The human 5-hydroxytryptamine 1A receptor differentially modulates phospholipase C and adenylyl cyclase activities

The Role of Serotonin (5-Hydroxytryptamine 1A and 1B ) Receptors in Prostate Cancer Cell Proliferation

PURPOSE

Serotonin (5-hydroxytryptamine), a monoamine neurotransmitter released by prostate neuroendocrine cells, has a fundamental role in tumor growth, differentiation and gene expression. We investigated the effect of 5-hydroxytryptamine and 5-hydroxytryptamine antagonists on the growth of prostate cancer cells and we identified 5-hydroxytryptamine receptor expression in PC3 cells and in human hormone refractory prostate cancer tissue.

MATERIALS AND METHODS

A total of 12 preparations of hormone refractory PC3 human prostate cancer cells were incubated with 5-hydroxytryptamine, or the 5-hydroxytryptamine receptor antagonists 5-hydroxytryptamine1A, 1B, 1D, 2, 3 or 4. After 72 hours cell viability was assessed using the crystal violet assay. PC3 cells treated with 5-hydroxytryptamine1A and 1B antagonists were investigated for apoptosis using flow cytometry. PC3 cells and sections of hormone refractory human prostate cancer tissue were studied by immunohistochemistry and Western blot analysis.

RESULTS

In PC3 cells 5-hydroxytryptamine caused dose dependent proliferation with a maximum increase of 15% in 12 preparations at a concentration of 10(-8) M at 72 hours compared to controls (p < 0.0001). At a concentration of 10(-4) M at 72 hours the 5HT1A antagonist NAN-190 hydrobromide and the 5-hydroxytryptamine1B antagonist SB224289 HCl (Tocris Laboratories, Bristol, United Kingdom) induced a 20% and 78% inhibitory effect, respectively, on PC3 cell growth compared to that in controls (p < 0.0001). In PC3 cells 5-hydroxytryptamine1A and 1B antagonists demonstrated apoptosis after 24 and 48 hours of incubation. Immunostaining for 5-hydroxytryptamine1A and 1B receptors was seen in PC3 cells and prostate cancer tissue. Western blot analysis demonstrated 5-hydroxytryptamine1A and 1B receptor proteins with 46 and 43 kDa bands, respectively.

CONCLUSIONS

In PC3 prostate cancer cells 5-hydroxytryptamine1A and to a greater extent 5-hydroxytryptamine1B antagonists significantly inhibit growth and induce apoptosis. To our knowledge growth inhibition caused by the 5-hydroxytryptamine1B antagonist SB224289 HCl is a novel finding, as is apoptosis caused by the 2 antagonists 5-hydroxytryptamine1A and 1B. This effect is most likely mediated via 5-hydroxytryptamine1A and 1B receptors. Therefore, our results imply that 5-hydroxytryptamine1A and in particular 5-hydroxytryptamine1B receptor antagonists warrant further investigations as potential anti-neoplastic agents.

Binding properties of dipropyltryptamine at the human 5-HT1a receptor

Dipropyltryptamine (DPT) is a synthetic indolealkylamine first characterized in the 1960s. Largely forgotten since the discovery of multiple serotonin receptor subtypes, some of the properties of DPT at the cloned human 5-HT1a receptor are described here. When [3H]8-OH-DPAT is bound to the receptor, DPT inhibits the interaction with an IC50 of 0.1 micromol/l. This interaction is shown to be competitive when double-reciprocal plots of the DPT/agonist interaction are analyzed. DPT's effects in the signal transduction system are complex. While DPT alone (0.1-1,000 micromol/l) activates Gi when both cAMP and gamma-S-GTP incorporation are measured, in the presence of 5-HT (0.1-10 micromol/l), DPT blocks the agonist effect. In combination, the findings suggest that DPT is a moderate affinity partial agonist at the human 5-HT1a receptor. These results provide evidence that DPT has potential as a versatile experimental tool at 5-HT1a receptors.

The recombinant 5-HT1A receptor: G protein coupling and signalling pathways

The 5-hydroxytryptamine 5-HT1A receptor was one of the first G protein coupled receptors whose cDNA and gene were isolated by molecular cloning methods. Transfection of the cDNA of this receptor into cells previously bearing no 5-HT receptors has resulted in the acquisition of large amounts of information regarding potential signal transduction pathways linked to the receptor, correlations of receptor structure to its various functions, and pharmacological properties of the receptor. Transfection studies with the 5-HT1A receptor have generated critical new information that might otherwise have been elusive. This information notably includes the discovery of unsuspected novel signalling linkages, the elucidation of the mechanisms of receptor desensitization, the refinement of models of the receptor pharmacophore, and the development of silent receptor antagonists, among others. The current review summarizes the most important studies of the recombinant 5-HT1A receptor in the decade since the identification of its cDNA.

The human 5-HT1A receptor: comparison of its binding properties in transfected cells and cortical tissue

1. The binding characteristics of tritium labeled 8-hydroxy-dipropyl-aminotetralin, or [3H]8-OH-DPAT, to the serotonin1A (5-HT1A) receptor in the stably transfected HeLa cell clone HA6 and in human cortical tissue were examined and compared. 2. A series of kinetic studies of [3H]8-OH-DPAT binding to the transfected HA6 cell line demonstrated two components in both the association and the dissociation reactions. 3. In saturation experiments, at least two affinity states were unequivocally detected in the HA6 cell line and the human cortical tissue. Using isotopic dilutions, the binding isotherms were best fitted to a two-site model, and similar affinity values were obtained in both systems (KH approximately 1.1 nM and KL approximately 12-223 nM). 4. Most of the drugs used in competitions inhibited [3H]8-OH-DPAT binding, following a two-site model, and maintained their rank order of binding potency in both systems; that is, 5-HT > or = 8-OH-DPAT > buspirone > pindolol. Inconsistencies, however, were found for the antagonists NAN-190 and pindolol; only one inhibition constant was determined for HA6 cells, but two affinities were detected with cortical tissue. 5. The results indicate that, although data from binding studies using the cell expression system reflect, to a certain extent, those obtained with the cortical tissue, some discrepancies remained. 6. Finally, and in contrast with what is observed with the 5-HT1A receptor expressed in the HA6 cell line, it is possible that different receptors, or subtypes of one receptor, or even uptake sites normally expressed in cortical tissue, could interact with [3H]8-OH-DPAT or the competing drugs or both, thus leading to the observation of additional binding sites.

Functional characteristics of heterologously expressed 5-HT receptors

Over the past 10 years, molecular cloning has revealed the presence of 15 serotonin (5-hydroxytryptamine; 5-HT) receptor subtypes, which can be subdivided in seven subfamilies. Except for the 5-HT3 receptors, which are ligand-gated ion channels, all 5-HT receptors belong to the superfamily of G-protein-coupled receptors. The large multiplicity of 5-HT receptor subtypes has been suggested to be a direct result of the evolutionary age of the 5-HT system. Molecular information on G-protein-coupled 5-HT receptors is currently available for several mammalian species as well as for a limited number of invertebrate species (insects, molluscs). The aim of this review is to give an overview of all cloned 5-HT receptor subtypes belonging to the superfamily of G-protein-coupled receptors with specific emphasis on the pharmacological and signaling properties of the receptors upon expression in several heterologous expression systems.

Inhibition of cAMP accumulation via recombinant human serotonin 5-HT1A receptors: considerations on receptor effector coupling across systems

Inhibition of forskolin-stimulated cyclic AMP accumulation was measured in two stable HeLa cell lines HA6 and HA7 expressing different levels of recombinant human 5-HT1A receptors. These cells were studied previously to characterize another second messenger system activated by 5-HT1A receptors, i.e. calcium mobilization. The pharmacological characterization of the inhibition of cyclic AMP accumulation was made using agonists (5-HT, 8-OH-DPAT, buspirone, MDL 73005) and putative antagonists (SDZ 216-525, NAN-190, WAY-100135, pindolol, propranolol, WAY 100635). It is shown that 5-HT, 8-OH-DPAT, buspirone, MDL 73005 behaved as full (or nearly full) and potent agonists, whereas SDZ 216-525, NAN-190 and WAY-100135 displayed a limited (and similar) degree of intrinsic activity at human 5-HT1A receptors; on the other hand pindolol, propranolol and WAY 100635 behaved as "silent" antagonists. The effects were quantitatively and qualitatively very similar in both cells lines for all drugs tested, suggesting that the coupling between 5-HT1A receptors and inhibition of cyclic AMP accumulation in HeLa cells is very tight. There were, however, significant variations in both the level of agonism and the potency of a number of compounds when calcium mobilization and the inhibition of cyclic AMP accumulation were compared. Especially in HA7 cells which express lower receptor levels, a number of drugs failed to display agonism (e.g. buspirone or MDL 73005), whereas in HA6 cells they acted as partial agonists. Together, the data show that functional responses mediated by the same receptor can vary rather dramatically depending on receptor density and/or on the effector system involved. Interestingly, 5-HT1A receptor-mediated inhibition of adenylate cyclase activity measured in calf hippocampal membranes shows very similar degrees of potency and intrinsic activity for a number of compounds that have been tested on the inhibition of cyclic AMP accumulation in HeLa cells, suggesting that the very tight coupling observed in the recombinant system may apply to native 5-HT1A receptors.