ATM, BRCA1, and Aurora A: Mechanisms of G2/M Checkpoint Control in Human Embryonic Stem Cells

When cultured in vitro, human embryonic stem cells (hESCs) acquire genetic abnormalities that have slowed their therapeutic use. As hESCs have a “leaky” G1/S boundary, the pressure of ensuring genetic integrity falls on the G2/M checkpoint, which can be activated by failed chromosomal decatenation (among other stimuli). It is hypothesized that hESCs have a deficient decatenation checkpoint, but little data supports this. Evidence suggests that the ataxia telangiectasia mutated (ATM) kinase controls the G2/M decatenation and DNA damage checkpoints, though previous reports are conflicting on this point. My work demonstrates that inhibition of decatenation activates ATM and arrests hESCs in G2. Pharmacologic inhibition of ATM (ATMi) abrogates this arrest, allowing hESCs to enter mitosis. Live cell imaging studies reveal that ATMi increases the time it takes to complete mitosis. Culture of cells under ATMi causes a gain of DNA content, which is reversed once ATMi is relieved. BRCA1, a known target of ATM, is also involved in the G2/M checkpoint. Experimental evidence reveals that activated ATM phosphorylates BRCA1, preventing Aurora A from interacting with and phosphorylating BRCA1 on S308, a modification necessary for mitotic entry. Together, this data illuminates a novel pathway by which ATM activation mediates G2 arrest in hESCs

Redox cell signaling and hepatic progenitor cells.

Hepatic diseases are widespread in the world and organ transplantation is currently the only treatment for liver failure. New cell-based approaches have been considered, since stem cells may represent a possible source to treat liver diseases. Acute and chronic liver diseases are characterized by high production of reactive oxygen and nitrogen species, with consequent oxidative modifications of cellular macromolecules and alteration of signaling pathways, metabolism and cell cycle. Although considered harmful molecules, reactive species are involved in cell growth and differentiation processes, modulating the activity of transcription factors, which take part in stemness/proliferation. It is conceivable that redox balance may regulate the development of hepatic progenitor cells, function and survival in synchrony with metabolism during chronic liver diseases. This review aims to summarize diverse redox-sensitive signaling pathways involved in stem cell fate, highlighting the important role of hepatic progenitor cells as a possible source to treat end-stage liver disease for organ regeneration