Lunasin is an anti-mitotic agent that has been shown to inhibit cell cycle progression and tumor initiation by acting on the DNA of neoplastic cells. It is found in soybeans, oats and barley and is used in the treatment of cancer, high cholesterol, heart disease and rheumatoid arthritis (RA).
Several mechanisms have been proposed to explain the anti-mitotic activity of lunasin. One is its ability to bind and demethylate histone H3 and histone H4 N-terminal tail acetylation. Inhibition of H3 and H4 N-terminal tail acetylation is commonly associated with transcriptional silencing and downregulation of genes.
Another mechanism is its ability to upregulate the expression of chemopreventative genes by binding to hypoacetylated histone H4 and increasing the acetylation of Lysl6 residues on the H4 N-terminal tail. Inhibition of histone acetylation and hypoacetylated histones are also correlated with the inhibition of cell cycle progression and apoptosis.
Inhibition of H3 and H4 acetylation by lunasin can be explained in part by its chromatin-binding domain, which contains a short helical region homologous to that of other chromatin-binding proteins. This helical region is essential for the chromatin-binding activity of lunasin.
The helix of the lunasin helical structure is very tightly bound to the phosphate end of the acetylated histone, forming a hydrogen bond with the acetylated phosphate. The hydrogen bond formed between the acetylated phosphate of histone H4 and the helix of lunasin is an important step in the activation of this chromatin-binding activity.
Histone acetylation experiments using a set of recombinant histones showed that lunasin could bind to H4-Lysl6 with high affinity in vitro but not in vivo. However, when the secondary co-activator pCAF was added to the recombinant histones it was able to significantly increase the H4-Lysl6 acetylation.