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Characterization of melatonin receptors in human placental trophoblasts and prostate cancer

by Lau, Kai-wing

Abstract (Summary)
(Uncorrected OCR) Abstract of thesis entitled

Characterization of melatonin receptors in human placental trophoblasts and prostate cancer

Submitted by

KaiWingLAU

for the degree of Master of Philosophy at The University of Hong Kong

in October 2002

Previous studies have shown that the direct anti-proliferative signals of melatonin, a pineal gland neurohormone, could be transduced, in part, via MT 2 melatonin receptor subtype in placental choriocarcinoma JAr and possibly JEG-3 cells, and via MT1 receptor subtype in human prostate cancer LNCaP cells. In this study, human MT1 and MT2 melatonin receptors were expressed in 3A-Sub-E, a post-crisis SV40-transformed human placental trophoblast cell line previously found to express neither receptor subtypes, and were characterized by radioligand binding using 2-e25I]iodomelatonin. The effects of melatonin or 2-iodomelatonin (a melatonin membrane receptor agonist) on the proliferation of3A-Sub-E cells stably expressing either MT1 or MT2 membrane receptors were also examined. Furthermore, melatonin receptors in human prostate cancer tissue were characterized by 2-e25I]iodomelatonin binding.

Both melatonin and 2-iodomelatonin inhibited JAr cell proliferation. The proliferation of JEG-3 cells was inhibited by melatonin but not by 2-iodomelatonin. These results indicated that MT 2 receptor subtype contributed to the transduction of melatonin anti-proliferative signal in JAr cells, but not in JEG-3 cells. Both MT1 and MT2 receptor subtypes expressed in transfected 3A-Sub-E cells were saturable and of high affmity as assayed by 2-e25I]iodomelatonin binding. Moreover, 2-e25I]iodomelatonin binding to 3A-Sub-E-MT1 and 3A-Sub-E-MT2 cells was sensitive to GTPyS and pertussis toxin, indicating coupling of both receptor subtypes to Gi/o proteins. Pharmacological profiles of MT 1 and MT 2 receptor subtypes constructed from competition curves using melatonergic compounds were highly comparable to those reported for other transfected

cells or native tissues. Disappointingly, physiological and pharmacological concentrations of melatonin did not inhibit the proliferation of both 3A-Sub-E-MT1 and 3A-Sub-E-MT2 cells. The data suggested that expression of melatonin receptors alone in 3A-Sub-E cells did not confer sensitivity to melatonin anti-proliferation, even the receptors demonstrated similar pharmacological properties to endogenous receptors.

Notably, melatonin receptors were detected by 2-e25I]iodomelatonin binding in the membrane preparations of a human prostate cancer sample. Saturation study revealed a single class of high-affinity binding sites. Pharmacological profile of the melatonin receptors in the cancer sample was clearly more comparable to those ofMTl than to MT2 receptors expressed in transfected 3A-Sub-E or PC-3 cells. The expression of MT1 receptor protein In prostate cancer tissue was further confirmed by immunohistochemistry using anti-MTI receptor serum. As MT1 receptor has been shown to playa significant role in transducing the direct anti-proliferative actions of melatonin on human prostate cancer LNCaP cells, expression of MT1 receptors that can bind to melatonin in the human tissue studied indicates that the prostate cancer cells in this particular patient are very likely to respond to the oncostatic actions of melatonin. Given that melatonin has been reported to inhibit the growth of prostate cancer cells under androgen-deprived condition in vivo, it is therefore worth considering the possibility of using melatonin to treat this patient when his tumor relapses from castration-induced rermSSlOn.

Bibliographical Information:

Advisor:

School:The University of Hong Kong

School Location:China - Hong Kong SAR

Source Type:Master's Thesis

Keywords:melatonin receptors trophoblast prostate cancer

ISBN:

Date of Publication:01/01/2003

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