RAC3 more than a nuclear receptor coactivator: a key inhibitor of senescence that is downregulated in aging.

Receptor-associated coactivator 3 (RAC3) is a nuclear receptor coactivator usually overexpressed in tumors that exerts oncogenic functions in the cytoplasm and the nucleus. Although as part of its oncogenic actions it was previously identified as an inhibitor of apoptosis and autophagy, its expression is required in order to preserve the pluripotency and embryonic stem cell self-renewal. In this work we investigated its role in cellular senescence. We found that RAC3 overexpression in the nontumoral HEK293 cells inhibits the premature senescence induced by hydrogen peroxide or rapamycin. The mechanism involves not only the inhibition of autophagy early induced by these stimuli in the pathway to senescence, but also the increase in levels and nuclear localization of both the cell cycle suppressors p53/p21 and the longevity promoters FOXO1A, FOXO3A and SIRT1. Furthermore, we found that RAC3 overexpression is required in order to maintain the telomerase activity. In tumoral HeLa cells its activity was inhibited by depletion of RAC3 inducing replicative senescence. Moreover, we demonstrated that in vivo, levels of RAC3 are downregulated in the liver from aged as compared with young rats, whereas the levels of p21 are increased, correlating with the expected senescent cell contents in aged tissues. A similar downregulation of RAC3 was observed in the premature and replicative senescence of human fetal WI-38 cells and premature senescence of hepatocyte HepG2 cell line. Taken together, all these results demonstrate that RAC3 is an inhibitor of senescence whose downregulation in aged individuals could be probably a tumor suppressor mechanism, avoiding the clonal expansion of risky old cells having damaged DNA.

The p160 nuclear receptor co-activator RAC3 exerts an anti-apoptotic role through a cytoplasmatic action.

The p160 nuclear receptor co-activators represent a family of molecules, which are recruited by steroid nuclear receptors as well as other transcription factors that are overexpressed in several tumors. We investigated the role of one member of this family on the sensitivity of cells to apoptosis. We observed that overexpression of the RAC3 (receptor-associated co-activator-3) p160 co-activator inhibits hydrogen peroxide-induced cell death in human embryonic kidney 293 (HEK293) cells. The mechanism involves the activation of anti-apoptotic pathways mediated through enhanced nuclear factor kappa B (NF-kappaB) activity, inhibition of caspase-9 activation, diminished apoptotic-inducing factor (AIF) nuclear localization and a change in the activation pattern of several kinases, including an increase in both AKT and p38 kinase activities, and inhibition of ERK2. Moreover, RAC3 has been found associated with a protein complex containing AIF, Hsp90 and dynein, suggesting a role for the co-activator in the cytoplasmatic nuclear transport of these proteins associated with cytoskeleton. These results demonstrate that there are several molecular pathways that could be affected by their overexpression, including those not restricted to steroid regulation or the nuclear action of co-activators, which results in diminished sensitivity to apoptosis. Furthermore, this could represent one mechanism by which co-activators contribute to tumor development.