Rapid induction of histone hyperacetylation and cellular differentiation in human breast tumor cell lines following degradation of histone deacetylase-1

Quinidine inhibits proliferation and promotes cellular differentiation in h uman breast tumor epithelial cells. Previously we showed quinidine arrested MCF-7 cells in G(1) phase of the cell cycle and led to a G(1) to G(0) tran sition followed by apoptotic cell death, The present experiments demonstrat ed that MCF 7, MCF-7ras, T47D, MDA-MB-231, and MDA-MB-435 cells transiently differentiate before undergoing apoptosis in response to quinidine. The ce lls accumulated lipid droplets, and the cytokeratin 18 cytoskeleton was reo rganized, Hyperacetylated histone H4 appeared within 2 h of the addition of quinidine to the medium, and levels Were maximal by 24 h, Quinidine-treate d MCF-7 cells showed elevated p21(WAF1) hypophosphorylation and suppression of retinoblastoma protein, and down-regulation of cyclin D1, similar to th e cell cycle response observed with cells induced to differentiate by histo ne deacetylase inhibitors, trichostatin A, and trapoxin. Quinidine did not show evidence for direct inhibition of histone deacetylase enzymatic activi ty in vitro. HDAC1 was undetectable in MCF-7 cells 30 min after addition of quinidine to the growth medium. The proteasome inhibitors MG-132 and lacta cystin completely protected HDAC1 from the action of quinidine. We conclude that quinidine is a breast tumor cell differentiating agent that causes th e loss of HDAC1 via a proteasomal sensitive mechanism.