FIGURE 9.

The proteasome inhibitor MG132 blocks degradation of ERα by TPSF. A, shown is the time course of the effect of TPSF on ER levels. MCF-7 cells were plated as described under “Experimental Procedures.” After 4 days in MEM + 5% 1× CD-FBS, the medium was replaced with medium containing 10 nm E₂ with or without 10 μm TPSF. Cells were harvested at the indicated times, extracts prepared, and ERα protein levels were determined by Western blot as described under “Experimental Procedures.” B, MG132 reverses the down-regulation of ER by TPSF. Cells were treated as in panel A and maintained for 24 h in medium containing 10 nm E₂ with or without 10 μm and 10 μm MG132. Preparation of cell extracts and Western blotting were as described under “Experimental Procedures.”