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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