Effect of Se-methylselenocysteine (MSC) on phosphatidylinositol 3-kinase (PI3-K) activity in TM6 cells. Synchronized TM6 cells were treated with 100 μM MSC at 6 hours as described in Fig. 1. Another set of TM6 cells were pretreated with 1 μM wortmannin (WOR) for 30 min before the 1 hour stimulation with fresh DMEM/F12 medium containing growth factors (5 ng/ml epidermal growth factor and 10 μg/ml insulin) and serum (2% adult bovine serum). PI3-K activity was performed on 500 μg of protein lysates as described in the Materials and methods section. The kinase activity in the control cells increased within 1 hour of stimulation, and was strongly inhibited by WOR. The PI3-K activity in MSC-treated cells at 16 and 24 hours were drastically lowered compared with that of the control cells. The data are a representative of experiments performed in triplicate for each time point.