Downregulation of the small GTPase ras-related nuclear protein accelerates cellular ageing.

The small GTPase Ran, Ras-related nuclear protein, plays important roles in multiple fundamental cellular functions such as nucleocytoplasmic transport, mitotic spindle assembly, and nuclear envelope formation, by binding to either GTP or GDP as a molecular switch. Although it has been clinically demonstrated that Ran is highly expressed in multiple types of cancer cells and specimens, the physiological significance of Ran expression levels is unknown.During the long-term culture of normal mammalian cells, we found that the endogenous Ran level gradually reduced in a passage-dependent manner. To examine the physiological significance of Ran reduction, we first performed small interfering RNA (siRNA)-mediated abrogation of Ran in human diploid fibroblasts.Ran-depleted cells showed several senescent phenotypes. Furthermore, we found that nuclear accumulation of importin alpha, which was also observed in cells treated with siRNA against CAS, a specific export factor for importin alpha, occurred in the Ran-depleted cells before the cells showed senescent phenotypes. Further, the CAS-depleted cells also exhibited cellular senescence. Indeed, importin alpha showed predominant nuclear localisation in a passage-dependent manner.Reduction in Ran levels causes cytoplasmic decrease and nuclear accumulation of importin alpha leading to cellular senescence in normal cells.The amount of intracellular Ran may be critically related to cell fate determination, such as malignant transformation and senescence. The cellular ageing process may proceed through gradual regression of Ran-dependent nucleocytoplasmic transport competency.