17-β-Estradiol-dependent regulation of somatostatin receptor subtype expression in the 7315b prolactin secreting rat pituitary tumor in vitro and in vivo

In the present study, we have investigated the role of estrogens in the regulation of somatostatin receptor subtype (sst) expression in 7315b PRL- secreting rat pituitary tumor cells in vitro and in vivo. sst were undetectable in freshly dispersed cells of the transplantable 7315b tumor. When 7315b cells were cultured in medium containing 10% FCS, the number of high affinity sst increased with prolonged culture time. However, when the medium was supplemented with 10% horse serum (HS) instead of FCS, no sst were detectable on 7315b cells even after three weeks of culturing. In contrast to HS, FCS contains high E2-levels (HS, 8 pM; FCS, 134 pM). The antiestrogen tamoxifen (0.5 μM) significantly inhibited the sst number to 50.5% of the value of untreated FCS-grown cells, suggesting that E2 stimulates sst expression in 7315b rat pituitary tumor cells. E2 (l0 nM) induced a rapid increase in sst number in HS-grown 7315b cells. Octreotide (1μM) significantly inhibited PRL release and the intracellular PRL concentration of 7315b cells that were cultured in medium supplemented with FCS or with HS + l0 nM E2 but not in HS alone. This indicates that the sst present on these cells are biologically active. RT-PCR analysis revealed that none of the five currently known sst subtypes were present in freshly dispersed 7315b pituitary tumor cells. The expression of sst2- and sst3- messenger RNA (mRNA) was unequivocally correlated to the presence of E2 because these sst subtypes were detected only in cells that were cultured for7 and 14 days in medium supplemented with FCS or with HS + 10 nM E2. sst1, sst4 and sst5 messenger RNA could not be detected. The 7315b tumor itself synthesizes and secretes huge amounts of PRL. The high PRL levels in tumor-bearing rats inhibit the ovarian E2-production. No detectable E2 levels could be measured in the serum of 7315b tumor-bearing rats. The sc administration of 20 μg/day E2-benzoate normalized the circulating E2 levels in 7315b tumor- bearing rats. Moreover, E2-treatment indeed induced sst expression in vivo as shown by ligand binding studies using membrane homogenates and [125I- Tyr3]-octreotide as radioligand and by autoradiography on tissue sections. In agreement with the in vitro studies, the expression of the sst2 subtype was established by RT-PCR analysis in 7315b tumors of E2-treated rats. However, in contrast to the in vitro studies. E2-treatment did not effectuate the expression of the sst3 subtype, suggesting that the in vitro stimulus of E2 is stronger. In conclusion: 1) sst2 and sst3 expression in the 7315b rat prolactinoma model is primarily dependent upon the presence of estrogens; 2) the antihormonal action of octreotide in 7315b tumor cells in vitro is mediated via the sst2 and/or sst3 subtypes; 3) the absence of sst expression in vivo can be explained by the hormonal environment of the 7315b tumor cells. The 7315b tumor cells in vivo may down-regulate their own receptor status via their host, because of the ensuing hyperprolactinemia results in a hypo-estrogenic state.</p

Somatostatin receptor manipulation

The expression of somatostatin receptors (ssts) on human tumours is the basis for the successful therapeutic and diagnostic application of (radiolabelled) somatostatin analogues. Manipulation (up-regulation) of sst expression might improve the uptake of radioligand in in vivo scintigraphy of human sst-positive tumours, as well as the potential success of radiotherapy using radiolabelled SRIF analogues. In clonal pituitary cell lines, agonist exposure (SRIF-14, SRIF-28, octreotide) has been shown to either reduce or increase sst (subtype) expression, suggesting cell-type-specific responsiveness. In addition, glucocorticoids and oestrogens were shown to down- and up-regulate, respectively, sst numbers. So far, little information is available with respect to sst (subtype) regulation in non-pituitary-derived cell types. We have found that sst expression in the model of the transplantable prolactin (PRL)-secreting rat pituitary tumour 7315b is mainly dependent upon the presence of oestradiol (E2), both in vivo and in vitro. This tumour is sst negative in vivo. In vitro, the addition of E2 induces sst expression (sst2 and sst3 subtypes). The in vivo administration of E2 (20 μg/day subcutaneously) to 7315b-tumour-bearing rats induces sst2 mRNA expression. The absence of sst expression in 7315b tumours in vivo may be due to the inhibition of ovarian E2 production by the high circulating PRL levels in the 7315b-prolactinoma-bearing rats. Indeed, no detectable E2 levels were found in the serum of 7315b-tumour-bearing rats. Taken together, our data suggest that the 7315b rat prolactinoma can indirectly manipulate (down-regulate) its own sst expression, in vivo, via its host. This experimental 7315b prolactinoma model might be representative for most untreated female prolactinoma patients. Clinically, patients with microprolactinomas do not benefit from octreotide treatment.</p

Somatostatin receptor manipulation

The expression of somatostatin receptors (ssts) on human tumours is the basis for the successful therapeutic and diagnostic application of (radiolabelled) somatostatin analogues. Manipulation (up-regulation) of sst expression might improve the uptake of radioligand in in vivo scintigraphy of human sst-positive tumours, as well as the potential success of radiotherapy using radiolabelled SRIF analogues. In clonal pituitary cell lines, agonist exposure (SRIF-14, SRIF-28, octreotide) has been shown to either reduce or increase sst (subtype) expression, suggesting cell-type-specific responsiveness. In addition, glucocorticoids and oestrogens were shown to down- and up-regulate, respectively, sst numbers. So far, little information is available with respect to sst (subtype) regulation in non-pituitary-derived cell types. We have found that sst expression in the model of the transplantable prolactin (PRL)-secreting rat pituitary tumour 7315b is mainly dependent upon the presence of oestradiol (E2), both in vivo and in vitro. This tumour is sst negative in vivo. In vitro, the addition of E2 induces sst expression (sst2 and sst3 subtypes). The in vivo administration of E2 (20 μg/day subcutaneously) to 7315b-tumour-bearing rats induces sst2 mRNA expression. The absence of sst expression in 7315b tumours in vivo may be due to the inhibition of ovarian E2 production by the high circulating PRL levels in the 7315b-prolactinoma-bearing rats. Indeed, no detectable E2 levels were found in the serum of 7315b-tumour-bearing rats. Taken together, our data suggest that the 7315b rat prolactinoma can indirectly manipulate (down-regulate) its own sst expression, in vivo, via its host. This experimental 7315b prolactinoma model might be representative for most untreated female prolactinoma patients. Clinically, patients with microprolactinomas do not benefit from octreotide treatment.</p