HDAC inhibitors down-regulate β5 proteasome subunit expression in proliferating WI-38 cells. (A) Proliferating WI-38 cells were treated with butyrate (BA) or TSA for 0, 24, 48, or 72 hours, as indicated. Cytosolic extracts were prepared and equivalent protein levels (determined by a Bradford assay) were analyzed by immunoblotting using an antibody specific for the β5 proteasome subunit. Actin served as a loading control. (B) Immunoblots from Figure 7A were quantified by optical densitometry. The graphical display shows the relative levels of the β5 subunit (normalized to actin) from the corresponding immunoblots (± standard error; n = 3). The decline in β5 levels was found to be significant at the indicated time points (**, p < 0.01).