Real-time evaluation of somatostatin subtype 2 receptor activity employing the technique of cytosensor microphysiometry

Somatostatin and dopamine-somatostatin multiple ligands directed towards somatostatin and dopamine receptors in pituitary adenomas

AIM

We report the comparative efficacy of octreotide, cabergoline and multiple ligands directed towards the different somatostatin subtypes (ssts), such as BIM-23A779 and SOM-230, and of chimeric analogs which bind both somatostatin and the dopamine D2 receptors (D2R), such as BIM-23A760 and BIM-23A781, in cell cultures from human growth hormone (GH)-secreting pituitary adenomas.

PROCEDURES

RT-PCR analysis of the quantitative expression of the different ssts and D2R mRNAs was performed on tumor fragments of 22 GH-secreting adenomas collected after surgery. Pharmacological studies, using the different ligands, were performed on cell cultures of such tumors.

RESULTS

sst2, sst5 and D2R were constantly coexpressed in all tumors, in variable amounts. The levels of expression of sst2 and D2R mRNAs were significantly correlated with the maximal GH suppression by either octreotide or cabergoline (p < 0.001). In each tumor tested, 3 patterns of response, in terms of GH suppression, were observed. GH secretion was preferentially inhibited by the sst2 preferential compound octreotide in 61% of the tumors. In 19% of the tumors, the maximal inhibition of GH release was achieved with the sst5 preferential compound BIM-23268. The dopamine analog cabergoline was the most effective inhibitor of GH secretion in 21% of cases. Among the compounds tested, the most potent inhibitors of GH secretion were the sst2, sst5, D2R chimeric compound BIM-23A760, followed by the sst universal ligand SOM-230.

CONCLUSIONS

The variable patterns of response to sst2, sst5 and dopamine D2 analogs may explain the greater efficacy of drugs which bind to the 3 receptors in suppressing GH secretion. The biological potency (EC50) and efficacy of the chimeric compound BIM-23A760 on GH secretion can be partly explained by its high affinity for sst2. The effect of multiple receptor activation on the functions of other pituitary tumor types, such as prolactinomas and corticotropinomas, is not presently analyzed, and the efficacy of multireceptor ligands remains to be elucidated.

Efficacy of chimeric molecules directed towards multiple somatostatin and dopamine receptors on inhibition of GH and prolactin secretion from GH-secreting pituitary adenomas classified as partially responsive to somatostatin analog therapy

OBJECTIVE

This study compared the potency of a somatostatin receptor (sstr)2-sstr5 analog, BIM-23244, of an sstr2-dopamine D2 receptor (sstr2-DAD2) molecule, BIM-23A387 and of new somatostatin-dopamine chimeric molecules with differing, enhanced affinities for sstr2, sstr5 and DAD2, BIM-23A758, BIM-23A760 and BIM-23A761, to suppress GH and prolactin (PRL) from 18 human GH adenomas that are partially responsive to octreotide or lanreotide.

MATERIALS AND METHODS

The sstr2, sstr5 and DAD2 mRNA levels were determined by RT-PCR. The effect of drugs was tested in cell cultures at various concentrations.

RESULTS

In all tumors, the sstr2, sstr5 and DAD2 mRNA levels were coexpressed (mean levels+/-s.e.m. 0.4+/-0.1, 5.3+/-1.9 and 2.0+/-0.4 copy/copy beta-glucuronidase). In 13 tumors, the maximal suppression of GH secretion produced by BIM-23A387 (30+/-3%) and BIM-23244 (28+/-3%) was greater than that produced by octreotide (23+/-3%). In six out of 13 tumors, BIM-23A758, BIM-23A760 and BIM- 23A761 produced greater maximal suppression of GH secretion than octreotide (33+/-5, 38+/-2 and 41+/-2 vs 24+/-2%). Their EC(50) values were 10, 2 and 4 pmol/l. BIM-23A761 was more effective than BIM-23A387 in GH suppression (41+/-2 vs 32+/-4%). The new chimeric molecules produced maximal PRL suppression greater than octreotide (62+/-8 to 74+/-5 vs 46+/-11%).

CONCLUSIONS

Novel dopamine-somatostatin chimeric molecules with differing, enhanced activity at sstr2, sstr5 and DAD2, consistently produced significatly greater suppression of GH and PRL than either octreotide or single-receptor-interacting ligands in tumors from patients classified as only partially responsive to octreotide therapy. The higher efficacy of the chimeric compounds was, at least partially, linked to their high affinity for sstr2 (IC50 1-10 pmol/l). The other mechanisms by which such molecules produce an enhanced inhibition of GH remain to be elucidated.

Somatostatin receptor subtype 1-selective activation reduces cell growth and calcitonin secretion in a human medullary thyroid carcinoma cell line

Medullary thyroid carcinoma (MTC) is a rare and aggressive tumor and so far medical therapy has provided inconclusive results. In the human MTC cell line TT, expressing all somatostatin (SST) receptor subtypes, cell proliferation decreases with SST and SST receptor subtype 2 (sst(2)), but not sst(5), selective agonist treatment, whereas calcitonin (CT) expression and secretion are reduced by SST, but not by sst(2) and sst(5) agonists. The effectiveness of two new SST analogs, BIM-23926 and BIM-23745, selectively interacting with sst(1), was investigated in the TT cell line. DNA synthesis is significantly reduced by BIM-23926 (27-40% at 10(-10)-10(-6)M) and BIM-23745 (32-90% at 10(-8)-10(-6)M). Viable cell number is also significantly reduced by both BIM-23926 (40% at 10(-12)-10(-6)M) and BIM-23745 ( approximately 40% at 10(-10)-10(-6)M). Treatment with sst(1)-selective agonists significantly reduces CT secretion and gene expression, with a reduction of CREB phosphorylation. These findings suggest that potent sst(1)-selective agonists could have a therapeutic role in MTC.

Biphasic response to human galanin of extracellular acidification in human Bowes melanoma cells

The metabolic response of galanin GAL1 receptor subtype, endogenously expressed in human Bowes melanoma (HBM) cells, was investigated. Cytosensor microphysiometry was used to determine the extracellular acidification rate. A biphasic response, consisting of a rapid increase in the extracellular acidification rate followed by a decrease below the basal level, was observed after perfusion with human galanin. The magnitude and the rate of onset of both phases were dependent on the galanin concentration. The increase in the extracellular acidification rate (maximum of 25% of basal level; -log(EC(50))=7.23+/-0.14) was transient, whereas the following decrease (maximum of 40% of basal level; -log(EC(50))=7.77+/-0.23) was sustained. The EC(50) values for the increase and decrease were in a similar range. After consecutive galanin administration, the magnitude of the response was the same as for the unexposed cells, indicating the absence of galanin receptor desensitization or internalization in HBM cells. Responses were blocked by pretreatment with pertussis toxin and phorbol-12-myristate-13-acetate (PMA), indicating a G-protein/protein kinase C signalling pathway. Our microphysiometry results show a biphasic response of the extracellular acidification rate mediated by the galanin receptor expressed in HBM cells which has not been described previously for any other endogenously expressed neuropeptide receptor.

Rational design of a peptide agonist that interacts selectively with the orphan receptor, bombesin receptor subtype 3

The orphan receptor, bombesin (Bn) receptor subtype 3 (BRS-3), shares high homology with bombesin receptors (neuromedin B receptor (NMB-R) and gastrin-releasing peptide receptor (GRP-R)). This receptor is widely distributed in the central nervous system and gastrointestinal tract; target disruption leads to obesity, diabetes, and hypertension, however, its role in physiological and pathological processes remain unknown due to lack of selective ligands or identification of its natural ligand. We have recently discovered (Mantey, S. A., Weber, H. C., Sainz, E., Akeson, M., Ryan, R. R. Pradhan, T. K., Searles, R. P., Spindel, E. R., Battey, J. F., Coy, D. H., and Jensen, R. T. (1997) J. Biol. Chem. 272, 26062-26071) that [d-Tyr(6),beta-Ala(11),Phe(13),Nle(14)]Bn-(6-14) has high affinity for BRS-3 and using this ligand showed BRS-3 has a unique pharmacology with high affinity for no known natural Bn peptides. However, use of this ligand is limited because it has high affinity for all known Bn receptors. In the present study we have attempted to identify BRS-3 selective ligands using a strategy of rational peptide design with the substitution of conformationally restricted amino acids into the prototype ligand [d-Tyr(6),beta-Ala(11),Phe(13),Nle(14)]Bn-(6-14) or its d-Phe(6) analogue. Each of the 22 peptides synthesized had binding affinities determined for hBRS-3, hGRPR, and hNMBR, and hBRS-3 selective ligands were tested for their ability to activate phospholipase C and increase inositol phosphates ([(3)H]inositol phosphate). Using this approach we have identified a number of BRS-3 selective ligands. These ligands functioned as receptor agonists and their binding affinities were reflected in their potencies for altering [(3)H]inositol phosphate. Two peptides with an (R)- or (S)-amino-3-phenylpropionic acid substitution for beta-Ala(11) in the prototype ligand had the highest selectivity for the hBRS-3 over the mammalian Bn receptors and did not interact with receptors for other gastrointestinal hormones/neurotransmitters. Molecular modeling demonstrated these two selective BRS-3 ligands had a unique conformation of the position 11 beta-amino acid. This selectivity was of sufficient magnitude that these should be useful in explaining the role of hBRS-3 activation in obesity, glucose homeostasis, hypertension, and other physiological or pathological processes.

Somatostatin receptor-mediated arachidonic acid mobilization: evidence for partial agonism of synthetic peptides

1. Somatostatin and the stable octapeptide analogues, octreotide and angiopeptin, were examined for their ability to stimulate the release of tritium from [(3)H]-arachidonic acid pre-loaded CHO-K1 cells expressing human recombinant sst(2), sst(3) or sst(5) receptors. 2. Somatostatin stimulated tritium release (pEC(50)) through the sst(2) (7.8+/-0.1) and sst(5) (7.3+/-0.2), but not the sst(3) receptor. Octreotide behaved as a full (sst(2) receptor) or partial agonist (sst(5) receptor), whereas angiopeptin behaved as a weak partial agonist at both receptor types. 3. Maximum responses to somatostatin through both receptor types were significantly reduced by pertussis toxin, whereas pEC(50) estimates were unaffected. 4. Inhibition of MEK1 or Src, but not PKA, PI 3-kinases or tyrosine kinases, by reportedly selective inhibitors reduced sst(2)-mediated responses by somatostatin, but not angiopeptin. A selective inhibitor of PKC (Ro-31-8220) reduced both somatostatin and angiopeptin responses. 5. These data provide further evidence for partial agonist activity of synthetic peptides of somatostatin. Furthermore, the somatostatin receptor signalling mechanisms which mediate arachidonic acid mobilization appear to be multiple and complex.

Effects of very high antibiotic concentrations on human intervertebral disc cell proliferation, viability, and metabolism in vitro

STUDY DESIGN

Four antibiotics commonly used during spinal surgery (cefazolin, gentamycin, cefamandole, and vancomycin) were tested for their effects on cultured human disc cells from the anulus.

OBJECTIVE

To determine the viability, proliferation, and metabolism of cells cultured from the human anulus after they were exposed to four antibiotics.

SUMMARY OF BACKGROUND DATA

Previous studies concerning the effect of antibiotics on the disc have used animal models or explanted discs, but little is understood about the effect of antibiotics on the proliferation, viability, and metabolism of cells from the anulus.

METHODS

In this study, 3H-thymidine incorporation, trypan blue exclusion, and cell metabolism were determined using cells from the human anulus grown in monolayer culture. The latter measurement used a cytosensor microphysiometer to monitor the rate at which cells acidified their microenvironment, an event that is proportional to cellular metabolism because it reflects the excretion over time of acidic products such as lactic acid from glycolysis and CO2 from cellular respiration.

RESULTS

After 48 hours of antibiotic exposure, cell viability was significantly lower as a result of all four antibiotics at the highest concentration tested. Cell proliferation was lower after exposure to cefazolin and cefamandole. During a 6-hour antibiotic exposure, anulus cells in the highest concentration of cefamandole or vancomycin displayed a significantly decreased rate of cell metabolism.

CONCLUSIONS

These findings show that high doses of antibiotics can have direct, deleterious effects on cultured disc cell survival, cell proliferation, and metabolic rates. Discitis is a serious primary or postoperative complication that often requires prolonged antibiotic treatment. Studies such as the current investigation with cultured cells from the anulus show the importance of a greater understanding concerning antibiotic effects on disc cell proliferation and metabolism.

Differential agonist activity of somatostatin and L-362855 at human recombinant sst4 receptors

1. The operational characteristics of somatostatin (SRIF) sst4 receptors are poorly understood. In this study, we have characterized human recombinant sst4 receptors expressed in CHO cells (CHOsst4) by radioligand binding and microphysiometry. 2. Increasing concentrations SRIF or other SRIF receptor ligands inhibited specific [125I]-Tyr11-SRIF binding in CHOsst4 cell membranes with respective pIC50 values of SRIF (8.82), L-362855 (7.40), BIM-23027 (<5.5) and MK-678 (<5.5). 3. These ligands displayed agonist activity, producing concentration-dependent increases in rates of extracellular acidification (EAR) with pEC50 values of SRIF (9.6) and L-362855 (8.0), respectively. BIM-23027 and MK-678 were at least 1000 times weaker than SRIF. The SRIF maximum was about 40% of that observed with L-362855. 4. In the presence of SRIF (0.1-1 nM), concentration-effect curves to L-362855 were displaced to the right with a progressive reduction in the L-362855 maximum. 5. When cells were only exposed to a single maximally effective concentration of SRIF or L-362855, there was no difference in the magnitude of the agonist-induced increase in EAR. However, a second agonist challenge, 30 min later showed that responses to SRIF but not L-362855 were markedly desensitized. 6. When concentration-effect curves to SRIF and L-362855 were obtained by combining data from cells exposed to only a single agonist concentration, SRIF (pEC50 9.2) was approximately 20 times more potent than L-362855 (pEC50 8.0) but the maxima were the same. Responses to both SRIF and L-362855 were abolished by pertussis toxin. 7. SRIF and L-362855-induced increases in EAR were inhibited by N-ethyl isopropyl amiloride (10 microM) but were not modified by inhibitors of PKC (Go-6976), MAP kinase (PD-98059), tyrosine kinase (genistein) or tyrosine phosphatase (sodium orthovanadate). 8. The results suggest that SRIF-induced increases in EAR in CHOsst4 cells involved activation of the Na+/H+ antiporter and were mediated via Gi/Go G proteins. Responses to SRIF, but not L-362855, were subject to marked desensitization which may be a consequence of differential activation of receptor-effector coupling pathways.