Effects of Proteasome Inhibitors on the Nucleolar Size of Porcine Oocytes

During the final stage of oocyte growth, the morphology of the oocyte nucleoli changes into a compact structure. The objective of this study was to determine the involvement of the proteasome, which is a large protein complex responsible for degrading intracellular proteins, in the nucleolar compaction. The mean nucleolar diameter of growing porcine oocytes (about 100 mu m in diameter) was larger than that of fully grown (120 mu m) oocytes (15.5 +/- 0.3 vs. 13.2 +/- 0.1 mu m, P<0.05). When fully grown oocytes were treated with proteasome inhibitors, MG132 (10 and 20 mu M) and lactacystin (100 and 200 mu M), the nucleolar diameter significantly increased from 12.9 mu m to 14.9-16.1 mu m. In contrast, transcription inhibitors, actinomycin D (0.8-8 mu M) and alpha-amanitin (10-100 mu M) reduced the nucleolar diameter of growing oocytes to 9.4-12.4 mu m. MG132 partially prevented this reduction in nucleolar diameter. These results suggest that the proteasome regulates the nucleolar size in porcine oocytes perhaps through the degradation of nucleolar proteins.