Rapamycin prevents strong phosphorylation of p53 on serine 46 and attenuates activation of the p53 pathway in A549 lung cancer cells exposed to actinomycin D

The activation of the p53 pathway by 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR), a molecule that mimics metabolic stress, is attenuated by rapamycin, an inhibitor of mTOR kinase, immunosuppressant, and cancer drug. Rapamycin also extends lifespan in experimental animals. Because AICAR is a relatively weak activator of p53, we investigated whether stimulation of p53 by the strong activator actinomycin D is also sensitive to the inhibitory effect of rapamycin. In A549 lung cancer cells, activation of p53 by actinomycin D was associated with phosphorylation of p53 on Ser46. Rapamycin inhibited the accumulation of phospho-Ser46 p53, attenuated upregulation of some p53 target genes, and altered cell-cycle progression. Moreover, in cells exposed to actinomycin D, rapamycin attenuated the accumulation of PML, a protein that in some conditions stimulates Ser46 phosphorylation. However, Ser46 phosphorylation was not diminished in PML-knockdown cells, suggesting that in our system PML does not play a major role in stimulating p53 phosphorylation on Ser46. Knockdown of p53 diminished the upregulation of PML by stress-inducing agents, consistent with the idea that PML is a p53-regulated gene. Our data suggest that the attenuation of p53 phosphorylation on Ser46 may play a significant role in the biological activity of anti-aging rapamycin.