Role of PIM1 in the activation of p53 induced by RPS19 deficiency in colon and prostate cancer cell lines

Aim of this thesis was to address a possible relationship between PIM1 and p53 during ribosomal stress caused by RPS19 depletion. Initially, we showed that, in colon (HCT116) and prostate cancer (LNCaP) cells, RPS19 depletion induces: (1) a decrease of PIM1 level and (2) p53 activation. Then, using PIM1 inhibition with specific siRNA and PIM1 overexpression with plasmid or lentiviral vectors, we showed that PIM1 regulates p53 level. In addition, we found that RPS19 depletion causes an increase of phospho-AMPK and a decrease of phospho-AKT levels. Interestingly, both proteins are known regulators of p53 and therefore could play a role in the PIM1-dependent regulation of p53. We found that AMPK is not involved in p53 regulation whereas AKT may affect p53 activation through MDM2. Moreover, we have also found that the decrease of phospho AKT in response to ribosomal stress is dependent on PIM1. In conclusion, a number of experiments in different cell lines, led us to hypothesize the following pathway: 1) Ribosomal stress, caused by the depletion of RPS19, induces a reduction of PIM1 level. 2) PIM1 decrease causes a reduction of AKT phosphorylation on Ser 473 which in turn inhibit the MDM2 ubiquitin ligase activity with a consequent increase in p53 level. 3) The increase in p53 level activates the MDM2 transcription, raising the level of the protein.