Nucleophosmin/B23: A multifunctional regulator that determines the fate of CCN2 mRNA

CCN2/CTGF is a multifunctional molecule that has been shown to play a central role in chondrocyte differentiation. During this process, the expression of ccn2 is tightly regulated to confer a maximal level at prehypertrophic - hypertrophic stages, in which the 3'-untranslated region (UTR) of the mRNA is critically involved in mediating its post-transcriptional regulation. In our previous studies, we found that a 40-kDa protein binding specifically to an RNA cis-element, 3'-100/50, in the 3'-UTR of the chicken ccn2 mRNA regulated the intracellular stability of the mRNA. The interaction of this 40-kDa protein with 3^′-100/50 was enhanced in proliferating chondrocytes, in which ccn2 mRNA is rapidly degraded; whereas a prolonged half life of ccn2 mRNA is observed in hypertrophic chondrocytes, where the interaction of the 40 kDa-protein and 3'-100/50 is diminished. Collectively, the data suggested that this 40-kDa protein acts as a ccn2-specific mRNA destabilizer during chondrocyte differentiation. In this present study we finally identified this 40-kDa protein as nucleophosmin (NPM)/B23. NPM is a nuclear-cytoplasmic shuttling protein that is characterized by its multiple functionality. This protein is known to be a histone chaperone, a regulator of ribosomal RNA transcription, as well as an RNA-binding post-transcriptional regulator of gene expression. In our hands, direct binding of NPM to 3'-100/50 was confirmed not only by RNA EMSA and UV crosslinking assays, but also by RNA immunoprecipitation analysis. By using recombinant chicken NPM, we could successfully reconstitute the post-transcriptional regulation of ccn2 by NPM in vitro and found that this regulation was more robust in chondrocytes than in fibroblasts. Furthermore, siRNA-mediated gene silencing of NPM in vivo clearly showed enhanced ccn2 gene expression and a prolonged half life of the ccn2 mRNA, confirming the functional property of NPM as a specific destabilizer of the ccn2 mRNA in living cells. The 5'-100/50 element, a target of NPM, is evolutionally conserved among vertebrate species. Therefore, we consider NPM to be a critical post-transcriptional regulator of ccn2 acting via 3'-UTR during endochondral ossification and possibly, in other physiological and pathological states as well.