The effects of melatonin, N‐acetylserotonin and serotonin on the growth and tyrosinase activity of SK‐Mel 23 and SK‐Mel 28 human melanoma cell lines were investigated. Binding assays were also performed to establish the nature of the binding site. SK‐Mel 28 cells were responsive to melatonin and its precursors, exhibiting a decrease in growth and an increase in tyrosinase activity after a 72 hr treatment. N‐acetylserotonin was as potent as melatonin, the minimal effective concentration (MEC, which is defined as the smallest concentration that elicits a measurable biological response, significantly different from control) being 10−8 m. Serotonin was the least potent (MEC  = 10−6 m). Both melatonin antagonists, prazosin and luzindole, exhibited no effect per se and reversed both responses to melatonin. SK‐Mel 23 cells, however, showed no significant responses to the indoleamines. Competition binding assays in SK‐Mel 28 cells demonstrated the presence of binding sites to 2‐[125 I]‐iodomelatonin, which was displaced by the unlabelled hormone, by both antagonists, and by N‐acetylserotonin. The curve adjustment of the displacement values with melatonin suggests the existence of two binding sites, with the following Ki values: 1.0 × 10−10 m and 6.5 × 10−6 m. Ki values for acetylserotonin, prazosin and luzindole were, respectively, 3.8 × 10−8 m, 1.2 × 10−8 m, and 8.3 × 10−6 m. Surprisingly, in SK‐Mel 23 cells, melatonin and luzindole were able to compete with the radioligand, with Ki values of 3.1 × 10−8 and 2.4 × 10−8 m, respectively. Our data suggest that SK‐Mel 28 cells probably possess high affinity binding sites to melatonin and, in addition, MT3 low affinity binding sites, because N‐acetylserotonin was as effective as the native hormone, and prazosin effectively blocked the actions of melatonin. Both sites are functional as demonstrated by the blockade promoted by both luzindole and prazosin on the proliferative and melanogenic responses. Although growth and tyrosinase activity of SK‐Mel 23 cells were not affected by melatonin or its precursors, this cell line possesses high affinity binding sites, which may be non‐functional, or trigger responses other than the ones herein investigated.

Skins of Potamotrygon reticulatus are light in color in vitro, exhibiting punctate melanophores. α‐Melanocyte stimulating hormone (EC50 = 4.58 × 10–9 M) and prolactin (EC50 = 1.44 × 10–9 M) darken the skins in a dose‐dependent manner. The endothelins ET‐1, ET‐2 and ET‐3, and the purines, ATP, and uracil triphosphate (UTP) were not able to induce either skin lightening or darkening. Forskolin and the calcium ionophore A23187 promoted a dose‐dependent darkening response, whereas N2, 2′‐O‐dibutyryl guanosine 3′‐5′‐cyclic monophosphate (db cyclic GMP), phorbol‐12‐myristate‐13‐acetate (TPA), and 1‐oleoyl‐2‐acetyl‐sn‐glycerol (OAG) were ineffective. The maximal response obtained with the calcium ionophore A23187 was only 76% of maximal darkening. These results indicate that the cyclic adenosine 3′‐5′‐monophosphate (cAMP) pathway is probably involved in the pigment dispersion of P. reticulatus melanophores. Other experiments should be done to further investigate how cytosolic calcium may be physiologically increased, and the existence of a putative cross‐talk between calcium and cAMP signals. In conclusion, the only hormones effective on P. reticulatus melanophores were prolactin and α‐MSH. No aggregating agent has been shown to antagonize these actions. Prolactin effect on elasmobranch melanophores adds a novel physiological role to this ancient hormone. J. Exp. Zool. 284:485–491, 1999. © 1999 Wiley‐Liss, Inc.

The biological effects of catecholamines in mammalian pigment cells are poorly understood. Our previous results showed the presence of alpha(1)-adrenoceptors in SK-Mel 23 human melanoma cells. The aims of this work were to (1) characterize catecholamine effects on proliferation, tyrosinase activity and expression, (2) identify the alpha(1)-adrenoceptor subtypes, and (3) verify whether chronic norepinephrine (NE) treatment modified the types and/or pharmacological characteristics of adrenoceptors present in SK-Mel 23 human melanoma cells. Cells treated with the alpha(1)-adrenergic agonist, phenylephrine (PHE, 10(-5) or 10(-4) M), for 24-72 h, exhibited decreased cell proliferation and enhanced tyrosinase activity, but unaltered tyrosinase expression as compared with the control. The proliferation and tyrosinase activity responses were inhibited by the alpha(1)-adrenergic antagonist prazosin, suggesting they were evoked by alpha(1)-adrenoceptors. The presence of actinomycin D, a transcription inhibitor, did not diminish PHE-induced effects. RT-PCR assays, followed by cloning and sequencing, demonstrated the presence of alpha(1A)- and alpha(1B)-adrenoceptor subtypes. NE-treated cells (24 or 72 h) were used in competition assays, and showed no significant change in the competition curves of alpha(1)-adrenoceptors as compared with control curves. Other adrenoceptor subtypes were not identified in these cells, and NE pretreatment did not induce their expression. In conclusion, the activation of SK-Mel 23 human melanoma alpha(1)-adrenoceptors elicit biological effects, such as proliferation decrease and tyrosinase activity increase. Desensitization or expression of other adrenoceptor subtypes after chronic NE treatment were not observed.

Organic molecules with DNA-damage ability are of great potential in the development of medicine, toxicology, biochemistry, organic chemistry, biotechnology and gene therapy. DNA damaging agents have historically played a central role in cancer therapy. Even as new approaches to cancer therapy become available, it seems likely that there will be a continued need for the study and development of novel DNA damaging cytotoxins. These agents will see continued use due to their well-establised role in treating various types of cancer and because many of the new approaches to cancer treatment such as immunoteraphy and modulation of the cell cycle are most effective when used in combination with traditional cytotoxins. It is commonly believed that natural products which display potent biological activity are results of natural selection. DNA-damaging natural products frequently possess potent cytotoxic, cytostatic or mutagenic properties and, in nature, may serve as either offensive or defensive weapon in the struggle for survival. Natural products constitute a vast library of organic compounds that can serve as a useful force. A practical reason for the longstanding interest in DNA-damaging natural products is the fact that the cytotoxic or cytostatic effects of these agents sometimes endow them with useful medicinal properties, especially as potential anticancer therapeutics. Several DNA-damaging natural products are currently in use for the treatment of various cancers and others have served as lead compounds in the development of therapeutic agents. Many anti-cancer agents work by alkylating DNA while others destroy DNA by radical chemistry, starting either (i) by abstracting a hydrogen atom from a deoxyribose sugar or (ii) by adding to the alkene pi bond in a base.DNA-damaging agents can be placed into four chemical categories: Intercalators, alkylating agents, DNA strand breakers and groove binders. In this review, DNA damage mechanisms of anti-cancer drugs are

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Adults of Rana catesbeiana maintained for 4 days in 12:12 light/dark regimen exhibited a rhythmic color change of 24 hr. Under constant light, however, the rhythm disappeared, and the reflectance values gradually became greater, that is the animals became lighter. Under constant darkness, the rhythm was also abolished, but the animals tended to a darker color. On black background the skin darkening proceeded at a faster rate as compared to the skin lightening of animals adapting to a white background. The difference in color change rate suggests that the darkening responses are probably mediated by an increase in a circulating hormone, whereas skin lightening probably results from the serum level decrease of the same hormone. Most certainly, this hormone is alpha-MSH, as the in vitro assays demonstrated its high potency as a full darkening agonist (EC50 = 9 x 10(-10) M). Prolactin (EC50 = 7.7 x 10(-8) M) and endothelins 2 (EC50 = 1.3 x 10(-6) M) and 3 (EC50 = 4.8 x 10(-7) M) were also full agonists, but 100- to 1000-fold less potent than alpha-MSH. Isoproterenol, in the absence or presence of dibenamine, and endothelin-1 also elicited darkening responses in a dose-related manner, but reaching only 23% and 35% of the maximal darkening, respectively. Isoproterenol darkening effect was completely blocked by propranolol, confirming its action through beta-adrenoceptors. These results, taken together with the lack of lightening activity of norepinephrine on alpha-MSH-darkened skins, suggest that R. catesbeiana melanophores do not possess very active beta-adrenoceptors and lack alpha-adrenoceptors. On the other hand, the lightening agonist melatonin elicited only half-maximal dose-dependent reversal of MSH-induced darkening. Our results suggest that the chromatic rhythm is not endogenous, and most likely is determined by the light/dark cycle effect on alpha-MSH secretion.

Vertebrates have a central clock and also several peripheral clocks. Light responses might result from the integration of light signals by these clocks. The dermal melanophores of Xenopus laevis have a photoreceptor molecule denominated melanopsin (OPN4x). The mechanisms of the circadian clock involve positive and negative feedback. We hypothesize that these dermal melanophores also present peripheral clock characteristics. Using quantitative PCR, we analyzed the pattern of temporal expression of Opn4x and the clock genes Per1, Per2, Bmal1, and Clock in these cells subjected to a 14-h light:10-h dark (14L:10D) regime or constant darkness (DD). Also, in view of the physiological role of melatonin in the dermal melanophores of X. laevis, we determined whether melatonin modulates the expression of these clock genes. These genes show a time-dependent expression pattern when these cells are exposed to 14L:10D, which differs from the pattern observed under DD. Cells kept in DD for 5 days exhibited overall increased mRNA expression for Opn4x and Clock, and a lower expression for Per1, Per2, and Bmal1. When the cells were kept in DD for 5 days and treated with melatonin for 1 h, 24 h before extraction, the mRNA levels tended to decrease for Opn4x and Clock, did not change for Bmal1, and increased for Per1 and Per2 at different Zeitgeber times (ZT). Although these data are limited to one-day data collection, and therefore preliminary, we suggest that the dermal melanophores of X. laevis might have some characteristics of a peripheral clock, and that melatonin modulates, to a certain extent, melanopsin and clock gene expression.

In vivo and in vitro assays were performed with S91 murine melanoma cells aiming to investigate the effects of testosterone and photoperiod on tumor growth and melanogenesis (tyrosinase activity). In vivo assays were performed by inducing melanoma tumors in castrated mice receiving increasing concentrations of testosterone and submitted to varying photoperiod regimens. The results demonstrated that the increase of melanin content was higher in animals submitted to the longest days, thus demonstrating the importance of photoperiod length in melanin synthesis. Increase in tumor growth and protein content was observed in testosterone-treated animals submitted to 12L:12D; in testosterone-treated animals submitted to 4L:20D and 20L:4D tumor growth was significantly smaller. In S91 cultured cells, testosterone increased cell proliferation and reduced tyrosinase activity in a dose-dependent manner. Radioactive binding assays demonstrated that the hormone was acting through low affinity testosterone receptors, since the presence of aromatase inhibitor did not affect the binding assay in a statistically significant way, and all the in vitro experiments were performed in the presence of the inhibitor. Our in vivo data added to the in vitro results corroborate the hypothesis that S91 melanoma cells directly respond to testosterone and that this effect is modulated by light.