Abstract During reprogramming of somatic cells, heightened proliferation is one of the earliest changes observed. While other early events such as mesenchymal-to-epithelial transition have been well studied, the mechanisms by which the cell cycle switches from a slow cycling state to a faster cycling state are still incompletely understood. To investigate the role of Oct-3/4 in this early feature of reprogramming, we created a 4-Hydroxytamoxifen dependent Oct-3/4 Estrogen Receptor fusion (OctER). We show that OctER can substitute for Oct-3/4 to reprogram mouse embryonic fibroblasts to induced pluripotent stem cells. While over-expression of OctER or Klf4 individually did not affect cell proliferation, in combination, these factors hasten the cell cycle, in a tamoxifen dose-dependent manner, supporting a key role for OctER. Oct-3/4 + Klf4 increased proliferation by enhancing expression of Cyclin A2. We verified occupancy of endogenous Oct-3/4 and Klf4 at bioinformatically identified binding sites in the Cyclin A2 promoter in mouse embryonic stem cells (mESC). Using inducible OctER along with Klf4, we show dose-dependent induction of Cyclin A2 promoter-reporter activity and mRNA levels. Taken together, our results provide further evidence of the interdependence of pluripotency and the rapid cell cycle seen in mESC, and identify CyclinA2 as a key early target.

Inducible expression of Oct-3/4 reveals synergy with Klf4 in targeting Cyclin A2 to enhance proliferation during early reprogramming