Osteoclastogenic activity of translationally-controlled tumor protein (TCTP) with reciprocal repression of p21

Translationally-controlled tumor protein (TCTP) plays a role in a number of cellular processes, but there is limited information about its function in cell differentiation. Previous observations of a twofold induction of TCTP mRNA during osteoclast differentiation prompted us to investigate its involvement in osteoclast differentiation. The osteoclastogenicity of TCTP gradually expressed during osteoclast differentiation was confirmed in mouse and human cells using loss-of-function studies and TCTP heterogeneous mice and transgenic mice. Higher expression ratios of TCTP to p21 could represent TCTP-mediated phenotypic induction of osteoclast differentiation accompanied by p21 down-regulation, attenuating the proliferation of osteoclast precursor cells.     

Control of translational repression by protein-protein interactions.

The coat protein of the RNA bacteriophage MS2 is a translational repressor and interacts with a specific RNA stem-loop to inhibit translation of the viral replicase gene. As part of an effort to dissect genetically its RNA binding function, mutations were identified in the coat protein sequence that suppress mutational defects in the translational operator. Each of the mutants displayed a super-repressor phenotype, repressing translation from the wild-type and a variety of mutant operators better than did the wild-type coat protein. At least one mutant probably binds RNA more tightly than wild-type. The other mutants, however, were defective for assembly of virus-like particles, and self-associated predominantly as dimers. It is proposed that this assembly defect accounts for their super-repressor characteristics, since failure to assemble into virus-like particles elevates the effective concentration of repressor dimers. This hypothesis is supported by the observation that deletion of thirteen amino acids known to be important for assembly of dimers into capsids also resulted in the same assembly defect and in super-repressor activity. A second class of assembly defects is also described. Deletion of two amino acids from the C-terminus of coat protein resulted in failure to form capsids, most of the coat protein having the apparent molecular weight expected of trimers. This mutant (dl-8) was completely defective for repressor activity, probably because of an inability to form dimers. These results point out the inter-dependence of the structural and regulatory functions of coat protein.     

Cyclin D-cdk6 complex is targeted by p21(WAF) in growth-arrested lymphoma cells

Normal human B lymphocytes are sensitive to the growth-inhibitory action of transforming growth factor beta1 (TGFbeta1) whereas malignant B lymphoma cells are mostly resistant to TGFbeta1 effects. We examined the phosphorylation status of retinoblastoma protein and the activity of G(1) cyclin-dependent kinases (cdk) in TGFbeta1-sensitive malignant follicular lymphoma cells during the TGFbeta1 treatment. The kinase activity of cdk2, cdk4, and cdk6 was significantly reduced and hypophosphorylation of pRb on serine 795 (S795) and threonine 373 (T373) was observed. We examined the composition of cdk complexes and the level of cdk inhibitors to explain the inhibitory action of TGFbeta1 toward cdk activity. Both cdk4 and cdk6 were notably dissociated from cyclin D cofactors, while cyclin E-cdk2 complexes remained coupled in TGFbeta1-treated cells. TGFbeta1-induced growth arrest was associated with notably increased binding of p21(WAF1) to cdk4 and cdk6. No induction of cdk-inhibitor molecules of INK family was observed in TGFbeta1-treated DoHH2 cells. As shown, TGFbeta1-induced growth arrest of malignant B cells was associated with the activation of CIP/KIP family members of cdk inhibitors.