Serotonin (5-hydroxytryptamine, 5-HT) has been described as a mitogen it) a variety of cell types and carcinomas. it exerts its mitogenic effect by interacting with a wide range of 5-HT receptor types. Certain studies suggest that some selective serotonin re-uptake inhibitors promote breast cancer in animals and humans. This Study attempts to clarify the role of serotonin in promoting the growth of neoplastic mammary cells. Expression of the 5-HT2A scrotoninergic receptor Subtype in MCF-7 cells was determined by RT-PCR, Western blotting, and immunofluorescence analysis. The mitogenic effect of 5-HT on MCF-7 cells was determined by means of the MTT proliferation assay. We have demonstrated that the 5-HT2A receptor Subtype is fully expressed in the MCF-7 human breast cancer cell line, in terms of encoding mRNA and receptor protein. Automated sequencing has confirmed that the 5-HT2A receptor present in this cell line is identical to the 5-HT2A receptor found in human platelets and in human cerebral cortex. Furthermore, this receptor was found by immunofluorescence to be on the plasma membrane. MTT proliferation assays revealed that 5-HT and DO1, a selective 5-HT2A receptor subtype agonist, stimulated MCF-7 cell. These results indicate that 5-HT plays a mitogenic role in neoplastic mammary cells. Our data also indicate that 5-HT exerts this positive growth effect on MCF-7 cells through, in part, the 5-HT2A receptor subtype, which is fully expressed in this cell line. (c) 2006 Elsevier Inc. All rights reserved.