Prolonged proteasome inhibition cyclically upregulates Oct3/4 and Nanog gene expression, but reduces induced pluripotent stem cell colony formation.

There is ample evidence that the ubiquitin-proteasome system is an important regulator of transcription and its activity is necessary for maintaining pluripotency and promoting cellular reprogramming. Moreover, proteasome activity contributes to maintaining the open chromatin structure found in pluripotent stem cells, acting as a transcriptional inhibitor at specific gene loci generally associated with differentiation. The current study was designed to understand further the role of proteasome inhibition in reprogramming and its ability to modulate endogenous expression of pluripotency-related genes and induced pluripotent stem cells (iPSCs) colony formation. Herein, we demonstrate that acute combinatorial treatment with the proteasome inhibitors MG101 or MG132 and the histone deacetylase (HDAC) inhibitor valproic acid (VPA) increases gene expression of the pluripotency marker Oct3/4, and that MG101 alone is as effective as VPA in the induction of Oct3/4 mRNA expression in fibroblasts. Prolonged proteasome inhibition cyclically upregulates gene expression of Oct3/4 and Nanog, but reduces colony formation in the presence of the iPSC induction cocktail. In conclusion, our results demonstrate that the 26S proteasome is an essential modulator in the reprogramming process. Its inhibition enhances expression of pluripotency-related genes; however, efficient colony formation requires proteasome activity. Therefore, discovery of small molecules that increase proteasome activity might lead to more efficient cell reprogramming and generation of pluripotent cells.