Novel extraribosomal function of human ribosomal protein rpL3

The nucleolus appears to be involved in additional cellular functions that may not be directly related to ribosome subunit biogenesis. In fact, perturbation of ribosome assembly has recently emerged as a relevant cell cycle arrest or apoptosis promoting pathway, relying on the dynamic sequestration or release of proteins in response to stress stimuli. In particular, it has been reported that upon nucleolar stress a subset of ribosomal proteins can be delocalized in the nucleoplasm where they can bind to MDM2, activating therefore p53 and its major target p21 to promote cell cycle arrest or apoptosis depending on the particular stress and cellular conditions. An increasing number of proteins have been proposed as positive regulators of p21 expression. It has been demonstrated that various protein factors involved in ribosome biogenesis such as nucleophosmin (NPM) can regulate p21 expression p53-independently at transcriptional and post-translational levels. In a previous study, we have reported a direct protein-protein interaction between NPM and the ribosomal protein rpL3, required for the autoregulatory circuit of the rpL3 expression. We wondered whether this interaction could occur also in another context and whether rpL3 could be involved in the regulation of p21 expression. In order to verify this hypothesis, we first analyzed changes in p21 protein levels in p53-null Calu-6 cells upon alteration of rpL3 production. We observed that the enforced expression of the rpL3 protein resulted in a dose-dependent increasing of p21 protein amount. In addition, we found that the rpL3 overexpression, associated with p21 upregulation, activates the apoptotic pathway through mitochondria in Calu-6 cells. Interestingly, we detected that the induction of apoptosis upon rpL3 overexpression was abrogated by p21 silencing, indicating that the mitochondrial apoptosis activated by rpL3 overexpression could be p21-dependent. The results of these experiments will be discussed