Effect of HDAC inhibitor vorinostat in Hodgkin lymphoma (HL) cell lines. (A) Cells were incubated with 0.1 to 5 μM of vorinostat for 6 hours and whole cell lysates were examined for histone 3 acetylation (Ac-histone 3) using Western blot analysis. During this time frame, histone acetylation was observed using concentrations of 2 μM or higher. (B) Cells were incubated with 5 μM of vorinostat for 0.5 to 6 hours demonstrated that histone acetylation is achieved as early as 0.5 hours of incubation and lasted for at least 6 hours. (C) Vorinostat (5 μM) induced p21 expression as early as 6 hours and lasted for up to 48 hours. (D) Vorinostat induced antiproliferative effects in HL cell lines as determined by the MTS cell proliferation assay. This effect was prominent after 48 hours in culture and was dose dependent. Each value is the mean of 3 independent experiments performed in triplicate (± SEM). (E) The antiproliferative effect of vorinostat was associated with an increase in the G₂/M cell-cycle fraction, especially when higher concentrations were used. The G2M fraction was higher after 48 hours of incubation (bottom panel) compared with 24 hours of incubation (top panel). Results are the means of 3 independent experiments.