(-)-Epigallocatechin-3-gallate and DZNep reduce polycomb protein level via a proteasome-dependent mechanism in skin cancer cells

Polycomb group (PcG) protein-dependent histone methylation and ubiquitination drives chromatin compaction leading to reduced tumor suppressor expression and increased cancer cell survival. Green tea polyphenols and S-adenosylhomocysteine (AdoHcy) hydrolase inhibitors are important candidate chemopreventive agents. Previous studies indicate that (-)-epigallocatechin-3-gallate (EGCG), a potent green tea polyphenol, suppresses PcG protein level and skin cancer cell survival. Inhibition of AdoHcy hydrolase with 3-deazaneplanocin A (DZNep) inhibits methyltransferases by reducing methyl group availability. In the present study, we examine the impact of EGCG and DZNep cotreatment on skin cancer cell function. EGCG and DZNep, independently and in combination, reduce the level of PcG proteins including Ezh2, eed, Suz12, Mel18 and Bmi-1. This is associated with reduced H3K27me3 and H2AK119ub formation, histone modifications associated with closed chromatin. Histone deacetylase 1 level is also reduced and acetylated H3 formation is increased. These changes are associated with increased tumor suppressor expression and reduced cell survival and are partially reversed by vector-mediated maintenance of Bmi-1 level. The reduction in PcG protein level is associated with increased ubiquitination and is reversed by proteasome inhibitors, suggesting proteasome-associated degradation.