17-BETA-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 th e regulation of somatostatin receptor subtype (sat) expression in 7315 b PRL-secreting rat pituitary tumor cells in vitro and in uiuo. sst we re 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) i nstead of FCS, no sst were detectable on 7315b cells even after three weeks of culturing. in contrast to HS, FCS contains high E(2)-levels ( HS, 8 pM; FCS, 134 pM). The antiestrogen tamoxifen (0.5 mu M) signific antly inhibited the sst number to 50.5% of the value of untreated FCS- grown cells, suggesting that E(2) stimulates sst expression in 7315b r at pituitary tumor cells. E(2) (10 nM) induced a rapid increase in sat number in HS-grown 7315b cells. Octreotide (1 mu M) significantly inh ibited PRL release and the intracellular PRL concentration of 7315b ce lls that were cultured in medium supplemented with FCS or with HS + 10 nM E(2) but not in HS alone. This indicates that the sst present on t hese cells are biologically active. RT-PCR analysis revealed that none of the five currently known sat subtypes were present in freshly disp ersed 7315b pituitary tumor cells. The expression of sst(2)- and sst(3 )- messenger RNA (mRNA) was unequivocally correlated to the presence o f E(2) because these sst subtypes were detected only in cells that wer e cultured for 7 and 14 days in medium supplemented with FCS or with R S + 10 nM E(2). sst(1), sst(4) and sst(5) messenger RNA could not be d etected. The 7315b tumor itself synthesizes and secretes huge amounts of PRL. The high PRL levels in tumor-bearing rats inhibit the ovarian E(2)-production. No detectable E(2) levels could be measured in the se rum of 7315b tumor-bearing rats. The sc administration of 20 mu g/day E(2)-benzoate normalized the circulating E(2) levels in 7315b tumor-be aring rats. Moreover, E(2)-treatment indeed induced sst expression in vivo as shown by ligand binding studies using membrane homogenates and [I-125-Tyr(3)]-octreotide as radioligand and by autoradiography on ti ssue sections. In agreement with the in vitro studies, the expression of the sst(2) subtype was established by RT-PCR analysis in 7315b tumo rs of E(2)-treated rats. However, in contrast to the in vitro studies, E(2)-treatment did not effectuate the expression of the sst(3) subtyp e, suggesting that the in vitro stimulus of E(2) is stronger. In concl usion: 1) sst(2) and sat(3) expression in the 7315b rat prolactinoma m odel is primarily dependent upon the presence of estrogens; 2) the ant ihormonal action of octreotide in 7315b tumor cells in. vitro is media ted via the sst(2) and/or sat(3) subtypes; 3) the absence of sat expre ssion in vivo can be explained by the hormonal environment of the 7315 b tumor cells. The 7315b tumor cells in vivo may downregulate their ow n receptor status via their host, because of the ensuing hyperprolacti nemia results in a hypo-estrogenic state.