Effects of melatonin on N-nitroso-N-methylurea-induced carcinogenesis in rats and mutagenesis in vitro (Ames test and COMET assay) Sergei A. Musatov a, b , Vladimir N. Anisimov b, * , Ve Âronique Andre  c , Carole Vigreux c , Thierry Godard c , Franc Ëois Sichel c a Department of Medical Biology and Genetics, I.P. Pavlov St. Petersburg State Medical University, St. Petersburg, Russia b Laboratory of Carcinogenesis and Ageing, N.N. Petrov Research Institute of Oncology, 68, Leningradskaya Str., Pesochny-2, St. Petersburg, 189646, Russia c UPRES-EA1772 and CJF INSERM 96-03, Universite  de Caen, 14032 Caen Cedex and Laboratoire de Cance Ârologie Experimentale, Centre FranC Ë ois Baclesse, 14076 Caen Cedex 05, France Received 10 September 1998; accepted 18 November 1998 Abstract The effect of melatonin, an indole hormone of the pineal gland, on the initiation of N-nitroso-N-methylurea (NMU)-induced carcinogenesis in rats and mutagenesis in vitro has been investigated. Two-month-old female LIO rats (groups 1 and 2) were exposed to a single injection of NMU (50 mg/kg of body weight, i.v.). Rats from group 2 were given melatonin orally (20 mg/l) from 18:00 to 09:00 h over 3 days (2 days before and 1 day after NMU injection). Animals from group 1 (control) were administered the solvent (ethanol/water, 1:1000). Rats were followed up to natural death or were sacri®ced when moribund. Tumors developed both in rats treated with NMU alone (50.0%) and in rats exposed to NMU plus melatonin (34.8%). The percentage of malignant tumor-bearing rats in group 2 (21.7%) was lower (P , 0:02) than that in the other group (41.7%). Melatonin also decreased the multiplicity of malignant tumors 1.3-fold and reduced the incidence of malignancies in some organs. Two in vitro tests were used for mutagenesis studies: the Ames test (strains TA 100 and TA 102 of Salmonella typhimurium) and the Single Cell Gel Electrophoresis assay (SCGE assay or COMET assay) performed on CHOK1 cells. Melatonin itself revealed no genotoxic effect in either of the tests. No protective action of melatonin (at doses of up to 2 mmol/ plate) towards NMU was found in the Ames test. In contrast, in the SCGE assay a slight, but statistically signi®cant (P , 0:001), dose-related anticlastogenic effect of melatonin (10 210 ±10 27 M) was observed. Thus, our data indicate that melatonin may act as an anti-initiating hormone in NMU-induced carcinogenesis and possess anticlastogenic activity towards NMU in CHOK1 cells. q 1999 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Melatonin; Ames test; DNA damage; SCGE assay; COMET assay; Chinese hamster ovary cells; N-Nitroso-N-methylurea; Carci- nogenesis; Rat 1. Introduction Melatonin, an indole hormone secreted in verte- brates mainly in the pineal gland, is known to modu- late the growth of spontaneous and induced neoplasms Cancer Letters 138 (1999) 37±44 0304-3835/99/$ - see front matter q 1999 Elsevier Science Ireland Ltd. All rights reserved. PII: S0304-3835(98)00365-6 * Corresponding author. Tel.: 17-812-4378607; fax: 17-812- 4378947. E-mail address: anisimov@anisimov.spb.ru (V.N. Anisimov)