Repression of the Aryl Hydrocarbon Receptor Is Required to Maintain Mitotic Progression and Prevent Loss of Pluripotency of Embryonic Stem Cells

Lack of cell cycle checkpoints and uninterrupted passage through S-phase continuously renew the embryonic stem (ES) cell population and maintain pluripotency. Here, we show that to regulate mitotic progression and pluripotency ES cells must keep the aryl hydrocarbon receptor (AHR), an environmental sensor and transcriptional regulator, in a persistent state of repression. This repression, however, is not always absolute, causing the AHR to fluctuate between reversible states of expression and repression, with a fraction of the cells escaping repression at any one time. Cells that escape AHR repression exhibit reduced levels of the pluripotency factors OCT4 and SOX2 and show an extended mitotic traverse time due to AHR-dependent MID1 repression and the subsequent disruption of the MID1-PP2A-CDC25B-CDK1 signaling pathway that regulates mitosis. Unlike the bulk of the cell population that differentiates into cardiomyo-cytes upon stimulation, AHR-expressing ES cells restrict cardiogenesis and commit to a neuroglia cell fate. It appears that the untimely expression of the Ahr gene needs to be repressed to maintain ES cell mitotic progression and prevent premature loss of pluripotency.