Inhibition of GSK3 does not alter cyclin D1 levels: (A) MRC5 cells were treated for 3 hrs with MG132 alone, or in combination with 25 mM LiCl. The cells were then fixed and the level of cyclin D1 (left), or the level of phospho-Thr-286 cyclin D1 (right), for each cell was determined by image analysis and plotted against its level of DNA (with BrdU positive cells indicated as small, closed circles). (B) In an analogous experiment, MRC5 cells were treated with the indicated concentration of LiCl together with MG132. After 3 hrs the average level of phospho-Thr-286 cyclin D1 in each cell cycle phase was determined for each LiCl concentration indicated (0, 12.5, of 25 mM). (C) NIH3T3 cells were treated with MG132 together with 25 mM valproate (V), with 25 μM GSK3 inhibitor II (II), or with no treatment as a control (C). The average levels of phospho-Thr-286 cyclin D1 in each cell cycle phase of each treatment is shown.