Fig. 2
From: The cell cycle as a brake for β-cell regeneration from embryonic stem cells
Cell cycle regulation of ES cells (a) and β cells (b). Upregulated components are indicated in red and inhibited proteins are indicated in blue. Levels of the Polycomb protein enhancer of zeste homologue 2 (EZH2) are increased during cell proliferation, which leads to a decrease in levels of the cell cycle inhibitors p21 and p16INK4A. a In ES cells, Cdk2–cyclin E is considered the primary cell cycle complex and, once activated, it phosphorylates retinoblastoma protein (pRB). Once pRB has been phosphorylated, E2F is released. The released E2F is then free to act as a transcription factor and it subsequently binds to DNA promoter regions and activates the expression of proteins required in the next stages of the cell cycle and in DNA replication. b In β cells, Cdk4–cyclin D is considered the primary cell cycle complex and is responsible for pRb phosphorylation