Cancer Letters

Cancer Letters

Volume 151, Issue 2, 14 April 2000, Pages 133-143
Cancer Letters

Melatonin decreases cell proliferation and transformation in a melatonin receptor-dependent manner

https://doi.org/10.1016/S0304-3835(99)00394-8Get rights and content

Abstract

There are conflicting claims for the role of melatonin in oncogenesis. In addition, the mechanism(s) underlying melatonin's effects in oncogenic processes is (are) unknown. In this study, the effects of melatonin exposure on cell proliferation and transformation were assessed in NIH3T3 cells transfected with either the human mt1 (NIH-mt1) or MT2 (NIH-MT2) melatonin receptors. The effects of melatonin exposure on proliferation was assessed by direct cell counts and [3H]thymidine uptake assays. The effect of chronic melatonin pretreatment on transformation was assessed by focus assays. In both NIH-mt1 and NIH-MT2 cells, melatonin pretreatment decreased cell proliferation and transformation. Control (NIH-neo) cells did not show this effect. However, as revealed by the [3H]thymidine uptake assays, an increase in DNA synthesis occurred in NIH-mt1 cells, whereas no increase occurred in the NIH-MT2 or NIH-neo cells. Upon examination of melatonin receptors, a decrease in the function of both mt1 and MT2 receptors occurred. These data suggest that perhaps an attenuation of receptor-mediated processes are involved in the anti-proliferative and anti-transformation capabilities of melatonin in NIH3T3 cells. In addition, based on the [3H]thymidine assays, receptor mediated signal transduction mechanisms may slow the growth of cells via actions on the cell cycle. The results from this study shed new insight on the putative mechanisms underlying melatonin's effects on cell proliferation and transformation and lends support for a protective role of melatonin in oncogenesis.

Introduction

The effects of melatonin in oncogenesis is unclear, however, a majority of the studies conclude that the hormone has a protective role in the modulation of cancer [1], [2], [3], [4]. It is believed, though not conclusively proven, that there may be a link between the function of the pineal gland (which synthesizes and secretes melatonin) and tumor formation [5], [6], [7]. That is, in patients afflicted with certain types of cancers including leukemia, lymphoma, breast, or melanoma, hypertrophy of the pineal gland occurs [8]. Also, in many studies, those afflicted with cancer show higher daily blood levels of melatonin as compared to healthy individuals [9], [10], [11]. These studies suggest that melatonin synthesis and secretion from the pineal gland is increased in response to the development of cancer and, therefore, acts as a protective mechanism to control neoplasia [9], [10]. Although not tested experimentally, it is suggested that a feedback mechanism may occur between the pineal gland and proliferating cells. It is possible that the proliferating cells, but not resting cells, may be releasing (unknown) factors that trigger melatonin synthesis and/or secretion from the pineal gland [9]. However, this is merely speculation and further studies need to be performed to test whether this occurs.

The effects of melatonin on tumor formation in rats show similar results as humans. Pinealectomized rats have a higher incidence of dimethylbenzanthracene (DMBA)-induced tumor formation compared to intact rats. In addition, administration of melatonin to intact or pinealectomized rats reduces the incidence of DMBA-induced tumor formation when compared to rats given DMBA alone [12], [13]. Thus, melatonin may reduce the formation of tumors.

The mechanism underlying melatonin's inhibitory effects in oncogenesis may operate through its effects on cell proliferation. In one study using rats afflicted with R3327H Dunning prostatic adenocarcinoma, injection with melatonin results in a decrease in tumor weight and an increase in the doubling time of the tumor [14]. Similarly, melatonin exposure decreases the proliferation of SK-N-SH neuroblastoma cells [15], rat hepatoma AH130 cells [16], PC12 cells [17], human M-6 malignant melanoma cells [3], MCF-7 human breast cancer cells [2], [18], [19], and in fibroblasts derived from the skin of patients affected by systemic sclerosis [20]. However, decreases in cell proliferation may not always underlie the oncostatic actions of hormones or drugs. For example, treatment of Syrian hamster embryo (SHE) cells with estrogens (17β-estradiol and diethylstilbestrol) and anti-estrogens (tamoxifen, toremifene and ICI 164,134) increases the number of colony forming units (indicative of cellular transformation), without a concomitant increase in cell number [21]. Recently, however, it has been shown that melatonin decreases the invasive and metastatic properties of human MCF-7 breast cancer cells [22].

Even though numerous studies suggest that melatonin may play a protective role in oncogenesis, there is evidence to support an adverse effect of melatonin within the body. For example, administration of pharmacological doses of melatonin to C3H/He female mice results in their premature death due to the formation of reproductive tract tumors [23]. Furthermore, administration of melatonin during the morning has a stimulatory effect on tumor formation in mice bearing fibrosarcoma ascties or Ehrlich solid tumors, while late afternoon administration has an inhibitory effect [5].

Although these effects of melatonin within the body are unclear, they are most likely mediated through receptors. Activation of the mt1 melatonin receptor (formerly known as Mel1a) [24] by melatonin results in an inhibition of forskolin-induced cAMP formation [24], [25], [26], an inhibition of protein kinase A [25], [27] and an inhibition of the phosphorylation of cyclic AMP response element binding protein [25], [27]. Similarly, activation of the MT2 melatonin receptor (formerly known as Mel1b) [28] results in an inhibition of cAMP accumulation [28]. In support of melatonin receptor-mediated effects in oncogenesis, it is known that certain cancerous cell lines contain melatonin receptors including human MCF-7 breast cancer cells [29], mouse N1E115 neuroblastoma cells (unpublished data) and human B(E)C2 neuroblastoma cells (unpublished data). Besides plasma membrane-bound receptors, nuclear receptors may also be involved in melatonin's effects on oncogenesis as reviewed [30]. However, how these nuclear receptors mediate such effects remains unclear. Thus, melatonin, acting through its receptor(s) and ultimately via its signal transduction pathways may underlie the oncostatic effects of melatonin on cells.

To date, it is difficult to ascertain whether such effects of melatonin are mediated through receptors due to: (1) the unavailability of high-affinity and subtype-selective melatonin receptor antagonists and (2) because melatonin can penetrate cells [31], and produce its effects in a receptor-independent manner [32]. Since melatonin has the ability to exert its effects with or without a receptor, melatonin's effects on oncogenic mechanisms are difficult to interpret. Until selective antagonists are developed, another approach to use to study the mechanisms underlying melatonin's effects on cell proliferation and transformation is by use of transfected cell lines as models. Therefore, the direct effects of melatonin exposure on cells either containing no melatonin receptors (receptor-independent effects) or on cells expressing defined melatonin receptor subtypes (receptor-dependent effects) can be determined.

In the past, most of the research performed on cellular transformation or proliferation used, as their models, humans, animals or cells already in a cancerous state [3], [16], [22], [29]. Since melatonin may be able to promote oncogenesis in normal animals [23] and also because healthy individuals take melatonin to promote good health, it is necessary to examine the effects of pharmacological levels of melatonin in ‘normal’ tissue or cells. As a result, the goal of our study was to examine the effects of pharmacological concentrations of melatonin on cellular transformation and proliferation in a non-cancerous cell line and to determine whether melatonin receptors played a role.

Section snippets

Development of the cell lines

In this study, NIH3T3 cells were chosen because they display a well-characterized phenotype when transformed [33], [34]. Their signal transduction capabilities are also well-defined as others have used these cells for studying muscarinic cholinoceptors [34], serotonin 5HT2C receptors [35] and melatonin receptors [36]. At the beginning of each experiment, NIH-3T3 cells were initially grown to 60% confluence in 10-cm dishes in Dulbecco's modified Eagle's medium (DMEM), (Gibco-BRL, Grand Island,

Characterization of NIH3T3 cells transfected with the human mt1 and MT2 melatonin receptor

Expression of the human mt1 melatonin receptor in NIH3T3 cells did not change the pharmacology or function of these receptors when compared to receptors expressed in other tissues or cells [37] (Table 1). Similarly, expression of the human MT2 melatonin receptor in NIH3T3 cells yielded similar pharmacology and function of the MT2 receptor previously reported [36] (Table 1). In NIH-neo cells, no total specific binding of 2-[125I]iodomelatonin occurred and no inhibition of forskolin-induced cAMP

Discussion

This study has demonstrated that chronic pretreatment of ‘normal’ cells transfected with each of the melatonin receptor subtypes with pharmacological concentrations of melatonin: (1) reduced cellular transformation, (2) reduced cell proliferation, and (3) either reduced or stimulated DNA synthesis depending upon which melatonin receptor subtype was expressed in NIH3T3 cells. Thus, the effect of melatonin pretreatment on these processes was found to be receptor-dependent. In addition, these

Acknowledgements

The authors would like to thank Ms Ann Gorman for her help in the cell viability and autoradiographic studies. This work was supported by Faculty Start-up funds to PAW-E and by NIH R15NS37672 to MAM.

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