 to 150 +/- 56 mU/l, p < 0.05). During treatment, no change in TSH and PRL response either to TRH or dopamine infusion was observed. In conclusion, we demonstrated that treatment with low dose of acitretin induced a series of hormonal modifications that, in addition to a mild and transient reduction of TSH levels, included a persistent reduction of FT3, probably due to changes in thyroid hormone metabolism, and a decrease in PRL levels.

The dopamine receptor subtype 2 (D2R) promoter contains a functional retinoic acid response element involved in the control of D2R expression. The aim of the study was to evaluate the effect of 9-cis retinoic acid (9-cis RA) on D2R protein expression in human pituitary adenomas and GH3 cell line. Treatment with 9-cis RA (100 nM for 48 hrs) caused a 109 +/- 32% increase of basal D2R levels in five of eight growth hormone (GH)-secreting adenomas (GH-omas), a 129 +/- 28% increase in 7 of 11 nonfunctioning adenomas, and no effect in two resistant prolactinomas by Western blotting. The lack of D2R induction in some tumors was not associated with a different pattern of retinoid x receptor (RXR) and retinoic acid receptor (RAR) isoform expression that was similar in all tumors by immunohistochemistry. While the induction of D2R did not affect the slight but significant inhibitory effect exerted by dopamine (10 nM) on in vitro GH release by GH-oma cultured cells, in pituitary GH3 cell lines cis-9 RA enhanced the dopamine-induced inhibition of in vitro GH release (% inhibition: 16 +/- 2 versus 26 +/- 5, P < 0.05), cell proliferation (25 +/- 2% versus 44 +/- 5%, P < 0.05) and cell viability (16 +/- 0.8% versus 29 +/- 1%, P < 0.05), likely by activating caspase-3 (28 +/- 3% versus basal, P < 0.05). In conclusion, this study provides novel evidence for a permissive role of retinoids on the expression of D2R in a good proportion of pituitary tumors and on the generation of pro-apoptotic signals in GH3 cell line.