vilofasen

Vilofasen, commonly translated as the G0 phase, is a quiescent state in the animal cell cycle where cells are not actively preparing for DNA replication. It represents a reversible exit from the active cell cycle (G1, S, G2, M). Many differentiated cells spend most of their life in G0; some cells can re-enter the cycle later, while others enter a permanent G0 during differentiation.

Entry into G0 is triggered by withdrawal of mitogenic signals, nutrient limitation, contact inhibition, and cellular differentiation programs. Molecularly, cells in G0 show reduced activity of cyclin-dependent kinases, increased levels of CDK inhibitors such as p27 Kip1, p21 Cip1, and p16 INK4a, and hypophosphorylated RB. Transcription of S-phase and mitotic genes is downregulated, and metabolism is reduced relative to proliferating cells.

Exit from G0 and re-entry into the cell cycle requires revival of mitogenic signaling, leading to cyclin D/CDK4/6 activation, RB phosphorylation, and release of E2F transcription factors. This reactivates transcription of genes necessary for G1 progression and S-phase entry. The duration of G0 can vary from hours to years, depending on tissue type and physiological conditions.

Significance is considerable in development, tissue homeostasis, and aging. In cancer, cells frequently bypass G0 control, contributing to unregulated proliferation. G0 is distinct from senescence, which is usually a permanent arrest associated with distinct metabolic and secretory phenotypes.

Examples include mature neurons and skeletal muscle cells that reside in G0, while hepatocytes and lymphocytes can re-enter the cycle from G0 upon tissue demand or immune activation.