Mimosine is a cell-specific antagonist of folate metabolism.

Iron deficiency and iron chelators are known to alter folate metabolism in mammals, but the underlying biochemical mechanisms have not been established. Although many studies have demonstrated that the iron chelators mimosine and deferoxamine inhibit DNA replication in mammalian cells, their mechanism of action remains controversial. The effects of mimosine on folate metabolism were investigated in human MCF-7 cells and SH-SY5Y neuroblastoma. Our findings indicate that mimosine is a folate antagonist and that its effects are cell-specific. MCF-7 cells cultured in the presence of 350 microm mimosine were growth-arrested, whereas mimosine had no effect on SH-SY5Y cell proliferation. Mimosine altered the distribution of folate cofactor forms in MCF-7 cells, indicating that mimosine targets folate metabolism. However, mimosine does not influence folate metabolism in SH-SY5Y neuroblastoma. The effect of mimosine on folate metabolism is associated with decreased cytoplasmic serine hydroxymethyltransferase (cSHMT) expression in MCF-7 cells but not in SH-SY5Y cells. MCF-7 cells exposed to mimosine for 24 h have a 95% reduction in cSHMT protein, and cSHMT promoter activity is reduced over 95%. Transcription of the cSHMT gene is also inhibited by deferoxamine in MCF-7 cells, indicating that mimosine inhibits cSHMT transcription by chelating iron. Analyses of mimosine-resistant MCF-7 cell lines demonstrate that although the effect of mimosine on cell cycle is independent of its effects on cSHMT expression, it inhibits both processes through a common regulatory mechanism.

Molecular cloning, characterization and alternative splicing of the human cytoplasmic serine hydroxymethyltransferase gene.

The human cytoplasmic serine hydroxymethyltransferase (CSHMT) gene was isolated, sequenced and its expression characterized in human MCF-7 mammary carcinoma and SH_5Y5Y neuroblastoma cells. The 23-kb gene contains 12 introns and 13 exons; all splice junctions conform to the gt/ag rule. The open reading frame is interrupted by 10 introns, two of which are positionally conserved within the human mitochondrial SHMT gene. The gene is expressed with 330 nucleotides of 5' untranslated message within three exons. The 5' promoter region does not contain a consensus TATA, and primer extension and 5'-RACE studies suggest that transcription initiation occurs at multiple sites. Consensus motifs for several regulatory proteins, including SP1, mammary and neuronal-specific elements, NF1, a Y-box, and two steroid hormone response elements, are present within the first 408 nucleotides of the 5' promoter region. The human gene is expressed as multiple splice variants in both the 5' untranslated region and within the open reading frame, all due to exon excision. The splicing pattern is cell-specific. At least six CSHMT mRNA splice forms are present in MCF-7 cells; the gene is expressed as a full-length message as well as splice forms that lack exon(s) 2, 9 and 10. In 5Y cells, the predominant form of the message lacks exon 2, which encodes part of the 5' untranslated region, but does not contain deletions within the open reading frame. Western analysis suggests that the CSHMT gene is expressed as a single full-length protein in 5Y cells, but as multiple forms in MCF-7 cells. Multiple tissue Northern blots suggest that the CSHMT message levels and alternative splicing patterns display tissue-specific variations.