Growth-inhibitory activity of melatonin on human androgen-independent DU 145 prostate cancer cells

Glutathione depletion-induced apoptosis of Ha-ras-transformed NIH3T3 cells can be prevented by melatonin

It is well known that intracellular antioxidant glutathione (GSH) plays major roles in the maintenance of redox status and defense of oxidative stress. Ras, a small GTP-binding protein, may send growth-stimulating message to the nucleus through downstream Rac oncoprotein and superoxide (O(2*-)). These findings led us to investigate the effects of GSH and melatonin, a free-radical scavenger, on Ras-Rac-O(2*-)-related growth signal transduction. Our results demonstrate that overexpression of the inducible Ha-ras oncogene by isopropyl-beta-D-thiogalactoside (IPTG) increases the levels of reactive oxygen species (ROS, including O(2*-) and hydrogen peroxide (H(2)O(2))) and GSH in an Ha-ras-transformed NIH/3T3 fibroblast cell line. On the contrary, melatonin significantly suppresses ras-triggered cell growth by inhibiting the increase of ROS and GSH. Moreover, severe apoptosis of this transformed cell line occurred when the cell redox balance between ROS and GSH was dramatically changed in the presence of IPTG and L-buthionine-[S,R]-sulfoximine (BSO, a specific inhibitor of GSH synthetase). That BSO-induced cell apoptosis needs Ras to increase the ROS level was demonstrated by the free-radical scavenger melatonin. It effectively blocked cell apoptosis, but cell growth was also slowed without affecting Ras expression. Based on our studies, two approaches can be applied to treating ras-related cancers. One is utilizing melatonin to suppress cancer cell proliferation, and the other is utilizing BSO to induce cancer-cell apoptosis. Cotreatment of ras-related cancer cells with melatonin and BSO stops cell growth as well as apoptosis. Whether these cancer cells will undergo further regression or become recurrent merits investigation.

Melatonin and prostate cancer cell proliferation: interplay with castration, epidermal growth factor, and androgen sensitivity

BACKGROUND

Potential modulatory effects of melatonin on the proliferation of androgen-sensitive LNCaP and androgen-insensitive PC-3 and DU 145 prostate cancer cells were reported recently. In this study, we investigated the effects of combined melatonin and castration on LNCaP tumor growth in vivo, the interactions between melatonin and epidermal growth factor (EGF) on LNCaP cell proliferation, and melatonin actions on the proliferation of PC-3 and DU 145 cells.

METHODS

Tumor development and growth in castrated nude mice inoculated with LNCaP cells or in intact animals inoculated with DU 145 cells, with or without daily melatonin treatment, were monitored by observation and caliper measurement. MT(1) receptor expression in native or transfected prostate cancer cell lines was examined by immunocytochemistry or 2-[(125)I]iodomelatonin binding. Cyclin D1 expression in LNCaP cells was assessed by Western blotting, and cell proliferation was measured by thymidine incorporation and/or cell count.

RESULTS

Melatonin treatment was associated with further decreases in LNCaP tumor incidence and growth rate in castrated nude mice. Melatonin and 2-iodomelatonin (a melatonin receptor agonist) attenuated EGF-stimulated increases in LNCaP cell proliferation and cyclin D1 levels. Melatonin had no effect on the proliferation or growth of MT(1) receptor-expressing DU 145 cells, and of PC-3 cells in which MT(1) receptor protein was undetectable. The proliferation of transfected PC-3 cells expressing MT(1) receptor was unaffected by 2-iodomelatonin.

CONCLUSION

Together with previous data, the present results indicate synergistic action of melatonin and castration in inhibiting the growth of androgen-sensitive LNCaP tumor. Androgen-sensitive prostate cancer cell proliferation may be modulated by opposite changes in cyclin D1 levels induced by activated MT(1) and EGF receptors. In androgen-insensitive prostate cancer cells, MT(1) receptor-mediated signal transduction may become defective not only through changes in membrane receptor protein expression and/or functions, but also by means of alterations in downstream postreceptor signaling events.

Nuclear exclusion of the androgen receptor by melatonin

Androgen receptors (AR) play a crucial role in androgen-mediated processes and prostate cancer progression. The pineal hormone melatonin attenuates the androgen-dependent growth of benign and cancer prostate epithelial cells in vitro and may reverse clinical resistance to androgen ablation therapy in patients progressing on gonadotropin releasing hormone (GnRH) analogue. Where along the AR cascade does melatonin act remains to be determined. The effects of melatonin on AR localization, level and activity were assessed using androgen-insensitive prostate carcinoma PC3 cells stably transfected with a wild-type AR-expressing vector (PC3-AR).AR was localized to the PC3-AR cell nucleus in the absence of dihydrotestosterone (DHT). Melatonin caused a robust exclusion of the AR from the cell nucleus to the cytoplasm. The nuclear export inhibitor, leptomycin B prevented this process. The exclusion was selective since melatonin had no such effect on the nuclear localization of estrogen receptors alpha (ERalpha) in these cells. Melatonin also caused nuclear exclusion of the AR in the presence of DHT. In addition, it attenuated androgen induced reporter gene activity in PC3 cells co-transfected with the human AR and AR reporter plasmids. Elevated androgen concentrations counteracted melatonin's effects. Melatonin did not decrease AR level or androgen binding in the cells. The nuclear localization of the AR is a hallmark of its cellular activity. These data point to AR nuclear exclusion as a possible mechanism to attenuate androgen responses in target tissues.