P21-dependent G(1) arrest with downregulation of cyclin D1 and upregulation of cyclin E by the histone deacetylase inhibitor FR901228

Depsipeptide, FR901228, a novel cyclic peptide inhibitor of histone deacety lase with a unique cytotoxicity profile is currently in phase I clinical tr ials. Here we demonstrate that, in addition to G2/M arrest, FR901228 causes G1 arrest with Rb hypophosphorylation. In vitro kinase assays demonstrated no direct inhibition of CDK activity, however, an inhibition was observed in CDKs extracted from cells exposed to FR901228. Cyclin D1 protein disappe ared between 6 and 12 hours after treatment with FR901228, whereas cyclin E was upregulated. While it did not induce wt p53, FR901228 did induce p21(W AF1/CIP1) in a p53-independent manner. Cell clones lacking p21 were not arr ested in G1 phase, but continued DNA synthesis and were arrested in G2/M ph ase following FR901228 treatment. Finally, FR901228 blunted ERK2/MAPK activ ation by EGF whereas early signal transduction events remained intact since overall cellular tyrosine phosphorylation after EGF stimulation was unaffe cted. Thus, FR901228, while not directly inhibiting kinase activity, causes cyclin D1 downregulation and a p53-independent p21 induction, leading to i nhibition of CDK and dephosphorylation of Rb resulting in growth arrest in the early G1 phase. In contrast to the GI arrest, the G2/M arrest is p21-in dependent, but is associated with significant cytotoxicity. (C) 2000 Cancer Research Campaign.