Tellimagrandin I Decreases Cyclin B1 Protein Levels through SP1 in MDA‐MB‐231 Cells

Consumption of walnuts has been shown to inhibit MDA‐MB‐231 breast cancer xenograft growth in nude mice. However, identification of compounds and mechanism of action have not been identified. We recently reported that a walnut extract treatment of the triple‐negative breast cancer cell type, MDA‐MB‐231 cells, could a) arrest the cells in G2/M and decreased cyclin B1 levels, b) decrease mitochondrial membrane potential and release cytochrome c from the mitochondria, and c) increase phosphotidyl serine flipping in the cell membrane, and that these events were associated with cell growth inhibition and apoptotic death. Over‐expression of Cyclin B1 has been shown in breast cancers that lack estrogen receptor, progesterone receptor, and HER2 (i.e. the triple‐negative breast cancer) and is a potential emerging biomarker as well as therapeutic target. In an attempt to conduct detailed mechanistic studies with active compounds from walnut extracts, we were able to identify numerous ellagitannins which induced cell death in breast cancer cells. Of the ellagitannins isolated, tellimagrandin I (TMI) showed the strongest cell death. We have gone on further to characterize the mechanism of action of TMI. We have performed time and dose response analysis of TMI on MDA‐MB‐231 cells for cell death and show that TMI has an IC50 of 60 μM. TMI is also able to reduce Cyclin B1 mRNA and protein levels similar to a walnut extract. We see that mRNA levels are reduced relatively quickly (within 1 hour) while protein levels begin to be reduced at a much later time suggesting that the effects are transcriptional. We have identified other cell cycle specific mRNAs that are also reduced by TMI. A careful examination of all mRNAs that are reduced show that they all have SP1 binding sites. Interestingly, we show that SP1 mRNA levels are also reduced upon TMI treatment in a similar fashion as Cyclin B1. Taken together our results suggest that TMI is able to reduce Cyclin B1 through inhibition of SP1 in MDA‐MB‐231 cells.Support or Funding InformationThis work was supported by a grant from the California Walnut Commission to BW.